Air pollution consists of gas and particle contaminants
that are present in the atmosphere. Gaseous pollutants
include SO2 , NOx , ozone, carbon monoxide (CO),
volatile organic compounds (VOCs), certain toxic air
pollutants, and some gaseous forms of metals. Particle
pollution (PM2.5 and PM10) includes a mixture of
compounds. The majority of these compounds can be
grouped into five categories: sulfate, nitrate, elemental
(black) carbon, organic carbon, and crustal material.