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Atmospheric pollutants II
Oxides of nitrogen
Emissions are primarily in the form of NO, which is oxidised by ozone (O3) from nitric oxide to NO2. Nitrogen dioxide (NO2) is the primary concern for effects on health. The various oxides of nitrogen can also react with hydrocarbons in the atmosphere to contribute to photochemical smog. Oxides of nitrogen can also affect ecologically sensitive sites through deposition, causing acidification. The term nitrogen oxide can be used to refer to any of these oxides (oxygen compounds) of nitrogen: Nitric oxide (NO); Nitrogen dioxide (NO2); Dinitrogen oxide (N2O) (Nitrous oxide); Dinitrogen trioxide (N2O3); Dinitrogen tetroxide (N2O4); Dinitrogen pentoxide (N2O5). A mixture is often formed in chemical reactions that produce nitrogen oxides, with the proportions depending on the specific reaction and the conditions it is performed in. When dissolved in atmospheric moisture the result can be acid rain which can damage both trees and entire forest ecosystems.In atmospheric science the term NOx is used to mean the total concentration of NO plus NO2. During daylight NO and NO2 are in equilibrium with the ratio NO/NO2 determined by the intensity of sunshine (which converts NO2 to NO) and ozone (which reacts with NO to give back NO2). Three primary sources of Oxides of nitrogen formation in combustion processes are: prompt NO, fuel NO and thermal NO. Thermal NOx formation is recognized as the most relevant source when combusting natural gas.
Acid rain
Acid rain is defined as rain with a pH of below 5.6. Normal rain has a pH of slightly under 6, which is slightly acidic. This natural acidity is caused by dissolved carbon dioxide. Acid rain is caused by sulfur from impurities in fossil fuels and nitrogen from the air combining with oxygen to form sulfur dioxide and nitrogen oxides. These diffuse into the atmosphere and react with air to form sulfuric and nitric acids which are soluble and fall with the rain. Some The resulting increased acidity in soil has proven to be harmful to vegetation. Principal sources are industrial and power-generating plants and transportation vehicles. The gases may be carried hundreds of miles in the atmosphere before they are converted to acids and deposited. Since the industrial revolution, emissions of sulfur and nitrogen oxides to the atmosphere have increased. Industrial and energy-generating facilities that burn fossil fuels, primarily coal, are the principal sources of increased sulfur oxides. The toxic ions released due to acid rain form the greatest threat to humans.
Organic pollutants
Volatile organic compounds are organic chemical compounds that have high enough vapour pressures under normal conditions to significantly vaporize and enter the atmosphere. A wide range of carbon-based molecules, such as aldehydes, are volatile organic compounds. Common artifical sources of volatile organic compounds include paint thinners, dry cleaning solvents, and some constituents of petroleum fuels. Volatile organic compounds are an important outdoor air pollutant. In this field they are often divided up into the separate categories of methane (CH4) and non-methane. Methane is an extremely efficient greenhouse gas which may contribute to enhanced global warming. Within the non-methanes, benzene may lead to leukaemia through prolonged exposure. 1,3-butadiene is another dangerous compound which is often associated with industrial uses. Volatile organic compounds are often used in paint, plastics, and cosmetics.
Photochemical smog
Photochemical smog is caused when two kinds of air pollution combine in the presence of sunlight. The first kind is the particulates and nitric oxides from the exhaust of fossil fuel-burning engines in cars and coal power plants. The second kind is the emission of volatile organic compounds from paints, solvents, and other chemicals. Smog is a problem in a number of cities and continues to harm life. High levels of smog aggravate and even cause human respiratory problems.
