| 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. |