ACID RAIN

Short desdription about acid rain
Along with some chief air pollutants like aluminium, cadmium, lead, zinc and arsenic, oxides of sulphur and nitrogen lead to the formation of acid rain which causes acid rain or black rain.
Unpolluted or normal rain has a Ph of 5.6; the acidity is due to the presence of CO2 in air. But acid rain has Ph value of 4 to4.5 which is mainly due to SO2 (Oxides of sulphur) and oxides of nitrogen present in atmosphere. Problem of acid rain the world started only after the industrial revolution and after the extensive use of fossil fuels.
Acid precipitation or acid rain is a process of deposition of acid gases (SO2, nitrogen oxide) from the atmosphere on land in the form of precipitation or rain. It thus increases H-ion concentration of precipitation. During acid rain fall acidic gases mix with water vapors forming the respective acids.

CAUSES OF ACID RAIN:

The incomplete combustion of hydrocarbons is very harmful. It gives the effect of black snow covering the hill tops.
Oxides of sulphur and nitrogen (SO2, NO, NO2) are the main air pollutants produced mainly by the combustion of fossil fuels for power generation. In the industrial area the concentration of SO2 may reach to that extent that it damages the plants and some objects forming sulphuric acid. The sulphur dioxide present in the atmosphere reacts with water and forms sulphuric acid which later falls as acid rain.
In the atmosphere, SO2, NO2 do not remain in gaseous state for a long time ratehr they react with moisture to form respective acids (H2SO4 and HNO3) which then dissolve in wter vapour in the atmosphere and fall on the earth as acid rain or may remain in atmosphere in the form of clouds or fogs. These acidic oxides may even undergo physical and chemical transformations to produce toxic agents. About 60-70% of acids in the atmosphere are derived by the oxidation and hydrolysis of SO2 and H2S and rest 30-40% from various nitrogen compounds and other compounds.

EFFECT &CONSEQUENCE OF ACID RAIN:

1. Effect on biochemical cycle: The major impact of acid rain is on the nitrogen and sulphur cycle (biogeochemical cycle). Nitrogen cycle in the atmosphere is more sensitive to acidification than the other major nutrient cycles. PH level of soil and water affects the survival of Rhizobium bacteria and free living bacteria. It also affects dentrification and volatilization of ammonia.
2. Effects on aquatic and terrestial flora and fauna: The acid rain raises the acidity of water of pond and lakes. This acidification kills many bacteria, cynobacteria, algae, zooplanktons, etc. thus disrupting the ecological balance. Acid rain is responsible for top-drying of the forest trees. Calcium and Potassium nutrients also leach away from soil by acids. Soil acidity affects the activity of soil organisms. There is a direct effect of acid on fish. The acid mobilizes metal like aluminium (Al) from the surrounding soil which then enters into lakes by run off. When combined with high acidity, Al becomes toxic to fishes. Other heavy metals like Zn, Cd, Pb, Mg, Cu, etc also become toxic if acidified.
3. Health hazards: Acid rain is responsible for health hazards like cough, irritation of throat, bronchitis, asthma, diarrhoea and other respiratory and cardiac problems.
4. Corrodes the metals: Buildings, monuments, statue, bridges, fences, railways, etc are corroded by the acid rains.
5. Drifting action: Sometimes oxides are produced at one place and carried elsewhere where precipitation occurs and acids are formed. Drifted SO2 and NO2 cause acid rain in some big cities.