EFFECTS OF FLUID-DYNAMICAL STIRRING AND MIXING ON THE DEACTIVATION OFSTRATOSPHERIC CHLORINE

We quantify the sensitivity to mixing of chlorine deactivation and ozo ne depletion in simplified models of the northern hemisphere, middle-l atitude lower stratosphere. A photochemical box model augmented by a v olume exchange model of mixing was used to identify situations for whi ch effects of ClOx deactivation through mixing with NOx-rich air are a t least as important as photochemical effects alone. In one simulation representative of a filament of high-latitude, ClOx-activated air mix ing with low-latitude air, the O-3 depletion rate was found to be 0.28 7%, 0.127%, or 0.08% day(-1), for a volume exchange rate of 0, 0.06, o r 0.48 day(-1), respectively. If we take the first rate as representat ive of Lagrangian models, the second rate as typical of the lower stra tosphere, and the third rate as typical of those grid-based models tha t do not resolve the real mixing length scales, then our results sugge st that Lagrangian models that do not represent mixing processes can, in certain circumstances, be in error by as much or more than grid-bas ed models. The hox model results allowed the formulation of a simplifi ed, yet reasonably accurate reaction scheme for mixing-induced chlorin e deactivation which was then implemented in a two-dimensional model o f quasi-horizontal transport along isentropic surfaces. The two-dimens ional model represents mixing by an effective horizontal diffusivity t hat accounts for vertical diffusivity and horizontal strain, and for a dvective transport it uses winds from lower stratospheric observationa l analyses. Mixing-induced chlorine deactivation was found to exhibit substantial sensitivity to the effective diffusivity of the two-dimens ional model. The associated O-3 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 depend s on other details of the wind fields and the NO, concentration field. We suggest that O-3 loss in low-resolution models is sensitive to per turbations in the Ne? field, whereas, by contrast, O-3 loss in the low er stratosphere is probably far less sensitive to such perturbations. To obtain estimates of O-3 less over 11 days chat are not sensitive to the diffusivity employed, it was necessary to use effective horizonta l diffusivities D-H < 10(5) m(2) s(-1), corresponding to horizontal fe atures of about 200 km and hence requiring spatial resolution of about 40 km, which is much higher resolution than routinely employed. The r esults from the box model and the two-dimensional model are combined t o assess the importance of mixing-induced chlorine deactivation relati ve to photolysis-induced deactivation.