The Southern Ocean is the dominant marine sink for anthropogenic carbon, absorbing around 40% of carbon emitted since industrialisation, but it is a remote and challenging region to measure. Sparsity of observational data is the main cause of uncertainty in air-sea carbon flux in the Southern Ocean. Year-round observations of CO2 mixing ratios can aid understanding of air-sea flux in this critical region and provide valuable insight into how the carbon sink is changing over time as well as its seasonal and interannual variability. This work presents ten years of high frequency in situ carbon dioxide mixing ratios measured from two coastal Antarctic research stations; Halley, operated by the British Antarctic Survey, and the German research station, Neumayer. This data set provides a rare long-term measurement of CO2 in the Southern Ocean region, allowing annual growth rates, seasonal changes and interannual variability to be studied. The mean annual growth rate was calculated to be ~2.4 ppm year-1 between 2013 and 2022. The coastal location of these stations mean they are ideally placed to explore air-sea CO2 exchange in the Southern Ocean. Both the Halley and Neumayer records show short-term fluctuations in CO2 mixing ratios during the summer, with up to ~0.5 ppm decreases in CO2 over the course of a day, about one fifth of the average annual growth rate. Air mass trajectory analysis carried out using Hysplit with ERA5 meteorological data, suggests that these decreases in CO2 correspond to periods where the air sampled has spent time over the Southern Ocean, suggesting CO2 uptake has occurred. This work explores the possible drivers for the short-term variability in CO2 mixing ratios, focusing on the role of ocean uptake in the summer.

The Southern Ocean plays a critical role in modulating excess atmospheric carbon dioxide, accounting for roughly 40% of global ocean anthropogenic CO2 uptake since industrialisation. Given its significance in the global carbon cycle, understanding the Southern Ocean carbon sink is important but studies show high uncertainties in the magnitude and evolution of this carbon sink. The Southern Ocean is a remote and challenging region to measure, and the resulting sparsity of observational data is the main cause of uncertainty in air-sea carbon flux in the region. Long term, high-temporal-frequency data sets especially are rare for the Southern Ocean, but these can give valuable insights into the carbon cycle processes occurring in the region.This work presents ten years of high-temporal-frequency in situ atmospheric carbon dioxide mixing ratios measured from two coastal Antarctic research stations; Halley, operated by the British Antarctic Survey, and the German research station, Neumayer. The coastal location of these stations means they are ideally placed to explore air-sea CO2 exchange over the Southern Ocean. Both the Halley and Neumayer records show short-term fluctuations in CO2 mixing ratios during austral summer, with over ~0.5 ppm decreases in CO2 sometimes observed over the course of a day - about one fifth of the average annual growth rate (~2.4 ppm per year-1 for this 10-year record). Analysis of air mass trajectories reveal that these fluctuations in CO2 occur when the sampled air has spent considerable time in contact with the Southern Ocean, suggesting CO2 uptake has occurred, leading to the reduced CO2 mixing ratios observed.We present an in-depth analysis of the drivers of the short-term variability observed during austral summer, including the role of mixing height, sea-ice coverage, wind speed and biology. Observational data represent an important tool with which to tease out key factors determining Southern Ocean CO2 uptake, and thus in assessing how uptake may evolve in the future.

In order to reduce the uncertainty of aerosol radiative forcing in global climate models, we need to better understand natural aerosol sources which are important to constrain the current and pre-industrial climate. Here, we analyse particle number size distributions (PNSDs) collected during a year (2015) across four coastal and inland Antarctic research bases (Halley, Marambio, Dome C and King Sejong). We utilise k-means cluster analysis to separate the PNSD data into six main categories. “Nucleation” and “bursting” PNSDs occur 28 %–48 % of the time between sites, most commonly at the coastal sites of Marambio and King Sejong where air masses mostly come from the west and travel over extensive regions of sea ice, marginal ice and open ocean and likely arise from new particle formation. “Aitken high”, “Aitken low” and “bimodal” PNSDs occur 37 %–68 % of the time, most commonly at Dome C on the Antarctic Plateau, and likely arise from atmospheric transport and ageing from aerosol originating likely in both the coastal boundary layer and free troposphere. “Pristine” PNSDs with low aerosol concentrations occur 12 %–45 % of the time, most commonly at Halley, located at low altitudes and far from the coastal melting ice and influenced by air masses from the west. Not only the sea spray primary aerosols and gas to particle secondary aerosol sources, but also the different air masses impacting the research stations should be kept in mind when deliberating upon different aerosol precursor sources across research stations. We infer that both primary and secondary components from pelagic and sympagic regions strongly contribute to the annual seasonal cycle of Antarctic aerosols. Our simultaneous aerosol measurements stress the importance of the variation in atmospheric biogeochemistry across the Antarctic region.

Sea-air methane flux was measured directly by the eddy-covariance method across approximately 60,000 km of Arctic and Antarctic cruises during a number of summers. The Arctic Ocean (north of 60 degrees N, between 20 degrees W and 50 degrees E) and Southern Ocean (south of 50 degrees S, between 70 degrees W and 30 degrees E) are found to be on-shelf sources of atmospheric methane with mean sea-air fluxes of 9.17 +/- 2.91 (SEM (standard error of the mean)) mu mol m-2 d-1 and 8.98 +/- 0.91 mu mol m-2 d-1, respectively. Off-shelf, this region of the Arctic Ocean is found to be a source of methane (mean flux of 2.39 +/- 0.68 mu mol m-2 d-1), while this region of the Southern Ocean is found to be a methane sink (mean flux of -0.77 +/- 0.37 mu mol m-2 d-1). The highest fluxes observed are found around west Svalbard, South Georgia, and South Shetland Islands and Bransfield Strait; areas with evidence of the presence of methane flares emanating from the seabed. Hence, this study may provide evidence of direct emission of seabed methane to the atmosphere in both the Arctic and Antarctic. Comparing with previous studies, the results of this study may indicate an increase in sea-air flux of methane in areas with seafloor seepage over timescales of several decades. As climate change exacerbates rising water temperatures, continued monitoring of methane release from polar oceans into the future is crucial. The amount of methane released from oceans into the atmosphere is uncertain. Most oceanic methane is stored in the seabed and can escape into the water at seafloor seeps, but the extent to which it escapes into the atmosphere remains unclear. This study uses a relatively new method, eddy-covariance, to measure sea-air methane fluxes during Arctic and Antarctic cruises. This is the first time this technique has been applied to sea-air methane fluxes in both polar oceans. Our findings show that on-shelf regions of the Arctic and Southern Oceans release methane into the atmosphere, with average fluxes of 9.17 +/- 2.91 mu mol m-2 d-1 and 8.98 +/- 0.91 mu mol m-2 d-1, respectively. We also identified areas with significant methane release in regions where methane has been found seeping into the water from the seabed. This study provides potential evidence that methane from seabed seeps may be directly emitted into the atmosphere in both the Arctic and Antarctic. Comparing with earlier studies, there is indication that the amount of methane released has increased over the last decades. As climate change drives increasing water temperatures, there is a potential for increased methane release from the seabed into the atmosphere, therefore on-going observations of methane release from polar oceans are necessary. Coastal regions of the Arctic Ocean and Southern Ocean are found to be sources of atmospheric methane The ocean releases elevated quantities of methane where seabed methane seeps have been observed in both the Arctic and Southern Oceans This study suggests a possible rise in methane release from oceanic areas with seabed seeps over last decades compared with a prior study

Atmospheric methane (CH4) concentrations have more than doubled since the beginning of the industrial era, making methane the second most important anthropogenic greenhouse gas after carbon dioxide (CO2). Fossil fuel extraction is one of the major anthropogenic methane sources as it is estimated to account for 22 % of global CH4 emissions. However, studies indicate that inventories underestimate emissions arising from the oil and gas industry. In two airborne field campaigns carried out in spring 2018 and 2019 offshore gas facilities in the Southern North Sea were probed. A total of nine research flights were conducted to characterize platform emissions. The Twin Otter research aircraft, operated by the British Antarctic Survey, was equipped with a high-precision 10 Hz analyzer (Picarro) to continuously measure CH4 and CO2. In order to identify fossil fuel emissions ethane (C2H6) was simultaneously measured with a 1 Hz TILDAS instrument (Aerodyne Research, Inc). On offshore oil and gas platforms methane is emitted by leakage, venting or flaring. To catch the methane plume, stacked transects were flown downwind of single platforms or platform complexes. Methane fluxes were calculated for six British and four Dutch facilities using the mass balance method. Correlations with C2H6 and CO2 were found with the latter indicating partly combusted methane from flaring. Uncertainties of fluxes arise mainly due to uncertainty of the wind measurement and the plume height. The calculated fluxes were compared to emissions reported to inventories (UK National Atmospheric Emissions Inventory (NAEI), UK Environmental and Emissions Monitoring System database (EEMS), Scarpelli inventory (2016)) and individually reported emissions from Dutch operators.

Antarctica and the Southern Ocean (SO) are the most pristine areas of the globe and represent ideal places to investigate aerosol–climate interactions in an unperturbed atmosphere. In this study, we present submicrometer aerosol (PM1) source apportionment for two sample sets collected in parallel at the British Antarctic Survey stations of Signy and Halley during the austral summer of 2018–2019. Water-soluble organic matter (WSOM) is a major aerosol component at both sites (37 % and 29 % of water-soluble PM1, on average, at Signy and Halley, respectively). Remarkable differences between pelagic (open-ocean) and sympagic (influenced by sea ice) air mass histories and related aerosol sources are found. The application of factor analysis techniques to series of spectra obtained by means of proton-nuclear magnetic resonance (H-NMR) spectroscopy on the samples allows the identification of five organic aerosol (OA) sources: two primary organic aerosol (POA) types, characterized by sugars, polyols, and degradation products of lipids and associated with open-ocean and sympagic/coastal waters, respectively; two secondary organic aerosol (SOA) types, one enriched in methanesulfonic acid (MSA) and dimethylamine (DMA) and associated with pelagic waters and the other characterized by trimethylamine (TMA) and linked to sympagic environments; and a fifth component of unclear origin, possibly associated with the atmospheric aging of primary emissions. Overall, our results strongly indicate that the emissions from sympagic and pelagic ecosystems affect the variability in the submicrometer aerosol composition in the study area, with atmospheric circulation establishing marked latitudinal gradients only for some of the aerosol components (e.g., the sympagic components) while distributing the others (e.g., pelagic and/or aged components) both in maritime and inner Antarctic regions.

<p>In the polar regions, the usual OH radical formation pathway (ozone photolysis and reaction of O(<sup>1</sup>D) with H<sub>2</sub>O) is limited by the low water vapour concentration. However, gases emitted from the snowpack can be pre-cursors of HO<em><sub>x</sub></em> radicals and ozone, thereby controlling the oxidising capacity of the lower atmosphere above remote snow-covered regions.</p> <p>Snowpack photolysis of nitrate and the resulting emissions of the reactive nitrogen species NO<em><sub>x</sub></em> and HONO can lead to OH production through rapid cycling of RO<sub>2</sub> → HO<sub>2</sub> → OH and photolysis of HONO. Research into reactive nitrogen species in polar environments has focused on NO<em><sub>x</sub></em>, with far fewer investigations into HONO. Previous studies of HONO in the polar boundary layer and snowpack interstitial air suggest a photolytic snowpack source but the exact mechanism for HONO production is poorly understood; photochemical models of HONO sources and sinks often cannot be reconciled with the measured HONO concentrations.</p> <p>A LOng Path Absorption Photometer (LOPAP) was used to investigate the net HONO flux density above snow in the Clean Air Sector at Halley VI Research Station in coastal Antarctica during Austral summer 2021/22. We present amount fraction measurements of HONO in ambient air, as well as measurements of the HONO flux density between the snow and atmosphere by the flux-gradient method. The potential snowpack reactions driving this HONO release are discussed, as well as the implications of these measurements for the HO<em><sub>x</sub></em> budget. These findings help further our understanding of the atmospheric budget of reactive nitrogen and highlight the significant effects snow surfaces can have on the atmospheric chemistry in the boundary layer above.</p>

<p>Atmospheric methane (CH<sub>4</sub>) is a potent greenhouse gas with natural and anthropogenic sources. Concentrations have been significantly increasing over the past few decades, which poses a problem for future climate change goals. The contribution of oceans to the global atmospheric CH<sub>4</sub> cycle is largely uncertain. It is accepted that oceans act as a small net source of atmospheric CH<sub>4</sub>. As the polar regions are warming faster than the global average, it is important that we can better quantify CH<sub>4</sub> emissions from the polar oceans. </p> <p><br />In this study, we combine various forms of shipborne data (ambient atmospheric methane concentrations, sea-air CH<sub>4</sub> fluxes and isotopic composition of atmospheric CH<sub>4</sub>) taken during cruises in the Arctic and Southern Oceans to present a more complete picture of atmospheric CH<sub>4</sub> above polar oceans, including addressing the question of how much the oceanic component is contributing towards the atmospheric budget in these regions. Measurements  were made around the Barents Sea and Greenland Sea in the Arctic, and in the Atlantic sector of the Southern Ocean, including the Scotia Sea.</p> <p><br />Sea-air CH<sub>4</sub> fluxes are measured using the eddy covariance method; indeed, this the first study to use this technique to directly measure how much CH<sub>4</sub> is released from the ocean into the atmosphere in both the Southern and Arctic Oceans. Atmospheric CH<sub>4</sub> measurements are then investigated in order to understand the impact that CH<sub>4</sub> released from the ocean has on the atmospheric burden. We also measure the isotopic composition of CH<sub>4</sub> (δ<sup>2</sup>H and δ<sup>13</sup>C) in air samples taken onboard polar cruises, to understand the sources of atmospheric CH<sub>4</sub> above these oceans. The isotope measurements can indicate if the CH<sub>4</sub> comes from a biogenic or thermogenic source, which can help determine if anthropogenic or natural processes are behind the production.</p> <p><br />We investigate the potential sources of CH<sub>4</sub> released by the polar ocean by looking at areas of known seabed CH<sub>4</sub> seepages, investigating phytoplankton abundance, and investigating the isotopic composition of atmospheric CH<sub>4</sub> in areas of elevated CH<sub>4</sub>. </p> <p><br />We find that the region of the Arctic Ocean investigated in this study is a slight atmospheric CH<sub>4</sub> source in boreal summer, while the region of the Southern Ocean investigated is a CH<sub>4</sub> source in areas of shallower water/continental shelves and a CH<sub>4</sub> sink in region of open ocean, in austral summer. This finding is consistent with previous studies that have detected seabed CH<sub>4</sub> emission. Seabed CH<sub>4</sub> seepage at shallower depths is more likely to penetrate the sea-air interface, while CH<sub>4</sub> produced at the seabed at deeper depths gets oxidised as it travels through the water column, making it less likely to reach the surface . We also find evidence of localised “hot spots” of methane emission which will be described. </p>

Measurements of atmospheric nitrous acid (HONO) amount fraction and flux density above snow were carried out using a long-path absorption photometer at Halley station in coastal Antarctica between 22 January and 3 February 2022. The mean ±1σ HONO amount fraction was (2.1 ± 1.5) pmol mol−1 and showed a diurnal cycle (range of 1.0–3.2 pmol mol−1) with a maximum at solar noon. These HONO amount fractions are generally lower than have been observed at other Antarctic locations. The flux density of HONO from the snow, measured between 31 January and 1 February 2022, was between 0.5 and 3.4×1012 m-2s-1 and showed a decrease during the night. The measured flux density is close to the calculated HONO production rate from photolysis of nitrate present in the snow. A simple box model of HONO sources and sinks showed that the flux of HONO from the snow makes a >10 times larger contribution to the HONO budget than its formation through the reaction of OH and NO. Ratios of these HONO amount fractions to NOx measurements made in summer 2005 are low (0.15–0.35), which we take as an indication of our measurements being comparatively free from interferences. Further calculations suggest that HONO photolysis could produce up to 12 pmolmol-1h-1 of OH, approximately half that produced by ozone photolysis, which highlights the importance of HONO snow emissions as an OH source in the atmospheric boundary layer above Antarctic snowpacks.

The British Antarctic Survey (BAS) operates one of the most remote, advanced, and scientifically important research stations on the Antarctic continent – Halley. Located on the floating Brunt ice shelf, the station has provided meteorological and atmospheric observations since it was established in 1956. However, in the face of glaciological uncertainty, Halley Research Station had to close for the first time in its history during winter 2017. To overcome the subsequent data loss from the unmanned research station, engineering and science teams at BAS began automating the station.In 2018-19, the Halley automation project began with scientific equipment adapted and the installation of an innovative micro-turbine electrical generator. Science experiments ran uninterrupted throughout the nine-month winter period, with the station preserving core science data streams such as Meteorology and Ozone Monitoring, Tropospheric Chemistry and Climate, and Space Weather and Upper Atmospheric Observations. The system proved its ability to withstand the Antarctic environment during the 2019 winter; unaffected by ambient temperatures as low as -55˚C and winds gusting up to 70 knots.Work is ongoing to automate and reinstate the long-term atmospheric monitoring experiments at Halley. In December 2021, a new automated CO2 and CH4 analyser was installed in Halley’s Clean Air Sector (CAS) laboratory which will run continuously over the coming Antarctic winter. Halley’s coastal location provides an ideal platform to explore air-sea CO2 exchange in the Southern Ocean region. The Southern Ocean is a globally important carbon sink, estimated to account for ~75% of global ocean CO2 uptake but a sparsity of observations in the region has contributed to uncertainty around the inter-annual and seasonal nature of the Southern Ocean sink.CO2 mixing ratios have been measured at Halley at high temporal resolution since 2013. Before the installation of the new autonomous system at Halley, measurements were relocated to the German coastal Antarctic research station, Neumayer, at the end of 2017. Both the Halley and Neumayer records show short-term variability in CO2 mixing ratios during the summer, with up to ~0.5 ppb decreases in CO2 over the course of a day, about 1/6 of the average annual growth rate. Trajectory analysis suggests that these decreases in mixing ratio correspond to periods where the air sampled has spent time over the Southern Ocean, suggesting CO2 uptake has occurred. This work will explore the possible drivers for the short-term variability in CO2 mixing ratios. An overview of the automation work carried out so far at Halley and plans for future seasons will also be presented.

Atmospheric aerosols are important drivers of Arctic climate change through aerosol-cloud-climate interactions. However, large uncertainties remain on the sources and processes controlling particle numbers in both fine and coarse modes. Here, we applied a receptor model and an explainable machine learning technique to understand the sources and drivers of particle numbers from 10 nm to 20 μm in Svalbard. Nucleation, biogenic, secondary, anthropogenic, mineral dust, sea salt and blowing snow aerosols and their major environmental drivers were identified. Our results show that the monthly variations in particles are highly size/source dependent and regulated by meteorology. Secondary and nucleation aerosols are the largest contributors to potential cloud condensation nuclei (CCN, particle number with a diameter larger than 40 nm as a proxy) in the Arctic. Nonlinear responses to temperature were found for biogenic, local dust particles and potential CCN, highlighting the importance of melting sea ice and snow. These results indicate that the aerosol factors will respond to rapid Arctic warming differently and in a nonlinear fashion.

Methane (CH 4 ) mole fractions from the large semiseasonal Llanos de Moxos wetlands (∼70,000 km 2 ) in northern Bolivia were measured by aircraft flights and ground sampling during early March 2019 (late wet season). Daily fluxes of CH 4 determined from the measurements using box models and inverse modeling were between 168 (± 50) and 456 (± 145) mg CH 4 ⋅m −2 ⋅d −1 for the areas overflown, very high compared with those of previous Amazon basin studies. If the seasonality of the CH 4 emissions is comparable to other parts of the Amazon Basin, the region could contribute as much as 8% of annual Amazonian CH 4 emissions.

The Southern Ocean plays a fundamental role in in the global carbon cycle and is estimated to absorb ~40% of anthropogenic carbon-dioxide (CO2) emissions. Recent studies have highlighted the potentially large decadal variability of this uptake, and the uncertainties associated with estimates derived from different ocean carbon measurement technologies. The majority of these estimates of Southern Ocean CO2 uptake are commonly derived from ‘bottom-up’ analyses of oceanic carbon measurements. An independent means of estimating air-sea CO2 fluxes is provided by ‘top-down’ analyses, which employ inverse methods or data assimilation techniques combining atmospheric CO2 measurements with numerical transport model analyses. Robust regional flux estimates from such top-down methods require an atmospheric observational network with sufficient spatial coverage. At present, however, long-term measurements of atmospheric CO2 are only available at a limited number of sites in the Southern Ocean region. Given this sparse atmospheric sampling there is an urgent need for expansion of the current Southern Ocean atmospheric CO2 measurement network. The British Antarctic Survey has identified a number of locations (including the sub-Antarctic and South Atlantic Islands of Tristan da Cunha, South Georgia and the Falklands) where new systems for long-term observations of CO2 could be established. In this analysis we present results from a set of Observing System Sampling Experiments (OSSEs) using the GEOS-Chem atmospheric transport model, in combination with the Local Ensemble Transform Kalman Filter method (Chen et al. 2021) to identify the effectiveness of these locations towards providing improved constraints on Southern Ocean air-sea fluxes. Our assessment of potential sampling sites is derived from metrics quantifying the uncertainty reduction of regional oceanic CO2 flux estimates. References Chen et al. (2021) Variability of North Atlantic CO2 fluxes for the 2000–2017 period estimated from atmospheric inverse analyses. Biogeosciences, 18 (15). pp. 4549-4570. ISSN 1726-4189.

We report methane isotopologue data from aircraft and ground measurements in Africa and South America. Aircraft campaigns sampled strong methane fluxes over tropical papyrus wetlands in the Nile, Congo and Zambezi basins, herbaceous wetlands in Bolivian southern Amazonia, and over fires in African woodland, cropland and savannah grassland. Measured methane δ13CCH4 isotopic signatures were in the range −55 to −49‰ for emissions from equatorial Nile wetlands and agricultural areas, but widely −60 ± 1‰ from Upper Congo and Zambezi wetlands. Very similar δ13CCH4 signatures were measured over the Amazonian wetlands of NE Bolivia (around −59‰) and the overall δ13CCH4 signature from outer tropical wetlands in the southern Upper Congo and Upper Amazon drainage plotted together was −59 ± 2‰. These results were more negative than expected. For African cattle, δ13CCH4 values were around −60 to −50‰. Isotopic ratios in methane emitted by tropical fires depended on the C3 : C4 ratio of the biomass fuel. In smoke from tropical C3 dry forest fires in Senegal, δ13CCH4 values were around −28‰. By contrast, African C4 tropical grass fire δ13CCH4 values were −16 to −12‰. Methane from urban landfills in Zambia and Zimbabwe, which have frequent waste fires, had δ13CCH4 around −37 to −36‰. These new isotopic values help improve isotopic constraints on global methane budget models because atmospheric δ13CCH4 values predicted by global atmospheric models are highly sensitive to the δ13CCH4 isotopic signatures applied to tropical wetland emissions. Field and aircraft campaigns also observed widespread regional smoke pollution over Africa, in both the wet and dry seasons, and large urban pollution plumes. The work highlights the need to understand tropical greenhouse gas emissions in order to meet the goals of the UNFCCC Paris Agreement, and to help reduce air pollution over wide regions of Africa. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.

We present 20-year flask sample records of atmospheric CO2, δ(O2/N2), and atmospheric potential oxygen (APO) from the stations Lutjewad (the Netherlands) and Mace Head (Ireland), and a 3-year record from Halley station (Antarctica). We include details of our calibration procedures and the stability of our calibration scale over time, which we estimate to be 3 per meg over the 11 years of calibration, and our compatibility with the international Scripps O2 scale. The measurement records from Lutjewad and Mace Head show similar long-term trends during the period 2002–2018 of 2.31 ± 0.07 ppm yr−1 for CO2 and −21.2 ± 0.8 per meg yr−1 for δ(O2/N2) at Lutjewad, and 2.22 ± 0.04 ppm yr−1 for CO2 and −21.3 ± 0.9 per meg yr−1 for δ(O2/N2) at Mace Head. They also show a similar δ(O2/N2) seasonal cycle with an amplitude of 54 ± 4 per meg at Lutjewad and 61 ± 5 per meg at Mace Head, while the CO2 seasonal amplitude at Lutjewad (16.8 ± 0.5 ppm) is slightly higher than that at Mace Head (14.8 ± 0.3 ppm). We show that the observed long-term trends and seasonal cycles are in good agreement with the measurements from various other stations, especially the measurements from the Weybourne Atmospheric Observatory (United Kingdom). However, there are remarkable differences in the progression of annual trends between the Mace Head and Lutjewad records for δ(O2/N2) and APO, which might in part be caused by sampling differences, but also by environmental effects, such as North Atlantic Ocean oxygen ventilation changes to which Mace Head is more sensitive. The Halley record shows clear trends and seasonality in δ(O2/N2) and APO, the latter agreeing especially well with continuous measurements at the same location made by the University of East Anglia (UEA), while CO2 and δ(O2/N2) present slight disagreements, most likely caused by small leakages during sampling. From our 2002–2018 records, we find a good agreement with Global Carbon Budget 2021 (Friedlingstein et al. (2021) for the global ocean carbon sink: 2.1 ± 0.8 PgC yr−1, based on the Lutjewad record. The data presented in this work are available at https://doi.org/10.18160/qq7d-t060 (Nguyen et al., 2021).

The atmospheric methane (CH 4 ) burden is rising sharply, but the causes are still not well understood. One factor of uncertainty is the importance of tropical CH 4 emissions into the global mix. Isotopic signatures of major sources remain poorly constrained, despite their usefulness in constraining the global methane budget. Here, a collection of new δ 13 C CH 4 signatures is presented for a range of tropical wetlands and rice fields determined from air samples collected during campaigns from 2016 to 2020. Long-term monitoring of δ 13 C CH 4 in ambient air has been conducted at the Chacaltaya observatory, Bolivia and Southern Botswana. Both long-term records are dominated by biogenic CH 4 sources, with isotopic signatures expected from wetland sources. From the longer-term Bolivian record, a seasonal isotopic shift is observed corresponding to wetland extent suggesting that there is input of relatively isotopically light CH 4 to the atmosphere during periods of reduced wetland extent. This new data expands the geographical extent and range of measurements of tropical wetland and rice δ 13 C CH 4 sources and hints at significant seasonal variation in tropical wetland δ 13 C CH 4 signatures which may be important to capture in future global and regional models. This article is part of a discussion meeting issue ‘Rising methane: is warming feeding warming? (part 2)’.

Perfluoroalkyl acids (PFAAs) are synthetic chemicals with a variety of industrial and consumer applications that are now widely distributed in the global environment. Here, we report the measurement of six perfluorocarboxylates (PFCA, C4-C9) in a firn (granular compressed snow) core collected from a non-coastal, high-altitude site in Dronning Maud Land in Eastern Antarctica. Snow accumulation of the extracted core dated from 1958 to 2017, a period coinciding with the advent, use, and geographical shift in the global industrial production of poly/perfluoroalkylated substances, including PFAA. We observed increasing PFCA accumulation in snow over this time period, with chemical fluxes peaking in 2009-2013 for perfluorooctanoate (PFOA, C8) and nonanoate (PFNA, C9) with little evidence of a decline in these chemicals despite supposed recent global curtailments in their production. In contrast, the levels of perfluorobutanoate (PFBA, C4) increased markedly since 2000, with the highest fluxes in the uppermost snow layers. These findings are consistent with those previously made in the Arctic and can be attributed to chlorofluorocarbon replacements (e.g., hydrofluoroethers) as an inadvertent consequence of global regulation.

The North Sea is Europe's key oil and gas (O&G) basin with the output currently meeting 3 %–4 % of global oil supply. Despite this, there are few observational constraints on the nature of atmospheric emissions from this region, with most information derived from bottom-up inventory estimates. This study reports on airborne measurements of volatile organic compounds (VOCs) emitted from O&G-producing regions in the North Sea. VOC source emission signatures for the primary extraction products from offshore fields (oil, gas, condensate, mixed) were determined in four geographic regions. Measured iso-pentane to n-pentane (iC5 / nC5) ratios were 0.89–1.24 for all regions, used as a confirmatory indicator of O&G activities. Light alkanes (ethane, propane, butane, pentane) were the dominant species emitted in all four regions; however, total OH reactivity was dominated by unsaturated species, such as 1,3-butadiene, despite their relatively low abundance. Benzene to toluene ratios indicated the influence of possible terrestrial combustion sources of emissions in the southern, gas-producing region of the North Sea, seen only during south or south-westerly wind episodes. However, all other regions showed a characteristic signature of O&G operations. Correlations between ethane (C2H6) and methane (CH4) confirmed O&G production to be the primary CH4 source. The enhancement ratio (ΔC2H6/ΔCH4) ranged between 0.03–0.18, indicating a spatial dependence on emissions with both wet and dry CH4 emission sources. The excess mole fraction demonstrated that deepwater oil extraction resulted in a greater proportion of emissions of higher carbon number alkanes relative to CH4, whereas gas extraction, typically from shallow waters, resulted in a less complex mix of emissions dominated by CH4. The VOC source profiles measured were similar to those in the UK National Atmospheric Emissions Inventory (NAEI) for oil production, with consistency between the molar ratios of light alkanes to propane. The largest discrepancies between observations and the inventory were for mono-aromatic compounds, highlighting that these species are not currently fully captured in the inventory. These results demonstrate the applicability of VOC measurements to distinguish unique sources within the O&G sector and give an overview of VOC speciation over the North Sea.

Methane is the second-most important anthropogenic greenhouse gas after carbon dioxide. The atmospheric burden is rising rapidly. CH4 growth from about 720 ppb in pre-industrial times to nearly 1900 ppb now has predominantly been caused by human activity. This is proportionately a much greater rise than the increase in CO2. With a direct heating impact of about 0.5 Wm−2 and around 1 Wm−2 if indirect impacts are included, the climate warming consequences of anthropogenic methane emissions are very important. To consider recent methane studies in the UK, and related work by others internationally, the Royal Society scheduled a Discussion Meeting in October 2020. That meeting was postponed because of the Covid-19 pandemic, but rescheduled to 4–7 October 2021. ‘MOYA’, the UK’s Natural Environment Research Council’s research program on the Global Methane Budget (2016–2021), was a wide-ranging study of all aspects of atmospheric methane. The work included in situ measurement, for example, measuring 13C in methane, at remote locations such as Ascension Island and a wide range of field campaigns on the ground and in aircraft. This research has been complemented by satellite remote sensing and ground-based measurement of the vertical methane column. Linked to MOYA was ZWAMPS, studying the Upper Congo and Zambezi wetlands of Zambia. Modelling studies have been wide-ranging, including trajectory-based studies and