- Primary air pollutants: Pollutants that are
formed and emitted directly from particular
sources. Examples are particulates, carbon
monoxide, nitrogen oxide, and sulfur oxide.
- Secondary air pollutants: Pollutants that are
formed in the lower atmosphere by chemical
reactions. The two examples are ozone and secondary
organic aerosol (haze).
- Secondary pollutants are harder to control
because they have different ways of synthesizing
and the formation are not well understood. They
form naturally in the environment and cause
problems like photochemical smog.
- PM2.5 (also known as fine fraction
particles) is generally defined as those particles
with an aerodynamic diameter of 2.5 microns or
less. Sources of fine particles include all types
of combustion activities (motor vehicles, power
plants, wood burning, etc.) and certain industrial
- PM10-2.5(also known as coarse
fraction particles) is generally defined as those
particles with an aerodynamic diameter greater than
2.5 microns, but equal to or less than a nominal 10
microns. Sources of coarse particles include
crushing or grinding operations, and dust from
paved or unpaved roads.
- Other PM may be formed in the air from the
chemical change of gases. They are indirectly
formed when gases from burning fuels react with
sunlight and water vapor. The sources can be from
fuel combustion in motor vehicles, at power plants,
and in other industrial processes.