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Introduction

The chemically quasi-inert gas is widely used to characterize stratospheric transport, which is important to know for estimating the global burden of greenhouse gases or substances harmful to the ozone layer. As exhibits a quasi-linear and strong growth in its mixing ratio in the troposphere (Maiss and Levin, 1994) a mean age of stratospheric air can be deduced from the time lag since the troposphere last showed the mixing ratio measured in the stratosphere (see eg. Hall and Plumb, 1994; Harnisch et al., 1996; Hall and Waugh, 1998; Volk et al., 1997). Small deviations from linearity of the trend can be corrected in determining the mean age (Hall and Waugh, 1998; Volk et al., 1997). Generally, the derived maximum mean age of air in the stratosphere spans 4-5 years in the tropics up to about 10 years for polar winter observations. Recently, mesospheric loss of was discussed to explain obvious discrepancies between age determinations with versus other trace gases (Strunk et al., 2000; Harnisch et al., 1998) and discrepancies between circulation models and observations which consistently show lower age values than observed (Hall and Waugh, 1998). In their study Hall and Waugh used a range of constant loss rates above 60 km, which were compatible with the chemical lifetimes given in literature and showed that mesospheric loss, when unaccounted for, causes an overestimation of the mean age by up to 65 percent at subarctic latitudes and a height of 30 km. However, this effect strongly depends on the loss rates assumed.

In order to make more realistic comparisons with the observations, a simplified chemistry of is included in a 3-D model of the middle atmosphere based on meteorological analyses. The results are then compared with the profiles observed.

Chemistry of in the middle atmosphere