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

Abstract

The stratospheric inorganic bromine (Br) burden arising from the degradation of brominated very short-lived organic substances (VSL) 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 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 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 BrONO 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 (O), nitrogen dioxide (NO), total inorganic chlorine (Cl), and the efficiency of heterogeneous reactions of bromine reservoirs (mostly BrONO and HBr) occurring on ice crystals.