Where do aerosols come from?
Aerosols can either be directly emitted into the atmosphere (primary aerosols) or can form from the reaction and condensation of gaseous chemicals already existing in the atmosphere (secondary aerosols).[iv] The major natural sources of aerosols are dust, volcanic activity and forest fires, which contribute to primary and/or secondary aerosol formation. Globally, about 10% of aerosols are anthropogenic in origin. Most of this 10% is in the northern hemisphere, particularly in the area of industry, overgrazed grasslands and slash-and-burn agriculture.[v]Anthropogenic sources of aerosols include energy production from fossil fuels, vehicle emissions, and residential heating. Anthropogenic sources contribute to primary and secondary aerosol formation. For example: automobiles generate particles directly, and also emit organic compounds that contribute to the formation of secondary aerosols. [vi] Once in the atmosphere, aerosols may be transported extensively or linger in a single area. Th
