Aerosols are suspensions in air of microscopic solid or liquid particles.
They are emitted into the atmosphere both naturally (e.g., sea spray, volcanoes, forest fires, dust) and as a result of human activities (e.g., fossil fuel combustion, biomass/refuse burning, land use changes). They are also created and modified by chemical processes in the atmosphere.
Aerosols consist of many different chemicals. Their composition and size affects how they absorb or scatter light, whether they are liquids or solids, whether they are acidic, neutral, or basic, how much water vapor they absorb, how effectively they act to form cloud droplets, and how long they reside in the atmosphere.
The average lifetime of aerosols in the atmosphere is about a week. This lifetime is much too short for them to mix all around the Earth and hence aerosol concentrations are highest near their sources.
Aerosol particles profoundly affect Earth’s climate. They mostly reflect sunlight, and thus cool Earth’s surface. They therefore counter the effects of greenhouse gases, which absorb the heat escaping from Earth’s surface and thus heat Earth’s surface and lower atmosphere. The cooling by human-induced aerosols is roughly comparable to the heating by human-induced carbon dioxide. However, dark colored aerosols (soot) absorb sunlight and heat the atmosphere.
Aerosols also modify cloud amount, cloud distribution, and cloud properties such as cloud brightness. Aerosols may also influence precipitation amount, distribution, and frequency. Aerosols decrease visibility by scattering and absorbing sunlight. Aerosol deposition to the surface alters ecosystems by changing acid and nutrient balances.
The major aerosol types are sulfate, black carbon, organic carbon, mineral dust and sea salt.
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