Atmospheric oxidizing capacity in autumn Beijing: Analysis of the O3 and PM2.5 episodes based on observation-based model.

Atmospheric oxidizing capacity (AOC) is the fundamental driving factors of chemistry process (e.g., the formation of ozone (O3) and secondary organic aerosols (SOA)) in the troposphere. However, accurate quantification of AOC still remains uncertainty. In this study, a comprehensive field campaign was conducted during autumn 2019 in downtown of Beijing, where O3 and PM2.5 episodes had been experienced successively. The observation-based model (OBM) is used to quantify the AOC at O3 and PM2.5 episodes. The strong intensity of AOC is found at O3 and PM2.5 episodes, and hydroxyl radical (OH) is the dominating daytime oxidant for both episodes. The photolysis of O3 is main source of OH at O3 episode; the photolysis of nitrous acid (HONO) and formaldehyde (HCHO) plays important role in OH formation at PM2.5 episode. The radicals loss routines vary according to precursor pollutants, resulting in different types of air pollution. O3 budgets and sensitivity analysis indicates that O3 production is transition regime (both VOC and NOx-limited) at O3 episode. The heterogeneous reaction of hydroperoxy radicals (HO2) on aerosol surfaces has significant influence on OH and O3 production rates. The HO2 uptake coefficient (γHO2) is the determining factor and required accurate measurement in real atmospheric environment. Our findings could provide the important bases for coordinated control of PM2.5 and O3 pollution.

Effect of Different Combustion Processes on Atmospheric Nitrous Acid Formation Mechanisms: A Winter Comparative Observation in Urban, Suburban and Rural Areas of the North China Plain.

Atmospheric nitrous acid (HONO) is a dominant precursor of hydroxyl (OH) radicals, and its formation mechanisms are still controversial. Few studies have simultaneously explored effects of different combustion processes on HONO sources. Hereby, synchronous HONO measurement in urban (BJ), suburban (XH) and rural (DBT) areas with different combustion processes is performed in the North China Plain in winter. A box model is utilized to analyze HONO formation mechanisms. HONO concentration is the highest at the DBT site (2.51 ± 1.90 ppb), followed by the XH (2.18 ± 1.95 ppb) and BJ (1.17 ± 1.20 ppb) sites. Vehicle exhaust and coal combustion significantly contribute to nocturnal HONO at urban and rural sites, respectively. During a stagnant pollution period, the NO+OH reaction and combustion emissions are more crucial to HONO in urban and rural areas; meanwhile, the heterogeneous reaction of NO2 is more significant in suburban areas. Moreover, the production rate of OH from HONO photolysis is about 2 orders of magnitude higher than that from ozone photolysis. Consequently, vehicle exhaust and coal combustion can effectively emit HONO, further causing environmental pollution and health risks. It is necessary to expand the implementation of the clean energy transition policy in China, especially in areas with substantial coal combustion.

Sources of ambient non-methane hydrocarbon compounds and their impacts on O3 formation during autumn, Beijing.

The field observation of 54 non-methane hydrocarbon compounds (NMHCs) was conducted from September 1 to October 20 in 2020 during autumn in Haidian District, Beijing. The mean concentration of total NMHCs was 29.81 ± 11.39 ppbv during this period, and alkanes were the major components. There were typical festival effects of NMHCs with lower concentration during the National Day. Alkenes and aromatics were the dominant groups in ozone formation potential (OFP) and OH radical loss rate (LOH). The positive matrix factorization (PMF) running results revealed that vehicular exhaust became the biggest source in urban areas, followed by liquefied petroleum gas (LPG) usage, solvent usage, and fuel evaporation. The box model coupled with master chemical mechanism (MCM) was applied to study the impacts of different NMHCs sources on ozone (O3) formation in an O3 episode. The simulation results indicated that reducing NMHCs concentration could effectively suppress O3 formation. Moreover, reducing traffic-related emissions of NMHCs was an effective way to control O3 pollution at an urban site in Beijing.

Formation mechanisms of nitrous acid (HONO) during the haze and non-haze periods in Beijing, China.

As an important precursor of hydroxyl radical (OH), nitrous acid (HONO) plays a significant role in atmospheric chemistry. Here, an observation of HONO and relevant air pollutants in an urban site of Beijing from 14 to 28 April, 2017 was performed. Two distinct peaks of HONO concentrations occurred during the observation. In contrast, the concentration of particulate matter in the first period (period Ⅰ) was significantly higher than that in the second period (period Ⅱ). Comparing to HONO sources in the two periods, we found that the direct vehicle emission was an essential source of the ambient HONO during both periods at night, especially in period Ⅱ. The heterogeneous reaction of NO2 was the dominant source in period Ⅰ, while the homogeneous reaction of NO with OH was more critical source at night in period Ⅱ. In the daytime, the heterogeneous reaction of NO2 was a significant source and was confirmed by the good correlation coefficients (R2) between the unknown sources (Punknown) with NO2, PM2.5, NO2 × PM2.5 in period Ⅰ. Moreover, when solar radiation and OH radicals were considered to explore unknown sources in the daytime, the enhanced correlation of Punknown with photolysis rate of NO2 and OH ( [Formula: see text]  × OH) were 0.93 in period Ⅰ, 0.95 in period Ⅱ. These excellent correlation coefficients suggested that the unknown sources released HONO highly related to the solar radiation and the variation of OH radicals.

Insights into air pollution chemistry and sulphate formation from nitrous acid (HONO) measurements during haze events in Beijing.

Wintertime urban air pollution in many global megacities is characterised by episodic rapid increase in particulate matter concentrations associated with elevated relative humidity - so-called haze episodes, which have become characteristic of cities such as Beijing. Atmospheric chemistry within haze combines gas- and condensed-phase chemical processes, leading to the growth in secondary species such as sulphate aerosols. Here, we integrate observations of reactive gas phase species (HONO, OH, NOx) and time-resolved aerosol composition, to explore observational constraints on the mechanisms responsible for sulphate growth during the onset of haze events. We show that HONO abundance is dominated by established fast gas-phase photochemistry, but the consideration of the additional formation potentially associated with condensed-phase oxidation of S species by aqueous NO2 leading to NO2- production and hence HONO release, improves agreement between observed and calculated gas-phase HONO levels. This conclusion is highly dependent upon aerosol pH, ionic strength and particularly the parameterisation employed for S(iv) oxidation kinetics, for which an upper limit is derived.

Photocatalytic Oxidation of SO2 by TiO2: Aerosol Formation and the Key Role of Gaseous Reactive Oxygen Species.

Photocatalytic materials are proved to effectively eliminate gaseous pollutants and are widely used in the environment. However, as one of the rare experiments focusing on their influence on secondary aerosol formation generated in the gas phase (SAg), our study demonstrated the high-yield SAg formation in the photocatalysis process. In this study, the photodegradation of SO2 by TiO2 under various relative humidity (RH) conditions was deeply explored with multiple methods. Unexpectedly, H2SO4 aerosols (SAg-H2SO4) in yields of 10.10-32.64% were observed under the studied RH conditions for the first time. Gaseous •OH and H2O2 generated from the oxidation of H2O and reduction of O2 by TiO2 were directly detected in the photocatalysis process, and they were identified as the determining factor for SAg-H2SO4 formation. The formation of SAg-H2SO4 was also influenced by RH, the heterogeneous reaction of SO2, and the uptake of H2SO4. The role of the released gaseous •OH and H2O2 on atmospheric chemistry was proved to be unignorable by adopting the obtained parameters into the real environment. These findings provided direct experimental evidence of secondary pollution in the photocatalysis process and are of great significance to the field of atmospheric environment and photocatalytic materials.

Aerosol Promotes Peroxyacetyl Nitrate Formation During Winter in the North China Plain.

Peroxyacetyl nitrate (PAN) is an important indicator for photochemical pollution, formed similar to ozone in the photochemistry of certain volatile organic compounds (VOCs) in the presence of nitrogen oxides, and has displayed surprisingly high concentrations during wintertime that were better correlated to particulate rather than ozone concentrations, for which the reasons remained unknown. In this study, wintertime observations of PAN, VOCs, PM2.5, HONO, and various trace gases were investigated to find the relationship between aerosols and wintertime PAN formation. Wintertime photochemical pollution was affirmed by the high PAN concentrations (average: 1.2 ± 1.1 ppb, maximum: 7.1 ppb), despite low ozone concentrations. PAN concentrations were determined by its oxygenated VOC (OVOC) precursor concentrations and the NO/NO2 ratios and can be well parameterized based on the understanding of their chemical relationship. Data analysis and box modeling results suggest that PAN formation was mostly contributed by VOC aging processes involving OH oxidation or photolysis rather than ozonolysis pathways. Heterogeneous reactions on aerosols have supplied key photochemical oxidants such as HONO, which produced OH radicals upon photolysis, promoting OVOC formation and thereby enhancing PAN production, explaining the observed PM2.5-OVOC-PAN intercorrelation. In turn, parts of these OVOCs might participate in the formation of secondary organic aerosol, further aggravating haze pollution as a feedback. Low wintertime temperatures enable the long-range transport of PAN to downwind regions, and how that will impact their oxidation capacity and photochemical pollution requires further assessment in future studies.

Heterogeneous Formation of HONO Catalyzed by CO2.

Gas-phase nitrous acid (HONO) is a major precursor of hydroxyl radicals that dominate atmospheric oxidizing capacity. Nevertheless, pathways of HONO formation remain to be explored. This study unveiled an important CO2-catalysis mechanism of HONO formation, using Born-Oppenheimer molecular dynamics simulations and free-energy samplings. In the mechanism, HCO3- formed from CO2 hydrolysis reacts with NO2 dimers to produce HONO at water surfaces, and simultaneously, itself reconverts back to CO2 via intermediates OC(O)ONO- and HOC(O)ONO. A flow system experiment was performed to confirm the new mechanism, which indicated that HONO concentrations with CO2 injections were increased by 29.4-68.5%. The new mechanism can be extended to other humid surfaces. Therefore, this study unveiled a previously overlooked vital role of CO2 that catalyzes formation of HONO and affects atmospheric oxidizing capacity.

Contribution of Vehicle Emission and NO2 Surface Conversion to Nitrous Acid (HONO) in Urban Environments: Implications from Tests in a Tunnel.

Nitrous acid (HONO) is an important photochemical precursor to hydroxyl radicals particularly in an urban atmosphere, yet its primary emission and secondary production are often poorly constrained. Here, we measured HONO and nitrogen oxides (NOx) at both the inlet and the outlet in a busy urban tunnel (>30 000 vehicles per day) in south China. Multiple linear regression revealed that 73.9% of the inlet-outlet incremental HONO concentration was explained by NO2 surface conversion, while the rest was directly emitted from vehicles with an average HONO/NOx ratio of 1.31 ± 0.87%, which was higher than that from previous tunnel studies. The uptake coefficient of NO2, γ(NO2), on the tunnel surfaces was calculated to be (7.01 ± 0.02) × 10-5, much higher than that widely used in models. As tunnel surfaces are typical of urban surfaces in the wall and road materials, the dominance of HONO from surface reactions in the poorly lit urban tunnel demonstrated the importance of NO2 conversion on urban surfaces, instead of NO2 conversion on the aerosol surface, for both daytime and night-time HONO even in polluted ambient air. The higher γ(NO2) on urban surfaces and the elevated HONO/NOx ratio from this study can help explain the missing HONO sources in urban areas.

Important Oxidants and Their Impact on the Environmental Effects of Aerosols.

Oxidants are central species in the atmosphere, where they not only determine secondary particle formation but also impact human health and climate change. In general, they are unstable, highly reactive, and recyclable and have been studied in field observations, laboratory studies, and model simulations. The most widely investigated oxidants, such as OH radicals, O3, and Cl atom, HONO, NO3, N2O5, and Criegee Intermediates (CIs) have attracted more attention recently. Furthermore, secondary particles formed in the oxidations processes impact the particle physicochemical properties, such as hygroscopicity and optical properties and therefore impact the atmospheric radiation balance. Therefore, the newest investigation results of important oxidants (HONO, NO3, N2O5, and CIs) are reviewed in this manuscript, and the environmental effects of secondary particles formed through corresponding oxidation processes are also stated. Furthermore, some perspectives are further discussed in the article.

Reaction mechanism and kinetics of Criegee intermediate and hydroperoxymethyl formate.

The reaction mechanism and kinetics of the simplest Criegee intermediate CH2OO reaction with hydroperoxymethyl formate (HPMF) was investigated at high-level quantum chemistry calculations. HPMF has two reactive functional groups, -C(O)OH and -OOH. The calculated results of thermodynamic data and rate constants indicated that the insertion reactions of CH2OO with -OOH group of HPMF were more favorable than the reactions of CH2OO with -C(O)OH group. The calculated overall rate constant was 2.33 × 10-13 cm3/(molecule⋅sec) at 298 K and the rate constants decreased as the temperature increased from 200 to 480 K. In addition, we also proved the polymerization reaction mechanism between CH2OO and -OOH of HPMF. This theoretical study interpreted the previous experimental results, and supplied the structures of the intermediate products that couldn't be detected during the experiment.

Light absorption properties and potential sources of brown carbon in Fenwei Plain during 2018-2019.

A distinctive kind of organic carbon aerosol that could absorb ultraviolet-visible radiation is called brown carbon (BrC), which has an important positive influence on radiative budget and climate change. In this work, we reported the absorption properties and potential source of BrC based on a seven-wavelength aethalometer in the winter of 2018-2019 at an urban site of Sanmenxia in Fenwei Plain in central China. Specifically, the mean value of BrC absorption coefficient was 59.6 ± 36.0 Mm-1 at 370 nm and contributed 37.7% to total absorption, which made a significant impact on visibility and regional environment. Absorption coefficients of BrC showed double-peak pattern, and BrC had shown small fluctuations under haze days compared with clean days. As for the sources of BrC, BrC absorption coefficients expressed strong correlations with element carbon aerosols and primary organic carbon aerosols, indicating that most of BrC originated from primary emissions. The linear correlations between trace metal elements (K, As, Fe, Mn, Zn, and Pb) and BrC absorption coefficients further referred that the major sources of BrC were primary emissions, like coal burning, biomass burning, and vehicle emissions. The moderate relationship between BrC absorption coefficients and secondary organic aerosols suggested that secondary production of BrC also played an important role. The 120 hr backward air mass trajectories analysis and concentration-weighted trajectories analysis were also used to investigate potential sources of BrC in and around this area, which inferred most parts of BrC were derived from local emissions.

Different HONO Sources for Three Layers at the Urban Area of Beijing.

Gaseous nitrous acid (HONO) is a crucial precursor of the hydroxyl (OH) radical, which is a "detergent" in the atmosphere. Nowadays, HONO formation mechanisms at polluted urban areas are controversial, which restricts the understanding of atmospheric oxidative capacity and radical cycling. Herein, multiday vertical observation of HONO and NOx was simultaneously performed at three heights at the urban area of Beijing for the first time. The vertical distribution of HONO was often unexpected, and it had the highest HONO concentration at 120 m, followed by those at 8 and 240 m. 0D box model simulations suggest that ground and aerosol surfaces might play similar roles in NO2 conversion at 8 m during the whole measurement. NO2 conversion on aerosol surfaces was the most important HONO source aloft during haze days. At daytime, a strong missing HONO source unexpectedly existed in the urban aloft, and it was relevant to solar radiation and consumed OH.

Kinetic and mechanism studies of the ozonolysis of three unsaturated ketones.

Reaction rate constants and products of 1-octen-3-one, 3-octen-2-one and 4-hexen-3-one with ozone were studied in a 100-L fluorinated ethylene propylene (FEP) Teflon film bag using absolute rate method at 298 ± 1 K and atmospheric pressure. The rate constants were (1.09 ± 0.12) × 10-17, (3.48 ± 0.36) × 10-17 and (5.70 ± 0.60) × 10-17 cm3/(molecule⋅sec), respectively. According to the obtained rate constants, the effects of carbonyl were discussed. The carbonyl group in ß position has a net withdrawing effect with respect to an olefinic bond, then causing the decline of rate constants. The quantum chemical calculation was used to explain the results of rate constants. The products of ozonolysis were mainly aldehydes, which have significant influence on the formation of SOA, and hence play an important role in the atmosphere. In this work, we detected the main products of reaction and proposed the reaction mechanism by combining the results of quantum chemical calculations. Atmospheric lifetime for three unsaturated ketones reacted with ozone was 36.4, 11.4 and 6.9 hr for 1-octen-3-one, 3-octen-2-one and 4-hexen-3-one, respectively.

Effect of Titanium Dioxide on Secondary Organic Aerosol Formation.

Secondary organic aerosol (SOA), a dominant air pollutant in many countries, threatens the lives of millions of people. Extensive efforts have been invested in studying the formation mechanisms and influence factors of SOA. As promising materials in eliminating air pollutants, the role of photocatalytic materials in SOA formation is unclear. In this study, TiO2 was employed to explore its impact on SOA formation during the photooxidation of m-xylene with NO x in a smog chamber. We found that the presence of TiO2 strongly suppressed SOA formation. The yields of SOA in the photooxidation experiments of m-xylene with NO x were 0.3-4%, whereas negligible SOA was formed when TiO2 was added. When ((NH4)2SO4) was introduced as seed particles, the presence of TiO2 decreased the yields of SOA from 0.3-6% to 0.3-1.6%. The sharply decreased concentrations of reactive carbonyl compounds were the direct cause of the suppression effect of TiO2 on SOA formation. However, the suppression effect was influenced by the addition of seed particles and the initial concentration of NO x. Reaction mechanisms of the photocatalysis of m-xylene with and without NO x were proposed.

Heterogeneous Reaction of HCOOH on NaCl Particles at Different Relative Humidities.

The contribution of volatile organic acids to chloride depletion still remains unclear under ambient conditions in the coast and inland. In this work, the heterogeneous reaction of HCOOH on the NaCl surface at a series of relative humidities (RHs) was investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The formate was found to be formed on NaCl surface under dry and wet conditions, accompanied by the corresponding chloride depletion. The adsorbed HCOOH and the formation of formate on NaCl surface decreased with increasing RH below 30% RH. The adsorbed HCOOH decreased, while the formation of formate increased with enhanced RH at 45-70% RH. The variation in the formation of formate with RH suggests that chloride depletion may undergo similar changes. Additionally, the mechanism and kinetics for uptake of HCOOH on NaCl surface at various RHs were discussed and analyzed. Our results highlight the role of heterogeneous chemistry of volatile organic acid in the chloride depletion of NaCl in the coast and inland.

Kinetic study of the gas-phase reaction of O3 with three unsaturated alcohols.

Rate constants for the reactions of ozone with 1-octen-3-ol, 1-nonen-3-ol and 1-nonen-4-ol have been determined at 298±1K and atmospheric pressure for the first time. The experiments were performed in a 100-L FEP Teflon film bag using absolute rate method; the rate constants were (1.91±0.19)×10-17, (1.89±0.20)×10-17, and (0.83±0.08)×10-17cm3/(molecule·sec) for 1-octen-3-ol, 1-nonen-3-ol, and 1-nonen-4-ol, respectively. The rate constants have been compared with those of unsaturated alcohols structural homologs, and used to estimate the reaction reactivity. The electronegativity of carbon-carbon double bond was calculated by atomic charges analysis. The calculated results show that the electronic effect of the lone pair electrons of hydroxyl oxygen is the main cause of the difference in rate coefficient. According to the obtained rate constants, the atmospheric lifetimes of studied unsaturated alcohols were also estimated, which indicates that the reaction with ozone is an important loss pathway in the atmosphere, especially in polluted areas.

Enhanced Light Scattering of Secondary Organic Aerosols by Multiphase Reactions.

Secondary organic aerosol (SOA) plays a pivotal role in visibility and radiative forcing, both of which are intrinsically linked to the refractive index (RI). While previous studies have focused on the RI of SOA from traditional formation processes, the effect of multiphase reactions on the RI has not been considered. Here, we investigate the effects of multiphase processes on the RI and light-extinction of m-xylene-derived SOA, a common type of anthropogenic SOA. We find that multiphase reactions in the presence of liquid water lead to the formation of oligomers from intermediate products such as glyoxal and methylglyoxal, resulting in a large enhancement in the RI and light-scattering of this SOA. These reactions will result in increases in light-scattering efficiency and direct radiative forcing of approximately 20%-90%. These findings improve our understanding of SOA optical properties and have significant implications for evaluating the impacts of SOA on the rapid formation of regional haze, global radiative balance, and climate change.

Temperature dependence of the heterogeneous uptake of acrylic acid on Arizona test dust.

In this study, the temperature dependence of the heterogeneous uptake of acrylic acid on Arizona test dust (ATD) has been investigated within a temperature range of 255-315K using a Knudsen cell reactor. Combined with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiment, it was found that acrylic acid could adsorb on ATD via surface OH groups and convert to carboxylate on the particle surface. The kinetics study suggests that the initial true uptake coefficient (γt) of acrylic acid on ATD decreases from (4.02±0.12)×10-5 to (1.73±0.05)×10-5 with a temperature increase from 255 to 315K. According to the temperature dependence of uptake coefficients, the enthalpy (ΔHobs) and entropy (ΔSobs) of uptake processes were determined to be -(9.60±0.38) KJ/mol and -(121.55±1.33) J·K/mol, respectively. The activation energy for desorption (Edes) was calculated to be (14.57±0.60) KJ/mol. These results indicated that the heterogeneous uptake of acrylic acid on ATD surface was sensitive to temperature. The heterogeneous uptake on ATD could affect the concentration of acrylic acid in the atmosphere, especially at low temperature.

Mechanism and Kinetics of Heterogeneous Reactions of Unsaturated Organic Acids on α-Al2 O3 and CaCO3.

Heterogeneous reactions have a vital role in the atmosphere due to their significant effects on the evolution of atmospheric aerosols, which in turn contribute to air pollution. However, the mechanism and kinetics of these processes involving unsaturated organic acids, important types of volatile organic compounds, are still unclear. In this work, the heterogeneous uptake of two representative atmospheric unsaturated organic acids (acrylic acid and methacrylic acid) on mineral aerosols including α-Al2 O3 and CaCO3 are investigated using a Knudsen cell reactor and an in situ diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) reactor. The corresponding reaction pathways are proposed from the DRIFTS analysis. In addition, the initial uptake coefficients of unsaturated organic acids and their heterogeneous fate are obtained for the first time. Our results suggest that heterogeneous reactions on α-Al2 O3 and CaCO3 can be important sinks for acrylic acid and methacrylic acid, as well as possible contributors to the organic coating found on atmospheric aerosols, especially in high-pollution events.