Seasonal evolution of ozone-methane correlations in the polar regions

Recently Müller et al (1996) used the evolution of a measured relationship between ozone and methane in the Arctic polar vortex to quantify the Arctic ozone loss due to chemistry alone. Their analysis assumes several things. One assumption is that a compact correlation exists when the initial measurement is made. This requires either that a correlation exists when the polar vortex is formed, which seems unlikely due to the short lifetime of ozone under sunlight conditions, or that a correlation forms in the early winter inside an isolated dark vortex. A second assumption is that a compact correlation is maintained even after the rapid destruction of ozone.

There are not sufficiently extensive simultaneous measurements of ozone and methane to determine whether a well-defined correlation exists. Therefore data from a comprehensive middle atmosphere GCM (the Canadian Middle Atmosphere Model) are used to provide a surrogate dataset. The Antarctic is examined first because it provides an ideal environment for compact correlations to occur since the vortex is much more stable than in the Arctic.

The results suggest that although there is no correlation between ozone and methane when the vortex is formed, there is sufficient time for one to form through early winter. Throughout the late winter and spring the isolation of the polar air mass is examined by using particle trajectory calculations. This allows ozone-methane correlations to be calculated for air that has remained wholly within the polar vortex. The ozone loss inferred from this method can be compared with direct results from the model equations. The situation for the Arctic vortex is much less clear due to the lower stability of the vortex. The Arctic results are contrasted with those of the Antarctic.