Recent measurements of enhanced ozone depletion in springtime in the Antarctic circumpolar vortex1 were accompanied by observations of unusual behaviour of stratospheric aerosol2. Enhanced concentrations of condensation nuclei (CN) were observed above 20km. Some photochemical models predict elevated hydroxyl radical (OH) concentrations in the vortex. Here I point out that high OH concentrations provide a sufficient source of CN through reaction with carbonyl sulphide (OCS) to account for the observed increase in CN mass. Other mechanisms for CN enhancement are inconsistent with timescales or the vertical structure inferred from aerosol observations. The CN enhancement provides partial support for photochemical models of ozone depletion in the vortex.
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