Modeling the inorganic bromine partitioning in the tropical tropopause layer over the eastern and western Pacific Ocean

Abstract. The stratospheric inorganic bromine (Br_y) burden arising from the degradation of brominated very short-lived organic substances (VSL_org) and its partitioning between reactive and reservoir species is needed for a comprehensive assessment of the ozone depletion potential of brominated trace gases. Here we present modeled inorganic bromine abundances over the Pacific tropical tropopause based on aircraft observations of VSL_org from two campaigns of the Airborne Tropical TRopopause EXperiment (ATTREX 2013, carried out over the eastern Pacific, and ATTREX 2014, carried out over the western Pacific) and chemistry-climate simulations (along ATTREX flight tracks) using the specific meteorology prevailing. Using the Community Atmosphere Model with Chemistry (CAM-Chem) we model that BrO and Br are the daytime dominant species. Integrated across all ATTREX flights, BrO represents ∼ 43 and 48 % of daytime Br_y abundance at 17 km over the western and eastern Pacific, respectively. The results also show zones where Br / BrO > 1 depending on the solar zenith angle (SZA), ozone concentration, and temperature. On the other hand, BrCl and BrONO₂ were found to be the dominant nighttime species with ∼  61 and 56 % of abundance at 17 km over the western and eastern Pacific, respectively. The western-to-eastern differences in the partitioning of inorganic bromine are explained by different abundances of ozone (O₃), nitrogen dioxide (NO₂), total inorganic chlorine (Cl_y), and the efficiency of heterogeneous reactions of bromine reservoirs (mostly BrONO₂ and HBr) occurring on ice crystals.