Department of Earth System Science

A model is presented for the chemical mixing of stratospheric air over spatial scales from tens of kilometers to meters. Photochemistry, molecular diffusion, and strain (the stretching of air parcels due to wind shear) are combined into a single one-dimensional model. The model is applied to the case in which chemically perturbed air parcels from the Antarctic stratosphere are transported to mid-latitudes and strained into thin ribbon-like filaments until they are diffusively mixed with the ambient stratosphere. We find that the parcels may be treated as evolving in chemical isolation until the final mixing. When parcels reach a transverse thickness of 50–100 m in the lower stratosphere, they are rapidly dispersed by the combination of molecular diffusion and strain. The rapidity of the final mixing implies a lower limit to the vertical scales of inhomogeneities observed in the lower stratosphere. For this sensitivity study we consider four types of Antarctic air: a control case representing unprocessed polar air; heterogeneous processing by polar stratospheric clouds (PSCs) that has repartitioned the Cl_x and NO_y families; processing that also includes denitrification and dehydration; and all processing plus 90% ozone depletion. Large abundances of ClO, resulting initially from heterogeneous processing of stratospheric air on PSCs, are sustained by extensive denitrification. (One exception is the case of Antarctic air with major ozone depletion in which ClO is converted rapidly to HCl upon release of small amounts of NO_x as a result of the extremely nonlinear Cl_x-NO_y chemical system.) ClO concentrations in the mid-latitude stratosphere should be enhanced by as much as a factor of 5 due to the mixing of air processed around the Antarctic vortex and will remain elevated for most of the following season. Chemical propagation of the Antarctic ozone hole occurs in two phases: rapid loss of ozone in the heterogeneously processed parcels as they evolve in isolation, and more slowly, a relative recovery of ozone over the following months. Another important effect is the transport of denitrified Antarctic air reducing NO_x and hence the total catalytic destruction of ozone throughout the southern mid-latitudes. In Antarctic air that has already been depleted of ozone within the vortex, little additional loss occurs during transport, and the propagation of chemically perturbed air acts partially to offset the deficit at mid-latitudes caused by dynamical dilution of the ozone hole. In air which has not experienced substantial ozone loss, chemical propagation can generate a net ozone deficit of order 2–3% at mid-latitudes.