Biogenic Production of Reactive Bromocarbons: New Field Data and sea-air Fluxes in the Atlantic Ocean

Biogenic bromine production by phytoplankton and macroalgae is thought to represent an important link between ocean biology, climate and atmospheric composition. Models of atmospheric bromine chemistry suggest that natural sources of bromocarbons such as CHBr3 and CH2Br2 may account for up to 30% of stratospheric and tropospheric O3 depletion. However, at present these models are limited by the accuracy to which the bromine source terms can be described. In particular, simultaneous measurements of ocean surface water and marine boundary layer bromocarbon concentrations are lacking, limiting the ability to estimate sea to air fluxes to a reasonable degree of accuracy. Furthermore, little is known regarding the factors that control biogenic bromine production, or the temporal and spatial variability of the bromine source term at the regional scale. We present new data from two research cruises during which we measured a range of bromocarbons, including CHBr3, CH2Br2 and CH2IBr, in both surface seawater and the marine boundary layer using two GC-MS systems. The first cruise was to the North Eastern Atlantic (latitudinal range 53-59°N) in summer 2006, while the second cruise was to the Tropical and Subtropical Atlantic and the Mauritanian Upwelling (latitudinal range 16-30°N) in spring 2007. Concentration data and resulting sea air fluxes generally decrease in the order coastal > shelf > upwelling ~ open ocean. Although a broad trend of elevated seawater concentrations in waters with high chlorophyll a (phytoplankton productivity proxy) is observed, the relationship is not simple. We explore this complex relationship between phytoplankton and bromocarbon production in more detail, examining changes in phytoplankton assemblage and health as indicated by cell counts and pigment distributions. We then use these relationships to present a revised regional estimate for the North Atlantic sea to air flux of biogenic bromine.