Abstract. Closed-to-open cell mixed-phase cloud transitions within marine cold air outbreaks subjected to strong turbulent surface fluxes remain poorly understood despite their importance to high-latitude climate. The Cold-Air outbreak Experiment in the Sub-Arctic Region (CAESAR) research aircraft sampled closed-cells with cloud condensation nuclei concentrations surpassing 680 cm-3, decreasing to 90 cm-3 across a transition to open-cells. The aerosol likely originated from Siberian industrial emissions. With fetch, liquid water paths (LWPs) increase from 120 g m-2 to 270 g m-2 and cloud-top effective diameters increase from 10 μm to 16 μm, coincident with more riming. Ice particle number concentrations (Ni) are generally 2 L-1 or less, but exceed ice nucleating particle number concentrations by 100x. As the cloud-top inversion weakens and the boundary layer deepens further, ice precipitation co-exists with lidar-observed surface cold pools, modulated by entrainment events, juxtaposed with surface-based plumes of warm moist air. Open-cells contain isolated LWP peaks surpassing 500 g m-2 collocated with strong updrafts, adjacent to glaciated cloud. Ni surpasses 10 L-1 at cloud temperatures < -15 °C. Precipitation shafts contain abundant large graupel (> 5 mm diameter) with liquid-equivalent precipitation intensities reaching 3 mm hr-1 developing cold pools with virtual potential temperature depressions reaching 1.3 K. Nonetheless, buoyancy fluxes of 200-250 W m-2 prevent sub-cloud decoupling. The updrafts supporting liquid water production occur at the upwind edge of the cold pools. This case expands the observations needed to better understand mixed-phase Arctic cloud processes.

Residential solid fuel combustion is a significant source of products of incomplete combustion (PICs), raising concerns about air pollution and public health. While fuel and stove effects on PIC emission are well-studied, the role of oxygen levels at different altitudes is less understood. This study quantitatively evaluated the influence of oxygen levels (18.2%-26.6%) on emission factors (EFs) of fine particulate matter (PM2.5), organic carbon (OC), elemental carbon (EC), carbon monoxide (CO), nitrogen oxides (NOx), and sulfur dioxide (SO2), as well as on the oxidative potential (OP) of PM2.5 from residential biomass combustion. The PIC emissions were significantly influenced by oxygen levels, with EFs of PM2.5, OC, EC, and CO at 18.2% oxygen levels being 6.9-49 g/kg, 0.39-5.7 g/kg, 0.050-0.76 g/kg, and 65.9-244 g/kg, respectively, significantly higher than those at 20.4%-26.6%, by up to 90%, consistent with reduced combustion efficiency under oxygen-deficient conditions. Notably, oxygen-enriched combustion (>24.4%) may increase PM2.5, EC, SO2, and particularly NOx emissions, likely due to enhanced combustion temperature. The effect of oxygen levels on OP is fuel-dependent, with no consistent trend observed across different biomass types. Additionally, differences in PIC emissions among fuels were amplified at oxygen-deficient combustion due to varying sensitivity, emphasizing the need to consider fuel-specific responses in mitigation strategies, particularly in high-altitude regions. The ratio of char-EC/soot-EC remained consistent across oxygen levels, serving as a more reliable indicator in source apportionment than the OC/EC ratios. These findings offer valuable insights for designing altitude-specific emission control strategies and improving the understanding of oxygen effects on PIC emissions. Plain Language Summary This study examines how oxygen availability at different altitudes affects pollutant emissions and particle toxicity from residential biomass burning. We burned multiple biomass fuels under controlled oxygen levels (18.2%-26.6%) to simulate conditions from high altitude (2,800 m) to sea level. Under oxygen-deficient conditions (18.2%, simulating 2,800 m), emissions of products of incomplete combustion (PICs) such as fine particulate matter (PM2.5), organic carbon (OC), and carbon monoxide (CO) increased substantially. Additionally, oxygen-enriched combustion (>24.4%) may increase some pollutant emissions (in particular nitrogen oxides). Combustion at a moderate oxygen level (22.3%-24.4%, 700-1,400 m) appeared to offer a better balance with lower PICs emissions. The oxidative potential of PM2.5 depended on oxygen conditions and fuel type, implying fuel- and altitude-specific health impacts. The char-EC (elemental carbon)/soot-EC ratio remains stable across oxygen levels, offering a more reliable source indicator than OC/EC ratios. Overall, these findings improve our understanding of altitude-dependent emission behavior and support a more targeted air quality management in highland regions.

The Arctic is warming at more than twice the global average rate, a phenomenon known as Arctic amplification (Rantanen et al., 2022). In addition to greenhouse gases, short-lived climate forcers play a critical role in modulating Arctic climate through their impacts on radiation, cloud properties, and the surface energy balance (e.g. AMAP, 2015, 2021). Among these forcers, elemental carbon (EC) is of particular importance due to its strong light-absorbing properties and its ability to reduce surface albedo when deposited on snow and ice. Furthermore, aged EC particles transported to the Arctic can act as cloud condensation nuclei, influencing cloud microphysical processes and thereby modifying Arctic radiative forcing and climate feedbacks.In this study, we investigate long-term trends in EC concentrations and their potential drivers in the high Arctic using 16 years of continuous EC measurements from the Villum Research Station in northeast Greenland. We combine in situ observations with Lagrangian transport modelling and back-trajectory analyses to assess the relative contributions of changes in source-region emissions, transport pathway variability, and wet scavenging processes to the observed EC trends. Robust non-parametric statistical methods are applied to assess monotonic trends over the full observational period and before 2020, enabling a systematic comparison between the declining and stagnating phases. This integrated observational–modelling framework provides new constraints on the processes controlling EC variability in the Arctic and advances our understanding of how anthropogenic emission reductions are reflected in Arctic atmospheric composition under a rapidly evolving climate.

Environmentally persistent free radicals (EPFRs) can generate reactive oxygen species, leading to adverse health effects, with residential biomass burning recognized as a primary source. However, a significant knowledge gap remains regarding the effects of oxygen levels on EPFR emissions, a critical factor in emission dynamics and highly relevant for populations at varying altitudes. This study explored emission factors (EFs), formation mechanisms, and properties of EPFRs from biomass burning under varying oxygen proportions (18.2%-26.6%), simulating altitudes from sea level to 2,800 m. Significantly higher EPFR emissions from biomass burning were observed under lower oxygen proportion due to reduced combustion efficiency (r = -0.789, p < 0.001). EPFR emissions from wood, straw, and corn cob combustion at 18.2% oxygen proportion were up to 29, 9.0, and 1.8 times higher than those observed under higher oxygen proportions (20.4%-22.3%), respectively. The lower sensitivity of corn cob combustion to oxygen variations suggested its suitability for high-altitude regions. At 22.3%-26.6% oxygen proportions, EFEPFRs showed no significant variations, likely due to higher combustion temperatures promoting electron transfer, while radical diversity continued to increase. Notably, combustion under higher oxygen proportions might form more reactive oxygen-centered EPFRs, raising potential toxicity risks and highlighting the trade-off between emission reductions and changes in physicochemical properties. EPFRs formation, mediated by polycyclic aromatic hydrocarbons (PAHs) and trace element, was governed primarily by their intrinsic physicochemical properties rather than concentrations, with medium molecular weight PAHs identified as key precursors. This study provides critical oxygen-specific insights into EPFR formation and a valuable reference for mitigating associated health risks. Plain Language Summary Burning biomass, such as wood, straw, or corn cobs, in homes releases harmful pollutants called environmentally persistent free radicals (EPFRs), which damage health by creating reactive oxygen species. This study simulated biomass burning at altitudes from sea level to 2,800 m by adjusting oxygen levels from 18.2% to 26.6% to examine how oxygen impacts EPFR emissions. We found that burning biomass at higher altitudes could produce much more EPFRs due to less efficient combustion. For example, burning wood, straw, and corn cobs at 18.2% oxygen (simulating 2,800 m) released up to 29, 9.0, and 1.8 times more EPFRs, respectively, compared to 20.4%-22.3% oxygen (1,400-2,100 m). Interestingly, corn cobs were less affected by oxygen changes, making them a better option for high-altitude areas. Moreover, at oxygen levels above 22.3% (sea level to 1,400 m), oxygen variation had little effect on EPFR emissions. Notably, at lower altitudes, EPFRs may be more chemically reactive and harmful. This highlights that while reducing total emissions is important, changes in pollutant properties under different conditions also require attention. Our findings offer valuable insights into developing pollutant control solutions, crucial for high-altitude populations who face greater vulnerability due to low oxygen levels and heavy biomass use.

Abstract. The NASA airborne Arctic Radiation-Cloud-aerosol-Surface-Interaction Experiment (ARCSIX) collected a unique data set providing a near-simultaneous characterization of radiative fluxes, surface, cloud, and aerosol particle properties to address science questions on the surface radiation budget, the processes governing the cloud lifecycle, atmospheric composition, and the interactions between the surface and atmosphere. The overarching goal of ARCSIX was to quantify the contributions of surface, clouds, aerosol particles, and precipitation to summer sea ice melt. ARCSIX consisted of two deployments in 2024 (Spring: 2024-05-28 through 2024-06-13 and Summer: 2024-07-25 through 2024-08-15) to capture pre- and post-melt conditions. ARCSIX provided coordinated remote sensing and in situ sampling using three aircraft in a high-flyer/low-flyer configuration. The NASA G-III served as the high-flying remote sensing platform with two lower flying in situ and near-target remote sensor observing platforms, NASA P-3B and SPEC Inc. Learjet. ARCSIX data are well-suited to improve satellite remote sensing capabilities in the Arctic. ARCSIX included an array of sea ice mass balance buoys deployed in the Lincoln Sea that were regularly overflown during the campaign. ARCSIX research flights spanned the Baffin Bay, Lincoln Sea, west and north of the Canadian Archipelago, and the Greenland north and northeast coasts. During the spring deployment, 19 research flights took place covering 114 flight hours: 10 flights and 68 hours by the P-3B and nine flights and 46 hours by the G-III. During summer, 24 research flights covered 136 flight hours: nine flights and 75 hours by the P-3B, five flights and 26 hours by the G-III, and 10 flights and 35 hours by the Learjet. A total of 13 coordinated flights with 2+ aircraft were carried out. This paper describes the ARCSIX flight strategy, instrumentation, and data set access, and usage details. ARCSIX data are publicly available at https://doi.org/10.5067/SUBORBITAL/ARCSIX/DATA001.

Substantial uncertainties remain in numerical models incorporating different planetary boundary layer (PBL) schemes when it comes to reproducing the detailed thermodynamic structure of the convective boundary layer (CBL), particularly for the level of neutral stability (zn), at which statically unstable lower CBL begins to transit into slightly stable upper CBL. Using multi-year radiosonde data from 12 stations and large-eddy simulation (LES), we examined the detailed CBL thermodynamic structure and processes over the Tibetan Plateau, particularly focusing on the impact of surface heating and entrainment on zn. The results indicated that the values of zn spatially ranged within 0.16-0.38zi on the plateau, with zi representing the CBL depth, and zn was higher in the southwestern region and lower in the southeastern region. Surface-/entrainment-induced large-scale thermals (corresponding to nonlocal fluxes) tended to suppress/elevate zn, due to warm air penetrating into the upper/ lower CBL, whereas small-scale eddies (corresponding to local fluxes) exert the opposite effect on zn. The LES results indicated that zn increased before 08:00 Local Time (about 80 min after sunrise), as surface-induced small eddies dominated during the early stage of CBL development. After this time, zn decreased as large-scale surfaceinduced thermals became more active. This improved understanding provides guidance for further improvement of PBL schemes. Plain language summary: Convective boundary layer (CBL) over the Tibetan Plateau plays an important role in the climate system in East Asia. Nevertheless, detailed CBL structure and the impact factors over the plateau have not been fully examined yet. Combining the multi-year radiosonde fine-resolution profiles of potential temperature (theta) with the large eddy simulation (LES), we investigate the CBL structure, particularly the position of neutral point (zn), a transition level separating the statically unstable lower CBL from the slightly statically stable upper CBL. We also examine the impacts of surface heating and entrainment process on detailed CBL structure. theta profiles exhibit a three-layer structure, with zn spatially varying over the plateau. Different eddies with different sizes initiated from the surface and from the entrainment zone at the CBL top synergistically affect the evolution of the CBL and the altitude of zn. This improved understanding of the detail CBL structure and the relevant physical processes provides a new angle to evaluate and calibrate numerical weather prediction (NWP) models with PBL schemes.

The Arctic is warming up to 4 times faster than the global average, leading to rapid ice melting and consequently, a drastic change of the sources and processing of aerosols and their impact on clouds. Monitoring of these changes over the Arctic is extremely sparse, especially in the most remote regions where harsh conditions make it difficult to carry out even simple measurements. To address these knowledge gaps and develop better and new methods of remote sensing of aerosols and clouds, the CleanCloud project carried out the field campaign CLeancloud Arctic VIllum ExpeRiment (CLAVIER) at Villum Research Station (VRS) in northeast Greenland to study aerosol-cloud interaction (ACI) using in-situ surface and remote sensing as well as airborne measurements.CLAVIER covered two phase; spring (April) and summer (July/August) 2024, each lasting for one month. We have employed the existing in-situ surface aerosol monitoring at VRS, which includes a Scanning Mobility Particle Sizer (SMPS), a Cloud Condensation Nuclei Counter (CCNC), a High-Volume Sampler (HVS), a Nephelometer, an Aethalometer, a Neutral cluster and Air Ion Spectrometer (NAIS), a wind lidar and a ceilometer. During CLAVIER, the site was additionally equipped with a AeRosol aerosol-cloud lIdar System (ARIS lidar) and a Wideband Integrated Bioaerosol Sensor (WIBS-5/NEO) to provide realtime measurement of aerosols and fluorescent particles to infer the presence of bioaerosols and their potential contribution to Ice Nucelating Particles (INP). In addition, a W-band Cloud Doppler Radar (WProf) and a tethered balloon (Helikite) was operated during the spring phase. The helikite was equipped with aerosol and cloud instrumentation, including a Portable Optical Particle Spectrometer (POPS), a Miniaturized Scanning Electrical Mobility Sizer (mSEMS), a Single-channel tricolor absorption photometer (STAP) and a miniaturized Cloud Droplet Analyzer (miniCDA), and a filter sampler with the new nano-electromechanical membrane FTIR (NEMS-FTIR) technique. A second tethered balloon was also employed for meteorological and flux measurements. In the summer phase, a Proton-Transfer-Reaction Mass Spectrometry (PTR-MS) was used to measure VOCs online and cartridge sampling was performed for offline sampling of VOCs, as well as a WELAS (white-light aerosol spectrometer) for size distribution of larger sizes and the newest aethalometer AE36s. Finally, summertime measurements were also coordinated with the NASA ARCSIX aircraft mission for clousure experiments. In order to get a better understanding of the processes related to aerosol-cloud interactions, several modelling activities were and are being carried out for the CLAVIER period. These include the Flexible Particle Dispersion Model (FLEXPART), the WRF-SIP model to study in detail the secondary ice production in clouds, OpenIFSv48 global model to simulate the aerosol composition and forcing during the campaign, and finally, the FLEXPART-SOSAA framework and the ADCHEM model to study in detail the aerosol chemistry and impacts on CCN.This presentation will provide an overview of these activities and some preliminary results.

Biomass burning (BB) significantly influences cloud condensation nuclei (CCN) concentrations over the southeastern Atlantic; however, aerosol hygroscopicity (kappa) - a key factor for CCN activation - remains poorly constrained during the BB season. This study investigates kappa variability using in situ measurements from Ascension Island during the 2016 and 2017 BB seasons. Results show substantial monthly variability, with kappa values lowest in August and increasing through October. On average, kappa was significantly higher in 2017 (similar to 0.55) than in 2016 (similar to 0.33), suggesting that the aerosols in 2017 were more hygroscopic and more easily activated as CCN. Sulfate and sea salt were the two dominant contributors to kappa and the primary drivers of its interannual variability. During the 2017 BB season, sulfate - the major inorganic component - accounted for similar to 34 % of the submicron aerosol mass, while sea salt, estimated via kappa-closure analysis, contributed similar to 17 %. The higher kappa in 2017 was largely attributed to increased sea salt, likely driven by stronger marine winds. Approximately 67 % of sulfate was linked to BB emissions. Variations in BB combustion efficiency, modulated by regional meteorology, influenced sulfate fraction and thus kappa values. Specifically, higher relative humidity and lower wind speeds over BB source regions in 2017 favored smoldering combustion, explaining the higher sulfate fraction. Overall, the observed interannual differences in aerosol hygroscopicity reflect the combined impacts of BB combustion characteristics and sea salt emissions, underscoring the critical roles of both BB and marine aerosol sources in regulating aerosol-cloud interactions over the southeastern Atlantic.

The residential sector in China is a major contributor to light-absorbing carbonaceous aerosols, including black carbon and brown carbon, which have significant impacts on climate change. This study developed a province-level inventory of optical emissions of carbonaceous aerosols from the residential sector in China from 1960 to 2019. The inventory was based on activity data from the PKU-GEMS database and absorption emission factors derived from laboratory-based combustion experiments, which reduced uncertainties associated with traditional mass-based methods relying on mass absorption efficiency. The dataset provided annual light absorption at the ultraviolet region (370 nm) and the infrared region (880 nm), offering valuable insights into the spatial and temporal trends of optical emissions from residential carbonaceous aerosols. This inventory would support more accurate evaluations of radiative forcing impacts of carbonaceous aerosols.

The northern region of China is not only a sensitive area for global climate change and a key region with prominent monsoon climate, but also a “hotspot” for global land-atmosphere coupling. Terrain and geomorphology in this area are complex with a large spatiotemporal variation in land surface characteristics, and the climate dynamics of land-atmosphere interaction is relatively significant. In addition, affected by interactions between circulation systems in the mid-to high latitudes and low latitudes, atmospheric circulations in this area are relatively active, which makes it easy to induce extreme meteorological events such as droughts, sand storms, rainstorms, and hail. In view of this, from the perspective of scientific innovation, the main research works in the field of land-air interaction in northern China since this century are systematically summarized. Seven new research advancements have been outlined, including the comprehensive observational and experimental system of land-atmosphere interaction in northern China, the spatiotemporal changes in physical quantities involved in land surface processes and their responses to summer monsoon, the response characteristics of land surface evapotranspiration to climate warming, land surface process parameters and parameterization schemes, the mechanism of land surface energy and water imbalance, the spatiotemporal changes and influence mechanisms of atmospheric boundary layer, and the relationship of land-atmosphere interaction with weather and climate. Based on the research progress summarized in this paper and the cutting-edge international study trend, we propose six key breakthroughs in the future for the study in this field: (1) the study should be based on the implementation and development of a new meteorologically integrated operational observation system that can observe and test conventional land-atmosphere interaction, (2) we need to improve our understanding of multi-interface exchange processes involved in land-atmosphere interaction, (3) mechanism study in the multi-scale land-atmosphere coupling process will be strengthened, (4) we need to deepen our understanding of the characteristics of land-atmosphere interaction in the specific environment of northern China, (5) the impact of land-atmosphere interaction on extreme weather and climate will be revealed, (6) multiple complicated feedback mechanisms between land-atmosphere interaction and climate warming will be explored. The information given in this paper will provide a scientific reference as well as a roadmap to promote land-atmosphere interaction study in northern China in the future.

Aerosol-Cloud Interactions (ACI) play an important role in the hydrological cycle and are strong modulators of cloud radiative forcing and climate. Nevertheless, they remain poorly understood and constrained despite decades of research, because many processes and feedbacks are highly uncertain and are challenging to describe in regional and global climate models. Even less understood is the role of natural aerosol and ACI in a post-fossil future, where anthropogenic emissions is vastly reduced but emerging “natural” aerosol sources modulated by anthropogenic climate change (biomass burning, bioaerosols, dust) will dominate. The CleanCloud project aims to address these uncertainties and as part of its activities carries out major observational field campaigns at climate hot spots (Arctic, Mediterranean) to better constrain ACI processes, and, evaluate, improve and develop new remote sensing algorithms for studying aerosols, clouds and ACI.The first CleanCloud campaign was based at the Villum Research Station (81.6° N, 16.6° W) in North Greenland, with in-situ and remote sensing measurements, and consisted of two phases, one during the spring (16 March – 13 April) and one during summer (16 July – 13 August) of 2024, in collaboration with the NASA ARCSIX aircraft mission. The second campaign, named “Cleancloud Helmos OrograPhic site experimeNt (CHOPIN)”, is ongoing and is anticipated to last for 6 months, starting from 1 October at Mt.Helmos (38.0o N, 22.2o E) in the Peloponnese, Greece. A series of in situ and remote sensing measurements were distributed at 6 sites along the lee side of Mt. Helmos, 4 at the Kalavrita ski Center’s parking lot (altitude ~ 1690 m), 1 at the foothills (altitude ~ 1747 m) and the Helmos Hellenic Atmospheric Aerosol and Climate Change station ((HAC)2) at the mountaintop (altitude ~ 2314 m) constrain almost every aspect of the aerosols, clouds and their interaction in the region – and especially in the orographic clouds that form at the (HAC)2 station.Here, we present results from these two campaigns to examine the cloud (e.g., droplet number concentration & size) and aerosol microphysical characteristics (size distribution, CCN concentrations, chemical composition, bioaerosol number concentration and type) and cloud-scale dynamical forcing (vertical velocity) to understand their contribution to ACI processes. Radiosondes to derive the vertical structure of the atmosphere, Lidar systems and sun photometers were used to determine the presence of aerosol amount, their altitude and type (bioaerosol, dust, pollution, biomass burning) as well as the aerosol optical and columnar microphysical properties, Doppler lidars for turbulence and cloud-scale dynamics, radars to obtain the microphysical properties of the clouds, and finally satellites to retrieve the spatio-temporal evolution of the clouds. Additionally, in the case of CHOPIN campaign, cloud probes and cloud samplers were used to perform in-cloud sampling and to obtain the cloud microphysical properties. Thus, the use of this synergistic approach enables us to perform closure studies and to improve our current retrievals to predict cloud properties. These extensive field campaigns will aid in developing new ACI-related retrieval algorithms, development/improvement of parameterizations, and the ESA EarthCARE calibration/validation activities.

The coronavirus disease COVID-19 is primarily transmitted through person-to-person contact, but meteorological conditions may influence its spread and severity. High levels of indoor ozone are known to inactivate the virus, yet the impact of low-level tropospheric ozone remains unclear. We thus hypothesized that tropospheric ozone, influenced by seasonal conditions, may mitigate viral spread. We studied the influence of ozone concentration, temperature, and humidity on the COVID-19 reproduction number in three large Israeli cities during 2020–2021. The effect of these parameters was also analyzed in laboratory experiments on viral inactivation. Field results show that in winter, under low temperature and low humidity, the COVID-19 reproduction number decreases with an increase in ozone concentration. In contrast, in the summer, under high temperature and high humidity, the COVID-19 reproduction number increases weakly with an increase in ozone concentration. This seasonal variation is attributed to ozone’s dual effects. Indeed, in winter, ozone inactivates the virus, whereas in summer, ozone primarily impacts human respiratory health, which indirectly favors COVID-19 transmission. Nonetheless, experimental results did not fully align with the field survey, showing increased virus inactivation with an increase in temperature.

Detailed convective boundary layer (CBL) structure and the impact factors over the Tibetan Plateau has not been clearly understood, particularly for the level of neutral stability (zn), at which statically unstable lower CBL begins to transit into slightly stable upper CBL. Substantial uncertainties still exist in numerical models with different planetary boundary layer (PBL) schemes to reproduce such detailed structure. In this study, detailed CBL structure and processes over the Tibetan Plateau are examined using multi-year radiosonde data and large-eddy simulation (LES), particularly focusing on the impact of surface heating and entrainment on zn. The results indicated that the values of zn spatially ranged within 0.16–0.38zi on the plateau, with zi representing the CBL depth, and zn was higher in the southwestern region and lower in the southeastern region. Surface-/entrainment-induced large-scale thermals (corresponding to nonlocal fluxes) tended to suppress/elevate zn, due to warm turbulence penetrating into the upper/lower CBL, whereas small-scale eddies (corresponding to local fluxes) played an opposite role on modifying zn. The LES results suggested that zn increased before 08:00 Local Time (about 80 minutes after sunrise) because surface-induced small eddies dominated during the early stage of CBL growth and zn decreased afterwards as large-scale surface-induced thermals became more active. These improved understanding provides guidance for further improvement of PBL schemes.

Residential solid fuel combustion significantly impacts air quality and human health. Pelletized biomass fuels are promoted as a cleaner alternative, particularly for those who cannot afford the high costs of gas/electricity, but their emission characteristics and potential effects remain poorly understood. The present laboratory-based study evaluated pollution emissions from pelletized biomass burning, including CH4 (methane), NMHC (nonmethane hydrocarbon compounds), CO, SO2, NOx, PM2.5 (particulate matter with an aerodynamic diameter ≤2.5 μm), OC (organic carbon), EC (element carbon), PAHs (polycyclic aromatic hydrocarbons), EPFRs (environmentally persistent free radicals), and OP (oxidative potential) of PM2.5, and compared with those from raw biomass burning. For most targets, except for SO2 and NOx, the mass-based emission factors for pelletized biomass were 62-96% lower than those for raw biomass. SO2 and NOx levels were negatively correlated with other air pollutants (p < 0.05). Based on real-world daily consumption data, this study estimated that households using pelletized biomass could achieve significant reductions (51-95%) in emissions of CH4, NMHC, CO, PM2.5, OC, EC, PAHs, and EPFRs compared to those using raw biomass, while the differences in emissions of NOx and SO2 were statistically insignificant. The reduction rate of benzo(a)pyrene-equivalent emissions was only 16%, much lower than the reduction in the total PAH mass (78%). This is primarily attributed to the more PAHs with high toxic potentials, such as dibenz(a,h)anthracene, in the pelletized biomass emissions. Consequently, impacts on human health associated with PAHs might be overestimated if only the mass of total PAHs was counted. The OP of particles from the pellet burning was also significantly lower than that from raw biomass by 96%. The results suggested that pelletized biomass could be a transitional substitution option that can significantly improve air quality and mitigate human exposure.

The CMA-ChemRA (China Regional Weakly Coupled Chemical-Weather Reanalysis System) was developped using China's first-generation global atmospheric reanalysis product (CRA-40) as initial fields and boundary conditions, coupled with the WRF-Chem atmospheric chemical model and the WRFDA/3DVar assimilation system. By constructing a joint background error covariance matrix, CMA-ChemRA achieves weak coupling between atmospheric chemistry and meteorological variables, enabling simultaneous assimilation of diverse data sources, including hourly observations from ground stations, wind profilers, upper-air soundings, aircraft reports, and atmospheric composition measurements. To extend the dataset to periods before 2013 when China lacked PM2.5 observations, the system incorporates a reconstructed PM2.5 dataset derived by AI from visibility inversion alongside various emission inventories. The CMA-ChemRA system produces a reanalysis product from 2007 to the present, with a spatial resolution of 15 km and an hourly temporal resolution. It includes three-dimensional isobaric and near-surface layers for 6 key elements PM2.5, PM10, O3, SO2, NO2, and CO, as well as meteorological variables. This product is updated in near real-time, with a 50-min lag for forecast updates. Evaluation of the system shows substantial improvements in accuracy, with significant reductions in root mean square error (RMSE) for the six elements in the near-surface atmospheric layer post-assimilation. The model's depiction of ground-level PM2.5 concentrations aligns well with independent observational data across five urban regions, showing a narrow RMSE range of 15.5 to 32.8 μg/m3. Additionally, CMA-ChemRA demonstrates strong performance in capturing the evolution of dust storms and pollution events, particularly in accurately modeling PM2.5 concentrations during severe pollution episodes. Our innovative approach in constructing a joint background error covariance matrix and the resulting high-resolution, real-time updating CMA-ChemRA product. This represents significant advancement in the field of atmospheric and chemical weather reanalysis. The product serves as an crucial tool for environmental monitoring and forecasting in China.

AbstractDeserts play an important role in the climate system, which is closely associated with the emission and transport of dust aerosols. Based on the intensive observation experiment in the Taklimakan Desert, the potential physical processes between the deep convective boundary layer (CBL) and dust emission are revealed in this study. Deep CBL enables the formation of clouds in the late afternoon, leading to significant cooling of surface. Large‐scale buoyant coherent structures thereby transform into the mechanical coherent structures confined near the surface. The responses promote the earlier occurrence of low‐level jet (LLJ) than in cloudless conditions, which allows the downward transport of LLJ momentum and substantially increases surface wind. Therefore, dust emission is initiated by strong wind at dusk and lasts for several hours. The results are useful to predict dust emissions and improve our understanding of distinctive boundary‐layer processes in desert regions.

The Taklimakan Desert is one of key climate regions in East Asia, both highly influencing and highly sensitive to local/regional climate change. Based on a comprehensive observation experiment from 1 to 31 May 2022 in the hinterland of the Taklimakan Desert, the characteristics and mechanisms of turbulence intermittency are investigated in this study, with the purpose to correct turbulent fluxes. Using an improved algorithm to decompose turbulence and submeso motions, two intermittency regimes are recognized in the Taklimakan Desert, namely, D and T intermittency and onD intermittency. The former occurs under strongly stable conditions, characterized by the coexistence of dynamic and thermodynamic turbulence intermittency. The latter occurs under strongly unstable conditions and represents only dynamic turbulence intermittency. Physically, the D and T intermittency regime is related to submeso waves, whereas the onD regime is caused by the horizontal convergence/divergence of convective circulations. With the influence of intermittency and submeso motions, the observed turbulent statistics deviate from reality, which would mask the similarity relationships. To overcome the problem, turbulent statistics are corrected by removing submeso components from original fluctuations. The effectiveness of this method is demonstrated based on the flux-gradient relationships. It is also suggested that, for a big dataset, the impact of onD intermittency can be simply corrected by a correction factor while that of D and T intermittency cannot. The results of this study are helpful to develop the parameterization of turbulent exchange processes in the Taklimakan Desert, which is significant to improve the accuracy of weather forecasting and climate prediction.

Under the framework of the United Nations Sustainable Development Goals (SDGs), mitigating global PM2.5 exposure inequality has become a worldwide task. However, the relationship between PM2.5 exposure inequality and the SDGs remains poorly understood. In this study, we evaluated the spatial distribution of global PM2.5 exposure inequality from 2000 to 2019 on a global scale and investigated the interactions between PM2.5 exposure inequality and the SDGs. The results indicate that global PM2.5 exposure inequality is increasingly worsening. Compared with high-income and low-income regions, middle- and lower-income regions have higher levels of PM2.5 exposure inequality and face greater risks of PM2.5 exposure and associated health impacts. Our analysis of the relationship between PM2.5 exposure inequality and the SDGs reveals that the positive impact of air-SDGs related to air pollution and health, such as SDG3, SDG5, SDG8, and SDG13, on global PM2.5 exposure inequality is becoming increasingly significant. In contrast, the negative impact of SDG2, SDG7, and SDG10 on global PM2.5 exposure inequality has been growing annually. These findings provide valuable insights into the complex interactions between PM2.5 exposure inequality and the SDGs and offer a reference for future efforts to build healthier and more equitable urban air environments.

Abstract. Biomass burning (BB) is an important source of cloud condensation nuclei in the Southeast Atlantic (SEA). The formation of cloud droplets depends on aerosol hygroscopicity (κ), but its variations during the BB season are poorly understood. In this study, we investigate κ during the 2016 and 2017 BB seasons using 18 months of in situ observations on Ascension Island. The results show that κ varied monthly, reaching a low in August and increasing from September to October. The mean κ was 0.33 in 2016 and 0.55 in 2017, showing a significant difference. The changes in κ were mainly associated with differences in the mass fraction of sulfate aerosol. Source attribution showed that about 67 % of the sulfate-containing particles originated from BB, suggesting that BB is the main driver of the changes in sulfate aerosol. The ratio of black carbon to excess carbon monoxide (BC/ΔCO), used to indicate BB combustion conditions, correlated well with κ and the sulfate mass fraction: higher ratios (more flaming combustion) reduced the sulfate mass fraction and thus a lower κ. Therefore, the observed lower BC/ΔCO ratios in 2017 explain the higher κ values, suggesting less flaming combustion in that year. Meteorological changes in 2017, including lower wind speeds and higher relative humidity in Africa, may contributed to the altered combustion conditions, explaining the lower BC/ΔCO and higher κ in 2017. Overall, this study highlights the critical role of BB in understanding the sulfate budget, aerosol hygroscopicity, and CCN concentration in the marine boundary of the SEA.