CIESIN Thematic Guides

Ozone Depletion Processes

A complex interplay of chemistry, dynamics, and radiation lead to conditions conducive to significant ozone loss in the polar regions. The sequence of events leading to the springtime depletion of ozone is initiated by the onset of polar night, when high latitude regions receive no sunlight. The inclination of the Earth's orbit at about 23.5deg. causes the polar regions to experience continual darkness during their winter season. The air above the pole cools and a vortex is formed that isolates the colder region from the lower latitudes. Schoeberl and Hartmann (1991) provide a detailed description on the formation and evolution of the polar vortex in "The Dynamics of the Stratospheric Polar Vortex and Its Relation to Springtime Ozone Depletion."

Creation of the vortex sets the stage for the rapid depletion of ozone by catalytic cycles. A catalytic cycle is a series of reactions in which a chemical family or a particular species is depleted, leaving the catalyst unaffected. The odd-oxygen family, for example, is composed of ozone (O3) and atomic oxygen (O). In the presence of a chlorine atom, the net result is the conversion of an oxygen atom and ozone molecule to two molecules of molecular oxygen (O2). Chlorofluorocarbons (CFCs) themselves are not involved in the catalytic process; upon reaching the stratosphere, they are subject to higher levels of ultraviolet radiation that decompose the CFC and release atomic chlorine. The basic set of reactions that define the catalytic cycle involving chlorine and odd-oxygen appear below:

Cl + O3 ==> ClO + O2

ClO + O ==> Cl + O2

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net result: O3 + O ===> 2O2

Chlorine (Cl) is initially removed by reaction with ozone to form chlorine monoxide (ClO) in the first equation, but it is regenerated through reaction of ClO with an oxygen atom (O) in the second equation. The net result of the two reactions is the depletion of ozone and atomic oxygen.

The catalytic cycle involving chlorine and ozone was not discovered until 1973, and not until 1985 did Farman, Gardiner, and Shanklin report loss of large amounts of ozone over Halley Bay, Antarctica, in "Large Losses of Total Ozone in Antarctica Reveal Seasonal ClOx/NOx Interaction." The suspected cause of the depletion was catalytic cycles involving chlorine and nitrogen. In further studies detailed in "Polar Stratospheric Clouds and Ozone Depletion," Toon and Turco (1991) describe the multiphase processes involving Polar Stratospheric Clouds (PSCs) and chlorine and nitrogen compounds that were found to be involved in the extensive ozone loss. Field studies in 1987, which involved flights from South America over the Antarctic continent, showed the role of chlorine monoxide (ClO) in the ozone depletion picture. Anderson, Brune, and Proffitt (1989) describe a clear anti-correlation between chlorine monoxide and ozone concentration--that is, an increase in chlorine monoxide correlates to a decrease in ozone--inside the polar vortex in "Ozone Destruction by Chlorine Radicals Within the Antarctic Vortex." Depletions of greater than 50 percent of total atmospheric ozone have been observed over the Antarctic continent.

Similar processes lead to depletion in the Arctic, but to a lesser degree. Temperatures are higher due to the variability of the polar vortex, which limits formation of PSCs. The potential for significant depletion in the Arctic does exist, however. In "Arctic Measurements Indicate the Chilly Prospect of Ozone Depletion," Levi (1992) describes measurements taken from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) platform in 1992 that show concentrations of chlorine monoxide in the Arctic similar to those found in the Antarctic vortex. The breakup of the polar vortex following polar springtime leads to mixing of ozone-poor air parcels down to lower latitude regions. Brune et al. (1992) outline this process, along with other possible scenarios leading to observed decreases in mid-latitude ozone, in the chapter "Stratospheric Processes" of the Scientific Assessment of Ozone.

Multiphase reactions involving aerosol particles are another source of ozone loss that can occur at all latitudes. Hofmann and Solomon (1989) analyze the El Chichon eruption in 1983 and show ozone destruction in areas of higher aerosol concentration in "Ozone Destruction through Heterogeneous Chemistry Following the Eruption of El Chichon." The process is thought to be similar to the Polar Stratospheric Cloud (PSC) scenario: Aerosol particles act as a base for multiphase reactions, leading to ozone loss. Hofmann et al. (1992) report that additional ozone loss over Antarctica in 1991 beyond the depletion caused by PSCs may be attributed to this process following the eruption of Mt. Hudson in "Observation and Possible Causes of New Ozone Depletion in Antarctica in 1991."