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Aerosols are pool of airborne solid or liquid particles, with a typical size between 0.011 and 11 nm and suspended in the atmosphere for at least several hours. Aerosols may be of either natural or anthropogenic origin. Aerosols may influence climate in two ways: directly through scattering and absorbing radiation, and indirectly through acting as condensation nuclei for cloud formation or modifying the optical properties and lifetime of clouds. The term has also come to be associated, incorrectly, with the propellant used in "aerosol sprays".
Little substances suspended in the air. Some occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. Human activities, such as the burning of fossil fuels and the alteration of natural surface cover, also generate aerosols. Averaged over the globe, aerosols made by human activities currently account for about 10 percent of the total amount of aerosols in our atmosphere. Most of that 10 percent is concentrated in the Northern Hemisphere, especially downwind of industrial sites, slash-and-burn agricultural regions, and overgrazed grasslands.
Aerosols affect regional and global atmosphere. We have yet to accurately quantify the relative impacts on climate of natural aerosols and those of human cause. Moreover, we do not know in what regions of the planet the amount of atmospheric aerosol is increasing, is reducing, and is remaining almost constant. Overall, we are even unsure whether aerosols are warming or cooling our planet.
Earth's surface immediately below aerosols tends to remain cool because of aerosols. 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 of earth. The degree of this cooling effect depends on the size and composition of the aerosol particles, as well as the reflective properties of the primary 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.
It is also believed that aerosols have an indirect effect on climate by changing properties of clouds. Undeniably, if there were no aerosols in the climate, there would be no clouds. It is very difficult to form clouds without small aerosol particles acting as basics to start the formation of cloud droplets. As aerosol meditation 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, thickness, and rainfall amounts.
Scientists expect that if there are more aerosols then 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. This has two outcomes-- clouds with smaller drops reflect more sunlight and such clouds last longer, because it takes more time for small drops to unite 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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