Most of this gas is released from coal-burning power plants and other industrial sources, such as smelters, industrial boilers, and oil refineries. Sulfates are the largest contributor to haze in the eastern U. S., due to the region's large number of coal-fired power plants. In humid environments, sulfate particles grow rapidly to a size that are very efficient at scattering light, thereby exacerbating the problem in the East. 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.
Nitrate particles form in the air from nitrogen oxide gas.
Like sulfates, nitrates scatter more light in humid environments. Elemental carbon particles are very similar to soot. They are smaller than most other particles and tend to absorb rather than scatter light. These particles are emitted directly into the air from virtually all combustion activities, but are especially prevalent in diesel exhaust and smoke from the burning of wood and wastes. Crustal material is very similar to dust.