Given its unique properties as a radioactive chemically inert gas, radon can act as a valuable atmospheric tracer, for evaluating the performance of atmospheric transport models to calculate the sources of trace gases to the atmosphere. A radon flux map is the scientific starting point for simulating atmospheric radon concentrations using atmospheric transport models. As such, it is important to assess the available high resolution radon flux maps to ensure that simulated concentrations can be accurately interpreted. The spatial fluxes of radon primarily depend on soil and rock types, while temporal variations are influenced by soil moisture content.The recent advancements in generating two high-resolution radon flux maps for Europe using two different soil moisture reanalysis, GLDAS Noah and the ERA5 maps1, have significantly enhanced our understanding of radon flux dynamics. Yet, the radon flux values diverge notably between these two maps and sometimes these variations can be substantial, with differences as large as the absolute radon flux itself.In our work, two available versions of European radon flux maps are coupled with two Lagranian particle dispersion models – the Met Office’s Numerical Atmospheric Modelling Environment (NAME) and the FLEXPART model – are used to simulate radon concentrations measured at four tall tower sites in the United Kingdom: Heathfield, Ridge Hill, Tacolneston and Weybourne. We calculate the differences between the modelled radon concentrations to the observed radon concentrations at these sites and use this to investigate the sensitivity of two radon flux maps: GLDAS Noah and ERA5. References: 12022: https://doi.org/10.18160/2ST9-3NAD

Atmospheric trace gas measurements can be used to independently assess national greenhouse gas inventories through inverse modelling. Here, atmospheric nitrous oxide (N2O) measurements are used to derive monthly U.K. N2O emissions for 2013-2022 – using the InTEM and RHIME inverse methods – and Swiss N2O emissions for 2017-2022 – using the ELRIS inverse method. We find mean U.K. emissions of 90.5±23.0 and 111.7±32.1 Gg N2O yr-1 for 2013-2022 and corresponding trends of -0.68±0.48 and -2.10±0.72 Gg N2O yr-2, respectively, derived using InTEM and RHIME. The 2013-2022 mean U.K. N2O emissions as reported by the U.K. National Atmospheric Emissions Inventory were relatively constant at 74 Gg N2O yr-1 across this period, which is 14-33% smaller than the U.K. emissions derived from atmospheric data. Top-down Swiss emissions of 10.8±3.8 Gg N2O yr-1 derived using atmospheric measurements were very comparable to those reported in the Swiss National Inventory: 11.5 (8.3 to 14.9) Gg N2O yr-1 over 2017-2021. Pronounced seasonal N2O emissions cycles are inferred in the U.K. and Swiss data with similar seasonal magnitudes observed in both countries. In the U.K., the primary seasonal peak occurs in the spring with a second smaller peak occurring in the late summer for certain years. The springtime peak has a long seasonal decline that contrasts with the sharp rise and fall of N2O emissions estimated from the bottom-up U.K. Emissions Model (UKEM). Similarly, Swiss seasonal N2O emissions peak during the summer with a second smaller peak also occurring in the late summer/early autumn for certain years. Bayesian inference is used to minimize the U.K. seasonal cycle mismatch between the average top-down (atmospheric data-based) and UKEM bottom-up (process model and inventory-based) seasonal emissions at a sub-sector level. Increasing agricultural manure management and decreasing synthetic fertiliser N2O emissions reduces some of the discrepancy between the average U.K. top-down and bottom-up seasonal cycles. Other possibilities could also explain these discrepancies, such as missing emissions from NH3 deposition, but these require further investigation.

HFC-134a is the most prevalent hydrofluorocarbon used as a replacement for ozone-depleting CFCs and HCFCs. Due to its high global warming potential, it is regulated under various European and global frameworks, underscoring the importance of tracking its emissions. Emissions derived by the commonly used, bottom-up, methodology are affected by a certain degree of uncertainty. The bottom-up estimates can be aided with an independent top-down estimate based on atmospheric observations combined with an atmospheric transport model. This study presents HFC-134a emissions for Europe, with a specific focus on Italy, from 2008 to 2023. The emissions were estimated using a Bayesian inversion methodology, based on atmospheric observations collected at four European stations. Our analysis reveals a slightly increasing trend in HFC-134a emissions for Italy from 2008 to 2015 of 0.17 Gg yr^-1 , followed by a steady decrease thereafter, highlighting the effect of European regulation on fluorinated gases that came into force in 2014. We observed a reduction in HFC-134a emissions in the Po Basin inferred from the inversion method for 2020, likely due to mobility restrictions imposed during the COVID-19 pandemic. The observed mild seasonality in emissions may be partly attributed to higher air-conditioning activity during summer. Comparison with the Italian National Emission Inventory indicates an improvement in iterative bottom-up estimates, with the 2024 inventory emission trend post-2015 aligning closely with our inversion results. This study emphasises the need for collaboration between the two independent approaches to enhance the accuracy of emission estimates. Such cooperation is crucial to narrowing the gap in quantifying emissions of potent greenhouse gases and effectively assessing the progress of international policies and regulations.

We present a protocol to improve confidence in reported radon activity concentrations, facilitating direct site-to-site comparisons and integration with co-located greenhouse gas (GHG) measurements within a network of three independently managed observatories in the UK. Translating spot measurements of atmospheric GHG amount fractions into regional flux estimates (“top-down” analysis) is usually performed with atmospheric transport models (ATMs), which calculate the sensitivity of regional emissions to changes in observed GHGs at a finite number of locations. However, the uncertainty of regional emissions is closely linked to ATM uncertainties. Radon, emitted naturally from the land surface, can be used as a tracer of atmospheric transport and mixing to independently evaluate the performance of such models. To accomplish this, the radon measurements need to have a comparable precision to the GHGs at the modelled temporal resolution. Australian Nuclear Science and Technology Organisation (ANSTO) dual-flow-loop two-filter radon detectors provide output every 30 min. The measurement accuracy at this temporal resolution depends on the characterization and removal of instrumental background, the calibration procedure, and response time correction. Consequently, unless these steps are standardized, measurement precision may differ between sites. Here we describe standardized approaches regarding (1) instrument maintenance, (2) quality control of the raw data stream, (3) determination and removal of the instrumental background, (4) calibration methods, and (5) response time correction (by deconvolution). Furthermore, we assign uncertainties for each reported 30 min radon estimate (assuming these steps have been followed) and validate the final result through comparison of diurnal and sub-diurnal radon characteristics with co-located GHG measurements. While derived for a network of UK observatories, the proposed standardized protocol could be equally applied to two-filter dual-flow-loop radon observations across larger networks, such as the Integrated Carbon Observation System (ICOS) or the Global Atmosphere Watch (GAW) baseline network.

Atmospheric transport model (ATM) uncertainty continues to be a significant constraining factor in making confident top-down (inverse model based) GHG emission estimates. Despite its importance, accurately gauging model uncertainty and capturing its temporal fluctuations remains a challenge. Inversion frameworks typically involve an empirical selection of data to be assimilated whereby only the data from periods where the ATM has the lowest uncertainties are used for the inversion. There are numerous data filtering methods, that often depend on modelled parameters (mixing height, wind speed, potential temperature), which could result in data selection bias.To address this, we present analysis of radon measurements, a natural radioactive noble gas with simple and well-constrained source and sink. Radon’s unique characteristics make it an ideal tracer to study the transport and mixing of air and thus has potential to act as an independent metric to evaluate ATM performance. A new approach involves utilising measured and modelled radon (calculated using the Met Office Numerical Atmospheric Modelling Environment (NAME) dispersion model and radon flux map) to classify the ATM output uncertainty as either high (poor performance) or low (the best performance). This approach could be universally applied to any location measuring radon from a single inlet height and in conjunction with any other dispersion modelling scenarios. To evaluate the effectiveness of the radon selection method, we assess the methane (CH4) emissions across the UK using four tall tower sites (part of the Deriving Emissions linked to Climate Change - DECC network): Heathfield, Ridge Hill, Tacolneston and Weybourne. The CH4 emissions are estimated by the Met Office’s inversion modelling system – Inversion Technique for Emission Modelling (InTEM). We will compare how emissions sensitivity varies between our radon-based approach and the current selection method, which relies on model parameters and the vertical gradient of CH4 measurements. This comparative analysis aims to demonstrate the potential advantages of using radon as a tool for improving the accuracy of ATM performance assessments in GHG emission estimates.

Greenhouse gas (GHG) measurements are crucial for understanding climate change. Therefore, the World Meteorological Organisation (WMO) identified them as critical for global monitoring. There is a need for an infrastructure that can provide traceable atmospheric measurements to underpin the fulfilment of internationally agreed emissions reduction targets. High accuracy gas Reference Materials (RMs) are required to underpin GHG composition measurements for long term temporal and spatial trend analysis. These are prepared by compressing whole-air into high pressure cylinders for direct use following measurement by a central calibration laboratory (CCL) to provide traceability to international scales. A span of GHG amount fractions targeted for assessing instrument linearity can be made by addition of pure gases or removal from whole-air. Synthetic RMs, traceable to the SI, prepared gravimetrically from individual components to a target amount fraction, offer a potential alternative way to compare instrument performance without the difficulties of preparing certified whole air standards. However, use of synthetic RMs present their own challenges, namely in adequately matching the matrix to atmospheric composition in order to prevent measurement biases from pressure broadening when using techniques such as Cavity Ring Down Spectroscopy (CRDS). The UK-DECC network is comprised of tall tower monitoring sites measuring amount fractions of major GHGs using whole-air calibrated CRDS instruments. Whole-air RMs have been previously used to understand site-to-site differences and study possible instrumental reasons for differences. However, the use of synthetic-air RMs to quantify variations between monitoring sites in reported amount fractions has not been reported previously for this network. In this work a set of synthetic-air and whole-air RMs containing CH4, CO2 and N2O were prepared and measured in an intercomparison at UK-DECC network sites. The results show promise for the use of synthetic RMs in efficiently explaining measurement offsets across a network. Synthetic RMs, prepared appropriately, offer a potentially cost effective and more convenient route to help quality control atmospheric monitoring programmes.

Hydrofluorocarbons (HFCs) are potent greenhouse gases whose global abundance continues to rise and subsequently warm the Earth. Southern China is a rapidly developing region that has experienced a sharp increase in its HFC consumption. Here, we present the first high-frequency HFC observations in Southern China from 2022 to 2023, analyzing the atmospheric mole fractions of four HFCs (HFC-134a, HFC-32, HFC-125, and HFC-143a) and using inverse modeling to estimate their emissions in Southern China. We find that HFC emissions in Southern China are primarily concentrated in Jiangsu, Zhejiang, and Guangdong, with Jiangsu having the highest HFC-134a emissions (4.1 ± 0.5 Gg yr-1, ± 1 standard deviation). HFC-125 and HFC-32 emissions are elevated in Anhui, Jiangsu, and Guangdong, while HFC-143a emissions are predominantly in Jiangsu and Zhejiang. From 2022-2023, HFC emissions in the Pearl River Delta are expected to increase, while in the Yangtze River Delta, HFC-134a, HFC-125, and HFC-32 emissions are 94.2% ± 54.6%, 200.9% ± 28.7%, and 187.5% ± 24.2% higher than 2012-2016 levels, respectively. The rise in HFC consumption and the delayed emissions from HFC banks in Southern China highlight the necessity of estimating HFC emissions. Our findings will support local emission reduction policies and contribute to global climate change efforts.

Sulfur hexafluoride (), nitrogen trifluoride (), and three perfluorocarbons (PFCs; , , and ‐) are perfluorinated greenhouse gases (PF‐GHGs) with long atmospheric lifetimes and high global warming potentials. Using high‐frequency observations and a Bayesian inversion framework, we assess 2021–2023 PF‐GHG emissions in southeastern China, a rapidly industrializing region. Total PF‐GHG emissions rise from 69.50 (55.16–84.97) in 2021 to 96.19 (69.53–127.14) Mt ‐eq in 2023, accounting for 21.74% of global total PF‐GHG emissions in 2023. emissions nearly double, reaching 0.94 (0.69–1.25) Gg , with an annual growth rate of 40.38%, likely driven by semiconductor industry expansion. (51.75%) and (30.86%) dominate 3‐year PF‐GHG growth. ‐ shows strong seasonality, with a potential winter increase linked to HCFC‐22 feedstock use. Seasonal peaks align with winter electricity demand, while variations suggest potential links with semiconductor industry.

Sulfur hexafluoride (SF 6 ) is a potent greenhouse gas. Here we use long-term atmospheric observations to determine SF 6 emissions from China between 2011 and 2021, which are used to evaluate the Chinese national SF 6 emission inventory and to better understand the global SF 6 budget. SF 6 emissions in China substantially increased from 2.6 (2.3-2.7, 68% uncertainty) Gg yr −1 in 2011 to 5.1 (4.8-5.4) Gg yr −1 in 2021. The increase from China is larger than the global total emissions rise, implying that it has offset falling emissions from other countries. Emissions in the less-populated western regions of China, which have potentially not been well quantified in previous measurement-based estimates, contribute significantly to the national SF 6 emissions, likely due to substantial power generation and transmission in that area. The CO 2 -eq emissions of SF 6 in China in 2021 were 125 (117-132) million tonnes (Mt), comparable to the national total CO 2 emissions of several countries such as the Netherlands or Nigeria. The increasing SF 6 emissions offset some of the CO 2 reductions achieved through transitioning to renewable energy in the power industry, and might hinder progress towards achieving China’s goal of carbon neutrality by 2060 if no concrete control measures are implemented.

Just under three hundred thousand hourly O3 observations from the Mace Head, Ireland atmospheric monitoring station have been assembled into a complete dataset covering the thirty five-year period from April 1, 1987 through to May 31, 2022. Of these, seventy thousand hourly observations were assigned to baseline air masses over the 422 months in the study. Annual mean baseline mixing ratios rose from 34 ppb in 1988 to a peak of 42 ppb in 1999 before declining to 39 ppb in 2021. Monthly mean baseline mixing ratios reached a peak of 53 ppb in April 1999. Baseline O3 mixing ratios exhibit a marked seasonal cycle, with maxima in April and minima in August. A detailed examination of the differences between the baseline and complete monthly mean O3 mixing ratios revealed a set of monthly differences with some striking long-term changes. Wintertime differences were dominated by a series of O3 depletion events in which, during the early years of the study, reduced ozone to near zero in long-range transport events. European NOx emission reductions have reduced significantly the number and severity of these wintertime O3 depletion events. Summertime differences were dominated by a series of regional photochemical episodes. In the early years of the record, the regional photochemical episodes were intense enough to raise the monthly means in the complete record above the baseline means. European volatile organic compound (VOC) and NOx precursor emission reductions have dramatically reduced the number and intensity of these events. Episodic peak O3 levels have also fallen steadily as a result of concerted, European action on regional VOC and NOx emissions. Subsets of the baseline and complete ozone datasets between 2017 and 2022 were scrutinised for possible COVID-19 impacts. Baseline ozone levels were lower than expected during 2020 and 2021 but the statistical significance of these impacts is difficult to judge, and reduced stratosphere-troposphere exchange rather than COVID-19 emission reductions may be the cause.

Nitrogen trifluoride (NF3) is a potent and long-lived greenhouse gas that is widely used in the manufacture of semiconductors, photovoltaic cells, and flat panel displays. Using atmospheric observations from eight monitoring stations from the Advanced Global Atmospheric Gases Experiment (AGAGE) and inverse modeling with a global 3-D atmospheric chemical transport model (GEOS-Chem), we quantify global and regional NF3 emission from 2015 to 2021. We find that global emissions have grown from 1.93 +/- 0.58 Gg yr(-1) (+/- one standard deviation) in 2015 to 3.38 +/- 0.61 Gg yr(-1) in 2021, with an average annual increase of 10% yr(-1). The available observations allow us to attribute significant emissions to China (0.93 +/- 0.15 Gg yr(-1) in 2015 and 1.53 +/- 0.20 Gg yr(-1) in 2021) and South Korea (0.38 +/- 0.07 Gg yr-1 to 0.65 +/- 0.10 Gg yr(-1)). East Asia contributes around 73% of the global NF3 emission increase from 2015 to 2021: approximately 41% of the increase is from emissions from China (with Taiwan included), 19% from South Korea, and 13% from Japan. For Japan, which is the only one of these three countries to submit annual NF3 emissions to UNFCCC, our bottom-up and top-down estimates are higher than reported. With increasing demand for electronics, especially flat panel displays, emissions are expected to further increase in the future.

U.K. top-down nitrous oxide (N2O) emissions estimates for 2013-2022 Top-down emissions were derived using the InTEM (Manning et al., 2021) and RHIME (Ganesan et al., 2014) inverse models with atmospheric N2O mole fraction measurements from the U.K. DECC network (O'Doherty et al., 2020). This dataset provides monthly top-down emissions estimates for the U.K. (land-only, sea-only, land+sea components) with 68% confidence interval ranges. A priori total monthly N2O emissions created by the U.K. Emissions Model are also provided. This dataset was used in the publication "Combining top-down and bottom-up approaches to evaluate recent trends and seasonal patterns in U.K. N2O emissions" that has been submitted to JGR: Atmospheres by Saboya et al. Further information about the generation of this dataset and its implications can be found in Saboya et al. (2024). Please note InTEM emissions data hold a Crown Copyright.

HFC-125 and HFC-32 are fluorinated greenhouse gases of great concern due to their high GWPs and increasing background atmospheric concentrations. Long-term atmospheric observations of HFC-125 and HFC-32 were carried out in four representative cities of China (Beijing, Guangzhou, Hangzhou, and Lanzhou) from January 2012 to October 2019. Overall, the annual mean atmospheric concentrations of HFC-125 and HFC-32 both showed increasing trends, with average rates of 4.8 ppt yr(-1) and 7.9 ppt yr(-1). The average concentrations of HFC-125 and HFC-32 in urban areas were significantly higher than those in suburban areas. Significant differences in atmospheric concentrations of the two HFCs were observed among the four cities. HFC-125 and HFC-32 emissions were estimated accordingly, averaging 6.2 Gg yr(-1) (23.6 Mt. CO2-eq) and 5.7 Gg yr(-1) (4.3 Mt. CO2-eq) during 2012 and 2019 and growing at rates of 0.8 Gg yr(-1) (3.1 Mt. CO2-eq) and 0.8 Gg yr(-1) (0.6 Mt. CO2-eq), respectively, with an increasing contribution to global radiative forcing. The bottom-up inventories of HFC-125 and HFC-32 in the four cities increased annually from 2012 to 2019, with the highest emissions in Beijing, while the top-down emissions fluctuated during the research period. Synopsis: The atmospheric concentrations of HFC-125 and HFC-32 were measured from 2012 to 2019 in four representative cities of China. Both HFC emissions at national and city levels were estimated using observationbased and inventory methods.

Atmospheric methane (CH4) is the second-most-important anthropogenic greenhouse gas and has a 20-year global warming potential 82 times greater than carbon dioxide (CO2). Anthropogenic sources account for ∼ 60 % of global CH4 emissions, of which 20 % come from oil and gas exploration, production and distribution. High-resolution satellite-based imaging spectrometers are becoming important tools for detecting and monitoring CH4 point source emissions, aiding mitigation. However, validation of these satellite measurements, such as those from the commercial GHGSat satellite constellation, has so far not been documented for active leaks. Here we present the monitoring and quantification, by GHGSat's satellites, of the CH4 emissions from an active gas leak from a downstream natural gas distribution pipeline near Cheltenham, UK, in the spring and summer of 2023 and provide the first validation of the satellite-derived emission estimates using surface-based mobile greenhouse gas surveys. We also use a Lagrangian transport model, the UK Met Office's Numerical Atmospheric-dispersion Modelling Environment (NAME), to estimate the flux from both satellite- and ground-based observation methods and assess the leak's contribution to observed concentrations at a local tall tower site (30 km away). We find GHGSat's emission estimates to be in broad agreement with those made from the in situ measurements. During the study period (March–June 2023) GHGSat's emission estimates are 236–1357 kg CH4 h−1, whereas the mobile surface measurements are 634–846 kg CH4 h−1. The large variability is likely down to variations in flow through the pipe and engineering works across the 11-week period. Modelled flux estimates in NAME are 181–1243 kg CH4 h−1, which are lower than the satellite- and mobile-survey-derived fluxes but are within the uncertainty. After detecting the leak in March 2023, the local utility company was contacted, and the leak was fixed by mid-June 2023. Our results demonstrate that GHGSat's observations can produce flux estimates that broadly agree with surface-based mobile measurements. Validating the accuracy of the information provided by targeted, high-resolution satellite monitoring shows how it can play an important role in identifying emission sources, including unplanned fugitive releases that are inherently challenging to identify, track, and estimate their impact and duration. Rapid, widespread access to such data to inform local action to address fugitive emission sources across the oil and gas supply chain could play a significant role in reducing anthropogenic contributions to climate change.

Top-down evaluation of the UK’s methane, nitrous oxide and halogenated greenhouse gas emissions has been possible since the 1990s, firstly due to measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE), combined later with the UK national-scale Deriving Emissions linked to Climate Change (UK DECC) network. Here, we show that carbon dioxide-equivalent emissions inferred from observations of these gases have declined more slowly than stated in the UK’s National Inventory Report (NIR); a decline of approximately 50 Tg/yr CO2-e is found between 1990 and 2021, compared to approximately 100 Tg/yr CO2-e in the inventory. This difference, which is driven largely by a smaller-than-reported reduction in methane emissions, suggests that the UK may be approximately 3 years behind its stated progress toward net-zero. This paper will describe the evolution of greenhouse gas monitoring in the UK, including an overview of the first decade of national-scale emissions estimation from the UK DECC network. It will show how top-down emissions are calculated, and how atmospheric observation-based estimates are used, in close collaboration with inventory teams, to improve the national inventory. Finally, it will discuss the UK’s plans for a prototype “operational” emissions evaluation system, the Greenhouse gas Emissions Measurement and Modelling Advancement (GEMMA).

Methyl bromide (CH3Br) is an important ozone-depleting substance whose use is regulated under the Montreal Protocol. Quantifying emissions on the national scale is required to assess compliance with the Montreal Protocol and thereby ensure the timely recovery of the ozone layer. However, the spatial-temporal patterns of China's national CH3Br emissions remain unclear. Here we estimate the national emissions of CH3Br in China during 2011-2020 using atmospheric observations at 10 sites across China combined with an inversion technique (top-down) and compare those with an updated inventory of identified emission sources (bottom-up). Measured CH3Br mole fractions are enhanced well above the background mole fractions, especially at sites in eastern China. Top-down emission estimates exceed bottom-up estimates by 5.5 +/- 1.4 gigagrams per year, with the largest fraction (60%) of observationally derived CH3Br emissions arising from underestimated or unidentified emissions sources. This study shows the potential impacts of the unaccounted emissions on stratospheric ozone depletion, with implications for the Montreal Protocol. Methyl bromide (CH3Br) is an important ozone-depleting substance whose use is regulated under the Montreal Protocol. However, the spatial-temporal patterns of China's national CH3Br emissions remain unclear. Here, the authors find that China's top-down emission estimates exceed bottom-up estimates by 60%.

The recovery of the ozone layer relies on decreasing atmospheric mixing ratios of ozone-depleting substances (ODSs), including chlorofluorocarbons (CFCs). A significant decline in the mixing ratio of trichlorofluoromethane (CFC-11 or CCl3F ), the second most abundant CFC, has been observed since the mid-1990s. However, a slowdown in the decline after 2012 indicates a rise in emissions, particularly in Eastern Asia. Ground-based observations are lacking in southeastern China, limiting a thorough evaluation of CFC-11 levels and emissions in this region. A new Advanced Global Atmospheric Gases Experiment background station was established at Xichong (XCG), Shenzhen, China, to provide high-frequency continuous in situ observations. The annual mean CFC-11 mixing ratio, recorded from May 2022 to April 2023, is 221.64 +/- 2.29 ppt. When compared with a monthly (MHD) or daily (MLO) observation, this value is found to be 0.45% to 5.36% higher than the northern hemispheric background. With the inverse modeling and interspecies correlation method, we estimate CFC-11 emissions in southeastern China between 1.23 +/- 0.25 Gg yr-1 and 1.58 +/- 0.21 Gg yr-1, in line with the bottom-up estimation of 1.50 Gg yr-1. Results indicate that CFC-11 emissions in the Pearl River Delta region have returned to levels before 2010, aligning with regional and global trends. Observations from XCG would compensate for the deficiency of CFC-11 measurements in southeastern China, paving the road for ODS studies in this region and beyond.

Top-down evaluation of the UK’s methane, nitrous oxide and halogenated greenhouse gas emissions has been possible since the 1990s, firstly due to measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE), combined later with the UK national-scale Deriving Emissions linked to Climate Change (UK DECC) network. Here, we show that carbon dioxide-equivalent emissions inferred from observations of these gases have declined more slowly than stated in the UK’s National Inventory Report (NIR); a decline of approximately 50 Tg/yr CO2-e is found between 1990 and 2021, compared to approximately 100 Tg/yr CO2-e in the inventory. This difference, which is driven largely by a smaller-than-reported reduction in methane emissions, suggests that the UK may be approximately 3 years behind its stated progress toward net-zero. This paper will describe the evolution of greenhouse gas monitoring in the UK, including an overview of the first decade of national-scale emissions estimation from the UK DECC network. It will show how top-down emissions are calculated, and how atmospheric observation-based estimates are used, in close collaboration with inventory teams, to improve the national inventory. Finally, it will discuss the UK’s plans for a prototype “operational” emissions evaluation system, the Greenhouse gas Emissions Measurement and Modelling Advancement (GEMMA).

The hydroxyl radical (OH) largely determines the atmosphere's oxidative capacity and, thus, the lifetimes of numerous trace gases, including methane (CH4). Hitherto, observation-based approaches for estimating the atmospheric oxidative capacity have primarily relied on using methyl chloroform (MCF), but as the atmospheric abundance of MCF has declined, the uncertainties associated with this method have increased. In this study, we examine the use of five hydrofluorocarbons (HFCs) (HFC-134a, HFC-152a, HFC-365mfc, HFC-245fa, and HFC-32) in multi-species inversions, which assimilate three HFCs simultaneously, as an alternative method to estimate atmospheric OH. We find robust estimates of OH regardless of which combination of the three HFCs are used in the inversions. Our results show that OH has remained fairly stable during our study period from 2004 to 2021, with variations of < 2 % and no significant trend. Inversions including HFC-32 and HFC-152a (the shortest-lived species) indicate a small reduction in OH in 2020 (1.6±0.9 % relative to the mean over 2004–2021 and 0.6±0.9 % lower than in 2019), but considering all inversions, the reduction was only 0.5±1.1 %, and OH was at a similar level to that in 2019.

AbstractHFC-23 (trifluoromethane) is a potent greenhouse gas released to the atmosphere primarily as a by-product of HCFC-22 (chlorodifluoromethane) synthesis. Since 2020, the Kigali Amendment to the Montreal Protocol has required Parties to destroy their HFC-23 emissions to the extent possible. Here, we present updated HFC-23 emissions estimated from atmospheric observations. Globally, emissions fell to 14.0 ± 0.9 Gg yr-1 in 2023 from their maximum in 2019 of 17.3 ± 0.8 Gg yr-1, but remained five times higher than reported in 2021. Atmospheric observation-based emissions for eastern China, the world’s largest HCFC-22 producer, were also found to be substantially higher than 2020-2022 reported emissions. We estimate that potential HFC-23 sources not directly linked to HCFC-22 production explain only a minor, albeit highly uncertain, fraction of this discrepancy. Our findings suggest that HFC-23 emissions have not been destroyed to the extent reported by the Parties since the implementation of the Kigali Amendment.