Direct effect
Aerosols tend to cause cooling of the Earth's surface immediately below them. Because most aerosols reflect sunlight back into space, they have a "direct“ cooling effect by reducing the amount of solar radiation that reaches the surface.
The magnitude of this cooling effect depends on the size and composition of the aerosol particles, as well as the reflective properties of the underlying surface.
It is thought that aerosol cooling may partially offset expected global warming that is attributed to increases in the amount of carbon dioxide from human activity.
The direct effect is the mechanism by which aerosols scatter and absorb shortwave and longwave radiation, thereby altering the radiative balance of the Earth-atmosphere system.
Aerosols with diameters between about 0.1 and 2 μm scatter the most light per unit mass. The direct radiative effect of aerosols is sensitive to the single scattering albedo ωo. For example, a change in ωo from 0.9 to 0.8 can often change the sign of the direct effect, depending on the Albedo of the underlying surface and the altitude of the aerosols.
Aerosols with diameters between about 0.1 and 2 μm scatter the most light per unit mass. The direct radiative effect of aerosols is sensitive to the single scattering albedo ωo. For example, a change in ωo from 0.9 to 0.8 can often change the sign of the direct effect, depending on the Albedo of the underlying surface and the altitude of the aerosols.