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

Abstract. The role of halogen species (e.g., Br, Cl) in the troposphere of polar regions has been investigated since the discovery of their importance for boundary layer ozone destruction in the polar spring about 25 years ago. Halogen species take part in an auto-catalytic chemical reaction cycle, which releases Br₂ and BrCl from the sea salt aerosols, fresh sea ice or snowpack, leading to ozone depletion. In this study, three different chemical reaction schemes are investigated: a bromine-only reaction scheme, which then is subsequently extended to include nitrogen-containing compounds and chlorine species and corresponding chemical reactions. The importance of specific reactions and their rate constants is identified by a sensitivity analysis.

The heterogeneous reaction rates are parameterized by considering the aerodynamic resistance, a reactive surface ratio, β, i.e., the ratio of reactive surface area to total ground surface area, and the boundary layer height, L_mix. It is found that for β = 1, a substantial ozone decrease occurs after five days and ozone depletion lasts for 40 h for L_mix = 200 m. For about β ≥ 20, the time required for major ozone depletion ([O₃] < 4 ppb) to occur becomes independent of the height of the boundary layer, and for β = 100 it approaches two days, 28 h of which are attributable to the induction and 20 h to the depletion time.

In polar regions, a small amount of NO_x may exist, which stems from nitrate contained in the snow, and may have a strong impact on the ozone depletion. Therefore, the role of nitrogen-containing species on the ozone depletion rate is studied. The results show that the NO_x concentrations are influenced by different chemical reactions over different time periods. During ozone depletion, the reaction cycle involving the BrONO₂ hydrolysis is dominant. A critical value of 0.0004 of the uptake coefficient of the BrONO₂ hydrolysis reaction at the aerosol and saline surfaces is identified, beyond which the existence of NO_x species accelerates the ozone depletion event, whereas for lower values, deceleration occurs.