Elucidating the Mechanism on the Transition-Metal Ion-Synergetic-Catalyzed Oxidation of SO2 with Implications for Sulfate Formation in Beijing Haze.

Currently, atmospheric sulfate aerosols cannot be predicted reliably by numerical models because the pathways and kinetics of sulfate formation are unclear. Here, we systematically investigated the synergetic catalyzing role of transition-metal ions (TMIs, Fe3+/Mn2+) in the oxidation of SO2 by O2 on aerosols using chamber experiments. Our results showed that the synergetic effect of TMIs is critically dependent on aerosol pH due to the solubility of Fe(III) species sensitive to the aqueous phase acidity, which is effective only under pH < 3 conditions. The sulfate formation rate on aerosols is 2 orders of magnitude larger than that in bulk solution and increases significantly on smaller aerosols, suggesting that such a synergetic-catalyzed oxidation occurs on the aerosol surface. The kinetic reaction rate can be described as R = k*[H+]-2.95[Mn(II)][Fe(III)][S(IV)] (pH ≤ 3.0). We found that TMI-synergetic-catalyzed oxidation is the dominant pathway of sulfate formation in Beijing when haze particles are very acidic, while heterogeneous oxidation of SO2 by NO2 is the most important pathway when haze particles are weakly acidic. Our work for the first time clarified the role and kinetics of TMI-synergetic-catalyzed oxidation of SO2 by O2 in haze periods, which can be parameterized into models for future studies of sulfate formation.

Abundant NH3 in China Enhances Atmospheric HONO Production by Promoting the Heterogeneous Reaction of SO2 with NO2.

High levels of HONO have frequently been observed in Chinese haze periods and underestimated by current models due to some unknown sources and formation mechanisms. Combining lab-chamber simulations and field measurements in Xi'an and Beijing, China, we found that NH3 can significantly promote HONO formation via the reduction-oxidation of SO2 with NO2 in the aqueous phase of hygroscopic particles (e.g., NaCl). Concentrations of HONO formed in the aerosol phase showed an exponential increase (R2 = 0.91) with NH3 levels under the chamber conditions and a linear growth with NH3 levels in the two Chinese cities. The uptake coefficient of NO2 on NaCl particles ranged from 2.0 × 10-5 to 1.7 × 10-4, 3-4 orders of magnitude larger than that on water droplets. Our results further showed that HONO formed from the aerosol phase accounted for 4-33% of the total in the chamber, indicating that aerosol-phase formation is an important source of HONO in China, especially in haze periods. Since NH3, SO2, and NO2 abundantly coexist in China, the positive effect of NH3 on HONO formation could enhance the atmospheric oxidizing capacity in the country, causing severe secondary aerosol pollution. Our work suggests that NH3 emission control is imperative for mitigating air pollution in China.

Study on the Initiation of Interface Crack in Rock Joints.

The interfacial fracture of rock joints is an important although easily ignored issue in jointed rock engineering. To conduct this study, an interface crack model of rock joints was proposed. By analyzing the ratio of stress intensity factor to fracture toughness, the fracture mode of the interface crack was studied. Based on the Mohr-Coulomb criterion, an interface fracture criterion considering T-stress was established. To verify the proposed fracture criterion, laboratory and numerical tests were conducted. Finally, the effect of relative critical size α, internal friction angle φ and cohesion c on the initiation of an interface crack was comprehensively discussed. It is concluded that the proposed fracture criterion can predit the initiation of the interface cracks properly. With an increase in cohesion c, mode II fracture toughness KIIC also clearly increases. When the absolute value of KI is small, the effect of α is much larger than that of φ. In addition, with an increase in the absolute value of the mode I stress intensity factor, the φ of the joint plays a more important role in the initiation of the interface crack.

Aerosol liquid water content of PM2.5 and its influencing factors in Beijing, China.

Aerosol liquid water content (ALWC) has important influences on atmospheric radiation and aerosol chemical processes. In this work, the changes in ALWC of PM2.5 were investigated over four seasons based on hourly monitoring of inorganic water-soluble ions and their gaseous precursors using the thermodynamic model ISORROPIA II. The results showed that the ALWC concentrations exhibited pronounced seasonal (autumn > summer > spring > winter) and diurnal variation characteristics. The sensitivity tests indicated that ALWC depended strongly on TSO4 (total sulfate (gas and aerosols) expressed as equivalent H2SO4), followed by TNO3 (total nitrate (gas and aerosols) expressed as equivalent HNO3). The relatively low concentration of TCl (total chloride (gas and aerosols) expressed as equivalent HCl) limit its importance in the atmosphere. ALWC showed exponential growth features as a function RH in all four seasons. RH became the most influential factor on the variation of ALWC when RH exceeded 80% in all seasons. The seasonal average data showed that the ALWC increased from 2.92 µg·m-3 to 75.83 µg·m-3 when ambient RH increased from 30% to 90%, the associated sulfate, nitrate, and ammonium (abbreviated as SNA) mass fraction in PM2.5 rose from 0.39 to 0.58 in the atmosphere. The ALWC facilitated the formation of SNA through gas-particle conversion and partitioning. The self-amplifying processes between ALWC and SNA enhanced aerosol formation. By modeling ALWC under different seasonal atmospheric scenarios, it was found that reductions in chemical species could reduce ALWC concentrations in different degrees. Based on the current emission conditions, controlling excess NH3 emission could effectively reduce ALWC to a maximum of 45.71% in summer, indicating that NH3 control was crucial for reducing ALWC and PM2.5 concentrations under high levels of SO42- and NO3-.

Smog chamber simulation on heterogeneous reaction of O3 and NO2 on black carbon under various relative humidity conditions.

In this study, heterogeneous formation of nitrate from O3 reaction with NO2 on black carbon (BC) and KCl-treated BC surface in the presence of NH3 was simulated under 30-90% RH conditions by using a laboratory smog chamber. We found that O3 and NO2 in the chamber quickly reacted into N2O5 in the gas phase, which subsequently hydrolyzed into HNO3 and further neutralized with NH3 into NH4NO3 on the BC surface, along with a small amount of N2O5 decomposed into NO and NO2 through a reaction with the BC surface active site. Meanwhile, the fractal BC aggregates restructured and condensed to spherical particles during the NH4NO3 coating process. Compared to that during the exposure to NO2 or O3 alone, the presence of strong signals of CH2O+, CH2O2+ and CH4NO+ during the simultaneous exposure to both NO2 and O3 suggested a synergetic oxidizing effect of NO2 and O3, which significantly activated the BC surface by forming carbonyl, carboxylic and nitro groups, promoted the adsorption of water vapor onto the BC surface and enhanced the NH4NO3 formation. Under <75 ± 2% RH conditions the coating process of NH4NO3 on the BC surface consisted of a diffusion of N2O5 onto the surface and a subsequent hydrolysis, due to the limited number of water molecules adsorbed. However, under 90 ± 2% RH conditions N2O5 directly hydrolyzed on the aqueous phase of the BC surface due to the multilayer water molecules adsorbed, which caused an instant NH4NO3 formation on the surface without any delay. The coating rate of NH4NO3 on KCl-treated BC particles was 3-4 times faster than that on the pure BC particles at the initial stage, indicating an increasing formation of NH4NO3, mainly due to an enhanced hygroscopicity of BC by KCl salts.

Enhancing effect of NO2 on the formation of light-absorbing secondary organic aerosols from toluene photooxidation.

Aromatic hydrocarbons are one of the major precursors of atmospheric brown carbon (BrC) and both abundantly co-exist with NOx in the urban atmosphere especially in winter haze period. However, the impact of NOx on the formation of BrC derived from aromatic hydrocarbons is still not fully understood. In this study, the yield and light absorption of secondary organic aerosols (SOA) from toluene photooxidation under various nitrogen oxides (NO2) levels were investigated by using a 5 m3 photooxidation smog chamber. A trend of increase at first and then decrease in the SOA yield with an increasing NO2 concentration was observed. The acid-catalyzed heterogeneous reactions lead to the increase of SOA yield in the low-NO2 regime. The formation of low-volatility species might be suppressed at high-NO2 conditions is responsible for the decreased SOA yield. In contrast, light absorption and mass absorption coefficient (MAC) of the toluene-derived SOA continuously increased with the increasing NO2 concentrations. HR-ToF-AMS results showed that nitrogen-containing organic compounds (NOCs) are the main species that lead to the increase of the SOA light absorption. The ratio of CHN family to the total NOCs, which are derived from the nitro compounds, also increased dominantly with the increasing NO2 levels and accounted for more than half of the total NOCs when the NO2 concentration increased to 495 ppbv, indicating that nitro compounds rather than organic nitrates are the major light-absorbing species and preferably formed in the toluene oxidation process.

Influence of COVID-19 lockdown overlapping Chinese Spring Festival on household PM2.5 in rural Chinese homes.

During the 2019 novel coronavirus (COVID-19) pandemic, many countries took strong lockdown policy to reduce disease spreading, resulting in mitigating the ambient air pollution due to less traffic and industrial emissions. However, limited studies focused on the household air pollution especially in rural area, the potential risk induced by indoor air pollution exposure was unknown during this period. This field study continuously measured real-time PM2.5 levels in kitchen, living room, and outdoor in the normal days (Period-1) and the days of COVID-19 lockdown overlapping the Chinese Spring Festival (Period-2) in rural homes in China. The average daily PM2.5 concentrations increased by 17.4 and 5.1 µg/m3 in kitchen and living room during Period-2, respectively, which may be due to more fuel consumption for cooking and heating caused by larger family sizes than those during the normal days. The ambient PM2.5 concentration in rural areas in Period-2 decreased by 6.7 µg/m3 compared to the Period-1, less than the drop in urban areas (26.8 µg/m3). An increase of mass fraction of very fine particles in ambient air was observed during lockdown overlapping annual festival days, which could be explained by the residential solid fuel burning. Due to higher indoor air pollution level and longer time spent in indoor environments, daily personal exposure to PM2.5 was 134 ± 40 µg/m3 in Period-2, which was significantly higher than that during in Period-1 (126 ± 27 µg/m3, p < 0.05). The increase of personal PM2.5 exposure during Period-2 could potentially have negative impact on human health, indicating further investigations should be performed to estimate the health impact of global COVID-19 lockdown on community, especially in rural homes using solid fuels as the routine fuels.

Rapid sulfate formation from synergetic oxidation of SO2 by O3 and NO2 under ammonia-rich conditions: Implications for the explosive growth of atmospheric PM2.5 during haze events in China.

Extremely high levels of atmospheric sulfate aerosols have still frequently occurred in China especially in winter haze periods and often been underestimated by models due to some missing formation mechanisms. Here we investigated the heterogeneous reaction dynamics of SO2 oxidation by the abundantly co-existing O3 and NO2 in the urban atmosphere of China by using a laboratory smog chamber simulation technique. Our results showed that with an increase of NH3 concentrations from 0.05 ppm to 1.5 ppm, SO2 oxidation by O3 can be greatly promoted and lead to an exponential increase of diameter growth factor (GF) of particles in the chamber from 1.29 to 1.98 for NaCl seeds and from 1.20 to 1.60 for (NH4)2SO4 seeds, along with an increasing uptake coefficient (γ) of SO2 from 4.47 × 10-5 to 1.52 × 10-4 on NaCl seeds and from 2.32 × 10-5 to 5.74 × 10-5 on (NH4)2SO4 seeds, respectively. The heterogeneous production of sulfate from oxidation of SO2 under NH3-rich conditions by O3 and NO2 mixture in the chamber was 2.0-3.5 times the sum of sulfate from SO2 oxidations by O3 and NO2, suggesting a strongly synergetic effect of the mixed oxidants on the heterogeneous oxidation of SO2, which can cause rapid formation of (NH4)2SO4 and NH4NO3 and is responsible for the explosive growth of PM2.5 in the winter haze period of China. Our chamber results further showed that such synergetic process is only efficient under NH3-rich conditions, clearly indicating that the combined controls on O3, NOx and NH3 are necessary for further mitigating the PM2.5 pollution in China.

Enhanced aqueous-phase formation of secondary organic aerosols due to the regional biomass burning over North China Plain.

This study reveals the impact of biomass burning (BB) on secondary organic aerosols (SOA) formation in the North China Plain (NCP). Filter samples were analyzed for secondary inorganic aerosols (SIA), oxalic acid (C2) and related aqueous-phase SOA compounds (aqSOA), stable carbon isotope composition of C2 (δ13C(C2)) and aerosol liquid water content (ALWC). Based on the PM2.5 loadings, BB tracer concentrations, wildfire spots and air-mass back trajectories, we distinguished two episodes from the whole campaign, Episode I and Episode II, which were characteristic of regional and local BB, respectively. The abundances of PM2.5 and organic matter in the two events were comparable, but concentrations and fractions of SIA, aqSOA during Episode I were much higher than those during Episode II, along with heavier δ13C(C2), suggesting an enhanced aqSOA formation in the earlier period. We found that the enhancement of aqSOA formation during Episode I was caused by an increased ALWC, which was mainly driven by SIA during the regional BB event. Our work showed that intensive burning of crop residue in East Asia can sharply enhance aqSOA production on a large scale, which may have a significant impact on the regional climate and human health.

[Pollution Characteristics and Source Apportionment of n-Alkanes and PAHs in Summertime PM2.5 at Background Site of Yangtze River Delta].

To investigate the pollution characteristics and sources of organic aerosols at a background site of the Yangtze River Delta, day- and night- PM2.5 samples were collected from May 30th to August 15th, 2018 in Chongming Island, China and measured for their normal alkanes (n-alkanes) and polycyclic aromatic hydrocarbons (PAHs) content employing a GC-MS technique. Concentrations of PM2.5, n-alkanes, and PAHs during the entire sampling period were (33±21) µg·m-3, (26±44) ng·m-3, and (0.76±1.0) ng·m-3, respectively. During the entire campaign, 35% of the collected PM2.5 samples were of a particle loading larger than the first grade of the China National Air Quality Standard (35 µg·m-3), suggesting that further mitigation with respect to air pollution in Chongming Island remains imperative. In the period with a PM2.5 concentration higher than 35 µg·m-3, which was classified as the pollution period, concentrations of n-alkanes and PAHs were one order of magnitude higher than those in the period with PM2.5 less than 15 µg·m-3, which was classified as the clean period. During the entire campaign, OC was higher in the daytime than in the nighttime, mainly due to the daytime photooxidation that enhanced the formation of secondary organic aerosols. During the pollution period, concentrations of EC and other pollutants were higher in the nighttime than in daytime, mainly due to the transport of the inland pollutants by the nighttime land breeze. Such a diurnal difference was not observed for the pollutants in clean periods, mainly due to the relatively clean breeze from East China Sea that diluted the air pollution. Diagnostic ratios showed that 67% of n-alkanes in PM2.5 was derived from fossil fuel combustion. PMF analysis further showed that during the pollution period, vehicle exhausts and industrial emissions were the largest sources of PAHs, both accounting for 51% of the total in PM2.5. In contrast, during the clean periods ship emissions were the largest source, contributing about 45% of the total PAHs, exceeding the sum (38%) of vehicle and industrial emissions.

Non-agricultural sources dominate the atmospheric NH3 in Xi'an, a megacity in the semi-arid region of China.

Ammonia (NH3), as a dominant alkaline gas in the atmosphere, plays a vital role in Chinese urban haze formation process, but its source in urban areas of China is controversial. To identify the sources of urban NH3 in the semi-arid region of East Asia, real-time measurements of NH3 and NH4+ of PM2.5 in the urban atmosphere of Xi'an, inland China during the winter and summer of 2017 were performed and their stable nitrogen isotope composition were analyzed. NH3 was 38.0 ± 9.4 µg/m3 in the summer, which is 1.5 times higher than that in the winter. Concentration of NH3 in both seasons well correlated with that of PAHs in PM2.5 and the mass ratio of (BbF + BeP + IP + BghiP) to the total PAHs, suggesting that fossil fuel combustion is an important source of NH3 in Xi'an. Moreover, diurnal variation pattern of NH3 was consistent with that of CO in the summer, peaking in the morning and evening rush hours, respectively, further indicating an importance of the contribution of traffic emissions to NH3 in the city. Based on the source apportionment by using isotope mixing model, we found that 66.4% and 62.5% of NH3 in the urban atmosphere were contributed by non-agricultural sources in the summer and winter, respectively. Our work revealed that non-agricultural sources dominate the atmospheric NH3 of Xi'an, where haze pollution is still severe, and suggested that emission controls of non-agricultural NH3 could be an effective way to mitigate the air pollution problem in the semi-arid region of East Asia.

Chemical characteristics of airborne particles in Xi'an, inland China during dust storm episodes: Implications for heterogeneous formation of ammonium nitrate and enhancement of N-deposition.

To identify the sources and heterogeneous reactions of sulfate and nitrate with dust in the atmosphere, airborne particles in Xi'an, inland China during the spring of 2017 were collected and measured for chemical compositions, along with a laboratory simulation of the heterogeneous formation of ammonium nitrate on the dust surface. Our results showed that concentrations of Ca2+, Na+ and Cl- in the TSP samples were enhanced in the dust events, with the values of 41.8, 5.4 and 4.0 µg m-3, respectively, while NO3- (7.1 µg m-3) and NH4+ (2.4 µg m-3) remarkably decreased, compared to those in the non-dust periods. During the dust events, NH4+ correlated only with NO3- (R2 = 0.52) and abundantly occurred in the coarse mode (>2.1 µm), in contrast to that in the non-dust periods, which well correlated with sulfate and nitrate and enriched in the fine mode (<2.1 µm). SO42- in Xi'an during the dust events existed mostly as gypsum (CaSO4·2H2O) and mirabilite (Na2SO4·10H2O) and dominated in the coarse mode, suggesting that they were directly transported from the upwind Gobi Desert region. Our laboratory simulation results showed that during the long-range transport hygroscopic salts in the Gobi dust such as mirabilite can absorb water vapor and form a liquid phase on the particle surface, then gaseous NH3 and HNO3 partition into the aqueous phase and form NH4NO3, resulting in the strong correlation of NH4+ with NO3- and their accumulation on dust particles. The dry deposition flux of total inorganic nitrogen (NH4+ + NO3-) in Xi'an during the dust events was 0.97 mg-N m-2 d-1 and 37% higher than that in the non-dust periods. Such a significant enhanced N-deposition is ascribed to the heterogeneous formation of NH4NO3 on the dust particle surface, which has been ignored and should be included in future model simulations.

Novel Soluble Dietary Fiber-Tannin Self-Assembled Film: A Promising Protein Protective Material.

In this experiment, a natural promising protein protective film was fabricated through soluble dietary fiber (SDF)-tannin nanocluster self-assembly. FT-IR, XRD, and DSC tests were employed to investigate the interaction between the SDF and tannins before and after cross-linking induced by calcium ion. On the other hand, referring to the SEM and TEM results, the self-assembly process of the protein protective film could be indicated as follows: first, calcium ion, with its cross-ability, served as the "nucleus"; SDF and tannins were combined to prepare the nanoscale SDF-tannin clusters; then, the clusters were homogeneously deposited on the surface of protein to form a protective film by self-assembling hydrogen bond between tannin component of clusters as "adhesive" and protein in aqueous solutions under very mild conditions. Film thickness could also be controlled by tannin of different concentrations ranging from 114 to 1384 µm. Antibacterial test and in vitro cytotoxicity test proved that the film had a broad spectrum of antimicrobial properties and excellent cell biocompatibility, respectively, which might open up new applications in the food preservation and biomedical fields.