Environment of Earth

September 16, 2009

Atmospheric aerosols

Filed under: Environment — gargpk @ 3:02 pm
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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.

__________________________________________________________________________

Class                   Size range (mm)                Impact area

____________________________________________________

(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.

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