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

[摘要] Atmospheric chloromethane (CH₃Cl) plays an important role instratospheric ozone destruction, but many uncertainties still existregarding strengths of both sources and sinks and the processes leading toformation of this naturally occurring gas. Recent work has identified anovel chemical origin for CH₃Cl, which can explain its production in avariety of terrestrial environments: the widespread structural component ofplants, pectin, reacts readily with chloride ion to form CH₃Cl at bothambient and elevated temperatures (Hamilton et al., 2003). It has beenproposed that this abiotic chloride methylation process in terrestrialenvironments could be responsible for formation of a large proportion ofatmospheric CH₃Cl. However, more information is required to determinethe global importance of this new source and its contribution to theatmospheric CH₃Cl budget.A potentially powerful tool in studying the atmospheric CH₃Cl budget isthe use of stable carbon isotope ratios. In an accompanying paper it isreported that the reaction of CH₃Cl with OH radical, the dominant sinkfor atmospheric CH₃Cl, is accompanied by an unexpectedly largefractionation factor (Gola et al., 2005). Another recently published studyshows that CH₃Cl formed by the abiotic methylation process at ambienttemperatures has a unique stable carbon isotope signature, extremelydepleted in ¹³C, unequivocally distinguishing it from all other knownsources (Keppler et al., 2004). Using these findings together with dataexisting in the literature, we here present three scenarios for an isotopicmass balance for atmospheric CH₃Cl. Our calculations provide strongsupport for the proposal that the largest source of atmospheric CH₃Cl(1800 to 2500 Gg yr^-1) is the abiotic methylation of chloride interrestrial ecosytems, primarily located in tropical and subtropical areaswhere turnover of biomass is highest. Furthermore our calculations alsoindicate that the microbial soil sink for CH₃Cl is likely to be muchlarger (>1000 Gg yr^-1) than that previously assumed.

[学科分类] 大气科学