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Clouds and aerosol particles play important roles in the chemistry of the polar winter stratosphere. Under extremely cold conditions their surfaces host heterogeneous chemical reactions, which—depending on temperature and aerosol loading—may lead to a very fast transformation of chlorine containing reservoir compounds into active, ozone destroying chlorine. Furthermore, polar stratospheric clouds (PSCs) may generate large nitric-acid-containing particles, which sediment rapidly, thereby denitrifying the stratospheric air and disabling reactive nitrogen species to deactivate the ozone-destroying chlorine. Both processes, heterogeneous chemistry and denitrification provide the basis for chlorine-catalyzed ozone destruction, which under sustained cold conditions leads to the ozone hole over the Antarctic and substantial ozone depletion over the Arctic. While the principles of these polar ozone loss mechanisms have long been recognized, very recently new measurements and modeling of PSCs have cast serious doubts on our understanding of some of the most fundamental processes of PSC formation and heterogeneous chemistry. One important open question is whether the nucleation of nitric acid hydrates and subsequent denitrification are governed predominantly by ice-assisted or ice-free processes. A number of previously discussed processes appear to be unlikely from today's perspective, such as homogeneous nucleation of hydrates in supercooled liquid solution droplets, nucleation of hydrates in glassy aerosols or pseudo-heterogeneous nucleation at a droplet/vapor interface. However, while heterogeneous nucleation of hydrates on water ice is accepted in the light of laboratory experiments and field observations, an unidentified ice-free heterogeneous nucleation process, e.g. on dust particles, appears to be required in order to explain a number of very recent field observations, in particular by the downward-looking LiDAR on the CALIPSO satellite in the Arctic winter 2009–2010. This result is perplexing and reverses the previous understanding of PSC formation, because there is currently no conclusive laboratory evidence supporting this finding. There are also open issues concerning chlorine activation on solid and liquid PSC and cold stratospheric aerosol particles. In particular, activation reaction rates on nitric acid hydrate particles retain an uncertainty of almost two orders of magnitude. However, while this uncertainty remains a physico-chemical flaw, it appears not to hamper our ability to understand and predict chlorine activation and ozone loss, as liquid particles are usually the dominant chlorine processors. Liquid phase heterogeneous and multi-phase chemistry is much less uncertain. It appears that under many circumstances, the formation of PSCs is not even required for rapid chlorine activation, rather the heterogeneous chemistry on sufficiently cold background aerosol ensures the dissolution and activation of chlorine reservoir species such as HCl.

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