Analysis of the chemical kinetic mechanisms of ozone depletion and halogen release in the polar troposphere based on numerical modeling and field observation

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The role of reactive halogen species (RHS, e.g., Br, Br2, BrO, Cl, Cl2, ClO, OClO, etc.) in the polar troposphere has been investigated since the discovery of their importance for boundary layer ozone destruction in the polar spring. RHS take part in an auto-catalytic chemical reaction cycle, which releases Br2 and BrCl from sea salt aerosol, fresh sea ice or snowpack, leading to ozone depletion. In the first project period, three chemical reaction schemes were investigated in a zero dimensional model and compared to field observations: (1) a bromine-only reaction scheme (2) an extended Br scheme including nitrogen-containing compounds and (3) a further extended scheme additionally including chlorine species and corresponding chemical reactions. We also studied the importance of particular reactions and their rate constants by a sensitivity analysis, leading to a skeletal reaction mechanism for use in one-dimensional and three-dimensional configurations, which were set up to clarify the ozone depletion event.Despite good progress leading to considerably better insight into the mechanism of polar tropospheric halogen chemistry, many open questions remain. Therefore, we apply for the continuation of the project, which addresses the following questions: What is the influence of meteorological conditions on a local scale (e.g. wind speed, boundary layer height, vertical temperature profile) and on synoptic scales, in particular, the role of meteorological fronts on halogen activation. What is the role of atmospheric composition, i.e. aerosol (explicitly include the aerosol phase in the model so that the microphysics, which may affect the species concentration and temperature, can be investigated.) and in particular, NOX (N2O5 chemistry) in halogen activation? How is the seasonal evolution of Br-activation and O3 loss? Are there differences between Arctic and Antarctic (and extra-polar) halogen activation events with respect to their duration, halogen composition, etc.? If so, what are the reasons for these differences? What are the prerequisites and factors actually triggering the halogen release? Which role does the surface type (e.g. fresh sea ice, snow covering surface, open leads, frost flowers, etc.) play and how does it interact with the gas phase? The presence of IO can largely enhance the efficiency of ozone destruction by halogens, what is the mechanism? Moreover, a series of technical improvements to the model will be made: In the 3D model, due to the relative long simulation time, it is impossible to use a very fine grid close to the ground surface. However, by extending the box model to a 1D model with a prescribed turbulent diffusivity, it is feasible to use a very fine mesh near the surface. We will also investigate whether different sub-grid models affect the computational results in the large eddy simulation (LES).