The chemistry influencing ODEs in the Polar Boundary Layer in spring: a model study

Abstract. Near-total depletions of ozone have been observed in the Arctic spring since the mid 1980s. The autocatalytic cycles involving reactive halogens are now recognized to be of main importance for Ozone Depletion Events (ODEs) in the Polar Boundary Layer (PBL). We present sensitivity studies using the model MISTRA in the box-model mode on the influence of chemical species on these ozone depletion processes. In order to test the sensitivity of the chemistry under polar conditions, we compared base runs undergoing fluxes of either Br₂, BrCl, or Cl₂ to induce ozone depletions, with similar runs including a modification of the chemical conditions. The role of HCHO, H₂O₂, DMS, Cl₂, C₂H₄, C₂H₆, HONO, NO₂, and RONO₂ was investigated. Cases with elevated mixing ratios of HCHO, H₂O₂, DMS, Cl₂, and HONO induced a shift in bromine speciation from Br/BrO to HOBr/HBr, while high mixing ratios of C₂H₆ induced a shift from HOBr/HBr to Br/BrO. Cases with elevated mixing ratios of HONO, NO₂, and RONO₂ induced a shift to BrNO₂/BrONO₂. The shifts from Br/BrO to HOBr/HBr accelerated the aerosol debromination, but also increased the total amount of deposited bromine at the surface (mainly via increased deposition of HOBr). These shifts to HOBr/HBr also hindered the BrO self-reaction. In these cases, the ozone depletion was slowed down, where increases in H₂O₂ and HONO had the greatest effect. The tests with increased mixing ratios of C₂H₄ highlighted the decrease in HO_x which reduced the production of HOBr from bromine radicals. In addition, the direct reaction of C₂H₄ with bromine atoms led to less available reactive bromine. The aerosol debromination was therefore strongly reduced. Ozone levels were highly affected by the chemistry of C₂H₄. Cl₂-induced ozone depletions were found unrealistic compared to field measurements due to the rapid production of CH₃O₂, HO_x, and ROOH which rapidly convert reactive chlorine to HCl in a "chlorine counter-cycle". This counter-cycle efficiently reduces the concentration of reactive halogens in the boundary layer. Depending on the relative bromine and chlorine mixing ratios, the production of CH₃O₂, HO_x, and ROOH from the counter-cycle can significantly affect the bromine chemistry. Therefore, the presence of both bromine and chlorine in the air may unexpectedly lead to a slow down in ozone destruction. For all NO_y species studied (HONO, NO₂, RONO₂) the chemistry is characterized by an increased bromine deposition on snow reducing the amount of reactive bromine in the air. Ozone is less depleted under conditions of high mixing ratios of NO_x. The production of HNO₃ led to the acid displacement of HCl, and the release of chlorine out of salt aerosols (Cl₂ or BrCl) increased.