Rainfall is normally somewhat acidic with a pH of 5.6 to 6.0. However, as a result of air pollution effects much stronger acidities are often observed in polluted air. Some of the major atmospheric pollutant by industrial emission are sulphur dioxide (S02) and nitrogen oxides (NO and N02).
When particles of these substances form condensation nuclei, a dilute solution of sulphuric or nitric acid is produced as the acid get dissolved in water.
The primary pollutant S02 undergoes further reaction in sunlight to form S03 (sulphur trioxide).
S03 which is produced from S02 in the presence of sunlight and oxygen is called secondary pollutant. Subsequently, S03 reacts with moisture to form sulphuric acid
S03 + H20 -» H2SO4
Nitrogen oxide (NO) reacts with the oxygen in the ambient air to form N02. N02 gets converted to nitric acid in presence of water.
4N02 + 2H20 + 02 NO 4HNO,
The biological effects of these acidification changes on the flora and fauna of lakes air severe. Overall, the damage becomes noticeable below pH 6.6 and only few resistant species survive below pH6.
The effect of acid rain on terrestrial and aquatic environments is determined by
1. The acidity of the precipitation and
2. The geology of the soils and rocks in the area.
i. The ecosystem of a stream or lake may be severely affected when its pH falls below 5.
ii. Total biomass in such systems is reduced from two to ten times because few organisms can tolerate acid and because the acid may liberate toxic forms of trace metals.
iii. The diversity of species also decreases.
iv. Acidic conditions affect the reproductive capabilities of nearly all fish, resulting in slow decline-of fish populations over time periods as long as ten years.