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

Emerging investigator series: secondary organic aerosol formation from photooxidation of acyclic terpenes in an oxidation flow reactor

One major challenge in predicting secondary organic aerosol (SOA) formation in the atmosphere is incomplete representation of biogenic volatile organic compounds (BVOCs) emitted from plants, particularly those that are emitted as a result of stress - a condition that is becoming more frequent in a rapidly changing climate. One of the most common types of BVOCs emitted by plants in response to environmental stress are acyclic terpenes. In this work, SOA is generated from the photooxidation of acyclic terpenes in an oxidation flow reactor and compared to SOA production from a reference cyclic terpene - α-pinene. The acyclic terpenes used as SOA precursors included β-myrcene, β-ocimene, and linalool. Results showed that oxidation of all acyclic terpenes had lower SOA yields measured after 4 days photochemical age, in comparison to α-pinene. However, there was also evidence that the condensed organic products that formed, while a smaller amount overall, had a higher oligomeric content. In particular, β-ocimene SOA had higher oligomeric content than all the other chemical systems studied. SOA composition data from ultra-high performance liquid chromatography with electrospray ionization mass spectrometry (UHPLC-ESI-MS) was combined with mechanistic modeling using the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) to explore chemical mechanisms that could lead to this oligomer formation. Calculations based on composition data suggested that β-ocimene SOA was more viscous with a higher glass transition temperature than other SOA generated from acyclic terpene oxidation. This was attributed to a higher oligomeric content compared to other SOA systems studied. These results contribute to novel chemical insights about SOA formation from acyclic terpenes and relevant chemistry processes, highlighting the importance of improving underrepresented biogenic SOA formation in chemical transport models.

Interactions of peroxy radicals from monoterpene and isoprene oxidation simulated in the radical volatility basis set

Isoprene affects new particle formation rates in environments and experiments also containing monoterpenes. For the most part, isoprene reduces particle formation rates, but the reason is debated. It is proposed that due to its fast reaction with OH, isoprene may compete with larger monoterpenes for oxidants. However, by forming a large amount of peroxy-radicals (RO2), isoprene may also interfere with the formation of the nucleating species compared to a purely monoterpene system. We explore the RO2 cross reactions between monoterpene and isoprene oxidation products using the radical Volatility Basis Set (radical-VBS), a simplified reaction mechanism, comparing with observations from the CLOUD experiment at CERN. We find that isoprene interferes with covalently bound C20 dimers formed in the pure monoterpene system and consequently reduces the yields of the lowest volatility (Ultra Low Volatility Organic Carbon, ULVOC) VBS products. This in turn reduces nucleation rates, while having less of an effect on subsequent growth rates.

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.

Photooxidation-Initiated Aqueous-Phase Formation of Organic Peroxides: Delving into Formation Mechanisms

Formation of highly oxygenated molecules (HOMs) such as organic peroxides (ROOR, ROOH, and H2O2) is known to degrade food and organic matter. Gas-phase unimolecular autoxidation and bimolecular RO2 + HO2/RO2 reactions are prominently renowned mechanisms associated with the formation of peroxides. However, the reaction pathways and conditions favoring the generation of peroxides in the aqueous phase need to be evaluated. Here, we identified bulk aqueous-phase ROOHs in varying organic precursors, including a laboratory model compound and monoterpene oxidation products. Our results show that formation of ROOHs is suppressed at enhanced oxidant concentrations but exhibits complex trends at elevated precursor concentrations. Furthermore, we observed an exponential increase in the yield of ROOHs when UV light with longer wavelengths was used in the experiment, comparing UVA, UVB, and UVC. Water-soluble organic compounds represent a significant fraction of ambient cloud-water components (up to 500 μM). Thus, the reaction pathways facilitating the formation of HOMs (i.e., ROOHs) during the aqueous-phase oxidation of water-soluble species add to the climate and health burden of atmospheric particulate matter.

Uncovering the dominant contribution of intermediate volatility compounds in secondary organic aerosol formation from biomass-burning emissions

Organic vapors from biomass burning are a major source of secondary organic aerosols (SOAs). Previous smog chamber studies found that the SOA contributors in biomass-burning emissions are mainly volatile organic compounds (VOCs). While intermediate volatility organic compounds (IVOCs) are efficient SOA precursors and contribute a considerable fraction of biomass-burning emissions, their contribution to SOA formation has not been directly observed. Here, by deploying a newly-developed oxidation flow reactor to study SOA formation from wood burning, we find that IVOCs can contribute ∼70% of the formed SOA, i.e. >2 times more than VOCs. This previously missing SOA fraction is interpreted to be due to the high wall losses of semi-volatile oxidation products of IVOCs in smog chambers. The finding in this study reveals that SOA production from biomass burning is much higher than previously thought, and highlights the urgent need for more research on the IVOCs from biomass burning and potentially other emission sources.

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.

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.

Molecular and Structural Characterization of Isomeric Compounds in Atmospheric Organic Aerosol Using Ion Mobility-Mass Spectrometry

Secondary organic aerosol (SOA) formed through multiphase atmospheric chemistry makes up a large fraction of airborne particles. The chemical composition and molecular structures of SOA constituents vary between different emission sources and aging processes in the atmosphere, which complicates their identification. In this work, we employ drift tube ion mobility spectrometry with quadrupole time-of-flight mass spectrometry (IM-MS) detection for rapid gas-phase separation and multidimensional characterization of isomers in two biogenic SOAs produced from ozonolysis of isomeric monoterpenes, d-limonene (LSOA) and α-pinene (PSOA). SOA samples were ionized using electrospray ionization (ESI) and characterized using IM-MS in both positive and negative ionization modes. The IM-derived collision cross sections in nitrogen gas (^DTCCS_N2 ) for individual SOA components were obtained using multifield and single-field measurements. A novel application of IM multiplexing/high-resolution demultiplexing methodology was employed to increase sensitivity, improve peak shapes, and augment mobility baseline resolution, which revealed several isomeric structures for the measured ions. For LSOA and PSOA samples, we report significant structural differences of the isomer structures. Molecular structural calculations using density functional theory combined with the theoretical modeling of CCS values provide insights into the structural differences between LSOA and PSOA constituents. The average ^DTCCS_N2 values for monomeric SOA components observed as [M + Na]⁺ ions are 3-6% higher than those of their [M - H]^- counterparts. Meanwhile, dimeric and trimeric isomer components in both samples showed an inverse trend with the relevant values of [M - H]^- ions being 3-7% higher than their [M + Na]⁺ counterparts, respectively. The results indicate that the structures of Na⁺-coordinated oligomeric ions are more compact than those of the corresponding deprotonated species. The coordination with Na⁺ occurs on the oxygen atoms of the carbonyl groups leading to a compact configuration. Meanwhile, deprotonated molecules have higher ^DTCCS_N2 values due to their elongated structures in the gas phase. Therefore, ^DTCCS_N2 values of isomers in SOA mixtures depend strongly on the mode of ionization in ESI. Additionally, PSOA monomers and dimers exhibit larger ^DTCCS_N2 values (1-4%) than their LSOA counterparts owing to more rigid structures. A cyclobutane ring is present with functional groups pointing in opposite directions in PSOA compounds, as compared to noncyclic flexible LSOA structures, forming more compact ions in the gas phase. Lastly, we investigated the effects of direct photolysis on the chemical transformations of selected individual PSOA components. We use IM-MS to reveal structural changes associated with aerosol aging by photolysis. This study illustrates the detailed molecular and structural descriptors for the detection and annotation of structural isomers in complex SOA mixtures.

Effect of OH scavengers on the chemical composition of α-pinene secondary organic aerosol

OH scavengers are extensively used in studies of secondary organic aerosol (SOA) because they create an idealized environment where only a single oxidation pathway is occurring. Here, we present a detailed molecular characterization of SOA produced from α-pinene + O₃ with a variety of OH scavengers using the extractive electrospray time-of-flight mass spectrometer in our atmospheric simulation chamber, which is complemented by characterizing the gas phase composition in flow reactor experiments. Under our experimental conditions, radical chemistry largely controls the composition of SOA. Besides playing their desired role in suppressing the reaction of α-pinene with OH, OH scavengers alter the reaction pathways of radicals produced from α-pinene + O₃. This involves changing the HO₂ : RO₂ ratio, the identity of the RO₂ radicals present, and the RO₂ major sinks. As a result, the use of the OH scavengers has significant effects on the composition of SOA, including inclusions of scavenger molecules in SOA, the promotion of fragmentation reactions, and depletion of dimers formed via α-pinene RO₂-RO₂ reactions. To date fragmentation reactions and inclusion of OH scavenger products into secondary organic aerosol have not been reported in atmospheric simulation chamber studies. Therefore, care should be considered if and when to use an OH scavenger during experiments.

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.

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.

Exposure to naphthalene and β-pinene-derived secondary organic aerosol induced divergent changes in transcript levels of BEAS-2B cells

The health effects of exposure to secondary organic aerosols (SOAs) are still limited. Here, we investigated and compared the toxicities of soot particles (SP) coated with β-pinene SOA (SOAβPin-SP) and SP coated with naphthalene SOA (SOANap-SP) in a human bronchial epithelial cell line (BEAS-2B) residing at the air-liquid interface. SOAβPin-SP mostly contained oxygenated aliphatic compounds from β-pinene photooxidation, whereas SOANap-SP contained a significant fraction of oxygenated aromatic products under similar conditions. Following exposure, genome-wide transcriptome responses showed an Nrf2 oxidative stress response, particularly for SOANap-SP. Other signaling pathways, such as redox signaling, inflammatory signaling, and the involvement of matrix metalloproteinase, were identified to have a stronger impact following exposure to SOANap-SP. SOANap-SP also induced a stronger genotoxicity response than that of SOAβPin-SP. This study elucidated the mechanisms that govern SOA toxicity and showed that, compared to SOAs derived from a typical biogenic precursor, SOAs from a typical anthropogenic precursor have higher toxicological potency, which was accompanied with the activation of varied cellular mechanisms, such as aryl hydrocarbon receptor. This can be attributed to the difference in chemical composition; specifically, the aromatic compounds in the naphthalene-derived SOA had higher cytotoxic potential than that of the β-pinene-derived SOA.

Particle Formation from Photooxidation of αpinene, Limonene, and Myrcene

We present measurements of the effect of first-generation secondary organic aerosol (SOA) material on the growth of ∼10 nanometer diameter seed particles composed of sulfuric acid and water. Experiments were performed in an atmospheric pressure, vertically aligned flow reactor where OH was produced from HONO photolysis in the presence of either SO₂ or a monoterpene. For typical conditions, organic compounds at ∼300 ppbv are exposed to photooxidation for a time of ∼80 s at a [OH] of about 6 × 10⁶ cm^-3: thus, oxidation products have minimal OH exposure. The measured size changes of the sulfuric acid seed particles can then be attributed to the uptake of first-generation products. Along with descriptions of the apparatus and the procedure, the analysis to obtain SOA yields by comparing them to growth with H₂SO₄(g) is detailed. Results from photooxidation experiments of αpinene, limonene, and myrcene give SOA yields of 0.040, 0.084, and 0.16, respectively. These SOA yields roughly double with each addition of a double bond to the compound. The αpinene and limonene results are in accord with the results of many previous SOA experiments, while the myrcene SOA yield stands alone. Photooxidation of myrcene also led to significant nucleation, and the species responsible is comparable to H₂SO₄ at a 35% relative humidity in its nucleation capability.

Effect of Atmospheric Aging on Soot Particle Toxicity in Lung Cell Models at the Air–Liquid Interface: Differential Toxicological Impacts of Biogenic and Anthropogenic Secondary Organic Aerosols (SOAs)

Background:

Secondary organic aerosols (SOAs) formed from anthropogenic or biogenic gaseous precursors in the atmosphere substantially contribute to the ambient fine particulate matter [PM ≤2.5μm in aerodynamic diameter (PM2.5)] burden, which has been associated with adverse human health effects. However, there is only limited evidence on their differential toxicological impact.

Objectives:

We aimed to discriminate toxicological effects of aerosols generated by atmospheric aging on combustion soot particles (SPs) of gaseous biogenic (β-pinene) or anthropogenic (naphthalene) precursors in two different lung cell models exposed at the air–liquid interface (ALI).

Methods:

Mono- or cocultures of lung epithelial cells (A549) and endothelial cells (EA.hy926) were exposed at the ALI for 4 h to different aerosol concentrations of a photochemically aged mixture of primary combustion SP and β-pinene (SOAβPIN-SP) or naphthalene (SOANAP-SP). The internally mixed soot/SOA particles were comprehensively characterized in terms of their physical and chemical properties. We conducted toxicity tests to determine cytotoxicity, intracellular oxidative stress, primary and secondary genotoxicity, as well as inflammatory and angiogenic effects.

Results:

We observed considerable toxicity-related outcomes in cells treated with either SOA type. Greater adverse effects were measured for SOANAP-SP compared with SOAβPIN-SP in both cell models, whereas the nano-sized soot cores alone showed only minor effects. At the functional level, we found that SOANAP-SP augmented the secretion of malondialdehyde and interleukin-8 and may have induced the activation of endothelial cells in the coculture system. This activation was confirmed by comet assay, suggesting secondary genotoxicity and greater angiogenic potential. Chemical characterization of PM revealed distinct qualitative differences in the composition of the two secondary aerosol types.

Discussion:

In this study using A549 and EA.hy926 cells exposed at ALI, SOA compounds had greater toxicity than primary SPs. Photochemical aging of naphthalene was associated with the formation of more oxidized, more aromatic SOAs with a higher oxidative potential and toxicity compared with β-pinene. Thus, we conclude that the influence of atmospheric chemistry on the chemical PM composition plays a crucial role for the adverse health outcome of emissions. https://doi.org/10.1289/EHP9413

Theoretical study of the formation and nucleation mechanism of highly oxygenated multi-functional organic compounds produced by α-pinene

In recent years, highly oxygenated organic molecules (HOMs) derived from photochemical reactions of α-pinene, the most abundant monoterpene, have been considered as important precursors of biogenic particles. However, the specific reactions of HOMs remain largely unknown, especially the corresponding formation and nucleation mechanism in the nanoscale. In this study, we implemented quantum chemical calculations and molecular dynamics (MD) simulations to explore the mechanism of the formation of HOM monomers/dimers by ozonolysis and autoxidation of α-pinene. Furthermore, we investigated the mechanisms of HOMs with different oxygen-to‑carbon (O/C) ratios and functional groups participating in neutral and ion-induced nucleation. The results show that the formation of HOMs is hardly affected by water, sulfuric acid and ions. In the ion-induced nucleation, HOM can dominate the initial nucleation steps; however, in the neutral nucleation, HOMs are more likely to participate in the growth stage. In addition, the nucleation ability of HOM has a bearing on the O/C ratio and the types of the functional groups. The current calculations provide valuable insight into the formation mechanism of the pure organic particles at low sulfuric acid concentrations.

Gas-to-Particle Partitioning of Cyclohexene- and α-Pinene-Derived Highly Oxygenated Dimers Evaluated Using COSMOtherm

Oxidized organic compounds are expected to contribute to secondary organic aerosol (SOA) if they have sufficiently low volatilities. We estimated saturation vapor pressures and activity coefficients (at infinite dilution in water and a model water-insoluble organic phase) of cyclohexene- and α-pinene-derived accretion products, “dimers”, using the COSMOtherm19 program. We found that these two property estimates correlate with the number of hydrogen bond-donating functional groups and oxygen atoms in the compound. In contrast, when the number of H-bond donors is fixed, no clear differences are seen either between functional group types (e.g., OH or OOH as H-bond donors) or the formation mechanisms (e.g., gas-phase radical recombination vs liquid-phase closed-shell esterification). For the cyclohexene-derived dimers studied here, COSMOtherm19 predicts lower vapor pressures than the SIMPOL.1 group-contribution method in contrast to previous COSMOtherm estimates using older parameterizations and nonsystematic conformer sampling. The studied dimers can be classified as low, extremely low, or ultra-low-volatility organic compounds based on their estimated saturation mass concentrations. In the presence of aqueous and organic aerosol particles, all of the studied dimers are likely to partition into the particle phase and thereby contribute to SOA formation.

Superoxide Formation from Aqueous Reactions of Biogenic Secondary Organic Aerosols

Reactive oxygen species (ROS) play a central role in aqueous-phase processing and health effects of atmospheric aerosols. Although hydroxyl radical (^•OH) and hydrogen peroxide (H₂O₂) are regarded as major oxidants associated with secondary organic aerosols (SOA), the kinetics and reaction mechanisms of superoxide (O₂^•-) formation are rarely quantified and poorly understood. Here, we demonstrate a dominant formation of O₂^•- with molar yields of 0.01-0.03% from aqueous reactions of biogenic SOA generated by ^•OH photooxidation of isoprene, β-pinene, α-terpineol, and d-limonene. The temporal evolution of ^•OH and O₂^•- formation is elucidated by kinetic modeling with a cascade of aqueous reactions including the decomposition of organic hydroperoxides, ^•OH oxidation of primary or secondary alcohols, and unimolecular decomposition of α-hydroxyperoxyl radicals. Relative yields of various types of ROS reflect a relative abundance of organic hydroperoxides and alcohols contained in SOA. These findings and mechanistic understanding have important implications on the atmospheric fate of SOA and particle-phase reactions of highly oxygenated organic molecules as well as oxidative stress upon respiratory deposition.

Coupling Filter-Based Thermal Desorption Chemical Ionization Mass Spectrometry with Liquid Chromatography/Electrospray Ionization Mass Spectrometry for Molecular Analysis of Secondary Organic Aerosol

Filter-based thermal desorption (F-TD) techniques, such as the filter inlet for gases and aerosols, are widely employed to investigate the molecular composition and physicochemical properties of secondary organic aerosol (SOA). Here, we introduce an enhanced capability of F-TD through the combination of a customized F-TD inlet with chemical ionization mass spectrometry (CIMS) and ultraperformance liquid chromatography/electrospray ionization mass spectrometry (UPLC/ESI-MS). The utility of F-TD/CIMS + UPLC/ESI-MS is demonstrated by application to α-pinene ozonolysis SOA for which increased filter aerosol mass loading is shown to slow the evaporation rates of deposited compounds. Evidence for oligomer decomposition producing multimode F-TD/CIMS thermograms is provided by the measurement of the mass fraction remaining of monomeric and dimeric α-pinene oxidation products on the filter via UPLC/ESI-MS. In situ evaporation of aerosol particles suggests that α-pinene-derived hydroperoxides are thermally labile; thus, analysis of particle-phase (hydro)peroxides via F-TD may not be appropriate. A synthesized pinene-derived dimer ester (C₂₀H₃₂O₅) is found to be thermally stable up to 200 °C, whereas particle-phase dimers (C₁₉H₃₀O₅) are observed to form during F-TD analysis via thermally induced condensation of synthesized pinene-derived alcohols and diacids. The complementary F-TD/CIMS + UPLC/ESI-MS method offers previously inaccessible insight into the molecular composition and thermal desorption behavior of SOA that both clarifies and expands on analysis via traditional F-TD techniques.

Synthesis of Carboxylic Acid and Dimer Ester Surrogates to Constrain the Abundance and Distribution of Molecular Products in α-Pinene and β-Pinene Secondary Organic Aerosol

Liquid chromatography/negative electrospray ionization mass spectrometry [LC/(-)ESI-MS] is routinely employed to characterize the identity and abundance of molecular products in secondary organic aerosol (SOA) derived from monoterpene oxidation. Due to a lack of authentic standards, however, commercial terpenoic acids (e.g., cis-pinonic acid) are typically used as surrogates to quantify both monomeric and dimeric SOA constituents. Here, we synthesize a series of enantiopure, pinene-derived carboxylic acid and dimer ester homologues. We find that the (-)ESI efficiencies of the dimer esters are 19-36 times higher than that of cis-pinonic acid, demonstrating that the mass contribution of dimers to monoterpene SOA has been significantly overestimated in past studies. Using the measured (-)ESI efficiencies of the carboxylic acids and dimer esters as more representative surrogates, we determine that molecular products measureable by LC/(-)ESI-MS account for only 21.8 ± 2.6% and 18.9 ± 3.2% of the mass of SOA formed from ozonolysis of α-pinene and β-pinene, respectively. The 28-36 identified monomers (C_7-10H_10-18O_3-6) constitute 15.6-20.5% of total SOA mass, whereas only 1.3-3.3% of the SOA mass is attributable to the 46-62 identified dimers (C_15-19H_24-32O_4-11). The distribution of identified α-pinene and β-pinene SOA molecular products is examined as a function of carbon number (n_C), average carbon oxidation state (OS¯C), and volatility (C*). The observed order-of-magnitude difference in (-)ESI efficiency between monomers and dimers is expected to be broadly applicable to other biogenic and anthropogenic SOA systems analyzed via (-) or (+) LC/ESI-MS under various LC conditions, and demonstrates that the use of unrepresentative surrogates can lead to substantial systematic errors in quantitative LC/ESI-MS analyses of SOA.

Efficient Coupling of Reaction Pathways of Criegee Intermediates and Free Radicals in the Heterogeneous Ozonolysis of Alkenes

In the gas phase, ozonolysis of olefins is known to be a significant source of free radicals. However, for heterogeneous and condensed phase ozone reactions, the importance of reaction pathways that couple Criegee intermediates (CI) with hydroxyl (OH), alkoxy, and peroxy free radicals remains uncertain. Here we report experimental evidence for substantial free radical oxidation during the heterogeneous reaction of O₃ with cis-9-tricosene (Tri) aerosol. A kinetic model with three coupled submechanisms that include O₃, CI, and free radical reactions is used to explain how the observed Tri reactivity and its product distributions depend upon [O₃], [OH], and the presence of CI scavengers. During multiphase ozonolysis, the kinetic model predicts that only ∼30% of the alkene is actually consumed by O₃, while the remaining ∼70% is consumed by free radicals that cycle through pathways involving CI. These results reveal the importance of free radical oxidation during heterogeneous ozonolysis, which has been previously difficult to isolate due to the complex coupling of CI and OH reaction pathways.

Coupling an online ion conductivity measurement with the particle-into-liquid sampler: Evaluation and modeling using laboratory and field aerosol data

A particle-into-liquid sampler (PILS) was coupled to a flow-through conductivity cell to provide a continuous, nondestructive, online measurement in support of offline ion chromatography analysis. The conductivity measurement provides a rapid assessment of the total ion concentration augmenting slower batch-sample data from offline analysis and is developed primarily to assist airborne measurements, where fast time-response is essential. A conductivity model was developed for measured ions and excellent closure was derived for laboratory-generated aerosols (97% conductivity explained, R² > 0.99). The PILS-conductivity measurement was extensively tested throughout the NASA Cloud, Aerosol and Monsoon Processes: Philippines Experiment (CAMP²Ex) during nineteen research flights. A diverse range of ambient aerosol was sampled from biomass burning, fresh and aged urban pollution, and marine sources. Ambient aerosol did not exhibit the same degree of closure as the laboratory aerosol, with measured ions only accountable for 43% of the conductivity. The remaining fraction of the conductivity was examined in combination with ion charge balance and found to provide additional supporting information for diagnosing and modeling particle acidity. An urban plume case study was used to demonstrate the utility of the measurement for supplementing compositional data and augmenting the temporal capability of the PILS.

Photolysis Controls Atmospheric Budgets of Biogenic Secondary Organic Aerosol

Secondary organic aerosol (SOA) accounts for a large fraction of the tropospheric particulate matter. Although SOA production rates and mechanisms have been extensively investigated, loss pathways remain uncertain. Most large-scale chemistry and transport models account for mechanical deposition of SOA but not chemical losses such as photolysis. There is also a paucity of laboratory measurements of SOA photolysis, which limits how well photolytic losses can be modeled. Here, we show, through a combined experimental and modeling approach, that photolytic loss of SOA mass significantly alters SOA budget predictions. Using environmental chamber experiments at variable relative humidity between 0 and 60%, we find that SOA produced from several biogenic volatile organic compounds undergoes photolysis-induced mass loss at rates between 0 and 2.2 ± 0.4% of nitrogen dioxide (NO₂) photolysis, equivalent to average atmospheric lifetimes as short as 10 h. We incorporate our photolysis rates into a regional chemical transport model to test the sensitivity of predicted SOA mass concentrations to photolytic losses. The addition of photolysis causes a ∼50% reduction in biogenic SOA loadings over the Amazon, indicating that photolysis exerts a substantial control over the atmospheric SOA lifetime, with a likely dependence upon the SOA molecular composition and thus production mechanisms.

Resolving ambient organic aerosol formation and aging pathways with simultaneous molecular composition and volatility observations

Organic aerosol (OA) constitutes a significant fraction of atmospheric fine particle mass. However, the precursors and chemical processes responsible for a majority of OA are rarely conclusively identified. We use online observations of hundreds of simultaneously measured molecular components obtained from 15 laboratory OA formation experiments with constraints on their effective saturation vapor concentrations to attribute the VOC precursors and subsequent chemical pathways giving rise to the vast majority of OA mass measured in two forested regions. We find that precursors and chemical pathways regulating OA composition and volatility are dynamic over hours to days, with their variations driven by coupled interactions between multiple oxidants. The extent of physical and photochemical aging, and its modulation by NOx, were key to a uniquely comprehensive combined composition-volatility description of OA. Our findings thus provide some of the most complete mechanistic-level guidance to the development of OA descriptions in air quality and Earth system models.

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.

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.