Abstract. The efficacy of the climate intervention method known as cirrus cloud thinning (CCT) is difficult to evaluate in climate models, largely due to uncertainties governing the relative contributions of homogeneous and heterogeneous ice nucleation. Here we take a different approach by employing recent satellite retrievals from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) which provide estimates of the fraction of cirrus clouds dominated by homogeneous and heterogeneous ice nucleation and their associated physical properties. We employ a radiative transfer model (RTM) to quantify the cloud radiative effect for homogeneous and heterogeneous cirrus clouds at the top of atmosphere (TOA), Earth's surface, and within the atmosphere. The RTM experiments are initialized using cirrus microphysical profiles derived from CALIPSO retrievals for cirrus clouds dominated by homogeneous and heterogeneous ice nucleation across different regions (Arctic, Antarctic, and midlatitude) and surface types (ocean and land). We define two bounds: the lower bound assumes a full microphysical transition from the observed composition of homogeneous- and heterogeneous-dominated cirrus to only heterogeneous cirrus and production of new cirrus. The upper bound assumes production of new cirrus and that the atmospheric dynamics enables homogeneous freezing nucleation to occur regardless of the concentration of ice nucleating particles. Based on these bounds, we estimate an instantaneous surface effect ranging from −0.5 to +0.6 W m−2 and a TOA effect from −0.9 to +1.1 W m−2, respectively, showing the possibility of both cooling and warming. Recommendations are provided to improve the treatment of cirrus clouds in climate models.

Abstract. Arctic amplification manifests as a pervasive warming trend emerging over the past century in near-surface air temperature throughout the Arctic that is double the globally averaged temperature increase throughout most of the year. It results from complex processes involving oceanic, atmospheric and terrestrial components which require detailed study to discern roles of the fundamental processes involved to improve predictions of the Arctic environment. We report on signals that are beginning to emerge, on a timescale predicted by recent satellite remote sensing studies, from the unique 25 year record of detailed surface-based radiometer measurements obtained by the US Department of Energy Atmospheric Radiation Measurement (ARM) Facility North Slope of Alaska (NSA) site at Utqiaġvik, Alaska. Statistically significant warming trends are found at the site in the boreal fall, while a decrease in net radiation occurs in late summer. This decrease is driven primarily by the decrease in shortwave radiation resulting from increasing cloud liquid water path as observed by the microwave radiometer. The magnitude of the liquid water path trends is slightly larger than the root mean square error (RMSE) due to natural variability, while the magnitude of the net radiation trends is smaller than the RMSE natural variability, emphasizing that these signals are only just emerging. Analysis of prevailing meteorological regimes linking NSA with the Arctic Ocean and subarctic latitudes, and atmospheric rivers, suggests that specific changing circulation patterns are the primary driver for these summertime trends.

Abstract. Understanding the hygroscopicity and mixing state of atmospheric aerosol particles is crucial for accurately assessing their role in cloud formation and subsequent climate impacts. However, measurements in the lower free troposphere – a representative atmospheric layer characterizing regional background conditions in aerosol transport and atmospheric evolution – remain sparse, especially in regions influenced by both anthropogenic emissions and long-range transported air masses. This study adds further data on size-resolved hygroscopicity and mixing state measurements of aerosols at Mt. Hua (2060 m a.s.l., central China) during October–November 2021 using a Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA). Results reveal a clear size-dependence of aerosol hygroscopicity, with the mean hygroscopicity parameter (κmean) increased from 0.20 for 30 nm particles to 0.30 for 200 nm particles. The ambient submicron aerosols were primarily externally mixed, dominated by more-hygroscopic (MH) particles, with no significant diurnal variation, indicating minimal influence from boundary layer dynamics. Starting from 6 November, the combined influence of mineral dust and regional heating activities led to a notable reduction in aerosol hygroscopicity, especially in larger particles. This decline was driven by an increase in weakly hygroscopic components such as mineral dust, black carbon, and organic compounds, highlighting the impact of both long-range transport and regional emissions on aerosol composition. Notably, during episodes of striking high relative humidity (RH > 80 %) in Cluster 5, atmospheric aerosols containing mineral dust showed unexpected hygroscopic enhancement, suggesting in situ RH-driven chemical processing that increased aerosol hygroscopicity. Atmospheric aerosols at Mt. Hua displayed distinct hygroscopic properties compared to other high-altitude sites, underscoring regional differences in aerosol sources and free tropospheric processing. These findings advance our understanding of aerosol aging and processes in the lower free troposphere over central China, and offer crucial observational constraints for modeling aerosol–cloud interaction and regional climate impacts.

Abstract. Non-methane volatile organic compounds (NMVOCs) are key precursors of ozone and secondary organic aerosols. As one of the world’s largest NMVOC emitters, accurate emission inventories are essential for understanding and mitigating air pollution in China. Commonly-used inventories (e.g., MEIC) are largely based on bottom-up methods, which often fail to capture the spatiotemporal variability of NMVOC emissions, resulting in significant model-observation mismatches. This study evaluates the shape factor, filtered data volume, and monthly mean biases of OMI, OMPS, and TROPOMI formaldehyde products, with the latest OMPS and TROPOMI retrievals offering substantially higher effective spatiotemporal coverage. Monthly NMVOC emissions over China in 2020 are then optimized by independently assimilating formaldehyde retrievals either from OMPS or from TROPOMI, using a self-developed 4DEnVar assimilation emission inversion system. The OMPS- and TROPOMI-driven assimilation yields consistent seasonal and regional increments in NMVOC emissions in general, but distinctions are also notable. A consistency analysis is introduced to assess the reliability of these two posterior emissions. Highly consistent increments are obtained in the North China Plain (May–June), the Yangtze River Delta and Pearl River Delta (January–March, October–December), and the Sichuan Basin (January, June–December). These adjustments significantly improve surface ozone simulations, with 81.25 % of consistent cases demonstrating reduced biases and an average RMSE reduction of 24.7 %. These findings highlight the effectiveness of OMPS and TROPOMI formaldehyde assimilation, coupled with consistency analysis, in refining NMVOC emission estimates and enhancing ozone simulation accuracy. Similar promising results are achieved in the OMPS/TROPOMI-based NMVOC emission inversion in 2019.

Abstract. Atmospheric amines critically contribute to secondary aerosols formation via heterogeneous reactions, yet the molecular mechanisms governing heterogeneous amination chemistry of aerosols remain unclear. Here, we utilize an integrated tandem flow-tube system coupled with online ultrahigh-resolution mass spectrometry to elucidate the amination chemistry of ethylamine (EA) with representative organic aerosol components, including C20–C54 secondary ozonides (SOZs), C17–C27 carboxylic acids, and aldehydes. Our experiments provide evidence for the formation of four key intermediates: hydroxyl peroxyamines, amino hydroperoxides, peroxyamines, and amino ethers, which mediate SOZs conversion to hydroxyimines, amides, and imines. Furthermore, dihydroxylamines and hydroxylamines are identified as characteristic intermediates in carboxylic acids and aldehydes amination. Quantitative heterogeneous reactivity measurements (Δγeff) reveal that SOZs exhibit a pronounced inverse dependence on carbon chain length, e.g., C21 SOZ (Δγeff = 1.0 × 10−4) > C49 SOZ (Δγeff = 5.7 × 10−6), with consistently lower reactivity than acids and aldehydes, e.g., C17 acid (Δγeff = 2.3 × 10−4). The amination mechanism of SOZs is initiated by EA addition, followed by either hydroxyl peroxyamines-mediated dehydration yielding hydroxyimines and amides, or amino hydroperoxides-driven H2O2 elimination forming imines. For carboxylic acids and aldehydes, EA addition leads to dihydroxylamines and hydroxylamines formation, which subsequently dehydrate to produce amides and imines. These findings provide a mechanistic framework for understanding amine-driven aerosol aging processes that affects atmospheric chemistry, air quality, and climate systems.

Abstract. The sparse network of Arctic land-based stations results in significant data gaps for atmospheric methane (CH4), particularly in sea-ice covered regions. Ship-based measurements can complement these data, improving understanding of regional and seasonal CH4 variability. This study presents continuous atmospheric ship-borne recordings of CH4 concentration and isotopic composition above the open ocean and sea-ice surface during Leg 4 (June–July 2020) and Leg 5 (August–September 2020) of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in the central Arctic. Our measurements aim to enhance process understanding by identifying local emission sources and refining transport pathway and atmospheric mixing analysis. We compared three contamination-filtering methods and applied the Pollution Detection Algorithm to the raw data. Comparison with nearby land-based stations and their seasonal cycles suggests ship-borne data capture dynamic changes in CH4 sources, sinks, and transport processes, beyond seasonality. To unravel the underlying processes, we identified air mass transport pathways within the atmospheric boundary layer above the Arctic Ocean and their source areas using five-day backward trajectories from the LAGRANTO tool, based on ERA5 wind field data. Our analysis reveals that CH4 variability is driven by air masses predominantly influenced by open ocean and sea-ice-covered regions, with sea-ice dynamics imparting specific modifications along transport pathways. These findings underscore the importance of air mass transport and origin in shaping central Arctic CH4 variability. The study highlights the value of integrating ship-borne CH4 measurements with trajectory analysis to improve process-level understanding and support enhanced regional modelling.

Abstract. Assessing the impact of biomass burning (BB) emissions on tropospheric ozone is critical for understanding air pollution and climate interactions. BB emission inventories like Global Fire Emissions Database and Global Fire Assimilation System, typically based on sun-synchronous satellite observations, report emissions on daily, weekly or longer timescales with empirically derived factors generally used to overlay diurnal variations. To explore the sensitivity of tropospheric ozone to diurnal variability, we incorporated day-specific hourly BB variations inferred from geostationary satellite data into the GEOS-Chem atmospheric chemistry transport model. The simulations were compared with those using established inventories and evaluated against in situ and satellite observations. Simulations with real hourly-resolved emissions produce comparable surface ozone biases (−1.54 to +9.09 ppbv vs. −1.58 to +9.13 ppbv) and marginally higher correlations with TROPOMI nitrogen dioxide (r=0.80–0.89) and OMI ozone (r=0.80–0.94). Although the statistical improvements are limited, the geostationary-driven approach reveals pronounced regional ozone differences and mechanistic insights into the role of diurnal fire variability. Data-driven diurnal BB variations across Africa cause significant surface ozone changes (−8.57 to +21.88 ppbv) and alter tropospheric ozone columns by −0.41 to 1.09 DU, particularly in regions with intense fire activity like Angola and Zambia. These changes propagate globally, shifting regional OH concentrations by −4.4 % to +51.7 %. These findings emphasize the critical role of accurately describing diurnal BB variations in atmospheric models to better quantify its impacts on atmospheric composition, providing insights for Earth system model development and the use of geostationary-derived BB emissions datasets.

Abstract. Observations from airborne field campaigns are used to study the interplay between boundary-layer thermals and clouds in the trades. The size distributions of thermal and cloud-base chords inferred from turbulence and horizontal lidar-radar measurements are robustly described by the sum of two exponentials. Analytical calculations and statistical simulations show that the merging of objects is sufficient to explain the two exponentials, representing, respectively, the populations of merged- and unmerged-object chords. They also show how circulations induced by convective objects facilitate the merging process. The observed day-to-day variability of these populations at cloud base can thus be tied to the variability of thermal merging across the depth of the subcloud layer. Clouds rooted in unmerged thermals are small and shallow while those rooted in merged thermals are wider and deeper. An intricate interplay between thermal- and cloud-merging arises: when thermal merging is weak, thermal number density is high and cloud bases merge easily, leading to strong mesoscale mass fluxes and “Gravel” shallow mesoscale organizations. In contrast, when thermal merging is strong, clouds are fed by sparser but wider thermals, leading to longer cloud lifetimes but weaker cloud merging, weaker mesoscale mass fluxes, and “Flower” mesoscale organizations. This interplay between thermal- and cloud-merging imposes an upper bound on cloud coverage and suggests a negative feedback on the growth of mesoscale circulations. Thermal merging also controls observed size distributions of thermals in deep convective regimes. The merging process thus appears to be a fundamental player in the mesoscale organization of convection.

Abstract. A reliable quantification of greenhouse gas emissions is important for climate change mitigation strategies. Inverse methods based on observations and atmospheric transport simulations can support emission quantification at the national scale, yet, they are often limited by the observing systems, transport model uncertainties, and inversion methodologies. This two-part study introduces a system for observation-based, regional methane flux estimation. In the present Part 2, we apply this system to estimate German methane emissions in 2021. The numerical weather prediction model ICON with its ART module for trace gases is used to simulate the atmospheric transport while estimating uncertainties using a transport ensemble. We use a priori fluxes from national reporting to facilitate the validation of reported fluxes. Posterior fluxes are estimated with a modified synthesis inversion method introduced in Part 1, relying on in-situ observations. Compared to the a priori, we find a significant increase in methane emissions in Germany and in the Benelux. We estimate German methane emissions (32±19) % higher than the anthropogenic emissions in the national inventory, and our inversion method attributes this difference mainly to the agricultural sector, although separation from Land Use, Land Use Change and Forestry (LULUCF) as well as natural fluxes requires further research. The combination of an ensemble-enhanced numerical weather prediction model for atmospheric transport and good observation coverage paves the way to sector-specific, observation-based national emission estimates.

Abstract. We investigate the dynamics of the atmospheric Boundary Layer (BL) over the Atlantic Ocean, with a focus on the region surrounding Cabo Verde during the Joint Aeolus Tropical Atlantic Campaign (JATAC) and the ASKOS experiment, using a combination of ground-based PollyXT and Doppler lidars, satellite lidar data from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), radiosondes, and the model outputs of the Integrated Forecasting System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). The comparison of CALIPSO lidar results with ECMWF/IFS reanalysis for 2012–2022, revealed good agreement for BL top over open ocean regions but weaker relation over dust-affected areas of the African continent. In these regions, daytime CALIPSO retrievals typically indicate lower BL tops than ECMWF, while at night CALIPSO often detects aerosols within the residual layer, leading to higher estimates than the model. Observations in Cabo Verde highlight distinctive Marine Atmospheric Boundary Layer (MABL) characteristics, such as limited diurnal evolution, but also show the potential for BL heights to reach up to 1 km, driven by factors like strong winds that increase mechanical turbulence. Additionally, the technical and physical challenges in estimating the BL height using different datasets and methods are discussed, examining cases with different thermodynamical conditions and aerosol load that directly affect the dynamics of the BL. The findings underline the strengths and limitations of different observational and modeling approaches, and emphasizes on the importance of considering local meteorology and aerosol conditions when interpreting BL height.