The contribution of oceanic halocarbons to marine and free tropospheric air over the tropical West Pacific

Abstract. Emissions of halogenated very-short-lived substances (VSLSs) from the oceans contribute to the atmospheric halogen budget and affect tropospheric and stratospheric ozone. Here, we investigate the contribution of natural oceanic VSLS emissions to the marine atmospheric boundary layer (MABL) and their transport into the free troposphere (FT) over the tropical West Pacific. The study concentrates on bromoform, dibromomethane and methyl iodide measured on ship and aircraft during the SHIVA (Stratospheric Ozone: Halogen Impacts in a Varying Atmosphere) campaign in the South China and Sulu seas in November 2011. Elevated oceanic concentrations for bromoform, dibromomethane and methyl iodide of on average 19.9, 5.0 and 3.8 pmol L⁻¹, in particular close to Singapore and to the coast of Borneo, with high corresponding oceanic emissions of 1486, 405 and 433 pmol m⁻² h⁻¹ respectively, characterise this tropical region as a strong source of these compounds. Atmospheric mixing ratios in the MABL were unexpectedly relatively low with 2.08, 1.17 and 0.39 ppt for bromoform, dibromomethane and methyl iodide. We use meteorological and chemical ship and aircraft observations, FLEXPART trajectory calculations and source-loss estimates to identify the oceanic VSLS contribution to the MABL and to the FT. Our results show that the well-ventilated MABL and intense convection led to the low atmospheric mixing ratios in the MABL despite the high oceanic emissions. Up to 45 % of the accumulated bromoform in the FT above the region originates from the local South China Sea area, while dibromomethane is largely advected from distant source regions and the local ocean only contributes 20 %. The accumulated methyl iodide in the FT is higher than can be explained with local contributions. Possible reasons, uncertainties and consequences of our observations and model estimates are discussed.