Abstract
Abstract. Chloromethane (CH3Cl) is an important provider of chlorine to the stratosphere but detailed knowledge of its budget is missing. Stable isotope analysis is a potentially powerful tool to constrain CH3Cl flux estimates. The largest degree of isotope fractionation is expected to occur for deuterium in CH3Cl in the hydrogen abstraction reactions with its main sink reactant tropospheric OH and its minor sink reactant Cl atoms. We determined the isotope fractionation by stable hydrogen isotope analysis of the fraction of CH3Cl remaining after reaction with hydroxyl and chlorine radicals in a 3.5 m3 Teflon smog chamber at 293 ± 1 K. We measured the stable hydrogen isotope values of the unreacted CH3Cl using compound-specific thermal conversion isotope ratio mass spectrometry. The isotope fractionations of CH3Cl for the reactions with hydroxyl and chlorine radicals were found to be -264±45 and -280±11 ‰, respectively. For comparison, we performed similar experiments using methane (CH4) as the target compound with OH and obtained a fractionation constant of -205±6 ‰ which is in good agreement with values previously reported. The observed large kinetic isotope effects are helpful when employing isotopic analyses of CH3Cl in the atmosphere to improve our knowledge of its atmospheric budget.
Highlights
Chloromethane is the most abundant chlorine-containing trace gas in the Earth’s atmosphere, currently with a global mean mixing ratio of ∼ 540 ± 5 parts per trillion by volume and an atmospheric lifetime of 1.0–1.2 years (Carpenter et al, 2014)
Whilst we do not know the reasons for the discrepancies in the experimental ε values observed here and those reported by Sellevåg et al (2006), we suggest that they may be due to different measurement techniques employed in each of the studies
The ε value found in this study (−205 ± 6 ‰) was in good agreement with previous experimentally reported values conducted at similar temperatures
Summary
Chloromethane (often called methyl chloride) is the most abundant chlorine-containing trace gas in the Earth’s atmosphere, currently with a global mean mixing ratio of ∼ 540 ± 5 parts per trillion by volume (pptv) and an atmospheric lifetime of 1.0–1.2 years (Carpenter et al, 2014). The global emissions of CH3Cl have been estimated to be in the range of 4 to 5 Tg yr−1 (1 Tg = 1012 g) stemming from predominantly natural and anthropogenic sources (Montzka and Fraser, 2003; WMO, 2011; Carpenter et al, 2014). Current estimates of the CH3Cl global budget and the apportionment between sources and sinks are still highly uncertain. Known natural sources of CH3Cl include tropical plants (Yokouchi et al, 2002, 2007; Umezawa et al, 2015), wood-rotting fungi (Harper, 1985), oceans (Moore et al, 1996; Kolusu et al, 2017), plants of salt marshes (Rhew et al, 2000, 2003), aerated and flooded soil (Redeker et al, 2000; Keppler et al, 2000), senescent leaves and leaf litter (Hamilton et al, 2003; Derendorp et al, 2011) and wildfires. It has been reported that emissions from industrial sources, in China, might be much higher than previously assumed (Li et al, 2017)
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