A: Aerosols consist of tiny particles or droplets suspended in the air. Particles are produced by a variety of natural and human sources, ranging from sea salt formed by breaking waves in the ocean, to soot released by burning fossil fuels in engines.
Aerosol particles can “scatter” in two ways. First aerosol particles scatter light from the sun, which is the same way that a pool of water or a mirror reflects light in multiple directions. Some of this light goes back up into space, reducing the amount of heat that warms Earth’s surface.
Second, aerosol particles are scattered in the atmosphere by wind, storms and other air currents. They are transported and transformed in the atmosphere to be deposited back to the surface, mostly as rain and snow. Their time in the atmosphere is about one week, which is shorter than that of greenhouse gases. The effect of the particles is only in the lower atmosphere. They are important because there are so many sources of particles such as power plants and automobiles that continuously supply the atmosphere with new aerosols.
Even though such particles have a lifetime of only one week in the atmosphere, there are always a lot of them in the atmosphere of the Northern Hemisphere. This means they can have a major impact. For example, the many particles in the atmosphere cool the surface of Earth, making some places colder. Cooling of the ocean surface also reduces water evaporation into the air, and less water in the air may reduce rain and snow.
— Lynn Russell, professor of atmospheric chemistry, Center for Atmospheric Sciences