Secondary organic aerosol (SOA) represents a major component of urban air pollution. This study presents observational evidence from summer 2017 in urban Beijing, supported by model simulations and a case study from summer 2023, demonstrating the crucial role of nighttime organic nitrates (ONs) production in subsequent daytime SOA formation. Our measurements revealed that total reactive nitrogen compound (NO z ) concentrations exceeded 40 ppb at night, resulting from nitrate radical (NO3)-initiated oxidation of volatile organic compounds (VOCs) in the surface layer and through aloft production followed by downward transport. While these NO z existed primarily in the gas phase during nighttime, they underwent atmospheric aging processes the following day, significantly contributing to SOA growth and potentially new particle formation. Model simulations identified reactive terpenoids as the dominant VOC precursors for nighttime ON formation. These findings underscore the need for an improved understanding of nocturnal ONs production mechanisms given their substantial impact on daytime SOA production.

Despite recent improvements in air quality, wintertime PM2.5 pollution remains severe in northern China, with nitrate (NO3-) becoming an increasingly dominant component. However, the roles of aerosol aqueous conditions in regulating nitrate formation and loss are still poorly constrained. Here, 744 hourly PM2.5 samples collected during the winter of 2023-2024 in a typical northern Chinese city were analyzed to investigate the drivers of severe nitrate pollution. The mean PM2.5 concentration reached 106.0 +/- 84.4 mu g m(-3), with a maximum of 377.8 mu g m(-3). The NO3-/Total Ions (TI) ratio increased steadily to 43% in 2023, reflecting rising NO2 and effective SO2 emission reductions that shifted winter aerosol composition from sulfate-to nitrate-dominated. Four pollution episodes (PE1-PE4) were identified relative to a clean background period. Both PM2.5 and NO3-/TI ratios were strongly enhanced during all episodes. NO3- exhibited a strong dependence on aerosol liquid water content (ALWC) when ALWC <200 mu g m(-3), but this relationship weakened at higher ALWC, indicating a transition in dominant formation and loss pathways. Nitrate consistently dominated NH4+ neutralization. PE1 showed the highest aerosol pH but the lowest ALWC and nitrogen oxidation ratio, suggesting that more acidic conditions favor NO2 oxidation. At ALWC >200 mu g m(-3), gas-particle partitioning of nitrate approached quasi-equilibrium, while enhanced deposition indicated accelerated loss. Meanwhile, elevated ALWC increased the particle size through hygroscopic growth, reducing the PM1/PM2.5 ratio. These results demonstrate a threshold-dependent control of ALWC on nitrate production and removal, with ALWC = 200 mu g m(-3) representing a critical turning point for wintertime nitrate pollution.

Persistent odor nuisances in industrial areas and elevated exposure risks for neighboring communities can compromise the effectiveness of volatile organic compound (VOC) mitigation strategies. This two-year study (December 2020-November 2022) employed high-resolution monitoring to investigate odorant dynamics in China's national industrial parks, integrating online VOC measurements with odor activity value (OAV) and health risk assessments. Results revealed that up to 73.2% of total VOC concentrations (average 84.95 ppb) exhibited odor properties, with 45.9% (average 53.24 ppb) posing dual risks of olfactory irritation and health hazards. Total odorants displayed distinct afternoon concentration peaks and maintained high odor severity (OAV ∼ 3.34) across seasons. Approximately 59% of odorants were identified as carcinogenic, yet neither their concentrations nor perceived nuisance intensity reliably predicted workplace or community health risks. To bridge these gaps, we introduced a mass-OAV-risk coupling metric to prioritize control of odorous and pathogenic VOCs, including ethylene oxide, acetaldehyde, and methyl mercaptan. Discrepancies between interprovincial VOC transport patterns and localized OAV impacts further highlight the limitations of mass-based control strategies, necessitating odorant-specific regulations. This study advances industrial air quality management by demonstrating the critical need for compound-specific interventions that simultaneously mitigate olfactory nuisances and carcinogenic risks for surrounding communities, thereby refining VOC mitigation frameworks.

Oxygenated organic molecules (OOMs) are important precursors of secondary organic aerosol (SOA), yet their molecular characteristics and sources in megacities are not fully understood. Here, we characterize the molecular composition, summer-winter differences, and sources of gaseous OOMs in urban Beijing using an iodide chemical ionization mass spectrometer. Our results show that C4-C5 molecules, especially isoprene oxidation products, accounted for 48% of measured OOMs in summer, while CHO molecules and nitrophenols dominated in winter (∼60%). Most OOMs, particularly organic nitrates in the summer and organic acids in the winter, showed significant increases during polluted episodes. OOMs also increased significantly during summer heatwaves due to enhanced volatilization and precursor emissions. Source apportionment identified ten OOM factors, with isoprene-related factors contributing 43% of OOMs in summer, and nitrophenols and O4 organic acids prevailing in winter. The relationship between the OOMs and SOA indicated that more than half of summertime SOA is linked to isoprene-related oxidation, while nitrophenols and highly oxidized organic acids contributed most under elevated PM2.5 levels in winter. Emission controls during the 2022 Winter Olympic Games effectively reduced several OOMs groups, particularly organic acids, while increasing the formation of nitrogen-containing species. Overall, this work reveals the distinct summer-winter behavior of OOMs in Beijing and their varied roles in SOA formation under changing atmospheric and regulatory conditions.

Abstract. Understanding sulfur-nitrogen partitioning is essential for predicting secondary aerosol formation and mixing-state evolution, yet the mechanisms governing its cross-phase coupling remain poorly constrained. Here, we integrate single-particle mass spectrometry (SPA-MS) with air-pollutant and meteorological observations from two regional emission-control periods. We define three sulfur-to-nitrogen ratio metrics in the gas (gSNR), particle (pSNR), and number-based (nSNR) domains, and use causal inference and interpretable machine learning to identify their linkages and environmental drivers. The results reveal a stepwise propagation from precursor composition to particle chemistry and then to population mixing-state evolution. Although gSNR sets the first-order constraint on sulfur-nitrogen partitioning, the aerosol response is strongly particle-type dependent, with more pronounced sulfate enrichment in black-carbon-containing and organic-rich particles than in BC-free particles. Relative humidity (RH) emerges as the primary regulator of this coupling by modulating aerosol liquid water and phase transitions. Under dry conditions, the three SNR metrics diverge and aerosols remain largely externally mixed; under humid conditions, the metrics converge and aerosols evolve toward a more internally mixed state. Our results support the inclusion of RH- and particle-type-dependent parameterizations of cross-phase coupling and chemical heterogeneity in air-quality models.

Chemical reactions in atmospheric aqueous phases (cloud/fog and aerosol waters) are emerging as an important pathway to the formation of secondary organic aerosols (aqSOA). Many studies have investigated the aqSOA formation in the presence of light, while investigations regarding those in dark (absence of light) are rather limited, despite its likely significant role. Here, we review currently available key literatures regarding dark aqueous processing of organic precursors. The review begins with a summary of the chemical composition of atmospheric aqueous phases with a focus on the organics, then a brief introduction of the aqueous photochemistry (particularly those induced by the organic triplet excited states) and its comparison to the dark reactions. We then focus on the dark aqueous chemistry including the general bulk/interfacial reactions, two specifically important types of reactions (Fenton- and Maillard-like reactions), field observations, specific physicochemical and toxicological properties of dark aqSOA, key oxidants (nitrate radical, hydrogen peroxide, transition metal ions and dissolved oxygen) of dark aqueous reactions, as well as the environmental factors and reaction conditions that affect these reactions. These integrated findings from laboratory experiments and field measurements collectively improve our understanding on dark aqSOA, especially the effects on climate (potentially strong light-absorbing ability and hygroscopicity) and health (potentially toxic products). Nevertheless, significant knowledge gaps still exist, we then propose some key aspects that warrant further dedicated studies, for a better elucidation of the formation mechanisms, effects of dark aqSOA, therefore corresponding effective controls to aid air quality improvement, haze mitigation, and climate adaption.

Considering the impact of total carbon emission management policy in China, the coal-fired power plants (CFPP) must not only promote the large-scale deployment of zero-carbon/negative-carbon technologies in the long term, but also achieve staged carbon reduction targets in the short term through the optimization of operational-level emission reduction strategy. Since CCUS implementation in Chinese CFPP remains constrained by various factors, this study has developed an hourly plant-level optimization strategy that integrates load dispatching with dynamic coal blending as an operational-level synergistic strategy. The study examines the carbon benefit of the load dispatching strategy for the total carbon emission management system in CFPP, and compares it with the benefit of the operational-level synergistic strategy. The results indicate that the load dispatching optimization strategy can significantly reduce the carbon emissions of the unit. Compared with the traditional load distribution method, it can achieve a maximum net reduction of 5.8 tons of CO2/h. For each 1% increase in the load dispatched to high-efficiency units, the total carbon emissions of the plant can be reduced by 0.23%. Operationallevel synergistic strategy can stimulate the intrinsic emission reduction potential of CFPP. For the two 300 MW coal-fired units considered in this study, compared with the individual load dispatching optimization strategy, the operational-level synergistic strategy can achieve a net CO2 reduction of 11306.71 tons per year, and the average carbon emission factor (tCO2/MWh) of CFPP under the influence of the operational-level synergistic strategy has decreased by 0.87%. In addition, the carbon benefits of the operational-level synergistic strategy exhibit a high degree of sensitivity to fluctuations in carbon prices. With the increasing carbon price, the total carbon emissions and carbon emission factor of CFPP under the high carbon price scenario decrease by an average of 1.28% compared to the low carbon price scenario. The operational-level synergistic strategy can be widely applied to units with large load fluctuation ranges, such as peak load power generator sets. During the transition period of the industry towards long-term deep decarbonization, it provides an immediate and feasible emission reduction path for CFPP.

Nitrogen-containing organic compounds (NOCs) are important light-absorbing constituents of atmospheric PM2.5 and can substantially influence aerosol radiative forcing, air quality, and climate. Previous studies have mainly focused on the source apportionment and concentrations levels of NOCs, while the mechanisms governing their formation and particle-phase partitioning remain insufficiently constrained, particularly in tropical regions. Here, we aim to elucidate regional differences in NOCs characteristics in Myanmar, with emphasis on how relative humidity (RH) and precursor species influence their formation pathways. We report the first molecular-level spatio-temporal characterization of NOCs in Myanmar, identifying 1064 organic compounds in ESI- mode, with NOCs contributing 14 %-21 % of molecular formulas and 13 %-35 % of total mass. Organic nitrates (ONs) dominated CHON species across all sites, with higher abundances in Mandalay than in Yangon. Two ubiquitous nitrophenols, nitrocatechol (C6H5NO4) and dimethyl nitrocatechol (C8H9NO4), showed strong covariance but a distinct relationship of their particle-phase C8H9NO4 / C6H5NO4 ratio with RH. CHemistry with Aerosol Microphysics in Python (PyCHAM) box model simulations reveal that increasing RH enhances aerosol water content, to which C8H9NO4 and C6H5NO4 respond differently because of differences in their partitioning thermodynamics. Increased photochemistry in summertime further promotes C6H5NO4 formation. These two processes, in addition to gas-phase precursor concentration, can explain the observed RH relationship and demonstrate that the C8H9NO4 / C6H5NO4 ratio is sensitive, by comparable extents, to: partitioning thermodynamics, photochemistry and precursor supply. These findings provide new constraints on nitrophenol evolution in humid tropical environments and improve interpretation of NOC sources and aging processes, thereby supporting more accurate assessments of their regional and global radiative impacts.

Oxygenated organic molecules (OOMs) are critical intermediates in particle growth and secondary organic aerosol (SOA) formation in megacities. Here, we present comprehensive measurements of OOMs using three state-of-the-art mass spectrometers in Beijing winter. We demonstrate distinct differences in the extent of OOM formation and their contribution to particle growth under clean versus polluted conditions. The formation of organic nitrate (ON) OOMs shows a nonlinear dependence on NOx levels, with concentrations increasing as the NO/NO2 ratio rises from similar to 0.1 to 1. Under polluted conditions, daytime photochemical processes primarily drive OOM formation, while high-NO2 chemistry (NO/NO2 < 0.1) enhances ON production through nighttime oxidation under clean conditions. Aerosol growth model simulations reveal that low-volatility organic compounds account for 69-77% of particle growth in the 3-15 nm range, with SOA formation primarily driven by OOM condensation. Our results provide significant insights into urban air pollution dynamics, advancing our understanding of aerosol formation in megacities.

Extreme heatwaves increasingly amplify tropospheric ozone (O3) pollution, yet their quantitative contributions across China's megacity clusters remain poorly constrained. Here, we employed a machine learning-based framework integrating seven-year (2019–2025) observations and satellite data to isolate the August 2022 heatwave's contribution to regional O3 variation across Beijing-Tianjin-Hebei (BTH), Yangtze River Delta (YRD), Pearl River Delta (PRD), and Sichuan Basin (SCB). YRD and SCB exhibited afternoon (14:00–16:00 LT) O3 enhancements of 49–52 μg m–3 and ∼69 μg m–3 under sustained heat (19–25 days), with heatwave meteorology explaining 35% and 48% of August 2022 MDA8O3 variance, respectively. BTH and PRD showed 23–26 μg m–3 and 23–28 μg m–3 under intermittent conditions (2–5 days). O3 formation efficiency peaked at 37–38°C, corroborated by satellite-observed HCHO surges (21–27%). The heatwave further induced photochemical regime shifts, expanding NOx-sensitive conditions by 13–15% in the most heat-affected regions, namely the YRD and SCB. These findings demonstrate that heatwave duration critically determines regional O3 amplification magnitude, with compound meteorological-chemical feedbacks capable of offsetting multi-year emission reductions within weeks—underscoring the urgency of climate-adaptive air quality management.

Bare black carbon (BC) can be coated by other nonabsorbing components, inducing light absorption enhancement (Eabs) via the so-called "lensing effect." The coating components/processes, however, are complex in terms of both chemical composition and microphysical properties; therefore, Eabs is highly uncertain and inconsistent among different observations. Particle heterogeneity in composition is critical to the accurate estimation of Eabs; here, we quantified the impacts of various coating components on Eabs880 (Eabs at 880 nm, thus no impact from light-absorbing organics) by using observations from three megacities in China (Beijing, Nanjing, and Shanghai). Under all three scenarios with contrastingly different atmospheric conditions investigated here, low-volatility/highly oxygenated secondary organic aerosol (SOA) species and sulfate were consistently important contributors to Eabs880, while OA from both biomass burning and traffic appeared to be insignificant. The effects of semivolatile/less-oxygenated SOA species and nitrate varied largely, being highly dependent upon the meteorology and location. Specially, an industry-related OA resolved in Nanjing exhibited a notable contribution to Eabs880 thus its chemical and physical characteristics warrant future attention. Overall, our findings regarding the roles of specific sources in Eabs880 provide a direct and practically feasible guidance to effectively reduce BC pollution and its positive climate forcing.

Surface ozone, a major air pollutant with important implications for air quality, ecosystems, and climate, shows long-term trends shaped by both anthropogenic and climatic drivers. Here, we developed a machine learning-based approach, namely the fixed emission approximation (FEA), to decouple the effects of meteorological variability and anthropogenic emissions on summertime ozone trends in China under the clean air actions. Anthropogenic emissions drove an approximately +23.2 +/- 1.1 mu g m-3 increase in summer maximum daily 8 h average ozone during 2013-2017, followed by an approximately -4.6 +/- 1.5 mu g m-3 decrease between 2017 and 2020 in response to strengthened emission controls. In contrast, meteorological anomalies, including heatwaves and rainfall conditions, emerged as substantial drivers of ozone variability during 2020-2023. Satellite-derived formaldehyde-to-nitrogen dioxide ratios revealed widespread urban volatile organic compounds-limited regimes for ozone production, with a shift toward nitrogen oxides-limited sensitivity under influence of heatwaves. Extending the FEA framework to assess long-term climate influences from 1970 to 2023, we find that sustained climate warming has driven a substantial increase in urban summertime ozone in China. These results demonstrate that climate change was increasingly offsetting the benefits of emission reductions and highlight the need for integrated ozone mitigation strategies that jointly address emission controls and climate adaptation in a warming world.

China's carbon peaking and carbon neutrality strategy is reshaping the national industrial structure, yet its cascading effects on the transport sector-a critical economic pillar and major emission source-remain insufficiently understood, particularly regarding industrial-chain and regional transmission mechanisms. This study addresses this gap by quantifying how upstream and downstream industrial restructuring under carbonneutrality policies propagates into transport-sector output and emissions. Using a multi-regional Computable General Equilibrium model (IMED|CGE) covering China's 31 provinces, combined with structural path decomposition, we trace the industrial and spatial pathways through which policy-induced emission reductions are transmitted to the transport sector. Results show that carbon-neutrality policies trigger a systemic transformation of transport demand by restructuring upstream industries, reducing transport CO2 emissions by 43.1% in 2060. The contraction of energy-intensive sectors (e.g., coal mining, metal smelting) sharply reduces intermediate transport inputs (by 26.9 and 28.3 billion USD in 2060, respectively), curtailing high-carbon bulk freight demand. Conversely, the expansion of low-carbon services boosts transport demand by 87.0 billion USD, driving a structural shift toward high-value, lower-emission logistics. Transport electrification and energy-efficiency gains amplify these effects. Spatially, energy-exporting provinces (e.g., Shanxi and Inner Mongolia) achieve the largest emission reductions (up to 78.4%), with dominant pathways concentrated in energy-supply chains driven by inter-provincial exports. In economically diversified regions (e.g., Guangdong, Chongqing), reductions are primarily demand-side driven, linked to local construction and service activities. These findings suggest that differentiated strategies that prioritize upstream green energy supply and downstream low-carbon demand management are essential for a coordinated green transition in China's transport sector.

Secondary organic aerosol (SOA) represents a major component of urban air pollution. This study presents observational evidence from summer 2017 in urban Beijing, supported by model simulations and a case study from summer 2023, demonstrating the crucial role of nighttime organic nitrates (ONs) production in subsequent daytime SOA formation. Our measurements revealed that total reactive nitrogen compound (NOz) concentrations exceeded 40 ppb at night, resulting from nitrate radical (NO3)-initiated oxidation of volatile organic compounds (VOCs) in the surface layer and through aloft production followed by downward transport. While these NOz existed primarily in the gas phase during nighttime, they underwent atmospheric aging processes the following day, significantly contributing to SOA growth and potentially new particle formation. Model simulations identified reactive terpenoids as the dominant VOC precursors for nighttime ON formation. These findings underscore the need for an improved understanding of nocturnal ONs production mechanisms given their substantial impact on daytime SOA production.

Interprovincial electricity transmission is vital for decarbonizing China's power system, yet its impact on Grid Emission Factors (GEFs) remains underexplored. This study employs the Medium-to-Long-Term Provincial Grid Emission Factor Model (MLGEFM) to simulate provincial GEFs and carbon trajectories from 2020 to 2035 under four transmission scenarios. Results reveal significant national decarbonization, with the average GEF falling from 0.559 to 0.320 tCO2/MWh by 2035 under the Emission Targeted Scenario (ETS). However, transmission mechanisms exert heterogeneous regional impacts. While scale expansion reduces GEFs in central and southern China (e.g., -20.0% in Henan), it inadvertently raises GEFs in north and east China (e.g., +24.7% in Beijing). In contrast, structural optimization under ETS further reduces GEFs by 7.6% in Beijing and 13.1% in Shanghai. Although all scenarios achieve a national carbon peak around 2030, ETS achieves the optimal trajectory with a lower peak magnitude and steeper decline.

Black carbon is a global climate forcer due to its strong radiative absorption, which is highly sensitive to coating formation regulated by anthropogenic and biogenic emissions. However, how cross-regional biogenic sources modulate urban black carbon coating and radiative effects remains poorly understood. Here we integrate observations and model simulations to investigate such biogenic-anthropogenic interactions in eastern China. The results show that biogenic volatile organic compounds from vegetation-rich regions undergo atmospheric oxidation to produce oxygenated organic compounds, which are subsequently advected into downwind urban areas. These products enhance regional atmospheric oxidation capacity and supply additional precursors, thereby promoting secondary organic aerosol production. This biogenic-induced strengthening of regional photochemistry drives the formation of highly oxidized secondary organic aerosol coatings on black carbon and increases its fraction within the total particle population. Consequently, black carbon absorption efficiency increases more steeply with the coating carbon oxidation state under biogenic-rich conditions, yielding an average similar to 20% enhancement in radiative absorption from the lensing effect relative to biogenic-poor periods. Our findings reveal that cross-regional biogenic-anthropogenic interactions enhance both the formation and particle population fraction of secondary organic aerosol coatings on urban black carbon, potentially further amplifying its radiative effects as biogenic emissions increase under future warming scenarios.

Surface ozone (O3) pollution has significant health and environmental impacts. Clarifying the distribution of urban-rural disparities in ozone (URD-O3) is crucial for targeted O3 management. However, the evolution patterns and the driving factors of URD-O3 remain unclear. This study analyzed the spatiotemporal evolution of O3, emissions and meteorology, in Beijing-Tianjin-Hebei (BTH) and Yangtze River Delta (YRD) from 2013 to 2020 in China, and quantitatively evaluated their relative contributions to URD-O3. Results show a clear divergence in URD-O3 around 2017. Before 2017, urban O3 in BTH was lower than rural levels but exceeded rural levels subsequently, peaking at 1.18 mu g/m3 in 2020. In contrast, YRD consistently showed higher urban O3, where URD-O3 increased by 58.9% from 2013 to 2017 and stabilized thereafter. Moreover, the relative contributions of emissions in two regions are opposite. In BTH, the contribution of emissions to URD-O3 decreased from 40.1% to 20.2%, while it increased from 33.7% to 45.9% in YRD from 2013 to 2020. This result was attributed to the changes in O3 sensitivity, the URD-O3 sensitivity continuously narrowed in BTH, but widened in the YRD, altering the impact of emissions on O3 formation. This study highlights the importance of O3 sensitivity in O3 targeted governance.

Organic nitrates (ONs), including alkyl nitrates (RONO2) and peroxy nitrates (RO2NO2), are ubiquitous in the atmosphere. This review aims to clarify the critical role of ONs in connecting volatile organic compound oxidation, reactive nitrogen cycling, and secondary pollutant formation. We summarized recent advances in the formation mechanisms, measurement techniques, and atmospheric impacts of ONs, and identified areas for future research. ONs are primarily produced through VOC oxidation under NOx conditions, including daytime RO2 and NO reactions and nighttime NO3 initiated oxidation, with additional contributions from heterogeneous and multiphase processes. Advances in analytical techniques, from chromatography to real-time mass spectrometry such as AMS and CIMS, have enabled improved molecular-level detection of both gas-phase and particulate ONs. Observational studies further reveal strong regional variability. Overall, ONs are key intermediates linking VOC oxidation, reactive nitrogen cycling, and secondary pollutant formation in the atmosphere. Through their roles in SOA formation, ozone regulation, and brown carbon production, ONs influence air quality, atmospheric oxidation capacity, and climate processes. Continued advances in molecular-level observations and mechanistic understanding will be essential for improving atmospheric models and constraining the environmental impacts of ONs. Future progress will require better molecular-level constraints on ON formation and loss pathways, improved representation of multiphase chemistry and gas–particle partitioning in atmospheric models, and expanded observations across polluted, remote, and vertically resolved environments.

Metals are ubiquitous in the atmosphere, yet their interactions with brown carbon (BrC) are largely unclear. Here, we explored variations in the optical properties of aqueous extracts from ambient fine particulate matter (PM2.5) samples collected in Nanjing, following the addition of Mn2+, Cu2+, and Fe3+ across a concentration range (0–500 μM) and three pH levels (3.0, 5.0, and 6.5). Observations reveal that Mn2+-mediated BrC showed negligible changes, Cu2+ could slightly decrease the mass absorption efficiency (MAE) while increasing the absorption Ångström exponent (AAE) especially at elevated pH; in contrast, Fe3+ exerts a far more striking influence: at 500 μM, the MAE increased by a facroe of 2.7∼6.6. Influences of the three ions on fluorescent properties of BrC were less obvious; but again, Fe3+ exerted the largest impact by raising the proportion of low-oxygenated humic-like substances (HULIS) and lowering the relative abundance of highly oxidized HULIS. We further characterized the molecular compositions of the metal-mediated and original samples. The addition of Fe3+ again caused the largest reduction (“disappeared”) in number of identified molecules (up to 60% loss in negative ion mode), indicating that the formation of metal-organic (MO) complexes and/or that the degradation of BrC molecules by Fe3+ were stronger than Mn2+ and Cu2+. Additionally, we directly identified a few metal complexes (six dicarboxylic acids and one nitrophenol compound as ligands). Together with the “disappeared” species, these compounds are important in affecting the optical behaviors of atmospheric BrC in the presence of metals.