A: Aerosols are small particles, either solid or liquid, in the atmosphere. They can come from both natural sources and human activities.
Examples of natural aerosols include dust, ash, sea salt, volcanic aerosols and smoke from forest fires. Human-caused aerosols are generated from activities such as fossil fuel combustion from cars and power plants, the burning of plants for land clearing, and the disturbance of surface soils.
Depending on where they come from, individual aerosols have distinct chemical characteristics. They can also mix together in the atmosphere, so that dust, for instance, changes chemically after it passes over a smoggy area. As a result, the chemical composition of aerosols doesn’t remain constant but varies in time and location.
Averaged over the globe, human-caused aerosols account for about 10 percent of the total amount of aerosols in our atmosphere. Most of that is concentrated in the Northern Hemisphere, especially downwind of industrial sites, slash-and-burn agricultural regions, and overgrazed grasslands.
The remaining 90 percent of aerosols that originated from natural sources, however, do not necessarily have a greater or lesser impact on Earth’s atmosphere than human-induced aerosols, or vice versa.
Other properties of aerosols must be considered to determine their full effect. They include chemical composition, size, efficiency in reflecting or absorbing solar radiation, and their ability to act as cloud droplets or ice nuclei.
The complexities and complications of aerosols and their global effects are still ongoing research subjects in the atmospheric community.
--Yan Feng, postdoctoral research fellow, Center for Atmospheric Sciences