Effects of fluid‐dynamical stirring and mixing on the deactivation of stratospheric chlorine

Abstract

We quantify the sensitivity to mixing of chlorine deactivation and ozone depletion in simplified models of the northern hemisphere, middle‐latitude lower stratosphere. A photochemical box model augmented by a volume exchange model of mixing was used to identify situations for which effects of ClOx deactivation through mixing with NOx‐rich air are at least as important as photochemical effects alone. In one simulation representative of a filament of high‐latitude, ClOx‐activated air mixing with low‐latitude air, the O3 depletion rate was found to be 0.287%, 0.127%, or 0.08% day−1, for a volume exchange rate of 0, 0.06, or 0.48 day−1, respectively. If we take the first rate as representative of Lagrangian models, the second rate as typical of the lower stratosphere, and the third rate as typical of those grid‐based models that do not resolve the real mixing length scales, then our results suggest that Lagrangian models that do not represent mixing processes can, in certain circumstances, be in error by as much or more than grid‐based models. The box model results allowed the formulation of a simplified, yet reasonably accurate reaction scheme for mixing‐induced chlorine deactivation which was then implemented in a two‐dimensional model of quasi‐horizontal transport along isentropic surfaces. The two‐dimensional model represents mixing by an effective horizontal diffusivity that accounts for vertical diffusivity and horizontal strain, and for advective transport it uses winds from lower stratospheric observational analyses. Mixing‐induced chlorine deactivation was found to exhibit substantial sensitivity to the effective diffusivity of the two‐dimensional model. The associated O2 depletion from the Molina and Molina [1987] ClO‐dimer cycle was found to exhibit sensitivity in a number of different regimes. The sensitivity increases with time and also depends on other details of the wind fields and the NOx concentration field. We suggest that O3 loss in low‐resolution models is sensitive to perturbations in the NO2 field, whereas, by contrast, O3 loss in the lower stratosphere is probably far less sensitive to such perturbations. To obtain estimates of O3 loss over 11 days that are not sensitive to the diffusivity employed, it was necessary to use effective horizontal diffusivities DH < 105 m2 s−1, corresponding to horizontal features of about 200 km and hence requiring spatial resolution of about 40 km, which is much higher resolution than routinely employed. The results from the box model and the two‐dimensional model are combined to assess the importance of mixing‐induced chlorine deactivation relative to photolysis‐induced deactivation.