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. 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.
It enters the air from dirt roads, fields, and other open spaces as a result of wind, traffic, and other surface activities. Whereas other types of particles form from the condensation and growth of microscopic particles and gasses, crustal material results from the crushing and grinding of larger, earth-born material. Because it is difficult to reduce this material to microscopic sizes, crustal material tends to be larger than other particles and tends to fall from the air sooner, contributing less to the overall effect of haze. A uniform haze degrades visibility evenly across the horizon and from the ground to a height well above the highest features of the landscape. Uniform haze often travels long distances and covers large geographic area, in which case it is called a regional haze.