Rates and Yields of Unimolecular Reactions Producing Highly Oxidized Peroxy Radicals in the OH-Induced Autoxidation of α-Pinene, ß-Pinene, and Limonene.

Recent ambient atmospheric measurements have detected highly oxygenated organic molecules (HOMs) at many sites and are a consequence of autoxidation processes occurring at ambient temperatures. Monoterpenes in particular have a propensity to autoxidize although they exhibit a wide range of HOM yields, which may be due to a variety of reasons including reactions with different oxidants like OH and O3, differing hydrogen (H) atom transfer or peroxy radical cyclization rates, numbers of available reaction pathways, and/or energy loss processes for activated HO-monoterpene or O3-monoterpene adducts. In this work, the autoxidation mechanisms of (+)-α-pinene, (+)-ß-pinene, and (+)-limonene following initial OH oxidation and three successive O2 additions are examined using density functional theory (DFT) to understand what accounts for the disparity. Rates of different potential autoxidation pathways initiated by OH addition or abstraction reactions are quantified using transition-state theory (TST) and master equation approaches using the lowest-energy conformers. OH abstraction reactions do not appreciably influence HOM production in the pinenes and limit autoxidation for limonene because the subsequent autoxidation reactions are slow while OH addition reactions are found to be the main route to HOMs for all three monoterpenes. Generally, faster autoxidation rates are computed in later unimolecular reactions that produce RO7 radicals after OH addition (∼10 s-1 or greater) than rates for RO5 peroxy radical production (0.2-7 s-1). Mechanistic pathways that form RO7 peroxy radicals are similar for all three monoterpenes with a particular bicyclo RO7 radical involving a five-membered peroxide ring being favored for all three monoterpenes. The molar yields of RO7 radicals are 4.6% (+10.0/-2.4), 3.8% (+9.1/-2.6), and 7.6% (+13.1/-4.9) for α-pinene, ß-pinene, and limonene, respectively, at 298 K and 1 ppb of NO and only significantly decline at NO concentrations exceeding 10 ppb. The higher yield for limonene relative to the pinenes is predominantly a consequence of the initial oxidation step: OH adducts of the bicyclic pinenes have to use the excess energy after OH addition to break one of the rings and make the molecule more flexible for autoxidation although this process is inefficient, while one of the prominent OH adducts for monocyclic limonene does not have to do this and may add O2 immediately before autoxidizing further. These insights may be used to guide a better representation of these processes in atmospheric models because they affect particulate matter (PM), NOx, and ozone concentrations via enhanced production of low-volatility species, less early-generation NOx cycling, and altered organic nitrate production.

Quantifying wintertime O3 and NOx formation with relevance vector machines.

This paper uses a machine learning model called a relevance vector machine (RVM) to quantify ozone (O3) and nitrogen oxides (NOx) formation under wintertime conditions. Field study measurements were based on previous work described by Olson et al. (2019), where continuous measurements were reported from a wintertime field study in Utah. RVMs were formulated using either O3 or nitrogen dioxide (NO2) as the output variable. Values of the correlation coefficient (r2) between predicted and measured concentrations were 0.944 for O3 and 0.931 for NO2. RVMs are constructed from the observed measurements and result in sparse model formulations, meaning that only a subset of the data is used to approximate the entire dataset. For this study, the RVM with O3 as the output variable used only 20% of the measurement data while the RVM with NO2 used 16%. RVMs were then used as a predictive model to assess the importance of individual precursors. Using O3 as the output variable, increases in three species resulted in increased O3 concentrations: hydrogen peroxide (H2O2), dinitrogen pentoxide (N2O5), and molecular chlorine (Cl2). For the two termination products measured during the study, nitric acid (HNO3) and formic acid (CH2O2), no change in O3 concentration was observed. Using NO2 as the output variable, only increases in N2O5 resulted in increased NO2 concentrations.

Relative contributions of selected multigeneration products to chamber SOA formed from photooxidation of a range (C10-C17) of n-alkanes under high NO x conditions.

A series of chamber experiments was conducted to investigate the composition of secondary organic aerosol (SOA) following oxidation of a range of parent n-alkanes (C10-C17) in the presence of NO x . The relative contribution of selected species representing first, second, and higher generation products to SOA mass was measured using a high-resolution aerosol mass spectrometer. Gas chromatography was also used for a limited set of amenable species. Relative contributions varied substantially across the range of investigated alkanes reflecting slight changes in SOA composition. The contribution of first-generation cyclic hemiacetal is minimal toward the small end of the investigated range and gradually increase with n-alkane size. The relative contribution of second generation and higher nitrate-containing species, in contrast, decrease with an increased alkane size. A similar trend is observed for relative contribution of organonitrates to SOA. Finally, SOA yield and composition are sensitive to water vapor concentrations. This sensitivity is limited to a narrow range (dry to ~15% RH) with little, if any, impact above 15% suggesting that this impact may be negligible under ambient conditions. The impact of water vapor also appears to decrease with increasing alkane carbon number.

Data mining approaches to understanding the formation of secondary organic aerosol.

This research used data mining approaches to better understand factors affecting the formation of secondary organic aerosol (SOA). Although numerous laboratory and computational studies have been completed on SOA formation, it is still challenging to determine factors that most influence SOA formation. Experimental data were based on previous work described by Offenberg et al. (2017), where volume concentrations of SOA were measured in 139 laboratory experiments involving the oxidation of single hydrocarbons under different operating conditions. Three different data mining methods were used, including nearest neighbor, decision tree, and pattern mining. Both decision tree and pattern mining approaches identified similar chemical and experimental conditions that were important to SOA formation. Among these important factors included the number of methyl groups for the SOA precursor, the number of rings for the SOA precursor, and the presence of dinitrogen pentoxide (N2O5).

Monoterpenes are the largest source of summertime organic aerosol in the southeastern United States.

The chemical complexity of atmospheric organic aerosol (OA) has caused substantial uncertainties in understanding its origins and environmental impacts. Here, we provide constraints on OA origins through compositional characterization with molecular-level details. Our results suggest that secondary OA (SOA) from monoterpene oxidation accounts for approximately half of summertime fine OA in Centreville, AL, a forested area in the southeastern United States influenced by anthropogenic pollution. We find that different chemical processes involving nitrogen oxides, during days and nights, play a central role in determining the mass of monoterpene SOA produced. These findings elucidate the strong anthropogenic-biogenic interaction affecting ambient aerosol in the southeastern United States and point out the importance of reducing anthropogenic emissions, especially under a changing climate, where biogenic emissions will likely keep increasing.

Organic Hydroxy Acids as Highly Oxygenated Molecular (HOM) Tracers for Aged Isoprene Aerosol.

Highly oxygenated molecules (HOMs) are a class of compounds associated with secondary organic aerosols exhibiting high oxygen to carbon (O:C) ratios and often originating from the oxidation of biogenic compounds. Here, the photooxidation and ozonolysis of isoprene were examined under a range of conditions to identify HOM tracers for aged isoprene aerosol. The HOM tracers were identified as silylated derivatives by gas chromatography-mass spectrometry and by detecting their parent compounds by liquid chromatography-high resolution mass spectrometry. In addition to the previously observed methyltetrols and 2-methylglyceric acid, seven tracer compounds were identified, including 2-methyltartronic acid (MTtA), 2-methylerythronic acid (2MeTrA), 3-methylerythronic acid (3MeTrA), 2-methylthreonic acid (2MTrA), 3-methylthreonic acid (3MTrA), erythro-methyltartaric acid (e-MTA), and threo-methyltartaric acid (t-MTA). The molecular structures were confirmed with authentic standards synthesized in the laboratory. The presence of some of these HOMs in the gas and particle phases simultaneously provides evidence of their gas/particle partitioning. To determine the contributions of aged isoprene products to ambient aerosols, we analyzed ambient PM2.5 samples collected in the southeastern United States in summer 2003 and at two European monitoring stations located in Zielonka and Godów (Poland). Our findings show that methyltartaric acids (MTA) and 2- and 3-methylthreonic acids (and their stereoisomers) are representative of aged isoprene aerosol because they occur both in the laboratory chamber aerosol obtained and in ambient PM2.5. On the basis of gas chromatography-mass spectrometry (GC-MS) analysis, their concentrations were found to range from 0.04 ng for 3-methylthreonic acid to 6.3 ng m-3 for methyltartaric acid at the southeast site in Duke Forest, NC, USA.

Multigenerational Theoretical Study of Isoprene Peroxy Radical 1-5-Hydrogen Shift Reactions that Regenerate HO x Radicals and Produce Highly Oxidized Molecules.

A computational protocol is employed to glean new insight into the kinetics of several 1,5-hydrogen atom (H) shift reactions subsequent to first- and second-generation OH/O2 additions to isoprene. The M06-2X density functional was initially used with the Nudged Elastic Band (NEB) method to determine the potential energy surface of OH/O2 addition reactions, the 1,5-H shift reactions, and the fragmentation exit channels. The Master Equation Solver for Multi-Energy Well Reactions (MESMER) was applied to determine the rate constants for OH addition and the 1,5-H shifts. M06-2X was capable of quantifying the rate constants of OH addition to the first and second double bonds of isoprene with deviations less than 17% from the experimentally determined values. However, M06-2X underestimated the 1,5-H shift rate constants of second-generation isoprene peroxy radicals. Consequently, MN15, ωB97X-D, and CBS-QB3 methods were employed to compute average barrier heights to first- and second-generation 1,5-H shifts. In the first generation, the rate constants of H abstraction by ß-(1,2) and (4,3) isoprene hydroxy-peroxy radicals from the neighboring hydroxyl group are 1.1 × 10-3 and 2.4 × 10-3 s-1, respectively. These values are determined primarily by the barrier of the H shift reaction and, to a smaller albeit nonnegligible extent, by the stability of the resulting alkoxy radical and the exit barrier leading to C-C bond dissociation. In contrast, the average second-generation rate constant of 1,5-H shifts from H-R-OH sites to the peroxy radical is 1.8 × 10-1 s-1, with tunneling playing the significant role of increasing this value relative to first-generation 1,5-H shifts. Under low NO x conditions, first-generation isoprene oxidation reactions may recycle HO x at levels ranging from 10 to 30% due in large part to 1,5-H shifts, with the recycling efficiency being sensitive to HO2 concentrations and temperature. HO x recycling is expected to increase to levels beyond 80% in second-generation reactions of oxidized isoprene species because of isoprene epoxydiol (IEPOX) formation and further 1,5-H shifts that are kinetically favorable.

Time series analysis of wintertime O3 and NOx formation using vector autoregressions.

Concentrations of 11 species are reported from continuous measurements taken during a wintertime field study in Utah. Time series data for measured species generally displayed strong diurnal patterns. Six species show a diurnal pattern of daytime maximums (NO, NOy, O3, H2O2, CH2O2, and Cl2), while five species show a diurnal pattern of night time maximums (NO2, HONO, ClNO2, HNO3, and N2O5). Vector autoregression analyses were completed to better understand important species influencing the formation of O3 and NOx. For the species studied, r2 values of predicted versus measured concentrations ranged from 0.82-0.99. Fitting parameters for the autoregressive matrix, Π, indicated the importance of species precursors. In addition, values of fitting parameters for Π were relatively insensitive to data size, with variations generally <10%. Variable causation was quantified using the Granger causation method. Assuming O3 and NOx behave as chemical products, reactants (in order of importance) are as follows: H2O2, N2O5, HONO, and ClNO2.

Photochemical Conversion of Surrogate Emissions for Use in Toxicological Studies: Role of Particulate- and Gas-Phase Products.

The production of photochemical atmospheres under controlled conditions in an irradiation chamber permits the manipulation of parameters that influence the resulting air-pollutant chemistry and potential biological effects. To date, no studies have examined how contrasting atmospheres with a similar Air Quality Health Index (AQHI), but with differing ratios of criteria air pollutants, might differentially affect health end points. Here, we produced two atmospheres with similar AQHIs based on the final concentrations of ozone, nitrogen dioxide, and particulate matter (PM2.5). One simulated atmosphere (SA-PM) generated from irradiation of ∼23 ppmC gasoline, 5 ppmC α-pinene, 529 ppb NO, and 3 µg m-3 (NH4)2SO4 as a seed resulted in an average of 976 µg m-3 PM2.5, 326 ppb NO2, and 141 ppb O3 (AQHI 97.7). The other atmosphere (SA-O3) generated from 8 ppmC gasoline, 5 ppmC isoprene, 874 ppb NO, and 2 µg m-3 (NH4)2SO4 resulted in an average of 55 µg m-3 PM2.5, 643 ppb NO2, and 430 ppb O3 (AQHI of 99.8). Chemical speciation by gas chromatography showed that photo-oxidation degraded the organic precursors and promoted the de novo formation of secondary reaction products such as formaldehyde and acrolein. Further work in accompanying papers describe toxicological outcomes from the two distinct photochemical atmospheres.

Evaluation of an Air Quality Health Index for Predicting the Mutagenicity of Simulated Atmospheres.

No study has evaluated the mutagenicity of atmospheres with a calculated air quality health index (AQHI). Thus, we generated in a UV-light-containing reaction chamber two simulated atmospheres (SAs) with similar AQHIs but different proportions of criteria pollutants and evaluated them for mutagenicity in three Salmonella strains at the air-agar interface. We continuously injected into the chamber gasoline, nitric oxide, and ammonium sulfate, as well as either α-pinene to produce SA-PM, which had a high concentration of particulate matter (PM): 119 ppb ozone (O3), 321 ppb NO2, and 1007 µg/m3 PM2.5; or isoprene to produce SA-O3, which had a high ozone (O3) concentration: 415 ppb O3, 633 ppb NO2, and 55 µg/m3 PM2.5. Neither PM2.5 extracts, NO2, or O3 alone, nor nonphoto-oxidized mixtures were mutagenic or cytotoxic. Both photo-oxidized atmospheres were largely direct-acting base-substitution mutagens with similar mutagenic potencies in TA100 and TA104. The mutagenic potencies [(revertants/h)/(mgC/m3)] of SA-PM (4.3 ± 0.4) and SA-O3 (9.5 ± 1.3) in TA100 were significantly different ( P < 0.0001), but the mutation spectra were not ( P = 0.16), being ∼54% C → T and ∼46% C → A. Thus, the AQHI may have some predictive value for the mutagenicity of the gas phase of air.

Mutagenic atmospheres resulting from the photooxidation of aromatic hydrocarbon and NOx mixtures.

Although many volatile organic compounds (VOCs) are regulated to limit air pollution and the consequent health effects, the photooxidation products generally are not. Thus, we examined the mutagenicity in Salmonella TA100 of photochemical atmospheres generated in a steady-state atmospheric simulation chamber by irradiating mixtures of single aromatic VOCs, NOx, and ammonium sulfate seed aerosol in air. The 10 VOCs examined were benzene; toluene; ethylbenzene; o-, m-, and p-xylene; 1,2,4- and 1,3,5-trimethylbenzene; m-cresol; and naphthalene. Salmonella were exposed at the air-agar interface to the generated atmospheres for 1, 2, 4, 8, or 16 h. Dark-control exposures produced non-mutagenic atmospheres, illustrating that the gas-phase precursor VOCs were not mutagenic at the concentrations tested. Under irradiation, all but m-cresol and naphthalene produced mutagenic atmospheres, with potencies ranging from 2.0 (p-xylene) to 10.4 (ethylbenzene) revertants m3 mgC-1 h-1. The mutagenicity was due exclusively to direct-acting late-generation products of the photooxidation reactions. Gas-phase chemical analysis showed that a number of oxidized organic chemical species enhanced during the irradiated exposure experiments correlated (r ≥ 0.81) with the mutagenic potencies of the atmospheres. Molecular formulas assigned to these species indicated that they likely contained peroxy acid, aldehyde, alcohol, and other functionalities.

Light Absorption of Secondary Organic Aerosol: Composition and Contribution of Nitroaromatic Compounds.

Secondary organic aerosol (SOA) can affect the atmospheric radiation balance through absorbing light at shorter visible and UV wavelengths. However, the composition and optical properties of light-absorbing SOA is poorly understood. In this work, SOA filter samples were collected during individual chamber experiments conducted with three biogenic and eight aromatic volatile organic compound (VOC) precursors in the presence of NOX and H2O2. Compared with the SOA generated using the aromatic precursors, biogenic SOA generally exhibits negligible light absorption above 350 nm; the aromatic SOA generated in the presence of NOX shows stronger light absorption than that generated with H2O2. Fifteen nitroaromatic compound (NAC) chemical formulas were identified and quantified in SOA samples. Their contributions to the light absorption of sample extracts were also estimated. On average, the m-cresol/NOX SOA sample has the highest mass contribution from NACs (10.4 ± 6.74%, w/w), followed by naphthalene/NOX (6.41 ± 2.08%) and benzene/NOX (5.81 ± 3.82%) SOA. The average contributions of NACs to total light absorption were at least two times greater than their average mass contributions at 365 and 400 nm, revealing the potential use of chromophoric NACs as brown carbon (BrC) tracers in source apportionment and air quality modeling studies.

Predicting Thermal Behavior of Secondary Organic Aerosols.

Volume concentrations of secondary organic aerosol (SOA) are measured in 139 steady-state, single precursor hydrocarbon oxidation experiments after passing through a temperature controlled inlet. The response to change in temperature is well predicted through a feedforward Artificial Neural Network. The most parsimonious model, as indicated by Akaike's Information Criterion, Corrected (AIC,C), utilizes 11 input variables, a single hidden layer of 4 tanh activation function nodes, and a single linear output function. This model predicts thermal behavior of single precursor aerosols to less than ±5%, which is within the measurement uncertainty, while limiting the problem of overfitting. Prediction of thermal behavior of SOA can be achieved by a concise number of descriptors of the precursor hydrocarbon including the number of internal and external double bonds, number of methyl- and ethyl- functional groups, molecular weight, and number of ring structures, in addition to the volume of SOA formed, and an indicator of which of four oxidant precursors was used to initiate reactions (NOx photo-oxidation, photolysis of H2O2, ozonolysis, or thermal decomposition of N2O5). Additional input variables, such as chamber volumetric residence time, relative humidity, initial concentration of oxides of nitrogen, reacted hydrocarbon concentration, and further descriptors of the precursor hydrocarbon, including carbon number, number of oxygen atoms, and number of aromatic ring structures, lead to over fit models, and are unnecessary for an efficient, accurate predictive model of thermal behavior of SOA. This work indicates that predictive statistical modeling methods may be complementary to descriptive techniques for use in parametrization of air quality models.

Ozonolysis of α/ß-farnesene mixture: analysis of gas-phase and particulate reaction products.

Atmospheric oxidation of sesquiterpenes has been of considerable interest recently because of their likely contribution to ambient organic aerosol, but farnesene oxidation has been reported in only a few studies and with limited data. In the present study, a detailed chemical analysis of the organic fraction of gas and particle phases originating from the ozonolysis of a mixture of α-farnesene and ß-farnesene was carried out in a 14.5 m3 smog chamber. More than 80 organic compounds bearing OH functionality were detected for the first time in this system in the gas and particle phases. The major secondary organic aerosol (SOA) components included conjugated α-farnesene trienols, hydroxyl carboxylic acid and its corresponding lactones, C3-C7 linear dicarboxylic acids, and hydroxy/carbonyl/carboxylic compounds. Of particular importance was 5,6-dihydroxy-6-methylheptan-2-one (DHMHO), which was detected at high concentration. In the gas phase, the main species identified were trienols and their corresponding epoxides and diepoxides. Proposed reaction schemes are provided for selected compounds. A similar analysis was performed for ambient PM2.5 samples collected during summer 2013 as part of the SOAS to determine farnesene contributions to PM2.5. Gas chromatography-mass spectrometry analysis were consistent with the occurrence of several farnesene SOA compounds, indicating the potential impact of farnesene on the regional aerosol burden. The high abundance of DHMHO in chamber SOA and its presence in ambient PM2.5 is particularly important because to our knowledge it is specific to farnesene and therefore could serve as an indicator for farnesene emitted into ambient aerosol. In the absence of authentic standards, however, it is difficult to accurately quantify the contribution of SOA originating from farnesene to ambient PM2.5.

Constraints on primary and secondary particulate carbon sources using chemical tracer and 14C methods during CalNex-Bakersfield.

The present study investigates primary and secondary sources of organic carbon for Bakersfield, CA, USA as part of the 2010 CalNex study. The method used here involves integrated sampling that is designed to allow for detailed and specific chemical analysis of particulate matter (PM) in the Bakersfield airshed. To achieve this objective, filter samples were taken during thirty-four 23-hr periods between 19 May and 26 June 2010 and analyzed for organic tracers by gas chromatography - mass spectrometry (GC-MS). Contributions to organic carbon (OC) were determined by two organic tracer-based techniques: primary OC by chemical mass balance and secondary OC by a mass fraction method. Radiocarbon (14C) measurements of the total organic carbon were also made to determine the split between the modern and fossil carbon and thereby constrain unknown sources of OC not accounted for by either tracer-based attribution technique. From the analysis, OC contributions from four primary sources and four secondary sources were determined, which comprised three sources of modern carbon and five sources of fossil carbon. The major primary sources of OC were from vegetative detritus (9.8%), diesel (2.3%), gasoline (<1.0%), and lubricating oil impacted motor vehicle exhaust (30%); measured secondary sources resulted from isoprene (1.5%), α-pinene (<1.0%), toluene (<1.0%), and naphthalene (<1.0%, as an upper limit) contributions. The average observed organic carbon (OC) was 6.42 ± 2.33 µgC m-3. The 14C derived apportionment indicated that modern and fossil components were nearly equivalent on average; however, the fossil contribution ranged from 32-66% over the five week campaign. With the fossil primary and secondary sources aggregated, only 25% of the fossil organic carbon could not be attributed. Whereas, nearly 80% of the modern carbon could not be attributed to primary and secondary sources accessible to this analysis, which included tracers of biomass burning, vegetative detritus and secondary biogenic carbon. The results of the current study contributes source-based evaluation of the carbonaceous aerosol at CalNex Bakersfield.

Characterization of polar organosulfates in secondary organic aerosol from the green leaf volatile 3-Z-hexenal.

Evidence is provided that the green leaf volatile 3-Z-hexenal serves as a precursor for biogenic secondary organic aerosol through the formation of polar organosulfates (OSs) with molecular weight (MW) 226. The MW 226 C6-OSs were chemically elucidated, along with structurally similar MW 212 C5-OSs, whose biogenic precursor is likely related to 3-Z-hexenal but still remains unknown. The MW 226 and 212 OSs have a substantial abundance in ambient fine aerosol from K-puszta, Hungary, which is comparable to that of the isoprene-related MW 216 OSs, known to be formed through sulfation of C5-epoxydiols, second-generation gas-phase photooxidation products of isoprene. Using detailed interpretation of negative-ion electrospray ionization mass spectral data, the MW 226 compounds are assigned to isomeric sulfate esters of 3,4-dihydroxyhex-5-enoic acid with the sulfate group located at the C-3 or C-4 position. Two MW 212 compounds present in ambient fine aerosol are attributed to isomeric sulfate esters of 2,3-dihydroxypent-4-enoic acid, of which two are sulfated at C-3 and one is sulfated at C-2. The formation of the MW 226 OSs is tentatively explained through photooxidation of 3-Z-hexenal in the gas phase, resulting in an alkoxy radical, followed by a rearrangement and subsequent sulfation of the epoxy group in the particle phase.

2-hydroxyterpenylic acid: an oxygenated marker compound for α-pinene secondary organic aerosol in ambient fine aerosol.

An oxygenated MW 188 compound is commonly observed in substantial abundance in atmospheric aerosol samples and was proposed in previous studies as an α-pinene-related marker compound that is associated with aging processes. Owing to difficulties in producing this compound in sufficient amounts in laboratory studies and the occurrence of isobaric isomers, a complete assignment for individual MW 188 compounds could not be achieved in these studies. Results from a comprehensive mass spectrometric analysis are presented here to corroborate the proposed structure of the most abundant MW 188 compound as a 2-hydroxyterpenylic acid diastereoisomer with 2R,3R configuration. The application of collision-induced dissociation with liquid chromatography/electrospray ionization-ion trap mass spectrometry in both negative and positive ion modes, as well as chemical derivatization to methyl ester derivatives and analysis by the latter technique and gas chromatography/electron ionization mass spectrometry, enabled a comprehensive characterization of MW 188 isomers, including a detailed study of the fragmentation behavior using both mass spectrometric techniques. Furthermore, a MW 188 positional isomer, 4-hydroxyterpenylic acid, was tentatively identified, which also is of atmospheric relevance as it could be detected in ambient fine aerosol. Quantum chemical calculations were performed to support the diastereoisomeric assignment of the 2-hydroxyterpenylic acid isomers. Results from a time-resolved α-pinene photooxidation experiment show that the 2-hydroxyterpenylic acid 2R,3R diastereoisomer has a time profile distinctly different from that of 3-methyl-1,2,3-butanetricarboxylic acid, a marker for oxygenated (aged) secondary organic aerosol. This study presents a comprehensive chemical data set for a more complete structural characterization of hydroxyterpenylic acids in ambient fine aerosol, which sets the foundation to better understand the atmospheric fate of α-pinene in future studies.

Epoxide pathways improve model predictions of isoprene markers and reveal key role of acidity in aerosol formation.

Isoprene significantly contributes to organic aerosol in the southeastern United States where biogenic hydrocarbons mix with anthropogenic emissions. In this work, the Community Multiscale Air Quality model is updated to predict isoprene aerosol from epoxides produced under both high- and low-NOx conditions. The new aqueous aerosol pathways allow for explicit predictions of two key isoprene-derived species, 2-methyltetrols and 2-methylglyceric acid, that are more consistent with observations than estimates based on semivolatile partitioning. The new mechanism represents a significant source of organic carbon in the lower 2 km of the atmosphere and captures the abundance of 2-methyltetrols relative to organosulfates during the simulation period. For the parametrization considered here, a 25% reduction in SOx emissions effectively reduces isoprene aerosol, while a similar reduction in NOx leads to small increases in isoprene aerosol.

Yields and molecular composition of gas phase and secondary organic aerosol from the photooxidation of the volatile consumer product benzyl alcohol: formation of highly oxygenated and hydroxy nitroaromatic compounds.

Recently, volatile chemical products (VCPs) have been increasingly recognized as important precursors for secondary organic aerosol (SOA) and ozone in urban areas. However, their atmospheric chemistry, physical transformation, and their impact on climate, environment and human health remain poorly understood. Here, the yields and chemical composition at the molecular level of gas and particle phase products originating from the photooxidation of one of these VCPs, benzyl alcohol (BnOH), is reported. The SOA was generated in the presence of seed aerosol from nebulized ammonium sulfate solution in a 14.5 m3 smog chamber operated in flow mode. More than 50 organic compounds containing nitrogen and/or up to seven oxygen atoms were identified by mass spectrometry. While a detailed non-targeted analysis has been made, our primary focus has been to examine highly oxygenated and nitro-aromatic compounds. The major components include ring-opening products with high oxygen to carbon ratio (e.g., malic acid, tartaric acids, arabic acid, trihydroxy-oxo-pentanoic acids, and pentaric acid), and ring-retaining products (e.g., benzaldehyde, benzoic acid, catechol, 3-nitrobenzyl alcohol, 4-nitrocatechol, 2-hydroxy-5-nitrobenzyl alcohol, 2-nitrophloroglucinol, 3,4-dihydroxy-5-nitrobenzyl alcohol). The presence of some of these products in the gas and particle phases simultaneously provides evidence of their gas/particle partitioning. These oxygenated oxidation products made dominant contributions to the SOA particle composition in both low and high NOx systems. Yields, organic mass to organic carbon ratio, and proposed reaction schemes for selected compounds are provided. The aerosol yield was 5.2% for BnOH/H2O2 at SOA concentration of 52.9 µg m-3 and ranged between 1.7-8.1 % for BnOH/NOx at SOA concentration of 40.0-119.5 µg m-3.

Isoprene epoxydiols as precursors to secondary organic aerosol formation: acid-catalyzed reactive uptake studies with authentic compounds.

Isoprene epoxydiols (IEPOX), formed from the photooxidation of isoprene under low-NO(x) conditions, have recently been proposed as precursors of secondary organic aerosol (SOA) on the basis of mass spectrometric evidence. In the present study, IEPOX isomers were synthesized in high purity (>99%) to investigate their potential to form SOA via reactive uptake in a series of controlled dark chamber studies followed by reaction product analyses. IEPOX-derived SOA was substantially observed only in the presence of acidic aerosols, with conservative lower-bound yields of 4.7-6.4% for ß-IEPOX and 3.4-5.5% for δ-IEPOX, providing direct evidence for IEPOX isomers as precursors to isoprene SOA. These chamber studies demonstrate that IEPOX uptake explains the formation of known isoprene SOA tracers found in ambient aerosols, including 2-methyltetrols, C(5)-alkene triols, dimers, and IEPOX-derived organosulfates. Additionally, we show reactive uptake on the acidified sulfate aerosols supports a previously unreported acid-catalyzed intramolecular rearrangement of IEPOX to cis- and trans-3-methyltetrahydrofuran-3,4-diols (3-MeTHF-3,4-diols) in the particle phase. Analysis of these novel tracer compounds by aerosol mass spectrometry (AMS) suggests that they contribute to a unique factor resolved from positive matrix factorization (PMF) of AMS organic aerosol spectra collected from low-NO(x), isoprene-dominated regions influenced by the presence of acidic aerosols.