Scientific motivation, and co-ordination of HALO campaign ASCCI and provision of quality controlled merged data products

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The project ASCCI is a HALO mission in the Northern Hemisphere polar late winter to early spring which is proposed in the continuation of polar ozone measurement campaigns conducted with the HALO aircraft. Most notably these are the two HALO missions SOUTHTRAC in the Southern Hemisphere and POLSTRACC in the Northern Hemisphere. Especially the Northern Hemisphere shows very large year to year variability in polar stratospheric dynamics and therefore also in polar ozone. The first Arctic polar ozone related measurement campaign POLSTRACC took place in the winter 2015/2016, a winter characterized by record low temperatures until mid winter followed by a sudden stratospheric warming in February 2016. We therefore propose to perform a second polar winter campaign in the Northern Hemisphere to investigate the evolution of the dynamics of the polar vortex and ozone in the Northern Hemisphere high latitudes. As detailed in the proposal, and recently shown by Von der Gathen et al., (2021), climate change is expected to favour large seasonal loss of ozone in the Arctic in the future. It will thus be of importance to regularly revisit the Arctic lower stratosphere and investigate the ozone loss in a changing atmosphere. In contrast to mid-latitudes, modelled polar tropospheric ozone generally shows a significant low bias. It has been argued that this could be due both to chemistry (e.g. levels of tropospheric pollution and NOy influx from the stratosphere) as well as transport of ozone rich stratospheric air cold contribute to these discrepancies. While we propose to study the polar vortex and its chemical evolution, ASCCI will have a slightly stronger emphasis on the implications for the mid latitudes and for the Arctic troposphere. In particular the impact of polar ozone loss on spring time mid latitude ozone will be a focus of ASCCI as well as the chemical composition of the upper Arctic troposphere and in how far this can be linked to transport from the stratosphere and to tropospheric pollution. In this proposal we apply for the costs associated with the coodination of the campaign and costs associated for the production of final quality controlled merged data which include all observational data as well as meteorological and model data.