NEET Chemistry Notes Environmental Chemistry – Tropospheric Pollution
Tropospheric Pollution
Tropospheric Pollution
It is caused by gaseous pollutants and particulate matter.
Types of Tropospheric Pollutants
- C02— releases from decomposition of organic matter, from oceans and from fossil fuel combustion. Concentration – 330 ppm throughout troposphere.
- CO-— releases from decomposition of organic matter, industrial processes and from fuel combustion. Concentration – 1 to 50 ppm in urban traffic areas.
- CH4— releases from decomposition of organic matter, natural gas.
Concentration – 1 to 2 ppm throughout troposphere. - NO— releases from electrical discharges, internal combustion engines, combustion of organic matter. Concentration – 0.2 ppm in smog atmosphere.
- 03— releases from electrical discharges, diffusion from stratosphere, photochemical smog.
Concentration – 0.5 ppm in photochemical smog. - S02— releases from volcanic gases, forest fires, bacterial action, industrial processes such as roasting and from fossil fuel combustion.
Consequences of Tropospheric Pollution
- Green house effect is the heating of earth and its objects because of the trapping of IR radiations by C02 (mainly) and other green house gases like CH4,N0,03,CFCs and water vapours in the atmosphere. (In the absence of this effect, the earth would be converted into extremely cold planet.)
- Leakage of methyl isocyanate (MIC) gas caused death of approximately 3200 persons (Bhopal gas tragedy).
- Rain water due to the presence of dissolved carbon dioxide, normally has pH 5.6. When this pH value drops below 5, it is called acid rain (by Robert Augus). Oxides of nitrogen and sulphur are responsible for making rain acidic. In acid rain, the order of concentration of various acids is
H2SO4 >HNo3 >HCl - Acid rain damages the buildings and other structures. Moreover it corrodes metal pipes, results in several diseases.
Particulates
These are tiny pieces of solid or liquid matter associated with the earth’s atmosphere.
Pollutants
Particulates in atmosphere may be viable or non-viable. The viable particulates are the minute living organisms such as bacteria, fungi that are dispersed in atmosphere. Human beings are allergic to some of the fungi found in air. Mist, smoke, fumes and dust are non-viable particulates in atmosphere.
Types of Smog
- Classical and Photochemical Smog
Smog is the combination of smoke particles with tiny droplets of fog. S02 and particulate matter are main components of (London smog). It is mostly observed in cool humid climate. It is chemically reducing in nature. - Photochemical Smog (Los Angeles Smog)
It contains a mixture of primary pollutants such as nitrogen oxides, carbon monoxide and secondary pollutants such as o3 and HCHO. It occurs in warm, dry and sunny climate and are caused by the action of sunlight on nitrogen oxides and hydrocarbons. It is oxidising in nature.
Its formation can be shown as follows:
Stratospheric Pollution
It is mainly concerned with ozone layer depletion. In stratosphere, there is a region of high concentration of ozone (10 ppm), at a height of 23 km, called ozone layer. This layer does not allow the UV rays coming from the sun to reach on the earth. Thus, protects us from harmful effects of UV rays.
Ozone Depletion
Chlorofluorocarbons present in aerosols, air conditioning and refrigeration devices destroy ozone layer and reduce our protection against UV rays from the sun.
One molecule of CFC can destroy more than thousand molecules of o3 . This leads to the formation of ozone hole. CFCs are stable in lower atmosphere but when they reach the stratosphere, they split and become unstable by sunlight.
Ozone Depletion in Antarctica
Stable wind patterns in stratosphere are called polar vortex. It does not allow the o3 rich air to fill up the gap.
Remedial Measures
Industries should purify the smoke to a certain extent before releasing into air. They should use chimneys. Planting more and more trees is also a method to maintain the oxygen-carbon dioxide balance.