June 2009
Front Matter
Training
31
Physical Properties
•Physical properties of air toxics span the entire range of pollutants present in the atmosphere 
–As particles and gases and in semi-volatile form. 
–As both primary (directly emitted) and secondary (formed in the atmosphere).
–From mostly anthropogenic sources, but include some biogenic sources.
–Have a wide range of atmospheric lifetimes.
•Some air toxics such as VOCs (e.g., benzene and toluene) are precursors to ozone and particulate matter (PM); and other toxics such as heavy metals are components of PM.