The typical importance of each particle type in the eastern and western U.S. is shown in the figure to the right. Details on each particle type are provided below. Western U.S. Sulfate particles form in the air from sulfur dioxide gas. Most of this gas is released from coal-burning power plants and other industrial sources, such as smelters, industrial boilers, and oil refineries. In humid environments, sulfate particles grow rapidly to a size that is very efficient at scattering light, thereby exacerbating the problem in the East.
Nitrate particles form in the air from nitrogen oxide gas.
Like sulfates, nitrates scatter more light in humid environments. Organic carbon particles are emitted directly into the air and also form there as a reaction of various gaseous hydrocarbons. Gaseous hydrocarbons are also emitted naturally from trees and from fires, but these sources have only a small effect on overall visibility. Elemental carbon particles are very similar to soot. They are smaller than most other particles and tend to absorb rather than scatter light.