PM-2.5 Composition and Sources; PM-2.5 is composed of a mixture of particles directed emitted into the air and particles formed in the air from the chemical transformation of gaseous pollutants (secondary particles). The principal types of secondary particles are ammonium sulfate and nitrate formed in the air from gaseous emissions of sulfur dioxide (SO2) and oxides of nitrogen (NOx), reacting with ammonia (NH3). The main sources of SO2 are combustion of fossil fuels in boilers and the main sources of NOx are combustion of fossil fuel in boilers and mobile sources.
Emissions, Monitoring and Analysis Division Office of Air Quality Planning and Standards. 1997. PM-2.5 Composition and Sources. FACA National and Regional Strategies Workgroup