Numerical analysis of the chemical kinetic mechanisms of ozone depletion and halogen release in the polar troposphere

Abstract

Abstract. In recent years, the role of halogen species (e.g. Br, Cl) in the troposphere of polar regions is investigated after the discovery of their importance for boundary layer ozone destruction in the polar spring. Halogen species take part in an auto-catalytic chemical cycle including key self reactions. In this study, several chemical reaction schemes are investigated, and the importance of specific reactions and their rate constants is identified by a sensitivity analysis. A category of heterogeneous reactions related to HOBr activate halogen ions from sea salt aerosols, fresh sea ice or snow pack, driving the "bromine explosion". In the Arctic, a small amount of NOx may exist, which comes from nitrate contained in the snow, and this NOx may have a strong impact on ozone depletion. The heterogeneous reaction rates are parameterized by considering the aerodynamic resistance, a reactive surface ratio, β, i.e. ratio of reactive surface area to total ground surface area, and the boundary layer height, Lmix. It is found that for β = 1, the ozone depletion process starts after five days and lasts for 40 h for Lmix = 200 m. Ozone depletion duration becomes independent of the height of the boundary layer for about β≥20, and it approaches a value of two days for β=100. The role of nitrogen and chlorine containing species on the ozone depletion rate is studied. The calculation of the time integrated bromine and chlorine atom concentrations suggests a value in the order of 103 for the [Br] / [Cl] ratio, which reveals that atomic chlorine radicals have minor direct influence on the ozone depletion. The NOx concentrations are influenced by different chemical cycles over different time periods. During ozone depletion, the reaction cycle involving the BrONO2 hydrolysis is dominant. A critical value of 0.002 of the uptake coefficient of the BrONO2 hydrolysis reaction at the aerosol and saline surfaces is identified, beyond which the existence of NOx species accelerate the ozone depletion event – for lower values, deceleration occurs.