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Methyl bromide (CH3Br) is a potent ozone-depleting substance (ODS) that has both natural and anthropogenic sources. CH3Br has been used mainly for preplant soil fumigation, post-harvest grain and timber fumigation, and structural fumigation. Most non-quarantine and pre-shipment (non-QPS) uses were phased out by 2005 for non-Article 5 (developed) countries and by 2015 for Article 5 (developing) countries under the Montreal Protocol on Substances that Deplete the Ozone Layer; some uses have continued under critical-use exemptions (CUEs). Under the protocol, individual nations are required to report annual data on CH3Br production and consumption for quarantine-pre-shipment (QPS) uses, non-QPS uses, and CUEs to the United Nations Environment Programme (UNEP). In this study, we analyzed high-precision, in situ measurements of atmospheric mole fractions of CH3Br obtained at the Gosan station on Jeju Island, South Korea, from 2008 to 2019. The background mole fractions of CH3Br in the atmosphere at Gosan declined from 8.5±0.8gppt (parts per trillion) in 2008 to 7.4±0.6gppt in 2019 at a rate of -0.13±0.02gpptgyr-1. At Gosan, we also observed periods of persistent mole fractions (pollution events) elevated above the decreasing background in continental air masses from China. Statistical back-trajectory analyses showed that these pollution events are predominantly traced back to CH3Br emissions from eastern China. Using an interspecies correlation (ISC) method with the reference trace species CFC-11 (CCl3F), we estimate anthropogenic CH3Br emissions from eastern China at an average of 4.1±1.3gGggyr-1 in 2008-2019, approximately 2.9±1.3gGggyr-1 higher than the bottom-up emission estimates reported to UNEP. Possible non-fumigation CH3Br sources - rapeseed production and biomass burning - were assessed, and it was found that the discrepancy is most likely due to unreported or incorrectly reported QPS and non-QPS fumigation uses. These unreported anthropogenic emissions of CH3Br are confined to eastern China and account for 30g%-40g% of anthropogenic global CH3Br emissions. They are likely due to delays in the introduction of CH3Br alternatives, such as sulfuryl fluoride (SO2F2), heat, and irradiation, and a possible lack of industry awareness of the need for regulation of CH3Br production and use.

Abstract. Nitrous oxide is a potent greenhouse gas (GHG) and ozone-depleting substance, whose atmospheric abundance has risen throughout the contemporary record. In this work, we carry out the first global hierarchical Bayesian inversion to solve for nitrous oxide emissions, which includes prior emissions with truncated Gaussian distributions and Gaussian model errors, in order to examine the drivers of the atmospheric surface growth rate. We show that both emissions and climatic variability are key drivers of variations in the surface nitrous oxide growth rate between 2011 and 2020. We derive increasing global nitrous oxide emissions, which are mainly driven by emissions between 0 and 30∘ N, with the highest emissions recorded in 2020. Our mean global total emissions for 2011–2020 of 17.2 (16.7–17.7 at the 95 % credible intervals) Tg N yr−1, comprising of 12.0 (11.2–12.8) Tg N yr−1 from land and 5.2 (4.5–5.9) Tg N yr−1 from ocean, agrees well with previous studies, but we find that emissions are poorly constrained for some regions of the world, particularly for the oceans. The prior emissions used in this and other previous work exhibit a seasonal cycle in the extra-tropical Northern Hemisphere that is out of phase with the posterior solution, and there is a substantial zonal redistribution of emissions from the prior to the posterior. Correctly characterizing the uncertainties in the system, for example in the prior emission fields, is crucial for deriving posterior fluxes that are consistent with observations. In this hierarchical inversion, the model-measurement discrepancy and the prior flux uncertainty are informed by the data, rather than solely through “expert judgement”. We show cases where this framework provides different plausible adjustments to the prior fluxes compared to inversions using widely adopted, fixed uncertainty constraints.

Abstract. Emissions of the potent greenhouse gas perfluorocyclobutane(c-C4F8, PFC-318, octafluorocyclobutane) into the global atmosphereinferred from atmospheric measurements have been increasing sharply sincethe early 2000s. We find that these inferred emissions are highly correlatedwith the production of hydrochlorofluorocarbon-22 (HCFC-22, CHClF2) forfeedstock (FS) uses, because almost all HCFC-22 FS is pyrolyzed to produce(poly)tetrafluoroethylene ((P)TFE) and hexafluoropropylene (HFP), a processin which c-C4F8 is a known by-product, causing a significantfraction of global c-C4F8 emissions. We find a global emissionfactor of ∼0.003 kg c-C4F8 per kilogram of HCFC-22 FSpyrolyzed. Mitigation of these c-C4F8 emissions, e.g., throughprocess optimization, abatement, or different manufacturing processes, suchas refined methods of electrochemical fluorination and waste recycling,could reduce the climate impact of this industry. While it has been shownthat c-C4F8 emissions from developing countries dominate globalemissions, more atmospheric measurements and/or detailed process statisticsare needed to quantify c-C4F8 emissions at country to facilitylevels.

Global and regional atmospheric measurements and modeling can play key roles in discovering and quantifying unexpected nascent emissions of environmentally important substances. We focus here on three hydrochlorofluorocarbons (HCFCs) that are restricted by the Montreal Protocol because of their roles in stratospheric ozone depletion. Based on measurements of archived air samples and on in situ measurements at stations of the Advanced Global Atmospheric Gases Experiment (AGAGE) network, we report global abundances, trends, and regional enhancements for HCFC-132b ([Formula: see text]), which is newly discovered in the atmosphere, and updated results for HCFC-133a ([Formula: see text]) and HCFC-31 ([Formula: see text]ClF). No purposeful end-use is known for any of these compounds. We find that HCFC-132b appeared in the atmosphere 20 y ago and that its global emissions increased to 1.1 Gg⋅y-1 by 2019. Regional top-down emission estimates for East Asia, based on high-frequency measurements for 2016-2019, account for ∼95% of the global HCFC-132b emissions and for ∼80% of the global HCFC-133a emissions of 2.3 Gg⋅y-1 during this period. Global emissions of HCFC-31 for the same period are 0.71 Gg⋅y-1 Small European emissions of HCFC-132b and HCFC-133a, found in southeastern France, ceased in early 2017 when a fluorocarbon production facility in that area closed. Although unreported emissive end-uses cannot be ruled out, all three compounds are most likely emitted as intermediate by-products in chemical production pathways. Identification of harmful emissions to the atmosphere at an early stage can guide the effective development of global and regional environmental policy.

Abstract. We reconstruct atmospheric abundances of the potentgreenhouse gas c-C4F8 (perfluorocyclobutane, perfluorocarbonPFC-318) from measurements of in situ, archived, firn, and aircraft airsamples with precisions of ∼1 %–2 % reported on the SIO-14gravimetric calibration scale. Combined with inverse methods, we found near-zero atmospheric abundances from the early 1900s to the early 1960s, afterwhich they rose sharply, reaching 1.66 ppt (parts per trillion dry-air molefraction) in 2017. Global c-C4F8 emissions rose from near zero inthe 1960s to 1.2±0.1 (1σ) Gg yr−1 in the late 1970s tolate 1980s, then declined to 0.77±0.03 Gg yr−1 in the mid-1990sto early 2000s, followed by a rise since the early 2000s to 2.20±0.05 Gg yr−1 in 2017. These emissions are significantly larger thaninventory-based emission estimates. Estimated emissions from eastern Asiarose from 0.36 Gg yr−1 in 2010 to 0.73 Gg yr−1 in 2016 and 2017,31 % of global emissions, mostly from eastern China. We estimateemissions of 0.14 Gg yr−1 from northern and central India in 2016 andfind evidence for significant emissions from Russia. In contrast, recentemissions from northwestern Europe and Australia are estimated to be small(≤1 % each). We suggest that emissions from China, India, and Russiaare likely related to production of polytetrafluoroethylene (PTFE,“Teflon”) and other fluoropolymers and fluorochemicals that are based onthe pyrolysis of hydrochlorofluorocarbon HCFC-22 (CHClF2) in whichc-C4F8 is a known by-product. The semiconductor sector, wherec-C4F8 is used, is estimated to be a small source, at least inSouth Korea, Japan, Taiwan, and Europe. Without an obvious correlation withpopulation density, incineration of waste-containing fluoropolymers isprobably a minor source, and we find no evidence of emissions fromelectrolytic production of aluminum in Australia. While many possibleemissive uses of c-C4F8 are known and though we cannotcategorically exclude unknown sources, the start of significant emissionsmay well be related to the advent of commercial PTFE production in 1947.Process controls or abatement to reduce the c-C4F8 by-product wereprobably not in place in the early decades, explaining the increase inemissions in the 1960s and 1970s. With the advent of by-product reportingrequirements to the United Nations Framework Convention on Climate Change(UNFCCC) in the 1990s, concern about climate change and product stewardship,abatement, and perhaps the collection of c-C4F8 by-product for usein the semiconductor industry where it can be easily abated, it isconceivable that emissions in developed countries were stabilized and thenreduced, explaining the observed emission reduction in the 1980s and 1990s.Concurrently, production of PTFE in China began to increase rapidly. Withoutemission reduction requirements, it is plausible that global emissions todayare dominated by China and other developing countries. We predict thatc-C4F8 emissions will continue to rise and that c-C4F8will become the second most important emitted PFC in terms ofCO2-equivalent emissions within a year or two. The 2017 radiativeforcing of c-C4F8 (0.52 mW m−2) is small but emissions ofc-C4F8 and other PFCs, due to their very long atmosphericlifetimes, essentially permanently alter Earth's radiative budget and shouldbe reduced. Significant emissions inferred outside of the investigatedregions clearly show that observational capabilities and reportingrequirements need to be improved to understand global and country-scaleemissions of PFCs and other synthetic greenhouse gases and ozone-depletingsubstances.

The growth in global methane (CH4) concentration, which had been ongoing since the industrial revolution, stalled around the year 2000 before resuming globally in 2007. We evaluate the role of the hydroxyl radical (OH), the major CH4 sink, in the recent CH4 growth. We also examine the influence of systematic uncertainties in OH concentrations on CH4 emissions inferred from atmospheric observations. We use observations of 1,1,1-trichloroethane (CH3CCl3), which is lost primarily through reaction with OH, to estimate OH levels as well as CH3CC3 emissions, which have uncertainty that previously limited the accuracy of OH estimates. We find a 64-70% probability that a decline in OH has contributed to the post-2007 methane rise. Our median solution suggests that CH4 emissions increased relatively steadily during the late 1990s and early 2000s, after which growth was more modest. This solution obviates the need for a sudden statistically significant change in total CH4 emissions around the year 2007 to explain the atmospheric observations and can explain some of the decline in the atmospheric 13CH4/12CH4 ratio and the recent growth in C2H6 Our approach indicates that significant OH-related uncertainties in the CH4 budget remain, and we find that it is not possible to implicate, with a high degree of confidence, rapid global CH4 emissions changes as the primary driver of recent trends when our inferred OH trends and these uncertainties are considered.

We infer global and regional emissions of five of the most abundant hydrofluorocarbons (HFCs) using atmospheric measurements from the Advanced Global Atmospheric Gases Experiment and the National Institute for Environmental Studies, Japan, networks. We find that the total CO2-equivalent emissions of the five HFCs from countries that are required to provide detailed, annual reports to the United Nations Framework Convention on Climate Change (UNFCCC) increased from 198 (175-221) Tg-CO2-eq ⋅ y(-1) in 2007 to 275 (246-304) Tg-CO2-eq ⋅ y(-1) in 2012. These global warming potential-weighted aggregated emissions agree well with those reported to the UNFCCC throughout this period and indicate that the gap between reported emissions and global HFC emissions derived from atmospheric trends is almost entirely due to emissions from nonreporting countries. However, our measurement-based estimates of individual HFC species suggest that emissions, from reporting countries, of the most abundant HFC, HFC-134a, were only 79% (63-95%) of the UNFCCC inventory total, while other HFC emissions were significantly greater than the reported values. These results suggest that there are inaccuracies in the reporting methods for individual HFCs, which appear to cancel when aggregated together.

Methane is an important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since pre-industrial times. By reacting with hydroxyl radicals, methane reduces the oxidizing capacity of the atmosphere and generates ozone in the troposphere. Although most sources and sinks of methane have been identified, their relative contributions to atmospheric methane levels are highly uncertain. As such, the factors responsible for the observed stabilization of atmospheric methane levels in the early 2000s, and the renewed rise after 2006, remain unclear. Here, we construct decadal budgets for methane sources and sinks between 1980 and 2010, using a combination of atmospheric measurements and results from chemical transport models, ecosystem models, climate chemistry models and inventories of anthropogenic emissions. The resultant budgets suggest that data-driven approaches and ecosystem models overestimate total natural emissions. We build three contrasting emission scenarios-which differ in fossil fuel and microbial emissions-to explain the decadal variability in atmospheric methane levels detected, here and in previous studies, since 1985. Although uncertainties in emission trends do not allow definitive conclusions to be drawn, we show that the observed stabilization of methane levels between 1999 and 2006 can potentially be explained by decreasing-to-stable fossil fuel emissions, combined with stable-to-increasing microbial emissions. We show that a rise in natural wetland emissions and fossil fuel emissions probably accounts for the renewed increase in global methane levels after 2006, although the relative contribution of these two sources remains uncertain. © 2013 Macmillan Publishers Limited.

Trifluoromethane (CHF3, HFC-23), one of the most potent greenhouse gases among HFCs, is mainly emitted to the atmosphere as a by-product in the production of the ozone depleting legacy refrigerant and chemical feedstock chlorodifluoromethane (CHClF2, HCFC-22). A recent study on global HFC-23 emissions (Stanley et al., 2020) showed significant discrepancies over 2014–2017 between the increase in the observation-derived (top-down) emissions and the 87 % emission reduction expected from capture and destruction processes of HFC-23 at HCFC-22 production facilities mandated by national phase-out plans (bottom-up). However, the actual sources of the increased emissions were not identified. Here, we estimate the regional top-down emissions of HFC-23 for East Asia based on in situ measurements at Gosan, South Korea, and show that the HFC-23 emissions from eastern China have increased from 5.0 ± 0.4 Gg yr-1 in 2008 to 9.5 ± 1.0 Gg yr-1 in 2019. The continuous rise was contrary to the large emissions reduction reported since 2015 under the Chinese hydrochlorofluorocarbons production phase-out management plan (HPPMP). The magnitude of the mismatch between top-down and bottom-up estimates for 2015–2019 in eastern China was ~23.7 ± 3.6 Gg, which accounts for 47 ± 11 % of the global mismatch. Given the location of HCFC-22 production plants in eastern China and the fraction of regional to global HCFC-22 production capacities, the HFC-23 emissions rise in eastern China is most likely associated with known HCFC-22 production facilities and thus, observed discrepancies between top-down and bottom-up emissions could be attributed to unsuccessful factory level HFC-23 abatement and inaccurate quantification of emission reductions. [ABSTRACT FROM AUTHOR]