Rains in general cleanse the ground waters apart from recharging the water sources. Rain water is acidic with a pH of 5.6 to 5.7 This is because of the reaction of water with CO2 to give carbonic acid, which is a weak acid. This small amount of acidity is however beneficial to mankind. It is acidic enough to dissolve minerals and make them available to plant and animal life; yet not acidic enough to cause any adverse effect.
We know that in urban and industrial regions, the atmosphere is polluted with oxides of sulphur and nitrogen. We have also seen that SO2 and NO2 in the atmosphere in the presence of moisture bring about photochemical and chemical reactions, which results into sulfuric acid and nitric acid droplets respectively.
The falling rain and snow undergo the same reactions with the formation of huge quantities of sulfuric acid and nitric acid. Man-made and natural emissions of HCL gas also gets mixed in the falling rain.
Sulphuric acid, nitric acid and hydrochloric acid are all strong acids. Hence the pH of the rain water becomes less than about 4.5. this is popularly known as acid precipitation or acid rain.
The formation of acid droplets is catalysed by particulate lime, NH3 metal salt (Nacl) and metal oxides (MgO, ZnO, Mn2 O3, Fe2 O3). These species neutralize the acidity with the formation of sulfates and nitrates. This in turn increases the solubility of SO2 and NO2 in droplets. Both the acid droplets and salts are rapidly removed from the atmosphere by wet precipitation.
Analysis of acid rain samples showed the following species : H-+, NH4+, Na2+, Ca2+, Mg2+, SO42-, NO3-, AND CL-. These contribute from the three acids to the acid rain and mostly in the order H2SO4> HNO3> HCL.
Acid rain represents one of the important drawback of air pollution. They cause a number of diseases to human beings. Skin, hair and lungs will be affected immediately. The heavy metals in the acid rain pollute the drinking water sources which result in variety of health problems.
Acid rain directly damages leaves and shoot. Leaching of nutrients from leaves hinders the growth as well as survival of plants.
Acidification of soil affects microbial and soil fauna that are useful in nitrogen fixation. This also mobilizes unwanted minerals (e.g, Al) that are toxic.
Acidification of fresh water lakes makes them completely sterile. Aquatic flora and fauna are killed. Excess acid in their surroundings puts most fishes and amphibians under great physical stress.
Acid rains also corrode steel and other metal structures in factories increasing the chances of accidents.
Ancient buildings (e.g. TajMahal) and sculpture (several temples in Greece , Italy or India) have been carved from marble, limestone etc. are getting deformed by acid rains.
The extent of damage due to acid rain depends on topography, climate geology and human activity in the region.
The remedial measures to all the effects of acid rain are highly expensive and complicated. The only practical approach is to decrease the emission of oxides of sulphur and nitrogen.
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