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

Abstract

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, FLEX-PART 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.

Delivery of halogenated very short-lived substances from the west Indian Ocean to the stratosphere during the Asian summer monsoon

“…The Lagrangian model FLEXPART uses a convection scheme, described and evaluated by Forster et al (2007), to account for vertical transport. Using FLEXPART trajectories with ERA-Interim reanalysis, Fuhlbrügge et al (2016b) were able to simulate VSLS mixing ratios from the surface to the free troposphere up to 11 km above the tropical west Pacific in very good agreement with corresponding aircraft measurements applying a simple source-loss approach. Tegtmeier et al (2013) showed that the FLEXPART distribution of oceanic CH 3 I in the tropics agrees well with adjacent upper tropospheric and lower stratospheric aircraft measurements, thus increasing our confidence in the FLEXPART convection scheme and ERA-Interim velocities.…”

“…The average CHBr 3 emission during the OASIS campaign (910 pmol m −2 h −1 ) was larger than during most campaigns in tropical regions: 1.5 times larger than during TransBrom in the subtropical and tropical west Pacific (Tegtmeier et al, 2012), 1.2 times larger than during DRIVE in the tropical northeast Atlantic (Hepach et al, 2014), and 1.5 times larger than during MSM18/3 in the Atlantic equatorial upwelling (Hepach et al, 2015). Only the SHIVA campaign in the South China and Sulu seas yielded larger CHBr 3 emissions of 1486 pmol m −2 h −1 because of very high oceanic concentrations close to the coast (Fuhlbrügge et al, 2016b). The global open ocean estimate by Quack and Wallace (2003) Average CH 2 Br 2 emissions from the OASIS cruise (930 pmol m −2 h −1 ) are 2-6 times larger than the average cruise emissions listed in Table 3: TransBrom, DRIVE, MSM18/3, SHIVA, and M91.…”

“…During the SHIVA campaign in the South China Sea, high oceanic concentrations of bromoform produced mean emission rates that were higher than during OASIS. The SHIVA calculations show even higher transport efficiencies of 7.9 %, which lead to an entrainment of 48.4 nmol CHBr 3 (Table 5), because of the strong convective activity in that region during the time (Fuhlbrügge et al, 2016b). The MSM18/3 cruise in the equatorial Atlantic (Hepach et al, 2015) has the smallest emissions, entrainment, and a transport efficiency of 0.9 % (Table 5).…”

“…Despite its very short atmospheric lifetime, it can deliver iodine to the stratosphere in tropical regions (Solomon et al, 1994;Tegtmeier et al, 2013). Ship-based observations showed that bromocarbon emissions near coasts and in oceanic upwelling regions are generally higher than in the open ocean, because of macroalgal growth near coasts (Carpenter et al, 1999) and enhanced primary production in upwelling regions (Quack et al, 2007), while coastal anthropogenic sources also need to be considered (Quack and Wallace, 2003;Fuhlbrügge et al, 2016b). Measurements of VSLSs in the global oceans are sparse and the data show large variability.…”

Meteorological constraints on oceanic halocarbons above the Peruvian upwelling

“…Both could lead to either a shorter or longer residence time of the surface air masses within the MABL or below the trade inversion. However, Fuhlbrügge et al (2016) showed that differences in the MABL height of ERA-Interim and radiosonde observations affect the computed ODR only marginally. Further uncertainties may arise from spatial variations in the VSLS lifetimes and thus the chemical degradation of the compounds we used in this study.…”