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The reaction of CH ₄ with chlorine (Cl) radicals in the atmosphere is associated with an extraordinarily strong isotopic fractionation, where ¹2 CH ₄ reacts about 70 ‰ faster with Cl than ¹3 CH ₄ . Therefore, although the Cl-based sink of CH ₄ 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 CH ₄ 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 CH ₄ + Cl reaction in the atmosphere is the detection of the extremely ¹3 C-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 CH ₄ at the regional and global scale and its potential to detect and evaluate possible attempts to enhance CH ₄ removal from the atmosphere.

Prokaryotic and eukaryotic microscopic phototrophs (“microalgae”) can synthesize the potent greenhouse gas and ozone depleting pollutant nitrous oxide (N2O). However, we do not know how much microalgae contribute to aquatic N2O emissions because these organisms co-occur with prolific N2O producers like denitrifying and nitrifying bacteria. Here we demonstrate for the first time that microalgae produce distinct N2O isotopic signatures that will enable us to fill this knowledge gap. The eukaryotes Chlamydomonas reinhardtii and Chlorella vulgaris, and the prokaryote Microcystis aeruginosa synthesized N2O 265 – 755 nmol g-DW−1 h−1 when in darkness and supplied with 10 mM nitrite (NO2-). The N2O isotopic composition (δ15N, δ18O, and site preference, SP) of each species was determined using a modified off-axis integrated-cavity-output spectroscopy analyser with an offline sample purification and homogenisation system. The SP values differed between eukaryotic and prokaryotic algae (25.8 ± 0.3 ‰ and 24.1 ± 0.2 ‰ for C. reinhardtii and C. vulgaris, respectively vs 2.1 ± 3.0 ‰ for M. aeruginosa), as did bulk isotope values. Both values differ from SP produced by denitrifiers. This first characterization of the N2O isotopic fingerprints of microscopic phototrophs suggests that SP-N2O could be used to untangle algal, bacterial, and fungal N2O production pathways. As the presence of microalgae could influence N2O dynamics in aquatic ecosystems, field monitoring is also needed to establish the occurrence and significance of microalgal N2O synthesis under relevant conditions.

Methane (CH.sub.4) is a potent greenhouse gas; however, the causes of its growth since 2006 are a subject of debate. While measurements of CH.sub.4 mole fraction and carbon isotopic composition ([delta].sup.13 C-CH.sub.4) have been extensively used to investigate the global CH.sub.4 budget, the hydrogen isotopic composition ([delta]D-CH.sub.4) remains underutilised despite its unique sensitivity to source types and oxidation processes. Here, we assimilate a newly harmonised 35-year dataset of dual isotope measurements from high-latitude monitoring stations in both hemispheres within a two-box Bayesian inversion to quantify global CH.sub.4 sources and sinks. The model integrates prior emissions from five source categories based on global bottom-up inventories. Methane removal processes are represented by sink-specific kinetic isotope effects as tropospheric and stratospheric loss, and soil uptake.

Methane (CH4) is a potent greenhouse gas; however, the causes of its growth since 2006 are a subject of debate. While measurements of CH4 mole fraction and carbon isotopic composition (δ13C-CH4) have been extensively used to investigate the global CH4 budget, the hydrogen isotopic composition (δD-CH4) remains underutilised despite its unique sensitivity to source types and oxidation processes. Here, we assimilate a newly harmonised 35-year dataset of dual isotope measurements from high-latitude monitoring stations in both hemispheres within a two-box Bayesian inversion to quantify global CH4 sources and sinks. The model integrates prior emissions from five source categories based on global bottom-up inventories. Methane removal processes are represented by sink-specific kinetic isotope effects as tropospheric and stratospheric loss, and soil uptake. We find that the inclusion of δD-CH4 improves the model's ability to constrain emission apportionment between biogenic and thermogenic sources, particularly for fossil fuel emissions during the late 1990s and early 2000s, which affects CH4 lifetime estimate. CH4 increase post-2006 is driven mainly by rising wetland emissions, while fossil-fuel growth is modest, biomass burning declines, and agriculture and waste make smaller, regionalised contributions. The optimised inversion results favour a strong 13C kinetic isotope effect in total tropospheric CH4 removal and a net shortening of the NH lifetime of CH4 by 0.2 years. This study demonstrates the added value of incorporating δD-CH4 into inverse modelling frameworks and underscores the importance of long-term δD-CH4 measurements for advancing our understanding of CH4 biogeochemistry and its role in the global carbon cycle.

Establishing interlaboratory compatibility among measurements of stable isotope ratios of atmospheric methane (δ13C-CH4 and δD-CH4) is challenging. Significant offsets are common because laboratories have different ties to the VPDB or SMOW-SLAP scales. Umezawa et al. (2018) surveyed numerous comparison efforts for CH4 isotope measurements conducted from 2003 to 2017 and found scale offsets of up to 0.5 ‰ for δ13C-CH4 and 13 ‰ for δD-CH4 between laboratories. This exceeds the World Meteorological Organisation Global Atmospheric Watch (WMO-GAW) network compatibility targets of 0.02 ‰ and 1 ‰ considerably. We employ a method to establish scale offsets between laboratories using their reported CH4 isotope measurements on atmospheric samples. Our study includes data from eight laboratories with experience in high-precision isotope ratio mass spectrometry (IRMS) measurements for atmospheric CH4. The analysis relies exclusively on routine atmospheric measurements conducted by these laboratories at high-latitude stations in the Northern and Southern Hemispheres, where we assume each measurement represents sufficiently well-mixed air at the latitude for direct comparison. We use two methodologies for interlaboratory comparisons: (I) assessing differences between time-adjacent observation data and (II) smoothing the observed data using polynomial and harmonic functions before comparison. The results of both methods are consistent, and with a few exceptions, the overall average offsets between laboratories align well with those reported by Umezawa et al. (2018). This indicates that interlaboratory offsets remain robust over multi-year periods. The evaluation of routine measurements allows us to calculate the interlaboratory offsets from hundreds, in some cases thousands of measurements. Therefore, the uncertainty in the mean interlaboratory offset is not limited by the analytical error of a single analysis but by real atmospheric variability between the sampling dates and stations. Using the same method, we assess this uncertainty by investigating measurements from four high-latitude sites analysed by the INSTAAR laboratory. After applying the derived interlaboratory offsets, we present a harmonised time series for δ13C-CH4 and δD-CH4 at high northern and southern latitudes, covering the period from 1988 to 2023.

Secondary organic aerosol (SOA) plays a central role in air pollution and climate. However, the description of the sources and mechanisms leading to SOA is elusive despite decades of research. While stable isotope analysis is increasingly used to constrain sources of ambient aerosol, in many cases it is difficult to apply because neither the isotopic composition of aerosol precursors nor the fractionation of aerosol forming processes is well characterised. In this paper, SOA formation from ozonolysis of α-pinene – an important precursor and perhaps the best-known model system used in laboratory studies – was investigated using position-dependent and average determinations of 13C in α-pinene and advanced analysis of reaction products using thermal-desorption proton-transfer-reaction mass spectrometry (PTR-MS). The total carbon (TC) isotopic composition δ13C of the initial α-pinene was measured, and the δ13C of the specific carbon atom sites was determined using position-specific isotope analysis (PSIA). The PSIA analysis showed variations at individual positions from −6.9 to +10. 5 ‰ relative to the bulk composition. SOA was formed from α-pinene and ozone in a constant-flow chamber under dark, dry, and low-NOx conditions, with OH scavengers and in the absence of seed particles. The excess of ozone and long residence time in the flow chamber ensured that virtually all α-pinene had reacted. Product SOA was collected on two sequential quartz filters. The filters were analysed offline by heating them stepwise from 100 to 400 °C to desorb organic compounds that were (i) detected using PTR-MS for chemical analysis and to determine the O : C ratio, and (ii) converted to CO2 for 13C analysis. More than 400 ions in the mass range 39–800 Da were detected from the desorbed material and quantified using a PTR-MS. The largest amount desorbed at 150 °C. The O : C ratio of material from the front filter increased from 0.18 to 0.25 as the desorption temperature was raised from 100 to 250 °C. At temperatures above 250 °C, the O : C ratio of thermally desorbed material, presumably from oligomeric precursors, was constant. The observation of a number of components that occurred across the full range of desorption temperatures suggests that they are generated by thermal decomposition of oligomers. The isotopic composition of SOA was more or less independent of desorption temperature above 100 °C. TC analysis showed that SOA was enriched in 13C by 0.6–1.2 ‰ relative to the initial α-pinene. According to mass balance, gas-phase products will be depleted relative to the initial α-pinene. Accordingly, organic material on the back filters, which contain adsorbed gas-phase compounds, is depleted in 13C in TC by 0.7 ‰ relative to the initial α-pinene, and by 1.3 ‰ compared to SOA collected on the front filter. The observed difference in 13C between the gas and particle phases may arise from isotope-dependent changes in the branching ratios in the α-pinene + O3 reaction. Alternatively, some gas-phase products involve carbon atoms from highly enriched and depleted sites, as shown in the PSIA analysis, giving a non-kinetic origin to the observed fractionations. In either case, the present study suggests that the site-specific distribution of 13C in the source material itself governs the abundance of 13C in SOA.

The reaction of CH 4 with chlorine (Cl) radicals in the atmosphere is associated with an extraordinarily strong isotopic fractionation, where 12 CH 4 reacts about 70 ‰ faster with Cl than 13 CH 4 . Therefore, although the Cl-based sink of CH 4 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 CH 4 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 CH 4 + Cl reaction in the atmosphere is the detection of the extremely 13 C-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 CH 4 at the regional and global scale and its potential to detect and evaluate possible attempts to enhance CH 4 removal from the atmosphere.

We assess the performance of an isotope ratio infrared spectrometer (IRIS) to measure carbon (δ13C) and oxygen (δ18O) isotope ratios in atmospheric carbon dioxide (CO2) and report observations from a 26 d field deployment trial at Baring Head, New Zealand, NIWA's atmospheric observatory for Southern Ocean baseline air. Our study describes an operational method to improve the performance in comparison to previous publications on this analytical instrument. By using a calibration technique that reflected the principle of identical treatment of sample and reference gases, we achieved a reproducibility of 0.07 ‰ for δ13C-CO2 and 0.06 ‰ for δ18O-CO2 over multiple days. This performance is within the extended compatibility goal of 0.1 ‰ for both δ13C-CO2 and δ18O-CO2, which was recommended by the World Meteorological Organization (WMO). Further improvement in measurement performance is desirable to also meet the WMO network compatibility goals of 0.01 ‰ for δ13C-CO2 and 0.05 ‰ for δ18O-CO2, which is needed to resolve the small variability that is typical for background air observatories such as Baring Head. One objective of this study was to assess the capabilities and limitations of the IRIS analyser to resolve δ13C-CO2 and δ18O-CO2 variations under field conditions. Therefore, we selected multiple events within the 26 d record for Keeling plot analysis. This resolved the isotopic composition of endmembers with an uncertainty of ≤ 1 ‰ when the magnitude of CO2 signals is larger than 10 ppm. The uncertainty of the Keeling plot analysis strongly increased for smaller CO2 events (2–7 ppm), where the instrument performance is the limiting factor and may only allow for the distinction between very different endmembers, such as the role of terrestrial versus oceanic carbon cycle processes.

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 CH¬4 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.

Measurements from multiple laboratories have to be related to unifying and traceable reference material in order to be comparable. However, such fundamental reference materials are not available for isotope ratios in atmospheric methane, which led to misinterpretations of combined data sets in the past. We developed a method to produce a suite of synthetic CH4-in-air standard gases that can be used to unify methane isotope ratio measurements of laboratories in the atmospheric monitoring community. Therefore, we calibrated a suite of pure methane gases of different methanogenic origin against international referencing materials that define the VSMOW (Vienna Standard Mean Ocean Water) and VPDB (Vienna Pee Dee Belemnite) isotope scales. The isotope ratios of our pure methane gases range between −320 and +40 ‰ for δ2H–CH4 and between −70 and −40 ‰ for δ13C–CH4, enveloping the isotope ratios of tropospheric methane (about −85 and −47 ‰ for δ2H–CH4 and δ13C–CH4 respectively). Estimated uncertainties, including the full traceability chain, are < 1.5 ‰ and < 0.2 ‰ for δ2H and δ13C calibrations respectively. Aliquots of the calibrated pure methane gases have been diluted with methane-free air to atmospheric methane levels and filled into 5 L glass flasks. The synthetic CH4-in-air standards comprise atmospheric oxygen/nitrogen ratios as well as argon, krypton and nitrous oxide mole fractions to prevent gas-specific measurement artefacts. The resulting synthetic CH4-in-air standards are referred to as JRAS-M16 (Jena Reference Air Set – Methane 2016) and will be available to the atmospheric monitoring community. JRAS-M16 may be used as unifying isotope scale anchor for isotope ratio measurements in atmospheric methane, so that data sets can be merged into a consistent global data frame.