Aerosols

About three-quarters of all aerosols found in the Earth's atmosphere come from natural sources. The most important of these natural components are sea salt, soil and rock debris, products of volcanic emissions, smoke from forest fires, and solid and liquid particles formed by chemical reactions in the atmosphere. As an example of the last category, gaseous organic compounds released by plants are converted by solar energy in the atmosphere to liquid and solid compounds that may then become components of an aerosol. A number of nitrogen and sulfur compounds released into the atmosphere as the result of living and non-living changes undergo similar transformations.

Volcanic eruptions are major, if highly irregular, sources of atmospheric aerosols. The eruptions of Mount Hudson in Chile in August 1991, and Mount Pinatubo in the Philippines in June 1991, produced huge volumes of aerosols that had measurable effects on Earth's atmosphere.

The remaining atmospheric aerosols result from human actions. Some, such as the aerosols released from spray-can products, go directly to form aerosols in the atmosphere. Others undergo chemical changes similar to those associated with natural products. For example, oxides of nitrogen and sulfur produced during the combustion of fossil fuels may be converted to liquid or solid nitrates and sulfates, which are then incorporated into atmospheric aerosols.