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Figure 1 Thermal inversion
Figure 1 Thermal inversion
Figure 2 The Los Angeles Basin
Figure 2 The Los Angeles Basin

Smog is a result of physical and chemical activities between various pollutants released from smoke stacks in the presence of fog (thus the word smog). Classical smog refers mainly to plume of sulfur oxides and particulate matter generated from the combustion of coals and petroleum products. It is formed primarily in the early morning, during winter months, and in places of high humidity where condensation of water vapor over smoke particles is easier. The notorious London “fog” discussed above is a good example of this type of smog. Photochemical smog is very different from classical smog because it is formed under specific meteorological conditions and only when a large amount of sunlight is available. It forms when nitric oxides and hydrocarbons react in the presence of sunlight to produce ozone; this ozone can be further oxidized to produce nitric dioxide and other photochemical oxidants.

NO + 1/2 O2 NO2

NO2 + HC NO + O

O + O2 O3

The Los Angeles Basin is a prime location for photochemical activities because of the large number of cars and power stations, the availability of sunlight, and its proximity to water. Furthermore, the basin is surrounded on three sides by the San Fernando mountain range, which prevents the dispersion of the pollutants. To make the matter worse, Los Angeles has many days in which a thermal (or temperature) inversion layer blankets the basin and prevents the upward mixing of pollutants with atmospheric air. Thermal inversion refers to the condition where, as a result of a large amount of emissions from cars and smoke stacks, temperature increases rather than decreases with height (Figure 1). Under these conditions, a warm layer is sandwiched between a layer of cold air near the surface and another cold layer in the upper atmosphere. Thermal inversion is more severe when wind is calm and pollutants can stay in place for many hours or even days. The smaller the inversion’s height, the less volume is available where pollutants can mix and the greater their concentration. The situation can be visualized as having a pot (mountains) filled with water (oceans) and other ingredients (pollutants) with the lid closed (the inversion layer) to prepare a stew (photochemical smog). All that is missing is the heat, which is provided by the abundant Southern California sun (Figure 1).


(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Further Reading

Gore, A., An Inconvenient Truth, Penguin Books, 2007.

Roleff, T., Pollution: Opposing viewpoints, Greenhaven Press, 2000.

Walsh, P. J., Dudney, C. S., Copenhave, E. D., Indoor Air Quality, CRC Press, 1984.

Environmental Science and Technology, published by the American Chemical Society.

External Links

Environmental Protection Agency (

Occupational Safety and Health Administration (OSHA) (

Intergovernmental Panel on Climate Control (IPCC), (

United Nations Environment Programme (

World Health Organization (WHO) (