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Production and consumption of CFC-11 (trichlorofluoromethane, CCl3F), CFC-12 (dichlorodifluoromethane, CCl2F2) and CCl4 (carbon tetrachloride) are controlled under the regulations of the Montreal Protocol and have been phased out globally since 2010. Only CCl4 is still widely produced as a chemical feedstock. After 2010, emissions of CFC-11 and CFC-12 should therefore mostly originate from existing banks (e.g. from foams, mobile air conditioning units and refrigerators); however evidence has emerged of an increase in global emissions of CFC-11 in the last decade, some of which has not been fully accounted for. The motivation for this work was to assess the emissions of CFC-11, CFC-12 and CCl4 from western Europe. All countries in this region have been subject to the controls of the Montreal Protocol since the late 1980s and, as non-Article 5 Parties, have been prohibited from producing CFCs and CCl4 for dispersive use since 1996. Four different inverse modelling systems are used to estimate emissions of these gases from 2008 to 2021 using data from four atmospheric measurement stations: Mace Head (Ireland), Jungfraujoch (Switzerland), Monte Cimone (Italy) and Tacolneston (UK). The average of the four model studies found that western European emissions of CFC-11, CFC-12 and CCl4 between 2008 and 2021 were declining at 3.5 % yr−1 (2.7 % yr−1–4.8 % yr−1), 7.7 % yr−1 (6.3 % yr−1–8.0 % yr−1) and 4.4 % yr−1 (2.6 % yr−1–6.4 % yr−1), respectively. Even though the emissions were declining throughout the period, the area including northern France, Belgium, the Netherlands and Luxembourg showed consistently elevated emissions of CFC-11 compared with the surrounding regions. Emissions of CFC-12 were slightly elevated in the same region. CCl4 emissions were the highest in the south of France. France had the highest emissions of all three gases over the period 2008–2021. Emissions from western Europe (2008–2021) were on average 2.4 ± 0.4 Gg (CFC-11), 1.3 ± 0.3 Gg (CFC-12) and 0.9 ± 0.2 Gg (CCl4). Our estimated decline in emissions of CFC-11 is consistent with a western European bank release rate of 3.4 % (2.6 %–4.5 %). This study concludes that emissions of CFC-11, CFC-12 and CCl4 have all declined from 2008 to 2021 in western Europe. Therefore, no evidence is found that western European emissions contributed to the unexplained part of the global increase in atmospheric concentrations of CFC-11 observed in the last decade.

Long term measurement activities carried out at the WMO GAW global station of Monte Cimone (CMN) provide a useful insight into the characterization of the composition of the Southern European atmosphere. Since 2003, ozone depleting substances (ODSs) and halogenated Greenhouse Gases (GHGs) have been measured at CMN in the frame of the AGAGE (Advanced Global Atmospheric Gases Experiment) programme, with the aim of tracking progress towards the implementation of the international treaties on stratospheric ozone depletion and climate, and getting a better understanding of emissions both in terms of magnitude and localisation at the regional (EU) scale. CMN is often influenced by the advection of polluted air masses from the Po basin, one of the most polluted areas in Europe, as well as by the transport from other highly anthropised regions in Central EU. However, during the cold months and at night-time in the warm season, the site is representative of the free troposphere.By providing high quality, high frequency, continuous -almost uninterrupted- observations of ODSs (chlorofluorocarbons and hydrochlorofluorocarbons) and their radiatively active substitutes (hydrofluorocarbons) to modellers, the monitoring activities at CMN are crucial for improving the overall sensitivity of the inverse modelling techniques used to derive emissions through the so called “top-down” approach. Such approach represents an important quasi-independent cross-check of national GHG emission inventories submitted annually by the parties to the United Nations Framework Convention on Climate Change (UNFCCC). Here we will present results of the Bayesian inverse modelling technique used to derive emissions at the EU national scale based on the observations described above.

Global emissions of the ozone depleting gas 1,1-dichloro-1-fluoroethane (HCFC-141b, CH3CCl2F), derived from measurements of atmospheric mole fractions, have been rising between 2017–2021 despite a fall in reported production and consumption for dispersive uses. This study evaluates the possible drivers behind this renewed rise in emissions. HCFC-141b is a controlled substance under the Montreal Protocol, and its phase-out is currently underway, after a peak in reported consumption and production in developing countries (Article 5) in 2013. If reported production and consumption are correct, it suggests that the 2017–2021 rise is due to an increase in emissions from the bank when HCFC-141b containing appliances reach the end of their life, or from production of HCFC-141b not reported for dispersive uses. Regional emissions have been estimated between 2017–2020 for all regions where measurements have sufficient sensitivity to emissions. This includes the regions of northwestern Europe, east Asia, the USA and Australia, where emissions decreased by a total of 1.6 ± 3.9 Gg yr−1, compared to a mean global increase of 3.0 ± 1.2 Gg yr−1 over the same period. Collectively these regions only account for around a third of global emissions in 2020. Therefore we are not able to pinpoint the source regions or specific activities responsible for the recent global emission rise.

Inverse modeling is a widely used top-down method to infer greenhouse gas (GHG) emissions and their spatial distribution based on atmospheric observations. The errors associated with inverse modeling have multiple sources, such as observations and a-priori emission estimates, but they are often dominated by the transport model error. Here, we utilize the Lagrangian Particle Dispersion Model (LPDM) FLEXPART, driven by the meteorological fields of the regional numerical weather prediction model COSMO. The main source of errors in LPDMs is the turbulence diffusion parameterization and the meteorological fields. The latter are outputs of an Eulerian model. Recently, we introduced an improved parameterization scheme of the turbulence diffusion in FLEXPART, which significantly improves FLEXPART-COSMO simulations at 1 km resolution. We exploit F-gases measurements from two extended field campaigns on the Swiss Plateau (in Beromünster and Sottens) and we conduct both high- (1 km) and low-resolution (7 km) FLEXPART transport simulations that are then used in a Bayesian analytical inversion to estimate spatial emission distributions. Our results for four F-gases (HFC-134a, HFC-125, HFC-32, SF6) indicate that both high-resolution inversions and a dense measurement network significantly improve the ability to estimate the spatial distribution of emissions. Furthermore, the total emission estimates from the high-resolution inversions (351±44 Mg yr−1 for HFC-134a, 101±21 Mg yr−1 for HFC-125, 50±8 Mg yr−1 for HFC-32, 9.0±1.1 Mg yr−1 for SF6) are significantly higher compared to the low-resolution inversions (20–40 % increase) and result in total a-posteriori emission estimates that are closer to national inventory values as reported to the UNFCCC (10–20 % difference between high-resolution inversion estimates and inventory values compared to 30–40 % difference between the low-resolution inversion estimates and inventory values). Specifically, we attribute these improvements to a better representation of the atmospheric flow in complex terrain in the high-resolution model, partly induced by the more realistic topography. We further conduct numerous sensitivity inversions, varying different parameters and variables of our Bayesian inversion framework to explore the whole range of uncertainty in the inversion errors (e.g., inversion grid, spatial distribution of a-priori emissions, covariance parameters like baseline uncertainty and spatial correlation length, temporal resolution of the assimilated observations, observation network, seasonality of emissions). From the above-mentioned parameters, we find that the uncertainty of the mole fraction baseline and the spatial distribution of the a-priori emissions have the largest impact on the a-posteriori total emission estimates and their spatial distribution. This study is a step towards mitigating the errors associated with the transport models and better characterizing the uncertainty inherent in the inversion error. Improvements in the latter will facilitate the validation and standardization of the national GHG emission inventories and support policymakers.

National greenhouse gas inventories (GHGIs) are submitted annually to the United Nations Framework Convention on Climate Change (UNFCCC). They are estimated in compliance with Intergovernmental Panel on Climate Change (IPCC) methodological guidance using activity data, emission factors and facility-level measurements. For some sources, the outputs from these calculations are very uncertain. Inverse modelling techniques that use high-quality, long-term measurements of atmospheric gases have been developed to provide independent verification of national GHGIs. This is considered good practice by the IPCC as it helps national inventory compilers to verify reported emissions and to reduce emission uncertainty. Emission estimates from the InTEM (Inversion Technique for Emission Modelling) model are presented for the UK for the hydrofluorocarbons (HFCs) reported to the UNFCCC (HFC-125, HFC-134a, HFC-143a, HFC-152a, HFC-23, HFC-32, HFC-227ea, HFC-245fa, HFC-43-10mee and HFC-365mfc). These HFCs have high global warming potentials (GWPs), and the global background mole fractions of all but two are increasing, thus highlighting their relevance to the climate and a need for increasing the accuracy of emission estimation for regulatory purposes. This study presents evidence that the long-term annual increase in growth of HFC-134a has stopped and is now decreasing. For HFC-32 there is an early indication, its rapid global growth period has ended, and there is evidence that the annual increase in global growth for HFC-125 has slowed from 2018. The inverse modelling results indicate that the UK implementation of European Union regulation of HFC emissions has been successful in initiating a decline in UK emissions from 2018. Comparison of the total InTEM UK HFC emissions in 2020 with the average from 2009–2012 shows a drop of 35 %, indicating progress toward the target of a 79 % decrease in sales by 2030. The total InTEM HFC emission estimates (2008–2018) are on average 73 (62–83) % of, or 4.3 (2.7–5.9) Tg CO2-eq yr−1 lower than, the total HFC emission estimates from the UK GHGI. There are also significant discrepancies between the two estimates for the individual HFCs.

Perfluorocarbons (PFCs) are amongst the most potent greenhouse gases listed under the United Nations Framework Convention on Climate Change (UNFCCC). With atmospheric lifetimes in the order of thousands to tens of thousands of years, PFC emissions represent a permanent alteration to the global atmosphere on human timescales. While the industries responsible for the vast majority of these emissions – aluminium smelting and semi-conductor manufacturing – have made efficiency improvements and introduced abatement measures, the global mean mole fractions of three PFCs, namely tetrafluoromethane (CF4, PFC-14), hexafluoroethane (C2F6, PFC-116) and octafluoropropane (C3F8, PFC-218), continue to grow. In this study, we update baseline growth rates using in-situ high-frequency measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE) and, using data from four European stations, estimate PFC emissions for northwest Europe. The global growth rate of CF4 decreased from 1.3 ppt yr−1 in 1979 to 0.6 ppt yr−1 around 2010 followed by a renewed steady increase to 0.9 ppt yr−1 in 2018. For C2F6, the growth rate grew to a maximum of 0.125 ppt yr−1 around 1999, followed by a decline to a minimum of 0.075 ppt yr−1 in 2009, followed by weak growth thereafter. The C3F8 growth rate was around 0.007 ppt yr−1 until the early 1990s and then quickly grew to a maximum of 0.03 ppt yr−1 in 2003/04. Following a period of decline until 2012 to 0.015 ppt yr−1, the growth rate slowly increased again to ~ 0.017 ppt yr−1 in 2019. Unlike CF4 (and to a lesser extent C2F6), we observed no clear minimum associated with the 2008 financial crisis for C3F8. We used an inverse modelling framework to infer PFC emissions for northwest Europe. No statistically significant trend in regional emissions was observed for any of the PFCs assessed. For CF4, European emissions in early years were linked predominantly to the aluminium industry. However, we link large emissions in recent years to a chemical manufacturer in northwest Italy. Emissions of C2F6 are linked to a range of sources, including a semi-conductor manufacturer in Ireland and a cluster of smelters in Germany's Ruhr valley. In contrast, northwest European emissions of C3F8 are dominated by a single source in northwest England, raising the possibility of using emissions from this site for a tracer release experiment.

We report a 40-year history of SF6 atmospheric mole fractions measured at the Advanced Global Atmospheric Gases Experiment (AGAGE) monitoring sites, combined with archived air samples, to determine emission estimates from 1978 to 2018. Previously we reported a global emission rate of 7.3±0.6 Gg yr−1 in 2008 and over the past decade emissions have continued to increase by about 24 % to 9.04±0.35 Gg yr−1 in 2018. We show that changing patterns in SF6 consumption from developed (Kyoto Protocol Annex-1) to developing countries (non-Annex-1) and the rapid global expansion of the electric power industry, mainly in Asia, have increased the demand for SF6-insulated switchgear, circuit breakers, and transformers. The large bank of SF6 sequestered in this electrical equipment provides a substantial source of emissions from maintenance, replacement, and continuous leakage. Other emissive sources of SF6 occur from the magnesium, aluminium, and electronics industries as well as more minor industrial applications. More recently, reported emissions, including those from electrical equipment and metal industries, primarily in the Annex-1 countries, have declined steadily through substitution of alternative blanketing gases and technological improvements in less emissive equipment and more efficient industrial practices. Nevertheless, there are still demands for SF6 in Annex-1 countries due to economic growth, as well as continuing emissions from older equipment and additional emissions from newly installed SF6-insulated electrical equipment, although at low emission rates. In addition, in the non-Annex-1 countries, SF6 emissions have increased due to an expansion in the growth of the electrical power, metal, and electronics industries to support their continuing development. There is an annual difference of 2.5–5 Gg yr−1 (1990–2018) between our modelled top-down emissions and the UNFCCC-reported bottom-up emissions (United Nations Framework Convention on Climate Change), which we attempt to reconcile through analysis of the potential contribution of emissions from the various industrial applications which use SF6. We also investigate regional emissions in East Asia (China, S. Korea) and western Europe and their respective contributions to the global atmospheric SF6 inventory. On an average annual basis, our estimated emissions from the whole of China are approximately 10 times greater than emissions from western Europe. In 2018, our modelled Chinese and western European emissions accounted for ∼36 % and 3.1 %, respectively, of our global SF6 emissions estimate.

We have constructed an atmospheric inversion framework based on TM5-4DVAR to jointly assimilate measurements of methane and δ13C of methane in order to estimate source-specific methane emissions. Here we present global emission estimates from this framework for the period 1999–2016. We assimilate a newly constructed, multi-agency database of CH4 and δ13C measurements. We find that traditional CH4-only atmospheric inversions are unlikely to estimate emissions consistent with atmospheric δ13C data, and assimilating δ13C data is necessary to derive emissions consistent with both measurements. Our framework attributes ca. 85 % of the post-2007 growth in atmospheric methane to microbial sources, with about half of that coming from the tropics between 23.5∘ N and 23.5∘ S. This contradicts the attribution of the recent growth in the methane budget of the Global Carbon Project (GCP). We find that the GCP attribution is only consistent with our top-down estimate in the absence of δ13C data. We find that at global and continental scales, δ13C data can separate microbial from fossil methane emissions much better than CH4 data alone, and at smaller scales this ability is limited by the current δ13C measurement coverage. Finally, we find that the largest uncertainty in using δ13C data to separate different methane source types comes from our knowledge of atmospheric chemistry, specifically the distribution of tropospheric chlorine and the isotopic discrimination of the methane sink.

Verification of the nationally reported greenhouse gas (GHG) inventories using inverse modelling and atmospheric observations is considered to be best practice by the United Nations Framework Convention on Climate Change (UNFCCC). It allows for an independent assessment of the nationally reported GHG emissions using a comprehensively different approach to the inventory methods. Significant differences in the emissions estimated using the two approaches are a means of identifying areas worthy of further investigation. An inversion methodology called Inversion Technique for Emission Modelling (InTEM) has been developed that uses a non-negative least squares minimisation technique to determine the emission magnitude and distribution that most accurately reproduces the observations. By estimating the underlying baseline time series, atmospheric concentrations where the short-term impact of regional pollution has been removed, and by modelling where the air has passed over on route to the observation stations on a regional scale, estimates of UK emissions are made. In this study we use an extensive network of observations with six stations across the UK and six more in neighbouring countries. InTEM uses information from a Lagrangian dispersion model NAME (Numerical Atmospheric dispersion Modelling Environment), driven by three-dimensional, modelled meteorology, to understand how the air mixes during transport from the emission sources to observation points. The InTEM inversion results are submitted annually by the UK as part of their National Inventory Report to the UNFCCC. They are used within the UK inventory team to highlight areas for investigation and have led to significant improvements to the submitted UK inventory. The latest UK comparisons will be shown along with examples of how the inversion results have informed the inventory.

Significance We demonstrate the need to detect and track unexpected substances in the atmosphere and to locate their sources. Here, we report on three hydrochlorofluorocarbons (HCFCs) that have no known end-uses. HCFC-132b ( C H 2 C l C C l F 2 ) is newly discovered in the global atmosphere. We identify East Asia as the dominant source region for global emissions of this compound and of HCFC-133a ( C H 2 C l C F 3 ). We also quantify global emissions of HCFC-31 ( C H 2 ClF). These compounds are most likely emitted as intermediate by-products of chemical production processes. The early discovery and identification of such unexpected emissions can identify the related industrial practices and help to develop and manage environmental policies to reduce unwanted and potentially harmful emissions before the scale of the problem becomes more costly to mitigate.

The potent greenhouse gas sulfuryl fluoride (SO2F2) is increasingly used as a fumigant, replacing methyl bromide, whose structural and soil fumigation uses have been phased out under the Montreal Protocol. We use measurements on archived air samples and in situ observations from the Advanced Global Atmospheric Gases Experiment (AGAGE) and a box model of the global atmosphere to show a global increase of SO2F2 mole fraction from 0.3 ± 0.02 to 2.5 ± 0.08 ppt along with a global increase in emissions from 0.5 ± 0.4 Gg yr−1 to 2.9 ± 0.4 Gg yr−1 from 1978 to 2019. Based on a hybrid model incorporating bottom-up industry data and a top-down downscaling approach, we estimate the spatial distribution and trend in SO2F2 regional emissions between 2000 and 2019 and propose that the global emissions increase is driven by the growing use of SO2F2 in structural fumigation in North America and in postharvest treatment of grains and other agricultural products worldwide.

Dichloromethane (CH2Cl2) and perchloroethylene (C2Cl4) are chlorinated very short lived substances (Cl-VSLS) with anthropogenic sources. Recent studies highlight the increasing influence of such compounds, particularly CH2Cl2, on the stratospheric chlorine budget and therefore on ozone depletion. Here, a multiyear global-scale synthesis inversion was performed to optimize CH2Cl2 (2006–2017) and C2Cl4 (2007–2017) emissions. The approach combines long-term surface observations from global monitoring networks, output from a three-dimensional chemical transport model (TOMCAT), and novel bottom-up information on prior industry emissions. Our posterior results show an increase in global CH2Cl2 emissions from 637 ± 36 Gg yr−1 in 2006 to 1,171 ± 45 Gg yr−1 in 2017, with Asian emissions accounting for 68% and 89% of these totals, respectively. In absolute terms, Asian CH2Cl2 emissions increased annually by 51 Gg yr−1 over the study period, while European and North American emissions declined, indicating a continental-scale shift in emission distribution since the mid-2000s. For C2Cl4, we estimate a decrease in global emissions from 141 ± 14 Gg yr−1 in 2007 to 106 ± 12 Gg yr−1 in 2017. The time-varying posterior emissions offer significant improvements over the prior. Utilizing the posterior emissions leads to modeled tropospheric CH2Cl2 and C2Cl4 abundances and trends in good agreement to those observed (including independent observations to the inversion). A shorter C2Cl4 lifetime, from including an uncertain Cl sink, leads to larger global C2Cl4 emissions by a factor of ~1.5, which in some places improves model-measurement agreement. The sensitivity of our findings to assumptions in the inversion procedure, including CH2Cl2 oceanic emissions, is discussed.

We report a 40-year history of SF6 atmospheric mole fractions measured at the Advanced Global Atmospheric Gases Experiment (AGAGE) monitoring sites, combined with archived air samples to determine emission estimates from 1978–2018. Previously we reported a global emission rate of 7.3 ± 0.6 Gigagrams (Gg) yr−1 in 2008 and over the past decade emissions have continued to increase by about 24 % to 9.04 ± 0.35 Gg yr−1 in 2018. We show that changing patterns in SF6 consumption from developed (Kyoto Protocol Annex-1) to developing countries (non-Annex-1) and the rapid global expansion of the electric power industry, mainly in Asia, have increased the demand for SF6-insulated switchgear, circuit breakers and transformers. The large bank of SF6 sequestered in this electrical equipment provides a substantial source of emissions from maintenance, replacement and continuous leakage. Other emissive sources of SF6 occur from the magnesium, aluminium, electronics industries and more minor industrial applications. More recently, reported emissions, including those from electrical equipment and metal industries, primarily in the Annex-1 countries, have steadily declined through substitution of alternative blanketing gases and technological improvements in less emissive equipment and more efficient industrial practices. Conversely, in the non-Annex-1 countries SF6 emissions have increased due to an expansion in the growth of the electrical power, metal and electronics industries to support their development. There is an annual difference of 2.5–5 Gg yr−1 (1990–2018) between our modelled top-down emissions and the UNFCCC reported bottom-up emissions, which we attempt to reconcile through analysis of the potential contribution of emissions from the various industrial applications which use SF6. We also investigate regional emissions in East Asia (China, S. Korea) and Western Europe and their respective contributions to the global atmospheric SF6 inventory. On an average annual basis, our estimated emissions from the whole of China are approximately 10 times greater than emissions from Western Europe. In 2018, our modelled Chinese and Western European emissions accounted for ~ 36 % and 3.1 %, respectively, of our global SF6 emissions estimate.

Methyl Chloride (CH3Cl) is a chlorine-containing trace gas in the atmosphere contributing significantly to stratospheric ozone depletion. While the atmospheric CH3Cl emissions are predominantly caused by natural sources on the global budget, significant uncertainties still remain for the anthropogenic CH3Cl emission strengths. In summer 2007 an intensive field campaign within the ACTRIS-2 Project was hosted at the Mt. Cimone World Meteorological Organization/Global Atmosphere Watch global station (CMN, 44,17&deg, N, 10,68&deg, E, 2165 m a.s.l.). High-frequency and high precision in situ measurements of atmospheric CH3Cl revealed significant high-frequency variability superimposed on the seasonally varying regional background levels. The high-frequency CH3Cl variability was characterized by an evident cycle over 24 h with maxima during the afternoon which points towards a systematic role of thermal vertical transport of air-masses from the regional boundary layer. The temporal correlation analysis with specific tracers of anthropogenic activity (traffic, industry, petrochemical industry) together with bivariate analysis as a function of local wind regime suggested that, even if the role of natural marine emissions appears as predominant, the northern Italy boundary layer could potentially represent a non-negligible source of CH3Cl during summer. Since industrial production and use of CH3Cl have not been regulated under the Montreal Protocol (MP) or its successor amendments, continuous monitoring of CH3Cl outflow from the Po Basin is important to properly assess its anthropogenic emissions.

We present a global distribution of surface methane (CH4) emission estimates for 2000–2012 derived using the CarbonTracker Europe-CH4 (CTE-CH4) data assimilation system. In CTE-CH4, anthropogenic and biospheric CH4 emissions are simultaneously estimated based on constraints of global atmospheric in situ CH4 observations. The system was configured to either estimate only anthropogenic or biospheric sources per region, or to estimate both categories simultaneously. The latter increased the number of optimizable parameters from 62 to 78. In addition, the differences between two numerical schemes available to perform turbulent vertical mixing in the atmospheric transport model TM5 were examined. Together, the system configurations encompass important axes of uncertainty in inversions and allow us to examine the robustness of the flux estimates. The posterior emission estimates are further evaluated by comparing simulated atmospheric CH4 to surface in situ observations, vertical profiles of CH4 made by aircraft, remotely sensed dry-air total column-averaged mole fraction (XCH4) from the Total Carbon Column Observing Network (TCCON), and XCH4 from the Greenhouse gases Observing Satellite (GOSAT). The evaluation with non-assimilated observations shows that posterior XCH4 is better matched with the retrievals when the vertical mixing scheme with faster interhemispheric exchange is used. Estimated posterior mean total global emissions during 2000–2012 are 516 ± 51 Tg CH4 yr−1, with an increase of 18 Tg CH4 yr−1 from 2000–2006 to 2007–2012. The increase is mainly driven by an increase in emissions from South American temperate, Asian temperate and Asian tropical TransCom regions. In addition, the increase is hardly sensitive to different model configurations (

We employed a global high-resolution inverse model to optimize the CH4 emission using Greenhouse gas Observing Satellite (GOSAT) and surface observation data for a period from 2011−2017 for the two main source categories of anthropogenic and natural emissions. We used the Emission Database for Global Atmospheric Research (EDGAR v4.3.2) for anthropogenic methane emission and scaled them by country to match the national inventories reported to the United Nations Framework Convention on Climate Change (UNFCCC). Wetland and soil sink prior fluxes were simulated using the Vegetation Integrative Simulator of Trace gases (VISIT) model. Biomass burning prior fluxes were provided by the Global Fire Assimilation System (GFAS). We estimated a global total anthropogenic and natural methane emissions of 340.9 Tg CH4 yr−1 and 232.5 Tg CH4 yr−1, respectively. Country-scale analysis of the estimated anthropogenic emissions showed that all the top-emitting countries showed differences with their respective inventories to be within the uncertainty range of the inventories, confirming that the posterior anthropogenic emissions did not deviate from nationally reported values. Large countries, such as China, Russia, and the United States, had the mean estimated emission of 45.7 ± 8.6, 31.9 ± 7.8, and 29.8 ± 7.8 Tg CH4 yr−1, respectively. For natural wetland emissions, we estimated large emissions for Brazil (39.8 ± 12.4 Tg CH4 yr−1), the United States (25.9 ± 8.3 Tg CH4 yr−1), Russia (13.2 ± 9.3 Tg CH4 yr−1), India (12.3 ± 6.4 Tg CH4 yr−1), and Canada (12.2 ± 5.1 Tg CH4 yr−1). In both emission categories, the major emitting countries all had the model corrections to emissions within the uncertainty range of inventories. The advantages of the approach used in this study were: (1) use of high-resolution transport, useful for simulations near emission hotspots, (2) prior anthropogenic emissions adjusted to the UNFCCC reports, (3) combining surface and satellite observations, which improves the estimation of both natural and anthropogenic methane emissions over spatial scale of countries.

Very short‐lived substances (VSLS), including dichloromethane (CH2Cl2), chloroform (CHCl3), perchloroethylene (C2Cl4), and 1,2‐dichloroethane (C2H4Cl2), are a stratospheric chlorine source and therefore contribute to ozone depletion. We quantify stratospheric chlorine trends from these VSLS (VSLCltot) using a chemical transport model and atmospheric measurements, including novel high‐altitude aircraft data from the NASA VIRGAS (2015) and POSIDON (2016) missions. We estimate VSLCltot increased from 69 (±14) parts per trillion (ppt) Cl in 2000 to 111 (±22) ppt Cl in 2017, with >80% delivered to the stratosphere through source gas injection, and the remainder from product gases. The modeled evolution of chlorine source gas injection agrees well with historical aircraft data, which corroborate reported surface CH2Cl2 increases since the mid‐2000s. The relative contribution of VSLS to total stratospheric chlorine increased from ~2% in 2000 to ~3.4% in 2017, reflecting both VSLS growth and decreases in long‐lived halocarbons. We derive a mean VSLCltot growth rate of 3.8 (±0.3) ppt Cl/year between 2004 and 2017, though year‐to‐year growth rates are variable and were small or negative in the period 2015–2017. Whether this is a transient effect, or longer‐term stabilization, requires monitoring. In the upper stratosphere, the modeled rate of HCl decline (2004–2017) is −5.2% per decade with VSLS included, in good agreement to ACE satellite data (−4.8% per decade), and 15% slower than a model simulation without VSLS. Thus, VSLS have offset a portion of stratospheric chlorine reductions since the mid‐2000s., Key Points Stratospheric chlorine from very short‐lived substances increased by 3.8 ppt/year over 2004–2017, with a growth slowdown in 2015–2017Chlorine from short‐lived substances improves model representation of upper stratospheric HCl trendsShort‐lived chlorine offsets the 2004–2017 rate of upper stratospheric HCl decline by 15%

We report ground-based atmospheric measurements and emission estimates for the halons H-1211 (CBrClF2), H-1301 (CBrF3), and H-2402 (CBrF2CBrF2) from the AGAGE (Advanced Global Atmospheric Gases Experiment) and the National Oceanic and Atmospheric Administration global networks. We also include results from archived air samples in canisters and from polar firn in both hemispheres, thereby deriving an atmospheric record of nearly nine decades (1930s to present). All three halons were absent from the atmosphere until ∼1970, when their atmospheric burdens started to increase rapidly. In recent years H-1211 and H-2402 mole fractions have been declining, but H-1301 has continued to grow. High-frequency observations show continuing emissions of H-1211 and H-1301 near most AGAGE sites. For H-2402 the only emissions detected were derived from the region surrounding the Sea of Japan/East Sea. Based on our observations, we derive global emissions using two different inversion approaches. Emissions for H-1211 declined from a peak of 11 kt yr−1 (late 1990s) to 3.9 kt yr−1 at the end of our record (mean of 2013–2015), for H-1301 from 5.4 kt yr−1 (late 1980s) to 1.6 kt yr−1, and for H-2402 from 1.8 kt yr−1 (late 1980s) to 0.38 kt yr−1. Yearly summed halon emissions have decreased substantially; nevertheless, since 2000 they have accounted for ∼30% of the emissions of all major anthropogenic ozone depletion substances, when weighted by ozone depletion potentials.

High frequency, in situ observations from 11 globally distributed sites for the period 1994–2014 and archived air measurements dating from 1978 onward have been used to determine the global growth rate of 1,1-difluoroethane (HFC-152a, CH3CHF2). These observations have been combined with a range of atmospheric transport models to derive global emission estimates in a top-down approach. HFC-152a is a greenhouse gas with a short atmospheric lifetime of about 1.5 years. Since it does not contain chlorine or bromine, HFC-152a makes no direct contribution to the destruction of stratospheric ozone and is therefore used as a substitute for the ozone depleting chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). The concentration of HFC-152a has grown substantially since the first direct measurements in 1994, reaching a maximum annual global growth rate of 0.84 ± 0.05 ppt yr−1 in 2006, implying a substantial increase in emissions up to 2006. However, since 2007, the annual rate of growth has slowed to 0.38 ± 0.04 ppt yr−1 in 2010 with a further decline to an annual average rate of growth in 2013–2014 of −0.06 ± 0.05 ppt yr−1. The annual average Northern Hemisphere (NH) mole fraction in 1994 was 1.2 ppt rising to an annual average mole fraction of 10.1 ppt in 2014. Average annual mole fractions in the Southern Hemisphere (SH) in 1998 and 2014 were 0.84 and 4.5 ppt, respectively. We estimate global emissions of HFC-152a have risen from 7.3 ± 5.6 Gg yr−1 in 1994 to a maximum of 54.4 ± 17.1 Gg yr−1 in 2011, declining to 52.5 ± 20.1 Gg yr−1 in 2014 or 7.2 ± 2.8 Tg-CO2 eq yr−1. Analysis of mole fraction enhancements above regional background atmospheric levels suggests substantial emissions from North America, Asia, and Europe. Global HFC emissions (so called “bottom up” emissions) reported by the United Nations Framework Convention on Climate Change (UNFCCC) are based on cumulative national emission data reported to the UNFCCC, which in turn are based on national consumption data. There appears to be a significant underestimate ( > 20 Gg) of “bottom-up” reported emissions of HFC-152a, possibly arising from largely underestimated USA emissions and undeclared Asian emissions.

Since January 2010 continuous high-frequency in situ measurements of a range of anthropogenic Non-Methane Volatile Organic Compounds (NMVOCs) has been carried out at the World Meteorological Organisation Global Atmospheric Watch observatory at Mt. Cimone, on the highest peak of the Italian Northern Apennines, at the border between the Po Valley and the Mediterranean Basin. Five-year (2010-2014) time series of eleven NMVOCs, including aromatic and aliphatic species, have been analysed in order to derive average mixing ratios and detect annual and seasonal variability. Recent studies conducted in Europe, mainly in urban areas, have reported a decrease in atmospheric NMVOCs. Here we investigate how the decline in emissions, due to the implementation of air pollution policies, is reflected in the annual variability of NMVOC mixing ratios measured at a regional background location. Analysis of temporal trends for wellmixed conditions showed statistically significant decreases in ethyne, n-pentane and ethyl-benzene, while no significant trends were found for propane, butanes, i-pentane, toluene and xylenes. The seasonal variability of NMVOCs has been studied showing clear seasonal cycles for longer lived compounds and cycles with smaller seasonal amplitudes for shorterlived species. We used the propane time series to describe the seasonal cycle and to verify to what extent the mixing ratios of propane have been depleted by OH oxidation. We found that, during the summer, different transport times to the receptors and different source distribution are the main responsible for the relatively low integrated OH concentrations at Mt. Cimone.

HCFC-22 (CHClF2), a stratospherie ozone depleting substance and a powerful greenhouse gas, is the third most abundant anthropogenic halocarbon in the atmosphere. Primarily used in refrigeration and air conditioning systems, its global production and consumption have increased during the last 60 years, with the global increases in the last decade mainly attributable to developing countries. In 2007, an adjustment to the Montreal Protocol for Substances that Deplete the Ozone Layer called for an accelerated phase out of HCFCs, implying a 75% reduction (base year 1989) of HCFC production and consumption by 2010 in developed countries against the previous 65% reduction. In Europe HCFC-22 is continuously monitored at the two sites Mace Head (Ireland) and Monte Cimone (Italy). Combining atmospheric observations with a Bayesian inversion technique, we estimated fluxes of HCFC-22 from Europe and from eight macro-areas within it, over an 11-year period from January 2002 to December 2012, during which the accelerated restrictions on HCFCs production and consumption have entered into force. According to our study, the maximum emissions over the entire domain was in 2003 (38.2 +/- 4.7 Gg yr(-1)), and the minimum in 2012 (12.1 +/- 2.0 Gg yr(-1)); emissions continuously decreased between these years, except for secondary maxima in the 2008 and 2010. Despite such a decrease in regional emissions, background values of HCFC-22 measured at the two European stations over 2002-2012 are still increasing as a consequence of global emissions, in part from developing countries, with an average trend of ca 7.0 ppt yr(-1). However, the observations at the two European stations show also that since 2008 a decrease in the global growth rate has occurred. In general, our European emission estimates are in good agreement with those reported by previous studies that used different techniques. Since the currently dominant emission source of HCFC-22 is from banks, we assess the banks' size and their contribution to the total European emissions up to 2030, and we project a fast decrease approaching negligible emissions in the last five years of the considered period. Finally, inversions conducted over three month periods showed evidence for a seasonal cycle in emissions in regions in the Mediterranean basin but not outside it. Emissions derived from regions in the Mediterranean basin were ca. 25% higher in warmer months than in colder months. (C) 2015 The Authors. Published by Elsevier Ltd.

We present the organization, instrumentation, datasets, data interpretation, modeling, and accomplishments of the multinational global atmospheric measurement program AGAGE (Advanced Global Atmospheric Gases Experiment). AGAGE is distinguished by its capability to measure globally, at high frequency, and at multiple sites all the important species in the Montreal Protocol and all the important non-carbon-dioxide (non-CO[subscript 2]) gases assessed by the Intergovernmental Panel on Climate Change (CO[subscript 2] is also measured at several sites). The scientific objectives of AGAGE are important in furthering our understanding of global chemical and climatic phenomena. They are the following: (1) to accurately measure the temporal and spatial distributions of anthropogenic gases that contribute the majority of reactive halogen to the stratosphere and/or are strong infrared absorbers (chlorocarbons, chlorofluorocarbons – CFCs, bromocarbons, hydrochlorofluorocarbons – HCFCs, hydrofluorocarbons – HFCs and polyfluorinated compounds (perfluorocarbons – PFCs), nitrogen trifluoride – NF[subscript 3], sulfuryl fluoride – SO[subscript 2]F[subscript 2], and sulfur hexafluoride – SF[subscript 6]) and use these measurements to determine the global rates of their emission and/or destruction (i.e., lifetimes); (2) to accurately measure the global distributions and temporal behaviors and determine the sources and sinks of non-CO[subscript 2] biogenic–anthropogenic gases important to climate change and/or ozone depletion (methane – CH[subscript 4], nitrous oxide – N[subscript 2]O, carbon monoxide – CO, molecular hydrogen – H[subscript 2], methyl chloride – CH[subscript 3]Cl, and methyl bromide – CH[subscript 3]Br); (3) to identify new long-lived greenhouse and ozone-depleting gases (e.g., SO[subscript 2]F[subscript 2], NF[subscript 3], heavy PFCs (C[subscript 4]F[subscript 10], C[subscript 5]F[subscript 12], C[subscript 6]F[subscript 14], C[subscript 7]F[subscript 16], and C[subscript 8]F[subscript 18]) and hydrofluoroolefins (HFOs; e.g., CH[subscript 2] = CFCF[subscript 3]) have been identified in AGAGE), initiate the real-time monitoring of these new gases, and reconstruct their past histories from AGAGE, air archive, and firn air measurements; (4) to determine the average concentrations and trends of tropospheric hydroxyl radicals (OH) from the rates of destruction of atmospheric trichloroethane (CH[subscript 3]CCl[subscript 3]), HFCs, and HCFCs and estimates of their emissions; (5) to determine from atmospheric observations and estimates of their destruction rates the magnitudes and distributions by region of surface sources and sinks of all measured gases; (6) to provide accurate data on the global accumulation of many of these trace gases that are used to test the synoptic-, regional-, and global-scale circulations predicted by three-dimensional models; and (7) to provide global and regional measurements of methane, carbon monoxide, and molecular hydrogen and estimates of hydroxyl levels to test primary atmospheric oxidation pathways at midlatitudes and the tropics. Network Information and Data Repository: http://agage.mit.edu/data or http://cdiac.ess-dive.lbl.gov/ndps/alegage.html (https://doi.org/10.3334/CDIAC/atg.db1001)., United States. National Aeronautics and Space Administration (NAG5-12669), United States. National Aeronautics and Space Administration (NNX07AE89G), United States. National Aeronautics and Space Administration (NNX11AF17G), United States. National Aeronautics and Space Administration (NNX16AC98G)

Based on observations of three chlorofluorocarbons, CFC-13 (chlorotrifluoromethane), CFC-114 (dichlorotetrafluoroethane) and CFC-115 (chloropentafluoroethane) in atmospheric and firn samples, we reconstruct records of their tropospheric histories spanning nearly eight decades. These compounds were measured in polar firn air samples, in ambient air archived in canisters, and in-situ at the AGAGE (Advanced Global Atmospheric Gases Experiment) network and affiliated sites. Global emissions to the atmosphere are derived from these observations using an inversion based on a 12-box atmospheric transport model. For CFC-13, we provide the first comprehensive global analysis. This compound increased monotonically from its first appearance in the atmosphere in the late 1950s to a mean global abundance of 3.18 ppt (dry air mole fraction in parts-per-trillion, pmol mol−-1) in 2016. Its growth rate has decreased since the mid 1980s but has remained at a surprisingly high level of 0.02 ppt yr−1 since the late 2000s. CFC-114 increased from its appearance in the 1950s to a maximum of 16.6 ppt in the early 2000s, and has since slightly declined to 16.3 ppt in 2016. CFC-115 increased monotonically from its first appearance in the 1960s and reached a global mean mole fraction of 8.52 ppt in 2016. Growth rates of all three compounds over the past years are significantly larger than would be expected from zero emissions. Under the assumption of unaltered lifetimes and atmospheric transport patterns, we derive global emissions from our measurements, which have remained unexpectedly high in recent years: Mean yearly emissions for the last decade (2007–2016) of CFC-13 are at 0.48 ± 0.15 kt yr−1 (> 15 % of past peak emissions), of CFC-114 at 1.90 ± 0.84 kt yr−1 (~ 10 % of peak emissions), and of CFC-115 at 0.80 ± 0.50 kt yr−1 (> 5 % of peak emissions). Mean yearly emissions of CFC-115 for 2014–2016 are 1.08 ± 0.50 kt yr−1 and have more than doubled compared to 2009. Cumulative global emissions for CFC-114 derived from observations through 2016 exceed the global cumulative production derived from reported inventory data by > 10 % while those for CFC-115 agree well. We find CFC-13 emissions from aluminum smelters and impurities of CFC-115 in the refrigerant HFC-125 (CHF2CF3) but if extrapolated to global emissions neither of them can account for the lingering global emissions determined from the atmospheric observations. We also conduct regional inversions for the years 2012–2016 for the north-east Asian area using observations from the Korean Gosan AGAGE site and find significant emissions for CFC-114 and CFC-115, suggesting that a large fraction of their global emissions currently occur in north-eastern Asia and more specifically on the Chinese mainland.

Hydrofluorocarbons are powerful greenhouse gases developed by industry after the phase-out of the ozone depleting chlorofluorocarbons and hydrochlorofluorocarbons required by the Montreal Protocol. The climate benefit of reducing the emissions of hydrofluorocarbons has been widely recognised, leading to an amendment of the Montreal Protocol (Kigali Amendment) calling for developed countries to start to phase-down hydrofluorocarbons by 2019 and in developing countries to follow with a freeze between 2024 and 2028. In this way, nearly half a degree Celsius of warming would be avoided by the end of the century. Hydrofluorocarbons are also included in the basket of gases controlled under the Kyoto Protocol of the United Nations Framework Convention on Climate Change. Annex I parties to the Convention submit annual national greenhouse gas inventories based on a bottom-up approach, which relies on declared anthropogenic activities. Top-down methodologies, based on atmospheric measurements and modelling, can be used in support to the inventory compilation. In this study we used atmospheric data from four European sites combined with the FLEXPART dispersion model and a Bayesian inversion method, in order to derive emissions of nine individual hydrofluorocarbons from the whole European Geographic Domain and from twelve regions within it, then comparing our results with the annual emissions that the European countries submit every year to the United Nations Framework Convention on Climate Change, as well as with the bottom-up Emissions Database for Global Atmospheric Research. We found several discrepancies when considering the specific compounds and on the country level. However, an overall agreement is found when comparing European aggregated data, which between 2008 and 2014 are on average 84.2 +/- 28.0 Tg-CO2-eq.yr(-1) against the 95.1 Tg-CO2-eq.yr(-1) reported by UNFCCC in the same period. Therefore, in agreement with other studies, the gap on the global level between bottom-up estimates of Annex I countries and total global top-down emissions should be essentially due to emissions from non-reporting countries (non-Annex I). (C) 2017 Elsevier Ltd. All rights reserved.

We investigate the sensitivity of future space-borne lidar measurements to changes in surface methane emissions. We use surface methane observations from nine European ground stations, and a Lagrangian transport model to obtain surface methane emissions for 2010. Our inversion shows the strongest emissions from the Netherlands, the coalmines in Upper Silesia Poland, and wetlands in southern Finland. Our simulated methane surface concentration captures at least half of the daily variability in the observations, suggesting that the transport model is correctly simulating the regional transport pathways over Europe. With this tool we can perturb the surface fluxes and see the resulting changes in the simulated column methane measurements. For example, we show that future lidar instruments can detect a 50% reduction in methane emissions from the Netherlands and Germany, but only after averaging measurements on a monthly time scale.

Gobal methane emissions were estimated for 2000–2012 using the CarbonTracker Europe-CH4 (CTE-CH4) data assimilation system. In CTE-CH4, the anthropogenic and biosphere emissions of CH4 are simultaneously constrained by global atmospheric in-situ methane mole fraction observations. We use three configurations developed in Tsuruta et al. (2016) to assess the sensitivity of the CH4 flux estimates to (a) the number of unknown flux scaling factors to be optimized which in turn depends on the choice of underlying land-ecosystem map, and (b) on the parametrization of vertical mixing in the 30 atmospheric transport model TM5. The posterior emission estimates were evaluated by comparing simulations to surface in-situ observation sites, to profile observations made by aircraft, to dry air total column-averaged mole fractions (XCH4) observations from the Total Carbon Column Observing Network (TCCON), and to XCH4 retrievals from the Greenhouse gases Observing SATellite (GOSAT). Our estimated posterior mean global total emissions during 2000–2012 are 516 ± 51 Tg CH4 yr−1, and emission estimates during 2007–2012 are 18 Tg CH4 yr−1 greater than those from 2001–2006, mainly driven by an 35 increase in emissions from the south America temperate region, the Asia temperate region and Asia tropics. The sensitivity of the flux estimates to the underlying ecosystem map was large for the Asia temperate region and Australia, but not significant in the northern latitude regions, i.e. the north American boreal region, the north American temperate region and Europe. Instead, the posterior estimates for the northern latitude regions show larger sensitivity to the choice of convection scheme in TM5. The Gregory et al. (2000) mixing scheme with faster interhemispheric exchange leads to higher estimated CH4 emissions at northern latitudes, and lower emissions in southern latitudes, compared to the estimates using Tiedtke (1989) convection scheme. Our evaluation with non-assimilated observations showed that posterior mole fractions were better matched with the 5 observations when Gregory et al. (2000) convection scheme was used.

Carbon tetrachloride (CCl4) is a long-lived radiatively-active compound able to destroy stratospheric ozone. Due to its inclusion in the Montreal Protocol on Substances that Deplete the Ozone Layer, the last two decades have seen a sharp decrease in its large scale emissive use with a consequent decline of its atmospheric mole fractions. However, the Montreal Protocol restrictions do not apply to the use of carbon tetrachloride as feedstock for the production of other chemicals, implying the risk of fugitive emissions from the industry sector. The occurrence of such unintended emissions is suggested by a significant discrepancy between global emissions as derived by reported production and feedstock usage (bottom-up emissions), and those based on atmospheric observations (top-down emissions). In order to better constrain the atmospheric budget of carbon tetrachloride, several studies based on a combination of atmospheric observations and inverse modelling have been conducted in recent years in various regions of the world. This study is focused on the European scale and based on long-term high-frequency observations at three European sites, combined with a Bayesian inversion methodology. We estimated that average European emissions for 2006–2014 were 2.3 (± 0.8) Gg yr−1, with an average decreasing trend of 7.3 % per year. Our analysis identified France as the main source of emissions over the whole study period, with an average contribution to total European emissions of 25 %. The inversion was also able to allow the localisation of emission "hot-spots" in the domain, with major source areas in Southern France, Central England (UK) and Benelux (Belgium, The Netherlands, Luxembourg), where most of industrial scale production of basic organic chemicals are located. According to our results, European emissions correspond to 4.0 % of global emissions for 2006–2012. Together with other regional studies, our results allow a better constraint of the global budget of carbon tetrachloride and a better quantification of the gap between top-down and bottom-up estimates.

European emissions of nine representative halocarbons (CFC-11, CFC-12, Halon 1211, HCFC-141b, HCFC-142b, HCFC-22, HFC-125, HFC-134a, HFC-152a) are derived for the year 2009 by combining long-term observations in Switzerland, Italy, and Ireland with campaign measurements from Hungary. For the first time, halocarbon emissions over Eastern Europe are assessed by top-down methods, and these results are compared to Western European emissions. The employed inversion method builds on least-squares optimization linking atmospheric observations with calculations from the Lagrangian particle dispersion model FLEXPART. The aggregated halocarbon emissions over the study area are estimated at 125 (106-150) Tg of CO(2) equiv/y, of which the hydrofluorocarbons (HFCs) make up the most important fraction with 41% (31-52%). We find that chlorofluorocarbon (CFC) emissions from banks are still significant and account for 35% (27-43%) of total halocarbon emissions in Europe. The regional differences in per capita emissions are only small for the HFCs, while emissions of CFCs and hydrochlorofluorocarbons (HCFCs) tend to be higher in Western Europe compared to Eastern Europe. In total, the inferred per capita emissions are similar to estimates for China, but 3.5 (2.3-4.5) times lower than for the United States. Our study demonstrates the large benefits of adding a strategically well placed measurement site to the existing European observation network of halocarbons, as it extends the coverage of the inversion domain toward Eastern Europe and helps to better constrain the emissions over Central Europe.

South Asia is strongly influenced by the so-called Atmospheric Brown Cloud (ABC), a wide polluted layer extending from the Indian Ocean to the Himalayas during the winter and pre-monsoon seasons (November to April). This thick, grey-brown haze blanket substantially interacts with the incoming solar radiation, causing a cooling of the Earth's surface and a warming of the atmosphere, thus influencing the monsoon system and climate. In this area, the Himalayan region, particularly sensitive to climate change, offers a unique opportunity to detect global change processes and to analyse the influence of anthropogenic pollution on background atmospheric conditions through continuous monitoring activities. This paper provides a detailed description of the atmospheric conditions characterizing the high Himalayas, thanks to continuous observations begun in March 2006 at the Nepal Climate Observatory – Pyramid (NCO-P) located at 5079 m a.s.l. on the southern foothills of Mt. Everest, in the framework of ABC-UNEP and SHARE-Ev-K2-CNR projects. Besides giving an overview of the measurement site and experimental activities, the work presents an in-depth characterization of meteorological conditions and air-mass circulation at NCO-P during the first two years of activity (March 2006–February 2008). The mean values of atmospheric pressure, temperature and wind speed recorded at the site were: 551 hPa, −3.0 °C, 4.7 m s−1, respectively. The highest seasonal values of temperature (1.7 °C) and relative humidity (94%) were registered during the monsoon season, which was also characterized by thick clouds present in about 80% of the afternoon hours and by a frequency of cloud-free sky less than 10%. The lowest temperature and relative humidity values were registered during winter, −6.3 °C and 22%, respectively, the season being characterised by mainly cloud-free sky conditions and rare thick clouds. The summer monsoon influenced the rain precipitation (seasonal mean 237 mm), while wind was dominated by flows from the bottom of the valley (S-SW) and upper mountain (N-NE). In relation to seasonal weather conditions, the time series variability of black carbon and dust particles (optical active aerosols) and ozone (regional greenhouse gas) were analysed, as they are significant constituents of the Atmospheric Brown Cloud and strongly influence the atmospheric radiative forcing. The highest seasonal values of black carbon (BC), ozone (O3) and dust particles were observed during the pre-monsoon season (316.9 ng m−3, 60.9 ppbv, 0.37 cm−3, respectively), while the lowest concentrations occurred during the monsoon for BC and O3 (49.6 ng m−3 and 33.6 ppbv, respectively) and post-monsoon for dust particles (0.07 cm−3). The seasonal cycles of these compounds are influenced both by the local mountain wind system and by the three principal large-scale circulation regimes: Westerly, South-Westerly and Regional, as shown by the analysis of in-situ meteorological parameters and 5-day LAGRANTO back-trajectories. In particular, the analysis of data representative of synoptic-scale circulation showed that the highest median values (O3: 68 ppbv, BC: 124 ng m−3, dust particles: 0.44 cm−3, respectively) were related with air-masses from polluted and arid regions in the Indian subcontinent, as well as the Arabian Peninsula and Persian Gulf. Furthermore, it was documented that in 90% of pre-monsoon days the Khumbu valley represents a "direct channel" able to transport polluted air-masses from the Asian Brown Cloud up to NCO-P and to higher altitudes. On such days the average day-time BC concentration (625 ng m−3) was at least double that recorded on the remaining days, even if during some pollution hot spots BC daily values increased up to 1000 ng m−3. In this study, two years of Himalayan observation activities carried out at NCO-P, in conjunction with model circulation analyses, provide some of the first evidence that polluted air-masses linked to the Atmospheric Brown Cloud can reach the high Himalayas, in particular during the pre-monsoon season, influencing the pristine atmospheric composition.