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. More than 3 decades after the discovery of the ozone hole, theprocesses involved in its formation are believed to be understood ingreat detail. Current state-of-the-art models can reproducethe observed chemical composition in the springtime polar stratosphere,especially regarding the quantification of halogen-catalysed ozoneloss.However, we report here on a discrepancy between simulations andobservations during the less-well-studied period of the onset ofchlorine activation. During this period, which in the Antarctic isbetween May and July, model simulations significantly overestimateHCl, one of the key chemical species, inside the polar vortex duringpolar night. This HCl discrepancy is also observed in the Arctic.The discrepancy exists in different models to varying extents; here,we discuss three independent ones, the Chemical LagrangianModel of the Stratosphere (CLaMS) as well as the Eulerianmodels SD-WACCM (the specified dynamics version of the Whole AtmosphereCommunity Climate Model) and TOMCAT/SLIMCAT. The HCl discrepancy points tosome unknown process in the formulation of stratospheric chemistrythat is currently not represented in the models. We characterise the HCl discrepancy in space and time for theLagrangian chemistry–transport model CLaMS, in which HCl in thepolar vortex core stays about constant from June to August in theAntarctic, while the observations indicate a continuous HCl decreaseover this period. The somewhat smaller discrepancies in the Eulerianmodels SD-WACCM and TOMCAT/SLIMCAT are also presented. Numericaldiffusion in the transport scheme of the Eulerian models isidentified to be a likely cause for the inter-model differences.Although the missing process has not yet been identified, weinvestigate different hypotheses on the basis of the characteristicsof the discrepancy.An underestimated HCl uptake into the polar stratospheric cloud (PSC) particles that consistmainly of H2O and HNO3 cannot explain it due tothe temperature correlation of the discrepancy. Also, a directphotolysis of particulate HNO3 does not resolve the discrepancysince it would also cause changes in chlorine chemistry in latewinter which are not observed.The ionisation caused by galactic cosmic rays provides an additional NOx and HOx source that can explain only about 20 %of the discrepancy.However, the model simulations show that a hypotheticaldecomposition of particulate HNO3 by some other process notdependent on the solar elevation, e.g. involving galactic cosmicrays, may be a possible mechanism to resolve the HCl discrepancy.Since the discrepancy reported here occurs during the beginning ofthe chlorine activation period, where the ozone loss rates are small,there is only a minor impact of about 2 % on the overall ozonecolumn loss over the course of Antarctic winter and spring.

On the discrepancy of HCl processing in the core of the wintertime polar vortices