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Unless there are chemical reactions going in the opposite direction, most ()> in the atmosphere would be transformed into ozone in about 10,000 years. For many years it was thought that the return reactions from O3 to O^ would involve only oxygen allotropes (Chapman, 1930):

These reactions too cannot be influenced by humankind. Things changed, however, when it was realized that catalytic reactions could be more important than the previously mentioned Chapman reactions in converting O3 back to O?. First, it was hypothesized by Crutzen (1970) that NO and NO? could catalyze the destruction of ozone:

In the following year Johnston (1971) and Crutzen (1971) independently proposed that the nitric oxide emitted from the large fleets of supersonic transport aircraft, the SSTs -which were planned to be built in the United States, France, Britain, and the Soviet Union - could result in substantial ozone depletion. Only a few SSTs were ever built. However, a few years later, Molina and Rowland (1974) hypothesized that CI and CIO, released to the atmosphere from the photochemical decay of the chlorofluorocarbon gases (CFCI3 and CF2Q2), could deplete ozone by a similar chain of catalytic reactions as shown earlier with NO and NO^:

In particular, since the Second World W ar, the stratospheric abundance of chlorine-containing gases has increased strongly; consequently, the stratosphere now contains approximately six times more chlorine than the amount provided by methvlchloride (CH3CI), which is emitted from the oceans. Until 1985, it was thought that ozone destruction via the C10x catalytic cycle would take place primarily over the altitude range 30-45 km, whereas at lower elevations, where most ozone is located, much less ozone would be destroyed. However, observations reported in 1985 by researchers of the British Antarctic Survey (Farman et al., 1985) showed that the most dramatic ozone decreases were occurring during September-October principally in the lower layers of the stratosphere over Antarctica, a finding that was totally unexpected. Previously it was believed, and this is true in most situations, that below 30 km reactions between the NO* and C10K catalysts, producing hydrochloric acid (HCl) and chlorine nitrate

Rib O3 + hp ->0 + 02(X<1140nm) RS 0 4-0;, 2 O2

R6 NO + 0} NO2 4- 0> R1 Q$ + hv O 4- O2 R7 0 4- NO2 NO 4- 02

RI 0.î + hi> -*0 + 02(<1140nm) R9 CIO 4- O —► CI 4- 02

Maiepwan, 3amnmeHHbiii aetopcKUM npaBOM

(C;I0N02) via and

would strongly reduce the concentrations of the "ozone killers^ NOx and CIOt, thus protecting ozone from otherwise much stronger destruction. Through these reactions, the majority of stratospheric inorganic chlorine is mostly tied up as HCI and CIONO2, which do not react with ()?. That these favorable circumstances do not always exist became clear after Far man et al. (1985) discovered that average springtime (September-October) stratospheric ozone amounts above their research station Halley Bay on the Antarctic continent had been strongly decreasing year by year since the middle of the 1970s, Similar low O3 values had also been reported by Chubachi (1984) of the Japan Polar Research Institute. From balloon soundings it became clear that rapid and complete ozone loss was taking place within a month in the same height range, 14-22 km, where maximum ozone concentrations are usually found (Figure 1.3). The observations were a total surprise to the stratospheric ozone research community. Until 1985 it had been common wisdom that the ozone in this altitude region was chemically inert. Analyses of satellite observations showed that large ozone decrease occurred over much of Antarctica during the months of September and October. The big question was, how was this possible? After only a few years of intensive research the principal causes became clear. At very cold temperatures> less than around — Hi) C, which occur rp 1 ri t 1 t 11

Column y^ Variability rp 1 ri t 1 t 11

Column y^ Variability

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