Isotopic Characterization (2H, 13C, 37Cl, 81Br) of the Abiotic Sinks of Methyl Bromide and Methyl Chloride in Water and Implications for Future Studies.

Abstract

Methyl bromide and methyl chloride are each the largest natural sources of halogens to the stratosphere and hence substantially contribute to stratospheric ozone loss. The atmospheric budgets of both compounds are, despite considerable 10 efforts, still unbalanced with known sinks outweighing known sources. Stable isotope analysis may be capable to provide additional process level information and source differentiation of methyl halides and it is particularly powerful if isotopes of multiple elements in these compounds are measured. In the current study triple-elemental isotope analysis ( 2 H, 13 C, 37 Cl/ 81 Br) was applied to investigate the two main abiotic degradation processes of methyl halides (CH3X) in water: hydrolysis and halide exchange. Both nucleophilic substitution reactions caused large carbon and significant bromine isotope 15 effects in CH3Br and small secondary inverse hydrogen isotope effects. Calculated loss rates indicated that exchange with chloride (Cl -) may be a major abiotic sink for CH3Br in oceans whereas hydrolysis may contribute to degradation in freshwater and soils. For CH3Cl only hydrolysis was observed with large carbon and chlorine isotope effects and a secondary inverse hydrogen isotope effect. Halide exchange could not be detected for CH3Cl at ambient temperatures and may not be a significant sink for this compound. This study demonstrates, to our knowledge, the first triple-elemental isotope analyses of 20 CH3Cl and CH3Br. The presented results have important implications for source apportionment of tropospheric CH3X if viewed in conjunction with results from previous studies.

Dual C–Br Isotope Fractionation Indicates Distinct Reductive Dehalogenation Mechanisms of 1,2-Dibromoethane in Dehalococcoides- and Dehalogenimonas-Containing Cultures

“…Like 37 Cl (natural abundance of 24.22%), the heavy bromine stable isotope ( 81 Br) also has a high abundance (40.3%). 31 A previous study 32 showed that the Rayleigh equation can also be applied to calculate the isotopic fractionation of chlorine despite the higher natural abundance of 37 Cl compared to 13 C, and this equation was recently used in Br isotope fractionation studies with different compounds (e.g., 1,2-DBA, 11 methyl bromide, 33 and brominated ethenes 34 ). Calculation of C and Br apparent kinetic isotope effects (AKIEs) from estimated ε bulk values is presented in the Supporting Information, and their uncertainty was calculated by error propagation.…”