Aerosols occur in the atmosphere from natural causes; for instance, they are blown off the surface of deserts or dry regions. As a result of the eruption of Mt. Pinatubo in the Philippines in June 1991, considerable amounts of aerosol were added to the stratosphere that, for approximately 2 years, scattered solar radiation leading to a loss of radiation and a cooling at the surface.
As noted previously, human activities also affect the amount of aerosol in the atmosphere. The main direct effect of aerosols, such as sulfate particles, is the scattering of some solar radiation back to space, which tends to cool the earth’s surface. Aerosols can also influence the radiation budget by directly absorbing solar radiation, leading to local heating of the atmosphere, and, to a lesser extent, by absorbing and emitting thermal radiation. A further influence of aerosols is that many of them act as nuclei on which cloud droplets condense. A changed concentration therefore affects the number and size of droplets in a cloud and hence alters the reflection and the absorption of solar radiation by the cloud.
Because man-made aerosols typically remain in the atmosphere for only a few days before they are washed out by precipitation, they tend to be concentrated near their sources, such as industrial regions, adding complexity to climate change because they can help mask, at least temporarily, any global warming arising from increased greenhouse gases.