Formation of oligomers in secondary organic aerosol

Prolonged Dark Chemical Processes in Secondary Organic Aerosols on Filters and in Aqueous Solution

Secondary organic aerosol (SOA) represents a large fraction of atmospheric aerosol particles that significantly affect both the Earth's climate and human health. Laboratory-generated SOA or ambient particles are routinely collected on filters for a detailed chemical analysis. Such filter sampling is prone to artifactual changes in composition during collection, storage, sample workup, and analysis. In this study, we investigate the chemical composition differences in SOA generated in the laboratory, kept at room temperature as aqueous extracts or on filters, and analyzed in detail after a storage time of a day and up to 4 weeks using liquid chromatography coupled to high-resolution mass spectrometry. We observe significantly different temporal concentration changes for monomers and oligomers in both extracts and on filters. In SOA aqueous extracts, many monomers increase in concentration over time, while many dimers decay at the same time. In contrast, on filters, we observe a strong and persistent concentration increase of many dimers and a decrease of many monomers. This study highlights artifacts arising from SOA chemistry occurring during storage, which should be considered when detailed organic aerosol compositions are studied. The particle-phase reactions on filters can also serve as a model system for atmospheric particle aging processes.

Particle-phase accretion forms dimer esters in pinene secondary organic aerosol

Secondary organic aerosol (SOA) is ubiquitous in the atmosphere and plays a pivotal role in climate, air quality, and health. The production of low-volatility dimeric compounds through accretion reactions is a key aspect of SOA formation. However, despite extensive study, the structures and thus the formation mechanisms of dimers in SOA remain largely uncharacterized. In this work, we elucidate the structures of several major dimer esters in SOA from ozonolysis of α-pinene and β-pinene-substantial global SOA sources-through independent synthesis of authentic standards. We show that these dimer esters are formed in the particle phase and propose a mechanism of nucleophilic addition of alcohols to a cyclic acylperoxyhemiacetal. This chemistry likely represents a general pathway to dimeric compounds in ambient SOA.

Alkylperoxy radicals are responsible for the formation of oxygenated primary organic aerosol

Organic aerosol (OA) is an air pollutant ubiquitous in urban atmospheres. Urban OA is usually apportioned into primary OA (POA), mostly emitted by mobile sources, and secondary OA (SOA), which forms in the atmosphere due to oxidation of gas-phase precursors from anthropogenic and biogenic sources. By performing coordinated measurements in the particle phase and the gas phase, we show that the alkylperoxy radical chemistry that is responsible for low-temperature ignition also leads to the formation of oxygenated POA (OxyPOA). OxyPOA is distinct from POA emitted during high-temperature ignition and is chemically similar to SOA. We present evidence for the prevalence of OxyPOA in emissions of a spark-ignition engine and a next-generation advanced compression-ignition engine, highlighting the importance of understanding OxyPOA for predicting urban air pollution patterns in current and future atmospheres.

Organic synthesis in the study of terpene-derived oxidation products in the atmosphere

Covering: 1997 up to 2022Volatile biogenic terpenes involved in the formation of secondary organic aerosol (SOA) particles participate in rich atmospheric chemistry that impacts numerous aspects of the earth's complex climate system. Despite the importance of these species, understanding their fate in the atmosphere and determining their atmospherically-relevant properties has been limited by the availability of authentic standards and probe molecules. Advances in synthetic organic chemistry directly aimed at answering these questions have, however, led to exciting discoveries at the interface of chemistry and atmospheric science. Herein we provide a review of the literature regarding the synthesis of commercially unavailable authentic standards used to analyze the composition, properties, and mechanisms of SOA particles in the atmosphere.

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.

Review of the influencing factors of secondary organic aerosol formation and aging mechanism based on photochemical smog chamber simulation methods

The formation and aging mechanism of secondary organic aerosol (SOA) and its influencing factors have attracted increasing attention in recent years because of their effects on climate change, atmospheric quality and human health. However, there are still large errors between air quality model simulation results and field observations. The currently undetected components during the formation and aging of SOA due to the limitation of current monitoring techniques and the interactions among multiple SOA formation influencing factors might be the main reasons for the differences. In this paper, we present a detailed review of the complex dynamic physical and chemical processes and the corresponding influencing factors involved in SOA formation and aging. And all these results were mainly based the studies of photochemical smog chamber simulation. Although the properties of precursor volatile organic compounds (VOCs), oxidants (such as OH radicals), and atmospheric environmental factors (such as NO_x, SO₂, NH₃, light intensity, temperature, humidity and seed aerosols) jointly influence the products and yield of SOA, the nucleation and vapor pressure of these products were found to be the most fundamental aspects when interpreting the dynamics of the SOA formation and aging process. The development of techniques for measuring intermediate species in SOA generation processes and the study of SOA generation and aging mechanism in complex systems should be important topics of future SOA research.

Highly Acidic Conditions Drastically Alter the Chemical Composition and Absorption Coefficient of α-Pinene Secondary Organic Aerosol

Secondary organic aerosols (SOA), formed through the gas-phase oxidation of volatile organic compounds (VOCs), can reside in the atmosphere for many days. The formation of SOA takes place rapidly within hours after VOC emissions, but SOA can undergo much slower physical and chemical processes throughout their lifetime in the atmosphere. The acidity of atmospheric aerosols spans a wide range, with the most acidic particles having negative pH values, which can promote acid-catalyzed reactions. The goal of this work is to elucidate poorly understood mechanisms and rates of acid-catalyzed aging of mixtures of representative SOA compounds. SOA were generated by the ozonolysis of α-pinene in a continuous flow reactor and then collected using a foil substrate. SOA samples were extracted and aged by exposure to varying concentrations of aqueous H₂SO₄ for 1-2 days. Chemical analysis of fresh and aged samples was conducted using ultra-performance liquid chromatography coupled with photodiode array spectrophotomety and high-resolution mass spectrometry. In addition, UV-vis spectrophotometry and fluorescence spectrophotometry were used to examine the changes in optical properties before and after aging. We observed that SOA that aged in moderately acidic conditions (pH from 0 to 4) experienced small changes in composition, while SOA that aged in a highly acidic environment (pH from -1 to 0) experienced more dramatic changes in composition, including the formation of compounds containing sulfur. Additionally, at highly acidic conditions, light-absorbing and fluorescent compounds appeared, but their identities could not be ascertained due to their small relative abundance. This study shows that acidity is a major driver of SOA aging, resulting in a large change in the chemical composition and optical properties of aerosols in regions where high concentrations of H₂SO₄ persist, such as upper troposphere and lower stratosphere.

Suppression of the phenolic SOA formation in the presence of electrolytic inorganic seed

Phenolic compounds are largely attributed to wildfire gases and rapidly react with atmospheric oxidants to form persistent phenoxy free radicals, which influence atmospheric chemistry and secondary organic aerosol (SOA) formation. In this study, phenol or o-cresol was photochemically oxidized under various conditions (NO_x levels, humidity, and seed conditions) in an outdoor photochemical reactor. Unexpectedly, SOA growth of both phenols was suppressed in the presence of salted aqueous aerosol compared to non-seed SOA. This discovery is different from the typical SOA formation of aromatic or biogenic hydrocarbons, which show noticeably higher SOA yields via organic aqueous reactions. Phenol, o-cresol, and their phenolic products (e.g., catechols) are absorbed in aqueous aerosol and form phenoxy radicals via heterogeneous reactions under sunlight. The resulting phenoxy radicals are redistributed between the gas and particle phases. Gaseous phenoxy radicals quickly react with ozone to form phenyl peroxide radicals and regenerated through a NO_x cycle to retard phenol oxidation and SOA formation. The explicit oxidation mechanisms of phenol or o-cresol in the absence of aqueous phase were derived including the Master Chemical Mechanism (MCM v3.3.1) and the path for peroxy radical adducts originating from the addition of an OH radical to phenols to form low volatility products (e.g., multi-hydroxy aromatics). The resulting gas mechanisms of phenol or o-cresol were, then, applied to the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model to predict SOA formation via multiphase partitioning of organics and aerosol-phase oligomerization. The model well simulated chamber-generated phenolic SOA in absence of wet-inorganic seed, but significantly overestimated SOA mass in presence of wet seed. This study suggests that heterogeneous chemistry to form phenoxy radicals needs to be included to improve SOA prediction from phenols. The suppression of atmospheric oxidation due to phenoxy radicals in wet inorganic aerosol can explain the low SOA formation during wildfire episodes.

Ozonolysis of α-Pinene and Δ3-Carene Mixtures: Formation of Dimers with Two Precursors

The formation of secondary organic aerosol (SOA) from the structurally similar monoterpenes, α-pinene and Δ³-carene, differs substantially. The aerosol phase is already complex for a single precursor, and when mixtures are oxidized, products, e.g., dimers, may form between different volatile organic compounds (VOCs). This work investigates whether differences in SOA formation and properties from the oxidation of individual monoterpenes persist when a mixture of the monoterpenes is oxidized. Ozonolysis of α-pinene, Δ³-carene, and a 1:1 mixture of them was performed in the Aarhus University Research on Aerosol (AURA) atmospheric simulation chamber. Here, ∼100 ppb of monoterpene was oxidized by 200 ppb O₃ under dark conditions at 20 °C. The particle number concentration and particle mass concentration for ozonolysis of α-pinene exceed those from ozonolysis of Δ³-carene alone, while their mixture results in concentrations similar to α-pinene ozonolysis. Detailed offline analysis reveals evidence of VOC-cross-product dimers in SOA from ozonolysis of the monoterpene mixture: a VOC-cross-product dimer likely composed of the monomeric units cis-caric acid and 10-hydroxy-pinonic acid and a VOC-cross-product dimer ester likely from the monomeric units caronaldehyde and terpenylic acid were tentatively identified by liquid chromatography-mass spectrometry. To improve the understanding of chemical mechanisms determining SOA, it is relevant to identify VOC-cross-products.

Evidence for the coupling of refill liquids content and new particle formation in electronic cigarette vapors

The size and chemical content of particles in electronic cigarette vapors (e-vapors) dictate their fate in the human body. Understanding how particles in e-vapors are formed and their size is critical to identifying and mitigating the adverse consequences of vaping. Thermal decomposition and reactions of the refill liquid (e-liquid) components play a key role in new particles formation. Here we report the evolution of particle number concentration in e-vapors over time for variable mixtures of refill e-liquids and operating conditions. Particle with aerodynamic diameter < 300 nm accounted for up to 17% (or 780 μg/m³) of e-vapors particles. Two events of increasing particle number concentration were observed, 2-3 s after puff completion and a second 4-5 s later. The intensity of each event varied by the abundance of propylene glycol, glycerol, and flavorings in e-liquids. Propylene glycol and glycerol were associated with the first event. Flavorings containing aromatic and aliphatic unsaturated functional groups were strongly associated with the second event and to a lesser extent with the first one. The results indicate that particles in e-vapors may be formed through the heteromolecular condensation of propylene glycol, glycerol, and flavorings, including both parent chemicals and/or their thermal decomposition products.

Acylperoxy Radicals as Key Intermediates in the Formation of Dimeric Compounds in α-Pinene Secondary Organic Aerosol

High molecular weight dimeric compounds constitute a significant fraction of secondary organic aerosol (SOA) and have profound impacts on the properties and lifecycle of particles in the atmosphere. Although different formation mechanisms involving reactive intermediates and/or closed-shell monomeric species have been proposed for the particle-phase dimers, their relative importance remains in debate. Here, we report unambiguous experimental evidence of the important role of acyl organic peroxy radicals (RO₂) and a small but non-negligible contribution from stabilized Criegee intermediates (SCIs) in the formation of particle-phase dimers during ozonolysis of α-pinene, one of the most important precursors for biogenic SOA. Specifically, we find that acyl RO₂-involved reactions explain 50-80% of total oxygenated dimer signals (C₁₅-C₂₀, O/C ≥ 0.4) and 20-30% of the total less oxygenated (O/C < 0.4) dimer signals. In particular, they contribute to 70% of C₁₅-C₁₉ dimer ester formation, likely mainly via the decarboxylation of diacyl peroxides arising from acyl RO₂ cross-reactions. In comparison, SCIs play a minor role in the formation of C₁₅-C₁₉ dimer esters but react noticeably with the most abundant C₉ and C₁₀ carboxylic acids and/or carbonyl products to form C₁₉ and C₂₀ dimeric peroxides, which are prone to particle-phase transformation to form more stable dimers without the peroxide functionality. This work provides a clearer view of the formation pathways of particle-phase dimers from α-pinene oxidation and would help reduce the uncertainties in future atmospheric modeling of the budget, properties, and health and climate impacts of SOA.

Understanding the Evolution of Smoke Mass Extinction Efficiency Using Field Campaign Measurements

Aerosol mass extinction efficiency (MEE) is a key aerosol property used to connect aerosol optical properties with aerosol mass concentrations. Using measurements of smoke obtained during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign we find that mid-visible smoke MEE can change by a factor of 2-3 between fresh smoke (<2 hr old) and one-day-old smoke. While increases in aerosol size partially explain this trend, changes in the real part of the aerosol refractive index (real(n)) are necessary to provide closure assuming Mie theory. Real(n) estimates derived from multiple days of FIREX-AQ measurements increase with age (from 1.40 - 1.45 to 1.5-1.54 from fresh to one-day-old) and are found to be positively correlated with organic aerosol oxidation state and aerosol size, and negatively correlated with smoke volatility. Future laboratory, field, and modeling studies should focus on better understanding and parameterizing these relationships to fully represent smoke aging.

Molecular characterization of nitrogen-containing organic compounds in the winter North China Plain

The molecular characteristics of organic aerosols (OAs) in heavily polluted areas affected by coal combustion (CC) were investigated. In terms of relative abundance, the total nitrogen-containing organic compounds (NOC) accounted for about 61%-68% of all molecules detected in methanol-soluble organic carbon (MSOC) by LC - Q-TOF - MS. More than 85% of the CHON- formulas are nitro-aromatic compounds, which are generally considered to be secondary organic compounds, as evidenced by the lower degree of overlap of these substances in the atmospheric samples and CC samples. Some polycyclic aromatic compounds with 4 N and 1-2O and very low H/C and O/C ratio produced by CC are unstable and easily react to form compounds with higher degrees of saturation. Almost all of the CHON+ homologues detected in the CC samples were also found in the atmospheric samples, indicating that the large amount of CHON+ compounds produced by CC are stable during atmospheric processes. The CHN+ compounds produced by CC contain a certain amount of highly unsaturated compounds, among which 1 N-containing polycyclic aromatic hydrocarbons (1 N-PAHs) is stable in atmosphere and can serve as markers of CC.

Initial pH Governs Secondary Organic Aerosol Phase State and Morphology after Uptake of Isoprene Epoxydiols (IEPOX)

Aerosol acidity increases secondary organic aerosol (SOA) formed from the reactive uptake of isoprene-derived epoxydiols (IEPOX) by enhancing condensed-phase reactions within sulfate-containing submicron particles, leading to low-volatility organic products. However, the link between the initial aerosol acidity and the resulting physicochemical properties of IEPOX-derived SOA remains uncertain. Herein, we show distinct differences in the morphology, phase state, and chemical composition of individual organic-inorganic mixed particles after IEPOX uptake to ammonium sulfate particles with different initial atmospherically relevant acidities (pH = 1, 3, and 5). Physicochemical properties were characterized via atomic force microscopy coupled with photothermal infrared spectroscopy (AFM-PTIR) and Raman microspectroscopy. Compared to less acidic particles (pH 3 and 5), reactive uptake of IEPOX to the most acidic particles (pH 1) resulted in 50% more organosulfate formation, clearer phase separation (core-shell), and more irregularly shaped morphologies, suggesting that the organic phase transitioned to semisolid or solid. This study highlights that initial aerosol acidity may govern the subsequent aerosol physicochemical properties, such as viscosity and morphology, following the multiphase chemical reactions of IEPOX. These results can be used in future studies to improve model parameterizations of SOA formation from IEPOX and its properties, toward the goal of bridging predictions and atmospheric observations.

A review of secondary organic aerosols formation focusing on organosulfates and organic nitrates

Secondary organic aerosols (SOA) are crucial constitution of fine particulate matter (PM), which are mainly derived from photochemical oxidation products of primary organic matter and volatile organic compounds (VOCs), and can induce terrible impacts to human health, air quality and climate change. As we know, organosulfates (OSs) and organic nitrates (ON) are important contributors for SOA formation, which could be possibly produced through various pathways, resulting in extremely complex formation mechanism of SOA. Although plenty of research has been focused on the origins, spatial distribution and formation mechanisms of SOA, a comprehensive and systematic understanding of SOA formation in the atmosphere remains to be detailed explored, especially the most important OSs and ON dedications. Thus, in this review, we systematically summarize the recent research about origins and formation mechanisms of OSs and ON, and especially focus on their contribution to SOA, so as to have a clearer understanding of the origin, spatial distribution and formation principle of SOA. Importantly, we interpret the complex interaction with coexistence effect of SO_x and NO_x on SOA formation, and emphasize the future insights for SOA research to expect a more comprehensive theory and practice to alleviate SOA burden.

Roles of semivolatile and intermediate-volatility organic compounds in secondary organic aerosol formation and its implication: A review

Secondary organic aerosol (SOA) is a very important component of fine particulate matter (PM_2.5) in the atmosphere. However, the simulations of SOA, which could help to elucidate the detailed mechanism of SOA formation and quantify the roles of various precursors, remains unsatisfactory, as SOA levels are frequently underestimated. It has been found that the performance of SOA formation models can be significantly improved by incorporating the emission and evolution of semivolatile and intermediate-volatility organic compounds (S/IVOCs). In order to explore the roles of S/IVOCs in SOA formation, this study reviews some simulation models which could consider S/IVOCs for SOA formation as well as the development of emission inventories of S/IVOCs and S/IVOC modules for SOA formation. In addition, the future research directions for simulations of the effect of S/IVOCs on SOA formation are suggested.

Molecular Chirality and Cloud Activation Potentials of Dimeric α-Pinene Oxidation Products

The surface activity of ten atmospherically relevant α-pinene-derived dimers having varying terminal functional groups and backbone stereochemistry is reported. We find ∼10% differences in surface activity between diastereomers of the same dimer, demonstrating that surface activity depends upon backbone stereochemistry. Octanol-water (K_OW) and octanol-ammonium sulfate partitioning coefficient (K_OAS) measurements of our standards align well with the surface activity measurements, with the more surface-active dimers exhibiting increased hydrophobicity. Our findings establish a link between molecular chirality and cloud activation potential of secondary organic aerosol particles. Given the diurnal variations in enantiomeric excess of biogenic emissions, possible contributions of such a link to biosphere:atmosphere feedbacks as well as aerosol particle viscosity and phase separation are discussed.

Phase Behavior of Hydrocarbon-like Primary Organic Aerosol and Secondary Organic Aerosol Proxies Based on Their Elemental Oxygen-to-Carbon Ratio

A large fraction of atmospheric aerosols can be characterized as primary organic aerosol (POA) and secondary organic aerosol (SOA). Knowledge of the phase behavior, that is, the number and type of phases within internal POA + SOA mixtures, is crucial to predict their effect on climate and air quality. For example, if POA and SOA form a single phase, POA will enhance the formation of SOA by providing organic mass to absorb SOA precursors. Using microscopy, we studied the phase behavior of mixtures of SOA proxies and hydrocarbon-like POA proxies at relative humidity (RH) values of 90%, 45%, and below 5%. Internal mixtures of POA and SOA almost always formed two phases if the elemental oxygen-to-carbon ratio (O/C) of the POA was less than 0.11, which encompasses a large fraction of atmospheric hydrocarbon-like POA from fossil fuel combustion. SOA proxies mixed with POA proxies having 0.11 ≤ O/C ≤ 0.29 mostly resulted in particles with one liquid phase. However, two liquid phases were also observed, depending on the type of SOA and POA surrogates, and an increase in phase-separated particles was observed when increasing the RH in this O/C range. The results have implications for predicting atmospheric SOA formation and policy strategies to reduce SOA in urban environments.

Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds

The acidity of aqueous atmospheric solutions is a key parameter driving both the partitioning of semi-volatile acidic and basic trace gases and their aqueous-phase chemistry. In addition, the acidity of atmospheric aqueous phases, e.g., deliquesced aerosol particles, cloud, and fog droplets, is also dictated by aqueous-phase chemistry. These feedbacks between acidity and chemistry have crucial implications for the tropospheric lifetime of air pollutants, atmospheric composition, deposition to terrestrial and oceanic ecosystems, visibility, climate, and human health. Atmospheric research has made substantial progress in understanding feedbacks between acidity and multiphase chemistry during recent decades. This paper reviews the current state of knowledge on these feedbacks with a focus on aerosol and cloud systems, which involve both inorganic and organic aqueous-phase chemistry. Here, we describe the impacts of acidity on the phase partitioning of acidic and basic gases and buffering phenomena. Next, we review feedbacks of different acidity regimes on key chemical reaction mechanisms and kinetics, as well as uncertainties and chemical subsystems with incomplete information. Finally, we discuss atmospheric implications and highlight the need for future investigations, particularly with respect to reducing emissions of key acid precursors in a changing world, and the need for advancements in field and laboratory measurements and model tools.

Analytical methods for organosulfate detection in aerosol particles: Current status and future perspectives

Organosulfates (OSs) are well-known water-soluble constituents of atmospheric aerosol particles. They are formed from multiphase reactions between volatile organic compounds (VOCs) and their photooxidation products, and acidic sulfate originating from biogenic and anthropogenic sources in the atmosphere. Although the analytical procedures used to measure OSs, including sampling, pre-treatment, and instrumental detection, have advanced substantially in the last decade, there is still a need for accurate and standardized analysis procedures for the identification, quantification, and comparison of OSs in different regions. Additionally, there has no study focused on the health effects of OSs. This review outlines the analytical methods developed for OS detection during the last decade, highlighting both improvements and drawbacks. It also considers the future development of analytical methods for OS detection, and proposes the establishment of OSs screening method from the perspective of health effects to solve the problem of unknown health related OSs identification.

Online Characterization of Organic Aerosol by Condensational Growth into Aqueous Droplets Coupled with Droplet-Assisted Ionization

Online analysis of ultrafine (<100 nm diameter) particles was performed by sending the aerosol through a condensation growth chamber (CGC) to create micrometer-size aqueous droplets that were subsequently analyzed by mass spectrometry with droplet-assisted ionization (DAI). Three experiments are reported which illustrate key performance characteristics of the method and give insight into the ion formation process: size-selected cortisone particles, size-selected secondary organic aerosol (SOA) particles, and freshly nucleated SOA under atmospherically relevant conditions. In each case, SOA was produced by α-pinene ozonolysis. For size-selected cortisone particles between 30 and 90 nm diameter and SOA particles between 30 and 70 nm, the ion signal intensity was found to be approximately independent of particle size. This observation is attributed to the formation of aqueous droplets in the CGC whose size distribution is independent of the original particle size. A consequence of this behavior is that the sensitivity of molecular detection increases as the particle size decreases, and the method is particularly well suited for new particle formation studies under atmospherically relevant conditions. This aspect of the CGC-DAI method was illustrated by the online analysis of freshly nucleated SOA samples with median diameters, number concentrations, and mass concentrations on the order of 25 nm, 10⁴ cm^-3, 0.2 μg m^-3, respectively. Mass spectra of freshly nucleated SOA could be explained by condensation of highly oxidized molecules (HOMs) that subsequently reacted in the particle phase. Size-selected SOA showed increasing oligomerization with increasing particle size, which is consistent with established particle growth mechanisms.

Dark Iron-Catalyzed Reactions in Acidic and Viscous Aerosol Systems Efficiently Form Secondary Brown Carbon

Iron-driven secondary brown carbon formation reactions from water-soluble organics in cloud droplets and aerosols create insoluble and soluble products of emerging atmospheric importance. This work shows, for the first time, results on dark iron-catalyzed polymerization of catechol forming insoluble black polycatechol particles and colored water-soluble oligomers under conditions characteristic of viscous multicomponent aerosol systems with relatively high ionic strength (I = 1-12 m) and acidic pH (∼2). These systems contain ammonium sulfate (AS)/nitrate (AN) and C3-C5 dicarboxylic acids, namely, malonic, malic, succinic, and glutaric acids. Using dynamic light scattering (DLS) and ultra high pressure liquid chromatography-mass spectrometry (UHPLC-MS), we show results on the rate of particle growth/agglomeration and identity of soluble oligomeric reaction products. We found that increasing I above 1 m and adding diacids with oxygen-to-carbon molar ratio (O:C > 1) significantly reduced the rate of polycatechol formation/aggregation by a factor of 1.3 ± 0.4 in AS solution in the first 60 min of reaction time. Using AN, rates were too slow to be quantified using DLS, but particles formed after 24 h reaction time. These results were explained by the relative concentration and affinity of ligands to Fe(III). We also report detectable amounts of soluble and colored oligomers in reactions with a slow rate of polycatechol formation, including organonitrogen compounds. These results highlight that brown carbon formation from iron chemistry is efficient under a wide range of aerosol physical states and chemical composition.

Simulating the impact of gas-wall partitioning on SOA formation using the explicit gas mechanism integrated with aqueous reactions containing electrolytes

The impact of the Teflon reactor wall on secondary organic aerosol (SOA) formation was explicitly simulated by using the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model integrated with gas-wall partitioning (GWP). The formation of oxygenated semivolatile organic compounds (SVOCs) from the photooxidation of hydrocarbons (HC) was simulated by using an explicit gas-kinetic mechanism (MCM V3.3.1). In the model, SVOC's GWP and gas-particle partitioning onto preexisting particulate matter were kinetically treated with the absorption and desorption processes. The UNIPAR model streamlined aerosol growth via the oligomerization of reactive SVOCs in the organic phase and aqueous reactions in the inorganic phase. Two important GWP parameters, GWP coefficient (K_{w, i}) and the deposition rate constant (k_on_{w, i}) of SVOCs (i) to the wall were predicted by using a quantitative structure activity relationship (QSAR) employing SVOCs' physicochemical descriptors. This GWP model was then incorporated with the UNIPAR model in the DSMACC-KPP platform and simulated SOA chamber data. The three different HCs (toluene, 1,3,5-trimethylbenzene, and α-pinene) were photochemically oxidized in the presence of NO_x and inorganic seed aerosols in an outdoor photochemical smog chamber (UF-APHOR). The impact of GWP on SOA mass varied ranging from 9% to 71% with HCs, seed conditions, NO_x, and temperature. Toluene SOA in the absence of inorganic aerosol was the most sensitive to GWP. However, in the presence of wet-inorganic seed, the impact of GWP on SOA was smaller than that of non-seed SOA owing to rapid reactions of organic species in the aqueous phase. SOA mass can be significantly underestimated in the absence of wet-inorganic seed when the aerosol model employs parameters derived using SOA data with GWP artifacts.

Extreme Low-Temperature Combustion Chemistry: Ozone-Initiated Oxidation of Methyl Hexanoate

The accelerating chemical effect of ozone addition on the oxidation chemistry of methyl hexanoate [CH₃(CH₂)₄C(═O)OCH₃] was investigated over a temperature range from 460 to 940 K. Using an externally heated jet-stirred reactor at p = 700 Torr (residence time τ = 1.3 s, stoichiometry φ = 0.5, 80% argon dilution), we explored the relevant chemical pathways by employing molecular-beam mass spectrometry with electron and single-photon ionization to trace the temperature dependencies of key intermediates, including many hydroperoxides. In the absence of ozone, reactivity is observed in the so-called low-temperature chemistry (LTC) regime between 550 and 700 K, which is governed by hydroperoxides formed from sequential O₂ addition and isomerization reactions. At temperatures above 700 K, we observed the negative temperature coefficient (NTC) regime, in which the reactivity decreases with increasing temperatures, until near 800 K, where the reactivity increases again. Upon addition of ozone (1000 ppm), the overall reactivity of the system is dramatically changed due to the time scale of ozone decomposition in comparison to fuel oxidation time scales of the mixtures at different temperatures. While the LTC regime seems to be only slightly affected by the addition of ozone with respect to the identity and quantity of the observed intermediates, we observed an increased reactivity in the intermediate NTC temperature range. Furthermore, we observed experimental evidence for an additional oxidation regime in the range near 500 K, herein referred to as the extreme low-temperature chemistry (ELTC) regime. Experimental evidence and theoretical rate constant calculations indicate that this ELTC regime is likely to be initiated by H abstraction from methyl hexanoate via O atoms, which originate from thermal O₃ decomposition. The theoretical calculations show that the rate constants for methyl ester initiation via abstraction by O atoms increase dramatically with the size of the methyl ester, suggesting that ELTC is likely not important for the smaller methyl esters. Experimental evidence is provided indicating that, similar to the LTC regime, the chemistry in the ELTC regime is dominated by hydroperoxide chemistry. However, mass spectra recorded at various reactor temperatures and at different photon energies provide experimental evidence of some differences in chemical species between the ELTC and the LTC temperature ranges.

Characteristics and seasonal variations of high-molecular-weight oligomers in urban haze aerosols

Organic aerosols (OA) undergo sophisticated physiochemical processes in the atmosphere, playing a crucial role in extreme haze formations over the Northern China Plain. However, current understandings of the detailed composition and formation pathways are limited. In this study, high-molecular weight (HMW) species were observed in samples collected year-round in urban Beijing, especially in autumn and winter, during 2016-2017. The positive-ion-mode mass spectra of matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) showed that higher signal intensities were obtained in the mass-to-charge (m/z) ranges of 200-500 and 800-900, with repetitive mass difference patterns of m/z 12, 14, 16, and 18. This provided sound evidence that high-molecular-weight oligomers were generated as haze episodes became exacerbated. These oligomer signal intensities were enhanced in the presence of high relative humidity, aerosol water content, and PM_2.5 (particles with an aerodynamic diameter ≤ 2.5 μm) mass, proving that the multiphase reaction processes play a fundamental role in haze formation in Beijing. Our study can form a basis for improved air pollution mitigation measures aimed at OA to improve health outcomes.

Brown Carbon Production by Aqueous-Phase Interactions of Glyoxal and SO2

Oxalic acid and sulfate salts are major components of aerosol particles. Here, we explore the potential for their respective precursor species, glyoxal and SO₂, to form atmospheric brown carbon via aqueous-phase reactions in a series of bulk aqueous and flow chamber aerosol experiments. In bulk aqueous solutions, UV- and visible-light-absorbing products are observed at pH 3-4 and 5-6, respectively, with small but detectable yields of hydroxyquinone and polyketone products formed, especially at pH 6. Hydroxymethanesulfonate (HMS), C₂, and C₃ sulfonates are major products detected by electrospray ionization mass spectrometry (ESI-MS) at pH 5. Past studies have assumed that the reaction of formaldehyde and sulfite was the only atmospheric source of HMS. In flow chamber experiments involving sulfite aerosol and gas-phase glyoxal with only 1 min residence times, significant aerosol growth is observed. Rapid brown carbon formation is seen with aqueous aerosol particles at >80% relative humidity (RH). Brown carbon formation slows at 50-60% RH and when the aerosol particles are acidified with sulfuric acid but stops entirely only under dry conditions. This chemistry may therefore contribute to brown carbon production in cloud-processed pollution plumes as oxidizing volatile organic compounds (VOCs) interact with SO₂ and water.

Particle-Phase Diffusion Modulates Partitioning of Semivolatile Organic Compounds to Aged Secondary Organic Aerosol

The diffusivity of semivolatile organic compounds (SVOCs) in the bulk particle phase of a viscous atmospheric secondary organic aerosol (SOA) can have a profound impact on aerosol growth and size distribution dynamics. Here, we investigate the bulk diffusivity of SVOCs formed from photo-oxidation of isoprene as they partition to a bimodal aerosol consisting of an Aitken (potassium sulfate) and accumulation mode (aged α-pinene SOA) particles as a function of relative humidity (RH). The model analysis of the observed size distribution evolution shows that liquid-like diffusion coefficient values of D_b > 10^-10 cm² s^-1 fail to explain the growth of the Aitken mode. Instead, much lower values of D_b between 2.5 × 10^-15 cm² s^-1 at 32% RH and 8 × 10^-15 cm² s^-1 at 82% RH were needed to successfully reproduce the growth of both modes. The diffusivity within the aged α-pinene SOA remains appreciably slow even at 80% RH, resulting in hindered partitioning of SVOCs to large viscous particles and allowing smaller and relatively less viscous particles to effectively absorb the available SVOCs and grow much faster than would be possible otherwise. These results have important implications for modeling SOA formation and growth in the ambient atmosphere.

The fuel of atmospheric chemistry: Toward a complete description of reactive organic carbon

The Earth's atmosphere contains a multitude of emitted (primary) and chemically formed (secondary) gases and particles that degrade air quality and modulate the climate. Reactive organic carbon (ROC) species are the fuel of the chemistry of the atmosphere, dominating short-lived emissions, reactivity, and the secondary production of key species such as ozone, particulate matter, and carbon dioxide. Despite the central importance of ROC, the diversity and complexity of this class of species has been a longstanding obstacle to developing a comprehensive understanding of how the composition of our atmosphere, and the associated environmental implications, will evolve. Here, we characterize the role of ROC in atmospheric chemistry and the challenges inherent in measuring and modeling ROC, and highlight recent progress toward achieving mass closure for the complete description of atmospheric ROC.

Monoterpene and Sesquiterpene α-Hydroxy Organosulfates: Synthesis, MS/MS Characteristics, and Ambient Presence

Organosulfates (OSs) derived from biogenic volatile organic compounds are important compounds signifying interactions between anthropogenic sulfur pollution and natural emission. In this work, we substantially expand the OS standard library through the chemical synthesis of 26 α-hydroxy OS standards from eight monoterpenes (i.e., α- and β-pinene, limonene, sabinene, Δ³-carene, terpinolene, and α- and γ-terpinene) and two sesquiterpenes (i.e., α-humulene and β-caryophyllene). The sulfation of unsymmetrically substituted 1,2-diol intermediates produced a regioisomeric mixture of two OSs. The major regioisomeric OSs were isolated and purified for full NMR characterization, while the minor regioisomers could only be determined by liquid chromatograph-mass spectrometer (MS). The tandem mass spectra of the molecular ion formed through electrospray ionization confirmed the formation of abundant bisulfate ion fragments (m/z 97) and certain minor ion fragments characteristic of the carbon backbone. A knowledge of the MS/MS spectra and chromatographic retention times for authentic standards allows us to identify α-hydroxy OSs derived from six monoterpenes and β-caryophyllene in ambient samples. Notably, among two possible regioisomers of α-hydroxy OSs, we only detected the isomers with the sulfate group at the less substituted carbon position derived from α-pinene, limonene, sabinene, Δ³-carene, and terpinolene in the ambient samples. This observation sheds light on the atmospheric OS formation mechanisms.

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.

Synthesis and surface spectroscopy of α-pinene isotopologues and their corresponding secondary organic material

The synthesis and surface-specific spectroscopic analysis of α-pinene isotopologues and their corresponding secondary organic material is reported.

Atmospheric aerosol–cloud interactions remain among the least understood processes within the climate system, leaving large uncertainties in the prediction of future climates. In particular, the nature of the surfaces of aerosol particles formed from biogenic terpenes, such as α-pinene, is poorly understood despite the importance of surface phenomena in their formation, growth, radiative properties, and ultimate fate. Herein we report the coupling of a site-specific deuterium labeling strategy with vibrational sum frequency generation (SFG) spectroscopy to probe the surface C–H oscillators in α-pinene-derived secondary organic aerosol material (SOM) generated in an atmospheric flow tube reactor. Three α-pinene isotopologues with methylene bridge, bridgehead methine, allylic, and vinyl deuteration were synthesized and their vapor phase SFG spectra were compared to that of unlabeled α-pinene. Subsequent analysis of the SFG spectra of their corresponding SOM revealed that deuteration of the bridge methylene C–H oscillators present on the cyclobutane ring in α-pinene leads to a considerable signal intensity decrease (ca. 30–40%), meriting speculation that the cyclobutane moiety remains largely intact within the surface bound species present in the SOM formed upon α-pinene oxidation. These insights provide further clues as to the complexity of aerosol particle surfaces, and establish a framework for future investigations of the heterogeneous interactions between precursor terpenes and particle surfaces that lead to aerosol particle growth under dynamically changing conditions in the atmosphere.

Infrared Ion Spectroscopy of Environmental Organic Mixtures: Probing the Composition of α-Pinene Secondary Organic Aerosol

Characterizing the chemical composition of organic aerosols can elucidate aging mechanisms as well as the chemical and physical properties of the aerosol. However, the high chemical complexity and often low atmospheric abundance present a difficult analytical challenge. Milligrams or more of material may be needed for speciated spectroscopic analysis. In contrast, mass spectrometry provides a very sensitive platform but limited structural information. Here, we combine the strengths of mass spectrometry and infrared (IR) action spectroscopy to generate characteristic IR spectra of individual, mass-isolated ion populations. Soft ionization combined with in situ infrared ion spectroscopy, using the tunable free-electron laser FELIX, provides detailed information on molecular structures and functional groups. We apply this technique, along with quantum mechanical modeling, to characterize organic molecules in secondary organic aerosol (SOA) formed from the ozonolysis of α-pinene. Spectral overlap with a standard is used to identify cis-pinonic acid. We also demonstrate the characterization of isomers for multiple SOA products using both quantum mechanical computations and analyses of fragment ion spectra. These results demonstrate the detailed structural information on isolated ions obtained by combining mass spectrometry with fingerprint IR spectroscopy.

Effect of Oxalate and Sulfate on Iron-Catalyzed Secondary Brown Carbon Formation

Oxalate and sulfate are ubiquitous components of ambient aerosols with a high complexation affinity to iron. However, their effect on iron-driven secondary brown carbon formation in solution from soluble aromatic and aliphatic reagents was not studied. We report masses and hydrodynamic particle sizes of insoluble particles formed from the dark aqueous phase reaction of catechol, guaiacol, fumaric, and muconic acids with Fe(III) in the presence of oxalate or sulfate. Results show that oxalate decreases particle yield in solution from the reaction of Fe(III), with a stronger effect for guaiacol than catechol. For both compounds, the addition of sulfate results in the formation of more polydisperse (0.1-5 μm) and heavier particles than those from control experiments. Reactions with fumaric and muconic acids show that oxalate (not sulfate) and pH are determining factors in the efficiency of particle formation in solution. Polymerization reactions occur readily in the presence of sulfate in solution producing particles with iron-coordinated and/or pore-trapped sulfate anions. The addition of oxalate to the reactions of Fe(III) with all organics, except guaiacol, produced fewer and larger polymeric particles (>0.5 μm). These results imply that even in the presence of competing ligands, the formation of insoluble and colored particles from soluble organic precursors still dominates over the formation of soluble iron complexes.

Bouncier Particles at Night: Biogenic Secondary Organic Aerosol Chemistry and Sulfate Drive Diel Variations in the Aerosol Phase in a Mixed Forest

Aerosol phase state is critical for quantifying aerosol effects on climate and air quality. However, significant challenges remain in our ability to predict and quantify phase state during its evolution in the atmosphere. Herein, we demonstrate that aerosol phase (liquid, semisolid, solid) exhibits a diel cycle in a mixed forest environment, oscillating between a viscous, semisolid phase state at night and liquid phase state with phase separation during the day. The viscous nighttime particles existed despite higher relative humidity and were independently confirmed by bounce factor measurements and atomic force microscopy. High-resolution mass spectrometry shows the more viscous phase state at night is impacted by the formation of terpene-derived and higher molecular weight secondary organic aerosol (SOA) and smaller inorganic sulfate mass fractions. Larger daytime particulate sulfate mass fractions, as well as a predominance of lower molecular weight isoprene-derived SOA, lead to the liquid state of the daytime particles and phase separation after greater uptake of liquid water, despite the lower daytime relative humidity. The observed diel cycle of aerosol phase should provoke rethinking of the SOA atmospheric lifecycle, as it suggests diurnal variability in gas-particle partitioning and mixing time scales, which influence aerosol multiphase chemistry, lifetime, and climate impacts.

New application of direct analysis in real time high-resolution mass spectrometry for the untargeted analysis of fresh and aged secondary organic aerosols generated from monoterpenes

RATIONALE

Secondary organic aerosols (SOAs) represent a significant portion of total atmospheric aerosols. They are generated by the oxidation of volatile organic compounds (VOCs), and particularly biogenic VOCs (BVOCs). The analysis of such samples is usually performed by targeted methods that often require time-consuming preparation steps that can induce loss of compounds and/or sample contaminations.

METHODS

Recently, untargeted methods using high-resolution mass spectrometry (HRMS) have been successfully employed for a broad characterization of chemicals in SOAs. Herein we propose a new application of the direct analysis in real time (DART) ionization method combined with HRMS to quickly detect several hundred chemicals in SOAs collected on a quartz filter without sample preparation or separation techniques.

RESULTS

The reproducibility of measurements was good, with several hundred elemental compositions common to three different replicates. The relative standard deviations of the intensities of the chemical families ranged from 6% to 35%, with sufficient sensitivity to allow the unambiguous detection of 4 ng/mm² of pinic acid. The presence of oligomers and specific tracers was highlighted by MSⁿ (n ≤ 4) experiments, an achievement that is difficult to attain with other ultrahigh-resolution mass spectrometers. Contributions of this untargeted DART-HRMS method were illustrated by the analysis of fresh and aged SOAs from different gaseous precursors such as limonene, a β-pinene/limonene mixture or scots pines emissions.

CONCLUSIONS

The results show that it is possible to use DART-HRMS for the identification of tracers of specific aging reactions, or for the identification of aerosols from specific biogenic precursors.

Identification of the Criegee intermediate reaction network in ethylene ozonolysis: impact on energy conversion strategies and atmospheric chemistry

The reaction network of the simplest Criegee intermediate (CI) CH₂OO has been studied experimentally during the ozonolysis of ethylene.

Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry

Organic peroxy radicals (RO₂) are key intermediates in the atmospheric degradation of organic matter and fuel combustion, but to date, few direct studies of specific RO₂ in complex reaction systems exist, leading to large gaps in our understanding of their fate. We show, using direct, speciated measurements of a suite of RO₂ and gas-phase dimers from O₃-initiated oxidation of α-pinene, that ∼150 gaseous dimers (C_16-20H_24-34O_4-13) are primarily formed through RO₂ cross-reactions, with a typical rate constant of 0.75-2 × 10^-12 cm³ molecule^-1 s^-1 and a lower-limit dimer formation branching ratio of 4%. These findings imply a gaseous dimer yield that varies strongly with nitric oxide (NO) concentrations, of at least 0.2-2.5% by mole (0.5-6.6% by mass) for conditions typical of forested regions with low to moderate anthropogenic influence (i.e., ≤50-parts per trillion NO). Given their very low volatility, the gaseous C_16-20 dimers provide a potentially important organic medium for initial particle formation, and alone can explain 5-60% of α-pinene secondary organic aerosol mass yields measured at atmospherically relevant particle mass loadings. The responses of RO₂, dimers, and highly oxygenated multifunctional compounds (HOM) to reacted α-pinene concentration and NO imply that an average ∼20% of primary α-pinene RO₂ from OH reaction and 10% from ozonolysis autoxidize at 3-10 s^-1 and ≥1 s^-1, respectively, confirming both oxidation pathways produce HOM efficiently, even at higher NO concentrations typical of urban areas. Thus, gas-phase dimer formation and RO₂ autoxidation are ubiquitous sources of low-volatility organic compounds capable of driving atmospheric particle formation and growth.

Formation and Evolution of aqSOA from Aqueous-Phase Reactions of Phenolic Carbonyls: Comparison between Ammonium Sulfate and Ammonium Nitrate Solutions

We investigate the effects of sulfate and nitrate on the formation and evolution of secondary organic aerosol formed in the aqueous phase (aqSOA) from photooxidation of two phenolic carbonyls emitted from wood burning. AqSOA was formed efficiently from the photooxidation of both syringaldehyde (C₉H₁₀O₄) and acetosyringone (C₁₀H₁₂O₄) in ammonium sulfate and ammonium nitrate solutions, with mass yields ranging from 30% to 120%. Positive matrix factorization on the organic mass spectra acquired by an Aerosol Mass Spectrometer revealed a combination of functionalization, oligomerization, and fragmentation processes in the chemical evolution of aqSOA. Functionalization and oligomerization dominated in the first 4 h of reaction, with phenolic oligomers and their derivatives significantly contributing to aqSOA formation; and oxidation of the first-generation products led to an abundance of oxygenated ring-opening products. Degradation rates of syringaldehyde and acetosyringone in nitrate solutions were 1.5 and 3.5 times faster than rates in sulfate solutions, and aqSOA yields in nitrate experiments are twice as high as in sulfate experiments. Nitrate likely promoted the reactions because it is a photolytic source of OH radicals, while sulfate is not, highlighting the importance of aerosol-phase nitrate in the formation of aqSOA by facilitating the photooxidation of organic precursors.

Synergistic O3 + OH oxidation pathway to extremely low-volatility dimers revealed in β-pinene secondary organic aerosol

Dimeric compounds contribute significantly to the formation and growth of atmospheric secondary organic aerosol (SOA) derived from monoterpene oxidation. However, the mechanisms of dimer production, in particular the relevance of gas- vs. particle-phase chemistry, remain unclear. Here, through a combination of mass spectrometric, chromatographic, and synthetic techniques, we identify a suite of dimeric compounds (C_15-19H_24-32O_5-11) formed from concerted O₃ and OH oxidation of β-pinene (i.e., accretion of O₃- and OH-derived products/intermediates). These dimers account for an appreciable fraction (5.9-25.4%) of the β-pinene SOA mass and are designated as extremely low-volatility organic compounds. Certain dimers, characterized as covalent dimer esters, are conclusively shown to form through heterogeneous chemistry, while evidence of dimer production via gas-phase reactions is also presented. The formation of dimers through synergistic O₃ + OH oxidation represents a potentially significant, heretofore-unidentified source of low-volatility monoterpene SOA. This reactivity also suggests that the current treatment of SOA formation as a sum of products originating from the isolated oxidation of individual precursors fails to accurately reflect the complexity of oxidation pathways at play in the real atmosphere. Accounting for the role of synergistic oxidation in ambient SOA formation could help to resolve the discrepancy between the measured atmospheric burden of SOA and that predicted by regional air quality and global climate models.

Photolysis of glutaraldehyde in brine: A showcase study for removal of a common biocide in oil and gas produced water

Glutaraldehyde (GA) has been used extensively as a biocide in hydraulic fracturing fluid leading to its presence in oil and gas produced water. In this study, photolysis was used to degrade GA from brine solutions simulating produced water. Photolysis of GA was performed under ultraviolet (UV) irradiation. GA can be photolyzed by UV at all studied conditions with the efficiency ranging from 52 to 85% within one hour irradiation. Photolysis of GA followed pseudo-first order kinetics. A photolysis rate constant of GA at 0.1 mM in 200 g/L of salt at pH 7 was 0.0269 min^-1 with a quantum yield of 0.0549 under 224 W illumination. The degradation rate of GA increased with increasing incident light intensity and decreasing pH. Increasing initial GA concentration resulted in decreasing degradation rate of GA. The degradation of GA was affected by salt concentration. At lower salt concentrations, notable retardation of GA photodegradation rate was observed while at higher salt concentrations GA photodegradation was improved compared to those without salt. OH was more dominant in sample without salt than sample with salt suggesting different photolytic mechanisms, indirect and direct photolysis, respectively. Oligomers were identified as the main photoproducts of GA photolysis.

Nitrogen-Containing, Light-Absorbing Oligomers Produced in Aerosol Particles Exposed to Methylglyoxal, Photolysis, and Cloud Cycling

Aqueous methylglyoxal chemistry has often been implicated as an important source of oligomers in atmospheric aerosol. Here we report on chemical analysis of brown carbon aerosol particles collected from cloud cycling/photolysis chamber experiments, where gaseous methylglyoxal and methylamine interacted with glycine, ammonium, or methylammonium sulfate seed particles. Eighteen N-containing oligomers were identified in the particulate phase by liquid chromatography/diode array detection/electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry. Chemical formulas were determined and, for 6 major oligomer products, MS² fragmentation spectra were used to propose tentative structures and mechanisms. Electronic absorption spectra were calculated for six tentative product structures by an ab initio second order algebraic-diagrammatic-construction/density functional theory approach. For five structures, matching calculated and measured absorption spectra suggest that they are dominant light-absorbing species at their chromatographic retention times. Detected oligomers incorporated methylglyoxal and amines, as expected, but also pyruvic acid, hydroxyacetone, and significant quantities of acetaldehyde. The finding that ∼80% (by mass) of detected oligomers contained acetaldehyde, a methylglyoxal photolysis product, suggests that daytime methylglyoxal oligomer formation is dominated by radical addition mechanisms involving CH₃CO*. These mechanisms are evidently responsible for enhanced browning observed during photolytic cloud events.

Formation mechanism of secondary organic aerosol from ozonolysis of gasoline vehicle exhaust

Gasoline vehicles are a major source of anthropogenic secondary organic aerosols (SOAs). However, current models based on known precursors fail to explain the substantial SOAs from vehicle emissions due to the inadequate understanding of the formation mechanism. To provide more information on this issue, the formation of SOAs from ozonolysis of four light-duty gasoline vehicle exhaust systems was investigated with a vacuum ultraviolet photoionization mass spectrometer (VUV-PIMS). Remarkable SOAs formation was observed and the SOAs were primarily aliphatic alkenes. PI mass spectra of the SOAs from all vehicles exhibited similar spectral patterns (a regular mass group with m/z at 98, 112, 126 …). Interestingly, most carbonyl products of aliphatic alkenes observed as major gaseous products have specific molecular weights, and the main formation pathway of SOAs can be explained well using aldol condensation reactions of these carbonyls. This is a direct observation of the aldol condensation as a dominated pathway for SOAs formation, and the first report on the composition and formation mechanism of the SOAs from the ozonolysis of gasoline vehicle exhaust is given. The study reveals that low molecular weight alkenes may play a more significant role in vehicle-induced SOAs formation than previously believed. More importantly, the PI mass spectra of SOAs from vehicles show similarities to the field aerosol sample mass spectra, suggesting the possible significance of the aldol condensation reactions in ambient aerosol formation. Since carbonyls are a major degradation product of biogenic and anthropogenic VOCs through atmospheric oxidation processes, the mechanism proposed in this study can be applied more generally to explain aerosol formation from the oxidation of atmospheric hydrocarbons.

Growth Kinetics and Size Distribution Dynamics of Viscous Secondary Organic Aerosol

Low bulk diffusivity inside viscous semisolid atmospheric secondary organic aerosol (SOA) can prolong equilibration time scale, but its broader impacts on aerosol growth and size distribution dynamics are poorly understood. Here, we present quantitative insights into the effects of bulk diffusivity on the growth and evaporation kinetics of SOA formed under dry conditions from photooxidation of isoprene in the presence of a bimodal aerosol consisting of Aitken (ammonium sulfate) and accumulation (isoprene or α-pinene SOA) mode particles. Aerosol composition measurements and evaporation kinetics indicate that isoprene SOA is composed of several semivolatile organic compounds (SVOCs), with some reversibly reacting to form oligomers. Model analysis shows that liquid-like bulk diffusivities can be used to fit the observed evaporation kinetics of accumulation mode particles but fail to explain the growth kinetics of bimodal aerosol by significantly under-predicting the evolution of the Aitken mode. In contrast, the semisolid scenario successfully reproduces both evaporation and growth kinetics, with the interpretation that hindered partitioning of SVOCs into large viscous particles effectively promotes the growth of smaller particles that have shorter diffusion time scales. This effect has important implications for the growth of atmospheric ultrafine particles to climatically active sizes.

Spiers Memorial Lecture. Introductory lecture: chemistry in the urban atmosphere

The urban atmosphere is characterised by a multitude of complex processes. Gaseous and particulate components are continuously emitted into the atmosphere from many different sources. These components are then dispersed in the urban atmosphere via turbulent mixing. Numerous chemical reactions modify the gas phase chemistry on multiple time scales, producing secondary pollutants. Through partitioning, the chemical and physical properties of the aerosol particles are also constantly changing as a consequence of dispersion and gas phase chemistry. This review presents an overview of the involved processes, focusing on the contributions presented at this conference and putting them into a broader context. Advanced methods for aerosol source apportionment are presented as well, followed by some aspects of health effects related to air pollution.

Trends in the oxidation and relative volatility of chamber-generated secondary organic aerosol

The relationship between the oxidation state and relative volatility of secondary organic aerosol (SOA) from the oxidation of a wide range of hydrocarbons is investigated using a fast-stepping, scanning thermodenuder interfaced with a high-resolution time-of-flight aerosol mass spectrometer (AMS). SOA oxidation state varied widely across the investigated range of parent hydrocarbons but was relatively stable for replicate experiments using a single hydrocarbon precursor. On average, unit mass resolution indicators of SOA oxidation (e.g., AMS f 43 and f 44) are consistent with previously reported values. Linear regression of H:C vs. O:C obtained from parameterization of f 43 and f 44 and elemental analysis of high-resolution spectra in Van Krevelen space both yield a slope of ~-0.5 across different SOA types. A similar slope was obtained for a distinct subset of toluene/NO x reactions in which the integrated oxidant exposure was varied to alter oxidation. The relative volatility of different SOA types displays similar variability and is strongly correlated with SOA oxidation state (OS - ). On average, relatively low oxidation and volatility were observed for aliphatic alkene (including terpenes) and n-alkane SOA while the opposite is true for mono- and polycyclic aromatic hydrocarbon SOA. Effective enthalpy for total chamber aerosol obtained from volatility differential mobility analysis is also highly correlated withOS - c indicating a primary role for oxidation levels in determining the volatility of chamber SOA. Effective enthalpies for chamber SOA are substantially lower than those of neat organic standards but are on the order of those obtained for partially oligomerized glyoxal and methyl glyoxal.

Growth of organic films on indoor surfaces

We present a model for the growth of organic films on impermeable indoor surfaces. The model couples transport through a gas-side boundary layer adjacent to the surface with equilibrium partitioning of semivolatile organic compounds (SVOCs) between the gas phase and the surface film. Model predictions indicate that film growth would primarily be influenced by the gas-phase concentration of SVOCs with octanol-air partitioning (K_oa ) values in the approximate range 10≤log K_oa ≤13. Within the relevant range, SVOCs with lower values will equilibrate with the surface film more rapidly. Over time, the film becomes relatively enriched in species with higher log K_oa values, while the proportion of gas-phase SVOCs not in equilibrium with the film decreases. Given stable airborne SVOC concentrations, films grow at faster rates initially and then subsequently diminish to an almost steady growth rate. Once an SVOC is equilibrated with the film, its mass per unit film volume remains constant, while its mass per unit area increases in proportion to overall film thickness. The predictions of the conceptual model and its mathematical embodiment are generally consistent with results reported in the peer-reviewed literature.

The effect of sub-zero temperature on the formation and composition of secondary organic aerosol from ozonolysis of alpha-pinene

This study presents a newly constructed temperature controlled cold-room smog chamber at Aarhus University, Denmark. The chamber is herein utilized to study the effect of sub-zero temperature on the formation and chemical composition of secondary organic aerosol (SOA) from ozone initiated oxidation of α-pinene. The chemical composition of α-pinene SOA formed from dark ozonolysis of α-pinene at 293 K and 258 K was investigated using High-Resolution Time-of-Flight Aerosol Mass Spectrometry (HR-ToF-AMS) and Ultra-High Performance Liquid Chromatography/Electrospray Ionization Quadrupole Time-of-Flight Mass Spectrometry (UHPLC/ESI-qToF-MS). For comparison, an OH-initiated oxidation experiment was performed at 293 K. In ozonolysis experiments it was found that oxygen-to-carbon (O : C) ratios were higher in the particles formed at 293 K compared to 258 K. A total of 16 different organic acids and 30 dimers esters were quantified in the collected particles composing up to 34% of the total α-pinene SOA mass with increased mass fraction of carboxylic acids in particles from α-pinene ozonolysis at 258 K compared to 293 K. In contrast, dimer esters showed suppressed formation at the sub-zero reaction temperature, thus contributing 3% to SOA mass at 258 K while contributing 9% at 293 K. SOA formed in the OH-initiated oxidation of α-pinene at 293 K resulted in low concentrations of dimer esters supporting Criegee intermediates as a possible pathway to dimer ester formation. Vapour pressure estimates of the identified carboxylic acids and dimer esters are presented and show how otherwise semi-volatile carboxylic acids at sufficiently low temperatures may classify as low or even extremely low volatile organic compounds (ELVOC), thus may add to an enhanced particle formation observed at the sub-zero temperature through gas-to-particle conversion. The change in chemical composition of the SOA particles with temperature is ascribed to a combination of effects: the decreased vapour pressures and hence increased condensation of carboxylic acids from the gas phase to the particle phase along with suppressed formation of the high molecular weight dimer esters and different gas and particle phase chemistry results in particles of different chemical composition as a consequence of low reaction temperatures.