Impact of the marine atmospheric boundary layer conditions on VSLS abundances in the eastern tropical and subtropical North Atlantic Ocean

摘要

During the DRIVE~(Diurnal and Regional Variability of Halogen Emissions)ship campaign we investigated the variability of the halogenated veryshort-lived substances (VSLS) bromoform (CHBr₃), dibromomethane(CH₂Br₂) and methyl iodide (CH₃I) in the marine atmosphericboundary layer in the eastern tropical and subtropical North Atlantic Oceanduring May/June 2010. The highest VSLS mixing ratios were found near theMauritanian coast and close to Lisbon (Portugal). With backward trajectorieswe identified predominantly air masses from the open North Atlantic withsome coastal influence in the Mauritanian upwelling area, due to theprevailing NW winds. The maximum VSLS mixing ratios above the Mauritanianupwelling were 8.92 ppt for bromoform, 3.14 ppt for dibromomethane and 3.29 ppt for methyl iodide, with an observed maximum range of the daily mean upto 50% for bromoform, 26% for dibromomethane and 56% for methyliodide. The influence of various meteorological parameters – such as wind,surface air pressure, surface air and surface water temperature, humidityand marine atmospheric boundary layer (MABL) height – on VSLS concentrationsand fluxes was investigated. The strongest relationship was found betweenthe MABL height and bromoform, dibromomethane and methyl iodide abundances.Lowest MABL heights above the Mauritanian upwelling area coincide withhighest VSLS mixing ratios and vice versa above the open ocean. Significanthigh anti-correlations confirm this relationship for the whole cruise. Weconclude that especially above oceanic upwelling systems, in addition tosea–air fluxes, MABL height variations can influence atmospheric VSLS mixingratios, occasionally leading to elevated atmospheric abundances. This may addto the postulated missing VSLS sources in the Mauritanian upwelling region(Quack et al., 2007).