A number of natural phenomena contribute significantly to environmental concentrations of polluting substances on a global scale. For example, dusts may be windblown many thousands of miles from their point of origin. Forest, bush and grass fires caused by lightning release large quantities of smoke and trace gases into the atmosphere. Forests also release volatile hydrocarbons (terpenes) which, during sunny weather conditions, may participate in photochemical reactions. Substantial releases of sulphur dioxide and particulates are associated with volcanic emissions. Because some plumes penetrate the stratosphere, the atmospheric residence times of such substances may be long and in consequence the atmospheric radiation balance can be altered and world climate influenced (Whelpdale & Munn, 1976). Sea spray is a major source of atmospheric particulate matter and trace gases are sometimes transferred from the oceans to the atmosphere by evaporation. At times, however, adsorption from the atmosphere is the dominant process. For example, the oceans generally act as a sink for carbon dioxide, although during periods of intense phytoplankton blooming they may act as a source.
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