* Pollutants enter the air from various sources, but burning of fossil fuels contributes the most to air pollution. * Exhaust fumes from vehicles contains soot (tiny carbon particles), lead (from cars using unleaded petrol), carbon monoxide, oxides of nitrogen and hydrocarbons due to the incomplete combustion of fossil fuels. * Burning of fossil fuels in the combustion engines of vehicles and electrical power stations also releases large amount of carbon dioxide into the atmosphere. * Human activities such as open burning cause severe smog and haze. * Industrial plants and factories also pump large amount of carbon monoxide, oxides of nitrogen and sulphur dioxide into the atmosphere. * Both oxides of nitrogen and sulphur dioxide can combine with water vapour in the atmosphere to form nitric acid and sulphuric acid respectively. * Then, they fall back to the Earth as acid rain.
* Rainwater typically has a pH value or 5.6, due to the presence of dissolved carbon dioxide, which forms carbonic acid. * The pH of acid rain is, however, less then 5.0.
Major primary pollutants produced by human activity include: * Sulphur oxides (SO4) - especially sulfur dioxide, a chemical compound with the formula SO2. SO2 is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide. Further oxidation of SO2, usually in the presence of a catalyst such as NO2, forms H2SO4, and thus acid rain. This is one of the causes for concern over the environmental impact of the use of these fuels as power sources. * Nitrogen oxides (NO3) - especially nitrogen dioxide are emitted from high temperature combustion, and are also produced naturally during thunderstorms by electrical discharge. Can be seen as the brown haze dome above or plume downwind of cities. Nitrogen dioxide is the chemical compound with the formula NO2. It is one of the several nitrogen oxides. This reddish-brown toxic gas has a characteristic sharp, biting odor. NO2 is one of the most prominent air pollutants. * Carbon monoxide (CO)- is a colourless, odourless, non-irritating but very poisonous gas. It is a product by incomplete combustion of fuel such as natural gas, coal or wood. Vehicular exhaust is a major source of carbon monoxide. * Carbon dioxide (CO2) - a colourless, odourless, non-toxic greenhouse gas also associated with ocean acidification, emitted from sources such as combustion, cement production, and respiration. It is otherwise recycled in the atmosphere in the carbon cycle. * Volatile organic compounds - VOCs are an important outdoor air pollutant. In this field they are often divided into the separate categories of methane (CH4) and non-methane (NMVOCs). Methane is an extremely efficient greenhouse gas which contributes to enhanced global warming. Other hydrocarbon VOCs are also significant greenhouse gases via their role in creating ozone and in prolonging the life of methane in the atmosphere, although the effect varies depending on local air quality. Within the NMVOCs, the aromatic compounds benzene, toluene and xylene are suspected carcinogens and may lead to leukemia through prolonged exposure. 1,3-butadiene is another dangerous compound which is often associated with industrial uses. * Atmospheric particulate matter - Particulates, alternatively referred to as particulate matter (PM) or fine particles, are tiny particles of solid or liquid suspended in a gas. In contrast, aerosol refers to particles and the gas together. Sources of particulate matter can be manmade or natural. Some particulates occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil fuels in vehicles, power plants and various industrial processes also generate significant amounts of aerosols. Averaged over the globe, anthropogenic...
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