In the atmosphere, the sulphur and nitrogen oxides undergo complex chemical processes that transform them into their respective acids such as sulphuric acid and nitric acid.
The formation of acid rain involves the types of phases. These are:
(I) Gas Phase Reactions:
Gas phase reactions predominate in the ninety of emission sources. The oxides can be converted into acids in several ways in the pressure of catalyst such as photo chemically generated free radicals. These free radicals are formed under the influence of sunlight. Sunny weather accelerates the rate of conversion to the formation of acid. After sunset, the rate of conversion falls off rapidly.
(II) Liquid Phase Reactions:
Liquid phase reaction predominates of great distance from the source of emission. However, there is a great deal of controversy regarding the mechanism involved in such reactions. In case of formation of sulphuric acid rain, it is considered essential that the presence of strong oxidizing agents such as metal catalysts are important in the polluted areas.
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