Air-sea fluxes of biogenic bromine from the tropical and North Atlantic Ocean

Abstract

Abstract. Air-sea fluxes and bulk seawater and atmospheric concentrations of bromoform (CHBr 3 ) and dibromomethane (CH 2 Br 2 ) were measured during two research cruises in the northeast Atlantic (53-59 • N, June-July 2006) and tropical eastern Atlantic Ocean including over the African coastal upwelling system (16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) • N May-June 2007). Saturations and sea-air fluxes of these compounds generally decreased in the order coastal > upwelling > shelf > open ocean, and outside of coastal regions, a broad trend of elevated surface seawater concentrations with high chlorophyll-a was observed. We show that upwelling regions (coastal and equatorial) represent regional hot spots of bromocarbons, but are probably not of major significance globally, contributing at most a few percent of the total global emissions of CHBr 3 and CH 2 Br 2 . From limited data from eastern Atlantic coastlines, we tentatively suggest that globally, coastal oceans (depth <180 m) together contribute ∼2.5 (1.4-3.5) Gmol Br yr −1 of CHBr 3 , excluding influences from anthropogenic sources such as coastal power stations. This flux estimate is close to current estimates of the total open ocean source. We also show that the concentration ratio of CH 2 Br 2 /CHBr 3 in seawater is a strong function of concentration (and location), with a lower CH 2 Br 2 /CHBr 3 ratio found in coastal regions near to macroalgal sources.

Estimates of tropical bromoform emissions using an inversion method

“…While do not identify a specific local cause of these high mixing ratios, previous studies have typically found such levels of CHBr 3 when seaweed are nearby (e.g. Carpenter and Liss, 2000;Pyle et al, 2011). Aside from these periods, mixing ratios at Tawau are usually within the range 1-2 ppt.…”

“…To do this we briefly discuss two variables, ocean depth and surface chlorophyll a concentration, that have been related to CHBr 3 emissions in previous studies (e.g. Carpenter et al, 2009;Palmer and Reason, 2009;Ordóñez et al, 2012). Figure 9 shows PDFs for surface elevation (negative values give ocean depth) and chlorophyll a concentration for the whole of the tropics and for the "fine" grid cells.…”

“…In the "bottom-up" method local flux calculations are assumed to be representative of, and are extrapolated over, particular oceanic domains (e.g. Quack and Wallace, 2003;Carpenter and Liss, 2000;Butler et al, 2007;Ziska et al, 2013). In the "top-down" method an idealised pattern of emissions is typically assumed based on available observational evidence and the emission magnitude is then varied to match observations made in the "background" atmosphere (e.g.…”

Isotopic composition of polyhalomethanes from marine macrophytes – systematic effects of the halogen substituents on isotopic composition

“…However, the current emissions estimates for these climaterelevant compounds remain fairly uncertain owing to the large spatial and temporal variability in observed halocarbon mixing ratios and fluxes. This is, in particular, true for short-lived halocarbons such as iodomethane (CH 3 I) and bromoform (tribromomethane, CHBr 3 ), [8][9][10][11][12][13][14] with the largest uncertainties in the known sources being observed for coastal and near-shore emissions. [15] Despite the uncertainties in global halocarbon emission budgets, it is well accepted that in the marine realm, diverse autotrophic organisms contribute to halocarbon production.…”

“…A better understanding of the biogeochemical controls of the production-destruction equilibrium may substantially improve current emission estimates [10,12] and is of vital importance to address potential future changes. Carbon isotope signatures of individual halocarbons have been proposed as a valuable tool to distinguish between different sources, to obtain information on source and sink mechanisms and to refine the global budgets of CH 3 Br [35] and CH 3 Cl.…”