Rapid chlorine deactivation at very low ozone concentrations in the Antarctic stratosphere

The intensive catalytic chemical ozone loss cycles involving inorganic chlorine compounds are known to cause the ozone hole, that regularly forms in the Antarctic polar vortex each winter and spring.  One key point of the explanation of the ozone hole are heterogeneous reactions on the surface of Polar Stratospheric Cloud (PSC) particles, which are present in the polar stratosphere owing to the very low stratospheric temperatures. Ozone mixing ratios can reach values of a few ppbv which is below detection limit of ozone sonde observations.  Under these extreme conditions, fast chlorine deactivation into the reservoir HCl does occur even though polar stratospheric clouds are still present, that are normally causing chlorine activation. In this study we revisit this issue and investigate the occurring chlorine chemistry in more detail.  The explanation of the chemical mechanism of the fast net HCl formation is based on the automatic determination of reaction pathways by the Pathway Analysis Program (PAP) (Lehmann, 2004). The simulations of chemical composition are performed by the model CLaMS in box model mode along an ensemble of about 600 trajectories in the Antarctic spring 2018.  The simulated rapid complete chlorine deactivation into HCl in the presence of PSCs is in line with satellite  observations by the Microwave Limb Sounder (MLS).   ReferenceLehmann, R.: An algorithm for the determination of all significant pathways in chemical reaction systems, J. Atmos. Chem. 47, 45-78 (2004).