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Stratospheric Chemistry and Dynamics

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Stratospheric Chemistry and Dynamics

No part of the global environment has been disturbed by human activity as significantly as the stratosphere. In the upper stratosphere and lower mesosphere ozone is removed by catalytic cycles involving halogen oxides. In addition, a very substantial depletion of stratospheric ozone over Antarctica has been observed during springtime since the end of the 1970’s. This depletion is largely due to the emission of industrial chlorofluorocarbon gases (WMO 2003 and references therein). Also over the Arctic a major depletion of stratospheric ozone by about 100 DU (Dobson Units) has become obvious during springtime in the past decade. Surface reactions on liquid aerosols, nitric acid trihydrate (NAT) particles and ice particles are believed – via the activation of chlorine – to be primarily responsible for these changes. International regulatory measures, in the form of e.g. the Montreal Protocol, having now been taken to eliminate the production of chlorofluorocarbons by the end of the 20th century (WMO 1995). However the amount of stratospheric chlorine will reach its maximum at the beginning of the 21st century. A first recovery of the ozone layer is expected around 2010 (WMO 2003). The loss of ozone in the stratosphere is also affected, in a synergistic manner, by tropospheric emission of greenhouse gases (see figure 1-5). For example, the anthropogenic tropospheric concentrations of nitrous oxide and methane are increasing, leading to additional formation of stratospheric NOx and water vapour (H2O) and potentially enhancing the probability for formation of PSCs. Reactions on these clouds lead to the activation of chlorine radicals that are responsible for the formation of the ‘ozone hole’. Thus, even though the stratospheric chlorine content is expected to decline at the beginning of the 21st century, ozone depletion in the lower stratosphere at higher latitudes may not. (see fig. 1-5)


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fig. 1-5:

Schematic sketch of the interactions between stratospheric ozone and other atmospheric constituents and processes. Anthropogenic emissions are shown in green while other factors affecting the climate system (e.g., volcanoes) are shown in beige. Red arrows indicate where one species or process affects another. Feedbacks are shown with bold purple lines. For example, decreasing polar stratospheric temperatures increase ozone depletion. Reduced ozone then causes stratospheric cooling, creating a positive feedback. (Graphics after: NIWA)


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