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Chloromethane (CH3Cl) is the most abundant natural chlorinated organic compound in the atmosphere playing an important role in catalyzing stratospheric ozone loss. Vegetation emits the largest amounts of CH3Cl to the atmosphere but its source strength is highly uncertain leading also to large uncertainties in the global budget of CH3Cl. Triple-element stable isotope analysis may help to reduce uncertainties because it provides additional process-level information compared to conventional quantification methods. In this study we performed experiments to obtain a first triple-elemental isotopic fingerprint (2H, 13C, 37Cl) of CH3Cl emitted by a relevant plant species (royal fern, Osmunda regalis). Isotopic values of all three elements showed considerable differences compared to isotopic values of industrially manufactured CH3Cl which bodes well for future applications to distinguish individual sources. Isotopic analysis of potential precursors (rain, methoxy groups) of CH3Cl in plants revealed no measurable change of hydrogen and chlorine isotopic ratios during formation which may provide a simpler route to estimate the isotopic composition of CH3Cl emissions. Plant degradation experiments of CH3Cl were carried out with club moss (Selaginella kraussiana) revealing significant isotopic fractionation for all three elements. The fractionation pattern characterized by epsilon and lambda is inconsistent with known biotic dechlorination reactions indicating a yet unreported biotic degradation mechanism for CH3Cl. Overall, this study provides first insights into the triple-elemental isotopic fingerprint of plant emissions and degradation. The results may represent important input data for future isotope-based models to improve global budget estimates of CH3Cl and to explore the yet unknown degradation pathways.

Here, we review the contributions of Professor Barbara Sherwood Lollar to Compound Specific Isotope Analysis (CSIA) in contaminant hydrogeology and environmental chemistry. We first discuss the seminal work by Professor Sherwood Lollar’s lab on establishing CSIA as a quantitative tool for contaminant (bio)transformation at contaminated groundwater sites. We describe the critical research by her lab in the development and validation of sample collection techniques, single- and multi-element CSIA, and isotope data evaluation for reliable interpretations. We highlight the contributions of Professor Sherwood Lollar’s lab towards the development of best practices for the successful application of CSIA by industry to demonstrate the occurrence of (bio)transformation, identify (bio)transformation mechanisms, quantify the extent and rate of degradation and differentiate among potential contaminant sources. We then explore Professor Sherwood Lollar’s applications of CSIA to hydrogeology of the deep subsurface and the fast widening of the field to new environments (e.g., sediments), contaminants (e.g., chlorofluorocarbons, pesticides), and systems (e.g., plant, enzymes).

Transfer of organic compounds between aqueous and gaseous phases may change the isotopic composition which complicates the isotopic characterization of sources and transformation mechanisms in environmental samples. Studies investigating kinetic phase transfer of compounds dissolved in water (volatilization) are scarce, even though it presents an environmentally very relevant phase transfer scenario. In the current study, the occurrence of kinetic isotope fractionation (H-2/H-1, C-13/C-12, Cl-37/Cl-35) was investigated for two volatile organic compounds (trichloroethene, TCE and trichloromethane, TCM) during volatilization from water and gas-phase dissolution in water. In addition, experiments were also carried out at equilibrium conditions. The results indicated that volatilization of trichloromethane and trichloroethene from water, in contrast to pure phase evaporation, only caused small (chlorine) or negligible (hydrogen, carbon) isotope fractionation whereas for dissolution in water significant carbon isotope effects were found. At equilibrium conditions, hydrogen and carbon isotopes showed significant differences between dissolved and gaseous phase whereas small to insignificant differences were measured for chlorine isotopes. The results confirm the hypothesis that isotope effects during volatilization of organics from water are caused by transport inhibition in the aqueous phase. The consideration of gas-phase diffusion and vapor pressure isotope effects (Craig-Gordon model) could not reproduce the measured isotopic data. Overall, this study provides an overview of the most common kinetic and equilibrium partitioning scenarios and reports associated isotope effects. As such it illustrates under which environmental conditions isotopic signatures of chlorinated volatile organics may change, or remain constant, during transfer between surface waters and air.

Methyl bromide (CH3Br) and methyl chloride (CH3Cl) significantly contribute to stratospheric ozone depletion. The atmospheric budgets of both compounds are unbalanced with known degradation processes outweighing known emissions. Stable isotope analysis may be capable to identify and quantify emissions and to achieve a balanced budget. Degradation processes do, however, cause isotope fractionation in methyl halides after emission and hence knowledge about these processes is a crucial prerequisite for any isotopic mass balance approach. In the current study, triple-element isotope analysis (2H, 13C, 37Cl/81Br) was applied to investigate the two main abiotic degradation processes of methyl halides (CH3X) in fresh and seawater: hydrolysis and halide exchange. For CH3Br, nucleophilic attack by both H2O and Cl– caused significant primary carbon and bromine isotope effects accompanied by a secondary inverse hydrogen isotope effect. For CH3Cl only nucleophilic substitution by H2O was observed at significant rates causing large primary carbon and chlorine isotope effects and a secondary inverse hydrogen isotope effect. Observed dual-element isotope ratios differed slightly from literature values for microbial degradation in water and hugely from radical reactions in the troposphere. This bodes well for successfully distinguishing and quantifying degradation processes in atmospheric methyl halides using triple-element isotope analysis.

Methyl chloride and methyl bromide (CH3Cl and CH3Br) are the largest natural sources of chlorine and bromine, respectively, to the stratosphere, where they contribute to ozone depletion. We report the anthropogenic production of CH3Cl and CH3Br during breadbaking, and suggest this production is an abiotic process involving the methyl ester functional groups in pectin and lignin structural polymers of plant cells. Wide variations in baking styles allow only rough estimates of this flux of methyl halides on a global basis. A simple model suggests that CH3Br emissions from breadbaking likely peaked circa 1990 at approximately 200 tonnes per year (about 0.3% of industrial production), prior to restrictions on the dough conditioner potassium bromate. In contrast, CH3Cl emissions from breadbaking may be of similar magnitude as acknowledged present-day CH3Cl industrial emissions. Because the mechanisms involve functional groups and compounds widely found in plant materials, this type of methyl halide production may occur in other cooking techniques as well.

The isotopic composition of ozone depleting methyl halides may provide valuable information on the sources and sinks of these compounds. However, so far mostly stable carbon isotope analysis of methyl chloride and methyl bromide has been at­tempted. One obstacle to progress with their other isotope systems has been the challenge to obtain sufficiently large amounts of methyl halides to meet the detection limits of existing isotope analytical methods. A compound-specific technique using gas chromatography multiple-collector inductively-coupled plasma mass spectrometry (GC-MC-ICPMS) has in this work been developed to measure 81Br/79Br isotope ratios in CH3Br. This ICPMS method is three orders of magnitude more sensitive compared to existing gas source mass spectrometry methods (dual inlet and continuous flow) for Br isotope determination and thus opens up possibilities for applications toward ambient atmospheric samples to investigate the sources and sinks of methyl halides.

Methyl bromide is the most important natural bromine contributor to stratospheric ozone depletion, yet there are still large uncertainties regarding quantification of its sources and sinks. The stable bromine isotope composition of CH(3)Br is potentially a powerful tool to apportion its sources and to study both its transport and its reactive fate. A novel compound-specific method to measure (81)Br/(79)Br isotope ratios in CH3Br using gas chromatography hyphenated with inductively coupled plasma multiple-collector mass spectrometry (GC/MCICPMS) was developed. Sample amounts of >40 ng could bemeasured with a precision of 0.1 parts per thousand (1 sigma, n=3). The method results are reproducible over the long term as shown with 36 analyses acquired over 3 months, yielding a standard deviation ( 1s) better than 0.4 parts per thousand. This new method demonstrates for the first time Br isotope ratio determination in gaseous brominated samples. It is three orders of magnitude more sensitive than previously existing isotope ratio mass spectrometry methods for Br isotope determination of other organobromines, thus allowing applications towards ambient atmospheric samples., authorCount :7