Secondary Organic Aerosol Formation from Ambient Air at an Urban Site in Beijing: Effects of OH Exposure and Precursor Concentrations

Satellite NO2 Retrieval Complicated by Aerosol Composition over Global Urban Agglomerations: Seasonal Variations and Long-Term Trends (2001-2018)

Tropospheric nitrogen dioxide (NO2) poses a serious threat to the environmental quality and public health. Satellite NO2 observations have been continuously used to monitor NO2 variations and improve model performances. However, the accuracy of satellite NO2 retrieval depends on the knowledge of aerosol optical properties, in particular for urban agglomerations accompanied by significant changes in aerosol characteristics. In this study, we investigate the impacts of aerosol composition on tropospheric NO2 retrieval for an 18 year global data set from Global Ozone Monitoring Experiment (GOME)-series satellite sensors. With a focus on cloud-free scenes dominated by the presence of aerosols, individual aerosol composition affects the uncertainties of tropospheric NO2 columns through impacts on the aerosol loading amount, relative vertical distribution of aerosol and NO2, aerosol absorption properties, and surface albedo determination. Among aerosol compositions, secondary inorganic aerosol mostly dominates the NO2 uncertainty by up to 43.5% in urban agglomerations, while organic aerosols contribute significantly to the NO2 uncertainty by -8.9 to 37.3% during biomass burning seasons. The possible contrary influences from different aerosol species highlight the importance and complexity of aerosol correction on tropospheric NO2 retrieval and indicate the need for a full picture of aerosol properties. This is of particular importance for interpreting seasonal variations or long-term trends of tropospheric NO2 columns as well as for mitigating ozone and fine particulate matter pollution.

Role of WSOCs and pH on Ammonium Nitrate Aerosol Efflorescence: Insights into Secondary Aerosol Formation

Efflorescence of ammonium nitrate (AN) aerosols significantly impacts atmospheric secondary aerosol formation, climate, and human health. We investigated the effect of representative water-soluble organic compounds (WSOCs) (sucralose (SUC), glycerol (GLY), and citric acid (CA) on AN:WSOC aerosol efflorescence using vacuum Fourier transform infrared spectroscopy. Combining efflorescence relative humidity (ERH) measurements, heterogeneous nucleation rates, and model predictions, we found that aerosol viscosity, correlating with molecular diffusion, effectively predicted ERH variations among the AN:WSOC aerosols. WSOCs with higher viscosity (SUC and CA) hindered efflorescence, while GLY with a lower viscosity showed a minor effect. At a low AN:CA molar ratio (10:1), CA promoted ERH, likely due to CA crystallization. Increasing the droplet pH inhibited AN:CA aerosol efflorescence. In contrast, for AN:SUC and AN:GLY aerosols, efflorescence is pH-insensitive. With the addition of trivial sulfate, AN:SUC droplets exhibited two-stage efflorescence, coinciding with ammonium sulfate and AN efflorescence. Given the atmospheric abundance, the morphology, phase, and mixing state of nitrate aerosols are significant for atmospheric chemistry and physics. Our results suggest that AN:WSOCs aerosols can exist in the amorphous phase in the atmosphere, with efflorescence behavior depending on the aerosol composition, viscosity, pH, and the cation and anion interactions in a complex manner.

Combined Smog Chamber/Oxidation Flow Reactor Study on Aging of Secondary Organic Aerosol from Photooxidation of Aromatic Hydrocarbons

Secondary organic aerosol (SOA) is a significant component of atmospheric fine particulate matter (PM2.5), and their physicochemical properties can be significantly changed in the aging process. In this study, we used a combination consisting of a smog chamber (SC) and oxidation flow reactor (OFR) to investigate the continuous aging process of gas-phase organic intermediates and SOA formed from the photooxidation of toluene, a typical aromatic hydrocarbon. Our results showed that as the OH exposure increased from 2.6 × 1011 to 6.3 × 1011 molecules cm-3 s (equivalent aging time of 2.01-4.85 days), the SOA mass concentration (2.9 ± 0.05-28.7 ± 0.6 μg cm-3) and corrected SOA yield (0.073-0.26) were significantly enhanced. As the aging process proceeds, organic acids and multiple oxygen-containing oxidation products are continuously produced from the photochemical aging process of gas-phase organic intermediates (mainly semi-volatile and intermediate volatility species, S/IVOCs). The multigeneration oxidation products then partition to the aerosol phase, while functionalization of SOA rather than fragmentation dominated in the photochemical aging process, resulting in much higher SOA yield after aging compared to that in the SC. Our study indicates that SOA yields as a function of OH exposure should be considered in air quality models to improve SOA simulation, and thus accurately assess the impact on SOA properties and regional air quality.

Real time measurements of the secondary organic aerosol formation and aging from ambient air using an oxidation flow reactor in seoul during winter

Herein, the formation and aging processes of organic aerosol (OA) in urban Seoul, Korea, during winter were investigated using a high-resolution aerosol mass spectrometer (HR-ToF-AMS) and an oxidation flow reactor (OFR). The results demonstrated that the highest secondary OA (SOA) production (ΔOA = 3.44 μg m^-3 with a relative OA enhancement ratio (EROA) = 1.40) occurred at ∼2 eq. days of OH exposure. Particularly, higher SOA production was observed under the following atmospheric conditions: high relative humidity (RH) (>70%) and high PM₁ mass concentration (>50 μg m^-3), demonstrating that oxidation capacity, heterogeneous and aqueous phase reactions are important for further oxidation. Additionally, increased SOA formation occurs under both higher hydrocarbon-like OA and more oxidized OOA conditions. Further oxidation of both freshly emitted and aged and/or transported OA can be a remarkable further source of SOA in winter in Seoul and further downwind areas. In particular, the high mass concentration of MO-OOA in high total PM₁ would be an important indication that SOA formation could be accelerated by a heterogeneous reaction, necessitating additional investigations on the haze formation process.

VOC species controlling O3 formation in ambient air and their sources in Kaifeng, China

The concentration of ozone has been in a rising crescendo in the last decade while the fine particles (PM_2.5) is gradually decreasing but still at a high level in central China. Volatile organic compounds (VOCs) are the vital precursors of ozone and PM_2.5. A total of 101 VOC species were measured in four seasons at five sites from 2019 to 2021 in Kaifeng. VOC sources and geographic origin of sources were identified by the positive matrix factorization (PMF) model and the hybrid single-particle Lagrangian integrated trajectory transport model. The source-specific OH loss rates (L_OH) and ozone formation potential (OFP) were calculated to estimate the effects of each VOC source. The average mixing ratios of total VOCs (TVOC) were 43.15 parts per billion (ppb), of which the alkanes, alkenes, aromatics, halocarbons, and oxygenated VOCs respectively accounted for 49%, 12%, 11%, 14%, and 14%. Although the mixing ratios of alkenes were comparatively low, they played a dominant role in the L_OH and OFP, especially ethene (0.55 s^-1, 7%; 27.11 μg/m³, 10%) and 1,3-butadiene (0.74 s^-1, 10%; 12.52 μg/m³, 5%). The vehicle-related source which emitted considerable alkenes ranked as the foremost contributing factor (21%). Biomass burning was probably influenced by other cities in the western and southern Henan and other provinces, Shandong and Hebei.

Atmospheric Particle Hygroscopicity and the Influence by Oxidation State of Organic Aerosols in Urban Beijing

A hygroscopic tandem differential mobility analyser (H-TDMA) was used to observe the size-resolved hygroscopic characteristics of submicron particles in January and April 2018 in urban Beijing. The probability distribution of the hygroscopic growth factor (HGF-PDF) in winter and spring usually showed a bimodal pattern, with more hygroscopic mode (MH) being more dominant. The seasonal variation in particle hygroscopicity was related to the origin of air mass, which received polluted southerly air masses in spring and clean northwesterly air masses in winter. Particles showed stronger hygroscopic behaviour during heavy pollution episodes (HPEs) with elevated concentrations of secondary aerosols, especially higher mass fraction of nitrate, which were indicated using the PM_2.5 (particulate matter with diameter below 2.5 µm) mass concentration normalised by CO mass concentration. The hygroscopic parameter (κ) values were calculated using H-TDMA (κ_htdma) and chemical composition (κ_chem). The closure study showed that κ_chem was overestimated in winter afternoon when compared with κ_htdma, because the organic particle hygroscopic parameter (κ_org) was overestimated in the calculations. It was influenced by the presence of a high concentration of hydrocarbon-like organic aerosol (HOA) with a weak water uptake ability. A positive relationship was observed between κ_org and the ratio of oxygenated organic aerosol (OOA) and HOA, thereby indicating that the strong oxidation state enhanced the hygroscopicity of the particles. This study revealed the effect of local emission sources and secondary aerosol formation processes on particle hygroscopicity, which is of great significance for understanding the pollution formation mechanism in the North China Plain.

Dramatic decrease of secondary organic aerosol formation potential in Beijing: Important contribution from reduction of coal combustion emission

Secondary organic aerosol (SOA) formation originating from the emission of anthropogenic volatile organic compounds (VOCs) makes a significant contribution to fine particulate matter (PM_2.5) pollution in urban areas. Investigation on the SOA formation potential (SOAFP) can help us understand the contribution of different sources to SOA formation. To characterize the SOAFP of ambient air from anthropogenic VOCs in the urban area of Beijing, field observation was implemented using a twin oxidation flow reactor (Twin-OFRs) system in the winters of 2016 and 2017. Compared to the winter of 2016, the seasonal-average SOAFP in the winter of 2017 was found to decrease by about 74% (18.6 to 4.9 μg/m³), which is more than that of PM₁ (59%, 48.7 to 20.2 μg/m³), PM_2.5 (61%, 114.4 to 44.8 μg/m³) and CO (57%, 2.1 to 0.9 mg/m³) that mainly comes from the combustion of fossil fuels, suggesting complex affecting factors on SOAFP. The results of wind decomposition mathematical modeling showed that anthropogenic factors and favorable meteorological conditions both contributed significantly to the decrease in SOAFP. The reduction of emissions from scatter coal combustion, which is the key VOCs source for SOAFP, is probably the most important anthropogenic factor affecting SOAFP. In the winter of 2016, the ratio of benzene to toluene is 1.45 that was close to 1.54 representing coal combustion emission; however, it decreased dramatically to 1.05 in the winter of 2017, suggesting considerable reduction of VOC emissions from scatter coal combustion in the latter year due to the coal-to-gas transition in Beijing and surrounding regions. The SOAFP measured in this study considers all ambient VOCs that can react with OH radical, providing another representative method for estimating it. These results could be beneficial to understanding the factors driving SOAFP and its contribution to PM_2.5, especially in regions with high-intensity anthropogenic emissions. Synopsis: This study reported the sharp decline of secondary organic aerosol formation potential (SOAFP) between two consecutive winters in Beijing and analyzed the reasons.

Oxidation Flow Reactor Results in a Chinese Megacity Emphasize the Important Contribution of S/IVOCs to Ambient SOA Formation

Oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA) formation potential of ambient air in Guangzhou, China was investigated using a field-deployed oxidation flow reactor (OFR). The OFR was used to mimic hours to weeks of atmospheric exposure to hydroxyl (OH) radicals within the 2-3 min residence time. A comprehensive investigation on the variation of VOCs and OVOCs as a function of OH exposure is shown. Substantial formation of organic acids and nitrogen-containing OVOC species were observed. Maximum SOA formation in the OFR was observed following 1-4 equiv days' OH exposure. SOA produced from known/measured VOC/IVOC precursors such as single-ring aromatics and long-chain alkanes can account for 52-75% of measured SOA under low NO_x and 26-60% under high NO_x conditions based on laboratory SOA yield parametrizations. To our knowledge, this is the first time that the contribution (8-20%) of long-chain (C₈-C₂₀) alkane oxidation to OFR SOA formation was quantified from direct measurement. By additionally estimating contribution from unmeasured semivolatile and intermediate volatility compounds (S/IVOCs) that are committed with C₈-C₂₀ alkanes, 64-100% of the SOA formation observed in the OFR can be explained, signifying the important contribution of S/IVOCs such as large cyclic alkanes to ambient SOA.

Secondary organic aerosol formation and aging from ambient air in an oxidation flow reactor during wintertime in Beijing, China

Secondary organic aerosols (SOA) constitute a large fraction of atmospheric aerosols, yet our knowledge of the formation and aging processes of SOA in megacities of China is still limited. In this work, the formation and aging processes of SOA in winter in Beijing was investigated using a high-resolution aerosol mass spectrometer (AMS) and an oxidation flow reactor (OFR). Our results showed that the OA enhancement from OH aging peaked at ∼3.9 equivalent days with an average enhancement of 0.9 (±0.3) μg m^-3. Positive matrix factorization analysis of AMS-OFR data identified three primary OA (POA) and two SOA factors. While the concentrations of POA factors decreased as a function of photochemical age, the two SOA factors showed clear enhancements by 2.5 and 4.3 μg m^-3 at ∼3.9 and ∼2.6 days of equivalent photochemical age, respectively. The average contribution of SOA to the total OA was 47% in ambient air and 87% in OFR-oxidized ambient air. The elevated oxygen-to-carbon (O/C) ratio from 0.49 to 0.77-0.82 and the decreased hydrogen-to-carbon (H/C) from 1.37 to ∼1.1 highlighted the formation of more oxidized SOA during photochemical aging in winter in Beijing. The ubiquitous SOA enhancement as a function of OA levels indicated the significant formation potential of SOA in winter, and it varied differently among different episodes. In particular, we observed a maximum SOA enhancement of 38.6 μg m^-3 during a biomass burning event. This result demonstrates that photochemical oxidation of ubiquitous biomass burning emissions can be a large source of SOA in winter in North China Plain.

Characterization and source identification of submicron aerosol during serious haze pollution periods in Beijing

Submicron aerosol is of extensive concern not only due to its significant impact on air quality but also because it is detrimental to human health. In this study, we investigated the characteristics, sources and chemical processes of submicron aerosol based on real-time online measurements of submicron aerosols (NR-PM₁) during December 2015 at an urban site in Beijing. The average mass concentration of NR-PM₁ was 92.5±84.9 µg/m³, the hourly maximum was 459.1 µg/m³ during the entire observation. The organic aerosol (OA) (55%) was the largest contributor to NR-PM₁. The average mass concentration of PAHs was 0.217±0.247 µg/m³, exhibiting the highest concentration at night and the lowest levels in the daytime. The average mass concentration of organic nitrate was 2.52±2.36 µg/m³ and that of inorganic nitrate was 7.62±8.22 µg/m³, accounting for 36% and 64%, respectively, of the total nitrate mass. Positive matrix factorization (PMF) differentiated the OA into five chemical components including LV-OOA, SV-OOA, COA, HOA and CCOA, accounting for 22%, 16%, 13%, 25% and 24% respectively, of the total OA. The average NR-PM₁ mass concentration on the heavy polluted days (HPD) was 182.8±70.2 µg/m³, which was approximately 9 times that on clean days (CD). The enhanced secondary formation of SNA was evident on HPD, especially the rapid increase of sulfate (23%) and nitrate (19%).

Secondary organic aerosols produced from photochemical oxidation of secondarily evaporated biomass burning organic gases: Chemical composition, toxicity, optical properties, and climate effect

Biomass burning (BB) is an important source of primary organic aerosols (POA). These POA contain a significant fraction of semivolatile organic compounds, and can release them into the gas phase during the dilution process in transport. Such evaporated compounds were termed "secondarily evaporated BB organic gases (SBB-OGs)" to distinguish them from the more studied primary emissions. SBB-OGs contribute to the formation of secondary organic aerosols (SOA) through reactions with atmospheric oxidants, and thus may influence human health and the Earth's radiation budget. In this study, tar materials collected from wood pyrolysis were taken as proxies for POA from smoldering-phase BB and were used to release SBB-OGs constantly in the lab. OH-initiated oxidation of the SBB-OGs in the absence of NO_x was investigated using an oxidation flow reactor, and the chemical, optical, and toxicological properties of SOA were comprehensively characterized. Carbonyl compounds were the most abundant species in identified SOA species. Human lung epithelial cells exposed to an environmentally relevant dose of the most aged SOA did not exhibit detectable cell mortality. The oxidative potential of SOA was characterized with the dithiothreitol (DTT) assay, and its DTT consumption rate was 15.5 ± 0.5 pmol min^-1 μg^-1. The SOA present comparable light scattering to BB-POA, but have lower light absorption with imaginary refractive index less than 0.01 within the wavelength range of 360-600 nm. Calculations based on Mie theory show that pure airborne SOA with atmospherically relevant sizes of 50-400 nm have a cooling effect; when acting as the coating materials, these SOA can counteract the warming effect brought by airborne black carbon aerosol.

Secondary Organic Aerosol Formation of Fleet Vehicle Emissions in China: Potential Seasonality of Spatial Distributions

Vehicle emissions are an important source of urban particular matter. To investigate the secondary organic aerosol (SOA) formation potential of real-world vehicle emissions, we exposed on-road air in Beijing to hydroxyl radicals generated in an oxidation flow reactor (OFR) under high-NO_x conditions on-board a mobile laboratory and characterized SOA and their precursors with a suite of state-of-the-art instrumentation. The OFR produced 10-170 μg m^-3 of SOA with a maximum SOA formation potential of 39-50 μg m^-3 ppmv^-1 CO that occurred following an integrated OH exposure of (1.3-2.0) × 10¹¹ molecules cm^-3 s. The results indicate relatively shorter photochemical ages for maximum SOA production than previous OFR results obtained under low-NO_x conditions. Such timescales represent the balance of functionalization and fragmentation, possibly resulting in different spatial distributions of SOA in different seasons as the oxidant level changes. The detected precursors may explain as much as 13% of the observed SOA with the remaining plausibly contributed by the oxidation of undetected intermediate-volatility organic compounds. Extrapolation of the results suggests an annual SOA production rate of 0.78 Tg yr^-1 from mobile gasoline sources in China, highlighting the importance of effective regulation of gaseous vehicular precursors to improve air quality in the future.

Secondary Organic Aerosol Formation Potential from Ambient Air in Beijing: Effects of Atmospheric Oxidation Capacity at Different Pollution Levels

Secondary organic aerosol (SOA) plays a critical role in sustained haze pollution in megacities. Traditional observation of atmospheric aerosols usually analyzes the ambient organic aerosol (OA) but neglects the SOA formation potential (SOAFP) of precursors remaining in ambient air. Knowledge on SOAFP is still limited, especially in megacities suffering from frequent haze. In this study, the SOAFP of ambient air in urban Beijing was characterized at different pollution levels based on a two-year field observation using an oxidation flow reactor (OFR) system. Both OA and SOAFP increased as a function of ambient pollution level, in which increasing concentrations of precursor volatile organic compounds (VOCs) and decreasing atmospheric oxidation capacity were found to be the two main influencing factors. To address the role of the atmospheric oxidation capacity in SOAFP, a relative OA enhancement ratio (ER_OA = 1 + SOAFP/OA) and the elemental composition of the OA were investigated in this study. The results indicated that the atmospheric oxidation capacity was weakened and resulted in higher SOAFP on more polluted days. The relationship found between SOAFP and the atmospheric oxidation capacity could be helpful in understanding changes in SOA pollution with improving air quality in the megacities of developing countries.

Seasonal variation of dicarboxylic acids in PM2.5 in Beijing: Implications for the formation and aging processes of secondary organic aerosols

Dicarboxylic acids are a group of highly oxidized components, which can provide insights into the formation mechanism and aging process of secondary organic aerosols (SOA). Based on the 12-h day and night PM_2.5 samples collected in downtown Beijing in January, April, July and October of 2017, dicarboxylic acids and relevant components were measured to investigate their seasonal variation pattern and sources. High concentrations of the identified organic acids were observed, following the decreasing order of July > January > October > April. The high fractions of phthalic acid and maleic acid in January indicated severe aromatic SOA pollution during the sampling period in winter, and the high malonic acid to succinic acid and malic acid to succinic acid ratios in July suggested strong photochemical formation over the sampling period in summer. Based on the calculation of principle component analysis and multiple linear regression, water-soluble organic acids were mainly formed from the aerosol aging process during the sampling periods except for January, while water-soluble organic carbon (WSOC) mostly originated from combustion sources. Correlation analysis was conducted between the CO-normalized concentrations of organic acids and PM_2.5, O₃, as well as the meteorological parameters. The results suggested that gas-phase photooxidation contributed significantly to the formation of these organic acids during the entire sampling period, and the aqueous-phase process played an important role over the severe haze event in January. Our results also suggested that the intensity of photooxidation and the aging degree of SOA were enhanced along with the reduction of PM_2.5 in Beijing in recent years.

Particle growth with photochemical age from new particle formation to haze in the winter of Beijing, China

Secondary aerosol formation in the aging process of primary emission is the main reason for haze pollution in eastern China. Pollution evolution with photochemical age was studied for the first time at a comprehensive field observation station during winter in Beijing. The photochemical age was used as an estimate of the timescale attributed to the aging process and was estimated from the ratio of toluene to benzene in this study. A low photochemical age indicates a fresh emission. The photochemical age of air masses during new particle formation (NPF) days was lower than that on haze days. In general, the strongest NPF events, along with a peak of the formation rate of 1.5 nm (J_1.5) and 3 nm particles (J₃), were observed when the photochemical age was between 12 and 24 h while rarely took place with photochemical ages less than 12 h. When photochemical age was larger than 48 h, haze occurred and NPF was suppressed. The sources and sinks of nanoparticles had distinct relation with the photochemical age. Our results show that the condensation sink (CS) showed a valley with photochemical ages ranging from 12 to 24 h, while H₂SO₄ concentration showed no obvious trend with the photochemical age. The high concentrations of precursor vapours within an air mass lead to persistent nucleation with photochemical age ranging from 12 to 48 h in winter. Coincidently, the fast increase of PM_2.5 mass was also observed during this range of photochemical age. Noteworthy, CS increased with the photochemical age on NPF days only, which is the likely reason for the observation that the PM_2.5 mass increased faster with photochemical age on NPF days compared with other days. The evolution of particles with the photochemical age provides new insights into understanding how particles originating from NPF transform to haze pollution.

Atmospheric photochemistry and secondary aerosol formation of urban air in Lyon, France

Photochemical aging of volatile organic compounds (VOCs) in the atmosphere is an important source of secondary organic aerosol (SOA). To evaluate the formation potential of SOA at an urban site in Lyon (France), an outdoor experiment using a Potential Aerosol Mass (PAM) oxidation flow reactor (OFR) was conducted throughout entire days during January-February 2017. Diurnal variation of SOA formations and their correlation with OH radical exposure (OHexp), ambient pollutants (VOCs and particulate matters, PM), Relative Humidity (RH), and temperature were explored in this study. Ambient urban air was exposed to high concentration of OH radicals with OHexp in range of (0.2-1.2)×10¹² molecule/(cm³•sec), corresponding to several days to weeks of equivalent atmospheric photochemical aging. The results informed that urban air at Lyon has high potency to contribute to SOA, and these SOA productions were favored from OH radical photochemical oxidation rather than via ozonolysis. Maximum SOA formation (36 µg/m³) was obtained at OHexp of about 7.4 × 10¹¹molecule/(cm³•sec), equivalent to approximately 5 days of atmospheric oxidation. The correlation between SOA formation and ambient environment conditions (RH & temperature, VOCs and PM) was observed. It was the first time to estimate SOA formation potential from ambient air over a long period in urban environment of Lyon.

Oxidation Potential Reduction of Carbon Nanomaterials during Atmospheric-Relevant Aging: Role of Surface Coating

Carbon nanomaterials from various sources are the important component of PM_2.5 and have many adverse effects on human health. They are prone to interact with other pollutants and subsequently age, defined here as changes in chemical properties. In this work, we investigated the aging process of various carbon nanoparticle samples such as Special Black 4A, Printex U, single-walled carbon nanotubes, and hexane flame soot by ambient air and studied the evolution of their oxidation potential. We found that coatings of inorganic and organic species dominated the aging process of carbonaceous particles by ambient air. The amounts of disordered carbon and C-H functional groups of aged carbonaceous particles decreased during the aging process; meanwhile, the contents of sulfate and nitrate showed significant increases. In addition, the oxidation potential measured by the dithiothreitol assay remarkably declined as a function of aging time with ambient air evidently because of heterogeneous reactions between SO₂ and NO₂, as well as the coating with organic vapors. This work is important for understanding the oxidation potential changes of carbonaceous particles during atmospheric transport.

Probing the Reactivity of Singlet Oxygen with Cyclic Monoterpenes

Monoterpenes represent a class of hydrocarbons consisting of two isoprene units. Like many other terpenes, monoterpenes emerge mainly from vegetation, indicating their significance in both atmospheric chemistry and pharmaceutical and food industries. The atmospheric recycling of monoterpenes constitutes a major source of secondary organic aerosols. Therefore, this contribution focuses on the mechanism and kinetics of atmospheric oxidation of five dominant monoterpenes (i.e., limonene, α-pinene, β-pinene, sabinene, and camphene) by singlet oxygen. The reactions are initiated via the ene-type addition of singlet oxygen (O2 1Δg) to the electron-rich double bond, progressing favorably through the concerted reaction mechanisms. The physical analyses of the frontier molecular orbitals agree well with the thermodynamic properties of the selected reagents, and the computed reaction rate parameters. The reactivity of monoterpenes with O2 1Δg follows the order of α-pinene > sabinene > limonene > β-pinene > camphene, i.e., α-pinene and camphene retain the highest and lowest reactivity toward singlet oxygen, with rate expressions of k(T) (M-1 s-1) = 1.13 × 108 exp(-48(kJ)/RT(K)) and 6.93 × 108 exp(-139(kJ)/RT(K)), respectively. The effect of solvent on the primary reaction pathways triggers a slight reduction in energy, ranging between 12 and 34 kJ/mol.

Impacts of SO2, Relative Humidity, and Seed Acidity on Secondary Organic Aerosol Formation in the Ozonolysis of Butyl Vinyl Ether

Alkyl vinyl ethers are widely used as fuel additives. Despite this, their atmospheric chemistry and secondary organic aerosol (SOA) formation potentials are still not well-known under complex pollution conditions. In this work, we examined the impact of SO₂, relative humidity (RH), and particle acidity on the formation and oxidation state (OSc) of SOA from butyl vinyl ether (BVE) ozonolysis. Increasing SO₂ concentration produced a notable promotion of SOA formation and OSc due to the significant increase in H₂SO₄ particles and formation of more highly oxidized components. Increased RH in the presence of SO₂ appeared to promote, suppress, and dominate the formation and OSc of SOA in the dry range (1-10%), low RH range (10-42%), and moderate RH range (42-64%), respectively. This highlights the importance of competition between H₂O and SO₂ in reacting with the stabilized Criegee intermediate in BVE ozonolysis at ambient RH. Increased particle acidity mainly contributed to the change in chemical composition of BVE-dominated SOA but not to SOA formation. The results presented here extend previous analysis of BVE-derived SOA and further aid our understanding of SOA formation potential of BVE ozonolysis under highly complex pollution conditions.

Photochemical aging and secondary organic aerosols generated from limonene in an oxidation flow reactor

Oxidation flow reactors (OFRs) are increasingly used to study the formation and evolution of secondary organic aerosols (SOA) in the atmosphere. The OH/HO₂ and OH/O₃ ratios in OFRs are similar to tropospheric ratios. In the present work, we investigated the production of SOA generated by OH oxydation and ozonolysis of limonene in OFR as a function of OH exposure and O₃ exposure. The results are compared with those obtained from the simulation chambers. The precursor gas is exposed to OH concentrations ranging from 2.11 × 10⁸ to 1.91 × 10⁹ molec cm^-3, with an estimated exposure time in the OFR of 137 s. In the environmental chambers, the precursor was oxidized using OH concentrations between 2.10 × 10⁶ and 2.12 × 10⁷ molec cm^-3 over exposure times of several hours. In the overlapping OH exposure region, the highest SOA yields are obtained in the OFR, which is explained by the ozonolysis of limonene in the OFR. However, the yields decrease with the increase of OHexp in both systems.

Differences of the oxidation process and secondary organic aerosol formation at low and high precursor concentrations

Current atmospheric quality models usually underestimate the level of ambient secondary organic aerosol (SOA), one of the possible reasons is that the precursors at different concentrations may undergo different oxidation processes and further affect SOA formation. Therefore, there is a need to perform more chamber studies to disclose the influence. In this work, SOA formation over a wide range of initial precursor concentrations (tens of ppb to hundreds of ppb levels) was investigated in a 30 m³ indoor smog chamber, and mainly through the analysis of multiple generations of VOCs detected from HR-ToF-PTRMS to expound the difference in the oxidation process between low and high precursor concentrations. Compared to high initial concentrations, gas-phase intermediates formed at low concentrations had a higher intensity by about one order of magnitude, and the low-volatility compounds also had a higher formation potential due to the competition between semi-volatile intermediates and precursors with oxidants. In addition, the formed SOA was more oxidized with higher f₄₄ value (0.14 ± 0.02) and more relevant to real atmosphere than that formed at high concentrations. This work should help to deeply understand SOA formation and improve the performance of air quality models for SOA simulation.

Secondary Organic Aerosol Formation from Urban Roadside Air in Hong Kong

Motor vehicle emissions are an important but poorly constrained source of secondary organic aerosol (SOA). Here, we investigated in situ SOA formation from urban roadside air in Hong Kong during winter time using an oxidation flow reactor (OFR), with equivalent atmospheric oxidation ranging from several hours to several days. The campaign-average mass enhancement of OA, nitrate, sulfate, and ammonium upon OFR aging was 7.0, 7.2, 0.8, and 2.6 μg m^-3, respectively. To investigate the sources of SOA formation potential, we performed multilinear regression analysis between measured peak SOA concentrations from OFR and the concentrations of toluene that represent motor vehicle emissions and cooking OA from positive matrix factorization (PMF) analysis of ambient OA. Traffic-related SOA precursors contributed 92.3%, 92.4%, and 83.1% to the total SOA formation potential during morning rush hours, noon and early afternoon, and evening meal time, respectively. The SOA production factor (PF) was approximately 5.2 times of primary OA (POA) emission factor (EF) and the secondary particulate matter (PM) PF was approximately 2.6 times of primary particles EF. This study highlights the potential benefit of reducing secondary PM production from motor vehicle emissions in mitigating PM pollutions.

Emission characteristics of 99 NMVOCs in different seasonal days and the relationship with air quality parameters in Beijing, China

In recent years, the haze incidents have occurred frequently in China. Therefore, more attention should be taken in comprehensively determination and analysis of the extended-ranges of volatile organic compounds (VOCs). Here, up to 99 non-methane VOCs (NMVOCs), including not frequently reported partial halocarbons and oxygenated VOC (OVOC) species, were monitored in atmosphere of Beijing. The mean concentration of total NMVOC (TNMVOC) decreased in the order of winter polluted days (216.05 μg m^-3) > summer polluted days (127.01 μg m^-3) > summer normal days (95.63 μg m^-3) > winter normal days (50.25 μg m^-3). The ethane to n-butane, ethylene to 1-butene, BTEX, acetaldehyde, acetone, n-hexanal, dichloromethane, chloroform and 1,2-dichloroethane, were determined to be the main composition in their respective alkane, alkene, aromatic, OVOC and halocarbon classes. The minor propylbenzene, diethylbenzene, ethyltoluene, and trimethylbenzene isomer ratios were within the narrow range of 1.3-3.21. Generally, the most abundant NMVOCs were alkanes in winter but OVOCs in summer. TNMVOC significantly positively correlated with PM₁₀, PM_2.5, CO, RH, SO₂ (winter), NO₂ (winter), but negatively with windspeed, SSD and PRS (winter). The opposite correlation was observed between TNMVOC and O₃ in winter and summer. There was no meaningful correlation between TNMVOC and T, PRS (summer), SO₂ (summer) and NO₂ (summer). 3D surface graphs, built by MATLAB, were drawn to investigate the relationship between PM_2.5 and NMVOC taking air quality parameters into account. The PM_2.5 concentration increased non-linearly as TNMVOC concentration increased, with various surface graphs. Unlike other air quality parameters, O₃ affected the relationship differently between winter and summer. The findings presented herein may provide a new train of thought for occurrence of haze.