Reassessment of the global methyl bromide budget

The phaseout of methyl bromide and the subsequent atmospheric response constitutes a remarkable natural experiment that can be used to improve our understanding of the global budget and atmospheric lifetime of this ozone depleting substance. Since 1999, atmospheric levels of methyl bromide have steadily declined due to the reduction in anthropogenic emissions and the atmosphere is approaching a new quasi steady state that is closer to preindustrial levels (Saltzman et al., 2004). Yvon-Lewis et al. (2009) showed that the decline in atmospheric methyl bromide from 1999-2005 was likely influenced by a perturbation in biomass burning emissions due to the 1998 ENSO but was otherwise consistent with the prescribed phase-out. Hu et al. (2012) extended the analysis through 2010, and showed that the saturation state of the ocean had responded to the decline in atmospheric levels. Here we examine the changes in atmospheric methyl bromide that have occurred through 2018 using a multi-box atmospheric model coupled to a spatially resolved surface ocean model.

One of the puzzling aspects of the methyl bromide budget is an apparent imbalance between the known sources to the atmosphere and the estimated losses from the atmosphere. The question of whether this "missing source" is real or an artifact of underestimated uncertainties in other elements of the budget has never been resolved. Prior global methyl bromide models have utilized gas transfer parameterizations for methyl bromide that do not account for the fact that it is a moderately soluble gas. The oceans act as both a source and sink of atmospheric methyl bromide (Butler, 1994; Yvon-Lewis and Butler, 1996). Because the oceans are near equilibrium with respect to the atmosphere, small changes in atmospheric levels induced by changes in terrestrial emissions can exert a large effect on the net ocean/atmosphere flux. The phase-out of anthropogenic emissions resulted in the oceans changing from a large net sink during the 1990's to near zero in 2005. As the net ocean/atmosphere flux declined, model estimates of the atmospheric methyl bromide budget become less sensitive to uncertainty in the parameterization of air/sea gas exchange. In this study, we take advantage of this change in the saturation state of the oceans to reevaluate the sensitivity of the global methyl bromide budget to gas transfer parameterizations and examine the consequences for estimating the "unknown" source.