Recent trends in atmospheric methyl bromide: analysis of post-Montreal Protocol variability

Abstract

Abstract. The atmospheric methyl bromide (CH 3 Br) burden has declined in recent years, in response to the phaseout of agricultural and structural fumigation consumption under the amendments to the Montreal Protocol. The timing and magnitude of this decrease represents an opportunity to examine our current understanding of atmospheric CH 3 Br and its budget, response to the phaseout, and response to interannual variability in biomass burning and global OH. In this study, simulations obtained from a time-dependent global model of atmospheric CH 3 Br emissions and uptake are compared to observations from the NOAA flask network. The model includes a detailed gridded ocean model coupled to a time-dependant atmospheric 2-box model. The phaseout of CH 3 Br production for agricultural uses began in 1998, concurrent with the pulse in biomass burning associated with the 1998 El Niño. The combined effects of three factors (biomass burning, global OH, and anthropogenic phaseout) appear to explain most of the observed atmospheric methyl bromide variability over the 1997-2008 period. The global budget remains imbalanced, with a large missing source indicated. The missing source does not exhibit a systematic decline during the phaseout period, and therefore, is not the result of significantly underestimating non-QPS agricultural CH 3 Br emissions. The model results suggest that the oceans should be less undersaturated than before the phaseout began.

Stable bromine isotopic composition of methyl bromide released from plant matter

“…Although most of these source and sink processes appear to be relatively well investigated, albeit facing upscaling challenges common to most bottom-up approaches, the global budget shows an imbalance of 32 Gg/a corresponding to ca. 25% of the known annual emissions (Yvon-Lewis et al, 2009). Hence, sources are either underestimated and/or sinks are overestimated.…”

“…For methyl chloride ~20-25% of the estimated total budget might come from biomass burning and a similar amount from senescent plants and plant litter (Keppler et al, 2005;Saito and Yokouchi, 2008). For methyl bromide approximately 10% of the atmospheric budget might originate from biomass burning (Yvon-Lewis et al, 2009). While vegetation-based emissions of CH3Br have the potential to reduce the estimated imbalance between sources and sinks in the atmospheric budget, lowtemperature formation from senescent and dead plant matter has not yet been included in the global budget because quantification is difficult.…”

A High-Volume Cryosampler and Sample Purification System for Bromine Isotope Studies of Methyl Bromide*

“…Although the concentration of the methyl halides (CH 3 X) in the atmosphere is now well known, numerous natural sources and sinks have been identified, some quite recently-for example, fungi (Harper 1985), macroalgae (Laturnus 2001;Laturnus et al 1998;Manley and Dastoor 1987), peatlands (Khan et al 2012;Varner et al 1999), rice paddies (Khan et al 2011;Redeker et al 2002), salt marshes (Rhew et al 2000;Rhew and Maz eas 2010), oceans (Hu et al 2010;King et al 2000;Lu et al 2010;Singh et al 1983), tropical rain forests (Saito et al 2008), tundra (Rhew et al 2007;Teh et al 2009), fires (Andreae et al 1996), cattle (Williams et al 1999), and ants (Mead et al 2008). Thus, many questions and uncertainties in the global budgets of these gases remain (e.g., Butler 2000;Montzka et al 2011;Simmonds et al 2004;Yvon-Lewis et al 2009). …”

“…Since it is a controlled compound under the Copenhagen Amendment to the Montreal Protocol (UNEP 2012), the human contribution to atmospheric CH 3 Br is presumably now decreasing (Montzka et al 2011;Yvon-Lewis et al 2009). With dominant losses occurring in the troposphere via reaction with the hydroxyl (OH) radical, both gases have relatively short atmospheric lifetimes-0.8 yr for CH 3 Br and 1.0 yr for CH 3 Cl.…”

“…The samples were collected as described in section 2 above, transferred into the 2.58-L stainless steel storage cylinders, and analyzed within 1 week. However, it is known that the ambient concentrations of methyl halides may vary by a factor of 2 or more on a short time scale because of transient pollution or local source events and on a longer time scale because of seasonal variability (Yvon-Lewis et al 2009). Thus, there may have been considerable variability during our multihour sampling periods.…”