Chemistry of Secondary Organic Aerosol Formation from Reactions of Monoterpenes with OH Radicals in the Presence of NOx.

The oxidation of volatile organic compounds (VOCs), which are emitted to the atmosphere from natural and anthropogenic sources, leads to the formation of ozone and secondary organic aerosol (SOA) particles that impact air quality and climate. In the study reported here, we investigated the products of the reactions of five biogenic monoterpenes with OH radicals (an important daytime oxidant) under conditions that mimic the chemistry that occurs in polluted air, and developed mechanisms to explain their formation. Experiments were conducted in an environmental chamber, and information on the identity of gas-phase molecular products was obtained using online mass spectrometry, while liquid chromatography and two methods of functional group analysis were used to characterize the SOA composition. The gas-phase products of the reactions were similar to those formed in our previous studies of the reactions of these monoterpenes with NO3 radicals (an important nighttime oxidant), in that they all contained various combinations of nitrate, carbonyl, hydroxyl, ester, and ether groups. But in spite of this, less SOA was formed in OH/NOx reactions and it was composed of monomers, while SOA formed in NO3 radical reactions consisted of acetal and hemiacetal oligomers formed by particle-phase accretion reactions. In addition, it appeared that some monomers underwent particle-phase hydrolysis, whereas oligomers did not. These differences are due primarily to the arrangement of hydroxyl, carbonyl, nitrate, and ether groups in the monomers, which can in turn be explained by differences in OH and NO3 radical reaction mechanisms. The results provide insight into the impact of VOC structure on the amount and composition of SOA formed by atmospheric oxidation, which influence important aerosol properties such as volatility and hygroscopicity.

Secondary Organic Aerosol Mass Yields from NO3 Oxidation of α-Pinene and Δ-Carene: Effect of RO2 Radical Fate.

Dark chamber experiments were conducted to study the SOA formed from the oxidation of α-pinene and Δ-carene under different peroxy radical (RO2) fate regimes: RO2 + NO3, RO2 + RO2, and RO2 + HO2. SOA mass yields from α-pinene oxidation were <1 to ∼25% and strongly dependent on available OA mass up to ∼100 µg m-3. The strong yield dependence of α-pinene oxidation is driven by absorptive partitioning to OA and not by available surface area for condensation. Yields from Δ-carene + NO3 were consistently higher, ranging from ∼10-50% with some dependence on OA for <25 µg m-3. Explicit kinetic modeling including vapor wall losses was conducted to enable comparisons across VOC precursors and RO2 fate regimes and to determine atmospherically relevant yields. Furthermore, SOA yields were similar for each monoterpene across the nominal RO2 + NO3, RO2 + RO2, or RO2 + HO2 regimes; thus, the volatility basis sets (VBS) constructed were independent of the chemical regime. Elemental O/C ratios of ∼0.4-0.6 and nitrate/organic mass ratios of ∼0.15 were observed in the particle phase for both monoterpenes in all regimes, using aerosol mass spectrometer (AMS) measurements. An empirical relationship for estimating particle density using AMS-derived elemental ratios, previously reported in the literature for non-nitrate containing OA, was successfully adapted to organic nitrate-rich SOA. Observations from an NO3- chemical ionization mass spectrometer (NO3-CIMS) suggest that Δ-carene more readily forms low-volatility gas-phase highly oxygenated molecules (HOMs) than α-pinene, which primarily forms volatile and semivolatile species, when reacted with NO3, regardless of RO2 regime. The similar Δ-carene SOA yields across regimes, high O/C ratios, and presence of HOMs, suggest that unimolecular and multistep processes such as alkoxy radical isomerization and decomposition may play a role in the formation of SOA from Δ-carene + NO3. The scarcity of peroxide functional groups (on average, 14% of C10 groups carried a peroxide functional group in one test experiment in the RO2 + RO2 regime) appears to rule out a major role for autoxidation and organic peroxide (ROOH, ROOR) formation. The consistently substantially lower SOA yields observed for α-pinene + NO3 suggest such pathways are less available for this precursor. The marked and robust regime-independent difference in SOA yield from two different precursor monoterpenes suggests that in order to accurately model SOA production in forested regions the chemical mechanism must feature some distinction among different monoterpenes.

Gas- and Particle-Phase Products and Their Mechanisms of Formation from the Reaction of Δ-3-Carene with NO3 Radicals.

Monoterpenes are a major component of the large quantities of biogenic volatile organic compounds that are emitted to the atmosphere each year. They have a variety of structures, which influences their subsequent reactions with OH radicals, O3, or NO3 radicals and the tendency for these reactions to form secondary organic aerosol (SOA). Here we report the results of an environmental chamber study of the reaction of Δ-3-carene, an abundant unsaturated C10 bicyclic monoterpene, with NO3 radicals, a major nighttime oxidant. Gas- and particle-phase reaction products were analyzed in real time and offline by using mass spectrometry, gas and liquid chromatography, infrared spectroscopy, and derivatization-spectrophotometric methods. The results were used to identify and quantify functional groups and molecular products and to develop gas- and particle-phase reaction mechanisms to explain their formation. Identified gas-phase products were all first-generation ring-retaining and ring-opened compounds (ten C10 and one C9 monomers) with 2-4 functional groups and one C20 dinitrooxydialkyl peroxide dimer. Upon partitioning to the particle phase, the monomers reacted further to form oligomers consisting almost entirely of C20 acetal and hemiacetal dimers, with those formed from a hydroxynitrate and hydroxycarbonyl nitrate comprising more than 50% of the SOA mass. The SOA contained an average of 0.94, 0.71, 0.15, 0.11, 0.16, 0.13, and 7.80 nitrate, carbonyl, hydroxyl, carboxyl, ester, peroxide, and methylene groups per C10 monomer and was formed with a mass yield of 56%. These results have important similarities and differences to those obtained from a previous similar study of the reaction of ß-pinene and yield new insights into the effects of monoterpene structure on gas- and particle-phase reactions that can lead to the formation of a large variety of multifunctional products and significant amounts of SOA.

Comprehensive Analysis of Products and the Development of a Quantitative Mechanism for the OH Radical-Initiated Oxidation of 1-Alkenes in the Presence of NOx.

The reactions of 1-tetradecene and 1-pentadecene, the C14 and C15 linear 1-alkenes, with OH radicals in the presence of NOx were investigated in a series of environmental chamber experiments. Particle-phase ß-hydroxynitrates, dihydroxynitrates, dihydroxycarbonyls, and 1,4-hydroxynitrates and gas-phase aldehydes were sampled and then identified and quantified using a suite of offline analytical techniques that included derivatization, gas and liquid chromatography, and multiple types of mass spectrometry. Measured molar yields of products formed by OH radical addition to the C═C double bond, including ß-hydroxynitrates, dihydroxynitrates, dihydroxycarbonyls (which have not been previously directly quantified with high accuracy), and aldehydes were 0.125 ± 0.01, 0.048 ± 0.005, 0.240 ± 0.04, and 0.268 ± 0.03 (0.264 ± 0.02 and 0.271 ± 0.04 for the formaldehyde and tridecanal/tetradecanal co-products of ß-hydroxyalkoxy radical decomposition), respectively. These values give a total molar yield of 0.681 ± 0.05, which agrees very well with the results of kinetics measurements that indicate that the fraction of reaction that occurs by OH radical addition is 0.70. The yields were used to calculate branching ratios for all OH radical addition pathways, including a value of 0.18 for the formation of dihydroxynitrates from the reaction of dihydroxyperoxy radicals with NO and values of 0.47 and 0.53 for ß-hydroxyalkoxy radical decomposition and isomerization. The results were used with literature data on the yields of aldehydes measured for similar reactions of smaller alkenes, a model for the effect of carbon number on branching ratios for organic nitrate formation, and a mechanism for H atom abstraction derived from studies of linear alkanes to achieve a complete, quantitative gas-phase reaction mechanism for 1-alkenes. The results should also be useful for constructing mechanisms for more complex reactions of volatile organic compounds.

Quantification and source characterization of volatile organic compounds from exercising and application of chlorine-based cleaning products in a university athletic center.

Humans spend approximately 90% of their time indoors, impacting their own air quality through occupancy and activities. Human VOC emissions indoors from exercise are still relatively uncertain, and questions remain about emissions from chlorine-based cleaners. To investigate these and other issues, the ATHLETic center study of Indoor Chemistry (ATHLETIC) campaign was conducted in the weight room of the Dal Ward Athletic Center at the University of Colorado Boulder. Using a Vocus Proton-Transfer-Reaction Time-of-Flight Mass Spectrometer (Vocus PTR-TOF), an Aerodyne Gas Chromatograph (GC), an Iodide-Chemical Ionization Time-of-Flight Mass Spectrometer (I-CIMS), and Picarro cavity ringdown spectrometers, we alternated measurements between the weight room and supply air, allowing for determination of VOC, NH3 , H2 O, and CO2 emission rates per person (emission factors). Human-derived emission factors were higher than previous studies of measuring indoor air quality in rooms with individuals at rest and correlated with increased CO2 emission factors. Emission factors from personal care products (PCPs) were consistent with previous studies and typically decreased throughout the day. In addition, N-chloraldimines were observed in the gas phase after the exercise equipment was cleaned with a dichlor solution. The chloraldimines likely originated from reactions of free amino acids with HOCl on gym surfaces.

Quantification of alkenes on indoor surfaces and implications for chemical sources and sinks.

Indoor surfaces are known to support organic films, but their thickness, composition, and variability between environments remain poorly characterized. Alkenes are expected to be a significant component of these films, with the reaction with O3 being a major sink for O3 and source of airborne chemicals. Here, we present a sensitive, microscale, nanospectrophotometric method for quantifying the alkene (C=C bond) content of surface films and demonstrate its applicability in five studies relevant to indoor air chemistry. Collection efficiencies determined for a filter wipe method were ~64%, and the overall detection limit for monoalkenes was ~10 nmol m-2 . On average, painted walls and glass windows sampled across the University of Colorado Boulder campus were coated by ~4 nm thick films containing ~20% alkenes, and a simple calculation indicates that the lifetime for these alkenes due to reaction with O3 is ~1 hour, indicating that the films are highly dynamic. Measurements of alkenes in films of skin oil, pan-fried cooking oils, a terpene-containing cleaner, and on various surfaces in a closed classroom overnight (where carboxyl groups were also measured) provided insight into the effects of chemical and physical processes on film and air composition.

Measurements and modeling of absorptive partitioning of volatile organic compounds to painted surfaces.

Partitioning to surfaces is an important sink for volatile organic compounds (VOCs) indoors, but the mechanisms are not well understood or quantified. Here, a mass spectrometer was coupled to a portable surface reactor and a flow tube to measure partitioning of VOCs into paint films coated onto glass or wallboard, and their subsequent diffusion. A model was developed to extract values of the effective absorbing organic mass concentration of the film, Cw , which is a measure of absorption capacity, and VOC diffusion coefficients, Df , from VOC time profiles measured during film passivation and depassivation. Values of Cw agreed well with the value estimated from the paint film mass and flow tube air volume, and Df values (also measured using attenuated total reflectance-Fourier transform infrared spectroscopy) correlated well with VOC vapor saturation concentrations, C*, estimated using a group contribution method. The value of these relationships for estimating key parameters that control VOC partitioning into paint and the fate of VOCs indoors was demonstrated using a house model, which indicated that >50% of VOCs with C* ≤108  µg/m3 (C* of octane, hexanone, and propanol) that contacted a paint film of typical thickness fully permeated the film regardless of emission duration.

Time-Resolved Measurements of Indoor Chemical Emissions, Deposition, and Reactions in a University Art Museum.

A 6-week study was conducted at the University of Colorado Art Museum, during which volatile organic compounds (VOCs), carbon dioxide (CO2), ozone (O3), nitric oxide (NO), nitrogen dioxide (NO2), other trace gases, and submicron aerosol were measured continuously. These measurements were then analyzed using a box model to quantify the rates of major processes that transformed the composition of the air. VOC emission factors were quantified for museum occupants and their activities. The deposition of VOCs to surfaces was quantified across a range of VOC saturation vapor concentrations ( C*) and Henry's Law constants ( H) and determined to be a major sink for VOCs with C* < 108 µg m-3 and H > 102 M atm-1. The reaction rates of VOCs with O3, OH radicals, and nitrate (NO3) radicals were quantified, with unsaturated and saturated VOCs having oxidation lifetimes of >5 and >15 h, making deposition to surfaces and ventilation the dominant VOC sinks in the museum. O3 loss rates were quantified inside a museum gallery, where reactions with surfaces, NO, occupants, and NO2 accounted for 62%, 31%, 5%, and 2% of the O3 sink. The measured concentrations of acetic acid, formic acid, NO2, O3, particulate matter, sulfur dioxide, and total VOCs were below the guidelines for museums.

Budgets of Organic Carbon Composition and Oxidation in Indoor Air.

The chemical composition of indoor air at the University of Colorado, Boulder art museum was measured by a suite of gas- and particle-phase instruments. Over 80% of the total observed organic carbon (TOOC) mass (100 µg m-3) consisted of reduced compounds (carbon oxidation state, OSC < -0.5) with high volatility (log10 C* > 7) and low carbon number (nC < 6). The museum TOOC was compared to other indoor and outdoor locations, which increased according to the following trend: remote < rural ≤ urban < indoor ≤ megacity. The museum TOOC was comparable to a university classroom and 3× less than residential environments. Trends in the total reactive flux were remote < indoor < rural < urban < megacity. High volatile organic compound (VOC) concentrations compensated low oxidant concentrations indoors to result in an appreciable reactive flux. Total hydroxyl radical (OH), ozone (O3), nitrate radical (NO3), and chlorine atom (Cl) reactivities for each location followed a similar trend to TOOC. High human occupancy events increased all oxidant reactivities in the museum by 65-125%. The lifetimes of O3, NO3, OH, and Cl reactivities were 13 h, 15 h, 23 days, and 189 days, respectively, corresponding to over 88% of indoor VOC oxidant reactivity being consumed outdoors after ventilation.

Branching Ratios and Rate Constants for Decomposition and Isomerization of ß-Hydroxyalkoxy Radicals Formed from OH Radical-Initiated Reactions of C6-C13 2-Methyl-1-Alkenes in the Presence of NOx.

A series of C6-C13 2-methyl-1-alkenes were reacted with OH radicals in the presence of NOx in a Teflon environmental chamber, and molar yields of the 2-ketone products were measured using gas chromatography. Yields were corrected for secondary reactions with OH radicals and for gas-wall partitioning of the 2-methyl-1-alkene and 2-ketone, with the latter correction being determined from measurements of gas-wall partitioning of 2-ketone standards. Molar yields of 2-ketones decreased with increasing 2-methyl-1-alkene carbon number from a maximum of 0.82 for C6 to a minimum of 0.34 ± 0.02 for C9-C13, which after normalization for the fraction of reaction that occurred by OH radical addition to the C═C double bond (with the rest occurring by H atom abstraction) were 0.86 and 0.39 ± 0.01. These yields were combined with branching ratios determined previously for site-specific OH radical addition to the C═C double bond and for formation of ß-hydroxynitrates to determine branching ratios for decomposition and isomerization of ß-hydroxyalkoxy radicals. Branching ratios for decomposition decreased with increasing 2-methyl-1-alkene carbon number from a maximum of 0.97 for C6 to a minimum of 0.49 ± 0.01 for C9-C13, while the corresponding values for isomerization increased from 0.03 to 0.51 ± 0.01. The results were used to estimate absolute rate constants and activation energies for decomposition and isomerization and were also combined with previously measured yields of ß-hydroxynitrates, dihydroxynitrates, trihydroxynitrates, and H atom abstraction products to obtain yields of ∼75% for the C9-C13 reaction products, with the remainder likely being mostly dihydroxycarbonyls and trihydroxycarbonyls.

Identification and Quantification of 4-Nitrocatechol Formed from OH and NO3 Radical-Initiated Reactions of Catechol in Air in the Presence of NOx: Implications for Secondary Organic Aerosol Formation from Biomass Burning.

Catechol (1,2-benzenediol) is emitted from biomass burning and produced from a reaction of phenol with OH radicals. It has been suggested as an important secondary organic aerosol (SOA) precursor, but the mechanisms of gas-phase oxidation and SOA formation have not been investigated in detail. In this study, catechol was reacted with OH and NO3 radicals in the presence of NOx in an environmental chamber to simulate daytime and nighttime chemistry. These reactions produced SOA with exceptionally high mass yields of 1.34 ± 0.20 and 1.50 ± 0.20, respectively, reflecting the low volatility and high density of reaction products. The dominant SOA product, 4-nitrocatechol, for which an authentic standard is available, was identified through thermal desorption particle beam mass spectrometry and Fourier transform infrared spectroscopy and was quantified in filter samples by liquid chromatography using UV detection. Molar yields of 4-nitrocatechol were 0.30 ± 0.03 and 0.91 ± 0.06 for reactions with OH and NO3 radicals, and thermal desorption measurements of volatility indicate that it is semivolatile at typical atmospheric aerosol loadings, consistent with field studies that have observed it in aerosol particles. Formation of 4-nitrocatechol is initiated by abstraction of a phenolic H atom by an OH or NO3 radical to form a ß-hydroxyphenoxy/o-semiquinone radical, which then reacts with NO2 to form the final product.

Identification and Quantitation of Aerosol Products of the Reaction of ß-Pinene with NO3 Radicals and Implications for Gas- and Particle-Phase Reaction Mechanisms.

Substantial amounts of gas- and particle-phase organic nitrates have been reported in field studies of atmospheric chemistry conducted around the world, and it has been proposed that a significant fraction of these may be formed from the nighttime reaction of monoterpenes with NO3 radicals. In the study presented here, ß-pinene (a major global monoterpene emission) was reacted with NO3 radicals in an environmental chamber and the molecular and functional group composition of the resulting secondary organic aerosol (SOA) was determined using a variety of methods. Eight products, which comprised ∼95% of the SOA mass, were identified and quantified. More than 90% (by mass) of these consisted of acetal heterodimers and heterotrimers that were apparently formed through acid-catalyzed reactions in phase-separated particles. The molar yield of the major oligomer was 16.7%, and the yields of the other six and the single monomer ranged from 1.1% to 2.9%, for a total yield of 30.7%. From these analyses it was determined that the yields of the two major monomer building blocks were 25.9% and 23.6%, and that those of the other four ranged from 2.0% to 4.8%, for a total monomer yield of 62.4%. The measured SOA mass yield was 88.9% and the O/C, N/C, and H/C ratios, molecular weight, and density of the SOA calculated from the results of functional group analysis of the bulk SOA were 0.40, 0.11, 1.79, 217 g mol-1, and 1.21 g cm-3, respectively, similar to values estimated from results of molecular analysis. The results demonstrate the combined importance of RO2• + RO2• reactions, alkoxy radical decomposition and isomerization, and acid-catalyzed particle-phase reactions in the NO3 radical-initiated oxidation of ß-pinene and subsequent formation of SOA and should be useful for understanding reactions of other monoterpenes and for developing models for the laboratory and atmosphere.

Direct Measurements of Gas/Particle Partitioning and Mass Accommodation Coefficients in Environmental Chambers.

Secondary organic aerosols (SOA) are a major contributor to fine particulate mass and wield substantial influences on the Earth's climate and human health. Despite extensive research in recent years, many of the fundamental processes of SOA formation and evolution remain poorly understood. Most atmospheric aerosol models use gas/particle equilibrium partitioning theory as a default treatment of gas-aerosol transfer, despite questions about potentially large kinetic effects. We have conducted fundamental SOA formation experiments in a Teflon environmental chamber using a novel method. A simple chemical system produces a very fast burst of low-volatility gas-phase products, which are competitively taken up by liquid organic seed particles and Teflon chamber walls. Clear changes in the species time evolution with differing amounts of seed allow us to quantify the particle uptake processes. We reproduce gas- and aerosol-phase observations using a kinetic box model, from which we quantify the aerosol mass accommodation coefficient (α) as 0.7 on average, with values near unity especially for low volatility species. α appears to decrease as volatility increases. α has historically been a very difficult parameter to measure with reported values varying over 3 orders of magnitude. We use the experimentally constrained model to evaluate the correction factor (Φ) needed for chamber SOA mass yields due to losses of vapors to walls as a function of species volatility and particle condensational sink. Φ ranges from 1-4.

Gas-Phase Carboxylic Acids in a University Classroom: Abundance, Variability, and Sources.

Gas-phase carboxylic acids are ubiquitous in ambient air, yet their indoor occurrence and abundance are poorly characterized. To fill this gap, we measured gas-phase carboxylic acids in real-time inside and outside of a university classroom using a high-resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS) equipped with an acetate ion source. A wide variety of carboxylic acids were identified indoors and outdoors, including monoacids, diacids, hydroxy acids, carbonyl acids, and aromatic acids. An empirical parametrization was derived to estimate the sensitivity (ion counts per ppt of the analytes) of the HRToF-CIMS to the acids. The campaign-average concentration of carboxylic acids measured outdoors was 1.0 ppb, with the peak concentration occurring in daytime. The average indoor concentration of carboxylic acids was 6.8 ppb, of which 87% was contributed by formic and lactic acid. While carboxylic acids measured outdoors displayed a single daytime peak, those measured indoors displayed a daytime and a nighttime peak. Besides indoor sources such as off-gassing of building materials, evidence for acid production from indoor chemical reactions with ozone was found. In addition, some carboxylic acids measured indoors correlated to CO2 in daytime, suggesting that human occupants may contribute to their abundance either through direct emissions or surface reactions.

The Essential Role for Laboratory Studies in Atmospheric Chemistry.

Laboratory studies of atmospheric chemistry characterize the nature of atmospherically relevant processes down to the molecular level, providing fundamental information used to assess how human activities drive environmental phenomena such as climate change, urban air pollution, ecosystem health, indoor air quality, and stratospheric ozone depletion. Laboratory studies have a central role in addressing the incomplete fundamental knowledge of atmospheric chemistry. This article highlights the evolving science needs for this community and emphasizes how our knowledge is far from complete, hindering our ability to predict the future state of our atmosphere and to respond to emerging global environmental change issues. Laboratory studies provide rich opportunities to expand our understanding of the atmosphere via collaborative research with the modeling and field measurement communities, and with neighboring disciplines.

Hygroscopicity of Organic Compounds as a Function of Carbon Chain Length and Carboxyl, Hydroperoxy, and Carbonyl Functional Groups.

The albedo and microphysical properties of clouds are controlled in part by the hygroscopicity of particles serving as cloud condensation nuclei (CCN). Hygroscopicity of complex organic mixtures in the atmosphere varies widely and remains challenging to predict. Here we present new measurements characterizing the CCN activity of pure compounds in which carbon chain length and the numbers of hydroperoxy, carboxyl, and carbonyl functional groups were systematically varied to establish the contributions of these groups to organic aerosol apparent hygroscopicity. Apparent hygroscopicity decreased with carbon chain length and increased with polar functional groups in the order carboxyl > hydroperoxy > carbonyl. Activation diameters at different supersaturations deviated from the -3/2 slope in log-log space predicted by Köhler theory, suggesting that water solubility limits CCN activity of particles composed of weakly functionalized organic compounds. Results are compared to a functional group contribution model that predicts CCN activity of organic compounds. The model performed well for most compounds but underpredicted the CCN activity of hydroperoxy groups. New best-fit hydroperoxy group/water interaction parameters were derived from the available CCN data. These results may help improve estimates of the CCN activity of ambient organic aerosols from composition data.

Quantification of Gas-Wall Partitioning in Teflon Environmental Chambers Using Rapid Bursts of Low-Volatility Oxidized Species Generated in Situ.

Partitioning of gas-phase organic compounds to the walls of Teflon environmental chambers is a recently reported phenomenon than can affect the yields of reaction products and secondary organic aerosol (SOA) measured in laboratory experiments. Reported time scales for reaching gas-wall partitioning (GWP) equilibrium (τGWE) differ by up to 3 orders of magnitude, however, leading to predicted effects that vary from substantial to negligible. A new technique is demonstrated here in which semi- and low-volatility oxidized organic compounds (saturation concentration c* < 100 µg m(-3)) were photochemically generated in rapid bursts in situ in an 8 m(3) environmental chamber, and then their decay in the absence of aerosol was measured using a high-resolution chemical ionization mass spectrometer (CIMS) equipped with an "inlet-less" NO3(-) ion source. Measured τGWE were 7-13 min (rel. std. dev. 33%) for all compounds. The fraction of each compound that partitioned to the walls at equilibrium follows absorptive partitioning theory with an equivalent wall mass concentration in the range 0.3-10 mg m(-3). Measurements using a CIMS equipped with a standard ion-molecule reaction region showed large biases due to the contact of compounds with walls. On the basis of these results, a set of parameters is proposed for modeling GWP in chamber experiments.

Kinetics of Acid-Catalyzed Dehydration of Cyclic Hemiacetals in Organic Aerosol Particles in Equilibrium with Nitric Acid Vapor.

Previous studies have shown that 1,4-hydroxycarbonyls, which are often major products of the atmospheric oxidation of hydrocarbons, can undergo acid-catalyzed cyclization and dehydration in aerosol particles to form highly reactive unsaturated dihydrofurans. In this study the kinetics of dehydration of cyclic hemiacetals, the rate-limiting step in this process, was investigated in a series of environmental chamber experiments in which secondary organic aerosol (SOA) containing cyclic hemiacetals was formed from the reaction of n-pentadecane with OH radicals in dry air in the presence of HNO3. A particle beam mass spectrometer was used to monitor the formation and dehydration of cyclic hemiacetals in real time, and SOA and HNO3 were quantified in filter samples by gravimetric analysis and ion chromatography. Measured dehydration rate constants increased linearly with increasing concentration of HNO3 in the gas phase and in SOA, corresponding to catalytic rate constants of 0.27 h(-1) ppmv(-1) and 7.0 h(-1) M(-1), respectively. The measured Henry's law constant for partitioning of HNO3 into SOA was 3.7 × 10(4) M atm(-1), ∼25% of the value for dissolution into water, and the acid dissociation constant was estimated to be <8 × 10(-4), at least a factor of 10(4) less than that for HNO3 in water. The results indicate that HNO3 was only weakly dissociated in the SOA and that dehydration of cyclic hemiacetals was catalyzed by molecular HNO3 rather than by H(+). The Henry's law constant and kinetics relationships measured here can be used to improve mechanisms and models of SOA formation from the oxidation of hydrocarbons in dry air in the presence of NOx, which are conditions commonly used in laboratory studies. The fate of cyclic hemiacetals in the atmosphere, where the effects of higher relative humidity, organic/aqueous phase separation, and acid catalysis by molecular H2SO4 and/or H(+) are likely to be important, is discussed.

Effect of the Keto Group on Yields and Composition of Organic Aerosol Formed from OH Radical-Initiated Reactions of Ketones in the Presence of NOx.

Yields of secondary organic aerosol (SOA) were measured for OH radical-initiated reactions of the 2- through 6-dodecanone positional isomers and also n-dodecane and n-tetradecane in the presence of NOx. Yields decreased in the order n-tetradecane > dodecanone isomer average > n-dodecane, and the dodecanone isomer yields decreased as the keto group moved toward the center of the molecule, with 6-dodecanone being an exception. Trends in the yields can be explained by the effect of carbon number and keto group presence and position on product vapor pressures, and by the isomer-specific effects of the keto group on branching ratios for keto alkoxy radical isomerization, decomposition, and reaction with O2. Most importantly, results indicate that isomerization of keto alkoxy radicals via 1,5- and 1,6-H shifts are significantly hindered by the presence of a keto group whereas decomposition is enhanced. Analysis of particle composition indicates that the SOA products are similar for all isomers, and that compared to those formed from the corresponding reactions of alkanes the presence of a pre-existing keto group opens up additional heterogeneous/multiphase reaction pathways that can lead to the formation of new products. The results demonstrate that the presence of a keto group alters gas and particle phase chemistry and provide new insights into the potential effects of molecular structure on the products of the atmospheric oxidation of volatile organic compounds and subsequent formation of SOA.