New insight into the atmospheric chloromethane budget gained using stable carbon isotope ratios

Abstract. Atmospheric chloromethane (CH₃Cl) plays an important role in stratospheric ozone destruction, but many uncertainties still exist regarding strengths of both sources and sinks and the processes leading to formation of this naturally occurring gas. Recent work has identified a novel chemical origin for CH₃Cl, which can explain its production in a variety of terrestrial environments: the widespread structural component of plants, pectin, reacts readily with chloride ion to form CH₃Cl at both ambient and elevated temperatures (Hamilton et al., 2003). It has been proposed that this abiotic chloride methylation process in terrestrial environments could be responsible for formation of a large proportion of atmospheric CH₃Cl. However, more information is required to determine the global importance of this new source and its contribution to the atmospheric CH₃Cl budget.

A potentially powerful tool in studying the atmospheric CH₃Cl budget is the use of stable carbon isotope ratios. In an accompanying paper it is reported that the reaction of CH₃Cl with OH radical, the dominant sink for atmospheric CH₃Cl, is accompanied by an unexpectedly large fractionation factor (Gola et al., 2005). Another recently published study shows that CH₃Cl formed by the abiotic methylation process at ambient temperatures has a unique stable carbon isotope signature, extremely depleted in ¹³C, unequivocally distinguishing it from all other known sources (Keppler et al., 2004). Using these findings together with data existing in the literature, we here present three scenarios for an isotopic mass balance for atmospheric CH₃Cl. Our calculations provide strong support for the proposal that the largest source of atmospheric CH₃Cl (1800 to 2500 Gg yr^-1) is the abiotic methylation of chloride in terrestrial ecosytems, primarily located in tropical and subtropical areas where turnover of biomass is highest. Furthermore our calculations also indicate that the microbial soil sink for CH₃Cl is likely to be much larger (>1000 Gg yr^-1) than that previously assumed.