The replacement of methane with carbon dioxide in marine hydrates has garnered widespread attention due to its potential for energy recovery and carbon dioxide sequestration. This study investigated the replacement of methane with carbon dioxide in hydrates utilizing Raman spectroscopy. Raman spectroscopic analysis indicated uniform replacement efficiency throughout hydrate particle depth ranging from 0 to 200 mu m. Moreover, within the particle size range of 40-500 mu m, variations exhibited negligible influence on both the sequestration fraction and replacement factor. These findings demonstrated that the replacement reaction was primarily controlled by intrinsic reaction kinetics within the silica gel. Elevated temperatures and increased gas-hydrate volume ratios were observed to accelerate the replacement rate. This phenomenon indicated that the ultimate replacement rate was governed by the liberation of methane from hydrogen bond cages, rather than the incorporation of carbon dioxide into these cages. Elevated nitrogen proportions elevated gas production pressure to 11.9 MPa at 278.15 K; however, this simultaneously reduced methane purity and other displacement efficiency parameters. Replacement terminated when the partial pressure of methane exceeded the equilibrium pressure of methane hydrate. Three-step gas injection simultaneously enhanced recovery, purity, and carbon dioxide sequestration rate. This study provides a viable technological approach for commercial exploitation of combustible ice.

Secondary organic aerosols (SOA), a major constituent of fine particulate matter (PM2.5), influence atmospheric chemistry, climate, and public health. However, their formation processes remain insufficiently represented in atmospheric models due to their complexity. Here, we investigate SOA formation from representative aromatic hydrocarbons (toluene, m-xylene, and 1,3,5-trimethylbenzene) under varying precursor concentrations using controlled smog chamber experiments. Leveraging high-resolution mass spectrometry and machine learning approaches, we demonstrate that oxidation capacity (characterized by the cumulative OH exposure), modulated by precursor concentration, is the primary driver of SOA yield. A key and novel finding is that at lower precursor concentrations (∼10 ppb), which are more representative of real atmospheric conditions, the enhanced OH exposure per molecule promotes more efficient multigenerational oxidation. This process favors the production of low-volatility organic compounds, thereby substantially increasing SOA formation compared to that at higher precursor concentrations. These findings, validated by the machine learning analysis, provide mechanistic insights into SOA evolution and underscore the need for improved representation of multigenerational oxidation in atmospheric models to enhance air quality and climate predictions.

Ordinary Portland Cement (OPC), a common construction material globally, has a porous, rough surface that facilitates air pollutant deposition, yet its intrinsic pollutant removal capacity remains poorly quantified. This study systematically investigated the NO2 adsorption process, products, and influencing factors for OPC under laboratory conditions. When NO2 is exposed to OPC, a rapid initial uptake and a clear steady-state uptake capacity were observed. Thermogravimetric analysis indicates that the chemical steady-state adsorption of NO2 primarily stems from the reaction between Ca(OH)2 and NO2 within the top 2 mm of the OPC surface. The primary reaction product is Ca(NO3)2 and Ca(NO2)2, with negligible HONO release. Under otherwise constant conditions, relative humidity (RH) strongly influenced initial adsorption, the steady-state adsorption is virtually unaffected under different RHs. NO2 adsorption efficiency on OPC increased linearly with gas residence time and exposed OPC surface area but decreased with initial NO2 concentration. Further, environmental impact assessments of this absorption process were conducted. At the national scale, the absorption quantity of NO2 due to OPC is not substantial, while in specific environment, this absorption process may significantly alert NO2 concentrations. For example, in underground parking lot scenario, according to the Computational Fluid Dynamics (CFD) simulation, the OPC coating could reduce NO2 concentration over 30% under varying wind speeds and pollutant emission conditions. This work provides a comprehensive quantitative assessment on the NO2 adsorption potential of OPC and the fundamental scientific data for further developing building materials with self-cleaning functions.

The synergistic effect between NOx and SO2 on mineral dust surfaces plays a crucial role in atmospheric chemistry. However, the coupled reaction mechanism between NO and SO2 on dust surfaces, particularly concerning the effect of RH, remains inadequately understood. This work investigates the synergistic oxidation of NO and SO2 on α-Fe2O3 surfaces. Under dry conditions, SO2 significantly promotes NO uptake and NO2 generation. This promotion is attributed to SO2-promoted O2 activation and the formation of surface sulfate species, which can serve as new active sites to oxidize NO into NO2. The generated NO2 as intermediates facilitates SO2 oxidation, ultimately establishing a positive feedback cycle. Moreover, RH can affect NO reaction by influencing the interfacial oxidation of SO2. Specifically, surface adsorbed water inhibits the uptake of NO by interacting with generated surface sulfate species, while it can enhance the conversion of NO to NO2 by promoting heterogeneous oxidation of SO2 and O2 activation. More importantly, the presence of adsorbed H2O plays a significant role in the conversion of NO2 into HONO, and these generated reactive nitrogen species also promote sulfate formation under humid conditions. These findings elucidate the complex coupling mechanism of NO and SO2 on α-Fe2O3 particles, providing critical insights into the synergistic effects during severe air pollution episodes.

H2O2 is a vital atmospheric oxidant, while its photolysis in the troposphere is negligible in conventional view. For the first time, we find that gas-phase H2O2 can undergo heterogeneous photolysis to produce OH radicals on the surfaces of particles such as SiO2, Fe2O3, and Na2SO4 under solar irradiation, which further convert NO to produce HONO and NO2. It is demonstrated that acidity can inhibit the formation of HONO and promote the formation of NO2 significantly. Moreover, the heterogeneous photolysis of H2O2 is dependent on the reactivity of particles, with inert surfaces being more conducive to the generation of OH radicals. A MCM box model indicates that the HONO formation rate via the H2O2 heterogeneous reaction can reach 0.10 ppb·h-1 at 9 a.m., and the relative contribution to the total daytime HONO production can account for approximately 5% at 1 p.m. Given the ubiquitous presence of SiO2 in mineral dust particles, glass curtain walls, and terrestrial soils, these findings indicate that heterogeneous photolysis of H2O2 on the particle surface could be a potential source of daytime HONO. Our research also underscores the importance of atmospheric interfacial chemistry and its contribution to unknown sources of atmospheric oxidizing capacity.

Nitrous acid (HONO) is a key precursor of OH radical, initiating daytime photochemistry. But its sources remain highly uncertain, particularly in suburban mountain area affected by urban transport. In summer 2023, intensive observations were conducted at the Mangshan (MS, 659 m) site, located along the prevailing southerly pathway from urban Beijing, with the Institute of Chemistry Chinese Academy of Sciences (ICCAS, 59 m) in the city center as an urban reference. Elevated HONO concentrations were observed at MS (average: 0.42 ± 0.31 ppb; maximum: 1.62 ppb). Multiple lines of evidence demonstrated that high pollutants levels at MS were strongly influenced by emissions from urban area and mountain foot through the “urban-mountain” transport chain and upslope valley winds. Daytime source analysis revealed that NO + OH homogeneous reaction only explained ∼ 7 % of observed HONO, indicating strong additional sources (Pother, average: 0.94 ± 0.42 ppb/h; maximum: 2.35 ppb/h). Photo-enhanced NO2 heterogeneous reactions and nitrate photolysis represented additional sources. The majority of unexplained HONO (∼ 48.7 %) was attributed to transport and secondary formation on transport. HONO photolysis contributed 79.3 % to OH production at MS, higher than 73.6 % at ICCAS and exceeding values at other mountain sites. Overall, Beijing urban emissions significantly enhanced HONO and oxidation capacity in the suburban mountain area through the “urban-mountain” transport chain. This study provides important evidence for understanding pollution transport and oxidation processes under complex terrain, with direct implications for improving regional air quality modeling and formulating targeted pollution control strategies.

Atmospheric hydrogen peroxide (H2O2) is a critical oxidant that influences atmospheric chemistry, playing a pivotal role in the cycling of hydroperoxyl (HO2) and hydroxyl (OH) radicals, ozone (O3) formation, and sulfate aerosol production. However, the current understanding of its concentration level, sources and atmospheric effects are still poor. This study investigates the drivers of elevated H2O2 concentrations during autumn in Beijing using comprehensive field observations from October to November 2021. The averaged H2O2 concentration in the urban boundary layer of Beijing is 0.26 +/- 0.03 ppb, peaking at 16:00. By integrating Random Forest Regression and relative incremental reactivity analysis, we systematically examine the influence of meteorological factors, trace gases, and photochemical reactions on H2O2 concentrations. The result showed the significant contributions of temperature (T) and photochemical processes to H2O2 production, while identifying key inhibitors such as nitrogen monoxide (NO). Additionally, we explore the role of H2O2 in sulfate formation during haze pollution episodes, finding that although H2O2-mediated oxidation contributes to sulfate production, it is not the dominant pathway during the campaign. These findings underscore the complex interplay between meteorological factors, trace gases, and multiphase reactions in regulating H2O2 concentrations and cycling, providing valuable insights into the dynamics of atmospheric oxidation processes and offer guidance for mitigating air quality issues in urban boundary layer.

Nucleation mode particles (NMPs, less than 25 nm in diameter) are ubiquitous in the atmosphere and have a negative impact on human health and climate. New particle formation (NPF) from extremely low volatile vapors is the dominating source of NMPs globally, but direct emissions from on-road vehicles are also an important source in the urban boundary layer. However, quantifying the contribution of NMPs in the urban boundary layer from NPF and direct emissions is a challenge owing to the complex sourcing and evaluation processes of NMPs. Here, black carbon coating thickness, together with the quantity of a fingerprint organic aerosol marker related to on-road vehicle emissions (HOA, hydrocarbon-like organic aerosol), was utilized to distinguish and determine the two main sources of NMPs. Owing to the constrains of this approach, the influence of upward wind on NMP transportation was excluded. Statistical analysis showed that NPF was the dominant source of NMPs under NPF, non-NPF, and haze conditions, whereas direct vehicle emissions remained a relatively constant contributor.

Despite significant reductions of sulfur dioxide emissions in urban atmosphere globally, sulfate aerosol levels have not decreased proportionally with their precursors. Critical gaps persist in understanding sulfate sources and formation mechanisms, impairing the precision of air pollution control strategies and sulfate chemistry parameterization in atmospheric models. Here we show, using a unique 528-meter observation platform in Beijing coupled with large eddy simulations, that higher concentrations of sulfate aerosol than at the ground occur in the boundary layer aloft, and the high sulfate contributes to the ground by thermal-sheared turbulent mixing triggered by solar radiation. This transport mechanism better explains the characteristic forenoon sulfate peaks observed in 16 global urban measurement campaigns. Our results reveal a ubiquitous sulfate source from the boundary layer aloft in densely populated areas, which was a previously overlooked contributor to ground-level sulfate. We emphasize the need to incorporate this critical source in air quality mitigation policies. Ground-level sulfate in urban cities can be enhanced by transport down from the higher boundary layer, according to field measurements from an observation platform in Beijing.

Hydrate-based separation has attracted considerable attention for the upgrading of flue gas and biogas. In this study, water absorption, ice formation, and hydrate morphology in silica gels were investigated using a stereomicroscope. Brunauer-Emmett-Teller (BET) analysis was employed to characterize changes in pore volume before and after hydrate/ice formation, while powder X-ray diffraction (P-XRD) was used to determine the freezing behavior of water confined in silica gels. The results showed that water in saturated silica gels either froze or formed hydrates, leading to the complete disappearance of pore volume. This finding suggests that hydrate/ice films formed on the silica gel surface as a result of volume expansion. Water confined in silica gels froze only below 263 K. Hydrates of both flue gas and biogas successfully formed in silica gels with different pore sizes. Increasing pressure was more effective than decreasing temperature in promoting hydrate formation. For different silica gels and gas systems, a lower hydrate formation rate was associated with a higher CO2 concentration in the dissociated gas, a larger concentration difference between the gas and hydrate phases, and a higher separation factor. Smaller pore sizes further enhanced CO2 separation performance. The maximum increase in CO2 concentration between the two phases reached 53.97 mol%. This approach provides an effective pathway for separating low-concentration CO2 mixtures that are difficult to treat by conventional cryogenic liquefaction. The findings also offer valuable insights into the utilization of LNG cold energy for hydrate-based flue gas separation.

Air pollution in China exhibits complex characteristics, involving the coupling of multiple pollutants across various media and processes. This has led to PM2.5 and O3 concentrations far exceeding the World Health Organization (WHO) guideline values, posing significant risks to public health and safety. While this severe pollution presents a major environmental challenge, it also offers critical opportunities for advancing atmospheric science research. This review summarizes the research progress made by the Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences (RCEES-CAS) over the past decade in elucidating the causes of air pollution complex and developing control technologies for key pollutants. Significant breakthroughs in understanding air pollution mechanisms include: (1) elucidating the activation mechanism of O2 and the synergistic effects of multi pollutant gases transformations at interfaces; (2) revealing the critical role of aerosol surface reactions in enhancing atmospheric oxidation capacity and driving the explosive growth of secondary particulate matter. Building on these findings, He’s team together with domestic peers proposed the concept and research framework of "atmospheric haze chemistry". Distinct from the cloud chemistry of London smog and the photochemical processes of Los Angeles smog, this framework provides an updated theoretical foundation for understanding the unique mechanisms underlying China’s air pollution complex. In pollution control technology, RCEES-CAS has developed advanced solutions for key pollutants such as NOx, VOCs, NH3, and O3, employing methods including adsorption, catalytic decomposition, selective catalytic reduction (SCR), and catalytic oxidation. Several well-established technologies have been successfully implemented, making significant contributions to air pollution mitigation in China. Additionally, an integrated carbon-neutral fuel strategy has been explored, combining green hydrogen production, biomass valorization, and CO2 conversion. Finally, this review discusses the challenges and future prospects in advancing atmospheric haze chemistry theory and achieving sustained improvements in air quality.

PM2.5 pollution is one of the prominent environmental issues currently faced in China, influenced by various factors and showed significant spatial differences. In this study, the Light Gradient Boosting Machine (LightGBM) model was employed in combination with SHapley Additive exPlanation (SHAP) methods to explore the key impact factors (precursor emissions, meteorological conditions, geographical features and socioeconomic factors) on average annual PM2.5 levels from 2015 to 2022 at both city and grid levels in China. The results show that incorporating pollutant concentration into the model enhances its performance, with R2 improving significantly from 0.79 to 0.93, which underscores the importance of pollutant concentration and the outstanding predictive performance of the LightGBM algorithm. Further, after increasing the spatial resolution and applying a grid-level model, R2 further improves to 0.96 ∼ 0.99. SHAP analysis revealed that PM2.5 levels in urban areas are significantly influenced by pollutant concentration such as NO2, CO, and SO2, accounting for 49.3 % of the total impact. In contrast, the grid-based model highlights the dominant role of meteorological factors such as temperature and precipitation influencing PM2.5 levels in non-urban areas. Moreover, the model results also suggested that the PM2.5 pollution in Yangtze River Delta (YRD) and Pearl River Delta (PRD) are mainly controlled by primary emissions, while in Beijing-Tianjin-Hebei (BTH), Fenwei Plain (FWP) and Sichuan Basin (SCB), atmospheric oxidation capacity is a limiting factor. This study underscores the potential of machine learning in atmospheric pollution control and offers insights for developing more effective and region-specific PM2.5 control policies.

Organic aerosol (OA) and its constituent particulate organic nitrate (pON) are critical factors affecting air quality and climate, yet their sources and transformation processes remain poorly understood. Machine learning (ML) excels at identifying nonlinear relationships among features, and in this study, interpretable ML is employed to identify the key factors governing OA and pON formation during an autumn field campaign in Beijing. Results demonstrate that both aerosol liquid water content (ALWC) and aerosol surface area are two primary factors governing the formation of OA and pON. Specifically, OA formation was predominantly driven by ALWC that is associated with aqueous‐phase processes or gas‐liquid partitioning, particularly during severe pollution episodes. pON formation was constrained by aerosol surface area, indicating the vital contribution of gas‐to‐particle partitioning from low volatility vapors or interface processes of precursors. Our results provide new insights into OA formation mechanisms.

Hydrates can effectively store cooling energy and gases, showing great potential in the field of gas storage, separation and transportation. Three kinds of silica gel were used to strengthen the formation of methane hydrate. The structural and morphological characteristics of ice crystals, as well as the decomposition process of hydrates, were investigated utilizing a Powder X-ray Diffractometer, a stereoscopic microscope, a Differential Scanning Calorimeter, and a cryo-Scanning Electron Microscope. The results showed that the freezing point is transferred to 262.4 K in the pores of silica gel. Hexagonal ice and cubic ice are formed in the pores and on the surface, and ice film is formed on the surface. In the temperature of 253.1-268.1 K and the initial pressure of 4.0-6.0 MPa, the temperature increase or the pressure reduction enhance the methane dissolution and the initial methane hydrate formation rate. In the pore size of 9.31 nm-40.3 nm, with the increase of pore size, the lower the hydrate formation pressure, and the higher the gas consumption and consumption rate. Within the range of 40.3 nm-77.14 nm for pore size and 42.6 m2/g to 188.1 m2/g for specific surface area, the influence of these parameters on gas consumption and the rate of consumption during hydrate formation is minimal. The optimal conditions for methane hydrate formation are identified as 268.1 K, 4.0 MPa, utilizing silica gel type III. The gas storage ratio reached 50.09 V/V, and the final pressure was 2.5 MPa, which is advantageous for the application of hydrate-based gas storage and the utilization of cooling energy.

The activation of molecular oxygen on transition metal oxide surfaces plays a crucial role in atmospheric chemistry and heterogeneous catalysis; however, understanding the intricate mechanisms remains a challenge. In this study, we elucidate for the first time the role of sulfur dioxide (SO2) in enhancing oxygen activation on manganese oxide (γ-MnO2) surface, thereby facilitating the oxidation of nitric oxide (NO) to nitrogen dioxide (NO2). The theoretical calculation results further demonstrate that the adsorbed SO2 and the formed sulfate can promote the adsorption of O2 and the reactivity of surface reactive oxygen species toward NO oxidation on γ-MnO2 (110) surface, respectively. During SO2 with O2 reaction, the formation of surface sulfate and surface-active oxygen atoms serves as new active sites to facilitate the oxidation of NO to NO2. Comparative analysis of the energy profiles reveals that NO oxidation with SO2 can release more heat energy than that without SO2, indicating enhanced thermodynamic accessibility in the presence of SO2. Therefore, a novel mechanism for the sulfate-mediated activation of O2 is proposed, which sheds light on the synergistic effects of multiple pollutants in heterogeneous reactions and is significant for understanding NO oxidation both in the atmosphere and in exhaust treatment.

Oxygenated organic molecules (OOMs) originate from both direct emissions and secondary formation via the oxidation of volatile organic compounds (VOCs) emitted from biogenic and anthropogenic sources. OOMs are suggested to play a crucial role in the nucleation and growth of newly formed particles, which in turn influence climate, health and air quality. However, OOMs and the associated nucleation and growth of aerosols in the boundary layer aloft are not considered in atmospheric chemistry models owing to limited observation data. Here, by using unique measurements on the top of a 528 m building tower in Beijing, we show that a variety of OOMs existed, dominated by C6-7 and C5 compounds, which probably arose from multigeneration oxidation of aromatic compounds and isoprene, respectively. Consequently, five possible formation pathways of OOMs were identified using a machine learning approach combined with their diurnal patterns. Further analysis suggested that OOMs, together with sulfuric acid and ammonia, are highly involved in the formation of nanoparticles. Our results complement the current mechanism of ammonia-sulfuric acid nucleation in the boundary layer aloft of anthropogenically influenced regions, highlighting the important role of OOMs in the nucleation and growth of aerosol. We suggest that OOMs should be considered in atmospheric chemistry models for better prediction accuracy.

Carbonate is a common component of soil and coarse atmospheric particulate matter, while few studies have focused on its photochemical reactivity. We present a novel photoactivation mechanism of carbonate and O2 in the absence of additional potent oxidants. Under light conditions, carbonate ions (CO32-), O2, and H3O+/OH- could spontaneously form a complex and generate CO3- and HO2/O2- radicals through an electron transfer process. Under these conditions, the H2O2 production in 1 mM Na2CO3 solution was 0.37 ± 0.10 μg·L-1 and the cumulative concentration of OH radicals in 4 h was 34.6 nM. The proposed mechanism could be a potential source of atmospheric oxidizing capacity, and the reactive species produced by the CO32--O2 photoactivation process could promote secondary sulfate production. Our findings underscore the importance of carbonate atmospheric reactivity, especially at special reaction surfaces such as air-water interfaces and microdroplets.