Air‐sea flux of bromoform: Controls, rates, and implications

Abstract

[1] Bromoform (CHBr 3 ) is the largest single source of atmospheric organic bromine and therefore of importance as a source of reactive halogens to the troposphere and lower stratosphere. The sea-to-air flux, originating with macroalgal and planktonic sources, is the main source for atmospheric bromoform. We review bromoform's contribution to atmospheric chemistry, its atmospheric and oceanic distributions and its oceanic sources and sinks. We have reassessed oceanic emissions, based on published aqueous and airborne concentration data, global climatological parameters, and information concerning coastal and biogenic sources. The goals are to attempt an estimate of the global source strength and partly to identify key regions that require further investigation. The sea-to-air flux is spatially and temporally variable with tropical, subtropical and shelf waters identified as potentially important source regions. We obtain an annual global flux of bromoform of $10 Gmol Br yr À1 (3-22 Gmol Br yr À1 ). This estimate is associated with significant uncertainty, arising from data precision and coverage, choice of air-sea exchange parameterizations and model assumptions. Anthropogenic sources of $0.3 (to 1.1) Gmol Br yr À1 (as CHBr 3 ) can be locally significant, but are globally negligible. Our estimate of the global oceanic source is three to four times higher than recent estimates based on the modeling of atmospheric sinks. The reasons for this discrepancy could lie with the limited regional and temporal data available and the broad assumptions that underlie our flux calculations. Alternatively, atmospheric sink calculations, often made on the basis of background CHBr 3 levels, may neglect the influence of strong but highly localized sources (e.g., from some coastal and shelf regions). The strongly variable and poorly characterized source of CHBr 3 , together with its short atmospheric lifetime, complicates model-based estimation of the distribution of reactive Br resulting from its atmospheric degradation. An integrated program of marine and atmospheric observations, atmospheric modeling and mechanistic studies of oceanic bromoform production is required to better constrain present and future Br delivery to the atmosphere.

A tropical West Pacific OH minimum and implications for stratospheric composition

“…In particular, bromine species are mainly produced by natural biological activity in the oceans (Quack and Wallace, 2003;WMO, 2011). Sources include macroalgae (kelp, seaweed) and phytoplankton (Quack and Wallace, 2003).…”

“…In particular, bromine species are mainly produced by natural biological activity in the oceans (Quack and Wallace, 2003;WMO, 2011). Sources include macroalgae (kelp, seaweed) and phytoplankton (Quack and Wallace, 2003). Increasing seaweed farming in Southeast Asia, which has been proposed for carbon dioxide sequestration, could contribute to increasing emissions in the future (MartinezAviles et al, 2010).…”

How sensitive is the recovery of stratospheric ozone to changes in concentrations of very short-lived bromocarbons?

“…In the pre-industrial era, the VSLS contribution may have been as much as 50 % of the total atmospheric bromine loading. VSLS bromocarbons are mainly produced by macro-and micro-algae (Carpenter and Liss, 2000;Quack and Wallace, 2003;Yokouchi et al, 2005). CHBr 3 and CH 2 Br 2 are the dominant contributors to bromine VSLS species.…”

“…CHBr 3 and CH 2 Br 2 are the dominant contributors to bromine VSLS species. Although their tropospheric lifetime is relatively short (a few weeks for CHBr 3 and several months for CH 2 Br 2 ), they can be lifted effectively through deep convective systems and transported into the upper troposphere and/or lower stratosphere (UTLS) to make a significant contribution to the total ambient bromine (Sturges et al, 2000;Yang et al, 2005;Salawitch, 2006). Being short lived, there are large uncertainties in using atmospheric concentration measurements to estimate their global fluxes and their net contribution to the stratospheric bromine (Warwick et al, 2006;Liang et al, 2010;Pyle et al, 2011;Ordóñez et al, 2012;Ziska et al, 2013;Hossaini et al, 2013).…”

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.…”