Primary pollutants in L.A. smog consist mainly of combustion products from gasoline- or diesel-burning internal combustion engines. There is a significant source of ROGs from stationary industries and small businesses.

Secondary pollutants are products of reactions in the atmosphere. There are NOT directly emitted.

It is possible for secondary pollutants to be products of reactions between primary pollutants, between a primary pollutant and a secondary pollutant, between other secondary pollutants, and between normal atmospheric constituents and other reactants of all classes.

The main secondary pollutant is ozone; it is classified as an oxidizer because it likes to give up its third oxygen atom to other compounds. PAN is also an oxidizer, but is found in much much smaller quantities as ozone.

Mobile sources (such as automobiles) are the largest sources of CO to the atmosphere. Stationary industrial sources are the largest source of particulate matter (PM).

ROGs seem to be shared between the stationary and mobile sources (slightly higher portion for mobile sources in more recent data). This characteristic will become important to distinguish when we deal with the regulation of ozone in smog.