On the discrepancy of HCl processing in the dark polar vortices

More than three decades after the discovery of the ozone hole, the processes involved in its formation are believed to be understood in great detail. Current state-of-the-art models are able to reproduce the observed chemical composition in the springtime polar stratosphere, especially regarding the quantification of halogen-catalysed ozone loss. However, here we report on a discrepancy between simulations and observations during the less-well studied period of the onset of chlorine activation. During this period, which in the Antarctic is between May and July, model simulations significantly overestimate HCl, one of the key chemical species, inside the polar vortex during polar night. This HCl discrepancy is also observed in the Arctic and present, to varying extents, in three independent models, the Lagrangian chemistry transport model CLaMS as well as the Eulerian models WACCM and TOMCAT/SLIMCAT. The HCl discrepancy points to some unknown process in the formulation of stratospheric chemistry that is currently not represented in the models. Here we characterise the HCl discrepancy in space and time for the Lagrangian Chemistry Transport Model CLaMS, in which HCl in the polar vortex core stays about constant from June to August in the Antarctic while the observations indicate a continuous HCl decrease over this period. The somewhat smaller discrepancies in the models WACCM and TOMCAT/SLIMCAT are also presented. Numerical diffusion in the Eulerian models is identified to be a likely cause for the inter-model differences. Although the missing process has not yet been identified, we investigate different hypotheses on the basis of the characteristics of the discrepancy. An under-estimated uptake of HCl into the PSC particles that consist mainly of H 2 O and HNO 3 cannot explain the discrepancy due to the temperature correlation of the discrepancy. Also, a direct photolysis of particulate HNO 3 does not explain the discrepancy since it would also cause changes in late winter which are not observed. The ionisation caused by Galactic Cosmic Rays provides an additional NO x and HO x source that can explain only around 20 % of the discrepancy. A hypothetical decomposition of particulate HNO 3 by some other process not dependent on the solar elevation, e.g. involving Galactic Cosmic Rays, may be a possible mechanism to resolve the HCl discrepancy. Since the discrepancy reported here occurs during the beginning of the chlorine activation period, where the ozone loss rates are slow, there is only a minor impact of about 2 % on the overall ozone column loss over the course of Antarctic winter and spring.