The process of attempting to limit the amount of air pollution by regulating the emission of pollutants or their precursors. Control strategies are alternative long-term policies that could reduce air pollution, as projected using air-quality modeling. For primary pollutants such as sulfur dioxide, control strategies include burning cleaner low-sulfur coal, scrubbing sulfur dioxide from the combustion products before releasing them into the atmosphere, or changing to alternative fuels or processes. For secondary pollutants such as ozone that are not emitted directly but are created in the lower atmosphere by a complex series of chemical and photochemical reactions involving NOx and volatile hydrocarbons, control is achieved by changing the emissions of one or both of the primary reactants.