Mass spectrometric measurement of hydrogen isotope fractionation for the reactions of chloromethane with OH and Cl

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 deu5 terium 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 10 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 15 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 iso20 tope effects are helpful when employing isotopic analyses of CH3Cl in the atmosphere to improve our knowledge of its atmospheric budget.


Introduction
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 1.0-1.2years (Carpenter et al., 2014).The global emissions of CH 3 Cl have been estimated to be in the range of 4 to 5 Tg yr −1 (1 Tg = 10 12 g) stemming from predominantly natural but also anthropogenic sources (Montzka and Fraser, 2003;WMO, 2011;Carpenter et al., 2014).However, current estimates of the CH 3 Cl global budget and the apportionment between sources and sinks are still highly uncertain.Known natural sources of CH 3 Cl include tropical plants (Yokouchi et al., 2002(Yokouchi et al., , 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(Rhew et al., , 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.Anthropogenic CH 3 Cl release to the atmosphere comes from the combustion of coal and biomass with minor emissions from cattle (Williams et al., 1999) and humans (Keppler et al., 2017).In addition, it has been reported that emissions from industrial sources, particularly in China, might be much higher than previously assumed (Li et al., 2017).
Published by Copernicus Publications on behalf of the European Geosciences Union.
F. Keppler et al.: Mass spectrometric measurement of CH 3 Cl hydrogen isotope fractionation The dominant sink for atmospheric CH 3 Cl results from the reaction with photochemically produced hydroxyl radicals (OH), currently estimated at about 2.8 Tg yr −1 (Carpenter et al., 2014).Furthermore, in the marine boundary layer the reaction of CH 3 Cl with chlorine radicals (Cl) represents another sink estimated to account for up to 0.4 Tg yr −1 (Khalil et al., 1999;Montzka and Fraser, 2003).Microbial CH 3 Cl degradation in soils may be a relevant additional global sink (McAnulla et al., 2001;Harper et al., 2003;Miller et al., 2004;Jaeger et al., 2018a), but its impact on the global CH 3 Cl budget is still highly uncertain.The microbial CH 3 Cl soil sink strength has been estimated to range from 0.1 to 1.6 Tg yr −1 (Harper et al., 2003;Keppler et al., 2005;Carpenter et al., 2014).Moreover, small proportions of tropospheric CH 3 Cl are lost to the stratosphere (146 Gg yr −1 , 1 Gg = 10 9 g) and to cold polar oceans (370 Gg yr −1 ) though oceans in total are a net source (Carpenter et al., 2014).Loss of tropospheric CH 3 Cl to the stratosphere is a result of turbulent mixing and the transport process itself is not thought to cause substantial isotope fractionation (Thompson et al., 2002).
A potentially powerful tool in the investigation of the budgets of atmospheric volatile organic compounds is the use of stable isotope ratios (Brenninkmeijer et al., 2003;Gensch et al., 2014).The general approach is that the atmospheric isotope ratio of a compound (e.g., CH 3 Cl) is considered to be equal the sum of isotopic fluxes from all sources corrected for kinetic isotopic fractionations that happen in sink processes: where δ 2 H atm and δ 2 H source i are the hydrogen isotope values of CH 3 Cl in the atmosphere and of the different sources i in per mil.i and j are the CH 3 Cl flux fraction for each source and sink.ε j is the isotope fractionation of each sink j in per mil.
The isotopic composition of atmospheric compounds might be altered by the kinetic isotope effects of physical, chemical or biological loss processes.The kinetic isotope effect (KIE) is usually defined as where k 1 and k 2 are the reaction rate constants for loss of the lighter and the heavier isotopologues, respectively.The KIE is typically expressed as isotope fractionation ε (also termed isotope enrichment constant) or isotope fractionation constant α.
First approaches of an isotope mass balance regarding stable carbon isotopes of CH 3 Cl have been provided by Keppler et al. (2005) and Saito and Yokouchi (2008).Several studies have investigated the stable carbon isotope source signature of CH 3 Cl produced via biotic and abiotic processes; however, for a more detailed overview we refer readers to the studies of Keppler et al. (2005) and Saito and Yokouchi (2008).Moreover, researchers have measured the KIE of stable carbon isotopes of CH 3 Cl during oxidation and during biodegradation by bacterial isolates (Miller et al., 2001;Nadalig et al., 2013;Nadalig et al., 2014), and in soils under laboratory conditions (Miller et al., 2004;Jaeger et al., 2018a).The first, and so far, only available analysis of the KIE for reaction of CH 3 Cl with OH has been reported by Gola et al. (2005) and this revealed unexpectedly large stable carbon isotope fractionation.The experiments were carried out in a smog chamber using long path Fourier transform infrared spectroscopy (FTIR) detection.However, we consider it important to confirm this result using another measurement technique such as stable isotope ratio mass spectrometry (IRMS).
So far most isotopic investigations of CH 3 Cl have focused on stable carbon isotope measurements, but stable hydrogen isotope measurements including both sources and sinks of CH 3 Cl have also recently become available (Greule et al., 2012;Nadalig et al., 2013Nadalig et al., , 2014;;Jaeger et al., 2018a, b).Moreover, relative rate experiments have been carried out for three isotopologues of CH 3 Cl and their reactions with Cl and OH.The OH and Cl reaction rates of CH 2 DCl were measured by long-path FTIR spectroscopy relative to CH 3 Cl at 298 ± 2 K and 1 atm (Sellevåg et al., 2006; Table 1).
In this manuscript, using a 3.5 m 3 Teflon smog chamber and IRMS measurements, we present results from kinetic studies of the hydrogen isotope fractionation in the atmospheric OH and Cl loss processes of CH 3 Cl.Furthermore, we also measured the isotope fractionation for the reaction between methane (CH 4 ) and OH using a similar experimental design and compared this value with those from previous studies.

Smog chamber experiments with chloromethane
The isotope fractionation experiments were performed in a 3.5 m 3 Teflon smog chamber (fluorinated ethylene propylene, FEP 200A, DuPont, Wilmington, DE, USA) with initial CH 3 Cl mixing ratio of 5 to 10 parts per million by volume (ppmv).Atomic chlorine was generated via photolysis of molecular chlorine (Cl 2 ; Rießner Gase, 0.971 % Cl 2 in N 2 ) by a solar simulator with an actinic flux comparable to the sun in mid-summer in Germany (Bleicher et al., 2014).Hydroxyl radicals were generated via the photolysis of ozone (O 3 ) at 253.7 nm in the presence of water vapor (RH = 70 %; produced by double-distilled water in a threeneck bottle humidifier) and/or H 2 .To obtain efficient OH formation, a Philips TUV lamp T8 (55 W) was coated with Teflon film (FEP 200) and mounted inside the smog chamber.O 3 was monitored by a chemiluminescence analyzer (UPK 8001).The chamber was continuously flushed with purified, hydrocarbon-free zero air (zero air generator, cmc Instruments, < 1 ppbv of O 3 , < 500 pptv NO x , < 100 ppbv of CH 4 ) at a rate of 4 L min −1 to maintain a slight overpressure of 0.5-1 Pa logged with a differential pressure sensor (Kalinsky Elektronik DS1).The quality of the air inside the chamber in terms of possible contamination was controlled by monitoring NO and NO x (EcoPhysics CLD 88p, coupled with a photolytic converter, EcoPhysics PLC 860).Perfluorohexane (PFH) with an initial mixing ratio of ∼ 25 parts per billion by volume (ppbv) was used as an internal standard to correct the resulting concentrations for dilution.The temperature was set to 20 ± 1 • C and monitored, together with the relative humidity, by a Teflon-cased sensor (Rotronic, HC2-IC102).
To guarantee constant mixing and small temperature gradients, a Teflon fan was mounted and operated inside the chamber.More detailed specification of the smog chamber can be found elsewhere (e.g., Wittmer et al., 2015).The mixing ratios of CH 3 Cl and PFH were quantified by a Hewlett Packard HP 6890 gas chromatograph coupled to a MSD 5973 mass spectrometer (GC-MS, Agilent Technologies, Palo Alto, CA) with a time resolution of 15 min throughout the experiments.Two CH 3 Cl reference gases from Linde (1006 ± 12 ppmv diluted in N 2 ) and Scott (1 ppmv) were used for calibration.The abundance of CH 3 Cl relative to PFH was used to calculate the remaining fraction of CH 3 Cl (Eq.4).The relative standard deviation of this procedure was determined prior to each experiment and also during the control experiment and ranged between 1.3 and 1.9 %.Aliquots (5 mL) were withdrawn from the chamber with a gas-tight syringe, injected into a stream of He (30 mL min − 1) and directed to a pre-concentration unit that was attached to the GC-MS.The pre-concentration unit consisted of a simple eight-port valve (VICI Valco) equipped with two cryotraps made of fused silica, which were immersed in liquid nitrogen for trapping the analytes.Prior to each sample measurement, a gaseous standard (5 mL of 100 ppmv CH 3 Cl in N 2 ) was measured.Figure 1 shows the design of the smog chamber used in our experiments.In the CH 3 Cl and OH experiments (1 to 3) 2000 ppmv H 2 was used to scavenge chlorine atoms originating from the photolysis or oxidation of formyl chloride (HCOCl), which forms as an intermediate in the reaction cascade.Under the experimental conditions typically more than 70 % of the CH 3 Cl was degraded within 6 to 10 h.From each experiment (CH 3 Cl + OH and CH 3 Cl + Cl) 10 to 15 canister samples (2 L stainless steel, evacuated < 10 −4 mbar) were collected at regular time intervals for subsequent stable hydrogen isotope measurements at Heidelberg University.An overview of the experimental details (Table S1 in the Supplement) and control measurements is provided in the Supplement.

Smog chamber degradation experiments with methane
The CH 4 degradation experiments were carried out under the same conditions as the CH 3 Cl degradation experiments but without PFH as an internal standard.Instead we used the flushing flow rate of zero air to account for the dilution during the experiment.The initial CH 4 mixing ratio was 6 ppmv.Throughout these experiments CH 4 and CO 2 mixing ratios were monitored with a Picarro G225i cavity ring-down spectrometer directly connected to the chamber.For more details see information provided in the Supplement.
2.3 Stable hydrogen isotope analysis using isotope ratio mass spectrometry

Chloromethane
Stable hydrogen isotope ratios of CH 3 Cl were measured by an in-house cryogenic pre-concentration unit coupled to a Hewlett Packard HP 6890 gas chromatograph (Agilent Technologies, Palo Alto, CA) and an isotope ratio mass spectrometer (IRMS; Isoprime, Manchester, UK) as described in detail by Greule et al. (2012) factor, determined daily during this investigation (two different measurement periods), was in the range of 5.75-6.16(first period) and 8.90-9.21(second period).The mean precision based on replicate measurements (n = 6) of the CH 3 Cl working standard was 2.1 and 3.8 ‰ for the first and second measurement periods, respectively.Samples were analyzed 3 times (n = 3), and the standard deviations (SD) of the measurements were in the range of 1.2 to 103.8 ‰.Lowest SD were observed for samples with lowest δ 2 H values (∼ −140 ‰) and highest mixing ratios and higher SD for samples with highest δ 2 H values (∼ +800 ‰) and lowest mixing ratios.
Please note that the above-described one-point calibration of the δ 2 H data might be affected by an additional error ("scale compression") and particularly might affect the uncertainties of the very positive δ 2 H values. Unfortunately CH 3 Cl working standards with distinct isotopic signatures spanning the full range of measured δ 2 H values (−150 to ∼ +800 ‰) are not currently available to eliminate or minimize such an error.

Methane
Stable hydrogen isotope ratios of CH 4 were analyzed using an in-house cryogenic pre-concentration unit coupled to a Hewlett Packard HP 6890 gas chromatograph (Agilent Technologies, Palo Alto, CA) and an isotope ratio mass spectrometer (DeltaPlus XL, ThermoQuest Finnigan, Bremen, Ger-many).The working gas was the same as that used for δ 2 H analysis of CH 3 Cl (cf Sect.2.3.1).
All δ 2 H values obtained from analysis of CH 4 were corrected using two CH 4 working standards (isometric instruments, Victoria, Canada) calibrated against IAEA and NIST reference substances (not specified by the company).The calibrated δ 2 H values of the working standard in ‰ vs. V-SMOW were −144 ± 4 and −138 ± 4 ‰.
The H + 3 factor determined daily during the 2-week measurement period was in the range 2.38-2.43.The daily average precision based on replicate measurements of the CH 4 working standard was 4.9 ‰ (n = 7).Samples were analyzed 3 times (n = 3), and the SD of the measurements were in the range of 1.4 to 40.9 ‰.Lowest SD were observed for samples with lowest δ 2 H values (∼ −180 ‰) and highest mixing ratios and higher SD for samples with highest δ 2 H values (∼ +300 ‰) and lowest mixing ratios.

Kinetic isotope effect, fractionation constant α and the isotope enrichment constant ε
In this study the isotope fractionation constant α and the isotope enrichment constant ε are derived from the slope of the Rayleigh plot according to Clark and Fritz (1997) and Elsner et al. (2005) and Eq. ( 2): To correct for ongoing analyte dilution the remaining fraction f has been calculated as follows where c x0 and c xt are the mixing ratios of CH 3 Cl at time zero and time t and c i0 and c it are the respective concentrations of the internal standard PFH.

Results
Three experiments of CH 3 Cl degradation with OH were performed between 25 February 2014 and 3 February 2015.Under the experimental conditions (see Sect. 2 and Supplement) more than 70 % of the CH 3 Cl was degraded within 6 to 10 h.The results from these experiments are shown in Fig. 2. Both the trends of changes in δ 2 H values of CH 3 Cl as well as the remaining fraction of CH 3 Cl observed in the three independent experiments are in good agreement (Fig. 2a).The calculated ε values for experiments 1 to 3 are −264 ± 7, −219 ± 6 and −308 ± 8 ‰, respectively (Fig. 2b), with a correlation coefficient R 2 of the slope of the regression line of 0.99 for all three experiments.The CH 3 Cl degradation with Cl experiment was conducted on 18 February 2014.Here, over 90 % of CH 3 Cl was degraded during reaction with Cl radicals within 7 to 8 h (Fig. 3a).The calculated ε of experiment 3 is −280 ± 11 ‰ (Fig. 3b) with a correlation coefficient of the slope of the regression line of 0.99.Due to limited analytical resources it was not possible to repeat this experiment.The experiment to determine the isotope enrichment constant of the degradation of CH 4 by hydroxyl radicals was conducted on 2 February 2015.Over 80 % of CH 4 was degraded during reaction with OH radicals within 7 h (Fig. 4a).The calculated ε of experiment 4 is −205 ± 6 ‰ (Fig. 4b) with a correlation coefficient of the slope of the regression line of 0.99.

Discussion
Chloromethane reacts with both hydroxyl and chlorine radicals in the atmosphere.The first degradation step of CH 3 Cl in both reactions is the abstraction of a hydrogen atom to yield CH 2 Cl and H 2 O or HCl, respectively (Spence et al., 1976;Khalil and Rasmussen, 1999).In both reactions hydrogen is directly present in the reacting bond, and thus influenced by the so-called primary isotope effect (Elsner et al., 2005).Particularly for hydrogen these primary kinetic isotope effects are in general large as they involve a large change in relative mass of the atoms being abstracted.In the following we would like to discuss and compare our results with (i) previous work conducted by Sellevåg et al. (2006), (ii) with OH degradation experiments of CH 4 and (iii) with the very recent report of biochemical degradation of CH 3 Cl in soils and plants (Jaeger et al., 2018a, b).
Although our experimental results show relatively large hydrogen isotope fractionations with ε values of −264 ± 45 (mean result from three independent experiments ±SD) and −280 ± 11 ‰ (mean result from three replicate analytical measurements of the same sample ±SD) for reaction of CH 3 Cl with OH and Cl radicals, respectively, they are smaller than the isotope fractionations previously measured and theoretically calculated by Sellevåg et al. (2006;Table 1).These researchers employed smog chamber experiments at 298 K and used FTIR measurements to determine the stable hydrogen isotope fractionation of CH 3 Cl and reported ε values of −410 and −420 ‰ for the reaction of CH 3 Cl with OH and Cl radicals, respectively.They also performed theoretical calculations of ε for the reactions of CH 2 DCl with OH and Cl radicals and reported ε values in the range of −330 to −430 and −540 to −590 ‰ , respectively (Table 1).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.For further discussion regarding differences of the experimental and analytical design and protocols of the two studies we refer the reader to the Supplement.However, we also conducted similar smog chamber experiments for the degradation of CH 4 with hydroxyl radicals (see Sect. 2 and Fig. 4) and calculated an ε value of −205 ± 6 ‰ for the reaction of CH 4 with OH radicals at a temperature of 293 ± 1 K.In Table 1 we compare our results with those from a number of previous studies (Saueressig et al., 2001;Sellevåg et al., 2006;DeMore, 1993;Gierczak et al., 1997;Xiao et al., 1993), which were conducted at temperatures ranging from 277 to 298 K (Table 1).The ε values for the reaction of CH 4 with OH radicals from all studies ranged from −145 to −294 ‰ with a mean value of −229 ± 44 ‰ and the most negative ε value of −294 ± 18 ‰ reported by Sellevåg et al. (2006).The ε value found in this study (−205 ± 6 ‰) was in good agreement with previous experimentally reported values conducted at similar temperatures.This finding gave us confidence that our experimental design and the measurements made using GC-IRMS were reliable.
Compared to primary isotope effects, changes in bonding are much smaller in the case of secondary isotope effects, where positions adjacent to the reacting bond are only slightly affected by the proximity to the reaction center (Elsner et al., 2005;Kirsch, 1977).It was suggested that for the same element, secondary isotope effects are generally at least 1 order of magnitude smaller than primary isotope effects (Kirsch, 1977;Westaway, 1987;Merrigan et al., 1999).
We therefore compared our results from chemical degradation experiments with those from recently reported biochemical degradation experiments (Jaeger et al., 2018a, b).So far, the only known pathway for biochemical consumption of CH 3 Cl is corrinoid-and tetrahydrofolate-dependent and is termed cmu (abbreviation for chloro methane utilization).This pathway was characterized in detail for the aerobic facultative methylotrophic strain Methylobacterium extorquens CM4 (Vannelli et al., 1999) and involves genes that were also detected in several other chloromethane-degrading strains (Schafer et al., 2007;Nadalig et al., 2011Nadalig et al., , 2013)).Dur-ing degradation of CH 3 Cl the methyl group is transferred to a corrinoid cofactor by the protein CmuA.In this case the carbon-chlorine bond of CH 3 Cl is broken and thus since the hydrogen atoms are adjacent to the reacting bond only a secondary isotope effect would be expected.Indeed, the first ε values reported (Jaeger et al., 2018a, b) for CH 3 Cl biodegradation by different soils and plants (ferns) are in the range of −50 ± 13 and −8 ± 19 ‰, respectively, and thus show considerably smaller kinetic isotope effects than for chemical degradation of CH 3 Cl by OH and Cl radicals measured in either this study or reported by Sellevåg et al. (2006).

Conclusions and future perspectives
We have performed experiments to measure the hydrogen isotope fractionation of the remaining unreacted CH 3 Cl following its degradation by hydroxyl and chlorine radicals in a 3.5 m 3 Teflon smog chamber at 293 ± 1 K. δ 2 H values of CH 3 Cl were measured using GC-IRMS.The calculated isotope fractionations of CH 3 Cl for the reactions with hydroxyl and with chlorine radicals were found to be smaller than either the experimentally measured (by FTIR) or theoretical values reported by Sellevåg et al. (2006).We also performed degradation experiments of CH 4 using the same smog chamber facilities yielding an isotope enrichment constant for the reaction of CH 4 with hydroxyl radicals of −205 ± 6 ‰ which is in good agreement with previously reported results.Although stable hydrogen isotope measurements of CH 3 Cl sources are still scarce, some recent studies have reported first data on δ 2 H values of CH 3 Cl sources and ε values on sinks (Greule et al., 2012;Jaeger et al., 2018Jaeger et al., , 2018b;;Nadalig et al., 2013Nadalig et al., , 2014)).
We have summarized all available information regarding δ 2 H values of environmental CH 3 Cl sources and their estimated fluxes in Table 2. Furthermore, the strengths of known CH 3 Cl sinks and their associated isotope enrichment constants are presented in Table 3. Eventually Fig. 5 displays the global CH 3 Cl budget showing the known hydrogen isotope signatures of sources and isotope enrichment constants associated with sinks.
Our results suggest that stable hydrogen isotope measurements of both sources and sinks of CH 3 Cl and particularly the observed large kinetic isotope effect of the atmospheric CH 3 Cl sinks might strongly assist with the refinement of current models of the global atmospheric CH 3 Cl budget.In contrast to the large hydrogen fractionation of CH 3 Cl by chemical degradation of OH and Cl radicals, the isotope fractionation of CH 3 Cl biodegradation is in the range of an order of magnitude lower.This therefore has the possibility of improving our understanding of the global CH 3 Cl budget once the δ 2 H value of atmospheric CH 3 Cl has been measured.The stable hydrogen isotopic composition of tropospheric CH 3 Cl depends on the isotopic source signatures and the kinetic isowww.atmos-chem-phys.net/18/6625/2018/Atmos.Chem.Phys., 18, 6625-6635, 2018   a Values for sink strength (best estimate and full range) were taken from Carpenter and Reimann (2014), except for the value of the reaction with Cl radicals in marine boundary layer and for total sinks shown in parentheses which includes the potential sink strength by Cl radicals in marine boundary layer (Montzka and Fraser, 2003).b this study.c Sellevåg et al. (2006).d Thompson et al. (2002)  tope effects of the sinks, primarily the reaction with OH and consumption by soils and potentially plants.
Several attempts at modeling the global CH 3 Cl budget using stable carbon isotope ratios have already been made (Harper et al., 2001(Harper et al., , 2003;;Thompson et al., 2002;Keppler et al., 2005;Saito and Yokouchi, 2008) but there are still major uncertainties regarding source and sink strengths as well as the respective stable isotope signatures.Therefore, we now suggest combining our knowledge of stable carbon and hydrogen isotopes of CH 3 Cl in the environment.Such a two-dimensional (2-D) stable isotope approach of hydrogen and carbon can be used to better understand the processes of CH 3 Cl biodegradation and formation.Furthermore, when this approach is combined with CH 3 Cl flux estimates it could help to better constrain the strength of CH 3 Cl sinks and sources within the global CH 3 Cl budget (Nadalig et al., 2014;Jaeger et al., 2018b).
We note that currently no data are available for the δ 2 H value of atmospheric CH 3 Cl.Although it will be a massive analytical challenge to obtain this value, we strongly suspect that it would likely lead to a better-refined isotopic mass balance for atmospheric CH 3 Cl and thus to a better understanding of the global CH 3 Cl budget.
Data availability.The data used in this publication are available to the community and can be accessed by request to the corresponding author.
Competing interests.The authors declare that they have no conflict of interest.

Figure 1 .
Figure 1.Schematic of the experimental smog chamber.

∼
= (α − 1) • lnf = ε • lnf where R t and R 0 are the 2 H / 1 H ratios in CH 3 Cl or CH 4 at the different time points and time zero, respectively, and f is the remaining CH 3 Cl or CH 4 fraction at the different time points.Negative values of ε indicates that the remaining CH 3 Cl or CH 4 is enriched in the heavier isotope and corresponds to a α < 1, meaning that over the entire experiment, the heavier CH 2 DCl or CH 3 D reacts by this factor more slowly than the lighter CH 3 Cl or CH 4 .The kinetic isotope effect is then calculated as KIE = 1 α .

Figure 2 .
Figure 2. Reaction of CH 3 Cl and OH.Three independent experiments (triangles, dots and squares) were carried out using an initial mixing ratio of 5 to 10 ppmv CH 3 Cl.More than 70 % of the CH 3 Cl was degraded within 6 to 10 h.(a) Measured δ 2 H values (filled circles, triangles and squares) of CH 3 Cl versus residual fraction (open circles, triangles and squares) of CH 3 Cl (calculated from changes of CH 3 Cl and PFH).Error bars of δ 2 H value of CH 3 Cl indicate the standard deviation (SD) of the mean of three replicate measurements.Some error bars lie within the symbol.(b) Rayleigh plot (Eq.3).Error bars were calculated by error propagation including uncertainties in δ 2 H values of CH 3 Cl and the remaining fraction.Dashed lines represent 95 % confidence intervals of the linear regressions (bold lines).

Figure 3 .
Figure 3. Reaction of CH 3 Cl and Cl.Initial mixing ratio of CH 3 Cl was ∼ 10 ppmv.More than 90 % of the CH 3 Cl was degraded within 7 to 8 h.(a) Measured δ 2 H values (filled circles) of CH 3 Cl versus residual fraction (open diamonds) CH 3 Cl.Error bars of δ 2 H values of CH 3 Cl indicate the standard deviation (SD) of the mean of three replicate measurements.Some error bars lie within the symbol.(b) Rayleigh plot (Eq.3).Data are expressed as the mean ± standard error of the mean, n = 3. Error bars were calculated by error propagation including uncertainties in δ 2 H values of CH 3 Cl.Dashed lines represent 95 % confidence intervals of the linear regressions (bold line).

Figure 4 .
Figure 4. Reaction of CH 4 and OH.Initial mixing ratio of CH 4 was ∼ 6 ppmv.More than 80 % of the CH 4 was degraded within 7 h.(a) Measured δ 2 H values of CH 4 versus residual fraction of CH 4 .Error bars of δ 2 H values of CH 4 indicate the standard deviation (SD) of the mean of three replicate measurements.Some error bars lie within the symbol.(b) Rayleigh plot (Eq.3).Error bars were calculated by error propagation including uncertainties in δ 2 H values of CH 4 and the remaining fraction.Dashed lines represent 95 % confidence intervals of the linear regressions (bold line).

Figure 5 .
Figure 5. Schematic of major sources and sinks involved in the global CH 3 Cl cycle (modified after Keppler et al., 2005) with known (experimentally determined) corresponding δ 2 H values and isotope enrichment constants, respectively.Red straight and dashed lines of arrows indicate sources and sinks of CH 3 Cl, respectively.Size/thickness of arrows indicates strength of fluxes in the environment.Question marks indicate where currently no data exist.All values are given in per mil.

Table 1 .
Reported hydrogen isotope enrichment constants for the reaction of CH 3 Cl with OH radicals and with Cl atoms and the reaction of CH 4 with OH radicals.

Table 2 .
Known sources and strengths of tropospheric CH 3 Cl and corresponding δ 2 H values.
Nadalig et al. (2013)aeger et al. ( , 2018b))e (full range) were taken fromCarpenter and Reimann (2014), except for emissions associated with chemical production by the industry which are fromLi et al. (2017).The value shown for total sources in parentheses includes chemical production by the industry.bGreuleetal.2012;pleasenotethatallvaluesprovidedforCH 3 Cl released from dried plants at elevated temperatures have been corrected by −23 ‰ due to recalibration of the reference gas.cJaegeretal.(2018b).dLietal.(2017).etakenfromGreuleetal.(2012),Nadaligetal. (2013)andJaeger et al. (2018aJaeger et al. ( , 2018b)); please note that values provided byGreule et al. (2012)andNadalig et al. (2013)for CH 3 Cl from sources of the chemical industry have been corrected by −23 ‰ due to recalibration of the reference gas.f including mangroves, wetlands, rice paddies and shrublands.?denotes that no value has been provided.

Table 3 .
Known sinks of tropospheric CH 3 Cl and the mean isotope enrichment constant ε reported for each.