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

Bromoform (CHBr3) is the largest single source of organic bromine for the atmosphere and of importance for the reactive halogen budgets in troposphere and lower stratosphere. Its photochemical products may contribute 10-20% to inorganic bromine in the troposphere and >50% in the lower stratosphere. The sea-to-air flux, originating with macroalgae and plankton, is the main source for atmospheric bromoform. We address the relevance of bromoform to atmospheric chemistry, as well as oceanic source and sink processes. We have performed a reassessment of oceanic emissions, based on published aqueous and airborne concentration data, global climatological parameters, and incorporation of knowledge about bromoforms coastal and biogenic sources. The goal is partly to estimate the global source strength and partly to identify key regions that may contribute bromoform to the atmosphere and hence require further investigation. We have therefore calculated annual integrated fluxes of bromoform for different latitudes and oceanic biogeochemical provinces. We obtain a global flux in the order of 9-13 Gmol Br yr-1 (as CHBr3), which is still associated with significant uncertainties. Coastal, shelf and open ocean regions contribute 13-17%, 44-48% and 34-43% to our calculated total respectively. Anthropogenic sources of ~0.3 Gmol Br yr-1 (as CHBr3) are negligible globally. Our estimate of the global oceanic source is >3x higher than recent estimates based on the modelling of atmospheric sinks. Reasons for the discrepancy could lie with the limited regional and temporal data available and broad assumptions that underlie our flux calculations. Alternatively, atmospheric sink calculations, often calculated on the basis of background CHBr3 levels, may neglect the influence of strong but highly localised sources (e.g. from some coastal regions). We believe that our approach is useful for attempting a first approximation of the flux based on known factors affecting the oceanic bromoform distribution. In the future, as more data become available, alternative schemes should certainly be developed and tested. The sea-to- air flux is spatially and temporally variable with tropical, subtropical and shelf waters identified as potentially important source regions. The strongly variable and poorly characterised source, together with bromoform's 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 modelling and mechanistic studies of oceanic bromoform production is required to better constrain present and future Br delivery to the atmosphere.