Although the phenomenon of “acid rain” (more correctly acid deposition) was identified in Manchester, England, as long ago as 1852, and described more thoroughly in 1872, modern scientific research has been going on only since the mid-1950s. Public concern about the problem began in the late 1960s. Acid rain is an environmental hazard that is transponder in nature. Northeastern America, North Western Europe and India are facing an acute problem of acid rain. Acid rain has affected certain rivers, lakes, streams and forests in United Kingdom (UK), United States of America (USA), Germany and many other countries.
Acid rain literally means ‘the presence of excessive acids in rain waters’. Acid precipitation is a mixture of strong mineral acids sulphuric acid (H2SO4), nitric acid (HNO3) and in some locations, hydrochloric acid (HCl). It usually has a ph of less than 5.6, the value of distilled water in equilibrium with atmospheric carbon dioxide.
Acid in the Rain Water’s
Acid rain problem is a result of anthropogenic activities. Most acids come from cars, homes, industries and power stations but some share is contributed by natural sources such as volcanoes, swamps and planktons. The acid problem is basically associated with the transport and subsequent deposition of oxides of sulphur, nitrogen and their oxidative products. These are produced by combustion of fossil fuels, power plants, automobile exhausts and domestic fires etc.
Formation of Acid Rain
Acid rain is one of the forms of acid deposition which can either be wet or dry, acid rain, snow, dew, fog, frost and mist are the wet form of deposition, while dust particles containing sulphate and nitrates which settle on the ground is called dry deposition.
Wet Acid Rain
Coal, fuelwood or petroleum products have sulphur and nitrogen. These elements, when burnt in atmospheric oxygen,’ are converted into their respective oxides (SO2 and NO3), which are highly soluble in water. By anthropogenic and by natural sources, oxides of sulphur and nitrogen enter the atmosphere.
Reaction with Sulphur
S + O2 = SO2
2SO2 + O2 = 2SO3
Reaction with Nitrogen
NO + O3 = NO2 + O2
NO3 + NO2 = N2O5
When air is saturated with water droplets (humid conditions), N2O5 invariably reacts
with water, vapors to form droplets of HNO3.
N2O5 + H2O = 2HNO3
Besides some HNO2 is also formed
N2O3 + H2O = 2HNO2
SO3 in humid conditions forms droplets of H2SO4.
SO2 + 1/202O2 + H2O = H2SO4
HNO3 and H2SO4 thus formed combine with HCl to generate precipitation, which is commonly referred to as acid rain.
The primary reason for concern is that acid deposition acidifies streams, and taken on coarse, sandy soils low in lime: The effect is seen particularly in headwater areas and in wet montane environments, wherever sulphate loading from anthropogenic sources is strong. The chemical and physical consequences of lake acidification include, increased leaching of calcium from terrestrial soils, mobilization of heavy metals such as aluminium, zinc, and manganese and an increase in the transparency of lake waters. The biological consequences include market changes in communities of aquatic plants and animals, with a progressive lessening of their diversity.
The acid deposition may further impoverish forests soils, developed on sandy substrate poor in lime. As a consequence of accelerated leaching of nutrients, such as phosphorous, potassium, magnesium and calcium from these soils, forest productivity would eventually be reduced. Moreover, the acid sulphate particles that contribute to acid precipitation are in the size range that penetrates deep into the lung, and they may well exacerbate lung diseases and increase mortality rates.