The use of δ 13C in CO to determine removal of CH4 by Cl radicals in the atmosphere

The reaction of CH4 with chlorine (Cl) radicals in the atmosphere is associated with an extraordinarily strong isotopic fractionation, where 12CH4 reacts about 70 ‰ faster with Cl than 13CH4. Therefore, although the Cl-based sink of CH4 constitutes only a small contribution to its total removal rate, the uncertainty in this small sink has been identified as one of the two largest uncertainties of isotope-based CH4 source apportionment at the global scale. The uncertainty arises from the fact that Cl levels in the atmosphere are so low that they cannot be detected directly. One very sensitive indirect method to identify and quantify the CH4 + Cl reaction in the atmosphere is the detection of the extremely 13C-depleted reaction product carbon monoxide (CO) from this reaction. This article reviews the concept of this approach, its successful application in the atmosphere, its challenges and opportunities for identifying and quantifying Cl-based removal of CH4 at the regional and global scale and its potential to detect and evaluate possible attempts to enhance CH4 removal from the atmosphere.


Introduction
Methane (CH 4 ) is the second most important anthropogenic greenhouse gas, and its enhanced emissions are responsible for about half a degree of warming since pre-industrial times (IPCC AR6, 2022).Due to its relatively short lifetime in the atmosphere of about 10 years, CH 4 is considered an attractive emission mitigation target, and reduction in CH 4 levels could slow down global warming on a relative short term (Ocko et al 2021, Cael andGoodwin 2023).Therefore, more than 150 nations have joined the Global Methane Pledge to reduce anthropogenic CH 4 emissions by 30% by the year 2030, compared to 2020 levels.This is an ambitious goal given the wide range of sources from anthropogenic activities, which include exploration, transport, storage and use of fossil fuels, waste management, ruminant and rice agriculture and industrial combustion and non-combustion processes (Saunois et al 2020).Numerous activities in the past decades have addressed quantifying and reducing CH 4 emissions across all these sectors, e.g.(Brandt et al 2014, Alvarez et al 2018, Shindell et al 2021, Smith et al 2021, Stavropoulou et al 2023, Wang et al 2023).Nevertheless, CH 4 levels in the atmosphere are still increasing, and the growth rate has been particularly high in the post-2020 years (Lan et al 2023).
The causes of the multi-year variations in CH 4 over the last decades are still being debated, and different studies have attributed them to either increasing emissions from anthropogenic sources ( et al 2017, Turner et al 2017, Stevenson et al 2022).Key information comes from the temporal evolution of the carbon isotopic composition of CH 4 .The reversal of the temporal δ 13 C trend (Nisbet et al 2016, Schaefer et al 2016, Lan et al 2021) suggests that the recent rise of CH 4 is different from the rise before the year 2000, and that fossil sources may no longer be responsible for the increase.In this case, the ongoing rise of CH 4 likely originates from biogenic sources, including the possibility that we already see a feedback of the global climate system to ongoing global warming, which is likely more difficult to mitigate than anthropogenic sources (Nisbet et al 2023).Note, however, that Thanwerdas et al (2023) suggest that the δ 13 C trend reversal could be explained by a shift in the source signatures without a large reduction in the relative share of the fossil sources.Notwithstanding, Kleinen et al (2021) showed that increased emissions from natural sources may cause future CH 4 concentrations to rise higher than expected in current global projections.In this case, emission reductions from anthropogenic sources may not be adequate to reduce CH 4 sufficiently.
An additional possible way to reduce atmospheric CH 4 levels is the acceleration of its removal.Most of the CH 4 is removed by reaction with the hydroxyl radical (OH), with small contributions from soil uptake and removal by atomic oxygen and chlorine (Cl) radicals.The Cl-based sink in the troposphere is very difficult to quantify because Cl levels are so low that they cannot be measured directly.Thus they have to be quantified by indirect measurements of Cl precursors, either in combination with models of atmospheric photochemistry (Hossaini et al 2016) or by measurable effects that reactions with Cl imprint on more stable species.This paper discusses the potential, challenges and opportunities of using the carbon isotopic composition of CO, the product of atmospheric oxidation reactions of CH 4 , as a tool to quantify the contribution of Cl to the removal of CH 4 and to identify regions and processes where CH 4 is removed by Cl in the atmosphere.

Isotope-based detection and quantification of Cl
The basis of this method is the extraordinarily strong 13 C/ 12 C kinetic isotope effect in the reaction CH 4 + Cl.Saueressig et al (1995) discovered that in the range of tropospheric temperatures Cl reacts about 70‰ faster with 12 CH 4 than with 13 CH 4 , an astonishingly strong kinetic stable carbon isotope effect.Thus, wherever this reaction is significant, it leads to a 13 C enrichment in the remaining CH 4 , and a depletion in the CO produced.
The carbon isotopic composition of CO is quantified in the common delta (δ) notation as where 13 R is the 13 C/ 12 C ratio and VPDB is the international reference material Vienna PeeDeeBelemnite with 13 R VPDB = 0.011 180 (28) (Zhang et al 1990).
For CH 4 , δ 13 C CH4 is defined analogously.The kinetic fractionation factor α of a certain reaction is the ratio between the rate coefficients of the reactions of the different isotopologues.α also is equal to the isotope ratio of the instantaneously produced reaction product (here CO, assuming complete conversion to CO) relative to the one of the substrates (here CH 4 ), thus (2) For the reaction under consideration, when CH 4 (δ 13 C CH4 = −48‰) is removed via the Cl + CH 4 reaction (α Cl + CH4 = 0.935), the δ 13 C of the produced CO is Thus, if only 1 ppb of CO with this depleted signature is added to an ambient reservoir of 100 ppb of CO with δ 13 C CO amb = −25‰, the mixture will have a significantly different isotopic composition (4)

Methods
For  Brenninkmeijer 1993) required large air samples (hundreds of liters), because purified gases had to be inserted into the IRMS using dual inlet systems.The advantage of this was that many samples could also be analyzed for 14 CO using accelerator mass spectrometry (Brenninkmeijer 1993) (Van Herpen et al 2023).The reaction of the released Cl with CH 4 is calculated online in the model, and the effect on δ 13 C CO is then calculated knowing the isotopic composition of the ambient and the formed CO (equations ( 3) and ( 4)).We present previously unpublished data from these simulations for the southern hemisphere to evaluate the impact of the Cl from MDSA on δ 13 CO in Baring Head, New Zealand.

δ 13 C CO -based detection of Cl in the stratosphere
In the stratosphere, the reaction of CH 4 with Cl that is produced from photolysis of CFCs in the ozone layer contributes strongly to CH 4 removal, which affects the radiative effects of stratospheric CH 4 and water vapor (Saiz-Lopez et al 2023).This leads to a welldocumented and large 13 C enrichment with altitude and latitude (Wahlen et al 1987, Brenninkmeijer et al 1995, Sugawara et al 1997, Rice et al 2003, Röckmann et al 2011).These observations can be reproduced in atmospheric models when the laboratory-based kinetic isotope effect in the Cl + CH 4 reaction is included (McCarthy et al 2003, Eichinger et al 2015, Thanwerdas et al 2022, Chandra et al 2024), demonstrating that the effect is well understood.Brenninkmeijer et al (1996) were the first to show that the 13 C enrichment in CH 4 is accompanied by a very strong depletion of 13 C in stratospheric CO. Figure 1 shows extremely low δ 13 C CO values in stratospheric air when the CO mole fraction decreases below 35 ppb.The symbols are color-coded by the concentration of 14 CO, an ultra-trace molecule that is formed in the upper troposphere and lower stratosphere from 14 C produced by cosmic radiation (Mak et al 1994).Higher 14 CO levels indicate a higher stratospheric character of the air mass, and it is evident that δ 13 C CO drops very strongly as the stratospheric character of the air mass increases.

δ 13 C CO -based detection of Cl in the troposphere: ozone depletion events (ODEs)
Tropospheric Cl is much more difficult to detect and quantify than stratospheric Cl, because tropospheric Cl concentrations are extremely low, between 10 2 and 10 5 cm −3 .Nevertheless, the δ 13 C CO method has been successfully used to detect and quantify elevated Cl levels associated with ODEs (ozone depletion events) in the Arctic (Röckmann et al 1999).Figure 2 shows an example of an ODE, when O 3 levels drop from ambient values of ∼40 ppb to values near zero within hours.The black symbols show δ 13 C CO during normal O 3 , and the long-term trend reflects part of the seasonal cycle of δ 13 C CO .During ODE episodes, δ 13 C CO drops by about 1‰ relative to background levels (red circles in figure 1 relative to black circles, interpolated by black line).Note that the O 3 destruction during ODEs is largely caused by brominecatalyzed O 3 destruction (Herrmann et al 2022) with additional contributions from iodine (Benavent et al 2022).However, the depletion in δ 13 C CO (and hydrocarbon ratios (Jobson et al 1994, Ariya et al 1998)) confirms that also Cl is produced in these events at levels that facilitate oxidation of roughly 1 ppb of CH 4 upwind of the measurement location.

δ 13 C CO -based detection of Cl in the tropical troposphere
Are such negative δ 13 C CO deviations also observable at other locations where Cl levels increase temporarily?The number of CO isotope observations made in the past is rather limited, but more than 20 years ago, Mak et al (2003) reported unexpected Figure 1.δ 13 CCO versus CO mole fraction, with a colour code of 14 CO (in molecules cm −3 at standard temperature and pressure (STP)) for air samples collected on a C130 aircraft platform during flights in the southern polar lower stratosphere between New Zealand and Antarctica in October 1993.Adapted with permission from Brenninkmeijer et al (1996).© The American Geophysical Union.δ 13 C CO variability, including very depleted values, at Barbados.At other stations, much smoother seasonal evolutions and higher values of δ 13 C CO are generally observed (Brenninkmeijer 1993, Röckmann et al 1998, 2002, Mak and Kra 1999, Kato et al 2000, Mak et al 2000, Gros et al 2001).Mak et al (2003) already suggested that the low δ 13 C CO values could originate from the reaction Cl + CH 4 , but at that time no source was known which could produce the relevant amounts of Cl in this region.
Van Herpen et al (2023) recently suggested that Cl could be liberated photocatalytically when ironcontaining mineral dust mixes with sea spray aerosol above the Atlantic.When a corresponding Marine Dust -Sea spray Aerosol (MDSA) mechanism was implemented in a global atmospheric model, sufficient Cl was produced in the model to oxidize ppb-level quantities of CH 4 during the passage of the air parcel over the Atlantic.Figure 3 shows the expected deviation in δ 13 C CO originating from this 13 C-depleted CO source (equation ( 3)) compared to a model without the MDSA mechanism for April 1997 from van Herpen et al (2023).In the model, the strength and location of these isotope signals varies with season.
To further confirm the MDSA hypothesis, air sampling programs have been recently initiated on several Atlantic islands, and on commercial ships.According to the modeling of MDSA performed in van Herpen et al (2023), the δ 13 C CO depletion should be strongest at around 30 • W longitude, with negative shifts in δ 13 C CO of up to 4 ‰ (figure 3).The dust chemistry should lead to slightly lower δ 13 C CO depletions at the permanent stations compared to the ship track, based on the location of the aerosol dust plume during this period.
Figure 4 shows CO and δ 13 C CO data from air samples collected during two ship tracks that transected a large Sahara dust plume on 6-7 April 2023 (around 30 • W, 7-11 • N, Maersk Visby, back triangles) and 4-9 April 2023 (7-27 • W, 29-1 • N, Cap San Augustine, grey circles).We also include data from our fixed stations at Tenerife, Cape Verde and Barbados that were taken during the same period, and samples from the Southern Hemispheric station Baring Head.The air samples collected on the Visby between April 5 and 10 and by the San Augustine between April 6 and 9 were collected in a period associated with high levels of Saharan dust. Figure 4 shows that during these days the δ 13 C CO of samples collected on the Visby was much lower than the reference data from San Augustine, Barbados, Tenerife and Cape Verde.The Visby samples had relatively higher CO concentrations (figure 4(b)), likely due to an extra source from combustion, either technological or forest burning.This is evident from the high δ 18 O values that are indicative of combustion CO (Brenninkmeijer and Röckmann 1997).However, combustion derived CO cannot explain the 13 C depletion in these samples, as CO from combustion sources has a similar or higher 13 C content than the ambient CO (Brenninkmeijer et al 1999).CO in the dust-associated Visby samples between 5-10 April is depleted in 13 C by more than 2 ‰ (average value of −31.6 ‰ compared to the reference average of −29.4 ‰).Such low values are found in the Southern Hemisphere (see data from Baring Head in figure 4), but the CO mole fraction and δ 18 O values are too high to reflect Southern Hemispheric air.The addition of a very small amount of strongly 13 C-depleted CO from the Cl + CH 4 reaction in the middle of the Atlantic can explain the negative δ 13 C excursions in the Visby samples.The Cl + CH 4 signal is weaker at the island stations and the San Augustine locations that are further outside the dust plume, in line with the model prediction by van Herpen et al (2023).Allan et al (2001Allan et al ( , 2005Allan et al ( , 2007) ) used the seasonal covariation of CH 4 and δ 13 C CH4 (referred to as phase ellipses) at high southern latitudes to quantify the possible role of Cl.They showed that the slopes of the phase ellipses are much larger than the one expected from the kinetic isotope effect in the removal of CH 4 by OH (i.e.3.9‰ according to Saueressig et al (1995) and 5.4‰ according to Cantrell et al (1990)).They concluded that this can only be caused by a significant contribution of tropospheric Cl to the sink, with a seasonal amplitude of 6 × 10 3 cm −3 .From a similar analysis, Platt et al (2004) estimated that the contribution of Cl to the tropospheric CH 4 sink could be as high as 3.3%, or 19 Tg CH 4 yr −1 .A weakness of their argument is that the slope of the phase ellipse also exhibits strong interannual variation.In particular over the first years of the record, it varied from 17 ‰ in 1997 to 6 ‰ in 1999.This would correspond to a change in the Cl contribution of several percent, and if this was caused by Cl, it should also have affected the overall δ 13 C of atmospheric CH 4 to a degree that is incompatible with atmospheric observations.Gromov et al (2018) illustrated this issue from the perspective of the reaction product CO.They argue that large interannual variations in Cl-based oxidation of CH 4 in the Extra Tropical Southern Hemisphere (ETSH) would necessarily result in strong corresponding variations of δ 13 C CO as the CO produced in this reaction is strongly depleted in 13 C (equation ( 1)).However, corresponding variations in δ 13 C CO in the ETSH have not been observed.Interpreting the available atmospheric model results, they put an upper limit of n Cl = 0.9 × 10 3 cm −3 on the variation of mean Cl levels in the ETSH, far less than the levels proposed by Allan and co-workers.Moreover, they argue that a large Cl source in the ETSH would lead to even lower background δ 13 C CO values, which are already hard to reconcile with the understanding of the global CO cycle.

Global scale considerations
Nevertheless, the argument for high Cl based on the δ 13 C CH4 phase ellipses has been supported by recent model analyses of the temporal and spatial trends in δ 13 C CH4 , which can only be reproduced in atmospheric models that use a strong isotope fractionation in the removal of CH 4 (Lan et al 2021, Basu et al 2022, Thanwerdas et al 2022).This requires either a high fractionation in the OH sink or a large contribution of Cl to the total sink.Whitehill et al (2017) reported 6.1 ‰ for the fractionation in the OH sink, higher than the previous reported values of 3.9‰ (Saueressig et al 1995) or 5.4‰ (Cantrell et al 1990).
A stronger fractionation in the OH sink would require a small contribution from Cl, whereas a weaker fractionation in the OH sink would require a much larger contribution from Cl to explain the atmospheric observations.
Is it possible that a large source of Cl from MDSA near South America can provide enough Cl to explain the phase-ellipses observed by Allan et al (2001Allan et al ( , 2005Allan et al ( , 2007)), but not affect δ 13 C CO at the Baring Head and Scott Base sites in a corresponding way?We examined the CESM model output from van Herpen et al (2023), which included an extrapolation to the global scale.The model output shows an additional strong Cl source near South America, and a modest source of Cl near Australia.Between July 1996 and June 1998, the modelled monthly average n Cl in the southern hemisphere varied between 400 and 2200 atoms cm −3 , resulting in ∆n Cl exceeding 2 × 10 3 cm −3 , which is of the order of magnitude suggested by Allan et al (2007).At Baring Head, the model output shows a δ 13 C CO of less than 0.3 ‰, in line with the analysis made by Gromov et al (2018).The inter-annual variation in the model output is even less pronounced.Note, however, that the model setup used in van Herpen et al (2023) was not targeted at the global scale, in particular it does not include a polar halogen source module (Fernandez et al 2019).
These results suggest that a large regional Cl source, far away from the remote observatories in the SH could at the same time provide a high average [Cl] exposure to explain the phase ellipses for CH 4 , without causing a large effect on δ 13 C CO .In addition, a large and previously unaccounted-for source of Cl in the NH could potentially alleviate difficulties in modeling the rather small interhemispheric gradient in 13 C CO , which was previously attributed to a possibly unrealistically low yield of CO from CH 4 oxidation (Manning et al 1997, Bergamaschi et al 2000), one of the key parameters of the tropospheric CO budget that is yet rather uncertain (Gromov et al 2018).Further modeling is needed to confirm the global scale relevance and implications of this potentially large Cl source.

Implications
Measurements of the carbon isotopic composition of CH 4 have been widely used to quantify the relative contribution of emissions from different source sectors to the observed variations in the growth rate of CH 4 over the past decades (Nisbet et al 2016, 2019, 2023, Schaefer et al 2016, Worden et al 2017, Lan et al 2021, Basu et al 2022, Thanwerdas et al 2022).The kinetic isotope effect in the total removal of CH 4 is a key parameter influencing this partitioning.An error in the assumption of its value will invariably translate to an error in the partitioning between fossil fuel related and biogenic source categories.Basu et al (2022) identified the uncertainty in the fractionation (both related to the uncertainty in the fractionation of the CO + OH reaction, and the contribution of Cl) as the single most important parameter precluding a more reliable separation between the different source sectors.Consequently, it is important to reduce this uncertainty using new measurement techniques, such as δ 13 C CO .Finally, we note that measurements of δ 13 C CO may be a valuable tool to quantify the efficiency of possible future CH 4 mitigation policies that may involve accelerating Cl-based destruction of CH 4 .

Figure 2 .
Figure 2. O3 levels (blue line, left axis) drop from normal ambient values around 40 ppb to values near zero within hours during ODEs.Right axis: δ 13 C of CO during background conditions (black circles, interpolated by black solid line) and ODEs (red circles) in spring 1997.Adapted with permission fromRöckmann et al (1999).© The American Geophysical Union.

Figure 3 .
Figure 3.The spatial extent of the decrease in δ 13 CCO resulting from CH4 oxidation by Cl that is produced in the CESM model including a MDSA mechanism for April 1997 (monthly average) compared to the default model without this mechanism.This figure uses model simulations from van Herpen et al (2023).

Figure 4 .
Figure 4. (A) ship trajectories overlayed on a satellite image showing the April 2023 average aerosol optical depth (imagery produced by NASA based on data provided by the MODIS Atmosphere Science Team, NASA Goddard Space Flight Center).(B) ship data compared with fixed stations shows 13 C depletions for dust-associated locations.(C) δ 13 C(CO) vs. CO.(D) δ 18 O(CO) vs. CO.We only analyze northern hemisphere ship flasks, because the isotopic composition of CO in the Southern Hemisphere is very different (see data from Baring Head, New Zealand).
Chandra et al 2021, Zhang et al 2021) biogenic sources (Nisbet et al 2016, Schaefer et al 2016, Lan et al 2021, Basu et al 2022, Oh et al 2022), both categories (Worden et al 2017, Thanwerdas et al 2023) or changes in the removal by the main sink, reaction with OH (Rigby Lamarque et al 2012, Tilmes et al 2016, Li et al 2023, Saiz-Lopez et al 2023, Van Herpen et al 2023).The MDSA mechanism (Wittmer and Zetzsch 2016, Van Herpen et al 2023) involves photocatalytic cycling of iron ions (Fe(III)-Fe(II)) and Cl in mixed MDSAs.The parameters in the model were chosen to match observed aerosol composition measurements in the North Atlantic.Details are provided in Manning M R and Lowe D C 2000 Aircraft observations of δ 13 C of atmospheric methane over the Pacific in August 1991 and 1993: evidence of an enrichment in (CH4)-13 C in the Southern Hemisphere J. Geophys.Res.105 1329-35 Mak J E and Yang W B 1998 Technique for analysis of air samples for 13 C and 18 O in carbon monoxide via continuous-flow isotope ratio mass spectrometry Anal.Chem.70 5159-61 Manning M R, Brenninkmeijer C A M and Allan W 1997 Atmospheric carbon monoxide budget of the southern hemisphere: implications of 13 C/ 12 C measurements J. Geophys.Res.Atmos.102 10673-82 McCarthy M C, Boering K A, Rice A L, Tyler S C, Connell P and Atlas E 2003 Carbon and hydrogen isotopic compositions of stratospheric methane: 2. two-dimensional model results and implications for kinetic isotope effects J. Geophys.Res.108 4461 Merritt D A, Brand W A and Hayes J M 1994 Isotope-ratio-monitoring gas chromatography-mass spectrometry: methods for isotopic calibration Org.Geochem.21 573-83 Nisbet E G et al 2016 Rising atmospheric methane: 2007-2014 growth and isotopic shift Glob.Biogeochem.Cycles 30 1356-70 Nisbet E G et al 2019 Very strong atmospheric methane growth in the 4 years 2014-2017: implications for the Paris agreement Glob.Biogeochem.Cycles 33 318-42 Nisbet E G et al 2023 Atmospheric methane: comparison between methane's record in 2006-2022 and during glacial terminations Glob.Biogeochem.Cycles 37 e2023GB007875 Ocko I B, Sun T, Shindell D, Oppenheimer M, Hristov A N, Pacala S W, Mauzerall D L, Xu Y and Hamburg S P 2021 Acting rapidly to deploy readily available methane mitigation measures by sector can immediately slow global warming Environ.Res.Lett.16 054042 Oh Y et al 2022 Improved global wetland carbon isotopic signatures support post-2006 microbial methane emission increase Commun.Earth Environ. 3 159 Pathirana S L, Van Der Veen C, Popa M E and Röckmann T 2015 An analytical system for stable isotope analysis on carbon monoxide using continuous-flow isotope-ratio mass spectrometry Atmos.Meas.Tech.8 5315-24 Platt U, Allan W and Lowe D 2004 Hemispheric average Cl atom concentration from 13 C/ 12 C ratios in atmospheric methane Atmos.Chem.Phys. 4 2393-9 Rice A L, Tyler S C, McCarthy M C, Boering K A and Atlas E 2003 carbon and hydrogen isotopic compositions of stratospheric methane: 1. High-precision observations from the NASA ER-2 aircraft J. Geophys.Res.108 4460 Rigby M et al 2017 Role of atmospheric oxidation inrecent methane growth Proc.Natl.Acad.Sci.USA 114 5373-7 Röckmann T, Brass M, Borchers R and Engel A 2011 The isotopic composition of methane in the stratosphere: high-altitude