Photochemical Smog

What is smog ?

Photochemical smog, commonly known as smog,  is intense local pollution usually trapped by a thermal inversion. Before the age of the automobile, most smog came from burning coal and was so severe that in 19th-century London, street lights were turned on by noon because soot and smog darkened the midday sky. Burning gasoline in motor vehicles is the main source of smog in most regions today. Powered by sunlight, oxides of nitrogen and volatile organic compounds react in the atmosphere to produce photochemical smog. Smog contains ozone, a form of oxygen gas made up of molecules with three oxygen atoms rather than the normal two. Ozone in the lower atmosphere is a poison -- it damages vegetation, kills trees, irritates lung tissues, and attacks rubber. Environmental officials measure ozone to determine the severity of smog. When the ozone level is high, other pollutants, including carbon monoxide, are usually present at high levels as well .

Ground-level ozone forms as a result of a chemical reaction between several distinct forms of pollutants and sunlight. Two groups of chemical pollutants are involved: nitrogen oxides (NOx), and volatile organic compounds (VOC). When stagnant air masses linger over urban areas, the pollutants are held in place for long periods of time. Sunlight interacts with these pollutants, transforming them into ground-level ozone. The ozone remains in the lower atmosphere until weather systems flush out a given area and dissipate them. An 'episode' of ground-level ozone can last from several hours to several days. Episodes are particularly severe in cities with high concentrations of NOx and VOC during periods of warm weather.