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An aerosol is a suspension of tiny particles or droplets in the air, such as dust, mist, or fumes. These particles may be inhaled or absorbed by the skin, and can sometimes cause adverse health effects for humans. The aerosol particles can be from natural sources or from anthropogenic sources such as the use of various (gaseous, liquid, or solid) fuels, whether the fuels are from fossil fuel sources or from renewable energy sources such as biomass or waste. Aerosol particles – from whatever the source – play an important role in the climate system because of their direct interaction (absorption and scattering) with solar and terrestrial radiation, as well as through their influence on cloud processes and thereby, indirectly, on radiative fluxes.

The particle size of an aerosol is often determined by the process that generated the particle. Combustion particles usually start out in the range 0.01 to 0.05 micron but combine with each other (agglomerate) to form larger particles. Powder is broken down into smaller particles and released into the air; it is difficult to break down such particles smaller than 0.5 micron. Biological particles usually become airborne from liquid or powder forms, so these particles are usually larger than 0.5 micron.

Aerosol emissions (often collectively classed as particulate matter) from stationary combustion sources burning renewable fuels such as biomass, and waste are a significant source of primary particles smaller than 2.5 microns (often written as PM_2.5) in urban areas. Combustion-generated particles are generally smaller than geologically produced dust and have unique chemical composition and morphology. The emission of particle matter that contains transition metals, ultrafine particles, and soot are controlled by the fuel composition and the oxidant-temperature-mixing history from the flame to the stack. Particle surface area, number of ultrafine particles, bioavailable transition metals, polycyclic aromatic hydrocarbon derivatives (PAH, also referred to as polynuclear aromatic hydrocarbon derivatives, PNAs), and other particle-bound organic compounds are important than particle mass in determining the effects of air pollution. Wood smoke forms when wood is combusted and is made up of a complex mixture of gases and fine particles (particulate matter, PM). In addition to particle pollution, wood smoke contains several toxic harmful air pollutants including benzene, formaldehyde, acrolein, and polycyclic aromatic hydrocarbon derivatives.

Aerosol particles have a lifetime of up to several weeks in the troposphere and occur in highly variable concentrations A large proportion of the particles that influence cloud processes and the radiative balance is derived from gaseous sulfur-containing emissions.

See also: Pollution Control.