The Global Cycle of Atmospheric Methy Bromide -- What Next?

Atmospheric methyl bromide (CH₃Br) has received considerable attention and produced some controversy over the past decade, owing to its significant role in ozone depletion and its complex cycling in nature. Unlike other gases regulated by the Montreal Protocol on Substances that Deplete Stratospheric Ozone, methyl bromide has substantial natural sources and sinks at Earth's surface, in addition to its losses in the atmosphere. Consequently, estimating the sources and sinks and calculating the lifetime of this gas have not been as straightforward as for gases, such as the CFC's, of entirely anthropogenic origin and with losses only in the atmosphere. For almost ten years our understanding of the budget of this gas was one where sinks vastly outweighed sources and the lifetime kept getting shorter with the discovery of new sinks. Complicating the picture has been the absence of an atmospheric record in the early 1990s, just as industrial production of the gas was held to constant levels. Research necessarily focused on identifying and quantifying the sources and sinks of this gas in marine and terrestrial systems in an effort to understand the budget and to calculate the atmospheric lifetime of CH₃Br. Recently published records documenting the decline of CH₃Br in the atmosphere following reductions in production shed new light on this gas, allowing us to revise budgets and lifetimes with a new set of constraints. The latest data also open up additional questions regarding lifetime calculations and biogenic emissions and suggest new opportunities for research. Montzka, S. A., J. H. Butler, B. D. Hall, D. J. Mondeel, and J. W. Elkins, A decline in tropospheric organic bromine, Geophys. Res. Lett., 30(15), 1826, doi:10.1029/2003GL017745, 2003.