Machine-Learning-Based Estimation of Marine CHBr₃ Emissions around Asia and the Implication on Bromine Radicals and Ozone Depletion

Ocean-produced, very short-lived bromine substances, such as bromoform (CHBr3), can significantly affect the tropospheric and stratospheric bromine loading and ozone budget. However, large uncertainties exist for marine CHBr3 emission estimations, especially around Asia. In this study, we establish a new marine emission inventory of CHBr3, considering the physical and biogeochemical forces in the interaction of the ocean and atmosphere. The surface seawater concentrations are predicted based on expanded in situ CHBr3 measurements and the machine-learning approach. The marine CHBr3 emission is estimated to be 55.9 Gg Br yr–1 around Asia, accounting for 20.4% of the global total. Our estimate is 18.7 Gg Br yr–1 higher than the previously widely used bottom-up emission inventory, primarily due to discrepancies in the Western Pacific and the South China Sea. Higher estimated marine CHBr3 emissions significantly affect the stratospheric bromine radicals and ozone over Asia, leading to a maximum increase of 13.6% in Br and 12.2% in BrO concentrations and thus up to a 2.5 ppbv reduction in stratospheric ozone. This study highlights the need for additional marine CHBr3 observations in data-sparse regions to better quantify marine CHBr3 emissions and assess their potential atmospheric impacts. This new approach also provides a valuable model framework to calculate sea–air fluxes for other compounds of interest.