Reactions of Atmospheric Chemistry!
From natural and man-made sources gaseous and particulate matters are emitted mainly into the troposphere. The larger and heavier particulate matters settle down quickly after emission due to gravitational force. The finer and lighter particles remain suspended for a longer period. Some of these particles settle down while the others, particularly, the hygroscopic ones act as cloud forming nuclei and are precipitated along with rain.
The gaseous pollutants emitted into the troposphere mostly undergo chemical transformation due to oxidation reactions (either thermally or photo-chemically) and the transformed substances return back to the earth’s surface either as dry particle or along with rain. The substances which reach the ground as dry particles are referred to as ‘dry depositions’, whereas the substances which come down with rain or snow are termed as ‘wet depositions’.
When rain water contains dissolved acids it is referred to as acid rain. Normally rain contains some dissolved carbon dioxide (CO2) and has a pH level of about 5.6. When rain contains other dissolved acids and salts the pH may be around four. In very polluted atmospheric conditions rain water pH may be as low as three. The sinks of some common primary and secondary gaseous pollutants are listed in Table 2.5.
Solar radiation initiates dissociation of some gas molecules present in the troposphere and thereby produces some free radicals. These free radicals then carry on chain reactions with nitrogen oxides, carbon monoxide, hydrocarbons and sulphurous compounds.
Carbon bearing compounds are oxidized to CO2 H2O, aldehydes, acids, esters, etc. NOx is oxidized to nitrogen dioxide to nitric acid, and sulphur compounds to sulphur trioxide to sulphuric acid. The acids produced may combine with ammonia and air-borne alkaline particles to form salts. The probable intermediate steps through which some of the air-borne gaseous pollutants may undergo chemical transformation and the end products produced are listed hereunder.
Polluted urban atmosphere contains NOx, hydrocarbons and other pollutants. The NOx molecules absorb solar radiation and produce oxidants take O3, O; OH’. These oxidants oxidize the hydrocarbons to aldehydes, peroxides and per-nitrates. Nox are oxidized to nitric acid. Sulphur compounds are oxidized to sulphuric acid. Under favourable temperature and humid conditions the secondary pollutants produce smog.
Only high altitude aircrafts and rockets discharge their exhaust into the stratosphere directly. These exhausts may contain NOx, hydrocarbons, CO, CO2 and H2O. Some pollutants, which have not completely reacted in the troposphere and CFC, which is less reactive under tropospheric conditions, diffuse into the stratosphere from the troposphere.
All of these undergo chemical transformation by reacting with ozone (O3) and some free radicals produced photo chemically in the stratosphere. Some of the probable reactions through which transformations take place and the end products produced are listed hereunder.
I. Free Radical Formation:
III. Nitrogen Oxides:
IV. Chlorofluorocarbons (CFCs):
The transformed products diffuse back to the troposphere from where they are precipitated to the earth’s surfaces.