Atmospheric ozone chemistry

Article By:

Ravishankara, A. R. Aeronomy Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado.

Last reviewed:2000

DOI:https://doi.org/10.1036/1097-8542.YB000120

Ozone, O₃, an allotropic form of oxygen containing three atoms, is an important chemical constituent of the Earth's atmosphere (Fig. 1). Ozone serves different functions in various regions of the atmosphere. Its fractional concentration ranges from parts per billion (ppb) to parts per million (ppm), and reaches its maximum abundance between roughly 20 and 25 km (12 and 15 mi) in altitude. This region of the stratosphere, called the ozone layer, acts as an ultraviolet (UV) radiation shield for the Earth. In the troposphere, ozone concentration is lower; yet, ozone plays many critical roles. The photolysis of ozone is the main source of the highly reactive hydroxyl (OH^•) radical which acts as a cleansing agent in the troposphere. This radical allows for most emissions in the atmosphere to be removed. However, direct contact with large concentrations of ozone is toxic and is major cause of urban air pollution (smog). When present in high concentrations, ozone negatively affects the health of animals and plants, and also damages structures. Further, ozone can absorb infrared (IR) radiation at wavelengths not greatly affected by carbon dioxide (CO₂) and act as a greenhouse gas. Thus, increases in ozone in the troposphere, especially in the upper troposphere, can lead to a greater greenhouse effect. Ozone is needed in the stratosphere to protect biological organisms from the harmful effects of ultraviolet radiation and in the upper troposphere to cleanse the atmosphere. However, depletion of ozone in the stratosphere and increases in the troposphere will have deleterious effects.

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