Air pollutants impact a wide variety of vegetation, including crop plants, landscape plants, forest and native vegetation. There are many types and sources of pollutants. They can be broken down into general categories such as mobile and fixed sources and natural sources. Examples of mobile sources include cars, trucks, buses, trains and aircraft. Examples of fixed sources would be large industrial operations, refineries, numerous small industries which produce pollutant precursors, agricultural burning, dairy and cattle industries, and fertilizer production processes.
Natural sources can also play a significant role in pollutant production.
For example, volcanic activity can release significant amounts of pollutants into the atmosphere which can be distributed globally. Fugitive dust from nonvegetated areas or from agricultural operations are also important environmental concerns. Although there are naturally occurring atmospheric pollutants, human activity accounts for a signigicant proportion of these pollutants. Pollutants are formed in the atmosphere when chemical components are fueled by sunlight. During hot summer months a temperature inversion layer may form in the atmosphere which traps pollutants near the ground.