Particulate matter is a combination of fine solids such as dirt, soil dust, pollens, moulds, ashes, and soot; and aerosols that are formed in the atmosphere from gaseous combustion by-products such as volatile organic compounds, sulphur dioxide and nitrogen oxides. Particulate matter is unique among atmospheric constituents in that it is not defined on the basis of its chemical composition. It may include a broad range of chemical species, including: elemental and organic carbon compounds; oxides of silicon, aluminium and iron; trace metals; sulphates; nitrates and ammonia. It is further classified as primary (emitted directly into the atmosphere) or secondary (formed in the atmosphere through chemical and physical transformations). The principal gases involved in secondary particulate formation are sulphur dioxide (SO2), nitrogen oxides (NOx), volatile organic carbons (VOCs) and ammonia (NH3). Primary particles are found in both the fine and coarse fractions, whereas secondary particles, such as sulphates and nitrates, are found predominantly in the fine fraction. Both primary and secondary PM can result from either natural or (human anthropogenic sources). Particulate matter air pollution comes from such diverse sources as motor vehicles, wood-burning stoves and fireplaces, construction activity, agriculture, industrial smokestacks, wildfires and other burn activity, and windblown dust from open lands.
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