Articles | Volume 16, issue 12
Research article
21 Jun 2016
Research article |  | 21 Jun 2016

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

Steffen Fuhlbrügge, Birgit Quack, Susann Tegtmeier, Elliot Atlas, Helmke Hepach, Qiang Shi, Stefan Raimund, and Kirstin Krüger

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.

Short summary
This study presents a novel estimate for the contribution of oceanic VSLS emissions to the atmospheric boundary layer and free troposphere during the SHIVA-Sonne cruise in the South China and Sulu seas in 2011. While oceanic emissions of CHBr3 and CH3I showed a significant contribution to their atmospheric abundances, atmospheric CH2Br2 appeared to be largely advected. Convective activity in the region can furthermore lead to low VSLS boundary layer mixing ratios despite high oceanic emissions.
Final-revised paper