Environment of Earth

March 9, 2008


Filed under: Environment — gargpk @ 1:14 pm


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.


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.


SO2 and oxides of nitrogen (NOx) 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 NOx 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 NOx 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 NOx 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 NOx 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 NOx 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.


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



H+ (cation)

SO42- (anion)

NO3 (anion)

Cl (anion)

NH4+ (cation)

Ca2+ (cation)

K+ (cation)

Mg2+ (cation)

Na+ (cation)


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


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






HCOOH (molecule)

CH3COOH (molecule)



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 NOx 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 NOx 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 (SO42-) 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 NOx. 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 NOx coupled with photochemistry and physical dynamics of stratosphere.

Acid gases like SO2 and NOx 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:


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


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

2NO + O2 2NO2


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 NOx 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 SO42- 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. NOx 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:


SO2 + H2O <—- SO2.H2O

SO2.H2O H+ + HSO3

HSO3——— H+ + SO3

O2 + 2HSO3 ——- 2H+ + 2SO42- (Reaction slow without catalyst)

H2O2 + HSO3 ——– H+ + SO42- + H2O (Reaction is rapid)

O3 + HSO3 —— H+ + SO42- + O2 (Reaction is rapid if pH>4.5)


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 SO42- 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 NOx 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 SO42- 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 SO42- and NO3 in presence of cations in solutions than in presence of free H+. Ammonium seems to increase the formation of SO42- 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 SO42- 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 NOx 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 NOx 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 SO42- in that season. At night, conversion of SO2 to SO42- 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. NOx 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.


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.

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