Cloud-scale modelling of the impact of deep convection on the fate of oceanic bromoform in the troposphere: a case study over the west coast of Borneo

Abstract. Coastal oceans emit bromoform (CHBr₃) that can be transported rapidly to the upper troposphere by deep convection. In the troposphere, the spatial and vertical distribution of CHBr₃ and its product gases (PGs) depend on emissions, chemical processing, transport by large scale flow, convection, and associated washout. This paper presents a modelling study on the fate of CHBr₃ and its PGs in the troposphere. A case study at cloud scale was conducted along the west coast of Borneo, when several deep convective systems triggered in the afternoon and early evening of November 19^th 2011. These systems were sampled by the Falcon aircraft during the field campaign of the SHIVA project. We analyse these systems using a simulation with the cloud-resolving meteorological model C-CATT-BRAMS at 2 × 2 km resolution that describes transport, photochemistry, and washout of CHBr₃. We find that simulated CHBr₃ mixing ratios and the observed values in the boundary layer and the outflow of the convective systems agree. However, the model underestimates the background CHBr₃ mixing ratios in the upper troposphere, which suggests a missing source. An analysis of the simulated chemical speciation of bromine within and around each simulated convective system during the mature convective stage reveals that > 85 % of the bromine derived from CHBr₃ and its PGs is transported vertically to the point of convective detrainment in the form of CHBr₃ and that the remaining small fraction is in the form of organic PGs, principally insoluble brominated carbonyls produced from the photo-oxidation of CHBr₃. The model simulates that within the boundary layer and free troposphere, the inorganic PGs are only present in soluble forms, i.e., HBr, HOBr, and BrONO₂, and consequently, within the convective clouds, the inorganic PGs are almost entirely removed by wet scavenging. For the conditions of the simulated case study Br₂ plays no significant role in the vertical transport of bromine. This likely results from the small simulated quantities of inorganic bromine involved, the presence of HBr in large excess compared to HOBr and the less soluble BrO, and the relatively quick removal of soluble compounds within the convective column. This prevalence of HBr is a result of the wider simulated regional atmospheric composition whereby background tropospheric ozone levels are exceptionally low.