Errors of emission
Mobile source air toxics come from four sources. One: some toxics are present in fuel and are emitted into the air when the fuel evaporates or passes through the engine unburned. Benzene, for example, is a component of gasoline. Cars emit small quantities of benzene in unburned fuel, or as vapour when gasoline evaporates. Two: mobile source air toxics are formed through engine combustion processes. A significant amount of automotive benzene comes from the incomplete combustion of compounds in gasoline, such as toluene and xylene that are chemically very similar to benzene. Like benzene, these compounds occur naturally in petroleum and become more concentrated when petroleum is refined to produce high-octane gasoline. DPM and DEOG emissions, as well as formaldehyde, acetaldehyde, and 1,3-butadiene, are also by-products of incomplete combustion. Three: compounds like formaldehyde and acetaldehyde, are also formed through a secondary process when other mobile source pollutants undergo chemical reactions in the atmosphere. Four: metal air toxics result from engine wear or from impurities in oil or gasoline. They can also be present in fuel additives.
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