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(i) Natural sources:
♤ seas and oceans,
♤ volcanic eruptions,
♤ Biological processes in the soil e.g., Decomposition of organic matter.
(ii) Man-made sources:
♤ burning of coal (60% of SO2) and
♤ petroleum products (30% of SO2), and
♤ The smelting of metal sulfide ores to obtain the pure metals.
♤ Industrial production of Sulfuric acid in metallurgical, chemical and fertilizer industries.
(b) Nitrogen Natural sources:
♤ lightening,
♤ volcanic eruption, and
♤ Biological activity.
Anthropogenic sources:
♤ Forest fires
♤ Combustion of oil, coal, and gas
(c) Formic acid
♤ Biomass burning due to forest fires causes emission of formic acid (HCOOH) and formaldehyde (HCHO) into the atmosphere.
♤ Large fraction formaldehyde gets photo - oxidation and forms formic acid in the atmosphere.
These are three main compounds that cause acidification of rain in the atmosphere.
(d) Other Acids:
♤ Chlorine
♤ Phosphoric acid
♤ Hydrochloric acid (smokestacks).
♤ Carbon monoxide and carbon dioxide (automobiles). These become carbonic acid.
Does it occurs only in industrial areas alone?
SOX and NOX that create Acid Rain are often transported to distances far away from their points of origin by the wind so that the adverse effects of pollution are also experienced at place remote from the place of genesis. The problem is further compounded as the environmental damage caused by acid rain is not uniform, but is area- specific.