Nitrogen oxides get dissolved into water vapour to form nitric acid.These acids are dissolved in water falling down in the form of rain drops. This is called acid rain.
In industrialised countries of the world large quantity of gaseous pollutants is released into the atmosphere by the factories. The most common among the gaseous pollutants are the oxides of nitrogen and sulphur.
Air borne SO2 and SO3 produced naturally or from human activities such as combustion of fossil fuels etc. react with water or water vapour to form H2SO3 and H2SO4.
ADVERTISEMENTS:
SO2 + H2O → H2SO3
SO3 + H2O → H2SO4
Nitrogen oxides get dissolved into water vapour to form nitric acid.
N2O5 + H2O → 2HNO3
These acids are dissolved in water falling down in the form of rain drops. This is called acid rain.
When the pH of rain water or snow is less than 5.7 it is called ACID RAIN. Acid rain is man-made ecological event. It is mainly due to H2SO4 and HNO3. The ratio of acids varies depending upon the amounts of oxides of sulphur or nitrogen liberated into the atmosphere. On the earth surface acid rain increases the acidity of water and thus destroys aquatic vegetation and lowers the oxygen content of water.
ADVERTISEMENTS:
Sources of Acid Rains:
There are two main sources of acid rains:
1. Oxides of sulphur:
Sulphur dioxide (SO2) and sulphur trioxide (SO3) are released into the atmosphere by the burning of fossil fuels.
ADVERTISEMENTS:
2. Oxides of nitrogen:
Oxides of nitrogen are released into the atmosphere by the automobiles exhaust and other sources.
Acid rain problem has increased tremendously by industrialization. Burning of fossil fuel for energy production has contributed 60 – 70 per cent of the total SO2 liberated globally. Acid rain problem has become a global problem because these oxides can travel thousands of kilometres and during their journey they may undergo some physical and chemical transformations which may cause acid rains and produce hazardous products.
Formation of Acid Rain:
ADVERTISEMENTS:
1. Action of SO2 leading to acid rain. It is formed in two steps:
(a) Conversion of SO2 into SO3 by absorption of UV radiation
(b) Reaction of secondary pollutant SO3 with moisture to form sulphuric acid. Sulphuric acid dissolved in rain drops falls on the earth surface as acid rains.
2. Action of NO2 leading acid rain. NO2 acts to produce acid rain in two steps,
(a) Formation of NO2 and N2O5 by absorption of UV-radiation from the sun.
(b) Reaction of NO2 and N2O5 with atmospheric moisture to form nitric acid which falls down as acid rain
Factors Determining the Acidity:
Rain water gets acidified on its way down to the earth. Its acidity depends on the amounts of oxides of sulphur and nitrates available for dissolution in the water.
The acidified rain water is mixed with already existing water on the earth. The resulting pH after this mixing determines the pH of the aquatic environment.
This is influenced by the following factors:
(i) The neutralizing ability of the aquatic medium and
(ii) The acidity of rain water.
Generally, lakes and rivers show a poor neutralizing capacity because their low contents. Only the rocky areas having CaCO3 exhibit a buffer action and maintain a pH of about 5 – 6. But continued occurrence of acid rain is bound to lower the pH of the aquatic medium.
Consequences of Acid Rain:
1. Acid rain declourizes the leaf pigments thus causes chlorosis and necrosis of leaves, which adversely affect agricultural production.
2. It inhibits CO2 fixation and photorespiration and adversely affects the productivity of crops.
3. It inhibits seed germination and growth of seedlings. Alfalfa, barley, cotton, lettuce and spinach are most susceptible to acid rains.
4. It affects the enzymatic activities.
5. It may cause wilting of plants.
6. Acid rain increases the acidity of water bodies and consequently destroys the various aquatic vegetation and animals. Many fishes die in the lakes and their population are affected.
7. Acid rain causes irritation in the eyes and skin of human beings.
8. Since the acid rain water may dissolve some toxic metallic pollutants, it can directly enhance their toxic action and may pose a serious threat to plant and animal life.
9. Acid rains change the pH of soils and affect the plant growth in various ways.
10. Besides above, it may cause corrosion of many buildings, bridges, monuments, railings facings etc. This would result in putting off external surface and weakening of the structures.
Controlling acid rain:
There are several measures through which acid rain can be checked:
(i) Emission of sulphur dioxide and nitrogen oxides from industries and power plants may be checked by using pollution controlling devices.
(ii) Liming or addition of calcium carbonate should be done to neutralise the adverse effects of acidity.
(iii) The long-term solution to check acid rain is to reduce the emission of the chemicals that cause acid rain.
Hydrocarbons and other chemical pollutants:
Many hydrocarbons are known to be present in the atmosphere as pollutants. Natural sources of hydrocarbons are largely biological worldwide methane production mainly from anaerobic decay of organic matter probably amounts to one billion metric tons annually. Some plants produce volatile terpenes. In urban areas, ethylene is known to inhibit the plant growth. Open chain hydrocarbons affect human beings.
Aromatic compounds cause irritation and injury in the mucous membrane at 500 ppm. Hydrocarbons are of particular concern because of their involvement in the production of photochemical oxidants which cause irritation and other effects. Southwick (1976) has listed common chemical air pollutants, their main sources and pathological effects on man (Table 13.15).
Air pollution due to odours:
Many chemical compounds, perfumes, whiskies and wines, and many other rotten substances emit undesirable smells which pollute the local atmosphere. Sometimes only by odour the quality identification of particular chemical is made and many edible things are evaluated only through odours. In dairy industry also smelling and testing of milk and its products may be used to determine the quality of raw material. Such testing is made only by experts. For food and perfumery industries, correct odour is essential.
Odour is a smell which can be determined by the nose. According of Karolinska Institute Symposium on Environmental Health, a product for the activation of the sense of smell, an olfactory experience is called odour and any chemical compound which can stimulate the olfactory sense is called odour. Odours are described on the basis of both intensity and characters.
Odour is a physiological response of individual person. There are some instruments which can measure the physical or chemical characteristics of odour indirectly but these instruments are not better than human nose to determine the smell.
Other air pollutants:
A number of other minor emissions such as vapour resulted from cosmetics, the smoke from tobacco products, aeroallergens (spores and pollengrains) which cause allergy in some persons are also regarded as air pollutants. Industrial processes especially chemical industries produce special air pollutants such as H2SO4, HCL, formaldehyde, alcohols and many other chemicals. A particular air pollutant that has attracted special interest in the recent years is asbestos. Its occupational hazards are known since 1943. All the urban dwellers are exposed to asbestos pollution. Pesticides and radiations are other important air pollutants which are very dangerous for human health.