Photo-oxidation of Aromatic Hydrocarbons Produces Low-Volatility Organic Compounds

Real-Time Identification of Aerosol-Phase Carboxylic Acid Production Using Extractive Electrospray Ionization Mass Spectrometry

Comprehensive identification of aerosol sources and their constituent organic compounds requires aerosol-phase molecular-level characterization with a high time resolution. While real-time chemical characterization of aerosols is becoming increasingly common, information about functionalization and structure is typically obtained from offline methods. This study presents a method for determining the presence of carboxylic acid functional groups in real time using extractive electrospray ionization mass spectrometry based on measurements of [M - H + 2Na]+ adducts. The method is validated and characterized using standard compounds. A proof-of-concept application to α-pinene secondary organic aerosol (SOA) shows the ability to identify carboxylic acids even in complex mixtures. The real-time capability of the method allows for the observation of the production of carboxylic acids, likely formed in the particle phase on short time scales (<120 min). Our research explains previous findings of carboxylic acids being a significant component of SOA and a quick decrease in peroxide functionalization following SOA formation. We show that the formation of these acids is commensurate with the increase of dimers in the particle phase. Our results imply that SOA is in constant evolution through condensed-phase processes, which lower the volatility of the aerosol components and increase the available condensed mass for SOA growth and, therefore, aerosol mass loading in the atmosphere. Further work could aim to quantify the effect of particle-phase acid formation on the aerosol volatility distributions.

Fostering a Holistic Understanding of the Full Volatility Spectrum of Organic Compounds from Benzene Series Precursors through Mechanistic Modeling

A comprehensive understanding of the full volatility spectrum of organic oxidation products from the benzene series precursors is important to quantify the air quality and climate effects of secondary organic aerosol (SOA) and new particle formation (NPF). However, current models fail to capture the full volatility spectrum due to the absence of important reaction pathways. Here, we develop a novel unified model framework, the integrated two-dimensional volatility basis set (I2D-VBS), to simulate the full volatility spectrum of products from benzene series precursors by simultaneously representing first-generational oxidation, multigenerational aging, autoxidation, dimerization, nitrate formation, etc. The model successfully reproduces the volatility and O/C distributions of oxygenated organic molecules (OOMs) as well as the concentrations and the O/C of SOA over wide-ranging experimental conditions. In typical urban environments, autoxidation and multigenerational oxidation are the two main pathways for the formation of OOMs and SOA with similar contributions, but autoxidation contributes more to low-volatility products. NOx can reduce about two-thirds of OOMs and SOA, and most of the extremely low-volatility products compared to clean conditions, by suppressing dimerization and autoxidation. The I2D-VBS facilitates a holistic understanding of full volatility product formation, which helps fill the large gap in the predictions of organic NPF, particle growth, and SOA formation.

Molecular-Level Insights into the Relationship between Volatility of Organic Aerosol Constituents and PM2.5 Air Pollution Levels: A Study with Ultrahigh-Resolution Mass Spectrometry

Volatility of organic aerosols (OAs) significantly influences new particle formation and the occurrence of particulate air pollution. However, the relationship between the volatility of OA and the level of particulate air pollution (i.e., particulate matter concentration) is not well understood. In this study, we compared the chemical composition (identified by an ultrahigh-resolution Orbitrap mass spectrometer) and volatility (estimated based on a predeveloped parametrization method) of OAs in urban PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 μm) samples from seven German and Chinese cities, where the PM2.5 concentration ranged from a light (14 μg m-3) to heavy (319 μg m-3) pollution level. A large fraction (71-98%) of compounds in PM2.5 samples were attributable to intermediate-volatility organic compounds (IVOCs) and semivolatile organic compounds (SVOCs). The fraction of low-volatility organic compounds (LVOCs) and extremely low-volatility organic compounds (ELVOCs) decreased from clean (28%) to heavily polluted urban regions (2%), while that of IVOCs increased from 34 to 62%. We found that the average peak area-weighted volatility of organic compounds in different cities showed a logarithmic correlation with the average PM2.5 concentration, indicating that the volatility of urban OAs increases with the increase of air pollution level. Our results provide new insights into the relationship between OA volatility and PM pollution levels and deepen the understanding of urban air pollutant evolution.

Correlation gas chromatography and two-dimensional volatility basis methods to predict gas-particle partitioning for e-cigarette aerosols

E-cigarette aerosols contain a complex mixture of harmful and potentially harmful chemicals. Once released into the environment, they evolve and become new sources of indoor air pollutants that could pose a significant threat to both users and non-users. However, current understanding of the physicochemical properties of e-cigarette aerosol constituents that govern gas-particle partitioning in the atmosphere is limited, making it difficult to estimate the health risks associated with exposure. Here, we used correlation gas chromatography (C-GC) and two-dimensional volatility basis set (2D-VBS) methods to determine the vapor pressures and volatility for commonly reported toxic and irritating e-cigarette aerosol constituents. The vapor pressures of target compounds at 298 K were estimated from the Antoine-type linear relationship between the vapor pressure of reference standards and their retention times. Our C-GC results showed an overall positive correlation (R = 0.84) with estimates using the EPI (Estimation Programs Interface) Suite. The volatility calculated by 2D-VBS correlates well with the calculated vapor pressure from both C-GC (R = 0.82) and EPI Suite (R = 0.85). The volatility distribution also indicated fresh e-cigarette aerosol constituents are mainly more volatile organic compounds. Our case study revealed that low-vapor-pressure compounds (e.g., σ-dodecalactone, γ-decalactone, and maltol) become enriched in the e-cigarette aerosols within 2 hours following vaping emissions. Overall, these findings demonstrate the applicability of the C-GC and 2D-VBS methods for determining the physiochemical properties of e-cigarette aerosol constituents, which can aid in assessing the dynamic chemical composition of e-cigarette aerosols and exposures to vaping emissions in indoor environments.

Nitrate Radicals Suppress Biogenic New Particle Formation from Monoterpene Oxidation

Highly oxygenated organic molecules (HOMs) are a major source of new particles that affect the Earth's climate. HOM production from the oxidation of volatile organic compounds (VOCs) occurs during both the day and night and can lead to new particle formation (NPF). However, NPF involving organic vapors has been reported much more often during the daytime than during nighttime. Here, we show that the nitrate radicals (NO3), which arise predominantly at night, inhibit NPF during the oxidation of monoterpenes based on three lines of observational evidence: NPF experiments in the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber at CERN (European Organization for Nuclear Research), radical chemistry experiments using an oxidation flow reactor, and field observations in a wetland that occasionally exhibits nocturnal NPF. Nitrooxy-peroxy radicals formed from NO3 chemistry suppress the production of ultralow-volatility organic compounds (ULVOCs) responsible for biogenic NPF, which are covalently bound peroxy radical (RO2) dimer association products. The ULVOC yield of α-pinene in the presence of NO3 is one-fifth of that resulting from ozone chemistry alone. Even trace amounts of NO3 radicals, at sub-parts per trillion level, suppress the NPF rate by a factor of 4. Ambient observations further confirm that when NO3 chemistry is involved, monoterpene NPF is completely turned off. Our results explain the frequent absence of nocturnal biogenic NPF in monoterpene (α-pinene)-rich environments.

Modeling the Formation of Organic Compounds across Full Volatility Ranges and Their Contribution to Nanoparticle Growth in a Polluted Atmosphere

Nanoparticle growth influences atmospheric particles' climatic effects, and it is largely driven by low-volatility organic vapors. However, the magnitude and mechanism of organics' contribution to nanoparticle growth in polluted environments remain unclear because current observations and models cannot capture organics across full volatility ranges or track their formation chemistry. Here, we develop a mechanistic model that characterizes the full volatility spectrum of organic vapors and their contributions to nanoparticle growth by coupling advanced organic oxidation modeling and kinetic gas-particle partitioning. The model is applied to Nanjing, a typical polluted city, and it effectively captures the volatility distribution of low-volatility organics (with saturation vapor concentrations <0.3 μg/m3), thus accurately reproducing growth rates (GRs), with a 4.91% normalized mean bias. Simulations indicate that as particles grow from 4 to 40 nm, the relative fractions of GRs attributable to organics increase from 59 to 86%, with the remaining contribution from H2SO4 and its clusters. Aromatics contribute much to condensable organic vapors (∼37%), especially low-volatility vapors (∼61%), thus contributing the most to GRs (32-46%) as 4-40 nm particles grow. Alkanes also contribute 19-35% of GRs, while biogenic volatile organic compounds contribute minimally (<13%). Our model helps assess the climatic impacts of particles and predict future changes.

Insights into the Peroxide-Bicyclic Intermediate Pathway of Aromatic Photooxidation: Experimental Yields and NOx-Dependency of Ring-Opening and Ring-Retaining Products

Aromatic hydrocarbons are important contributors to the formation of ozone and secondary organic aerosols in urban environments. The different parallel pathways in aromatic oxidation, however, remain inadequately understood. Here, we investigated the production yields and chemical distributions of gas-phase tracer products during the photooxidation of alkylbenzenes at atmospheric OH levels with NOx present using high-resolution mass spectrometers. The peroxide-bicyclic intermediate pathway emerged as the major pathway in aromatic oxidation, accounting for 52.1 ± 12.6%, 66.1 ± 16.6%, and 81.4 ± 24.3% of the total OH oxidation of toluene, m-xylene, and 1,3,5-trimethylbenzene, respectively. Notably, the yields of bicyclic nitrates produced from the reactions of bicyclic peroxy radicals (BPRs) with NO were considerably lower (3-5 times) than what the current mechanism predicted. Alongside traditional ring-opening products formed through the bicyclic pathway (dicarbonyls and furanones), we identified a significant proportion of carbonyl olefinic acids generated via the 1,5-aldehydic H-shift occurring in subsequent reactions of BPRs + NO, contributing 4-7% of the carbon flow in aromatic oxidation. Moreover, the observed NOx-dependencies of ring-opening and ring-retaining product yields provide insights into the competitive nature of reactions involving BPRs with NO, HO2, and RO2, which determine the refined product distributions and offer an explanation for the discrepancies between the experimental and model-based results.

An overlooked oxidation mechanism of toluene: computational predictions and experimental validations

Secondary organic aerosols (SOAs) influence the Earth's climate and threaten human health. Aromatic hydrocarbons (AHs) are major precursors for SOA formation in the urban atmosphere. However, the revealed oxidation mechanism dramatically underestimates the contribution of AHs to SOA formation, strongly suggesting the importance of seeking additional oxidation pathways for SOA formation. Using toluene, the most abundant AHs, as a model system and the combination of quantum chemical method and field observations based on advanced mass spectrometry, we herein demonstrate that the second-generation oxidation of AHs can form novel epoxides (TEPOX) with high yield. Such TEPOX can further react with H2SO4 or HNO3 in the aerosol phase to form less-volatile compounds including novel non-aromatic and ring-retaining organosulfates or organonitrates through reactive uptakes, providing new candidates of AH-derived organosulfates or organonitrates for future ambient observation. With the newly revealed mechanism, the chemistry-aerosol box modeling revealed that the SOA yield of toluene oxidation can reach up to 0.35, much higher than 0.088 based on the original mechanism under the conditions of pH = 2 and 0.1 ppbv NO. This study opens a route for the formation of reactive uptake SOA precursors from AHs and significantly fills the current knowledge gap for SOA formation in the urban atmosphere.

Aqueous-Phase Reactions of Anthropogenic Emissions Lead to the High Chemodiversity of Atmospheric Nitrogen-Containing Compounds during the Haze Event

Nitrogen-containing organic compounds (NOCs), a type of important reactive-nitrogen species, are abundant in organic aerosols in haze events observed in Northern China. However, due to the complex nature of NOCs, the sources, formation, and influencing factors are still ambiguous. Here, the molecular composition of organic matters (OMs) in hourly PM2.5 samples collected during a haze event in Northern China was characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We found that CHON compounds (formulas containing C, H, O, and N atoms) dominated the OM fractions during the haze and showed high chemodiversity and transformability. Relying on the newly developed revised-workflow and oxidation-hydrolyzation knowledge for CHON compounds, 64% of the major aromatic CHON compounds (>80%) could be derived from the oxidization or hydrolyzation processes. Results from FT-ICR MS data analysis further showed that the aerosol liquid water (ALW)-involved aqueous-phase reactions are important for the molecular distribution of aromatic-CHON compounds besides the coal combustion, and the ALW-involved aromatic-CHON compound formation during daytime and nighttime was different. Our results improve the understanding of molecular composition, sources, and potential formation of CHON compounds, which can help to advance the understanding for the formation, evolution, and control of haze.

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.

Role of sesquiterpenes in biogenic new particle formation

Biogenic vapors form new particles in the atmosphere, affecting global climate. The contributions of monoterpenes and isoprene to new particle formation (NPF) have been extensively studied. However, sesquiterpenes have received little attention despite a potentially important role due to their high molecular weight. Via chamber experiments performed under atmospheric conditions, we report biogenic NPF resulting from the oxidation of pure mixtures of β-caryophyllene, α-pinene, and isoprene, which produces oxygenated compounds over a wide range of volatilities. We find that a class of vapors termed ultralow-volatility organic compounds (ULVOCs) are highly efficient nucleators and quantitatively determine NPF efficiency. When compared with a mixture of isoprene and monoterpene alone, adding only 2% sesquiterpene increases the ULVOC yield and doubles the formation rate. Thus, sesquiterpene emissions need to be included in assessments of global aerosol concentrations in pristine climates where biogenic NPF is expected to be a major source of cloud condensation nuclei.

Volatility Parametrization of Low-Volatile Components of Ambient Organic Aerosols Based on Molecular Formulas

Evaluating the volatility of organic compounds based solely on their molecular formulas would avoid tough demands in deriving molecular structures. Here, we deployed an iodide-adduct Long Time-of-Flight Chemical Ionization Mass Spectrometry (LToF-CIMS) combined with a Filter Inlet for Gases and AEROsols (FIGAERO) to investigate molecular formulas and thermograms of organic compounds on ambient particulate samples collected in the summer of 2021 in a suburban site of Shanghai and to estimate saturation vapor pressures of low- and semivolatile components of ambient organic aerosols. Then, a hierarchical cluster analysis and a subsequent classification of obtained clusters by similarity calculation were applied to the measured data set of molecular formulas and saturation vapor pressures of organic aerosols with at least a 2/3 appearance frequency, together with a similar data set collected at a rural site in the Beijing-Tianjin-Hebei region during the winter of 2018 (Ren et al., 2018), to classify all compounds into multiple groups. For each group of compounds, parametrizations between volatility and elemental composition were derived, and then relationships between each group of parameters and the mean O:C were established to achieve a volatility-molecular formula parametrization with the O:C as a key input. Statistical comparison of estimated volatilities of low-volatile organic compounds shows a much better performance of our parametrization than previous molecular formula-based ones.

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.

Organic Peroxides in Aerosol: Key Reactive Intermediates for Multiphase Processes in the Atmosphere

Organic peroxides (POs) are organic molecules with one or more peroxide (-O-O-) functional groups. POs are commonly regarded as chemically labile termination products from gas-phase radical chemistry and therefore serve as temporary reservoirs for oxidative radicals (HOx and ROx) in the atmosphere. Owing to their ubiquity, active gas-particle partitioning behavior, and reactivity, POs are key reactive intermediates in atmospheric multiphase processes determining the life cycle (formation, growth, and aging), climate, and health impacts of aerosol. However, there remain substantial gaps in the origin, molecular diversity, and fate of POs due to their complex nature and dynamic behavior. Here, we summarize the current understanding on atmospheric POs, with a focus on their identification and quantification, state-of-the-art analytical developments, molecular-level formation mechanisms, multiphase chemical transformation pathways, as well as environmental and health impacts. We find that interactions with SO2 and transition metal ions are generally the fast PO transformation pathways in atmospheric liquid water, with lifetimes estimated to be minutes to hours, while hydrolysis is particularly important for α-substituted hydroperoxides. Meanwhile, photolysis and thermolysis are likely minor sinks for POs. These multiphase PO transformation pathways are distinctly different from their gas-phase fates, such as photolysis and reaction with OH radicals, which highlights the need to understand the multiphase partitioning of POs. By summarizing the current advances and remaining challenges for the investigation of POs, we propose future research priorities regarding their origin, fate, and impacts in the atmosphere.

Enigma of Urban Gaseous Oxygenated Organic Molecules: Precursor Type, Role of NOx, and Degree of Oxygenation

Oxidation of volatile organic compounds (VOCs) forms oxygenated organic molecules (OOMs), which contribute to secondary pollution. Herein, we present measurement results of OOMs using chemical ionization mass spectrometry with nitrate as the reagent ion in Shanghai. Compared to those in forests and laboratory studies, OOMs detected at this urban site were of relatively lower degree of oxygenation. This was attributed to the high NO_x concentrations (∼44 ppb), which overall showed a suppression on the propagation reactions. As another result, a large fraction of nitrogenous OOMs (75%) was observed, and this fraction further increased to 84% under a high NO/VOC ratio. By applying a novel framework on OOM categorization and supported by VOC measurements, 50 and 32% OOMs were attributed to aromatic and aliphatic precursors, respectively. Furthermore, aromatic OOMs are more oxygenated (effective oxygen number, nO_eff = 4-6) than aliphatic ones (nO_eff = 3-4), which can be partly explained by the difference in initiation mechanisms and points to possible discrimination in termination reactions. This study highlights the roles of NO_x in OOM formation in urban areas, as well as the formation of nitrogenous products that might show discrimination between aromatic and aliphatic VOCs.

A neglected pathway for the accretion products formation in the atmosphere

Highly oxygenated organic molecules (HOM) formed by the autoxidation of α-pinene initiated by OH radicals play an important role in new particle formation. It is believed that the accretion products, ROOR´, formed by the self- and cross-reaction of peroxy radicals (RO₂ + R'O₂ reactions), have extremely low volatility and are more likely to participate in nucleation. However, the mechanism of ROOR´ formation has not been fully demonstrated by experiment or theoretical calculation. Herein, we propose a novel mechanism of RO₂ reacting with α-pinene (RO₂ + α-pinene reactions) that have much lower potential barriers and larger rate constants than the reaction of RO₂ with R'O₂, which explains the ROOR´ formation found in the mass spectrometry experiments. The ROOR´ resulting from the reaction of RO₂ with α-pinene can produce HOM dimers and trimers with a higher oxygen-to‑carbon (O/C) ratio through a autoxidation chain. We also demonstrated that the presence of NO_x and HO₂ radical will reduce the RO₂ concentration, but cannot completely inhibit the formation of HOM monomers and ROOR´. Even if one or both of RO₂ radicals are acyl peroxy radicals (RC(O)O₂), the potential barriers of the reactions between RC(O)O₂ and α-pinene (RC(O)O₂ + α-pinene reactions) are lower than that of RO₂ reacting with RC(O)O₂ (RO₂ + RC(O)O₂ reactions) or RC(O)O₂ self-reactions (RC(O)O₂ + RC(O)O₂ reactions). The current work revealed, for the first time, a mechanism of RO₂/RC(O)O₂ reacting with α-pinene in the atmosphere, which provides new insight into the atmospheric chemistry of accretion products as SOA precursors.

Smog Chamber Study on the Role of NOx in SOA and O3 Formation from Aromatic Hydrocarbons

China is facing dual pressures to reduce both PM_2.5 and O₃ pollution, the crucial precursors of which are NO_x and VOCs. In our study, the role of NO_x in both secondary organic aerosol (SOA, the important constituent of PM_2.5) and O₃ formation was examined in our 30 m³ indoor smog chamber. As revealed in the present study, the NO_x level can obviously affect the OH concentration and volatility distribution of gas-phase oxidation products and thus O₃ and SOA formation. Reducing the NO_x concentration to the NO_x-sensitive regime can inhibit O₃ formation (by 42%), resulting in the reduction of oxidation capacity, which suppresses the SOA formation (by 45%) by inhibiting the formation of O- and N-containing gas-phase oxidation products with low volatility. The contribution of these oxidation products to the formation of SOA was also estimated, and the results could substantially support the trend of SOA yield with NO_x at different VOC levels. The atmospheric implications of NO_x in the coordinated control of PM_2.5 and O₃ are also discussed.

Insufficient Condensable Organic Vapors Lead to Slow Growth of New Particles in an Urban Environment

Atmospheric new particle formation significantly affects global climate and air quality after newly formed particles grow above ∼50 nm. In polluted urban atmospheres with 1-3 orders of magnitude higher new particle formation rates than those in clean atmospheres, particle growth rates are comparable or even lower for reasons that were previously unclear. Here, we address the slow growth in urban Beijing with advanced measurements of the size-resolved molecular composition of nanoparticles using the thermal desorption chemical ionization mass spectrometer and the gas precursors using the nitrate CI-APi-ToF. A particle growth model combining condensational growth and particle-phase acid-base chemistry was developed to explore the growth mechanisms. The composition of 8-40 nm particles during new particle formation events in urban Beijing is dominated by organics (∼80%) and sulfate (∼13%), and the remainder is from base compounds, nitrate, and chloride. With the increase in particle sizes, the fraction of sulfate decreases, while that of the slow-desorbed organics, organic acids, and nitrate increases. The simulated size-resolved composition and growth rates are consistent with the measured results in most cases, and they both indicate that the condensational growth of organic vapors and H₂SO₄ is the major growth pathway and the particle-phase acid-base reactions play a minor role. In comparison to the high concentrations of gaseous sulfuric acid and amines that cause high formation rates, the concentration of condensable organic vapors is comparably lower under the high NO_x levels, while those of the relatively high-volatility nitrogen-containing oxidation products are higher. The insufficient condensable organic vapors lead to slow growth, which further causes low survival of the newly formed particles in urban environments. Thus, the low growth rates, to some extent, counteract the impact of the high formation rates on air quality and global climate in urban environments.

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.

Limited Secondary Organic Aerosol Production from Acyclic Oxygenated Volatile Chemical Products

Volatile chemical products (VCPs) have recently been identified as potentially important unconventional sources of secondary organic aerosol (SOA), in part due to the mitigation of conventional emissions such as vehicle exhaust. Here, we report measurements of SOA production in an oxidation flow reactor from a series of common VCPs containing oxygenated functional groups and at least one oxygen within the molecular backbone. These include two oxygenated aromatic species (phenoxyethanol and 1-phenoxy-2-propanol), two esters (butyl butyrate and butyl acetate), and four glycol ethers (carbitol, methyl carbitol, butyl carbitol, and hexyl carbitol). We measured gas- and particle-phase products with a suite of mass spectrometers and particle-sizing instruments. Only the aromatic VCPs produce SOA with substantial yields. For the acyclic VCPs, ether and ester functionality promotes fragmentation and hinders autoxidation, whereas aromatic rings drive SOA formation in spite of the presence of ether groups. Therefore, our results suggest that a potential strategy to reduce urban SOA from VCPs would be to reformulate consumer products to include less oxygenated aromatic compounds.

Secondary organic aerosol formation from monocyclic aromatic hydrocarbons: insights from laboratory studies

Monocyclic aromatic hydrocarbons (MAHs) are key anthropogenic pollutants and often dominate the volatile organic compound emissions and secondary organic aerosol (SOA) formation especially in the urban atmosphere. To evaluate the environmental impacts of SOA formed from the oxidation of MAHs (aromatic SOA), it is of great importance to elucidate their chemical composition, formation mechanism, and physicochemical properties under various atmospheric conditions. Here we seek to compile a common framework for the current studies on aromatic SOA formation and summarize the knowledge on what has been primarily learned from laboratory studies. This review begins with a brief summary of MAHs' emission characteristics, followed by an overview of atmospheric degradation mechanisms for MAHs as well as gas- and particle-phase reactions involving aromatic SOA formation. SOA formation processes highlighted in this review are complex and depend highly on environmental conditions, posing a substantial challenge for theoretical description of aromatic SOA formation. Therefore, the following issues are further discussed in detail: the response of gas-phase chemistry and aromatic SOA mass yield as well as composition to NO_x levels, particle-phase reactions and molecular characterization of aromatic SOA in the presence of acidic sulfate, and physicochemical processes of SOA formation involving gas- or particle-phase water. Building on this current understanding, available experimental studies on the effects of environmental conditions were explored. A brief description of the atmospheric importance of aromatic SOA including their optical properties and health influences is also presented. Finally, we highlight the current challenges in laboratory studies and outline directions for future aromatic SOA research.

Molecular Composition of Oxygenated Organic Molecules and Their Contributions to Organic Aerosol in Beijing

The understanding at a molecular level of ambient secondary organic aerosol (SOA) formation is hampered by poorly constrained formation mechanisms and insufficient analytical methods. Especially in developing countries, SOA related haze is a great concern due to its significant effects on climate and human health. We present simultaneous measurements of gas-phase volatile organic compounds (VOCs), oxygenated organic molecules (OOMs), and particle-phase SOA in Beijing. We show that condensation of the measured OOMs explains 26-39% of the organic aerosol mass growth, with the contribution of OOMs to SOA enhanced during severe haze episodes. Our novel results provide a quantitative molecular connection from anthropogenic emissions to condensable organic oxidation product vapors, their concentration in particle-phase SOA, and ultimately to haze formation.

Anthropogenic Volatile Organic Compound (AVOC) Autoxidation as a Source of Highly Oxygenated Organic Molecules (HOM)

Gas-phase hydrocarbon autoxidation is a rapid pathway for the production of in situ aerosol precursor compounds. It is a highway to molecular growth and lowering of vapor pressure, and it produces hydrogen-bonding functional groups that allow a molecule to bind into a substrate. It is the crucial process in the formation and growth of atmospheric secondary organic aerosol (SOA). Recently, the rapid gas-phase autoxidation of several volatile organic compounds (VOC) has been shown to yield highly oxygenated organic molecules (HOM). Most of the details on HOM formation have been obtained from biogenic monoterpenes and their surrogates, with cyclic structures and double bonds both found to strongly facilitate HOM formation, especially in ozonolysis reactions. Similar structural features in common aromatic compounds have been observed to facilitate high HOM formation yields, despite the lack of appreciable O3 reaction rates. Similarly, the recently observed autoxidation and subsequent HOM formation in the oxidation of saturated hydrocarbons cannot be initiated by O3 and require different mechanistic steps for initiating and propagating the autoxidation sequence. This Perspective reflects on these recent findings in the context of the direct aerosol precursor formation in urban atmospheres.

Contribution of Atmospheric Oxygenated Organic Compounds to Particle Growth in an Urban Environment

Gas-phase oxygenated organic molecules (OOMs) can contribute substantially to the growth of newly formed particles. However, the characteristics of OOMs and their contributions to particle growth rate are not well understood in urban areas, which have complex anthropogenic emissions and atmospheric conditions. We performed long-term measurement of gas-phase OOMs in urban Beijing during 2018-2019 using nitrate-based chemical ionization mass spectrometry. OOM concentrations showed clear seasonal variations, with the highest in the summer and the lowest in the winter. Correspondingly, calculated particle growth rates due to OOM condensation were highest in summer, followed by spring, autumn, and winter. One prominent feature of OOMs in this urban environment was a high fraction (∼75%) of nitrogen-containing OOMs. These nitrogen-containing OOMs contributed only 50-60% of the total growth rate led by OOM condensation, owing to their slightly higher volatility than non-nitrate OOMs. By comparing the calculated condensation growth rates and the observed particle growth rates, we showed that sulfuric acid and its clusters are the main contributors to the growth of sub-3 nm particles, with OOMs significantly promoting the growth of 3-25 nm particles. In wintertime Beijing, however, there are missing contributors to the growth of particles above 3 nm, which remain to be further investigated.

Molecular characterization of ultrafine particles using extractive electrospray time-of-flight mass spectrometry

Aerosol particles negatively affect human health while also having climatic relevance due to, for example, their ability to act as cloud condensation nuclei. Ultrafine particles (diameter D_p < 100 nm) typically comprise the largest fraction of the total number concentration, however, their chemical characterization is difficult because of their low mass. Using an extractive electrospray time-of-flight mass spectrometer (EESI-TOF), we characterize the molecular composition of freshly nucleated particles from naphthalene and β-caryophyllene oxidation products at the CLOUD chamber at CERN. We perform a detailed intercomparison of the organic aerosol chemical composition measured by the EESI-TOF and an iodide adduct chemical ionization mass spectrometer equipped with a filter inlet for gases and aerosols (FIGAERO-I-CIMS). We also use an aerosol growth model based on the condensation of organic vapors to show that the chemical composition measured by the EESI-TOF is consistent with the expected condensed oxidation products. This agreement could be further improved by constraining the EESI-TOF compound-specific sensitivity or considering condensed-phase processes. Our results show that the EESI-TOF can obtain the chemical composition of particles as small as 20 nm in diameter with mass loadings as low as hundreds of ng m⁻³ in real time. This was until now difficult to achieve, as other online instruments are often limited by size cutoffs, ionization/thermal fragmentation and/or semi-continuous sampling. Using real-time simultaneous gas- and particle-phase data, we discuss the condensation of naphthalene oxidation products on a molecular level.

Using real-time simultaneous gas- and particle-phase data, the condensation of naphthalene and β-caryophyllene oxidation products on a molecular level is discussed.

Acid-Base Clusters during Atmospheric New Particle Formation in Urban Beijing

Molecular clustering is the initial step of atmospheric new particle formation (NPF) that generates numerous secondary particles. Using two online mass spectrometers with and without a chemical ionization inlet, we characterized the neutral clusters and the naturally charged ion clusters during NPF periods in urban Beijing. In ion clusters, we observed pure sulfuric acid (SA) clusters, SA-amine clusters, SA-ammonia (NH3) clusters, and SA-amine-NH3 clusters. However, only SA clusters and SA-amine clusters were observed in the neutral form. Meanwhile, oxygenated organic molecule (OOM) clusters charged by a nitrate ion and a bisulfate ion were observed in ion clusters. Acid-base clusters correlate well with the occurrence of sub-3 nm particles, whereas OOM clusters do not. Moreover, with the increasing cluster size, amine fractions in ion acid-base clusters decrease, while NH3 fractions increase. This variation results from the reduced stability differences between SA-amine clusters and SA-NH3 clusters, which is supported by both quantum chemistry calculations and chamber experiments. The lower average number of dimethylamine (DMA) molecules in atmospheric ion clusters than the saturated value from controlled SA-DMA nucleation experiments suggests that there is insufficient DMA in urban Beijing to fully stabilize large SA clusters, and therefore, other basic molecules such as NH3 play an important role.

Multigeneration Production of Secondary Organic Aerosol from Toluene Photooxidation

Photooxidation of volatile organic compounds (VOCs) produces secondary organic aerosol (SOA) and light-absorbing brown carbon (BrC) via multiple reaction steps/pathways, reflecting significant chemical complexity relevant to gaseous oxidation and subsequent gas-to-particle conversion. Toluene is an important VOC under urban conditions, but the fundamental chemical mechanism leading to SOA formation remains uncertain. Here, we elucidate multigeneration SOA production from toluene by simultaneously tracking the evolutions of gas-phase oxidation and aerosol formation in a reaction chamber. Large size increase and browning of monodisperse sub-micrometer seed particles occur shortly after initiating oxidation by hydroxyl radical (OH) at 10-90% relative humidity (RH). The evolution in gaseous products and aerosol properties (size/density/optical properties) and chemical speciation of aerosol-phase products indicate that the aerosol growth and browning result from earlier generation products consisting dominantly of dicarbonyl and carboxylic functional groups. While volatile dicarbonyls engage in aqueous reactions to yield nonvolatile oligomers and light-absorbing nitrogen heterocycles/heterochains (in the presence of NH₃) at high RH, organic acids contribute to aerosol carboxylates via ionic dissociation or acid-base reaction in a wide RH range. We conclude that toluene contributes importantly to SOA/BrC formation from dicarbonyls and organic acids because of their prompt and high yields from photooxidation and unique functionalities for participation in aerosol-phase reactions.

Exploring the inorganic and organic nitrate aerosol formation regimes at a suburban site on the North China Plain

Nitrate-driven aerosol pollution frequently occurs during winter over the North China Plain (NCP). Extensive studies have focused on inorganic nitrate formation, but few have focused on organic nitrates in China, precluding a thorough understanding of the nitrogen cycle and nitrate aerosol formation. Here, the inorganic (NO_3,inorg) and organic nitrate (NO_3,org) formation regimes under aerosol liquid water (ALW) and aerosol acidity (pH) influences were investigated during winter over the NCP based on data derived from an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). The campaign-averaged concentration of the total nitrate was 5.3 μg m^-3, with a 13% contribution from NO_3,org, which exhibited a significantly decreased contribution with increasing haze episode evolution. The diurnal cycles of NO_3,inorg and NO_3,org were similar, with high concentrations during the nighttime at a high ALW level, revealing the important role of aqueous-phase processes. However, the correlations between the aerosol pH and NO_3,inorg (R² = 0.13, P < 0.01) and NO_3,org (R² = 0.63, P < 0.01) during polluted periods indicated a contrasting effect of aerosol pH on inorganic and organic nitrate formation. Our results provide a useful reference for smog chamber studies and promote a better understanding of organic nitrate formation via anthropogenic emissions.

Impact of Urban Pollution on Organic-Mediated New-Particle Formation and Particle Number Concentration in the Amazon Rainforest

A major challenge in assessing the impact of aerosols on climate change is to understand how human activities change aerosol loading and properties relative to the pristine/preindustrial baseline. Here, we combine chemical transport simulations and field measurements to investigate the effect of anthropogenic pollution from an isolated metropolis on the particle number concentration over the preindustrial-like Amazon rainforest through various new-particle formation (NPF) mechanisms and primary particle emissions. To represent organic-mediated NPF, we employ a state-of-the-art model that systematically simulates the formation chemistry and thermodynamics of extremely low volatility organic compounds, as well as their roles in NPF processes, and further update the model to improve organic NPF simulations under human-influenced conditions. Results show that urban pollution from the metropolis increases the particle number concentration by a factor of 5-25 over the downwind region (within 200 km from the city center) compared to background conditions. Our model indicates that NPF contributes over 70% of the total particle number in the downwind region except immediately adjacent to the sources. Among different NPF mechanisms, the ternary NPF involving organics and sulfuric acid overwhelmingly dominates. The improved understanding of particle formation mechanisms will help better quantify anthropogenic aerosol forcing from preindustrial times to the present day.

New Insights into the Radical Chemistry and Product Distribution in the OH-Initiated Oxidation of Benzene

Emissions of aromatic compounds cause air pollution and detrimental health effects. Here, we explore the reaction kinetics and products of key radicals in benzene photo-oxidation. After initial OH addition and reaction with O₂, the effective production rates of phenol and bicyclic peroxy radical (BCP-peroxy) are experimentally constrained at 295 K to be 420 ± 80 and 370 ± 70 s^-1, respectively. These rates lead to approximately 53% yield for phenol and 47% yield for BCP-peroxy under atmospheric conditions. The reaction of BCP-peroxy with NO produces bicyclic hydroxy nitrate with a branching ratio <0.2%, indicating efficient NO_x recycling. Similarly, the reaction of BCP-peroxy with HO₂ largely recycles HO_x, producing the corresponding bicyclic alkoxy radical (BCP-oxy). Because of the presence of C-C double bonds and multiple functional groups, the chemistry of BCP-oxy and other alkoxy radicals in the system is diverse. Experimental results suggest the aldehydic H-shift and ring-closure to produce an epoxide functionality could be competitive with classic decomposition of alkoxy radicals. These reactions are potential sources of highly oxygenated molecules. Finally, despite the large number of compounds observed in our study, we are unable to account for ∼20% of the carbon flow.