Abstract. An improved mechanistic model of mercury redox chemistry has recently been implemented in the GEOS-Chem model. In this study, GEOS-Chem simulations were compared to ambient measurements made during a high-oxidized mercury episode that originated in the free troposphere at a mountaintop site in Colorado, USA (40.455° N, −106.744° W, 3220 m above sea level). Measurements were collected with a dual channel atmospheric oxidized mercury measurement system that has been shown to accurately quantify oxidized mercury compounds in ambient air. The model and observations showed similar temporal trends for elemental and oxidized mercury (R2 of 0.54 to 0.79) and similar elemental mercury concentrations (normalized mean square error of 0.04 in the base model). However, the base model only produced 17 % of the maximum oxidized mercury observed in the dual channel system. In sensitivity tests with increased oxidation rates, the model still only produced, at most, 23 % of maximum observed oxidized mercury. In addition to underestimating net mercury oxidation, an analysis of elemental to oxidized mercury slopes indicated the model overestimated oxidized mercury deposition. An analysis of GEOS-Chem results from a separate study confirmed that while GEOS-Chem is able to simulate the range of measured oxidized mercury in low-oxidized mercury episodes and locations it consistently underestimates measured values during high-oxidized mercury periods at surface locations in western USA.

Abstract. Bicyclic peroxy radicals (BPRs) from aromatics hydrocarbons oxidation play increasingly recognized roles in the formation of secondary air pollutants. However, their reaction mechanisms remain poorly constrained, largely due to the lack of direct measurement techniques. In this study, we developed a method for quantitative measurement of BPRs using an iodide chemical ionization mass spectrometer (Vocus AIM). Following instrument optimization, the sensitivity for BPRs reached 0.3–0.6 ncps pptv−1, with a detection limit of ∼ 1 pptv and an uncertainty of ∼ 41 %. Our flow reactor experiments revealed that the bicyclic pathway dominates the OH-initiated oxidation of aromatics under low-NOx conditions, accounting for 57.0 % and 69.5 % of the oxidation products of toluene and m-xylene, respectively. Comparative analysis further demonstrated that conventional product-yield-based approaches underestimate the branching ratio of the bicyclic pathway by 4 %–9 % relative to direct BPR quantification. This discrepancy suggests the presence of unaccounted reaction channels in current chemical mechanisms, even when autoxidation and accretion reactions are considered. By directly quantifying BPRs, this study provides new insights into the atmospheric oxidation of aromatics and highlights the need for further mechanistic investigation. Moreover, the reaction-pathway-controlled quantification approach proposed here effectively reduces the challenges associated with measuring functionalized RO2 radicals and demonstrates strong potential for sensitive, speciated RO2 detection using Vocus AIM in both laboratory and ambient environments.

Abstract. Carbonyl sulfide (OCS) is consumed by vegetation during photosynthesis in a one-way hydrolysis reaction, making measuring OCS vegetative uptake a means of inferring and quantifying global gross primary production. Recent studies highlight that uncertainties in OCS surface fluxes remain high and OCS datasets with better spatial coverage are required, particularly from satellite. Here OCS profiles are retrieved using measured radiances from the Infrared Atmospheric Sounding Interferometer (IASI) instruments. We estimate total column amounts over oceanic and inland water regions for the example year 2018, using an optimal estimation scheme, using the University of Leicester IASI retrieval scheme (ULIRS) for selected microwindows in the 2000–2100 cm−1 wavenumber range. Information content exceeds one between ±50° latitude and a peak in vertical sensitivity around 6–10 km (500–300 hPa) in the troposphere. Diurnal variations are limited to ±2 %, showing larger total column amounts at the daytime overpass. The IASI OCS observations show a correlation of at least 0.74 at half the ground-based flask measurement sites compared. Results also agree with the University of Leeds TOMCAT 3-D chemical transport model simulations within ±5 % throughout most tropical regions. This study demonstrates the ability of the IASI instrument to detect OCS in the troposphere and observe a reasonable seasonal cycle indicative of being driven by photosynthesis. Further data acquisition is recommended to gain insight into inter-annual variability in OCS. This novel work will also help improve our understanding of the role of vegetation in the carbon cycle, particularly when utilised in inversion methods.

Abstract. Tropospheric ozone (O3) is a harmful secondary atmospheric pollutant and an important greenhouse gas. Multiple satellite records have shown conflicting long-term O3 trends across regions of the globe, including Europe. Here, we investigate lower-tropospheric sub-column O3 (LTCO3, surface – 450 hPa) records from three ultraviolet (UV) sounders produced by the Rutherford Appleton Laboratory (RAL): the Global Ozone Monitoring Experiment (GOME, 1996–2010), the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY, 2003–2011) and the Ozone Monitoring Instrument (OMI, 2005–2017). GOME and SCIAMACHY detect negative trends of approximately −0.2 DU yr−1, while OMI indicates a negligible trend. The TOMCAT 3-D chemical transport model was used to investigate processes driving simulated trends and to identify possible reasons for satellite trend discrepancies. The simulated LTCO3 trends were negligible (consistent with ozonesonde trends), even when spatiotemporally co-located to the satellite level-2 swath data and convolved by averaging kernels (i.e. a measure of the satellite retrieval vertical sensitivity). Model sensitivity experiments with the emissions or meteorology fixed to 2008 also showed negligible LTCO3 trends between 1996 and 2018, indicating that changes in emissions and meteorology had a limited impact on LTCO3 temporal evolution. Given the substantial decrease in air pollutant emissions, this was unexpected, while year-to-year variability dominated the meteorological influence on LTCO3. Finally, we find a negligible trend in the long-term stratosphere O3 flux into the free troposphere over this period arriving over Europe. Overall, our observational and modelling analysis indicates that European LTCO3 trends have been stable between 1996 and 2018.

Abstract. The complex distribution of CO2 in the upper troposphere and lower stratosphere (UTLS) results from the interplay of different processes and mechanisms. However, in such difficult-to-access regions of the atmosphere our understanding of the CO2 variability remains limited. Using vertical trace gas profiles derived from measurements with the balloon-based AirCore technique for validation, we investigate the UTLS and stratospheric CO2 distribution simulated with the ECHAM/MESSy Atmospheric Chemistry (EMAC) global chemistry-climate model. By simulating an artificial, deseasonalised CO2 tracer, we disentangle the CO2 seasonal signal from long-term trend and transport contribution. This approach allows us to study the CO2 seasonal cycle in a unique way in remote areas and on a global scale. Our results show that the tropospheric CO2 seasonal cycle propagates upwards into the lowermost stratosphere and is most modulated in the extra-tropics between 300–100 hPa, characterised by a 50 % amplitude dampening and a 4-month phase shift in the Northern Hemisphere mid-latitudes. During this propagation the seasonal cycle shape is also tilted, which is associated with the transport barrier related to the strength of the subtropical jet. In the stratosphere, we identified both, a vertical and a horizontal “tape recorder” of the CO2 seasonal cycle. Originating in the tropical tropopause region this imprint is linked to the upwelling and the shallow branch of the Brewer-Dobson-circulation. As the CO2 seasonal signal carries information about transport processes on different timescales, the newly introduced tracer is a very useful diagnostic tool and would also be a suitable metric for model intercomparisons.

Abstract. Biogenic isoprene emissions play an important role in air quality so it is important to quantify their response to extreme events such as drought. This study aims at constraining drought stress on biogenic isoprene emissions in South Korea using satellite formaldehyde (HCHO) column, the key product of isoprene oxidation, and a chemistry transport model (GEOS-Chem) with Model of Emissions of Gases and Aerosols from Nature version 2.1 (MEGAN2.1). It was found that the HCHO column from the Ozone Monitoring Instrument (OMI) increased by 5.4 % under the drought condition compared to the normal condition, but GEOS-Chem simulated a 20.23 % increase indicating an overestimation of isoprene emissions under drought. We implemented two existing drought stress algorithms in MEGAN2.1 and found they were not effective to reduce HCHO column biases in South Korea because those algorithms were proposed and developed for the Southeast United States (SE US). To improve this, we applied an iterative finite difference mass balance (IFDMB) method to estimate isoprene emissions using the OMI HCHO column. With this method, isoprene emissions were reduced by 60 % under the drought condition compared to those simulated by the standard MEGAN2.1 implemented in GEOS-Chem. The increase of HCHO column under the drought condition compared to the normal condition was also reduced to 10.71 %, which is comparable to that in the satellite retrievals. Based on isoprene emission difference between MEGAN2.1 and IFDMB, we developed the empirical equations to adjust isoprene emissions in South Korea that also improved model prediction of the secondary pollutant such as ozone.

Abstract. Understanding the urban-rural patterns and driving drivers behind the recent decrease in particulate matter (PM) pollution across eastern China is essential for assessing the efficacy of environmental policies and ensuring equitable health co-benefits. By employing an interpretable, end-to-end machine learning framework integrating satellite observations, meteorological factors, and auxiliary datasets, this study reveals changes in urban and rural PM pollution and the underlying drivers. During the period 2015–2023, the average decrease rates of PM10 and PM2.5 in eastern China were −4.02 ± 1.29 and −2.41 ± 0.91 µg m−3 yr−1, respectively. The rate of decrease in urban areas was higher than that in rural areas, which played a dominant role in PM reduction. Significant reductions in PM concentrations were observed in urban core areas, suburbs, towns and regions with high agricultural pressure. The interpretability analysis showed that temperature and interannual variability were the main drivers of PM pollution reduction. However, only interannual variability showed a significant decreasing trend in its effect on PM pollution, while other driving factors showed periodic variations. Furthermore, there were differences in the drivers of PM reduction between urban and rural areas, particularly with interannual variability in particular contributing to PM pollution reduction in urban areas, but having a lesser impact in most rural areas. This study reveals the urban-rural patterns of PM pollution reduction in eastern China, and highlights the need for differentiated air pollution control strategies in urban and rural areas.

Abstract. In September 2022, the METHANE-To-Go Africa (MTGA) scientific aircraft campaign, part of UNEP's International Methane Emissions Observatory (IMEO) Methane Science Studies, conducted the first methane (CH4) emissions measurements from the offshore oil and gas sector in West Africa. Using aircraft-based mass balance methods, we quantified trace gas emissions from all 57 Angolan offshore facilities, estimating total sector emissions and assessing 30 individual sites and 10 facility groups, thereby providing the first independent dataset for this previously unstudied region. Emissions were generally consistent across repeated measurements, but two facilities showed intermittent high emissions of 10 and 4 t h−1, significantly influencing total emissions. Older, shallow-water platforms emitted more than newer, deep-water floating production facilities. These findings suggest that production volume is a poor proxy for CH4 emissions, while age and maintenance status are more reliable indicators. Due to operational variability, regular monitoring remains essential, particularly for older facilities. We estimate total CH4 emissions at 16.9±5.3th-1, only 20 %–22 % of EDGAR and CAMS inventory estimates, but over twice the operator-reported values. Additional trace gas measurements (CO2, CO, C2H6, SO2, NOy, and aerosols) suggest CH4 stems primarily from fugitive emissions and venting, not combustion. The calculated carbon intensity of Angolan offshore oil and gas is 3.4±0.8 g CO2 eq MJ−1, with nearly equal contributions from CH4 and CO2. Notably, shallow-water platforms are CH4-dominated, while deep-water facilities mostly emit CO2. These results improve our understanding of greenhouse gas emissions from offshore production in a key oil- and gas-producing region.

Abstract. The diurnal susceptibility of clouds and their radiative properties to aerosols is examined during their Lagrangian transition from subtropical stratocumulus to shallow cumulus regimes. Using large-eddy simulations, we analyze the six-day evolution of an air mass along a 3800 km observed trajectory from the coast of Peru toward the equator. Pristine and polluted scenarios are simulated with forcing imposed from weather reanalysis. The polluted scenario exhibits stronger diurnal variations in cloud water, cloud fraction, and albedo, with enhanced nighttime entrainment and suppressed precipitation. The overall response of cloud properties and outgoing shortwave radiation to droplet number concentration follows a distinct diurnal pattern: strong positive cloud adjustments dominate at night and in the morning, while weak negative adjustments prevail in the afternoon. This cycle is driven by the competition between precipitation suppression, which enhances cloud water and coverage, and entrainment drying, which depletes them. In polluted conditions, enhanced entrainment leads to a deeper and more decoupled boundary layer that cannot be sustained by surface fluxes in the afternoon, resulting in negative cloud adjustments. The enhanced entrainment rate under polluted conditions is caused by the reduced sedimentation of cloud and precipitation water from the entrainment zone. While the Twomey effect dominates the diurnal average albedo response, the diurnal variation in the competing cloud adjustments lead to a near-neutral net susceptibility in the afternoon, highlighting the critical role of diurnally varying processes in aerosol-cloud interactions.

Abstract. Both size and mixing state of Black Carbon (BC) are critical climate-relevant physical parameters. It remains a challenge for ambient measurements to characterize their variability across different atmospheric conditions particularly in outflow regions. To investigate how BC's physical properties are determined in source regions and altered during transport, we conducted 23 flight measurements of BC, CO, and CO2 over the Yellow Sea from 2021 to 2022. The refractory BC mass concentration (MrBC) varied by up to two orders of magnitude between near sea surface and around 5 km above sea level, and Planetary Boundary Layer height-dependence of MrBC was stronger in winter than in spring. Smallest rBC's mass median diameter (MMD, 163.4 nm) observed in South Korea-sourced air indicated fresh urban emissions, whereas larger MMD, enhanced internal mixing, and higher rBC/CO slopes were exhibited in the air masses from North Korea and China, reflecting additional emissions from biomass and coal combustion. Both MMD and internal mixing tended to decrease with altitude, highlighting the wet scavenging effect during particle transport. When accumulated precipitation exceeded 1 mm, MrBC decreased by more than 50 %, with moderate decrease in MMD and internal mixing. As a result, overall BC transport efficiency declined to 1/e within 5.5 d. These observations reflect the complex effects of source region, seasonality, and wet removal on varying rBC distributions in the outflow region. These findings provide valuable observational constraints for improving model representations of the size distribution and mixing state of ambient BC particles in outflow regions.