Visible emissions can occur in many shades, but they are usually classified as either black or white emissions. Black emissions are made of particles that absorb light, causing a dark appearance. White emissions contain particles that reflect or scatter light, causing a light appearance.
Regardless of whether they are classified as black or white, most emissions consist of particles ranging from 0.1 µm (micrometer = one-millionth of a meter) to 200 µm in size. The average human hair is 50-70 µm in diameter.
Particulate Matter (PM) shown in reference to an average human hair.
Particle Types
Particles in visible emissions can be classified as one of the following particle types.
Solid particles usually larger than 1 µm. Dust is usually released into the air by mechanical agitation (grinding, screening, driving). Because of their larger size, dust particles tend to settle quickly.
Carbon particles that have attached to or absorbed other hydrocarbons. Soot is the principal cause of blackness in smoke and is the result of incomplete combustion of carbonaceous material. Particle size is usually 1 µm or less.
Pure fly ash is light brown in color and is the result of unburned material during combustion. It is a major constituent of smoke. In a process that has nearly complete combustion, fly ash is usually inorganic material. The amount of inorganic fly ash is dependent on the fuel’s ash content. Ash content is regulated in most fuel oils to decrease airborne fly ash.
Usually less than 1 µm, fumes consist of metal or metal oxides created when vapors from high-temperature processes condense.
Liquid droplets that have diameters of 2 to 200 µm.
Gases are not specific in shape or volume and most are not visible to the human eye. However, nitrogen dioxide (NO2) has a brownish yellow appearance and chlorine gas (Cl) is greenish yellow.
Vapor is the gaseous state of a substance that is liquid or solid at normal temperature and pressure. Vapors generally do not have color, but they can alter light.