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. Aerosols are also believed to have an “indirect” effect on climate by changing the properties of clouds. Indeed, if there were no aerosols in the atmosphere, clouds would be much less common. It is very difficult to form cloud droplets without small aerosol particles acting as “seeds” to start the formation of cloud droplets. According to theory, as aerosol concentration increases within a cloud, the water in the cloud gets spread over many more particles, each of which is correspondingly smaller. Smaller particles fall more slowly in the atmosphere and decrease the amount of rainfall. In this way, changing aerosols in the atmosphere can change the frequency of cloud occurrence, cloud thickness, and rainfall amounts. If there are more aerosols, scientists expect more cloud droplets to form. Since the total amount of condensed water in the cloud is not expected to change much, the average droplet must become smaller. This has two consequences—clouds with smaller droplets reflect more sunlight, and such clouds last longer, because it takes more time for small droplets to coalesce into drops that are large enough to fall to the ground. Both effects increase the amount of sunlight that is reflected to space without reaching the surface.
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