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Air pollutants formed by the action of sunlight on oxides of nitrogen and hydrocarbons.Any of the chemicals which enter into oxidation reactions in the presence of light or other radiant energy are also termed as photochmecial oxidant.Ozone, a natural constituent of the stratosphere formed by the photolysis of molecular oxygen, can be transported by atmospheric circulation into the lower atmosphere. Natural hydrocarbons including terpenes from trees and vegetation are also subject to photochemical reactions producing oxidants. These two processes are the natural sources of background ozone concentrations.

Ozone and peroxyacylnitrates are formed in the lower atmosphere by reactions between oxides of nitrogen and an array of photochemically reactive hydrocarbons. The chemical structure and reactivity of each organic hydrocarbon determines its importance in the formation of oxidants. Motor vehicles, space heating, power plants, and industrial processes are major sources of these oxidant precursors.

A balance between oxidizing and reducing agents would be maintained in the atmosphere, thus avoiding accumulation of ozone and other photochemical oxidants, if it were not for the photochemical degradation of hydrocarbons into peroxy radicals. Peroxy radicals rapidly convert nitric oxide to nitrogen dioxide, thus shifting the equilibrium towards ozone production during daylight. At night, emissions of nitric oxide into the atmosphere serve as a sink for ozone.

Welding and the manufacture of hydrogen peroxide are the main sources of occupational exposure to ozone. The use of ultraviolet lamps, electrostatic precipitators, or photocopying machines may also generate ozone.

1.1.3 Environmental concentrations and exposures

Since significant concentrations of oxidants in urban areas are generally restricted to a period of 4-6 h per day, oxidant or ozone data are most often reported in terms of maximum 1-h concentrations or in terms of the number of hours or days recorded with hourly concentrations exceeding a specified value. In isolated places far removed from sources of pollution, maximum hourly ozone concentrations of 100 µg/m³ (0.05 ppm) have been recorded. Transport of oxidants from urban areas for distances of 100-700 kilometres appears to be a widespread phenomenon, and 1-h ozone concentrations of 120 µg/m³ (0.06 ppm) or more have been observed in rural areas. In some large cities, maximum 1-h oxidant concentrations exceed 200 µg/m³ (0.1 ppm) on 5-30% of days, while in Los Angeles it is commonplace for maximum 1-h oxidant values to exceed 200 µg/m³ (0.1 ppm) on most days of the month between May and October.

Diurnal patterns in oxidant levels are an important feature of the urban environment and result from hourly changes in solar radiation and pollutant emission intensity. Maximum hourly ozone levels frequently occur around noon and are often preceded by peak concentrations of nitrogen dioxide. Concentrations of peroxyacetylnitrate are typically between 1/50th and 1/100th of those of ozone and, in general, closely follow temporal variations in ozone levels.

On a seasonal basis, oxidant concentrations tend to increase during the high temperature season, and the frequency of days on which oxidant concentrations exceed 200 µg/m³ (0.1 ppm) is greatest during this period.

Oxidant concentrations indoors tend to be lower than those outdoors, and are reduced by destructive reaction on material surfaces. They are also reduced by activities that generate nitric oxide such as smoking and cooking.

14:34, August 11, 2010 (UTC)115.118.250.113mda