Appendix B Depletion Of Stratospheric Ozone

More than 90% of the ozone in the atmosphere is concentrated in the stratosphere as a layer extending from about of 5-11 miles above the earth's surface up to about 30 miles. Unlike the tropospheric ozone content in the lower layers of the atmosphere that gives rise to urban smog and air pollution, this stratospheric "ozone layer" serves a crucial purpose. Although the concentration of ozone in this layer is quite low53 it functions as an effective shield against the harmful ultraviolet (UV) radiation in extra-terrestrial solar radiation from reaching the earth's surface. The stratospheric ozone layer completely filters out the UV-C (X < 290 nm), and most of the UV-B radiation (X = 290 nm to 315 nm), both particularly damaging wavelengths to living organisms, from the solar radiation passing through it. It has little impact on wavelengths longer than about 315 nm including UV-A radiation. The evolution of life on earth was only made possible by the effective shielding action afforded by the ozone layer. The ozone concentration in stratosphere varies strongly with latitude over the globe with the highest values in mid latitudes and at the poles. Lowest ozone levels are found in the tropical regions. Air currents cause a longitudinal variation in ozone levels as well.

Ozone is produced by the photolysis of oxygen (by X < 242 nm UV radiation) and is readily reconverted to oxygen by the Chapman reaction of ozone with atomic oxygen. A simplified scheme of the photochemical equilibrium process for ozone formation is as follows. The concentration of ozone in the stratosphere is determined by the rates of chemical reactions that produce and destroy ozone.

Reactive species such as NOx molecules, chlorine radicals, and hydrogen radicals present in the stratosphere can also cause the catalytic conversion of ozone to oxygen, in competition with the last reaction shown above. Reactions such as the following, therefore interfere with the equilibrium photochemical processes, reducing the concentration of ozone in the stratosphere. The net reaction is one where ozone is converted to oxygen and catalytic Cl species survives for further reaction. Particularly effective in this regard are halogen monoxides (ClO and BrO).

53 If the ozone layer over the US were compressed into a layer of temperature 0° C at 1 atmosphere pressure, it would have a thickness of only 3 mm. As a 0.01 mm thickness of an ozone layer is defined to be 1 Dobson Unit (DU), the average thickness of the ozone layer over the US is about 300 DU.

Table B.1 Some Common Ozone Depletion Substances Controlled Under the Montreal Protocol

Name

Chemical Name

Lifetime (yrs)

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