An online-coupled regional climate-chemistry-aerosol model (RIEMS-Chem) was developed and applied to investigate the impact of the aerosol mixing state on aerosol-radiation-meteorology feedback during the winter haze episode of 8-13 February 2020 over the North China Plain (NCP). Model validation demonstrates the overall good ability of the model in reproducing the meteorological variables, PM2.5 and its components, and aerosol optical properties. Aerosol optical depth (AOD) and single scattering albedo (SSA) simulated with the Maxwell-Garnett mixing assumption are closest to the AERONET observations, whereas external mixing tends to predict lower AOD and higher SSA, in contrast to core-shell mixing and homogeneous mixing, which predict lower SSA. The direct radiative effects (DREs) under various aerosol mixing states differ largely, with the percentage differences of 24% and 40% and a factor of three at the top of the atmosphere (TOA), at the surface and in the atmosphere, respectively, averaged over the haze episode. The sensitivity of the aerosol optical properties and DRE to the black carbon size distribution, coating fraction, and hygroscopic growth under Maxwell-Garnett mixing is also investigated, showing the remarkable effect of hygroscopic growth. During the most severe haze day in Beijing, the changes in air temperature and relative humidity at 2 m (T2, RH2) and wind speed at 10 m (WS10) induced by aerosol radiative feedback vary by -1.2-1.4 degrees C, 5.5-6.2%, and -0.28-0.37 m s-1, respectively, and the feedback-induced increase in PM2.5 concentration varies considerably from 40.3 mu g m-3 to 57.6 mu g m-3 across different mixing states. The radiative feedback leads to an increase in PM2.5 concentration by 25%, 30%, 30% and 36% under external mixing, Maxwell-Garnett mixing, core-shell mixing and homogeneous mixing states, respectively. It is noteworthy that the aerosol mixing state not only affects the magnitude but also the direction of near-surface air temperature change, depending on the relative magnitude of aerosol-induced atmospheric heating and DRE-induced cooling. The strongest atmospheric heating rate (AHR) under homogeneous mixing leads to an increase in T2 and planetary boundary layer height (PBLH) in portions of southern NCP, which tends to reduce the PM2.5 concentration. This study demonstrates the important role of radiative feedback in exacerbating air pollution and the significant impacts of aerosol mixing state on DRE, AHR and radiative feedback during the haze episode.

The spectral dependence of aerosol absorption, characterized by the absorption & Aring;ngstr & ouml;m exponent (AAE), strongly influences radiative effects, yet the relative importance of controlling factors remains poorly quantified. We integrate multisource observations with an interpretable machine-learning framework (Shapley Additive Explanations, SHAP) to disentangle the roles of chemical composition and particle size in predicting AAE and to evaluate radiative impacts. Field observation in Beijing reveal that near-surface AAE is predominantly influenced by higher fine mineral dust and water-soluble inorganic ions fractions. Multi-year columnar data identify dust loading as the dominant predictor, followed by carbonaceous aerosols. The fine-mode radius accounts for 29 % of size parameters cumulative importance and ranks closely with black carbon. SHAP diagnostics highlight that columnar AAE contributes to radiative forcing at the top of the atmosphere (TOA) comparably to single scattering albedo (SSA), while its impact is clearly weaker at the bottom of the atmosphere and in the atmosphere. These findings help clarify AAE determinants and reduce uncertainties in aerosol radiative effect assessments.

Accurate quantification of carbonaceous aerosol spectral absorption is crucial for reliable radiative forcing assessment, yet conventional separation of measured black carbon and brown carbon absorption in urban areas remains uncertain due to methodological assumptions. We developed a Tracer-Guided Least-Square (TGLS) method that uses chemical tracers to constrain the fitting and eliminates unrealistic assumptions to improve carbonaceous aerosol absorption quantification. Applying this method to a field measurement in Beijing winter 2023-2024 using a multi-wavelength thermal/optical carbon analyzer (model DRI 2015), which selectively measures carbonaceous aerosol and excludes mineral dust absorption, we retrieved an absorption Angstrom exponent (AAE) of 0.81 for black carbon and 4.89 for brown carbon. The quantified brown carbon absorption accounted for similar to 19% of the carbonaceous aerosol absorption at 405 nm, lower than most previous reports, consistent with the lower organic carbon levels observed during our campaign compared with earlier wintertime Beijing studies. Compared to the fixed black carbon AAE assumption of 1.0, the TGLS method significantly reduces carbonaceous aerosol spectral absorption attribution errors and improves radiative forcing estimates, especially for brown carbon. This is critical since the radiative forcing of brown carbon is over four times more sensitive to AAE variations than that of black carbon. A key limitation is that the present implementation of TGLS is directly applicable only to dust-insensitive measurements, and its extension to optical instruments sensing total aerosol absorption requires further development. This study advances carbonaceous aerosol absorption quantification, provides insights into source-dependent variability, and highlights critical implications for radiative forcing estimation.

Abstract. Aerosol hygroscopicity is a key property governing aerosol water uptake and aerosol–cloud interactions, yet the hygroscopicity of organic aerosol (OA) remains poorly constrained because of its chemical complexity and diverse emission sources. Here, OA hygroscopicity (κOA) was retrieved from comprehensive field observations at a rural site in eastern China using humidified light-scattering measurements and ZSR-based closure analysis. κOA exhibited pronounced temporal variability during winter, ranging from nearly zero to 0.49 with a mean value of 0.11 ± 0.11. Although OA was generally highly oxidized, κOA was only weakly correlated with the O:C ratio, indicating that bulk oxidation state alone cannot explain its variability. Instead, κOA increased systematically with biomass-burning influence. Atmospheric aging substantially enhanced κOA under biomass-burning-dominated conditions, whereas highly oxidized OA under weak biomass-burning influence remained weakly hygroscopic. These findings demonstrate that the hygroscopic response of OA to atmospheric aging is fundamentally source dependent and indicate that source-dependent parameterizations provide a more physically realistic framework than conventional oxidation-based approaches for representing OA hygroscopicity in atmospheric models.

The vertical distribution of aerosol is critical to understand the effect of aerosol on climate. This work explored chemical characteristics of PM2.5 at three urban and one mountain sites in Shiyan located in central China. Annual PM2.5 mass concentrations at the mountain site were 70% of urban levels. The total of sulfate, nitrate, and ammonium (SNA) dominated PM2.5, showing higher proportion (49.3%) at mountain site. SNA shows the homogeneous vertical distribution in spring, summer, and autumn with coefficient of divergences below 0.2, whereas most of components were more abundant at urban sites in winter. Higher nitrogen oxidation ratio (0.27) and ratio of secondary organic carbon to organic carbon (0.65) indicated that the aerosol in mountainous atmosphere exhibited a greater degree of aging compared to that at urban sites. These findings enhance our understanding of the vertical distribution of PM2.5 composition and provide a scientific foundation for regional atmospheric pollution control.

To what extent the new particle formation (NPF) contributed to the cloud condensation nuclei (CCN) remained unclear, especially at the boundary layer top (BLT) in polluted atmosphere. Based on measurements at a mountain-top background site in southeastern China during spring 2024, this study systematically investigates the nucleation mechanism and subsequent growth dynamics of NPF events under contrasting air masses, and quantifies their role as a source of CCN. Eight NPF events were observed, and three of them occurred in the polluted conditions (NPF-P) which associated with regional transportation while the rest five events appeared in the clean conditions (NPF-C). The average formation rate (J2.5: 2.4 cm-3s-1 vs. 0.7 cm-3s-1) and growth rate (GR: 6.8 nm h-1 vs. 5.5 nm h-1) were significantly higher in NPF-P events than in NPF-C events, alongside elevated concentrations of sulfuric acid and ammonia. The correlation between log J3 and [H2SO4], as well as theoretical simulations with the MALTE_BOX model, indicates that the enhanced nucleation in polluted conditions can be attributed to the participation of ammonia in stabilizing sulfuric acid-based clusters. In addition, much higher CCN enhancement factor was observed in NPF-P (EFCCN: 1.6 vs. 0.7 in NPF-C) due to the regional transported of anthropogenic pollutants from the urban cluster regions and their secondary transformation under enhanced atmospheric oxidation capacity. Furthermore, the duration of NPF-to-CCN conversion was quantified using a "Time Window (tau)", revealing that polluted conditions accelerated the conversion by 17.0 % (tau = 16.4 h vs. 19.8 h). Nitrate played an important role in maintaining a rapid particle growth rate, thereby shortening tau and enhancing CCN production from NPF - a process that can ultimately influence cloud microphysical properties by increasing the potential cloud droplet number concentration. These findings reveal that polluted air masses enhance both the efficiency and speed of CCN production at the BLT through elevated atmospheric oxidation capacity.

An online coupled regional climate-chemistry-aerosol model (RIEMS-Chem) was applied to investigate the direct radiative effect of crop residue burning (CRB) aerosols and feedbacks on meteorology during CRB events in June 2015. Model validation against observations demonstrates a generally good model ability in reproducing meteorological variables, PM2.5 and its chemical components, and aerosol optical depth (AOD). On average, CRB aerosols contributed approximately 35 50

Amid China's nationwide air quality initiatives, this study investigates PM2.5-associated health risks through high time-resolution measurements of PM2.5 mass concentration and ten trace elements (Pb, Zn, Cu, Mn, V, As, Co, Cr, Ni, Cd) at paired urban-rural sites in North China Plain before and during Air Quality Improvement Period (AQIP). While rural areas achieved greater PM2.5 reductions (42.9 % vs. urban 28.1 %), urban regions showed stronger declines in toxic elements (51.7 % vs. rural 41.0 %). Source-specific risk assessments revealed urban cancer risks (CR) and non-cancer risks (NCR) persistently dominated by industrial emissions (45.8-54.1 %), contrasting with dynamic rural patterns: NCR shifted from secondary aerosol dominance (47.8 % before AQIP) to vehicle emissions (67.3 % during AQIP), while CR transitioned from vehicle sources (35.4 %) to biomass/coal combustion (28.5 %). Notably, urban NCR exceeded rural levels by 60 % despite lower CR, reflecting distinct risk drivers-as pollution intensifies, urban risks amplified by secondary aerosols and industrial activities, versus rural risks driven by combined secondary processes, traffic, and combustion. These spatial divergences highlight the need for tiered strategies: prioritising industrial/secondary controls in cities versus integrated secondary/ traffic/biomass-coal management in rural areas. The observed decoupling between PM2.5 mass reductions and persistent elemental risks underscores the criticality of metal-specific emission regulations.

Abstract Hollow nanostructured microwave absorbers based on FeCo-PBA/PDA and coated with g-C3N4 were fabricated via a facile pyrolysis approach, yielding the FeCo@N-doped C/g-C3N4 hollow nanocubes. The phases, morphologies, and microwave absorption properties in 2–18 GHz of the nanocubes were systematically characterized to clarify the effect of the inner/outer dual interfaces on the electromagnetic properties at microwave frequencies. The results indicate that the g-C3N4 coating lowers the outer interfacial impedance of the N-doped carbon frameworks and facilitates microwave penetration. In contrast, the inner interface consists of the carbon matrix embedded with FeCo/C core/shell nanoparticles, with a high impedance enabling multiple scattering. Besides, the introduction of g-C3N4 decreases the carbon content in the composites, thereby depressing the overall dielectric attenuation. The sample with FeCo-PBA/PDA:melamine mass ratio of 1:0.5 demonstrates the optimal microwave absorbing performance at 16.04 GHz with a minimum RL of –55.98 dB and an ultra-thin thickness of 1.775 mm, as well as a maximum effective absorbing bandwidth (RL < –10dB) of 5.76 GHz at 1.9 mm. Furthermore, the competing effects of the inner/outer impedance and the polarization modulation, as well as the nanoarchitecture design, on the tunable microwave absorption performance were discussed.

Accurate estimation of gross primary productivity (GPP) is fundamental for understanding ecosystem carbon cycling. Solar-induced chlorophyll fluorescence (SIF) and carbonyl sulfide (COS) provide complementary proxies of photosynthesis, yet their relative performance across temporal scales and sky conditions remains uncertain. Using continuous eddy covariance COS fluxes and ground-based SIF observations in a rice paddy, we assess their relationships with GPP at half-hourly and daily scales under clear and cloudy conditions. SIF shows stronger correlations with GPP at the half-hourly scale, whereas COS–GPP associations strengthen after temporal aggregation, particularly under cloudy skies. Partial correlations indicate that SIF captures short-term variability, while COS becomes more informative at the daily scale. Integrating SIF and COS improves GPP estimation across conditions, especially at the daily scale and under cloudy skies. These results demonstrate scale-dependent complementarity and highlight the value of multi-proxy approaches for robust ecosystem GPP estimation.

This study evaluates an innovative system that can be used for Near Real-Time Source Apportionment (NRT model) providing results within minutes after the measurements across six Chinese cities: Beijing, Langfang, Shijiazhuang, Xi'an, Wuhan, and Chongqing during 2020-22. The system leverages the AXA instrumental setup (ACSM, Xact, Aethalometer) to integrate high-time-resolution data and provide detailed insights into major particulate matter (PM) sources and their contributions. Secondary PM components dominated across all sites, accounting for up to 66% of the total PM2.5 mass in some cities. Primary sources such as solid fuel combustion contributed approximately 10-30%, while episodic dust events were a major source in Langfang during specific periods. The system's performance was validated by strong correlations (R2 > 0.82) with results from optimized source apportionment analyses. Furthermore, robustness tests using reduced datasets (two thirds for training and one third for validation) confirmed the system's reliability and adaptability under dynamic monitoring conditions. In these tests, high correlations with the optimized source apportionment were achieved, indicating the operational reliability of the model. These findings underscore the NRT model's potential as a critical tool for real-time air quality management, enabling rapid identification of pollution sources and informing timely mitigation strategies to improve urban air-quality.

The light scattering properties of fine particulate matter (PM2.5) were investigated in a megacity on the North China Plain during periods of substantial PM2.5 variation. The results revealed a non-linear reduction in the scattering efficient (sigma(sp)) relative to decreases in PM2.5 concentrations. Ammonium nitrate (NH4NO3) was the primary contributor to sigma(sp), accounting for 34.7 %-43.3 % at both high and low PM2.5 levels. However, primary organic aerosol (POA) became the largest contributor to sigma(sp) (31.5 %) at medium PM2.5 levels, with secondary organic aerosol (SOA) rising sharply by 13.9 %, collectively sustaining sigma(sp) and limiting its reduction. The scattering & Aring;ngstr & ouml;m exponent (SAE) decreased from 2.1 +/- 0.9 (high PM2.5 levels) by 31.1 %-50.5 % at medium and low levels, indicating a shift towards larger particle sizes as pollution levels declined. Strong negative correlations between SAE and the organic carbon/elemental carbon (OC/EC) ratio (r = -0.193, p < 0.01) at medium PM2.5 levels, as well as the mass scattering efficient (MSE) (r = -0.656, p < 0.01), testified the critical role of SOA formation in enhancing particle size and sigma(sp) under improved air quality conditions. Regression models based solely on aerosol components underestimated sigma(sp), likely due to the exclusion of particle growth and SOA formation. Incorporating POA, SOA and SAE into sigma(sp) calculations significantly improved predictive accuracy by 7 %-21 %. These findings demonstrate that enhanced SOA formation and particle size growth increase MSE and effectively counteract reductions in sigma(sp) despite declining PM2.5 levels, thereby further hindering improvements in atmospheric visibility.

Aerosols over the Tibetan Plateau (TP) strongly influence regional climate and hydrological cycles. Here we investigate the size-resolved microphysical and optical properties of aerosols in an urban area of the northern TP using a tandem system of a differential mobility analyzer, a condensation particle counter, and a single particle soot photometer. Under the 2021 summer conditions, the average particle number size distribution follows a lognormal pattern, peaking at similar to 70 nm. Refractory black carbon (rBC) aerosols constitute 17.7% of the total particle population in the 100-750 nm mobility diameter (D-mob) range, with their proportion rising to over 50% for D-mob > 500 nm. Most rBC particles are externally mixed, while only 12.2% are thickly coated with non-refractory materials. Externally mixed rBC particles show strong non-sphericity, with a dynamic shape factor increasing from 1.8 at 115 nm to 2.8 at 750 nm, consistent with aggregate structures. In contrast, thickly coated rBC particles are nearly spherical, with coating thickness increasing with size. The total rBC mass estimated from size-resolved measurements closely matches bulk rBC mass directly measured. rBC-free particles exhibit slight non-sphericity, with shape factor positively correlated with refractive index, likely due to dust contributions. Bulk scattering coefficients derived from size-resolved data match those estimated under the well-mixed spherical assumption. However, the later scheme-lacking observational constraints on morphology and mixing state-overestimates absorption by over a factor of three, thereby underestimating the single-scattering albedo. These results provide key constraints for improving aerosol radiative forcing estimates and advancing understanding of aerosol-climate interactions over the TP.

The co-pyrolysis of coal and waste plastics is a viable method for achieving efficient and clean utilization of coal and waste and enhancing light aromatic hydrocarbon production. In this study, the co-pyrolysis of Pingshuo coal and polystyrene (PS) was carried out via rapid infrared heating in a fixed bed reactor with varying mixing ratios to investigate the product distribution, tar composition, and char characteristics. The results revealed that the increase of PS mixing ratio increased the interaction of the co-pyrolysis volatiles, which raised the tar and light aromatic hydrocarbons yield. Especially for the mixing ratio of coal/PS being 7:3, the content of styrene and ethylbenzene in tar is 31.5 % and 15.2 % higher than the theoretically calculated value, respectively. Moreover, co-pyrolysis promoted the interaction between volatiles to produce biphenyls such as 1,3-diphenylpropane. Electron paramagnetic resonance results indicated that the co-pyrolysis char exhibited higher amounts of methoxy ether and 1-3 cyclic pi quinone free radicals, which was consistent with the decrease of phenols in the tar.

Aerosol acidity (pH) plays a critical role in atmospheric chemical processes, secondary aerosol formation, and urban air quality. Based on five years of hourly observations (2019-2023) in subtropical Dongguan, this study investigates the variability and thermodynamic regulation of aerosol pH, with a focus on aerosol liquid water content (ALWC), hydrogen ion (H+) concentrations, and their interactions. Secondary inorganic aerosols (SIA), including NH4+, SO42-, NO3- and Cl-, accounted for 92 ± 4 % of total water-soluble inorganic ions (WSIIs), with sufficient total NH3 (TNH3) and non-volatile cations (NVCs) available to neutralize acidic species. Nevertheless, aerosol pH remained persistently acidic (2.43-2.77). Compared with 2019, aerosol pH declined slightly during and after the COVID-19 pandemic despite substantial reductions in WSIIs, primarily due to a marked decrease in ALWC (∼40 %) together with a moderate increase in H+ (∼20 %). Mechanistically, aerosol pH was jointly regulated by ALWC and H+, both influenced by the thermodynamic gas-particle partitioning of semi-volatile species. Warm, dry conditions suppressed ALWC and elevated H+, lowering pH, whereas cooler, humid conditions sustained higher ALWC and lower H+, raising pH. Thermodynamic simulations with fixed mean compositions further showed that the effects of temperature and relative humidity (RH) on aerosol pH were mediated by the gas-particle partitioning of ammonia. Specifically, the pH response was minimized when NH4+(aq)/TNH3 was ∼0.2-0.3, corresponding to the strongest buffering capacity, whereas ratios below ∼0.2 produced gas-dominated regimes highly sensitive to temperature, and ratios above ∼0.3 yielded aqueous-dominated regimes more sensitive to RH. These findings provide a robust observational constraint for predicting aerosol acidity and its meteorological dependence, offering new guidance for air-quality management in subtropical regions.

Atmospheric brown carbon (BrC) plays a significant role in global warming, yet the evolution of its optical properties during aging remains poorly understood, leading to substantial uncertainties in its climate effects. In this study, we investigate the aging process of BrC and its driving factors using laboratory-generated biomass burning emissions, including four types of straw and one type of wood. Upon OH oxidation, there exists a large increase in OA fraction after 2 d aging, followed by a minor increase during aging to 7 d. The particle growth is dominated by the change in OA content and thus shows a similar trend during aging. The mass absorption efficiency (MAE) of fresh BrC measured at 370 nm is 2.1–5.7 m2 g−1. A sharp decline in MAE is observed after 2 d aging, equally attributed to photobleaching and secondary organic aerosol formation. Although a negative correlation is observed between particle size and MAE, the reduction in MAE is mainly driven by the decline in the imaginary part (k) of BrC, with particle size playing a minor role. Combined with positive matrix factorization (PMF) analysis, the study reveals that oxygenated OA, characterized by higher O / C ratios but lower MAE, increases significantly with aging. In contrast, two hydrocarbon-like OA factors with lower O / C ratios and higher MAE decrease over time. These results emphasize the importance of categorizing BrC based on its MAE and atmospheric behavior in climate models.

Solar-induced chlorophyll fluorescence (SIF) has emerged as a valuable tool for estimating gross primary production (GPP). However, the mechanism linking SIF to GPP under waterlogging stress remains unclear. Here, we investigated the GPP-SIF relationship and their responses to waterlogging stress using three years of continuous ground measurements in a maize field. Our results revealed a significant decoupling in the GPP-SIF relationship under waterlogging stress, as evidenced by a decline in R2 values from 0.87 and 0.79 (non-waterlogging years: 2020 and 2021) to 0.20 (waterlogging year:2022), consistent with SCOPE model simulations. We examined the underlying mechanisms independently regulating SIF and GPP fluctuations. Our analysis suggested a considerable transition in dominating factors influencing both parameters, shifting from photosynthetically active radiation (PAR) under non-waterlogging conditions to soil water content (SWC) under waterlogging stress. Notably, we quantified the impact of elevated SWC on GPP and SIF, finding that the effect was more pronounced on GPP (62.41% reduction) than on SIF (54.3% reduction). We observed the weakened significance of SIF mutiscattering components induced by alterations in soil background spectra due to increased SWC affecting SIF radiative transfer processes. Complemented by SCOPE simulations, our analysis suggested that the significant decoupling of SIF and GPP physiological components, along with asymmetrical responses to SWC, collectively contribute to the reduced GPP-SIF relationship under waterlogging stress. Overall, our study provides valuable insights into GPP and SIF dynamics under waterlogging stress in a maize field, emphasizing the effectiveness of radiative transfer models for understanding plant photosynthetic responses to waterlogging stress.