Mobile-Source Emissions

In big cities, it is common for half the air pollution to be caused by "mobile-source" emissions, i.e., emissions from vehicles.  Fuels burn incompletely in motor vehicles.  Combustion by-products emitted into the air include soot, carbon monoxide (CO), nitrogen oxides (NOx), and volatile organic compounds (VOCs).  VOCs also can be released during the refueling of vehicles.  The federal Clean Air Act Amendments of 1990 require many gasoline stations in smoggy areas to install special nozzles on their gasoline pumps.  The new nozzles help to keep harmful gases from escaping into the air by recycling them back into the gas pump.  This process is called vapor recovery.

Besides mobile sources, air pollution is attributable mainly to industry, home and commercial energy use, use of yard equipment such as lawnmowers, and volatile substances such as charcoal lighter fluid, varnish, and paint.