Particulate matter (PM) is the term for particles and liquid droplets suspended in the air. Particles emitted directly into the air are known as "direct" or "primary" PM. Other particles are formed indirectly in the atmosphere from the chemical reaction of gaseous pollutants known as "precursors." Sources of direct PM and PM precursors include factories, power plants, vehicles, construction activity, and natural sources such as fires and windblown dust.
Particles can be emitted directly into the atmosphere (above) or formed indirectly through chemical reactions (below).