Stratospheric ozone formation and destruction

The formation of ozone is a photochemical process that uses the energy involved in light. The shorter the wavelength of light, the larger the amount of energy it

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Fig. 3.6 Mean October levels of total ozone above Halley Bay (76°S), Antarctica, since 1957. The 1986 value is anomalous due to deformation of the ozone hole, which left Halley Bay temporarily outside the circumpolar vortex (a tight, self-contained wind system). Dobson units represent the thickness of the ozone layer at sealevel temperature and pressure (where 1 Dobson unit is equivalent to 0.01 mm). Data courtesy of the British Antarctic Survey. Inset shows seasonally averaged (Sep.-Nov.) ozone partial pressure at about 17 km at 70°S. Data courtesy of G. Konig-Langlo.

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Fig. 3.6 Mean October levels of total ozone above Halley Bay (76°S), Antarctica, since 1957. The 1986 value is anomalous due to deformation of the ozone hole, which left Halley Bay temporarily outside the circumpolar vortex (a tight, self-contained wind system). Dobson units represent the thickness of the ozone layer at sealevel temperature and pressure (where 1 Dobson unit is equivalent to 0.01 mm). Data courtesy of the British Antarctic Survey. Inset shows seasonally averaged (Sep.-Nov.) ozone partial pressure at about 17 km at 70°S. Data courtesy of G. Konig-Langlo.

carries. It requires ultraviolet (UV) radiation of wavelength less than 242 nm to have sufficient energy to split the oxygen molecule (O2) apart:

The UV photon here is symbolized by hv. Once oxygen atoms (O) have been formed, they can react with O2.

The production of O3 by this photochemical process can be balanced against the reactions that destroy O3. The most important is photolysis:

together with an additional reaction describing the destruction process for oxygen atoms:

Note the presence of the 'third body' M, which carries away excess energy during the reaction. The third body would typically be 02 or a nitrogen molecule (N2). Without this 'third body' the 02 that formed might split apart again. Calculations that balance the production and destruction of 03 considering only reactions that involve the element oxygen (i.e. oxygen-only paths) give a fair description of the 03 observed in the stratosphere. The results of these calculations produce the correct shape for the vertical profile of 03 in the atmosphere and the peak 03 concentration occurs at the correct altitude, but the predicted concentrations are too high. This is because there are other pathways that destroy 03. Some involve hydrogen-containing species:

which sum:

Similar reactions can be written for nitrogen-containing species, for example nitric oxide (N°), which arises from supersonic aircraft, or nitrous oxide (N2°), which crosses the tropopause into the stratosphere:

and N2°(g) can enter reaction 3.40 via the initial step:

Reactions involving these species sum in such a way as to destroy °3 and atomic oxygen while restoring the °H or N° molecules. They can thus be regarded as catalysts for °3 destruction. In this case the catalysts are chemical species that facilitate a reaction, but undergo no net consumption or production in the reaction (see also Box 4.4). The important point of these catalytic reaction chains in the chemistry of stratospheric °3 is that a single pollutant molecule can be responsible for the destruction of a large number of °3 molecules.

It is now very well established that the most important of these catalytic reaction chains affecting polar ozone loss are the ones based around chlorine-containing species, as detailed below.

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