The main sources of VOCs are farming operations, pesticides and fertilizers, passenger vehicles, trucks, petroleum production facilities, surface coatings, solvents and evaporated petroleum products.

NOx comes from fuel combustion, both in motor vehicles and industrial processes. Power plants, factories, cars, trucks, and stationary engines all create NOx. Heavy duty diesel trucks release about 40% of all NOx emissions.

Particulate matter comes from farming operations, combustion of fossil fuels, burning garbage, agricultural burning, dirt roads, industrial crushing and grinding, and windblown dust, among other sources. Additionally, PM can be formed when chemicals such as NOx, sulfur dioxides, and others (including many of the same that form ozone) react and condense in the atmosphere.

For detailed information on sources of pollution in a specific region of California, visit the California Air Resources Board’s emission inventory website.