Comments to the revised version

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 ::: can : reproduce the observed chemical composition in the springtime polar stratosphere, especially regarding the quantification of halogen-catalysed ozone loss. However, here we report ::: we ::::: report :::: here : on a discrepancy between simulations and observations during the less-well studied period of the onset 5 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 Arcticand present, . :::: The :::::::::: discrepancy ::::: exists :: in ::::::: different :::::: models : to varying extents, in three independent models : ; ::: here ::: we ::::::: discuss ::::: three :::::::::: independent :::: ones, the Lagrangian chemistry transport model CLaMS as well as the Eulerian models WACCM ::::::::::: SD-WACCM and TOMCAT/SLIMCAT. The HCl discrepancy points to some unknown process in the formulation of 10 stratospheric chemistry that is currently not represented in the models. Here we ::: We : characterise the HCl discrepancy in space and time for the Lagrangian Chemistry Transport Model :::::::::::::::: 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 ::::::: Eulerian ::::::: models ::::::::::: SD-WACCM and TOMCAT/SLIMCAT are also presented. Numerical diffusion in the 15 ::::::: transport ::::::: scheme :: of ::: 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 :::::::::::: underestimated :::: HCl :::::: uptake into the PSC particles that consist mainly of H2O and HNO3 cannot explain the discrepancy :: it due to the temperature correlation of the discrepancy. Also, a direct photolysis of particulate HNO3 does not explain :::::: resolve the discrepancy since it would also cause changes in :::::: chlorine ::::::::: chemistry :: in : late winter which are not 20 observed. The ionisation caused by Galactic Cosmic Rays provides an additional NOx and HOx source that can explain only around :::: about : 20% of the discrepancy. A :::::::: However, ::: the ::::: model :::::::::: simulations :::: show :::: that : a : hypothetical decomposition of particulate HNO3 by some other process not dependent on the solar elevation, e.g. involving Galactic Cosmic Rays, may be a possible