Major Air Pollutants
The prime air pollutants and the sources of major pollutants in India are as follows:
Sulfur dioxide
Sulfur dioxide has an unpleasant odor that is detectable at concentrations greater than about 1 ppm, although above 3 ppm (parts per million) the sense of smell is rapidly lost. Its tropospheric concentrations range from less than 1 ppb (parts per billion) in locations very remote from industrial activity to 2 ppm in highly polluted areas.
Sulfur dioxide is a respiratory irritant and can cause shortness of breath, enhanced likelihood of lower respiratory tract illness and chronic lung disease. Even relatively short exposure to the higher concentrations found in polluted areas can cause temporary damage to human health. This pollutant is rarely found alone and its potency is frequently enhanced by synergistic interactions with other contaminants.
This gas also causes damage to plants. Clearly some plants are more susceptible than other. The principal natural sources of sulfur dioxide are volcanoes and biological activities.
Oxides of nitrogen
Three of the oxides of nitrogen are significant primary pollutants. These are nitrous oxide (dinitrogen oxide, N20), nitric oxide (NO) and nitrogen dioxide (NO2). The atmospheric behaviour and major sources of the first of these is somewhat different from that of the other two, hence this will be considered separately. Nitrous oxide is a greenhouse gas and therefore it contributes to global warming. In addition, it enters the stratosphere, where it produces nitric oxide (NO) and so contributes to the mechanisms that control the concentrations of ozone (O3).
The main sources of atmospheric nitrous oxide are probably the process of denitrification. This is the microbial reduction of nitrate (NO3) that occurs in soils and waters with low oxygen contents. This natural phenomenon is encouraged by the use of artificial nitrogenous fertilizers, particularly if they are applied to soils that contain high concentrations of organic matter and have fluctuating levels of aeration. Similarly, the establishment of oxygen-depleted conditions in water bodies by the discharge of nutrients and/or oxygen-demanding wastes also leads to increased rates of denitrification.
Hydrocarbons
As suggested by their name, hydrocarbons are compounds that contain carbon and hydrogen. The simplest of these compounds, methane (CH4), is best considered separately from the other hydrocarbons.
Methane
Natural sources generate substantial amount of methane. This is produced almost entirely by micro-organisms under anaerobic conditions, such as those prevailing in wetlands and in the intestines of ruminant animals. Human activity also produces an additional quantity of methane. This is primarily from the result of paddy or rice production, low-temperature biomass burning, cattle rearing, waste disposal and fossil fuel extraction.
Oxides of carbon
There are two oxides of carbon, carbon dioxide (CO2) and carbon monoxide (CO). Let us consider these in turn. Carbon dioxide is present in the troposphere at a concentration of about 360 ppm. Currently, this is increasing. This is a cause for concern as carbon dioxide is a major greenhouse gas, and hence it is a contributor to global warming.
Carbon monoxide is generated biologically (both on land and within the oceans). It is also produced as a consequence of the atmospheric oxidation of hydrocarbons and during the incomplete combustion of fossil fuels and biomass. This gas has harmful effects on human health.
Suspended particulates in the troposphere
A suspension of solid or liquid particles within the air is called an aerosol. Aerosols are generated by many natural processes including the aerial entrainment of soil, sea spray, pollen and spores by wind, volcanic activity and the burning of biomass. With the exception of the last one, these sources mainly generate coarse particles. These tend to rapidly return to the Earth’s surface by sedimentation (i.e. falling) or washout (i.e. removal below clouds by rain, snow, etc).
Human activity can increase the fluxes from some of these natural sources. For example, poor agricultural practices can lead to large-scale loss of soil by Wind erosion. There are also purely anthropogenic sources of suspended particulates. Particularly important in this regard is the burning of fossil fuels and biomass. Particulates generated by combustion are mainly fine. These settle much more slowly than the coarse particulates and therefore tend to reside in the atmosphere for longer. In general, they are removed by wet deposition or, if brought into contact with surfaces by situ within the atmosphere. For example, the aerial oxidation of sulfur dioxide produces significant quantities of sulphate particles.
Lead compounds
More than 90% of the lead in the atmosphere is of anthropogenic origin. This contamination is extremely widespread as evidenced by the sudden dramatic increase in the lead levels of Greenland snow during the latter part of the twentieth century.
