Clumped isotope effects during OH and Cl oxidation of methane

Abstract

23A series of experiments were carried out to determine the clumped ( 13 CH 3 D) 24 methane kinetic isotope effects during oxidation of methane by OH and Cl radicals, the 25 major sink reactions for atmospheric methane. Experiments were performed in a 100 L 26 quartz photochemical reactor, in which OH was produced from the reaction of O( 1 D) 27 (from O 3 photolysis) with H 2 O, and Cl was from photolysis of Cl 2 . Samples were taken 28 from the reaction cell and analyzed for methane ( 12 CH 4 , 12 CH 3 D, 13 CH 4 , 13 CH 3 D) 29 isotopologue ratios using tunable infrared differential laser absorption spectroscopy. 30Measured kinetic isotope effects for singly substituted species were consistent with 31 previous experimental studies. For doubly substituted methane, 13 CH 3 D, the observed 32 kinetic isotope effects closely follow the product of the kinetic isotope effects for the 13 C 33 and deuterium substituted species (i.e., 13,2 KIE = 13 KIE × 2 KIE). The deviation from this 34 relationship is 0.3‰ ± 1.2‰ and 3.5‰ ± 0.7‰ for OH and Cl oxidation, respectively. 35This is consistent with model calculations performed using quantum chemistry and 36 transition state theory. The OH and Cl reactions enrich the residual methane in the 37clumped isotopologue in open system reactions. In a closed system, however, this 38 effect is overtaken by the large D/H isotope effect, which causes the residual methane 39 to become anti-clumped relative to the initial methane. Based on these results, we 40 demonstrate that oxidation of methane by OH, the predominant oxidant for tropospheric 41 methane, will only have a minor (~0.3 ‰) impact on the clumped isotope signature 42 (Δ 13 CH 3 D, measured as a deviation from a stochastic distribution of isotopes) of 43 tropospheric methane. This paper shows that Δ 13 CH 3 D will provide constraints on 44 3 methane source strengths, and predicts that Δ 12 CH 2 D 2 can provide information on 45 methane sink strengths. 46 4

The geochemistry of methane isotopologues

“…1.9). The results and equations can be generalized to other major methane sinks (including oxidation by OH and Cl in the atmosphere; Whitehill et al, 2017), and to other isotopologues (e.g., 12 CH 2 D 2 ).…”

“…5.1) that shows several insights into the origin and fate of the methane stable isotopologues in the environment. As of the time of writing, methane clumped isotoplogue data have appeared in at least a dozen articles Stolper et al, 2014a;Stolper et al, 2014b;Inagaki et al, 2015;Stolper et al, 2015;Wang et al, 2015;Douglas et al, 2016;Lopes et al, 2016;Wang et al, 2016;Young et al, 2016;Whitehill et al, 2017;Young et al, 2017). The ∆ 13 CH 3 D data were shown to be independent of and complementary to δ 13 C and δD.…”

“…1.9). The results and equations can be generalized to other major methane sinks (including oxidation by OH and Cl in the atmosphere; Whitehill et al, 2017), and to other isotopologues (e.g., 12 CH 2 D 2 ).Research on the behavior of methane isotopologues like 13 CH 3 D have a natural alignment to many of the questions that are important and possibly unanswered in assessment of petroleum systems, particularly in poorly understood basins ( Fig. 1.10).…”

“…10‰ range , more precise isotopologue-specific measurements of methane in experiments conducted at natural abundance will be necessary in order to constrain clumped isotopologue fractionations in atmospheric contexts. These experiments have been conducted, and the results are reported in a companion article (Whitehill et al, 2017); a summary of their results are shown in Table 4.1. In the present study, γ values for aerobic methane oxidation were determined (1.0004 ± 0.0006, 2σ, Table 4.2).…”

“…In particular, because methane oxidation demonstrably produces nonequilibrium clumped isotopologue signatures in 89 Chapter 4. Aerobic oxidation of methane 9.14 ± 0.14 1.0004 ± 0.0006Anaerobic oxidation of methane (AOM) Holler et al (2009) 6.4 to 8.5Nitrite-dependent anaerobic methane oxidation Rasigraf et al (2012) 7.8 ± 0.8 et al (2001) 58.5 ± 6.6 Joelsson et al (2015; 0.980 ± 0.038 Whitehill et al (2017) 41.3 ± 8.3 0.9997 ± 0.0012 et al (2000) 5.51 Saueressig et al (1995; both closed and open systems considered in this study (Figs. 4.4 and 4.6, respectively), the out-of-equilibrium clumped isotopologue signatures in samples from Upper and Lower Mystic Lakes (Massachusetts, USA), Swamp Y (Massachusetts, USA), and The Cedars (California, USA) are considered again here , as well as a sample from a pond at Caltech for which a related parameter, the ∆ 18 value, was found to be in disequilibrium (Stolper et al, 2015).…”