PRODUCTIVITY OF GLOBAL PLANT COVERS

Average values of the productivity of natural plant covers of Earth have been derived by using various theoretical and numerical models and data from a variety of studies including empirical determinations of productivity in individual biogeographical zones.

Terrestrial plant covers

Yefimova (1979) has made use of quite precise relationships between productivity of natural plant cover and meteorological factors in calculating values of the productivity and the coefficient of utilisation of photosynthetically active radiation for each continent. Results of her calculations are given in the Table-1. The data shows that the average productivity per unit area for the five continents of Earth does not differ very much. In each of these continents, magnitude of productivity over large part of continental territory is greatly limited by insufficient moisture or heat. The continent of South America is exception to this general condition since climatic conditions over large part of its territory are favourable for plant life.

Table 1: Productivity and coefficients of utilization of photosynthetically active radiation in various continents of Earth. (Yefimova 1979)

Continent	Productivity (x109 tonnes)	Productivity (center per hectare)	Coefficient of utilization of photosynthetically active radiation (as %age of total over vegetative period)
Europe Asia Africa North America South America Australia (including islands of Oceania)	8.9 38.3 31.0 18.1 37.2 7.6	85 98 103 82 209 86	1.26 0.88 0.59 0.94 1.13 0.44

In Australia and Africa, coefficients of utilization of photosynthetically active radiation are lower than average. This can be attributed to insufficient moisture over large parts of these continents, which inhibits the complete utilization of available radiation by plant covers.

In Europe and South America, most favorable conditions for the development of plant life are found. In Europe, located at higher latitudes and exposed to less solar radiation, its utilization is relatively greater.

Smil (1985) gave estimates of the productivity and storage of biomass in major biomes of the Earth. These estimates are given in Table-2. Data in this table shows that there is not much difference in the area occupied by different types of ecosystems except wetlands that occupy smallest area on the Earth. However, productivity is highest in cultivated lands where one ton of biomass is produced per one ton of phytomass, followed by tropical and temperate grasslands where 0.5 ton of biomass is produced by each ton of phytomass. Next in productivity are tundra, deserts-semi-deserts and wetlands where 0.2 tonnes of biomass is produced per hectare from one ton of phytomass per hectare. These areas are followed by wetlands and shrub-lands where productivity is 0.13 tonnes per hectare. Tropical, temperate and boreal forest, though occupy almost same area on Earth, produce 0.067, 0.04 and 0.02 ton of biomass per tone of phytomass per hectare respectively. Despite these facts, most important on Earth are tropical, temperate and boreal forests that have the highest concentration of biomass on Earth (totaling about 750 tonnes per hectare). These ecosystems also have the highest total storage of biomass on Earth totaling about 850 x 10⁹ tonnes. Further, it may be noted that contribution to total biomass production is equal for tropical rainforests and tropical grasslands (20 x 10⁹ t/yr), followed by boreal forests and tropical grasslands (15 x 10⁹ t/yr) and temperate forests, woodlands-shrub-lands and temperate grasslands (10 x 10⁹ t/yr). Tundra and deserts have quite high average of net biomass production per unit area and also quite high weight of phytomass per unit area. Despite this they contribute very little to total global biomass production (1.0 – 2.0 x10⁹ t/yr). However, if total biomass storage in different types of ecosystems on Earth is considered, tropical rainforests, temperate forests and boreal forests are the most important storehouses of organic matter on Earth having 850×10⁹ tonnes of biomass. Woodland and shrub-lands having 75×109 tonnes and then tropical and temperate grasslands having 60×109 tonnes of biomass storage follow these.

Table 2: Area, productivity and storage of major global ecosystems. (Smil, 1985)

Ecosystem	Total area (x106km2)	Average net production (tonnes/ha)	Average phytomass (tonnes/ha)	Total production (x109tonnes/year)	Total storage (x109tonnes/year)
Tropical rainforest Temperate forests Boreal forests Woodland and shrub-land Tropical grasslands Temperate grasslands Cultivation Tundra Deserts and semi-deserts Wetlands Settlements and transport	10.0 10.0 15.0 10.0 10.0 10.0 15.0 10.0 20.0 5.0 5.0	20.0 10.0 10.0 10.0 10.0 10.0 10.0 1.0 1.0 15.0 5.0	300.0 250.0 200.0 75.0 20.0 20.0 10.0 5.0 5.0 75.0 5.0	20.0 10.0 15.0 10.0 20.0 10.0 15.0 1.0 2.0 8.0 3.0	300.0 250.0 300.0 75.0 40.0 20.0 15.0 5.0 10.0 40.0 3.0
Total				114.0	1058.0

Aquatic plant covers

There is much less data about productivity of autotrophic plant covers in water bodies as compared to that about terrestrial plant covers. However, the available data indicates that the seas and oceans have the greatest volume of organic matter produced by phytoplankton located in the 30-40 meters deep layer of hydrosphere. At greater depths, quantity of solar radiation is insufficient for active development of photosynthesis.

In general, the productivity of shelf zones is substantially less than open ocean. It may attain maximum values in small bodies of water possessing large quantities of minerals required by the plants. The overall value of productivity for the oceans is estimated to be about 55 billion tonnes per year i.e. approximately 15 centner per hectare. This last figure is less than 1/6^th of the average productivity per unit area on continents.

Thus the estimates show that the yearly volume of productivity for the Earth as a whole is approximately 200 billion tonnes i.e. about 40 calories per hectare. This corresponds to an energy expenditure of approximately 0.15 kcal/cm² per year. This is about 0.1% of the solar radiation reaching the Earth’s surface.

ORGANIC MATTER IN EARTH’S ENVIRONMENT

In the environment, organic matter is synthesized in its biotic component i.e. biosphere. Autotrophic organisms are the only organisms that can synthesize organic matter using solar radiation and mineral matter taken from atmosphere, hydrosphere and edaphosphere. Autotrophs synthesize organic matter either by photosynthesis or by chemosynthesis. While chemosynthesis is important for cycling of nitrogen and certain other processes in the environment, photosynthesis is the major process responsible for formation of organic matter in the environment. Autotrophic green plants, particularly land plants are most important from the point of view of photosynthetic production of organic matter. In the photosynthesis, carbon dioxide and water are used and a certain portion of short-wave solar radiation is absorbed and expended within the plant cover. In considering the role of plant cover of Earth in the global energy and water balance, it is necessary to consider the amount of solar radiation and water utilised by plants in production of biomass i.e. in photosynthesis. For this, the quantities that are calculated and studied are efficiency of photosynthesis and productivity of transpiration.

1. Efficiency of photosynthesis: It is the ratio of energy expenditure on the synthesis of biomass to the total quantity of solar energy absorbed by plant cover in an area. Many experimental studies have shown that this efficiency of photosynthesis is very modest and under normal conditions, usually does not exceed 0.1 to 1.0 percent. However, under very favorable conditions, it may increase to several percent.

2. Productivity of transpiration: It is the ratio of the amount of biomass produced to the quantity of water transpired by photosynthesizing plant cover. This productivity of transpiration usually ranges from 0.5 to 0.1 percent which indicates that photosynthesizing plants use very little water and abundant transpiration in them merely circulates the water in the environment.

Thus general low values of both the above quantities indicate that under natural conditions, plant cover assimilates only a negligible part of available energy and water resources i.e. there is substantial limitation on the use of natural resources in production of biomass in the environment. It is important to establish the causes of this limitation for the study of the relationship of productivity of plant cover to climatic factors. Experimental studies of maximum possible efficiency of photosynthesis in controlled environmental conditions when carbon dioxide of the atmosphere is fully utilized indicate that under such conditions, plants can assimilate 5% or more of the solar energy received and the productivity of transpiration also increases manifold. However, in natural conditions maximum possible photosynthesis and, therefore, the production of biomass is greatly limited by various factors other than the availability of resources.

PHOTOSYNTHESIS WITHIN TERRESTRIAL PLANT COVER

In nature, most of the photosynthesis takes place within the terrestrial plant cover in which different meteorological conditions exist at different levels. The efficiencies of photosynthesis at various levels within the plant cover are not same and are determined by particular microclimatic (meteorological) conditions prevailing at different levels. The microclimatic effects of a forest cover are explained in terms of:

1. Plant coverage characteristics: These characteristics depend upon:

1. 1. Density of dominant forms in the forest covers.

   2. Distribution of different forms in the forest covers.

2. Stratification characteristics of plant cover: These characteristics depend upon:

   1. Total vertical height of plant cover.

   2. Number of vertical strata in the plant cover.

   3. Morphological characteristics of each strata in the plant cover which are determined by branching pattern of plants, evergreen or deciduous nature of foliage, size, density, texture and orientation of leaves.

Importance of the above features can be judged from comparison of tropical and temperate forest plant covers. In tropical forests, average height of tall trees is 46-55 metres, species diversity is 40-100 species per hectare, stratification is strong with 4-5 strata, undergrowth is dense commonly with two upper foliage strata and lower strata being denser. In temperate forests, average height of tall trees is about 30 metres, species diversity is less than 20 species per hectare, stratification is poor with usually 2-3 strata which are almost continuous from low shrubs to top of trees.

In the study of photosynthesis at various levels within plant cover, averaging of the values of meteorological elements at one level along the horizontal line is appropriate and it makes it possible to exclude the influence of individual plants on the meteorological regime. By applying such averaging techniques, following conclusions have been established:

1. Microclimate within the plant cover may be represented by a series of vertically varying profiles of meteorological elements, particularly of solar radiation, water vapor pressure, air temperature, carbon dioxide concentration and wind speed.

2. The profiles of meteorological elements show diurnal and seasonal variations.

3. The average vertical flow of short wave and long-wave radiation, heat and water vapor within the layer of plant covers and the momentum of the system depend substantially on height.

Microclimatic profiles within plant cover

1. Solar radiation: Plant cover significantly changes the pattern of incoming and outgoing radiation. Short-wave reflectivity of area depends somewhat on the density and characteristics of the plant cover. The albedo of areas having coniferous forests is about 8-14 while that having deciduous forests is about 12-18. Albedo of semiarid savannas and woodlands is much higher.

Large amount of solar radiation is trapped within the foliage canopy e.g. Fagus sylvetica forest traps about 80% of incoming radiation in the top strata of canopy and less than 5% reaches the ground. Such trapping is more pronounced on sunny days.

The foliage canopy absorbs more short-wave radiation than long-wave infrared radiation e.g. in tropical forests of Nigeria, only 7.6% radiation of <0.5 m reaches the ground while 45.3% of radiation of >0.6 m reaches the forest floor.

Effect of the age of plant cover on the penetration of light into the plant cover can be judged by the observation that in Pinus sylvestris forest in Germany, percentage of light reaching the ground floor is about 50% at 1.3 year, only 7.0% at 20 years and again 35% at 130 years.

Penetration of solar radiation within the plant cover generally obeys Bougner-Lambert Law:

I = I_o e^-KL

Where, I = radiation intensity on a horizontal plane within the plant cover; I_o = radiation intensity on a horizontal plane above the plant cover; L = leaf area index; K = Extinction coefficient.

Extinction coefficient (K) is constant for a given species and is related to:

• Amount and type of leaf chlorophyll.
• Canopy architecture and
• Reflectivity of leaves. Its value lies between 0.3 and 0.5 for grass-type plant cover and approaches 1.0 for nearly horizontal leaves. Value of K shows inverse relationship to chlorophyll content and reflectivity of leaves.

In general, light penetration into plant cover depends upon the type of plants (particularly trees), spacing of plants, age of plants, crown density, height of plants (particularly trees) and time of year. Percent light reaching the forest floor in some types of forests is given below:

• Birch-beech forest 50-75%
• Pine forest 20-40%
• Spruce-fir forest 10-25%
• Tropical forest 0.1-0.01%

In deciduous forests, light penetration increases during leafless conditions.

Thus the intensity of solar radiation decreases exponentially from top of plant cover towards Earth’s surface due to absorption and radiation scattering by the surface of plants. The resulting radiation balance, therefore, also decreases in the same direction due to screening effect of plants.

1. Air temperature: During day time, heating of foliage canopy causes a convectional transfer of sensible heat and so air temperature within upper canopy may be higher than above the canopy or below. At night, the relationship is reversed as upper canopy layer of air is cooled by contact which are both losing heat by radiation and also transpiring slowly.

Modification of thermal environment is due to shelter from sun, blanketing at night, heat loss by evotranspiration, reduction in wind speed and obstruction to vertical airflow.

Blanketing causes lower maximum and higher minimum temperatures and causes lower mean monthly temperatures in tropical and temperate forests.

At sea level, mean monthly differences in air temperature in temperate forest may reach 2.2^OC in summer but only 0.1^OC in winters. In hot summers, this difference can be more than 2.8^OC.

In forests, which do not transpire greatly in summers e.g. forteto oak maquis of Mediterranean area, day temperatures in woods may cause mean monthly temperatures to be higher than in open.

Altitude in the same climatic zone may affect the degree of temperature decrease in temperate forests. At 1000 meters altitude, lowering of temperatures may be twice that at sea level.

Vertical stratification in plant cover modifies the thermal profile within it in complex ways. In tropical forests, dense foliage canopy heats up greatly during daytime and cools rapidly during night. It shows a much greater diurnal temperature range in denser canopy than in the lower strata. Whereas daily temperatures of second story are intermediate between those of the tree tops and undergrowth, the nocturnal minima are higher than either tree tops or undergrowth because the second story is insulated by trapped air both below and above.

1. Saturation water vapour pressure: This profile within plant cover shows close correspondence with temperature profile both during day and night. Forest temperatures differ strikingly from those in open and the forest water vapour pressures were found to be higher within an oak stand than outside it for every month except December.

At night, actual water vapor pressure almost reaches saturation as the air and canopies are cooled by radiation and convection. Some water vapor is transferred through transpiration from the canopy. During day, upper canopy is air heated by convection and water vapor pressure curve shifts much from saturation curve. Deficit between the two increases the downward and at quite lower level, actual water vapor curve inflects. Towards the bottom of canopy, it reapproaches saturation curve due to transpiration coupled with low air movement and low temperature towards base of plant cover.

The flow of water vapor within the plant cover increases with height because of the influence of transpiration by plants and the momentum of the system declines downwards from the plant cover’s upper boundary as a result of the inhibiting effect of plants on the movement of air. This effect is associated with the reduction in turbulent exchange within the layer of plant cover compared with higher air layers. The coefficient of turbulent exchange within the layer also declines towards Earth’s surface.

Humidity conditions within the plant cover are very much different from those outside it due to evotranspiration characteristics of the cover. It generally depends upon the type of plant cover, density of plant cover, structure of vertical stratification and temperature effects. Time of the day and season also affect evotranspiration and, therefore, humidity within the plant cover.

Evotranspiration generally increases with density of vegetation and within the plant cover, relative humidity may be 3-10% higher than outside. This effect is more pronounced in summers.

Rainforests have high transpiration and so have high humidity inside their plant cover. Mean annual relative humidity excess is reported to be 9.4% in beech, 8.6% in Pinus abies forest, 7.9% in larch forest and 3.9% in Pinus sylvestris forests.

In tropical forests, night exhibits complete saturation while in daytime, the humidity decreases with height.

1. Carbon dioxide: The profile of carbon dioxide concentration within plant cover shows much diurnal variation due to photosynthetic uptake of carbon dioxide during daytime and respiratory addition of this gas during night. Carbon dioxide concentration in soil is very low and its use by plants is spatially and temporally very inhomogeneous.

During daytime, CO₂ concentration decreases from upper canopy towards ground. It reaches a minimum point near middle of canopy. Below this point, CO₂ concentration rapidly increases towards ground and becomes equal to CO₂ concentration outside the canopy at a point that roughly corresponds to compensation light intensity. It reaches fairly high level at soil surface. This profile is due to photosynthetic depletion of carbon dioxide in upper canopy, equilibrium corresponding to compensation point lower in the canopy and respiratory addition of carbon dioxide from lowest shaded leaves and soil microorganisms.

In the night, concentration of CO₂ gradually increases towards ground level due to its respiratory addition.

1. Wind velocity: The profile of wind speed shows no strong change in day and night but overall wind speed is higher during daytime due to convectional effects. Wind profile within the canopy develops due to steady state boundary layer flow. The profile is logarithmic above the canopy and becomes exponential within the canopy. The zero plane displacement (D) depends on the height of plants. The roughness height (z_o) is a measure of community roughness and it is effectively the thickness of a laminar sublayer through which individual elements project. Value of z_o is related to height variation and spacing of individual elements which in the plant cover are plants. In extrapolation of logarithmic curve downwards, the zero velocity intercept is found to lie at the height D+z_o. If the canopy were rigid, it would have a constant value but variation of surface roughness depends on leaf flutter, movement of branches and leaf streamlining. These variations cause variations in value of z_o with wind speed. Surface frictional characteristics are entirely specified by D and z_o. The wind profile is major factor in establishment of profiles of saturation water pressure and carbon dioxide within the plant cover.

Lateral air movement is generally lesser within the plant cover than outside it. Even large variations in outside wind velocities do not affect airflow inside forest cover. Vertical stratification structure, leaf canopy architecture, density of stand and season have marked influence on wind velocity profile within a plant cover. For this reason, reduction in wind velocity within the forest cover is different in temperate and tropical forests. Reduction in wind speed from outer edge towards deep inside a forest is greater in tropical rainforests. In temperate European forests, wind velocity at outer edge of forest is reduced to 60-80%, 50% and 7% at points 30 m, 60 m and 120 m respectively deep inside the forest. In Brazilian evergreen forest, wind velocity of 2.2 m/second at the outer edge of forest is reduced to 0.5 m/second at 100 m deep inside forest while at 1000 m inside forest the wind velocity becomes negligible. In this forest, outside storm velocity of 28 m/second was reduced to 2 m/second at 11 km deep inside the forest.

1. Flow of water vapor and momentum of system: The flow of water vapor within the layer of plant cover increases with height because of the influence of transpiration by plants. The momentum of the system decreases downwards from upper boundary of plant cover towards ground level as a result of the inhibiting effect of plants on the movement of air. This effect is associated with the reduction in the turbulent exchange within the layer of plant cover compared with higher air layers. The coefficient of turbulent exchange within the layer also decreases towards ground level.

The theory of photosynthesis within plant cover and numerical models of this process developed in recent years are based on the general idea of a transition from photosynthesis within a single leaf to photosynthesis within a layer that is homogeneous horizontally but possesses different physical conditions at various heights. Application of the theory of photosynthesis within a layer of plant cover indicates following general conclusions:

1. When assimilation process is not very sensitive to different meteorological elements, total assimilation within the plant cover strongly depends on the radiation flux for low levels of radiation. For large values of radiation, the total assimilation is independent of radiation flux and becomes dependent on other factors particularly temperature.

2. Within he plant cover, increase in total assimilation with increase in inflow of CO₂ from soil is much slower than would occur if all the inflow of CO₂ from soil were to be expended on assimilation. This is because the inflow of CO₂ from soil first encounters the leaves located in shade, which are not able to photosynthesize intensively due to insufficient radiation. The general increase in CO₂ concentration produced by its inflow from below is compensated by a reduced contribution of CO₂ from above. Thus assimilation within plant cover is influenced very little by the upward inflow of CO2 from soil and is largely influenced by flow of CO₂ coming downwards from the atmosphere.

PRODUCTIVITY OF PLANT COVER

The productivity of plant cover () is the difference between total assimilation and the expenditure of organic matter on respiration within a plant cover. Thus the productivity of a particular plant cover depends on photosynthesis and respiration in it. The leaves of plants are the major organs association with both photosynthesis and respiration. Therefore, the productivity of plant cover substantially depends on the value of the index of leaf surface (leaf index) and decreases for both very small and very large values of this index. In view of this, the value of productivity of a plant cover is calculated for an optimal value of leaf index i.e. the value of this index that corresponds to the highest value of productivity. From various studies, it has been established that the parameters and factors that affect the photosynthesis within a plant cover also influence the productivity of plant cover. Thus the productivity of plant cover is mainly determined by parameters characterizing the properties of plant cover itself and the climate.

In general, following important points can be observed in relation to productivity of plant cover in nature:

1. The structure of plants in the plant cover continuously changes throughout their life cycles and photosynthetic activity of leaves is never optimal throughout the entire vegetative period of any plant.

2. Availability of mineral nutrients in nature is always less than optimally required for maximum possible photosynthesis.

3. Under natural conditions, water regime of soil is also not constantly maintained at optimally required level.

Thus the productivity of plant cover in real natural conditions is always less than theoretically possible maximum level due to complex interactions between a variety of biological, climatic and soil factors.

Climatic factors and productivity of plant cover

In conditions of sufficient moisture, two climatic factors i.e. photosynthetically active radiation and temperature are particularly important in relation to productivity of plant cover.

The influence of radiation and temperature on productivity of plant cover is quite complex. In real natural situations, radiation is always a factor whose value is a ‘minimum’ because radiation available to leaves in lower layers of canopy is always insufficient. Therefore, increase in radiation flux always results in increased productivity of plant cover.

With increase in temperature, the productivity of plant cover increases initially. After attaining a certain maximum value that depends on the value of radiation flux, productivity begins to decrease with further increase in temperature. Thus productivity of plant cover substantially decreases above a certain threshold value of temperature which is determined by the radiation flux.

WINDS AND WIND SYSTEMS

Wind  is  simply defined as air in motion. Local  winds are produced on a local scale by processes of heating and cooling  of lower air. Following two categories of local winds may be recog­nized.

(i) Katabatic winds: The  first category includes local winds in hilly or  moun­tainous regions, where on clear and clam nights, heat is rapidly lost  by ground radiation. This produces a layer of  cold,  dense air close to ground. A component of the force of gravity, acting in the downslope direction, causes this cold air to move down the mountain  sides, pouring like a liquid into ravines  and  thence down  the grade of the larger valley floors. Mountain breezes of this origin are of a variety termed katabatic winds. Particular­ly strong, persistent katabatic winds are felt on the great ice caps  of Greenland and Antarctica where the lower air layer  becomes intensely chilled. Certain occurrences of severe  blizzards in these regions are katabatic winds.

(ii) Convection winds: In  the second category are included land and  sea  breezes, which affect only a coastal belt a few km in width. Heated during the  day by ground radiation, the air over land  becomes lighter and  rises  to higher elevations. Somewhat cooler air over the adjoining water then flows land-ward to replace the rising warmer air  creating a pleasant sea breeze. At night, rapid  cooling  of the land results in cooler, denser air which descends and spreads seaward to create a land breeze. These daily alternations of  air flow are parts of simple convection systems in which flow of  air takes a circular pattern in vertical cross section. Land and sea breezes are limited to periods of generally warm, clear weather when regional wind flows is weak, but they form an important element of the summer climate along coasts.

Irrespective of whether there are pressure centers or belts, a  pressure gradient always exists, running from higher to lower pressure.  If isobars are closely placed, it indicates  that  the pressure  gradient is strong and pressure changes occur rapidly within a short horizontal distance. Widely placed isobars indicate a weak pressure gradient. Most of the widespread and per­sistent  winds of the earth are air movements set up in response to pressure differences. The pressure gradient force acts in  the direction  of pressure gradient and tends to start the air flow from  higher  to lower air pressure.  Strong pressur gradients cause strong winds and vice versa. Calm exists in the centers  of high pressures.

Coriolis force and geostrophic winds

If  the  earth  did not rotate upon its  axis,  winds  would follow the direction of pressure gradient. However, the rotation of earth upon its axis produces another force, the Coriolis force which  tends to turn the flow of air. The direction of action  of Coriolis  force is stated in the Ferrels’s Law‚ which states  that any  object  or fluid moving horizontally in the  Northern  hemi­sphere tends to be deflected to the right of its path of  motion, regardless of the compass direction of the path. In the  Southern hemisphere,  similar  deflection occurs towards the left  of  the path  of motion. The Coriolis force is absent at the equator  but increases  progressively poleward. It should be noted  especially that the compass direction is not of any consequence. If we  face down the direction of motion, turning will always be towards  the right hand in Northern hemisphere. Since the deflective force  is very  weak, it is normally apparent only in freely moving  fluids such  as air or water. Ocean currents patterns are, to some ex­tent, governed by it, and streams occasionally will show a  tend­ency  to undercut their right-hand banks in hemisphere. Driftwood floating in rivers at high latitudes in Northern  hemi­sphere, concentrates along the right-hand edge of the stream.

Applying these principles to the relation of winds to pressure,  the gradient force (acting in the direction  of  the pressure gradient) and the Coriolis force (acting to the right of the  path of flow) reach a balance or equilibrium only  when the wind has been turned to the point that it flows in the direction at  right angles to the pressure gradient i.e. parallel with the isobars. The ideal wind in this state of balance with respect to the  forces, is termed the geostrophic wind for cases in which the isobars are straight. In general, air flow at high altitudes parallels the isobars. The rule for the relation of wind to air pressure  in the Northern hemisphere states that:  Standing with back  to the wind, the low pressure will be found on  the left-hand side and high pressure on the right-hand side.

Between  the  ground level and altitude of  about  2000-3000 ft., still  another force modifies the direction of wind. This force is the friction of air with ground surface. This force acts in  such a way as to counteract, in part, the Coriolis force  and to  prevent  the wind from being deflected until  parallel  with isobars.  Instead, the wind blows obliquely across  the  isobars, the angle being from 20 to 45 degrees.

EARTH’S SURFACE WIND SYSTEMS

The  wind  systems present on the earth’s  surfaces  may  be categorized as following:

(1) Doldrums: In  the  equatorial trough of low pressure, intense solar heating causes the moist air to break into great  convection columns, so that there is a general rise of air. This  region, lying  roughly between 5 degrees N and 5 degrees S latitudes  was long known as the equatorial belt of variable winds and calms  or the doldrums. There are no prevailing surface winds here, but  a fair distribution of directions around the compass. Calms prevail as much as a third of the time. Violent thunderstorms with strong squall winds are common. Since this zone is located on a belt  of low  pressure, it has no strong pressure  gradients  to  induce persistent flow of wind.

(2) Trade wind belts: In  the north and south of the doldrums are the trade wind belts. These roughly cover the two zones lying between  latitudes 5 degrees and 30 degrees N and S. These winds are the result of a pressure  gradient from the subtropical belt of high pressure to the equatorial  trough of low pressure. In  the  Northern  hemi­sphere,  air moving towards equator is deflected by  the  earth’s rotation to flow southwestward. Thus the prevailing wind is from the northeast and the winds are termed northeast trade winds. In the Southern hemisphere, deflection of moving air towards left causes  the southeast trades. Trade winds have a high degree of steadiness and directional persistence. Most winds come from  one quarter of the compass.

The  systems of doldrums and trades shifts seasonally  north and  south,  through several degrees of latitudes  alongwith  the pressure  belts that cause them. Because of the large land areas of northern hemisphere, there is a tendency for these belts to be shifted  farther  north in summer (July) than  they  are  shifted south  in  winter (January). The trades are best  developed over Atlantic  and Pacific oceans, but are upset in the Indian Ocean region due to proximity of the great Asian land mass.

(3) Winds of horse latitudes: Regions  between latitudes 30 and 40 degrees in  both  hemi­spheres  have long been called the subtropical belts of  variable winds  and  clams or the horse latitudes.  These  coincide with subtropical high-pressure belts. However, these are not continu­ous belts and high-pressure areas are concentrated into  distinct centers  or cells located over the oceans. The apparent outward spiraling movement of air is directed equatorward into the east­erly  trade  wind system; poleward into the westerly  trade  wind system. The cells of high pressure are most strongly developed in the summer (January in Southern and July in Northern hemisphere). There is also a latitudinal shifting following the sun’s declina­tion. This amounts to less than 5 degrees in Southern hemisphere, but it is about 8 degrees for the strong Hawaiian high located in the north eastern Pacific.

Winds in these regions are distributed around a considerable range of compass directions. Calms prevail upto quarter  of  the time. The  cells  of high pressure have generally fair, clear weather,  with a strong tendency to dryness. Most of the  world’s great  deserts  lie in this zone and in the  adjacent  trade-wind belt.  An explanation of the dry, clear weather lies in the  fact that  the  high  pressure cells are centers  of  descending  air, settling  from higher levels of the atmosphere and spreading  out near the earth’s surface and the descending air becomes  increas­ingly dry.

(4) Westerlies: Between  the latitudes 35 and 60 degrees, both N and  S,  is the belt of westerlies or the prevailing westerly winds. Moving from  the subtropical high-pressure centers towards the  subpolar lows,  these surface winds blow from a southwesterly quarter in the Northern  hemisphere and from a  northwesterly  quarter in Southern  hemisphere. This generalization is somewhat  misleading because winds from polar direction are frequent and strong.  More accurately,  winds within the westerly wind belts blow from any direction  of the compass but the westerly components are defi­nitely  predominant. In these belts, storm winds are common cloudy days with continued precipitation are frequent. Weather is highly changeable.

In  Northern  hemisphere, land masses cause considerable disruption of the westerly wind belt but in Southern  hemisphere, there  is an almost unbroken belt of ocean between the  latitudes 40 and 60 degrees S. Therefore, in Southern hemisphere the  west­erlies gain great strength and persistence.

(5) Polar easterlies: The characteristic wind systems of the arctic and  antarctic latitudes  is  described as polar easterlies. In the Antarctic, where an ice-capped mass rests squarely upon the south pole and is surrounded by a vast oceanic expanse, polar easterlies show an outward  spiraling flow. Deflected to the left in Southern  hemi­sphere, the radial winds would spiral counterclockwise, producing a system of southeasterly winds.

Atmospheric aerosols

Apart from gases atmosphere contains many types of extremely small  and light matter suspended in it generally included  under the term aerosol. The word aerosol includes a wide range  of material  that  remains  suspended for a period of  time in the atmosphere  and usually refers to small solid and liquid  matter. Solid  aerosols are usually defined as particles or  particulates and  are distinct from dust which includes large pieces of  solid material (>0 m in diameter) which settle out of atmosphere  due to gravitation after short period of suspension. While effects of dust are limited locally, smaller aerosols can be transported  to long distances and affect air quality and climate on regional and global scales. Aerosols originate from two main sources and  are accordingly termed primary aerosols or secondary aerosols.

(i) Primary  aerosols: These include matter that has been  swept into  the atmosphere from the surface of Earth such  as  dry desert  plains, lake beds and beaches,  volcanic  eruptions, forest  fires, ocean surfaces, disintegration of meteors in atmosphere,  biological sources (e.g. bacteria,  pollen  and (fungi) etc. About 90 percent of these aerosols are found  in troposphere  while  they are also found in upper  layers  of atmosphere  also. Primary aerosols of size 2.0-20.0  um  are defined  as coarse aerosols while those 2.0 m  in diameter are defined as fine aerosols.

(ii) Secondary aerosols: These aerosols are formed after various types  of chemical conversion processes in atmosphere  which involve  gases,  other aerosols and atmospheric contents particularly the water vapor. Very little is know about  the details  of the chemistry of trace gases to aerosols.  These aerosols  are almost always less than 2.0 m in size at  the time of their initial formation when they are at  nucleation mode  (<0.1 m) but grow rapidly to accumulation mode  (upto 2.0  m).  General age of a layer of these aerosols  can  be determined  by  the  relative amount  of  nucleation  versus accumulation  sizes. The smaller aerosols coagulate  rapidly and  aerosols larger than accumulation mode are  efficiently removed from atmosphere by wet and dry processes and  depos­ited onto the Earth’s surface.

a) Sulfate aerosols: A large fractions of aerosols are  sulfate aerosols.  In  the nucleation stage, liquid droplet  of  sulfuric acid  grows rapidly to accumulation size and eventually forms a stable  non-reactive  particle  containing sulfate. Most often eventual result is ammonium sulfate in ages aerosols or ammonium bisulphate. Typical concentrations of sulfate aerosols are :

Remote background area – 1-2 g/cubic meter

Non-urban continental areas – <10 g/cubic meter

Urban areas under anthropogenic influence – >10 g/cubic meter

b) Nitrate  aerosols: Nitrate is another important component  of aerosols  and mainly comes from oxidation of nitrogen  gas. Most common compound in fine aerosol range is ammonium nitrate. It  is not  as stable as ammonium sulfate and its concentration is  con­trolled  by the relative abundance of ammonium, nitrate,  sulfate and the level of atmospheric temperature. Nitrate also exists  in coarse  aerosols as a reactive interchange between  crustal  ele­ments over the continents or sea salt (ammonium nitrate) over the ocean.

c) Other aerosols: Most other aerosols can be further classified into size components with their areas of impacts as given in the subsequnet Table-1.

Optical effects of aerosol particles

High concentration of particulate material in the  atmosphere is responsible for the visible hazes. Suspended material  can cause a range of rather unusual atmospheric phenomena such asblue  moons, green suns and green flashes or arcs  about  the sun or moon.

The distances between aerosol particles are generally greater than 10-100 particle radii and with such distances,  scattering  of light by particles is incoherent. Therefore, optical effects due to atmospheric aerosol particles are explained by light scattering.

Table-1: Properties of miscellaneous aerosol particles present in atmosphere.

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Class                   Size range (mm)                Impact area

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(i) Aerosol size

Aitken                             0.005-0.1              Air electricity

Large                              0.1-1.0                Suspended particulate

Giant                           1.0-15.0                 Suspended particulate

Dust                              >15.0                  Gravitational fallout

(ii) Aerosol type

Small ions                    <0.001                  Air electricity

Large ions                    0.005-0.5              Atmospheric chemistry

Haze                         0.08-2.0            Visibility, human respiratory problems

Mist & fog               1.0-20.0       Visibility, atmospheric chemistry

Cloud condensation

nuclei                    0.05-5.0        Cloud processes

Main aerosol          0.5-5.0  Visibility,atmospheric mass                 chemistry, cloud processes, human respiratory problems

__________________________________________________________________________

Reyleigh Law for unpolarized light applicable only to  particles of radius <0.03 m implies that scattered intensity will be proportional to r⁶/⁴ where r = radius of particle and  = wavelength of light. Blue colour of scattered light from  sky is  explained  in terms of effective scattering at  shorter wave-lengths as the scattered intensity is inverse function of wave-wavelength. Red colour of setting Sun is because  light passes  over a very long path through atmosphere and most of its blue region of spectrum is lost due to scattering.  Spec­tacular  sunsets after volcanic eruptions or bush-fires arise due to higher than normal concentrations of very fine particulate material in the atmosphere after such eruptions.

EFFECTS OF ACID RAINS, ACID FOG AND ACID MIST

Acid rains, acid fog and acid mist cause quite serious damages to natural and man-made things. These damages may be studied under following categories:

Effects on materials, buildings and man-made objects

The chemical weathering and corrosive processes of materials like coated and uncoated carbon steel, painted steel, galvanized steel, nickel-plated steel, iron, copper, nickel and other metals exposed to rains, fog and mist of acidic pH is speeded in various ways. Ferrous metals are particularly attacked by oxides of sulphur. The iron rusts, its surface becomes flaky and flakes fall off to expose more metal thus resulting in continued corrosion. As a consequence, the ion in the buildings, vehicles, railway stock and tracks, electrical and telecommunication installations etc. suffer badly. In areas having acidic rains, fog and mist, corrosion of zinc products may be ten times faster than in clean areas. Acid rain and mist also damage paint coatings and thus expose the underlying material for further damage. Acidification of surface and groundwater in the affected areas results in corrosion of submerged structures and thus submerged parts of bridges, dams, industrial equipment, water storage tanks and hydro-electric turbines are seriously damaged.

All types of buildings, especially those built of sandstone, limestone and marble are seriously damaged and their rate of decay in affected areas is often 2-3 times higher than in unaffected areas. Both stone and the mortar of buildings is affected by acid rains.

Limestone buildings are worst affected due to reaction of sulphur with calcium carbonate in presence of moisture forming calcium sulphate which is soluble and is washed away with rainwater. Soluble sulphates, nitrates, chlorides and other salts being washed over the surface of stonework crystallize within the stone when the surface water evaporates. The expansion of such salts during crystallization enlarges the cracks leading to crumbling (exfoliation) of stone surface. This, in turn, further exposes the fresh underlying stone to chemical corrosion by acid rains. Cement that has high lime content is seriously affected by acid rains while sandstone which has high silica content, is comparatively less damaged. Main visible effect of acid rains on sandstone is formation of hard, black surface coating on the exposed surface. Granite, light-coloured stones and bricks become darkened and black in acid rains. Brick-built structures are less vulnerable than stone-built structures and for this reason, historical monuments, buildings and sculptors which are mostly made up of stone and marble, are seriously damaged by acid rains. In India, acid rain damage is markedly evident on Taj Mahel of Agra, Red For and Jama Masjid of Delhi.

Continuous etching and washing away of the exterior surface of stained or unstained glass exposed to acid rains reduces the glass thickness and thus glass windows of most of most of the historical buildings in Europe are being damaged.

Effect on human health

Though wet acid deposition by acid rains has no direct effect on human health, it indirectly affects human beings. These indirect effects are not directly due to acid rain itself but are due to toxic heavy metals released by it in the environment. Acidification of soils results in release of heavy metals like Cu, Zn, Cd and Hg in the soil. These metals leach down to ground water and/or are washed down to rivers and lakes. The terrestrial plants absorb these metals from soil and aquatic plants from water. These metals thus enter the natural food chain and are passed on successively to higher trophic levels ultimately reaching human beings as plant or animal food. Human body accumulates these heavy metals over long periods of time and their concentration in human body may reach toxic levels causing various diseases. High heavy metal concentrations are known to cause osteomalacia (an uncommon bone disease)in adults and diorrhoea in babies.

Effects on freshwater aquatic flora and fauna

Dry and wet acid deposition over freshwater bodies like rivers and lakes results in serious damage to flora and fauna of these surface water bodies mainly in the following ways:

1. Reduction in water pH: In areas subjected to dry and wet acid deposition, unbuffered or poorly buffered surface freshwater bodies become acidified resulting in significant changes in its ecology. Critical pH level for most of the aquatic plant or animal species is 6.0. However, pH tolerance range varies amongst species as well as at different stages in the life cycle of the individuals of a species. The species that can tolerate and survive in quite wide range of pH values of water are termed tolerant species while species that can survive only within a very narrow range of water pH are termed sensitive species. Due to such differential pH sensitivity of different species and different age groups of same species, acidification of water results in marked changes in species composition, age structure and biodiversity of freshwater aquatic habitat.

   1. Change in species composition: In freshwater bodies of normal pH, a large number of species are normally present. With increasing acidity of aquatic habitat, the species more sensitive to low pH i.e. sensitive species begin to be eliminated from the area and tolerant species are left. Gradually the acidity-sensitive species become extinct and tolerant species occupy the habitat in their place. Thus the species composition of the freshwater habitat changes due to increased acidity. For example, among animal species, many acid-sensitive species of amphibians( e.g. frogs, toads), fishes (salmon, roach and minow etc.), snails etc. become extinct and tolerant species of Hemiptera and Heteroptera (e.g. water bugs) and Corixidae (water boatman) which can survive down to a pH of 3.4, survive and expand. Among phytoplankton, most of the species of green algae, diatoms and small floating hydrophytes disappear below the pH of 5.8. Diatom species have extremely species specific narrow pH tolerance ranges and the species composition of diatoms in freshwater bodies changes very rapidly in response to pH changes of the water.

   2. Reduction in biodiversity and food-web complexity: Most of the species of plants and animals found in freshwater ecosystem are acid-sensitive and very few of them acid-tolerant. With increasing acidification of water body, acid-sensitive species disappear and acid-tolerant species survive and spread. This results in highly reduced biodiversity in acidified freshwater bodies. Generally, the population sizes and then the number of species of angiosperm hydrophytes, algae, zooplankton, aquatic insects and fishes gradually decrease with increasing acidification of water body. The number of phytoplankton and snail species declines below pH of 5.5. Snail species completely disappear below pH of 5.2; zooplankton disappears below pH of 5.0 and fish species rapidly disappear below pH of 4.0. Though acidification affects of the animal species, impact on fish populations is quite dramatic. Many acidified lakes on Ontario in Canada have become totally fish-less while in many of the lakes, trout, wall-eye, burbot and small-mouth bass have disappeared. Many lakes above 610 meters altitude with pH below 6.0 in northeast U.S.A. have also become totally fish-less. Upto 20.000 acidified lakes in Sweden have been affected in varying degrees. About 9,000 lakes in southern Sweden and 1,400 lakes in southern Norway have few fish species left in them with roach, arctic char, trout and perch having disappeared following acidification of lakes. Generally, size and diversity of fish populations shows progressive decline below the pH of 6.0. Mass death of fish populations may also occur in lakes and rivers following acid surges induced by melting of acidified snow in upstream areas during spring season. As a result of the decrease in number of species, the complexity of food webs also decreases and the food webs gradually become simplified in acidified freshwater ecosystems.

   3. Change in age structure and population dynamics: Breadth of pH tolerance range varies between different stages of the individuals of the same species. Young and old members of a species are often more sensitive to low pH and, therefore, they disappear more rapidly than middle-aged individuals. For example, among animals, fishes and amphibians are especially sensitive to acidity during their early embryonic stages. Thus in acidified freshwater bodies, the number of young and old individuals of the species which can tolerate acidity to some extent, gradually declines and the number of middle-aged adults gradually increases. This alters the age structure of acid-tolerant species in fresh-water ecosystems. For example, progressive decline in frog population has been reported from many acidified Swedish lakes due to inhibition of egg-hatching and death of tadpoles. Stocks of salmon have considerably declined in many acidified lakes and rivers of south and southeast Norway and western coast of Sweden. The effects of acidification on the fishes are extremely rapid. However, in low levels of acidification, older fishes survive and grow bigger in size due to reduced competition for food as a result of rapid death of younger ones, This short-term increase in fish biomass is really a signal of the imminent decline of the population due to decline in its reproductive capacity. Thus change in age structure ultimately results in changes in the population dynamics of the species ultimately leading to adverse results.

   4. Change in rate of mineral cycling: The species of rooted hydrophytes being highly acid-sensitive, disappear while filamentous algae and moss Sphagnum being acid-tolerant, colonize the bed of acidified freshwater bodies. Fungi and bacteria that play important role in decomposition of dead organic matter are not acid-tolerant and, therefore, tend to disappear below pH of 5.5. The growth of acid-tolerant filamentous algae and moss in acidified freshwater bodies seals off the oxygen input and slows the decomposition of organic matter on the lake floors. This coupled with absence of decomposing bacteria and fungi results in very much reduced rate of decomposition of organic matter and its accumulation at the bottom of freashwater body. Thus valuable mineral nutrients become trapped in the undecomposed organic matter instead of being released again into the ecosystem by decomposition.

2. Increase in toxic metal ion concentration: A very damaging effect of acid rains is increase in the concentrations of heavy metals like Al, Cd, Hg, Mn, Fe and Zn in surface freshwater bodies. Acid deposition on soil and rocks in the catchement areas makes these metals in soils and rocks more soluble and mobile. Thus these released heavy metals are washed down to lakes, rivers and other surface freshwater bodies alongwith runoff water. Acidification of water bodies also mobilizes these metals from the beds into the water. These heavy metals are highly toxic to plants and animals. The metals are first taken up by aquatic plants, accumulated in their bodies and then passed on to higher trophic levels via food chains. At each level in the food chain, the concentration of toxic heavy metals increases due to their accumulation in the animal bodies over time (bio-magnification). When concentration of any metal crosses the critical threshold tolerance value in the body of an organism, it becomes toxic to that organism. Accumulation of toxic levels of metals in animal body has been show to be an important factor in reduction of population size of many aquatic species as well as predatory animals living close to water bodies.

   1. In high concentration, Aluminium can become complexed with phosphates in the water which are often the critical limiting factors in aquatic ecosystems because they are essential nutrients for phytoplankton and hydrophytes. Reduction in phosphate leads to reduced primary production in the freshwater ecosystem. This ultimately results in progressive decrease in the food supply and, therefore, decline in population sizes of consumer animal species in higher trophic levels.

   2. Birds like flycatchers nesting on the shores of acidic lakes eat Al-laden fish and end up with its high concentration in their bodies. Due to high Al-concentration, they produce eggs with soft or no shells and, therefore, only few eggs hatch successfully leading to decline in their population sizes. Aluminium is acutely toxic to fish at pH levels that are not normally harmful. Its concentration as low as 0.2 mg per liter kills the fishes. Though Al-poisoning interferes with normal reproduction of fishes, its more damaging effect is on the gills. Precipitation of Aluminium on the gills interferes with transport of oxygen and ions (e.g. Na⁺ and Cd²⁺) across gill membrane. Much mucus is excuded to combat the Aluminium collected on the gills which further inhibits uptake of oxygen and salts in gills. Disturbance of ionic regulation affects transport of gases between respiratory organs and the body tissues. This alongwith inhibition of oxygen-uptake, causes respiratory stress leading ultimately to death. Accumulation of Hg, Cd and Zn has also been shown to cause damage in various aquatic animal species.

Effects on terrestrial ecosystems

Acid deposition on land affects the forests and crops directly as well as indirectly through alteration of the chemistry and microbiology of soil. Though effects of acid deposition on crops have important economic consequences, the effects on forests have been very dramatic and ecologically damaging. However, the study of the effects of acid deposition on land is a very complex problem because of the following two factors:

1. There is a very wide range and large number of possible interactions between atmosphere, soil and plants in terrestrial ecosystems.

2. Effects of acid deposition on soil and vegetation take very long time (decades in case of trees) to reach detectable levels.

Despite the constraints mentioned above, studies have yielded much information about the effects of acid rains on various aspects of terrestrial ecosystems. These may be categorized as following:

Effects on soil chemistry: Following acid deposition, a series of complex chemical reactions take place in the soil. General consequences of these reactions are:

1. Increasing nutrient deficiency in the soil: In the acidified soil, basic cations are replaced by hydrogen and aluminium ions. These liberated cations are rapidly leached down and out of the soil solution alongwith sulphate from the acid input. Basic cations are essential plant nutrients, particularly the K⁺, Na⁺, Ca²⁺ and Mg²⁺ which are taken up by plants from the soil in quite large amounts (macronutrients). Loss of essential nutrient cations from the soil adversely affects the plant growth. Poorly buffered soils are highly susceptible to acid-induced nutrient deficiency e.g. soils of Swedish forests have shown progressively decreasing levels of K⁺, Na⁺, Ca²⁺ and Mg²⁺ over a ten-year period of acid deposition. Replacement of nutrient cations by hydrogen and aluminium ions further increases the soil acidity. Setting up a vicious cycle.

2. Mobilization and increase in heavy metal content of soil: Increase in soil acidity is often associated with increased soil concentration of toxic heavy metals. Most common such heavy metals are Al, Cd, Mn, Hg, Pb, Fe and Zn. In the soil of normal pH, these metals remain chemically ‘bound up’ in the soil. However, acidic pH of soil frees these metals and the mobilized metals can now rapidly spread throughout the soil alongwith natural flow of soil water.

3. Damage to mineral structure of soil: Soil acidification also increases the weathering of silicate minerals during liberation of metals and thus causes loss of mineral structure of the soil.

Effects on soil microbes: Acid deposition on land results in acidification of soil which causes damage to various decomposing bacterial and fungal populations in the soil. As a result, rate of decomposition of organic matter is slowed down and, therefore, the nutrient recycling in the ecosystem is blocked. Since return of essential nutrients back to the soil is blocked, the soil progressively becomes impoverished. Experimental studies have shown that soil acidity strongly reduces the decomposition of the litter of pine, spruce, birch and other cellulose-rich materials. Such reduction in decomposition of organic matter also results in reduced respiration of soil microbes including nitrogen-fixing bacteria and blue-green algae. This increases the levels of ammonia in the soil due to reduced mobilization of nutrients previously released by decomposition and the soil nitrate levels are considerably reduced due to ammonification. Such changes in the soil having pH below 3.0 bring about marked changes in the population sizes and species composition of soil microbes. For example, total abundance of acid-sensitive enchytraeids decreases and that of tolerant springtails increases. Further, soil acidification causes significant damage to other soil fauna also, particularly the earthworms. Reduced earthworm population markedly alters the soil structure and consequently the soil productivity is reduced.

Effects on terrestrial plants and ecosystem:

Effects on higher plants

All types of plants are adversely affected by acid rain and the damage is caused in two ways; firstly through shoot system, particularly the foliage which are directly exposed to acid rain, acid fog or acid mist and secondly through root system via deficiency of soil nutrients and toxicity of heavy metals in the acidified soil. Visible symptoms in plants can assume various forms depending on the character and level of acid deposition and the buffer capacity of the soil. The symptoms also vary between species and with the age of plant and tissue. Younger tissues and young plants are generally more susceptible to acid rain damage. In general, acid rain damage in plants is manifested as reduced plant growth and hence decline in yields, reduced canopy cover, reduced reproductive capacity etc.

1. Increased susceptibility to pathogens: Acid rains damage the surface cuticle of leaves and other plant organs and thereby make the plant more susceptible to attack by pathogenic fungi and bacteria which can now enter through the damaged surface.

2. Reduced growth: As discussed above, increasing soil acidity result in decreased availability of essential plant nutrients in the soil due to decreased nutrient cycling. Further, high aluminium released in soil following soil acidification has been reported to damage root hairs and thus adversely affect nutrient uptake. As a result of these, plants growing in land areas affected by acid deposition generally show poor growth. The availability of nutrients to the trees and other plants is also influenced by the exchange processes that take place on the surface of leaves. Ammonia and nitrogen landing on the leaf surface via acid deposition pass through the semi-permeable membranes of epidermal cells of leaves and are incorporated into the leaf tissue. This results in cation exchange in leaf tissues and the abundant plant nutrients present in leaf tissues such as K, Ca, Mg and S are leached and washed off the leaf surface. This foliar leaching due to acid deposition also causes depletion of essential plant nutrients and, therefore, reduced plant growth.

3. Foliar injury: Various visible leaf injury symptoms develop in leaves of plants growing in areas affected by acid rains. In general, visible leaf injury symptoms depend on the density of trichomes and stomata. Due to plasmolysis of palisade cells in leaves, structural damage in chloroplasts are common. In leaves of several species galls are produced in response to acid deposition.

4. Reduction in symbiotic balances: In the plants growing in land areas affected by acid deposition, formation of root nodules is drastically reduced and other symbiotic associations like ectotrophic and endotrophic mycorrhizae are also adversely affected.

5. Reduction in reproductive capacity: Decrease in flowering, reduced pollen germination, inhibition of pollen tube growth and inhibition of seed germination has been reported due to acid rains. In Norway spruce, Scots pine and Silver birth, seed germination is inhibited between pH 3.8 and 5.4. In these plants initial establishment of seedling is highly sensitive to soil pH and rapidly decreases below pH 4.2. All these effects of acid rains ultimately result in reduced reproductive success of the affected plants and, therefore, in reduced population size of the affected sensitive species. With gradual decrease in reproductive potential of affected species, the tolerant species gain upper hand and due to better reproductive success, gradually spread in the area.

Effects on lichens, algae and bryophytes

The lichens, algae and bryophytes growing on or in the soil in the areas affected by acid rains are also affected severely. Lichens drive their nutrients from the minerals falling on them with rainwater. Therefore, acid rain reduces the the availability of nutrients to lichens to a far greater extent than other plants. Rate of assimilation of nutrients in lichen thallus also varies with pH of rainwater. Further different species of lichens and bryophytic plants show different tolerance levels of rainwater acidity. Sensitive species are generally eliminated very early in the areas affected by acid rains and such areas become dominated by tolerant species of lichens and bryophytes. Thus acid rains alter the population abundance, species composition and diversity of lichen and bryophytic flora.

Effects on forests:

Damage to forests due to acid rains is a complex problem. The available evidence suggests that the damage is caused due to a combination of a variety of contributory factors in addition to acid deposition. Such factors include dry deposition of the oxides of sulphur and nitrogen, ozone, heavy metal content of soil, parasites and plant diseases, extreme climatic conditions like very high or very low rainfall and temperature extremes (particularly frost), site factors e.g. soil drainage, soil characteristics, general state of health and age of trees, surge of naturally produced acids, acid flushes (e.g. during spring snow-melt or after prolonged draught) and forest management practices. Such factors also contribute to damages to crops caused by acid rains. Ozone has been found to increase the vulnerability of trees to acid-induced damage by increasing their susceptibility to poisoning and nutrient loss. Ozone might play significant role at high altitudes where sunshine required for photo-chemical production of ozone is more intense. Vegetation above 10,000 ft. line in West Germany shows many damaged trees. Much of the damage to vegetation remains undetected until it reaches a critical, perhaps irreversible stage. Species of coniferous and deciduous trees generally exhibit much genetic variability in their populations due to which all the individuals of a species do not show equal sensitivity to acid rains. Such variability is particularly marked in Scots pine and Norway spruce. Further, different species in a forest have different dose-response relationship. All these factors make generalizations about the acid-rain induced damage to forests quite different. However, most evident effects of acid rains on forests have been observed in the form of Crown-dieback and Waldsterben.

1. Crown-dieback: In forest systems, damage to trees which is most extensive in West Germany, is spreading alarmingly throughout Europe and is gradually building up in U.S.A. and Canada. Visible damage tends to be concentrated in older and established trees and appears to be species-specific. Scots pine is the most sensitive species in which needles become shorter, duration of needles on the tree decreases from three to one year, top buds dry, annual growth of shoot decreases and shape of crown changes. Damaged conifers, in general, show yellowing of needles, loss of needles, distortion of branches, thinning of tree tops, injuries in bark, changes in trunks and damage to fine roots. In deciduous trees, main symptoms include discolouration of deformation of leaves, early shedding of leaves, bark injuries, death of tree tops and lack of natural regeneration. In extreme condition of damage, tree tops in all types of trees die earlier than the branches further downwards. This condition has been termed crown die-back.

2. Waldsterben: In 1980s, German scientists first observed the wasting disease of trees attributed to acid rains and termed it waldsterben which literally means ‘death of trees’ or ‘dying tree syndrome’ that blighted trees and forests. The extent and rate of spread of such damage is quite alarming in industrialized countries. By 1985 about 52% forest area was affected in West Germany and about 86% of woodland in East Germany showed such damage. The damage has also been found in forest of France, Switzerland, Sweden, Italy, Hungary, Poland, Czechoslowakia, Russia, U.K. Canada and U.S.A. Tree death occurs within five weeks of the appearance of first symptoms. Further, waldsterben affects young saplings as well as mature trees. In forests of areas affected by acid rains, first signs of damage were reported in Abies alba in early 1970s and in Picea abies by late 1970s. Pinus sylvestris and Fagus sylvetica were affected by early 1980s and the damage spread to other species like larch, red oak, maple, ash and rowan showing that disease affects almost all tree species. Greatest absolute damage was found in spruce and greatest relative damage occurred in silver fir in which over 87% of the trees were damaged. Three stages have been identified in this damage process:

1. Nitrates or nitrogen oxide in the acid rain initially provide soil nutrients and the trees grow more rapidly.

2. In next stage, soil progressively loses the ability to neutralize the increased acidity and the acids begin to accumulate and cause leaching of nutrient cations leading to slowing down of tree growth and yellowing or discolouration of needles or leaves. Sulphate combines with metals in soil and increases heavy metal concentration in the soil.

3. In the last stage, toxic aluminium is released at pH 4.2 leading to destruction of tree roots and deterioration of natural defense mechanisms of trees that prevent the entry of pathogenic bacteria, fungi and viruses. The trees thus gradually die due to nutrient deficiency, heavy metal toxicity and various pathogenic diseases.

Effects on ecosystem

Among terrestrial plants, the sequence of the sensitivity to acid rains is herbaceous dicots> woody dicots>monocots>conifers. The acid rain induced damage to trees, which are most important primary produces in the terrestrial ecosystems, reduces the food availability to animals in higher trophic levels. As a result, the population sizes of various animal species is adversely affected. In general, acid rains result in changes in relative abundance of populations in all the trophic levels and also the reduced species diversity of terrestrial ecosystems. In all the trophic levels, sensitive species are gradually eliminated and are replaced by tolerant species.

Acid rain

 The natural rainwater moving through atmosphere comes in contact with various chemicals produced and deposited in the atmosphere. These chemicals are produced by various natural (e.g. electrical nitrogen fixation due to electrical lightening), biological (e.g. release of gases in decay and decomposition of organic matter and other biological processes) and geological (e.g. volcanic eruptions and weathering of rocks) processes. Due to this contamination, natural rainwater in perfectly unpolluted areas is also somewhat acidic. The pH of normally clean or ‘pristine’ rainwater is generally agreed by scientists to be 5.6.If rainwater falling in an area has pH value below 5.6, it is called acid rain.

Recent measurements show that rain and snow having pH 4.3 or below fall regularly over many areas of heavily industrialized Northern hemisphere, specially North America, northern and western Europe. Sometimes individual storms under favourable conditions may have may have very low pH values. For example, in 1979, Kane in Pennsylvania, America recorded a rain of pH 2.7 and in same year, , Wheeling in West Virginia had rain of pH 1.5. In Britain, Pitlochry had a rainfall of pH 2.4 in 1974. The acid rains is caused by emission of large quantities of sulphur dioxide and oxides of nitrogen in the atmosphere due to burning of fossil fuel in various industrial and other activities of human beings. Allied to acid rains are phenomena of acid mist and acid fog, both of which come under the category of occult precipitation. The cause of acid mist or acid fog is high concentration of sulphates and nitrates in the form of fine aerosol particles (dust or soot) in wind-driven ground-level clouds which causes condensation of tiny water droplets around these particles. These droplets being tiny fraction of normal rain drops, do not fall as rain water but remain suspended in the atmosphere forming acid mist or acid fog.

The problem of acid rain has attracted worldwide attention only since 1980s. However, the term ‘acid rain’ was first used by first Alkali Inspector of Britain,, Robert Angus in 1872. His work largely remained ignored until 1950s when Canadian ecologist Dr. Eville Gorham undertook detailed studies of rainwater quality and its control in Lake District in north-west England. By mid 1960s, early damage symptoms of acid rains begun to appear in Scandinavia and Swedish worker Svente Oden begain a concerted scientific effort in 1967 to bring awareness about acid rain problem. He is considered to be the father of modern acid rain studies.

GEOGRAPHY OF ACID RAIN

Primary pollutants causing acid rain problem are blown over long distances by the wind and thus spreading the problem over whole of the Earth’s surface. However, till now most of pollutants responsible for acid rain problem are produced in the highly industrialized nations, the areas of the impact of acid rains are few, noticeable, few and predictable. Common properties observed in areas affected seriously by acid rain problem are:

1. Heavy concentration of industries producing pollutants responsible for acid rain problem.

2. Downwards flow of winds from pollutant-producing areas.

3. Upland-mountainous position of pollutant-producing areas having thin glaciated bedrock and high rainfall-snowfall.

4. Numerous lakes and streams and rich forest cover in pollutant-producing area.

Areas sharing the above common properties are termed acid rain hot spots and include many parts of Scandinavia, upland Britain, West Germany and many parts of Northern Europe. Across Atlantic, such areas include Nova Scotia, Canadian Shield around southern Ontario and Quebec, Adriaondack Mountains, Great Smoky Mountains, parts of Wisconsin and Minnesota, Pacific Northwest U.S.A., Colorado Rockies and Pine Barrens of New Jersey. Japanese islands are also included in this category.

In contrast to above areas, there are two types of safe areas where acid rains are not a problem at present. These areas include:

1. The areas located away from and not downwind of possible source areas and themselves having little polluting industrialization. These areas include almost all of southern hemisphere, tropics and parts of northern hemisphere e.g. northern Russia.

2. The areas that receive acid rains but have natural resistance to its damaging impact due to buffering capacity provided by the alkaline dust blown from the west. Actually alkaline rains have been reported in Sweden before 1960 in areas with limestone outcrops and cement manufacturing areas. Wind blown alkaline material can de derived from deserts (fine material brought over from Sahara and Gobi deserts has been reported), from wind erosion of top soil alkaline particulate pollutants e.g. soot from smoke-emitting chimneys and agricultural fertilizers.

Geologically, the areas most vulnerable to acid rains fall under three categories:

1. Glacial areas on granite and other highly siliceous bedrock e.g. quartzite, quartz sandstone, certain gneisses and on materials derived from these.

2. Areas with thick deposits of siliceous sands e.g. sand plains of Denmark and Netherlands.

3. Areas with relatively old, highly weathered and leached soils.

Areas having severest acid rain damage are glaciated Pre-Cambrian shield areas of Scandinavia, glaciated parts of upland Britain having thin soils, eastern Canada and resistant Canadian Shield and northwest U.S.A. Problems of acidification develop much acutely on granite and similar other resistant rocks.

Acid rain as global problem

Though at present acid rain problem is mainly concentrated in highly industrialized areas, the long-range transport of concerned air pollutants results in gradual globalization of the problem. As a result of slow transport of acid rain causing pollutants from heavily industrialized areas to areas till now free from this problem, the latter areas are also beginning to show acidification damage. Such damage has been reported from many developing nations like Zambia, South Africa, Malaysia, Venezuela, India and China. Most productive farmlands of China and India, paddy fields of South-east Asia and forests of Amazon in South America have soils which are highly susceptible to acidification.

Global dimension of acid rain problem was established beyond doubt in 1981 with discovery of Arctic haze. It is bluish-gray haze developing in Arctic areas similar to that frequently found over and downwind of large industrial areas in western Europe and eastern North America. Haze layers often cover a horizontal area of upto 1000 km and are caused by scattering of solar radiation by minute suspended particles in the atmosphere. These particles vary in the size range of 0.1-1.0 micrometer and mostly comprise of sulphate aerosols. These aerosols are transported by jet streams in upper atmosphere and may reach upto 8000 km away from their industrial sources. Hazes are found to be thickest in Alaska’s North Slope extending atleast to Norway. Hazes mainly affect visibility and are not as damaging as the smog.

CAUSE AND FORMATION OF ACID RAIN

SO₂ and oxides of nitrogen (NO_x) emitted into the atmosphere due to industrial, commercial and other anthropogenic activities are the basic cause of acid rain formation. Therefore, the problem of acid rains has accompanied the rise of emission of these gases into the atmosphere.

SO₂ is emitted from three principal man-made sources:

1. Combustion of coal produces about 60% of total SO2 emitted into atmosphere.

2. Combustion of petroleum products which adds 30% of total emission.

3. Industrial activities like smelting of iron, zinc, nickel, copper ores, manufacture of sulphuric acid and operation of acid concentrators in petroleum industry. These produce the remaining 10% of this gas.

Overall emission of oxides of nitrogen is small in comparison with SO₂, their importance in formation of acid rains is very high. Most of the oxides of nitrogen (NO₃, NO₂, NO etc.) are produced from:

1. Combustion of fossil fuels.

2. Industrial chimneys and thermal power stations.

3. Motor vehicles in urban areas.

Man-made sources of SO₂ and NO_x emission are point sources (e.g. thermal power stations and industrial chimneys) and the emission from these occurs as a plume of gases. The plume of gases emitted from high stacks usually travels downwind for about 12 km as a straight line without much dispersion. Afterwards, its shape evolves by diffusion and changes progressively downwind into a widening cone. The direction, speed, distance of travel of the plume and its dispersal and diffusion depend upon meteorological conditions such as direction, velocity and pattern of propelling wind, air temperature (especially the vertical temperature gradient), air turbulence and atmospheric stability. Under stable atmospheric conditions, for example, at night over land and during day over snow covered ground, there is very little vertical dispersal for very long distance and the acidification may occur at quite far away place from the source of emission.

Dispersal of the plume of SO₂ and NO_x occurs in the mixing layer of atmosphere that extends from ground level upto 1-2 km altitude. The dispersal is triggered by diffusion and atmospheric turbulence, normally between 5 to 25 km from the point of source. The rate of diffusion and mixing of oxides into air is faster when flow of air is turbulent. The lower portion of the dispersing cone of oxide plume first touches the ground level at about 5 km distance form the point source while middle and upper portions are thoroughly dispersed in the air leading to dilution and chemical transformation.

The deposition of pollutant oxides from the plume onto the ground is of two types: dry deposition and wet deposition.

1. Dry deposition: The acidic oxides deposited from the bottom of the plume between 5-25 km from the source in the form of gases and particles constitute the dry deposition. Though such deposition is not acid rain in strict sense, it produces acidification of soils and surface water bodies similar to acid rain. This dry deposition also causes direct SO₂ and NO_x poisoning of the vegetation. Dry deposition of sulphur and nitrogen oxides and undissolved acids on lakes and steams straightaway dissolve in the water and acidify the water bodies. Such dry deposition on land and on vegetation remains inactive till dew or rainfall when these dry deposited acids dissolve in the dew or rain water and form active acids. Such sudden addition of high concentration of acids into an otherwise stable environment causes acid shocks, acid flushes or acid surges. These terms indicate increasing levels of acidification and decreasing time period in which such acidification takes place. During winters, SO₂ and NO_x pollutants are dry-deposited on snow and ice in the catchment areas of many lakes and rivers. In the following spring season, when this snow and ice melt, the acids accumulated in the snow and ice over long period are suddenly released over a period of few days to a week causing acid surges in the lakes and streams.

2. Wet deposition: It is the deposition of acidic oxides of the plume over land or vegetation after being dissolved in the rainwater, snow or ice forming acid rains, acid snow, acid mist or acid fog. Today’s industrial chimneys are normally 100-300 meters high and, therefore, such wet deposition normally occurs beyond 25 km from the point source. The prevailing wind pattern and the length of time over which oxides are transported in the wind system is of great importance in the geographical distribution of acid rains. Longer the SO₂ and NO_x remain in the atmosphere, greater is the possibility of their transformation to produce sulphuric and nitric acids.

The practice of increasing the height of chimneys and installation of electrostatic precipitators to reduce the air pollution appears to have magnified the problem of acid deposition in two ways. Firstly, tall stacks of pollutant-emitting units now emit pollutant gases at much greater heights so that these gases are now dispersed over much wider areas increasing the geographical extent of acid deposition. Secondly, installation of electrostatic precipitators and other mechanisms to remove alkaline particulates in chimneys has resulted in increased emission of acidic gases. It is because prior to installation of such mechanisms, acidic gases were neutralized to a large extent by alkaline particulates being emitted alongwith them.

CHEMISTRY OF ACID RAINS

Strictly speaking acid rain is a term which indicates a wide variety of mixtures of acids and oxides in the rainwater. For example, rainwater of pH 4.5 may contain a high sulphur content, high nitrogen content or any combination of the two. Acidity of rainwater results from chemical transformations of a large number of acidic ions added to the atmosphere from natural sources (e.g. sea salts, volcanic emissions, biogenic emissions, soil etc.) and by human. Major such ions can be categorized as following:

1. Inorganic ions: These include trace metal ions which often act as catalysts to quicken the acidity processes. At coastal sites, corrections for the impact of seawater on rainwater quality have to made before accurate assessment of the role of land-based sources can be made. In individual locations, rainwater quality may be strongly influenced by local sources.

2. Organic ions: These are important alongwith local biogenic sources in affecting the precipitation quality, particularly in tropics.

Table 1. Major inorganic and organic ions and molecules affecting rainwater acidity.

Ion or molecule	General source	Comments
INORGANIC IONS   H+ (cation)   SO42- (anion)   NO3- (anion)   Cl- (anion) NH4+ (cation)   Ca2+ (cation)   K+ (cation)   Mg2+ (cation)   Na+ (cation)   INORGANIC CATALYSTS   H2O2 (molecule) O3 (molecule) Fe3+ (cation) Mn2+ (cation) NO2 (molecule)	Aqueous chemistry   Combustion of fossil fuels, ocean and soil processes Agriculture, fossil-fuel burning   Ocean, some industries Agriculture, decay processes, industry Soil, agriculture   Soil, agriculture   Ocean, soil, agriculture   Ocean, industry       Aqueous chemistry Atmospheric chemistry Soil, industry Industry Fossil-fuel burning	Amount directly proportional to rainwater acidity Strong acid, gas and liquid reactions Strong acid, gas and liquid reactions Acid, mainly gas reaction Neutralization of anions   With carbonates, act as buffer acidity With carbonates, act as buffer acidity With carbonates, act as buffer acidity With carbonates, act as buffer acidity     Major at all rainwater pH Minor Minor Minor Minor
ORGANIC MOLECULES   HCOOH (molecule) CH3COOH (molecule)	Vegetation Vegetation	Weak acid Weak acid

 

The steps involved in each chemical process contributing to rainwater acidity depict a multitude of pathways with many of the steps being reversible and many of the steps exhibiting highly complex chemistry. Thus the overall chemistry of acid rain is extremely complicated because of the very large number of chemical interactions involved. Moreover, exact chemical composition of acid rain is not same in every area. It varies from place to place depending upon the proportion of different oxides present and the chemical transformations they have undergone during their stay in the atmosphere. Although a variety of natural and man-made oxides contribute to rainwater acidity through variety of chemical pathways, most important pathways are those associated with two major acidic gases i.e. SO₂ and NO₂ added to atmosphere from various polluting sources. The complex pattern of acid deposition has following six stages:

1. The atmosphere receives SO₂ and NO_x from natural and man-made sources.

2. Some of these oxides fall on the ground as dry deposition within 5-25 km from their parent sources.

3. Formation of photo-oxidants like ozone, is stimulated in the atmosphere.

4. The photo-oxidants interact with SO₂ and NO_x to produce acids (H₂SO₄ and HNO₃) by oxidation.

5. The oxides of sulphur and nitrogen, photo-oxidants and other gases (including NH₃) dissolve in the cloud and rain-droplets to produce acids (H⁺ and NH4⁺) and sulphates (SO₄^2-) and nitrates (NO₃^-).

6. Acid rain containing ions of sulphate, nitrate ammonium and hydrogen falls as wet deposition.

The most important step in this chain of reactions is the catalytic conversion of SO₂ and NO_x. This may take from a few hours to a few days in the atmosphere and can not occur without photo-oxidants (precurssors). Ozone is the most readily available and abundant photo-oxidant in the atmosphere . Hydrocarbons and NO added to the atmosphere as pollutants are the two main precurssors of ozone. The acid rain is the final product of the loading of SO₂ and NO_x coupled with photochemistry and physical dynamics of stratosphere.

Acid gases like SO₂ and NO_x are transformed into dilute acids in the rainwater by following three major types of reactions:

1. Homogeneous gas-phase reactions: These reactions occur in the dry atmosphere associated with photolytic oxidation processes.

2. Homogeneous aqueous-phase reactions: These occur between individual species in a liquid medium such as cloud or raindrop.

3. Heterogeneous aqueous-phase reactions: These occur during adsorption of acid gases on solid surfaces and are extremely complex. These reactions probably assist in creating rainwater acidity but are not considered to be as important as other two types of reactions in the overall chemistry of acid rains.

The relative importance of any chemical process operating in the atmosphere depends strongly on the meteorological conditions such as the presence of clouds, relative humidity, intensity of solar radiation, temperature etc. Following two factors are crucial to the operation of each process:

1. Time available to complete secondary chemical reactions.

2. Availability of excited ions and catalysts to assist the reactions.

Homogeneous gas-phase chemistry

In dry atmosphere, most of the acid gas reactions leading to formation of acid ions such as sulphates and nitrates involve excited molecules, atoms, free radicals and sunlight. The OH radical is particularly important in such reactions. Following main such chemical pathways lead to eventual formation of sulphuric and nitric acids in rainwater:

1. SULPHUR DIOXIDE

Very slow reaction:

2SO₂ + O₂ ——- 2SO₃

Unstable compounds:

OH + SO₂ + M —– HOSO₂ + M

HOSO₂ + O₂ —- HO₂ +SO₃

(M = catalyst; often Fe³⁺ or Mn²⁺)

Very fast reaction:

SO₃ + H₂O — H₂SO₄

1. NITROGEN DIOXIDE

Very slow reactions: ( ppb concentrations are reached in many days)

2NO + O₂ — 2NO₂

or,

HO₂ + NO —– OH + NO₂

2NO₂ + H₂0 —— HNO₃ + HONO

Factors affecting homogeneous gas-phase reactions

1. Interfering substances: Oxidation of SO₂ and NO₂ in the atmosphere is relatively a slow process and there may be several substances causing interference along the way. For example, HOSO₂, which is a very unstable substance, may react with CO, NO, water vapour, various hydrocarbons and other chemical species and block the reaction described above.

2. Catalysts: The reactions between SO₂ and NO₂ with O₂ in the dry atmosphere are considered to be so slow without catalysts that the eventual output of acid is very small. Reactions with the addition of catalysts and free radicals are the main sources of ions leading to acidity of rainwater.

3. OH radical: Oxidation rates of SO₂ and NO_x in a cloud-free atmosphere are highly variable and strongly dependent on the concentration of OH radical. If concentration of OH radical is relatively high (on the order of 9×10⁶ mol cm^-3), oxidation of SO₂ to SO₄^2- is approximately 3.7 ⁺/_- 1.9 % per hour. Conversion of NO₂ to HONO₂ is much more rapid reaction; its rate being about 34 ⁺/_- 17% per hour. With lower OH concentrations, the conversion rate is reduced and SO₂ converts at a rate of about 0.7% per hour or about 16.4% per day. NO_x conversion is at much faster rate and the rates vary between 6.2% per hour and 100% per day. In winters, conversion rates are 0.12% and 1.1% per hour respectively. At night, when OH concentrations are at minimum, conversion rates are sharply reduced.

Homogeneous aqueous-phase reactions

The species of sulphur and nitrogen can be incorporated in liquid water droplets in several ways e.g. (I) they may have high solubility in water; (ii) they may attach through diffusional processes; (iii) they may be incorporated through impactations and collisions and (iv) acid aerosol species may act as nuclei for formation of water droplets. Most important aqueous –phase reactions in acid-rain chemistry are as following:

A. SULPHUR DIOXIDE

SO₂ + H₂O <—- SO₂.H₂O

SO₂.H₂O — H⁺ + HSO₃^-

HSO₃^- ——— H⁺ + SO₃^-

O2 + 2HSO₃^- ——- 2H⁺ + 2SO₄^2- (Reaction slow without catalyst)

H₂O₂ + HSO₃^- ——– H⁺ + SO₄^2- + H₂O (Reaction is rapid)

O₃ + HSO₃^- —— H⁺ + SO₄^2- + O₂ (Reaction is rapid if pH>4.5)

B. NITROGEN DIOXIDE

NO₂ + O₃ —– O₂ + NO₃

NO₃ + NO₂ + M <—— N₂O₅ + M (M = Catalyst; often Fe³⁺ or Mn²⁺)

N₂O₅ + H₂O ——- 2H⁺ + NO₃ + NO₂

Factors affecting homogeneous aqueous-phase reactions

1. Reaction medium: Conversion of acid ions is much faster when reaction medium is water. At the droplet scale, sequence of conversions might be:

   1. Initial diffusion of gas to the droplet interface.

   2. Transfer across the interface into the droplet.

   3. Swift aqueous-phase equilibrium.

   4. Aqueous-phase reactions and concurrent diffusion.

2. Catalysts: In liquid water, catalysts are very important in determining the speed of conversion process. Models using proper chemical conversion estimates indicate that, with the exception of H₂O₂, impact of other catalysts is highly dependent on the pH level in the water. If pH of the droplet is of the order of 5.0, then conversion rates are significantly increased in the presence of O₃, Fe³⁺, Mn²⁺ and other ions. However, at pH level of 4.5, trace metal ions contribute only about 1% per hour to the conversion process and the impact of ozone drops to about 10% per hour. At pH level of 4.0, trace metal ions have negligible impact and ozone adds only about 1% per hour to conversion process. This occurs because, in part, solubility of SO₂ in water decreases with increasing H⁺ concentration.

3. Hydrogen peroxide: It enhances the rate of conversion of SO₂ to SO₄^2- independently of the pH level in water droplets. H₂O₂ dominates the aqueous chemistry process and may increase the conversion rates to 100% per hour depending upon the cloud type, altitude and other meteorological conditions until it is fully exhausted. Afterwards, ozone becomes the dominant catalyst of conversion reactions. H₂O₂ is not important in the formation of NO₃^-. Favourable conditions for the formation of H₂O₂ are low NO_x concentrations and high concentrations of hydrocarbons and aldehydes in the atmosphere. The conditions favourable for ozone formation are unfavourable for H₂O₂ formation.

4. Cations in solution: The rates of formation of SO₄^2- and NO₃^- may be altered by cations in solutions, particularly by ammonium (NH₄⁺). The cations may increase the rate of oxidation of SO2 by more than an order of magnitude. Ammonia can dissolve as a gas in water droplets and thus directly reduce the rainwater acidity. Presence of extra cations enhances the impact of catalysts, especially at pH above 4.5. This results in formation of disproportionately high amounts of SO₄^2- and NO₃^- in presence of cations in solutions than in presence of free H⁺. Ammonium seems to increase the formation of SO₄^2- most in spring when concentrations of both NH₄⁺ and H⁺ are highest. It has been suggested that about 50% increase in NH₄⁺ in Europe since 1950s may have had some impact on the change in SO₄^2- in rainwater relative to H⁺. If soil dust rich in cations like Ca2+ and Mg2+ is loaded into the atmosphere, these cations neutralize the strong acids and the rainwater tends towards alkalinity. For example, in India, strong acidic ions in atmospheres around urban areas are heavily neutralized by such soil dust.

5. Formation of NO₃^-: In areas where concentrations of hydrogen peroxide and ozone are negligible, formation of NO₃^- can control the production of sulphuric acid in atmosphere. The H₂O₂ is not important in the formation of NO₃^-. Though very little is known about conversion of NO_x in aqueous environment, N₂O₅ is supposed to play important role and perhaps of NO₃^- is directly formed from it depending on the relative concentrations of NO_x and NO₃^-. In the night, reaction of oxides of nitrogen with ozone can produce significant amounts of NO₃^- because of the absence of its photochemical destruction.

6. Season and time of day: Season and time of the day have important impact on acidity of rainwater and cloud-water due to following important reasons:

1. Difference in pollutants and ions: There are generally different mixtures of pollutants and ions available for acid conversion at different times of day and in different seasons.

2. Difference in gas-phase reaction rates: In winter, available solar energy is weaker and, therefore, gas-phase chemical reactions are slower than in summer.

3. Difference in concentrations of catalysts: In winters, oxidation in clouds generally decreases because concentrations of appropriate catalysts are lower than in summers. For example, levels of H₂O₂ may be about 16 times higher in summers (about 4.8 ppbv) than in winters (about 0.3 ppbv). This high H₂O₂ concentration in summers enhances the formation of SO₄^2- in that season. At night, conversion of SO₂ to SO₄^2- may reach 10% per hour in good catalytic conditions such as low stratus clouds over water.

4. Difference in photochemistry: The photochemical production and destruction of chemical species in atmosphere depends on the availability and intensity of solar radiation and, therefore, may affect their concentrations during day and night. For example, concentration of NO₃^- increases considerably at night when it is not being destroyed photochemical reactions.

5. Types of clouds and precipitation: Mechanism of removal from the clouds may vary by the types of clouds and precipitation. In the clean background air of southern hemisphere, gas-phase and aqueous-phase reactions are almost equal in importance. However, in northern hemisphere, particularly in winter season, aqueous-phase reactions become dominant.

Chemistry of acid fog

More recently, measurements at sites in parts of Europe, California and eastern U.S.A. have shown that in most circumstances, acid fog and water in low clouds has a lower pH value than equivalent acidic rainwater. Average pH values of acid fog in areas of heavy air pollution are about 3.4 and range from 2.8 to over 5.0 On average, mean concentrations of H+ and acid ions are 3 to 7 times higher in fog-water than in equivalent rainwater. Acid fog-water also has higher concentration of anions and cations. There are following five main reasons for the above describe differences:

Fog being located nearer to the ground, is often exposed to higher pollutant concentrations for longer periods of time than the rainwater during below-cloud scavenging. This exposure allows more time for extensive aqueous-phase chemical processes to take place.

Smaller fog and mist particles saturate with gaseous pollutants more quickly than the larger raindrops, allowing greater aqueous ion production.

The smaller droplets in fog and mist have a greater combined surface area compared to raindrops. As a result, acid gas diffusion is enhanced and higher concentrations of the resultant ions are produced.

The fog remains in the air mass in which it is formed while precipitation is often associated with changing air masses in frontal situations when much of the gaseous and aerosol material in the atmosphere is removed.

Pollutant aerosols originating several hundred kilometers away often act as nuclei for fog or cloud droplets and enhance aqueous chemical processes. The size and number of water droplets formed and the resultant chemistry depend on the number of aerosol nuclei available in the cloud. Greater number of these generally produce smaller and more numerous fog droplets. Ion concentrations in mist tend to be lower than in fog because mist contains a lesser number of droplets and this limits the chemical reactions.

However, fog-water shows wide variations in ion concentrations between sites and events. In stable atmosphere, low altitude fog masses are more likely to interact with pollutant emissions near the surface e.g. NO_x from automobiles. On the other hand, mountain fogs occurring in a well mixed atmosphere and at times, isolated from low-altitude pollutant emissions due to inversions, tend to be cleaner having pH values of 5.0 and above. On minor scale, dew from polluted atmosphere can also be acidic with free H+ comprising about 80% of acidity while species of sulphur and nitrogen may contribute about 60% and 30% respectively to the acidity. 

HYDROSPHERE OF EARTH

All the water in its various forms, present in all thecomponents of Earth’s environment together constitutes the hydro­sphere. Most of the water is present as liquid water on the Earth’s surface and some liquid water is present underground. Apart of total water is present as snow or ice on the Earth’s surface while a substantial part of water in Earth’s environment is also present as water vapor in the atmosphere. General fea­tures of various parts of the Earth’s hydrosphere are as given below:

1. Oceans: Major part of water is present in the oceans of the Earth. Average depth of oceans is about 3.7 kilometers and about 1300 million cubic kilometers water is present in oceans.

2. Ice sheets: Substantial quantity of water, about 24 million cubic kilometers, is present as solid in the ice sheets of Earth. About 90% of the volume of such water is found in Antarctica.

3. Groundwater: About 24 million cubic kilometer water is present under the ground surface at depths of upto two kilometers.

4. Lakes and rivers: On the land surface, approximately 0.18 million cubic kilometer water is present in lakes while about 0.002 million cubic kilometer water is found in rivers.

5. Atmospheric moisture: The amount of water present as water vapor in the atmosphere is about 0.013 million cubic kilometer.

6. Biological water: In addition to above categories, about 0.001 million cubic kilometer water is contained in the bodies of living organisms.

The Earth did not have any hydrosphere in the beginning. It is thought that hydrosphere emerged as the result of processes taking place in the lithosphere. These processes released a substantial quantity of water vapor and juvenile waters during the geological history of Earth. Further, the amounts of water present in oceans, as ice and in atmosphere have fluctuated with appearance and disappearance of major periods of glaciations on the Earth. Palaeogeographic data indicates that the level of oceans on Earth had declined by more than 100 meters during the age of greatest glaciations during Quaternary period. It is estimated that if ice sheets present on Earth today were to melt completely, the level of oceans of Earth would rise by about 66 meters.

Most important feature of global environment of Earth is the hydrological cycle which determines the distribution of water on Earth’s surface and in the atmosphere. The water is evaporated from surface of water bodies like oceans, lakes, rivers etc. as water vapor into atmosphere. It is also absorbed by plants from soil and lost as vapor to atmosphere through transpiration. The water vapor condenses in the atmosphere to form precipitation and thus water is returned from the atmosphere to the surface of Earth. Cyclic movement of water in different components of the global environment is termed hydrological cycle.

OCEAN-WATER
Major portion of earth’s surface is covered with oceans. The water body in the oceans absorbs a large amount of solar radia­tion and has far reaching impact on the heat balance of earth. The water in oceans moves up and down as well as from one place to other. These movements of oceanic water result in transfer of heat from one place to other and have fundamental influence on various components of water and energy balance of land and oceans. The distribution of water balance components has major role in creating and maintaining the climatic and weather conditions in different regions of earth. Therefore, a brief discussion of various aspects of ocean water, ocean currents and waves has been given below.

COMPOSITION OF SEA WATER

Sea water may be described as a brine i.e. the solution of dissolved salts which have accumulated over past periods of geological time from the inflow of runoff water from the land masses. On the land masses, the salts have been formed by the process of weathering of rocks in which weak acids corrode and dissolve the rocks forming various minerals. Due to evaporation of water from oceans, the concentration of salts in the sea water rises resulting in rise of salinity

The composition of sea water results in important properties which are important in understanding its role in Earth’s environment. One way to describe the composition of sea water is to state the principal ingredients that would be required to make an artificial brine approximately like sea water. These ingredi­ents are listed below:

Table-2: Ingradients in ocean water.

Salt	gm salt per 1000 gm water
Sodium chloride (NaCl)	23
Magnesium chloride (MgCl2)	5
Sodium sulfate (Na2SO4)	4
Calcium chloride (CaCl2)	1
Potassium chloride (KCl)	0.7
With other minor ingredients to total	34.5

Of the various elements combined in these salts, Chlorine alone makes up 55% by weight of all the dissolved matter and Sodium 31%. In addition to elements of above listed five salts, less abundant but important are Bromine, Carbon, Strontium, Boron, Silicon and Fluorine. At least some traces of half of the known elements can be found in sea water. Small amounts of all the gases of the atmosphere are also present in dissolved form in the sea water. Chief among these are Nitrogen, Oxygen, Carbon dioxide, hydrogen and argon.

SALINITY OF SEA WATER

Salinity is described as the proportion of dissolved salts to pure water. It is usually stated in units of parts per thousand by weight and is designated by a special symbol ⁰/₀₀. The total figure 34.5 ⁰/₀₀ in the above table represents 3.45 percent. Salinity of sea water varies slightly from one place to other in the oceans. Where diluted by abundant rainfall, as in the equatorial oceans, the salinity may be between 34.5 and 35.0 ⁰/₀₀, whereas in the subtropical high-pressure belts, where extreme dryness prevails, the evaporation may increase the salin­ity of surface sea water upto 35.5 ⁰/₀₀.

DISSOLVED GASES

Most important gases dissolved in the oceanic water are oxygen and carbon dioxide. The quantity of oxygen dissolved in the ocean­ic water changes within wide boundaries depending on the tempera­ture, living activities and certain other factors. The concentra­tion of carbon dioxide dissolved in sea water also changes but such change is has negligible importance since the overall quan­tity of carbon dioxide dissolved in sea water is about sixty times more than its amount in atmosphere. Carbon dioxide in sea water is assimilated by autotrophic organisms during photosynthe­sis and enters the organic matter cycle. Part of such assimilated carbon dioxide returns back to ocean water by respiration and after death and decomposition of living organisms but a substan­tial part of such assimilated gas is deposited as carbonate sediments at the bottoms of oceans.

DENSITY OF WATER

The density of water is given in grams per cubic centimeter. The density of pure water is greatest at 4 degrees C. At this tempera­ture, one cubic centimeter of water weighs exactly one gram i.e. its density is 1.000. However, the density of sea water ranges from 1.027 to 1.028. Two factors determine the density of sea water: salinity and temperature. Density increases with salinity and with low temperature upto -2 degrees C.

Density of sea water is of prime importance in circulation of ocean waters because slight density differences causes water to move. Where density of sea water increases by lowering of temperature of evaporation at the surface, the water tends to sink displacing less dense water below it. Just like convection wind systems, such vertical movements of sea water are also described as convectional currents.

OCEAN CURRENTS

Surface winds and density differences are two most important factors in generating and controlling ocean currents. Another factor affecting ocean current is the configuration of ocean basins and coasts.

Virtually all of the important surface currents of the oceans are set in motion by prevailing winds. Energy of winds is transferred to sea water by the frictional drag of the air blowing over the water surface. The Coriolis force impels the water drift toward the right of its path of motion in Northern hemisphere and, therefore, the current at the water surface is in the direction approximately 45 degrees to the right of the wind direc­tion. Under the influence of winds, currents may tend to bank up the water close to the coast of a continent, in which case the force of gravity, tending to equalize the water level, will cause other currents to be set up.

Density differences in oceans arise from greater heating by insolation or greater cooling by radiation, in one place than another. Thus the surface water chilled in the arctic and polar seas sinks to the ocean floor and spreads equatorward displacing upward the warmer, less dense water. Density differences can also be set up due to salinity differences. A currents tends to flow from the area of low salinity to the area of higher salinity, but this flow is also deflected by Coriolis force through a right angle in Northern hemisphere so that the flow is actually paral­lel with the slope of density gradient between the two places.

Configuration of ocean basins and coasts also affects the ocean currents. Currents initially set up by winds impinge upon a coast and are locally deflected to a different path or are confined in straits or gulfs.

The combined action of wind and density differences sets up the global oceanic circulation system including not only horizontal motions but vertical upswelling and down-sinking motions also. Oceanic currents of a shallow surface water zone have strong climatic influence upon overlaying layer of atmosphere and, hence, have been briefly discussed below.

GENERALIZED SCHEME OF OCEAN CURRENTS

There are a number of defined and permanent oceanic currents involving different oceans at the global scale. They have important impact on the climatic conditions of different regions of earth. It is now well recognized that oceanic circulation involves the complex motions of water masses of different temperatures and salinity characteristics. Important such movements have been briefly described below. However, this account does not take into account the movements of water masses at different depths.

1. Most striking features of generalized oceanic currents are the gyrals. These are circular movements of sea water around the subtropical highs, centered about 25 to 30 degrees S.

2. Two equatorial currents marks the belt of trade winds. Whereas the trade winds blow to the southwest and northwest obliquely across the parallels of latitudes, the water movement of this current flows the parallels. Thus the currents are turnd at an angle of about 45 degrees with the prevailing surface winds, because of the deflective force of the earth’s rotation.

3. The equatorial countercurrent separates the north and south equatorial currents and flows in opposite direction to them. It is well developed in the Pacific, Atlantic and Indian oceans.

4. Along the west sides of the oceans in low latitudes, the equatorial currents turn poleward forming warm currents parallel­ing the coasts. Examples of such oceanic currents are Gulf Stream (Florida and Caribbean stream), Japan current (Kuroshio) and the Brazil current. These currents bring higher than average tempera­tures along the respective coasts.

5. The west-wind drift is the slow eastward movement of oceanic water over the zone of westerlies. It covers a broad belt between 35degrees and 45degrees in the Northern hemisphere and between 30-35degrees to 70-75 degrees in Southern hemisphere where open ocean exists in the higher latitudes.

6. The west-wind drift, upon approaching the east side of the ocean is deflected both south and north along the coast. This results in equatorward flow of cool current produced by upwelling of colder water from greater depths. The Humboldt current (Peru current) off the coast of Chile and Peru, Benguela current off the southwest African coast, California current off the west coast of U.S.A. and Canaries current off the Spanish and North African coast are such currents. These currents bring colder than average temperatures along the respective coasts.

7. The North Atlantic current is a relatively warm current formed in the northern eastern Atlantic Ocean due to poleward deflection of west-wind drift. The current spreads around the British Isles, into the North Sea and along the Norwegian coast bringing about warming effect in summers alongwith it. This effect is more pronounced in winters.

8. In the Northern hemisphere, where the polar sea is largely landlocked, cold water flows equatorward along the west side of the large straits connecting the Arctic oceans with the Atlantic basin. Three such cold currents are Kamchatka current flowing southward along the Kamchatka Peninsula and Kurile Islands, Labrador current moving south from the Baffin Bay area through Davis Strait to reach the coast of Newfoundland, Nova Scotia and New England.

9. In both north Atlantic and Pacific oceans, the Icelandic and Aleutian lows coincide in a very rough manner with two centers of counterclockwise circulation involving the cold arctic currents and the west-wind drifts.

10. The Antarctic region has a relatively simple current scheme. It consists of a single Antarctic circumpolar current moving clockwise around the Antarctic continent in latitudes 50 to 60 degrees S where the expanse of open ocean exists. well.

OCEAN WAVES

Almost all the waves in the oceans that can be seen and felt, are produced by wind. The energy of moving air is transferred to water wave motion and this can, in turn, be expended upon the coasts of the lands causing the landforms of erosion and deposition. Thus ocean waves have important role in global energetics and coastal environments. A brief description of the waves in deep water, their growth and decay is given below.

The ocean waves generated by wind belong to a type known as progressive oscillatory waves since wave form travels through the water and causes an oscillatory water motion. Following terms are used in description of waves:

1. Wave height: It is the vertical distance between the trough and the crest of the wave and is usually measured in feet or meters.

2. Wave length: It is the horizontal distance from trough to trough or crest to crest and is also stated in feet or meters.

3. Wave velocity: It is the speed at which wave advances through water and is given in feet or meter per second or in knots (nautical miles per hour).

4. Period: It is the time elapsed between successive passages of wave crests past a fixed point and is given in seconds.

In the progressive oscillatory wave a tiny particle, such as a drop of water or small floating object, completes one vertical circle, or orbit, with passage of each wave length. Particles move forward on the wave crest, backward in the wave trough. At the sea surface the orbit is of same diameter as the wave height, but dies out rapidly with depth.

In the long waves the water particles return to the same starting point at the completion of each orbit. Hence there is no net motion in the direction of the wind. Only the energy of the wave and its form are transmitted through the water. However, in case of steep, high waves the orbits are not perfect circles. The particle moves just a bit faster forward when on the crest than when it returns in the trough, so that at the end of each circuit the particle makes a slight advance. This produces a very slow surface drift in the direction in which waves are traveling. The rate of this drift is called mass transport velocity. Under favorable conditions, the flow may reach a velocity as high as two knots and will tend to raise the water level along a coast against which the waves are breaking. This motion is not the same as set up by wind friction.

Ocean waves are usually not simple parallel crests and troughs. Instead, they appear highly irregular in height and form because of the interference among several wave trains that are normally present. These trains are not only of different periods, but travel in slightly different directions, so as to intersect at many points. Where two wave crests intersect, the wave height is increased, forming a peak. Where two troughs intersect, the depression is accentuated.

Two forms are waves are usually recognized: wind waves and swell.

(a) Wind waves: These are waves that are being formed and active­ly maintained by the wind. These grow through two mechanisms:

(1) The direct push of wind upon the windward slope of wave drives it forward, just as with floating object.

(2) The skin drag of air flowing over the water surface exerts a pull in the direction of wave motion. Over the wave crest, where drag is strongest, the orbital movement is supplemented adding energy to the wave. In the trough, which is protected, drag is weaker, hence does not counteract the reverse orbital movement as strongly as it is assisted on the crests. This results in a steady increase in the wave height and wave length to some maximum point possible under given wind strength. The wind waves commonly reach speeds much faster than the winds that produce and sustain them. This condition is possible only through the mechanism of skin drag.

The maximum height to which wind waves can grow is controlled by three factors.

(i) Wind velocity: It is obviously a major factor since this determines the amount of energy that can be supplied to the wave.

(ii) Duration of wind: This determines whether or not the waves the opportunity to grow to maximum size.

(iii) Available expanse of water (fetch): This is important because the waves travel as they grow. If waves are developed in a very large body of water over a period of many hours, so that neither duration nor fetch are limiting factors, the maximum wave height varies as the square of the wind velocity i.e.

Wave height (ft.) = 0.026 x Wind velocity² (knots)s

This would represent the greatest waves to be expected.

Wind duration is important in the early stages of wave growth. Under strong winds, say 30 knots, waves will continue to grow for more than 32 hours although most rapid growth will be in the first 15 hours. Fetch may be an important limiting factor in small bays and straits but has no appreciable effect for water expanses greater than 1000 kilometers across.

(b) Swell: These consist of wind waves that have left the region where they were formed and are gradually dying out in a region of calm or lesser winds. As waves continue to grow, they not only increase their speed but also become longer i.e. their wave length increases. When they have passed beyond the region of strong winds that formed them, waves are transformed into a swell, consisting of very long, low waves of simple form and parallel, even crests. For each time that the swell has traveled a distance in nautical miles equivalent to its length in feet, the swell looses one-third of its height. The energy is lost by friction from air resistance.

SEISMIC SEA WAVES

When sudden displacements of large earth masses occur on the ocean floor, a series of waves is sent out across the ocean. The cause may be slippage along a fault, a volcanic eruption or a large submarine landslide. The waves thus produced are called the seismic sea waves or tsunami (Japanese). These waves are enormous in length (100 to 200 km) and the height of the waves upon reach­ing the shore is observed to be as great as 50 meters in many cases or may even be upto 100 meters in rare instances. In the deep ocean, wave height is only a foot or two and because their length is much greater than the height, such waves may pass unnoticed by observes in a ship at sea. The period of such waves may be 10 to 30 minutes and the velocity of travel of the wave form may be 450 to 800 km per hour. Upon reaching the shallow water of a coastline, a seismic sea wave has the effect of caus­ing an unusual rise of water level. The low areas are inundated and the wind waves which are superimposed upon them are able to break upon much higher grounds than normal.

SEA ICE ICEBERGS AND ICE ISLANDS

Large area in high latitudes of Arctic and Antarctic regions is characterized by presence of ice in various forms over the oceans. Sea ice, pack ice, icebergs and ice islands are important such forms of ice in these regions.

Sea ice is formed by direct freezing of ocean water. It begins to form when the surface water is cooled to temperatures of about -2⁰C and is limited in thickness to about 5 meters be­cause once the insulating layer of floating ice has been formed over the water, heat is supplied from the underlying water as rapidly as it is lost from upper surface. Surface zone of sea ice is composed of fresh water, the salt being excluded in the proc­ess of freezing.

Pack ice is the name given to the ice that completely covers the sea surface. Under the force of wind and currents, pack ice breaks up into individual patches which are termed ice floes. Narrow strips of open water between such floes are called leads. Where ice floes are forcibly brought together by winds, the ice margins buckle and turn upward into pressure ridges resembling walls or irregular hummocks.

The North Polar Sea, which is surrounded by land masses, is normally covered by pack ice throughout the year, although open leads are numerous in the summer. The relatively warmer North Atlantic drift maintains an ice-free zone off the northern coast of Norway. In Antarctica, a vast ocean bounds the sea ice zone on the equatorward margin. Because the ice floes can drift freely north into warmer waters, the Antarctic ice pack does not spread beyond about 60⁰S latitude in the cold season. In March, close to the end of the warm season, the ice margin shrinks to a narrow zone bordering the Antarctic continent.

Icebergs and ice islands differ from sea ice in origin and thickness. Icebergs are formed by breaking off or calving of the blocks from a valley glacier or tongue of an ice cap. These may be several hundred meters in thickness. Icebergs are only slight­ly less dense than the sea water and so these float very low in the sea water, about 5/6^th of the bulk lying below the water level. The ice in icebergs is fresh since it is formed of com­pacted and re-crystallized snow. In the Northern hemisphere, icebergs are derived mostly from glacier tongues of the Greenland icecap. They drift slowly south with Labrador and Greenland currents and may find their way into North Atlantic sea in the vicinity of Grand Banks of Newfoundland. Icebergs of Antarctic region are distinctly different from those of arctic region. Whereas those of arctic region are irregular in shape and , therefore, present rather peaked outlines above sea water, the Antarctic icebergs are commonly tabular in form with flat tops and steep cliff-like sides. This is because the tabular icebergs are parts of ice shelves, the great, floating plate-like exten­sions of the continental icecap. In dimensions, a large tabular iceberg of the Antarctic may be tens of kilometers broad and over 700 meters thick, with an ice wall rising 70-100 meters above sea level.

Ice islands of North Polar Sea are somewhat related to tabular icebergs of Antarctic in origin. These are huge plates of floating ice which may be 25 kilometers across and have an area of 300 to 400 square kilometers. The bordering ice cliff, 7 to 10 meters above sea level indicates an ice thickness of 70 meters or more. The few ice islands known are probably derived from a shelf of land-fast glacial ice attached to Ellesmere Island about 83degrees N latitude. The ice islands move slowly with the water drift of Polar Sea and a charting of their tracks reveals much about the circulation in that ocean.

FACTORS AFFECTING PLANT SENSTIVITY TO AIR POLLUTANTS

Pollutant	Factor	Sensitivity	Comment
C2H4 pollution	Tissue age	Epinasty in immature leaves. Other symptoms on oldest leaves first.	Tissues with high natural C2H4 are more sensitive.
C2H4 pollution	High temperature	Sensitivity increased
C2H4 pollution	Other pollutants	Effects inhibited by high levels of SO2 or CO2
Cl2 pollution	Bright sunshine	Sensitivity increased
Cl2 pollution	Tissue age	Little effect; in conifers, current year’s needles most sensitive	Immature leaves tolerant in some species.
Cl2 pollution	Wet leaves	No effect
Cl2 pollution	Drought	Sensitivity decreased
Cl2 pollution	Low temperature	Sensitivity decreased in pines	Symptoms take longer to develop.
Cl2 pollution	Plat age	Seedling less sensitive than oleder plants
HCl pollution	Tissue age	Young, fully expanded leaves most sensitive	Immature leaves tolerant.
HCl pollution	Plant age	Seedlings less sensitive than mature plants	Older plants become more tolerant.
HCl pollution	High relat. Humidity	Sensitivity increased
HCl pollution	Ca-deficit	Sensitivity decreased	Effect shown for Nasturtium. May be different for other species.
HCl pollution	Ca-excess	Sensitivity increased	Effect shown for Nasturtium. May be different for other species.
HCl pollution	Cl-deficit	Sensitivity increased	Effect shown for Nasturtium. May be different for other species.
HCl pollution	Cl-excess	Sensitivity increased	Effect shown for Nasturtium. May be different for other species.
HCl pollution	Mg-deficit	Sensitivity increased
NH3 pollution	Concentration	Variable	Some conifers sensitive at moderate but tolerant at high levels.
NH3 pollution	Tissue age	Little effect
NH3 pollution	Darkness	Variable
NH3 pollution	Drought	Sensitivity decreased
NH3 pollution	Wet leaves	Sensitivity increased	Symptoms develop faster.
NOx pollution	Ca-excess	Sensitivity decreased	Opposite effect in some species.
NOx pollution	Tissue age	Immature leaves/needles most sensitive
NOx pollution	Cultivar	Highly variable; especially in gladiolus & tomato	In gladiolus, sensitivity related to flower colour
NOx pollution	High relat. humidity	Sensitivity increased
NOx pollution	Low temperature	Sensitivity decreased; symptom expression delayed
NOx pollution	Drought	Sensitivity decreased; symptoms induced in conifers needles previously exposed.	Sensitivity increased in some fruit trees.
NOx pollution	N-deficit	Sensitivity decreased	Opposite effect in some species.
NOx pollution	Ca-deficit	Sensitivity decreased	Opposite effect in some species.
NOx pollution	N-excess	Sensitivity decreased	Opposite effect in some species.
NOx pollution	P-excess	Sensitivity increased	Opposite effect in some species.
NOx pollution	Other pollutants	Interaction with SO2, NO2, O2 & hydrocarbons	Response varies with concentrations and relative proportions.
NOx pollution	K-deficit	Sensitivity increased	Opposite effect in some species.
O3 pollution	K-excess	Variable sensitivity
O3 pollution	N-excess	Variable sensitivity
O3 pollution	Plant age	Young plants most sensitive
O3 pollution	Tissue age	Intermediate leaves usually most sensitive
O3 pollution	Darkness	Sensitivity decreased	Plants grown in low light are more sensitive. High light during exposure increases injury.
O3 pollution	Wet leaves	Variable sensitivity
O3 pollution	Drought	Sensitivity decreased
O3 pollution	Other pollutants	Interactions with SO2, NO2, PAN & heavy metals	Response varies with species, concentration & relative proportions
O3 pollution	High soil salinity	Sensitivity decreased
O3 pollution	High relat. Humidity	Sensitivity increased
O3 pollution	S-excess	Sensitivity decreased
O3 pollution	N-deficit	Variable sensitivity
O3 pollution	P-deficit	Sensitivity decreased
O3 pollution	Low temperature	Sensitivity decreased	Sensitivity decreases again above 30 degree C. Response varies according to dose.
O3 pollution	K-deficit	Variable sensitivity
PAN pollution	High relat. Humidity	No effect
PAN pollution	Tissue age	Young, rapidly expanding leaves most sensitive	Sensitivity strongly affected by physiological age, results in bands of damage.
PAN pollution	Other pollutants	Interactions with O3 & SO2	Response varies with concentrations & pollutant.
PAN pollution	Drought	Sensitivity decreased
PAN pollution	Time of day	Sensitivity more in morning than after noon
PAN pollution	Darkness	Injury eliminated. Sensitivity increases with increased light intensity	Presence of light before, during and after exposure must for injury to occur.
PAN pollution	Low temperature	Injury decreased
PAN pollution	Plant age	Young plants more sensitive
SO2 pollution	Other pollutant	Interaction with O3, NO2, HF	Response varies with concentrations and relative proportions.
SO2 pollution	Time of day	More sensitivity when sugar content low	In many plants in the morning.
SO2 pollution	High relat. Humidity	Sensitivity increased
SO2 pollution	Drought	Sensitivity decreased
SO2 pollution	High wind	Sensitivity increased
SO2 pollution	Wet leaves	Variable; may increase
SO2 pollution	Darkness	Sensitivity decreased	Some plants e.g. Potato not closing stomata at night may be unaffected.
SO2 pollution	Low temperature	Sensitivity decreased	Susceptibility to frost injury increased by SO2 exposure
SO2 pollution	Plant age	Seedlings more sensitive than older plants
SO2 pollution	Season	Grasses more sensitive in winter; conifers more in April/May than in July/August	Not vry important for very short exposures.
SO2 pollution	N-deficit	Sensitivity decreased
SO2 pollution	S-deficit	Sensitivity decreased
SO2 pollution	P-deficit	Sensitivity decreased
SO2 pollution	N-excess	Sensitivity decreased
SO2 pollution	K-deficit	Sensitivity increased
SO2 pollution	S-excess	Sensitivity increased
SO2 pollution	Tissue age	Most in young, fully expanded leaves
SO2 pollution	Ca-deficit	Sensitivity increased

HABITAT AND PLANT RESPONSE

Cause	Common name	Botanical name	Plant part	Symptom
Cold area	Pea	Pisum sativum	Leaf	Bronzing
Cold area	Bean	Phaseolus vulgaris	Leaf	Bronzing
Cold area	Spinach	Spinacea oleracea	Leaf	Bronzing
Dry area	Pine	Pinus sp.	Needle	Tip burn; sharp boundaary between necrotic and healthy tissue
Dry area	Spruce	Picea sp.	Needle	Tip burn; sharp boundaary between necrotic and healthy tissue
Dry area	Fir	Abies sp.	Needle	Tip burn; sharp boundaary between necrotic and healthy tissue
Dry area	Aspen	Populus tremula	Leaf	Tip necrosis with sharp boundary
Dry area	Plum	Prunus domestica	Leaf	Marginal/interveinal necrosis, boundary chlorotic & diffuse
Dry area	Wych elm	Ulmus glabra	Leaf	Tip necrosis with sharp boundary
Dry area	Willow	Salix sp.	Leaf	Tip necrosis with sharp boundary
Dry area	Cherry	Prunus avium	Leaf	Marginal/interveinal necrosis, boundary chlorotic & diffuse
Dry area	Oak	Quercus rubur (Q. pedunculata)	Leaf	Interveinal brown/bronzed lesions
Dry area	Apple	Malus sylvestris	Fruit	Dark lesions
Dry area	Cherry	Prunus avium	Fruit	Brown/black depression at tip
Dry area	Pear	Pyrus communis	Fruit	Brown/black depression at tip
Dry area	Tomato	Lycopersicum esculentum	Fruit	Blossom end rot
Water logged area	Lucerne (alfalfa)	Medicago sativa	Leaf	White stipple
Hot area	Onion	Allium cepa	Leaf	Tip necrosis; white/grey stipple
Hot area	Horse chestnut	Aesculus hippocastanum	Leaf	Marginal necrosis
Hot area	Lettuce	Lectuca sativa	Leaf	Brown stipple of veins
Hot area	Lucerne (alfalfa)	Medicago sativa	Leaf	White stipple
Cold area	Lucerne (alfalfa)	Medicago sativa	Leaf	Necrotic stipple
Cold area	Potato	Solanum tuberosum	Leaf	Black discolouration; necrotic stipple

NUTRIENT DEFICIENCY SYMPTOMS IN PLANTS

Cause	Common name	Botanical name	Plant part	Symptom
K-deficit	Pear	Pyrus communis	Leaf	Upward curling
K-deficit	Pea	Pisum sativum	Leaf	Brown stipple
K-deficit	Potato	Solanum tuberosum	Leaf	Bronzing of lower surface; marginal, later interveinal necrosis
K-deficit	Tomato	Lycopersicum esculentum	Leaf	Marginal, later interveinal necrosis
K-deficit	Lucerne (alfalfa)	Medicago sativa	Leaf	Marginal, later interveinal necrosis
P-deficit	Cabbage	Brassica oleracea var. capitata	Leaf	Red/purple discolouration
P-deficit	Lucerne (alfalfa)	Medicago sativa	Leaf	Red/purple discolouration
P-deficit	Potato	Solanum tuberosum	Leaf	Curling or rolling
S-deficit	Tomato	Lycopersicum esculentum	Leaf	Very pale new leaf; Bleached old leaf
Ca-deficit	Tulip	Tulipa gesneriana	Leaf	Necrosis
Ca-deficit	Tulip	Tulipa gesneriana	Plant	Wilting
Mg-deficit	Pear	Pyrus communis	Leaf	Black necrosis
Mg-deficit	Potato	Solanum tuberosum	Leaf	Brown stipple
Mg-deficit	Tomato	Lycopersicum esculentum	Leaf	Curling & brittleness
Mn-deficit	Oats	Avena sativa	Leaf	Grey discolouration
Mn-deficit	Wheat	Triticum vulgare	Leaf	White dashes
Mn-deficit	Wheat	Triticum vulgare	Plant	Pale floppy appearance
Mn-deficit	Barley	Hordeum vulgare	Plant	Pale floppy appearance
Mn-deficit	Barley	Hordeum vulgare	Leaf	White dashes
Zn-deficit	Lucerne (alfalfa)	Medicago sativa	Leaf	White stipple on lower & Bronzing of upper surface
Zn-deficit	Tomato	Lycopersicum esculentum	Leaf	Interveinal chlorosis, later necrosis
Mn-excess	Cauliflower	Brassica oleracea var. botrytis	Leaf	Brown/purple stipple
Mn-excess	Potato	Solanum tuberosum	Leaf/stem	Black stipple or streaks