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. Clouds with low aerosol concentration and a few large droplets do not scatter light well, and allow much of the Sun's light to pass through and reach the surface. The high aerosol concentrations in these clouds provide the nucleation points necessary for the formation of many small liquid water droplets. Indeed, if there were no aerosols in the atmosphere, there would be no clouds. 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 occurence, cloud thickness, and rainfall amounts. If there are more aerosols, scientists expect more cloud drops to form. Since the total amount of condensed water in the cloud is not expected to change much, the average drop must become smaller. Both effects increase the amount of sunlight that is reflected to space without reaching the surface.