the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Sep 2025
Stable chlorine isotope composition of chlorofluorocarbons and chloromethane in the troposphere
Abstract. Chloromethane (CH3Cl) and chlorofluorocarbons (CFCs) play an important role in stratospheric ozone loss. Whereas anthropogenic CFC production was banned, CH3Cl emissions originating primarily from natural sources cannot be regulated. Stable chlorine isotope analysis has the potential to provide valuable insights into the sources and fate of these compounds, but to date only one study has reported atmospheric δ(37Cl) measurements linked to the isotopic standard of chlorine (SMOC, standard mean ocean chloride). In the current study, we collected tropospheric air samples over the course of one year at an urban site (Leipzig, Germany) and extracted CH3Cl, CFC-12, CFC-11, CFC-113, and HCFC-22 for stable chlorine isotope analysis. CFCs showed minor δ(37Cl) variations throughout one year, consistent with long atmospheric lifetimes and ceased production. In contrast, CH3Cl displayed an annual cycle, with lower δ(37Cl) in spring and higher δ(37Cl) in late summer to early fall. This trend likely reflects seasonal variations: increased emission of 37Cl–depleted CH3Cl by oceans and vegetation in spring and enhanced degradation in summer and fall causing isotope fractionation und thus higher δ(37Cl). Back trajectory modeling suggested a vertical difference of δ(37Cl), conceivably due to mixing of freshly emitted and 37Cl–depleted CH3Cl in the boundary layer. Additionally, stratosphere–troposphere transport of partially degraded and 37Cl–enriched CH3Cl was hypothesized. While these interpretations remain preliminary until further confirmation, isotopic data reported here represents a crucial step toward establishing δ(37Cl) fingerprints, which are prerequisites for future source appointment and atmospheric modeling studies.
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Status: open (until 31 Oct 2025)
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RC1: 'Comment on egusphere-2025-3652', Anonymous Referee #1, 10 Oct 2025
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AC1: 'Reply on RC1: short comment on the issue of interferences by other chlorinated compounds', Axel Horst, 21 Oct 2025
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We would like to comment on the issue of potential interferences caused by other chlorinated compounds and their influence on measured isotope deltas of our target compounds as raised by the referee. We agree with the referee that checking for interfering compounds is crucial in such a study. In our case, potential interferences were scrutinized by, in a first step, sacrificing some atmospheric samples to optimize GC parameters to ensure full baseline separation of chromatographic peaks. Then, in a second step, mass spectra of each target peak were examined for interfering compounds by using GC ion trap MS. Figure 1a in the main manuscript shows the mass trace (35Cl) of the MC-ICP-MS with all target compounds well separated. Some additional small peaks are visible, which belong to other chlorinated compounds including the ones mentioned by the referee.
In the very unlikely case that compounds such as C2H3Cl, CFC-115, CFC-1113 are unnoticeably overlapped by our more abundant target compounds, the effect on the isotopic composition would be negligible. Mole ratios of most of these chemicals are below 10 pmol mol-1 reaching to about 25 pmol mol-1 for HCFC-141b. Considering the two chlorine atoms in HCFC-141b, this would represent at most 10 % of the chlorine in an MC-ICP-MS mass trace compared with CH3Cl, CFC12, and CFC-11. Considering a large hypothetical isotopic difference of 5 ‰ between two compounds A (10 %) and B (90%), the isotopic composition of the resulting mixture AB would only differ by 0.5 ‰ from compound B, which is still within the assumed uncertainty. Hence, we are very confident that interferences did not affect our results because their mole ratio in the atmosphere is too low to shift the isotope delta of the much more abundant target compounds.
Citation: https://doi.org/10.5194/egusphere-2025-3652-AC1
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AC1: 'Reply on RC1: short comment on the issue of interferences by other chlorinated compounds', Axel Horst, 21 Oct 2025
reply
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RC2: 'Comment on egusphere-2025-3652', Anonymous Referee #2, 11 Oct 2025
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This study presented a detailed analysis of stable chlorine isotopes in methyl chloride (CH3Cl), chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) sampled in ambient air and purchased from commercial suppliers. The authors developed a protocol for measuring the 37Cl/35Cl ratios in these chlorinated compounds in gaseous samples and estimating the 37Cl enrichment relative to the standard mean ocean chloride (SMOC) isotopic standard (δ(37Cl) SMOC) using GC-MS. Their measurements showed (1) the range of δ(37Cl) SMOC in chlorinated hydrocarbons from industrial sources and (2) the average and the variability of δ(37Cl) SMOC in chlorinated hydrocarbons in ambient air collected from a site in Leipzig, Germany. Their analysis suggested that the observed fluctuations of δ(37Cl) SMOC in CH3Cl in ambient air is controlled by the seasonality of biological activity near the Earth’s surface and stratosphere-to-tropospheric flux of air aloft.
I think the methodology developed and the measurements conducted in this study would be a valuable contribution to our understanding of the budgets and the dynamics of atmospheric chlorine. However, there are still several aspects in the experiment design and data interpretation need more clarification in my opinion. I hope the authors can address the following comments and questions before publication:
- Section 2.3 and Section 3.2: How much chlorinated hydrocarbons did you spike in the synthetic air samples? Are control 1, 2, 3 are triplicates? I am trying to understand the variability of recovery rate across different experiments but there is some missing information.
- Section 3.4.1: I generally agree with the authors that δ(37Cl) SMOC in CH3Cl shows a larger seasonality than CFCs and HCFCs. However, the measurements are quite noisy, which is especially evident in Figure 2. Would it possible to do some statistical tests to demonstrate the seasonal difference in δ(37Cl) SMOC in CH3Cl is statistically significant (e.g., compare the averages in summer versus the rest of year)?
- Section 3.4.2.: Higher δ(37Cl) SMOC in CH3Cl is observed in air parcels originated from the free troposphere. The authors explained this phenomenon by the intrusion of stratospheric air, where CH3Cl are more enriched in 37Cl due to the photolytic degradation in the stratosphere. I think this as a qualitative explanation is reasonable. However, we do not see a similar pattern in CFC-12, which also undergoes photolysis in the stratosphere (Figure 2). Why is that?
In Line 230-232, the author stated that “fractionation in CFCs is mainly caused by destruction of these compounds in the stratosphere but reaction rates are slow and resulting changes of mole fractions (< 3 pmol mol-1) are, according to our results, not reflected in δ(37Cl) over the course of one year”. Could the difference in atmospheric lifetime of CH3Cl and CFC-12 quantitively explain the lack of contrast in δ(37Cl) in CFC-12 in the free troposphere versus the boundary layer?
Technical comments:
- Line 11 and other locations: “linked to the SMOC scale” sounds rather weird. Replacing with something like “calibrated by/ relative to the SMOC standard”?
- Line 209 and other locations: “pure-phase” is an awkward word choice.
- Table 2: very confusing to read. Maybe separate the measurements for commercially available reagents and the ambient samples into two separate panels? Also please include the units of the isotopic measurements in the caption or the header.
- Figure 2: Make a similar plot for CFC-113 and put it in the appendix so that interested readers can visualize the data more easily.
Citation: https://doi.org/10.5194/egusphere-2025-3652-RC2
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This paper describes novel data on chlorine isotopic composition of major ozone-depleting substances and is therefore adequate in principle for the scope of ACP, as well as of interest to a broad readership. Its publication is recommendable once the comments below have been addressed appropriately.
l25-26 These are good references but a but outdated. Please use more recent literature alongside it.
l26 "Production and consumption"
l30-38 Again, there is more recent evidence (e.g., Western et al., Nat. Geosci., 2023), please also see the most recent WMO Scientific Assessment of Ozone Depletion for an overview on CH3Cl.
l65-67 There is also Thomas et al., 2021 (https://doi.org/10.5194/acp-21-6857-2021).
l79-85 How was the purity of those materials ascertained?
l99-101 What was the sampling altitude? Are there nearby sources of any of the substances investigated? Was the potential for collecting locally rather than regionally representative samples assessed?
l138-144 How about interferences from the large range of other chlorine-containing substances not shown here, such as C2H3Cl, CFC-115, CFC-1113 (C2F3Cl),HCFC-142b, C2H5Cl..? Given that the isotopic effects in question are so small, even low quantities of such substances may be able to induce biases in the results (especially since many of these substances have strong seasonalities). This is a serious limitation of the study and should at least be discussed.
l181-205 Where there any quality checks carried out to determine whether the long-term storage did not induce isotopic changes? This is especially important for CH3Cl as it is know to be less stable in concentration over longer time scales.
l233 Not just legacy sources as the HCFC phase-down in certain countries is delayed - see the Montreal Protocol.
l242-244 Again, see Thomas et al., 2021.
l293-296 This assumption is not valid. CH3Cl mole fractions, similar to other long-lived trace gases, is know to exhibit large variability in near-ground air, caused by, e.g., nearby sources (such as biomass burning or certain plants) and sinks, or turbulent boundary layer mixing. in the absence of mixing ratio measurements this is a limitation of this study and should be presented as such.