Application of roadside air purifiers in urban street canyons: A pilot-scale study in Hong Kong.

The implementation of roadside air purifiers has emerged as an effective active control measure to alleviate air pollution in urban street canyons. However, technical questions raised under real conditions remain challenging. In this study, we conducted a pilot-scale investigation involving seven units of self-designed roadside air purifiers in an urban street canyon in Hong Kong. The air cleaning effects were quantified with an air quality sensor network after rigorous quality control. The removal efficiencies of Nitrogen dioxide (NO2), Fine suspended particulates (PM2.5), Carbon monoxide (CO), and Nitric oxide (NO) were determined by comparing with simultaneously measured ambient concentrations, with hourly average efficiencies of 14.0 %-16.9 %, 3.5-10.0 %, 11.9 %-18.7 %, and 19.2 %-44.9 %, respectively. Generally, the purification effects presented variations depending on the ambient pollutants' levels. Higher ambient concentrations of NO2, PM2.5, CO correlated with increased purification effects, while NO presented the opposite trend. The influence of interval distance combined with spatial distribution indicated the operation of purifiers will induce local NO2 attenuation even at an interval distance of four meters. Statistical analysis delivered evidence the air cleaning ability exhibited optimal performance when relative humidity level is ranged from 70 % to 90 %, aligning with the prevailing conditions in Hong Kong. Additionally, improved purification effects were observed at the downwind direction, and their performance was enhanced when the wind speed exceeded 2.5 m/s. Moreover, we estimated the operational lifetime of the air purifiers to be approximately 130 days, offering crucial information regarding the filter replacement cycle. This work serves as a pioneering case study, showcasing the feasibility and deployment considerations of roadside air purifiers in effectively controlling air pollution in urban environments.

Abundant oxygenated volatile organic compounds and their contribution to photochemical pollution in subtropical Hong Kong.

Volatile organic compounds (VOCs), which are ubiquitous pollutants in the urban and regional atmosphere, promote the formation of ozone (O3) and secondary organic aerosols, thereby significantly affecting the air quality and human health. The ambient VOCs at a coastal suburban site in Hong Kong were continuously measured using proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) from November 2020 to December 2020. 83 VOC species, including 23 CxHy, 53 CxHyO1-3, and 7 nitrogen-containing species, were measured during the campaign, with a mean concentration of 36.75 ppb. Oxygenated VOCs (OVOCs) accounted for most (77.4%) of the measured species, including CxHyO1 (50.7%) and CxHyO2 (25.1%). The measured VOC species exhibited distinct temporal and diurnal variations. High concentrations of isoprene and OVOCs were measured in autumn with more active photochemistry, whereas large evening peaks of aromatics from local and regional primary emissions were prominent in winter. The OH reactivity and O3 formation potential (OFP) of key precursors were quantified. OVOCs contributed about half of the total OH reactivity and OFP, followed by alkenes and aromatics, and the contribution of aromatics increased significantly in winter. The potential source contribution function was used to investigate the potential source regions associated with high VOC concentrations. Through positive matrix factorization analysis, six major sources were identified based on fingerprint molecules. The contributions of biogenic sources and secondary formation to the observed species were notable in late autumn, whereas vehicle emissions and solid fuel combustion had higher contributions in winter. The findings highlight the important role of OVOCs in photochemical pollution and provide valuable insights for the development of effective pollution control strategies.

Comprehensive Assessment of Pollution Sources and Health Impacts in Suburban Area of Shanghai.

Shanghai, one of China's largest metropolises, faces significant environmental pollution challenges due to rapid economic development. Suburban areas of Shanghai are affected by both long-distance transport and local sources of pollutants. This study conducted an integrated analysis that links health-risk assessment of heavy metals and source apportionment of atmospheric constituents to distinguish the contributions of emission sources and the major sources of health risks. Source-apportionment analysis revealed that secondary sources had the greatest contribution to the local pollutants, indicating the significant influence of peripheral and long-distance transport. Health-risk assessment of Cr, Ni, As, and Cd revealed that local residents were exposed to respiratory health risks, in which Cr is the major contributor. This health risk was primarily associated with emissions from nearby industry-related sources. Our study highlights the significant effects of both long-distance transport and local source emissions on atmospheric composition and human health in large urban agglomerations. The findings can inform future efforts to develop more precise emission-reduction strategies and policy improvements to mitigate environmental pollution and protect public health.

Molecular Characterization of Oxygenated Organic Molecules and Their Dominating Roles in Particle Growth in Hong Kong.

Oxygenated organic molecules (OOMs) are critical intermediates linking volatile organic compound oxidation and secondary organic aerosol (SOA) formation. Yet, the understanding of OOM components, formation mechanism, and impacts are still limited, especially for urbanized regions with a cocktail of anthropogenic emissions. Herein, ambient measurements of OOMs were conducted at a regional background site in South China in 2018. The molecular characteristics of OOMs revealed dominant nitrogen-containing products, and the influences of different factors on OOM composition and oxidation state were elucidated. Positive matrix factorization analysis resolved the complex OOM species to factors featured with fingerprint species from different oxidation pathways. A new method was developed to identify the key functional groups of OOMs, which successfully classified the majority species into carbonyls (8%), hydroperoxides (7%), nitrates (17%), peroxyl nitrates (10%), dinitrates (13%), aromatic ring-retaining species (6%), and terpenes (7%). The volatility estimation of OOMs was improved based on their identified functional groups and was used to simulate the aerosol growth process contributed by the condensation of those low-volatile OOMs. The results demonstrate the predominant role of OOMs in contributing sub-100 nm particle growth and SOA formation and highlight the importance of dinitrates and anthropogenic products from multistep oxidation.

Volatile organic compounds at a roadside site in Hong Kong: Characteristics, chemical reactivity, and health risk assessment.

Volatile organic compounds (VOCs) and oxygenated VOCs (OVOCs) play important roles in atmospheric chemistry and are recognized as the major pollutants in roadside microenvironments of metropolitan Hong Kong, China. In this study, the ambient VOCs and OVOCs were intensively monitored at a roadside site in Hong Kong for one month during morning and evening rush hours. The emission characterizations, as well as ozone formation potentials (OFP) and hydroxyl radical (OH) loss rates (LOH) were determined. Results from the campaign showed that the average concentrations of detected VOCs/OVOCs ranged from 0.21 to 9.67 ppb, and higher toluene to benzene (T/B) ratio was observed during evening sections due to the variation of fuel types in vehicle fleets and mix of additional emission sources in this site. On average, OVOCs had much higher concentrations than the targeted VOC species. Acetone, formaldehyde, and acetaldehyde were the three most abundant species, while formaldehyde showed the highest contributions to both OFP (32.20 %) and LOH (16.80 %). Furthermore, potential health hazards with inhalation exposure to formaldehyde, acetaldehyde, propionaldehyde, methyl ethyl ketone (MEK), 1,3-butadiene, toluene, benzene, and acrylonitrile were found. These results reveal that it is imperative to implement efficient control measures to reduce vehicle emissions for both primary and secondary pollutants and to protect both roadside workers and pedestrians.

Long-term variation and evaluation of air quality across Hong Kong.

Study of Air Quality Objectives (AQOs) and long-term changes of air pollution plays a decisive role in formulating and refining pollution control strategies. In this study, 10-year variations of six major air pollutants were analyzed at seven monitoring sites in Hong Kong. The continuous decrease of annual averaged concentrations of NO2, SO2, CO, PM2.5 and PM10 and numbers of days with severe pollution conditions validated the efficiency of the series of air pollution control schemes implemented by the Hong Kong government. However, there is still a big gap to meet the ultimate targets described by the World Health Organization. Besides, the concentration of O3 at roadside and urban stations increased by 135% ± 25% and 37% ± 18% from 2011 to 2020, respectively, meanwhile the highest 8 hr averaged O3 concentration was observed as 294 µg/m3 at background station in 2020, which pointed out the increasing ozone pollution in Hong Kong. There was a great decrease in the annual times of air quality health index (AQHI) laying in "high", "very high" and "serious" categories from 2011 to 2020 with the decrease rate of 89.70%, 91.30% and 89.74% at roadside stations, and 79.03%, 95.98% and 72.73% at urban stations, respectively. Nevertheless, the number of days categorized as "high" or above at roadside station was twice more than that in the urban station during the past ten years. Thus, more policies and attentions should be given to the roadside air quality and its adverse health effect to pedestrians on street.

Increased contribution to PM2.5 from traffic-influenced road dust in Shanghai over recent years and predictable future.

Traffic contributes to fine particulate matter (PM2.5) in the atmosphere through engine exhaust emissions and road dust generation. However, the evolution of traffic related PM2.5 emission over recent years remains unclear, especially when various efforts to reduce emission e.g., aftertreatment technologies and high emission standards from China IV to China V, have been implemented. In this study, hourly elemental carbon (EC), a marker of primary engine exhaust emissions, and trace element of calcium (Ca), a marker of road dust, were measured at a nearby highway sampling site in Shanghai from 2016 to 2019. A random forest-based machine learning algorithm was applied to decouple the influences of meteorological variables on the measured EC and Ca, revealing the deweathered trend in exhaust emissions and road dust. After meteorological normalization, we showed that non-exhaust emissions, i.e., road dust from traffic, increased their fractional contribution to PM2.5 over recent years. In particular, road dust was found to be more important, as revealed by the deweathered trend of Ca fraction in PM2.5, increasing at 6.1% year-1, more than twice that of EC (2.9% year-1). This study suggests that while various efforts have been successful in reducing vehicular exhaust emissions, road dust will not abate at a similar rate. The results of this study provide insights into the trend of traffic-related emissions over recent years based on high temporal resolution monitoring data, with important implications for policymaking.

Pollution-Derived Br2 Boosts Oxidation Power of the Coastal Atmosphere.

The bromine atom (Br•) has been known to destroy ozone (O3) and accelerate the deposition of toxic mercury (Hg). However, its abundance and sources outside the polar regions are not well-known. Here, we report significant levels of molecular bromine (Br2)─a producer of Br•─observed at a coastal site in Hong Kong, with an average noontime mixing ratio of 5 ppt. Given the short lifetime of Br2 (∼1 min at noon), this finding reveals a large Br2 daytime source. On the basis of laboratory and field evidence, we show that the observed daytime Br2 is generated by the photodissociation of particulate nitrate (NO3-) and that the reactive uptake of dinitrogen pentoxide (N2O5) on aerosols is an important nighttime source. Model-calculated Br• concentrations are comparable with that of the OH radical─the primary oxidant in the troposphere, accounting for 24% of the oxidation of isoprene, a 13% increase in net O3 production, and a nearly 10-fold increase in the production rate of toxic HgII. Our findings reveal that reactive bromines play a larger role in the atmospheric chemistry and air quality of polluted coastal and maritime areas than previously thought. Our results also suggest that tightening the control of emissions of two conventional pollutants (NOx and SO2)─thereby decreasing the levels of nitrate and aerosol acidity─would alleviate halogen radical production and its adverse impact on air quality.

Slower than expected reduction in annual PM2.5 in Xi'an revealed by machine learning-based meteorological normalization.

To evaluate the effectiveness of air pollution control policies, trend analysis of the air pollutants is often performed. However, trend analysis of air pollutants over multiple years is complicated by the fact that changes in meteorology over time can also affect the levels of air pollutants in addition to changes in emissions or atmospheric chemistry. To decouple the meteorological effect, this study performed a trend analysis of the hourly fine particulate matter (PM2.5) observed at an urban background site in Xi'an city over 5 years from 2015 to 2019 using the machine learning algorithm. As a novel way of meteorological normalization, the meteorological parameters were used as constant input for 5 consecutive years. In this way, the impact of meteorological parameters was excluded, providing insights into the "real" changes in PM2.5 due to changes in emission strength or atmospheric chemistry. After meteorological normalization, a decreasing trend of -3.3 % year-1 (-1.9 µg m-3 year-1) in PM2.5 was seen, instead of -4.4 % year-1 from direct PM2.5 observation. Assuming the rate of -1.9 µg m-3 year-1 were kept constant for the next few decades in Xi'an, it would take approximately 25 years (in the year 2045) to reduce the annual PM2.5 level to 5 µg m-3, the new guideline value from World Health Organization. We also show that PM2.5 is primarily associated with anthropogenic emissions, which, underwent aqueous phase chemistry in winter and photochemical oxidation in summer as suggested by partial dependence of RH and Ox in different seasons. Therefore, reducing the anthropogenic secondary aerosol precursors at a higher rate, such as NOx and VOCs is expected to reduce the particulate pollution in this region more effectively than the current -3.3 % year-1 found in this study.

Effect of NO2 on nocturnal chemistry of isoprene: Gaseous oxygenated products and secondary organic aerosol formation.

As one of the most abundant non-methane hydrocarbon in the atmosphere, isoprene has attracted lots of attention on its oxidation processes and environmental effects. However, less is known about the nocturnal chemistry of isoprene with multiple oxidants coexisting in the atmosphere. Besides, though highly oxygenated molecules (HOMs) have recently been recognized to contribute to secondary organic aerosol (SOA) formation, the specific contribution of measured HOMs on SOA formation in isoprene oxidation has not been well established. In this study, the oxidation of isoprene was simulated under dark and various NO2/O3 conditions. Plenty of oxidation products were identified by combining two state-of-the-art time-of-flight mass spectrometers, and more species with high C and N numbers and low volatilities were detected under high NO2 conditions. The nocturnal oxidation of isoprene was found to be governed by synergic effects of multiple oxidants, including O3, NO3•, and •OH at the same time, and the oxidation proportions changed with NO2. NO2 promoted the formation of most N-containing products especially N2 products, because of the decisive role of NO3• on their formation. Nevertheless, some products such as C5H10O3-5, C5H11NO6, and C10H16N2O10,11 showed a better correlation with HO2NO2 rather than NO2/O3, indicating the importance of HO2• chemistry on the oxidation products formation. Though the concentration of measured oxygenated products was dominated by volatile and semi-volatile organic compounds, the low- and extremely low-volatile organic compounds contributed over 97 % to the SOA formation potential. However, challenges still exist in accurately simulating SOA formation from the measured oxygenated molecules to match the measurement, and further comprehensive characterization of oxidation products in both gas and aerosol phases at the molecular level is needed.

Photodissociation of particulate nitrate as a source of daytime tropospheric Cl2.

Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases. Despite extensive research on molecular chlorine (Cl2), a Cl precursor, in the polar atmosphere, its sources in other regions are still poorly understood. Here we report the daytime Cl2 concentrations of up to 1 ppbv observed in a coastal area of Hong Kong, revealing a large daytime source of Cl2 (2.7 pptv s-1 at noon). Field and laboratory experiments indicate that photodissociation of particulate nitrate by sunlight under acidic conditions (pH < 3.0) can activate chloride and account for the observed daytime Cl2 production. The high Cl2 concentrations significantly increased atmospheric oxidation. Given the ubiquitous existence of chloride, nitrate, and acidic aerosols, we propose that nitrate photolysis is a significant daytime chlorine source globally. This so far unaccounted for source of chlorine can have substantial impacts on atmospheric chemistry.

Metal-Organic Frameworks for NOx Adsorption and Their Applications in Separation, Sensing, Catalysis, and Biology.

Nitrogen oxide (NOx ) is a family of poisonous and highly reactive gases formed when fuel is burned at high temperatures during anthropogenic behavior. It is a strong oxidizing agent that significantly contributes to the ozone and smog in the atmosphere. Thus, NOx removal is important for the ecological environment upon which the civilization depends. In recent decades, metal-organic frameworks (MOFs) have been regarded as ideal candidates to address these issues because they form a reticular structure between proper inorganic and organic constituents with ultrahigh porosity and high internal surface area. These characteristics render them chemically adaptable for NOx adsorption, separation, sensing, and catalysis. In additional, MOFs enable potential nitric oxide (NO) delivery for the signaling of molecular NO in the human body. Herein, the different advantages of MOFs for coping with current environmental burdens and improving the habitable environment of humans on the basis of NOx adsorption are reviewed.

Secondary Formation and Impacts of Gaseous Nitro-Phenolic Compounds in the Continental Outflow Observed at a Background Site in South China.

Nitro-phenolic compounds (NPs) have attracted increasing attention because of their health risks and impacts on visibility, climate, and atmospheric chemistry. Despite many measurements of particulate NPs, the knowledge of their gaseous abundances, sources, atmospheric fates, and impacts remains incomplete. Here, 18 gaseous NPs were continuously measured with a time-of-flight chemical ionization mass spectrometer at a background site in South China in autumn and winter. Abundant NPs were observed in the continental outflows from East Asia, with a total concentration up to 122.1 pptv. Secondary formation from the transported aromatics dominated the observed NPs, with mono-NPs exhibiting photochemical daytime peaks and nighttime enrichments of di-NPs and Cl-substituted NPs. The budget analysis indicates that besides the •OH oxidation of aromatics, the NO3• oxidation also contributed significantly to the daytime mono-NPs, while the further oxidation of mono-NPs by NO3• dominated the nocturnal formation of di-NPs. Photolysis was the main daytime sink of NPs and produced substantial HONO, which would influence atmospheric oxidation capacity in downwind and background regions. This study provides quantitative insights on the formation and impacts of gaseous NPs in the continental outflow and highlights the role of NO3• chemistry in the secondary nitro-aromatics production that may facilitate regional pollution.

Exploring the photocatalytic conversion mechanism of gaseous formaldehyde degradation on TiO2-x-OV surface.

To understand the conversion mechanism of photocatalytic gaseous formaldehyde (HCHO) degradation, strontium (Sr)-doped TiO2-x-OV catalysts was designed and synthesized in this study, with comparable HCHO removal performance. Our results proved that foreign-element doping reduced Ti4+ to the lower oxidation state Ti(4- x)+, and that the internal charge kinetics was largely facilitated by the unbalanced electron distribution. Oxygen vacancies (OVs) were developed spontaneously to realize an electron-localized phenomenon in TiO2-x-OV, thereby boosting O2 adsorption and activation for the enhanced generation of reactive oxygen species (ROS). At the chemisorption stage, in-situ DRIFTS spectra and density functional theory calculation results revealed that surface adsorbed O2 (Oads) and lattice O (Olat) engaged in the isomerisation of HCHO to dioxymethylene (DOM) on TiO2-x-OV and TiO2, respectively. Time-resolved DRIFTS spectra under light irradiation revealed that the DOM was then converted to formate and thoroughly oxidized to CO2 and H2O in TiO2-x-OV. While bicarbonate byproducts were detected from DOM hydroxylation or possible side conversion of CO2 in TiO2, owing to insufficient consumption of surface hydroxyl. Our study enhances the understanding on the photocatalytic oxidation of HCHO, thereby promoting the practical application in indoor air purification.

Chemical characteristics and sources of nitrogen-containing organic compounds at a regional site in the North China Plain during the transition period of autumn and winter.

Organic nitrogen constitutes a significant fraction of the nitrogen budget in particulate matter (PM). However, the composition and sources of nitrogen-containing organic compounds (NOCs) in PM remain unclear currently in North China Plain (NCP), China. Rare local or regional studies on NOCs were conducted. In this study, ambient fine particles (PM2.5) were collected in Xianghe, a regional background site in NCP, from 26 October to 26 December 2017. The insights from this study include NOC molecule identification, concentration level, and NOC sources and origins. Specifically, we have identified and quantified >90 NOC species, with urea being the most abundant, accounting for 39.7 ± 4.7% of the total NOC followed by free amino acids (FAAs; 21.9 ± 1.5%), cyclic NOCs (15.3 ± 4.5%), amines (14.8 ± 1.5%), alkyl amides (5.8 ± 0.5%), isocyanates (1.7 ± 0.2%), and nitriles (1.1 ± 0.2%). The time series of FAAs was well correlated (r = 0.51-0.68, p < 0.01) with the organic marker of levoglucosan and was moderately correlated with Ox (r = 0.29-0.41, p < 0.01), suggesting biomass burning and secondary formation were important FAAs sources. We also show that amines can be oxidized and/or reacted by aqueous-phase processing to form secondary aerosols, which are further enhanced by the involvement of iron in the catalytic process. Using the receptor model of positive matrix factorization (PMF), six factors were identified including coal combustion, crustal sources, biomass burning, industry-related sources, traffic emissions, and secondary aerosols. Source apportionment of NOC shows biomass burning was the dominant factor, accounting for 31.8% of the total NOCs. This study provides a unique dataset of NOCs at this regional background site in the NCP, with the insights of NOC chemical composition and sources gained in this study being important for future NOC modeling as well as NOC health effects studies.

Photocatalytic Air Purification Using Functional Polymeric Carbon Nitrides.

The techniques for the production of the environment have received attention because of the increasing air pollution, which results in a negative impact on the living environment of mankind. Over the decades, burgeoning interest in polymeric carbon nitride (PCN) based photocatalysts for heterogeneous catalysis of air pollutants has been witnessed, which is improved by harvesting visible light, layered/defective structures, functional groups, suitable/adjustable band positions, and existing Lewis basic sites. PCN-based photocatalytic air purification can reduce the negative impacts of the emission of air pollutants and convert the undesirable and harmful materials into value-added or nontoxic, or low-toxic chemicals. However, based on previous reports, the systematic summary and analysis of PCN-based photocatalysts in the catalytic elimination of air pollutants have not been reported. The research progress of functional PCN-based composite materials as photocatalysts for the removal of air pollutants is reviewed here. The working mechanisms of each enhancement modification are elucidated and discussed on structures (nanostructure, molecular structue, and composite) regarding their effects on light-absorption/utilization, reactant adsorption, intermediate/product desorption, charge kinetics, and reactive oxygen species production. Perspectives related to further challenges and directions as well as design strategies of PCN-based photocatalysts in the heterogeneous catalysis of air pollutants are also provided.

Improved Oxygen Activation over a Carbon/Co3O4 Nanocomposite for Efficient Catalytic Oxidation of Formaldehyde at Room Temperature.

Oxygen activation is a key step in the catalytic oxidation of formaldehyde (HCHO) at room temperature. In this study, we synthesized a carbon/Co3O4 nanocomposite (C-Co3O4) as a solution to the insufficient capability of pristine Co3O4 (P-Co3O4) to activate oxygen for the first time. Oxygen activation was improved via carbon preventing the agglomeration of Co3O4 nanoparticles, resulting in small particles (approximately 7.7 nm) and more exposed active sites (oxygen vacancies and Co3+). The removal efficiency of C-Co3O4 for 1 ppm of HCHO remained above 90%, whereas P-Co3O4 was rapidly deactivated. In static tests, the CO2 selectivity of C-Co3O4 was close to 100%, far exceeding that of P-Co3O4 (42%). Various microscopic analyses indicated the formation and interaction of a composite structure between the C and Co3O4 interface. The carbon composite caused a disorder on the surface lattice of Co3O4, constructing more oxygen vacancies than P-Co3O4. Consequently, the surface reducibility of C-Co3O4 was improved, as was its ability to continuously activate oxygen and H2O into reactive oxygen species (ROS). We speculate that accelerated production of ROS helped rapidly degrade intermediates such as dioxymethylene, formate, and carbonate into CO2. In contrast, carbonate accumulation on P-Co3O4 surfaces containing less ROS may have caused P-Co3O4 inactivation. Compared with noble nanoparticles, this study provides a transition metal-based nanocomposite for HCHO oxidation with high efficiency, high selectivity, and low cost, which is meaningful for indoor air purification.

Chemical and toxicological characterization of particulate emissions from diesel vehicles.

This paper presents a detailed chemical and toxicological characterization of the diesel particulate matter (PM) emitted from diesel vehicles running on a chassis dynamometer under different driving conditions. Chemical analyses were performed to characterize the contents of organic carbon (OC), elemental carbon (EC), and 31 polycyclic aromatic hydrocarbons (PAHs) in the collected PM samples. The OC-EC analysis results revealed that PM emissions from diesel vehicles in this study were dominated by OC and that the emission of vehicles equipped with diesel particulate filters had high OC/EC ratios. The PAH analysis results revealed that 4- and 5-ring PAHs were the dominant PAHs in the OC fraction of the PM samples. Particle toxicity was evaluated through three toxicological markers in human A549 cells, namely (1) acellular 2,7-dichlorofluorescein (DCFH) for oxidative potential, (2) interleukin-6 (IL-6) for inflammation, and (3) glutathione (GSH) for antioxidation after exposure. Statistical analyses revealed that vehicle sizes have statistically significant effects on the concentrations of the markers. Correlation analysis between PAHs and toxicological markers revealed that significant correlations existed between specific compounds and markers. Our results can be used as a reference by policy makers to formulate emission control strategies and as a dataset for other modeling studies.

Facet Engineered α-MnO2 for Efficient Catalytic Ozonation of Odor CH3SH: Oxygen Vacancy-Induced Active Centers and Catalytic Mechanism.

The oxygen vacancy in MnO2 is normally proved as the reactive site for the catalytic ozonation, and acquiring a highly reactive crystal facet with abundant oxygen vacancy by facet engineering is advisable for boosting the catalytic activity. In this study, three facet-engineered α-MnO2 was prepared and successfully utilized for catalytic ozonation toward an odorous CH3SH. The as-synthesized 310-MnO2 exhibited superior activity in catalytic ozonation of CH3SH than that of 110-MnO2 and 100-MnO2, which could achieve 100% removal efficiency for 70 ppm of CH3SH within 20 min. The results of XPS, Raman, H2-TPR, and DFT calculation all prove that the (310) facets possess a higher surface energy than other facets can feature the construction of oxygen vacancies, thus facilitating the adsorption and activate O3 into intermediate peroxide species (O2-/O22-) and reactive oxygen species (•O2-/1O2) for eliminating adjacent CH3SH. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) revealed that the CH3SH molecular was chemisorbed on S atom to form CH3S-, which was further converted into intermediate CH3SO3- and finally oxidized into SO42- and CO32-/CO2 during the process. Attributed to the deep oxidation of CH3SH on 310-MnO2 via efficient cycling of active oxygen vacancies, the lifetime of 310-MnO2 can be extended to 2.5 h with limited loss of activity, while 110-MnO2 and 100-MnO2 were inactivated within 1 h. This study deepens the comprehension of facet-engineering in MnO2 and presents an efficient and portable catalyst to control odorous pollution.

In Situ Measurements of Molecular Markers Facilitate Understanding of Dynamic Sources of Atmospheric Organic Aerosols.

Reducing the amount of organic aerosol (OA) is crucial to mitigation of particulate pollution in China. We present time and air-origin dependent variations of OA markers and source contributions at a regionally urban background site in South China. The continental air contained primary OA markers indicative of source categories, such as levoglucosan, fatty acids, and oleic acid. Secondary OA (SOA) markers derived from isoprene and monoterpenes also exhibited higher concentrations in continental air, due to more emissions of their precursors from terrestrial ecosystems and facilitation of anthropogenic sulfate for monoterpenes SOA. The marine air and continental-marine mixed air had more abundant hydroxyl dicarboxylic acids (OHDCA), with anthropogenic unsaturated organics as potential precursors. However, OHDCA formation in continental air was likely attributable to both biogenic and anthropogenic precursors. The production efficiency of OHDCA was highest in marine air, related to the presence of sulfur dioxide and/or organic precursors in ship emissions. Regional biomass burning (BB) was identified as the largest contributor of OA in continental air, with contributions fluctuating from 8% to 74%. In contrast, anthropogenic SOA accounted for the highest fraction of OA in marine (37 ± 4%) and mixed air (31 ± 3%), overriding the contributions from BB. This study demonstrates the utility of molecular markers for discerning OA pollution sources in the offshore marine atmosphere, where continental and marine air pollutants interact and atmospheric oxidative capacity may be enhanced.