Sources of pollutants

Carbon monoxide is produced from the partial oxidation of carbon-containing compounds; it forms in preference to the more usual carbon dioxide when there is a reduced availability of oxygen.

Carbon monoxide is a major atmospheric pollutant in urban areas, chiefly from exhaust of internal combution engines, but also from improper burning of various other fuels.



NO is unstable with respect to O2 and N2, although this conversion is very slow at ambient temperatures in the absence of a catalyst. Its synthesis from molecular nitrogen and oxygen requires elevated temperatures, >1000°C. A major natural source is lightning. The use of internal combustion engines has drastically increased the presence of nitric oxide in the environment as well.

Nitrogen dioxide is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:

2O2 + N2 → 2 NO2

The most important sources of NO2 are internal combustion engines , thermal power stations and, to a lesser extent, pulp mills. Atmospheric nuclear tests are also a source of nitrogen dioxide, which is responsible for the reddish colour of mushroom clouds.



SO2 is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide.