Stratospheric ozone

(meteorology) Atmospheric ozone that is relatively concentrated in the lower stratosphere in a layer between 9 and 18 miles (15 and 30 kilometers) above the earth's surface, and plays a critical role for the biosphere by absorbing the damaging ultraviolet radiation with wavelengths 320 nanometers and lower. Also known as ozone layer.

While ozone is found in trace quantities throughout the atmosphere, the largest concentrations are located in the lower stratosphere in a layer between 9 and 18 mi (15 and 30 km). Atmospheric ozone plays a critical role for the biosphere by absorbing the ultraviolet radiation with wavelength (λ) 240–320 nanometers. This radiation is lethal to simple unicellular organisms (algae, bacteria, protozoa) and to the surface cells of higher plants and animals. It also damages the genetic material of cells and is responsible for sunburn in human skin. The incidence of skin cancer has been statistically correlated with the observed surface intensity of ultraviolet wavelength 290–320 nm, which is not totally absorbed by the ozone layer. See also Ozone; Stratosphere; Ultraviolet radiation (biology).

Ozone also plays an important role in photochemical smog and in the purging of trace species from the lower atmosphere. Furthermore, it heats the upper atmosphere by absorbing solar ultraviolet and visible radiation (λ < 710 nm) and thermal infrared radiation (λ ≃ 9.6 micrometers). As a consequence, the temperature increases steadily from about −60°F (220 K) at the tropopause (5–10 mi or 8–16 km altitude) to about 45°F (280 K) at the stratopause (30 mi or 50 km altitude). This ozone heating provides the major energy source for driving the circulation of the upper stratosphere and mesosphere. See also Atmospheric general circulation; Tropopause.

Above about 19 mi (30 km), molecular oxygen (O₂) is dissociated to free oxygen atoms (O) during the daytime by ultraviolet photons, (hν), as shown in reaction (1). The oxygen atoms produced then form ozone (O₃) by reaction (2),
1

2
where M is an arbitrary molecule required to conserve energy and momentum in the reaction. Ozone has a short lifetime during the day because of photodissociation, as shown in reaction (3).
3
However, except above 54 mi (90 km), where O₂ begins to become a minor component of the atmosphere, reaction (3) does not lead to a net destruction of ozone. Instead the O is almost exclusively converted back to O₃ by reaction (2). If the odd oxygen concentration is defined as the sum of the O₃ and O concentrations, then odd oxygen is produced by reaction (1). It can be seen that reactions (2) and (3) do not affect the odd oxygen concentrations but merely define the ratio of O to O₃. Because the rate of reaction (2) decreases with altitude while that for reaction (3) increases, most of the odd oxygen below 36 mi (60 km) is in the form of O₃ while above 36 mi (60 km) it is in the form of O. The reaction that is responsible for a small fraction of the odd oxygen removal rate is shown as reaction (4).
4
A significant fraction of the removal is caused by the presence of chemical radicals [such as nitric oxide (NO), chlorine (Cl), bromine (Br), hydrogen (H), or hydroxyl (OH)], which serve to catalyze reaction (4) (see illustration).

Principal chemical cycles in the stratosphere. The destruction of ozone is affected by the presence of radicals which are produced by photolysis or oxidation of source gases. Chemical reservoirs are relatively stable but are removed from the stratosphere by transport toward the troposphere and rain-out.

The discovery in the mid-1980s of an ozone hole over Antarctica, which could not be explained by the classic theory of ozone and had not been predicted by earlier chemical models, led to many speculations concerning the causes of this event, which can be observed each year in September and October. As suggested by experimental and observational evidence, heterogeneous reactions on the surface of liquid or solid particles that produce Cl₂, HOCl, and ClNO₂ gas, and the subsequent rapid photolysis of these molecules, produces chlorine radicals (Cl, ClO) which in turn lead to the destruction of ozone in the lower stratosphere by a catalytic cycle [reactions (5)–(7)].
5

6

7
Solar radiation is needed for these processes to occur.

Sites on which the reactions producing Cl₂, HOCl, and ClNO₂ can occur are provided by the surface of ice crystals in polar stratospheric clouds (PSCs). These clouds are formed between 8 and 14 mi (12 and 22 km) when the temperature drops below approximately −123°F (187 K). Other types of particles are observed at temperatures above the frost point of −123°F (187 K). These particles provide additional surface area for these reactions to occur. Clouds are observed at high latitudes in winter. Because the winter temperatures are typically 20–30°F (10–15 K) colder in the Antarctic than in the Arctic, their frequency of occurrence is highest in the Southern Hemisphere. Thus, the formation of the springtime ozone hole over Antarctica is explained by the activation of chlorine and the catalytic destruction of O₃ which takes place during September, when the polar regions are sunlit but the air is still cold and isolated from midlatitude air by a strong polar vortex. Satellite observations made since the 1970s suggest that total ozone in the Arctic has been abnormally low during the 1990s, probably in relation to the exceptionally cold winter tempratures in the Arctic lower stratosphere recorded during that decade.