Smoke, smog, ash, and soot are all forms of atmospheric particulate matter that are based on size and often divided into 5 categories:
- Suspended Particulate Matter (SPM)
- Thoracic & Respirable Particles Inhalable Corse particles ranging from 2.5 to 10 µm (PM10 – PM2.5)
- Fine Fraction Particles contains sizes from 0.1 µm to 2.5 µm (PM2.5 – PM0.1)
- Ultrafine Particles are smaller than 0.1 µm (PM0.1)
Coarse particles are produced by the mechanical break-up of larger solid particles. The coarse fraction can include dust from roads, agricultural processes, uncovered soil or mining operations, as well as non-combustible materials released when burning fossil fuels. Pollen grains, mold spores, and plant and insect parts can also contribute to the coarse fraction. Finally, evaporation of sea spray can produce large particles near coasts.
Most of the total mass of airborne particulate matter is usually made up of fine particles ranging from 0.1 to 2.5 µm. Fine particles are largely formed from gases.
Ultrafine particles often contribute only a few percent to the total mass, though they are the most numerous, representing over 90% of the number of particles. Ultrafine particles are formed by nucleation, which is the initial stage in which gas becomes a particle. These particles can grow up to a size of 1 µm either through condensation, when additional gas condensates on the particles, or through coagulation, when two or more particles combine to form a larger particle. Particles produced by the intermediate reactions of gases in the atmosphere are called secondary particles.
Soot as an airborne contaminant in the environment has many various sources, all of which are results of some form of pyrolysis. Soot is a mass of impure carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolysis fuel particles such as coal, cenospheres, charred wood, and petroleum coke that may become airborne during pyrolysis and that are more properly identified as cokes or chars. Soot is also referred to as Black carbon (BC), or carbon black, or elemental carbon (EC).