Photoproduction of reactive intermediates from dissolved organic matter in coastal seawater around an urban metropolis in South China: Characterization and predictive modeling.

Chromophoric dissolved organic matter (CDOM) is an important photochemical precursor to reactive intermediates (RIs) (e.g., excited triplet states of chromophoric dissolved organic matter (3CDOM⁎), hydroxyl radicals (·OH), and singlet oxygen (1O2)) in aquatic systems to drive the photodegradation of contaminants. There have been limited studies on the photoproduction of RIs in coastal seawater CDOM in Asia, which impedes our ability to model the lifetimes and fates of contaminants in these coastal seawater systems. Hong Kong is an urban metropolis in South China, whose coastal seawater is susceptible to anthropogenic activities from the surrounding areas and the nearby Pearl River. We investigated the photoproduction of RIs in seawater around Hong Kong during the wet vs. dry season. Higher intensities of fluorescent components, dissolved organic carbon concentration ([DOC]), apparent quantum yields of RIs (ΦRIs), and steady-state concentrations of photogenerated RIs ([RIs]ss) were observed for samples collected in the areas closest to the Pearl River during the wet season. Lower humification degrees and ΦRIs but higher intensities of fluorescent components and [RIs]ss were generally observed for the wet season samples compared to the dry season samples. Statistical analysis revealed strong significant correlations (Spearman |r| > 0.6, p < 0.05) between ΦRIs and the absorbance properties (including the absorbance ratio E2:E3, spectral slope coefficients S350-400, and spectral slope ratio SR) of CDOM, and between [RIs]ss and the quantity-reflected properties (including the fluorescence intensity of humic-like components) of CDOM. Our modeling analyses combining orthogonal partial least squares and stepwise multiple linear regression showed excellent prediction strengths for [1O2]ss and [3CDOM⁎]ss (R2adj > 0.7) when [DOC] and the chemical and optical properties of CDOM were used as predictor variables. These modeling results demonstrate the feasibility of predicting the concentrations and quantum yields of RIs in seawater around Hong Kong, and potentially other coastal cities in South China, from easily measurable chemical and optical properties.

Emerging investigator series: aqueous photooxidation of live bacteria with hydroxyl radicals under cloud-like conditions: insights into the production and transformation of biological and organic matter originating from bioaerosols.

Live bacteria in clouds are exposed to free radicals such as the hydroxyl radical (˙OH), which is the main driver of many photochemical processes. While the ˙OH photooxidation of organic matter in clouds has been widely studied, equivalent investigations on the ˙OH photooxidation of bioaerosols are limited. Little is known about the daytime encounters between ˙OH and live bacteria in clouds. Here we investigated the aqueous ˙OH photooxidation of four bacterial strains, B. subtilis, P. putida, E. hormaechei B0910, and E. hormaechei pf0910, in microcosms composed of artificial cloud water that mimicked the chemical composition of cloud water in Hong Kong. The survival rates for the four bacterial strains decreased to zero within 6 hours during exposure to 1 × 10-16 M of ˙OH under artificial sunlight. Bacterial cell damage and lysis released biological and organic compounds, which were subsequently oxidized by ˙OH. The molecular weights of some of these biological and organic compounds were >50 kDa. The O/C, H/C, and N/C ratios increased at the initial onset of photooxidation. As the photooxidation progressed, there were few changes in the H/C and N/C, whereas the O/C continued to increase for hours after all the bacterial cells had died. The increase in the O/C was due to functionalization and fragmentation reactions, which increased the O content and decreased the C content, respectively. In particular, fragmentation reactions played key roles in transforming biological and organic compounds. Fragmentation reactions cleaved the C-C bonds of carbon backbones of higher molecular weight proteinaceous-like matter to form a variety of lower molecular weight compounds, including HULIS of molecular weight <3 kDa and highly oxygenated organic compounds of molecular weight <1.2 kDa. Overall, our results provided new insights at the process level into how daytime reactive interactions between live bacteria and ˙OH in clouds contribute to the formation and transformation of organic matter.

Kinetics of the nitrate-mediated photooxidation of monocarboxylic acids in the aqueous phase.

The photooxidation of organic compounds by hydroxyl radicals (·OH) in atmospheric aqueous phases contributes to both the formation and aging of secondary organic aerosols (SOAs), which usually include carboxylic acids. Hydrogen peroxide (H2O2) and inorganic nitrate are two important ·OH photochemical sources in atmospheric aqueous phases. The aqueous phase pH is an important factor that not only controls the dissociation of carboxylic acids and consequently their ·OH reactivities, but also the production of ·OH and other reactive species from the photolysis of some ·OH photochemical precursors, particularly inorganic nitrate. While many studies have reported on the aqueous pH-dependent photodegradation rates of carboxylic acids with ·OH produced by H2O2 photolysis, the aqueous pH-dependent photodegradation rates of carboxylic acids with ·OH produced by inorganic nitrate photolysis have not been studied. In this work, we investigated the pH-dependent (pH 2 to 7) aqueous photooxidation of formic acid (FA), glycolic acid (GA), and pyruvic acid (PA) initiated by the photolysis of ammonium nitrate (NH4NO3). The observed reaction rates of the three carboxylic acids were controlled by the [NH4NO3]/[carboxylic acid] concentration ratio. Higher [NH4NO3]/[carboxylic acid] concentration ratios resulted in faster photodegradation rates, which could be attributed to the higher concentrations of ·OH produced from the photolysis of higher concentrations of NH4NO3. In addition, the observed photodegradation rates of the three carboxylic acids strongly depended on the pH. The highest photodegradation rate was observed at pH 4 for FA, whereas the highest photodegradation rates were observed at pH 2 for GA and PA. The observed pH-dependent FA and GA photodegradation rates were due to the combined effects of the pH-dependent ·OH formation from NH4NO3 photolysis and the differences in ·OH reactivities of dissociated vs. undissociated FA and GA. In contrast, the observed pH-dependent PA photodegradation rate was due primarily to the pH-dependent decarboxylation of PA initiated by light. These results highlight how the aqueous phase pH and inorganic nitrate photolysis can combine to influence the degradation rates of carboxylic acids, which can have significant implications for how the atmospheric fates of carboxylic acids are modeled for regions with substantial concentrations of inorganic nitrate in cloud water and aqueous aerosols.

pH affects the aqueous-phase nitrate-mediated photooxidation of phenolic compounds: implications for brown carbon formation and evolution.

Brown carbon (BrC) is known to have important impacts on atmospheric chemistry and climate. Phenolic compounds are a prominent class of BrC precursors that are emitted in large quantities from biomass burning and fossil fuel combustion. Inorganic nitrate is a ubiquitous component of atmospheric aqueous phases such as cloudwater, fog, and aqueous aerosols. The photolysis of inorganic nitrate can lead to BrC formation via the photonitration of phenolic compounds in the aqueous phase. However, the acidity of the atmospheric aqueous phase adds complexity to these photonitration processes and needs to be considered when investigating BrC formation from the nitrate-mediated photooxidation of phenolic compounds. In this study, we investigated the influence of pH on the formation and evolution of BrC from the aqueous-phase photooxidation of guaiacol, catechol, 5-nitroguaiacol, and 4-nitrocatechol initiated by inorganic nitrate photolysis. The reaction rates, BrC composition and quantities were found to depend on the aqueous phase pH. Guaiacol, catechol, and 5-nitroguaiacol reacted substantially faster at lower pH. In contrast, 4-nitrocatechol reacted at slower rates at lower pH. For all four phenolic compounds, the initial stages of photooxidation resulted in an increase in light absorption (i.e., photo-enhancement) in the near-UV and visible range due to the formation of light absorbing products formed via the addition of nitro and/or hydroxyl groups to the phenolic compound. Greater photo-enhancement was observed at lower pH during the nitrate-mediated photooxidation of guaiacol and catechol. In contrast, greater photo-enhancement was observed at higher pH during the nitrate-mediated photooxidation of 5-nitroguaiacol and 4-nitrocatechol. This indicated that the effect that the aqueous phase pH has on the composition and yields of BrC formed is not universal, and will depend on the initial phenolic compound. These results provide new insights into how the atmospheric aqueous phase acidity influences the reactivities of different phenolic compounds and BrC formation/evolution during photooxidation initiated by inorganic nitrate photolysis, which will have significant implications for how the atmospheric fates of phenolic compounds and BrC formation/evolution are modeled for areas with high levels of inorganic nitrate.

Environmental photochemistry of organic UV filter butyl methoxydibenzoylmethane: Implications for photochemical fate in surface waters.

With the widespread use of sunscreen and other personal care products, organic ultraviolet filters (OUVFs) have become widely detected in the aquatic environment. Direct and indirect photolysis are important transformation pathways of OUVFs in aquatic environments, so their transformation products (TPs) are also chemicals of concern. Butyl methoxydibenzoylmethane (BMDBM) is one of the most commonly used OUVFs worldwide due to its ability to absorb ultraviolet light across a wide range of wavelengths, and it is ubiquitously detected in aquatic environments. In this study, we investigated the photodegradation of BMDBM through direct photolysis and hydroxyl radical (•OH) photooxidation. TPs were identified using ultrahigh performance liquid chromatography-high resolution mass spectrometry, and reaction mechanisms were proposed. Our results showed that the photodegradation rates for both enol and keto tautomer forms of BMDBM during direct photolysis and •OH photooxidation were similar. The formation of TPs resulted from α-cleavage and decarbonylation reactions involving the keto form of BMDBM. Comparisons of the kinetic data and TPs revealed that the direct photolysis mechanism was a significant sink for BMDBM even during •OH photooxidation. Evaluations of environmental properties based on the predicted physicochemical properties of BMDBM and TPs suggests that some of the TPs will have higher mobility than BMDBM. The quantitative structure-activity relationship (QSAR) approach was used to evaluate the ecotoxicity of BMDBM and the identified TPs. Most TPs were found to be less ecotoxic than BMDBM; however, TPs that had a diphenyl ring structure could be more ecotoxic than BMDBM. Overall, this study provides new insights into the photochemical behavior and ecotoxicity of BMDBM and its TPs, which are important for assessing the fate, persistence, accumulation, and adverse impacts of these compounds in aquatic environments.

Low-Molecular-Weight Carboxylic Acids in the Southeastern U.S.: Formation, Partitioning, and Implications for Organic Aerosol Aging.

While carboxylic acids are important components in both particle and gas phases in the atmosphere, their sources and partitioning are not fully understood. In this study, we present real-time measurements of both particle- and gas-phase concentrations for five of the most common and abundant low-molecular-weight carboxylic acids (LMWCA) in a rural region in the southeastern U.S. in Fall 2016. Through comparison with secondary organic aerosol (SOA) tracers, we find that isoprene was the most important local precursor for all five LMWCA but via different pathways. We propose that monocarboxylic acids (formic and acetic acids) were mainly formed through gas-phase photochemical reactions, while dicarboxylic acids (oxalic, malonic, and succinic acids) were predominantly from aqueous processing. Unexpectedly high concentrations of particle-phase formic and acetic acids (in the form of formate and acetate, respectively) were observed and likely the components of long-range transport organic aerosol (OA), decoupled from their gas-phase counterparts. In addition, an extraordinarily strong correlation (R2 = 0.90) was observed between a particulate LMWCA and aged SOA, which we tentatively attribute to boundary layer dynamics.

Particle Size Distribution Dynamics Can Help Constrain the Phase State of Secondary Organic Aerosol.

Particle phase state is a property of atmospheric aerosols that has important implications for the formation, evolution, and gas/particle partitioning of secondary organic aerosol (SOA). In this work, we use a size-resolved chemistry and microphysics model (Statistical Oxidation Model coupled to the TwO Moment Aerosol Sectional (SOM-TOMAS)), updated to include an explicit treatment of particle phase state, to constrain the bulk diffusion coefficient (Db) of SOA produced from α-pinene ozonolysis. By leveraging data from laboratory experiments performed in the absence of a seed and under dry conditions, we find that the Db for SOA can be constrained ((1-7) × 10-15 cm2 s-1 in these experiments) by simultaneously reproducing the time-varying SOA mass concentrations and the evolution of the particle size distribution. Another version of our model that used the predicted SOA composition to calculate the glass-transition temperature, viscosity, and, ultimately, Db (∼10-15 cm2 s-1) of the SOA was able to reproduce the mass and size distribution measurements when we included oligomer formation (oligomers accounted for about a fifth of the SOA mass). Our work highlights the potential of a size-resolved SOA model to constrain the particle phase state of SOA using historical measurements of the evolution of the particle size distribution.

Secondary Organic Aerosol (SOA) from Nitrate Radical Oxidation of Monoterpenes: Effects of Temperature, Dilution, and Humidity on Aerosol Formation, Mixing, and Evaporation.

Nitrate radical (NO3) oxidation of biogenic volatile organic compounds (BVOC) is important for nighttime secondary organic aerosol (SOA) formation. SOA produced at night may evaporate the following morning due to increasing temperatures or dilution of semivolatile compounds. We isothermally dilute the oxidation products from the limonene+NO3 reaction at 25 °C and observe negligible evaporation of organic aerosol via dilution. The SOA yields from limonene+NO3 are approximately constant (∼174%) at 25 °C and range from 81 to 148% at 40 °C. Based on the difference in yields between the two temperatures, we calculated an effective enthalpy of vaporization of 117-237 kJ mol-1. The aerosol yields at 40 °C can be as much as 50% lower compared to 25 °C. However, when aerosol formed at 25 °C is heated to 40 °C, only about 20% of the aerosol evaporates, which could indicate a resistance to aerosol evaporation. To better understand this, we probe the possibility that SOA from limonene+NO3 and ß-pinene+NO3 reactions is highly viscous. We demonstrate that particle morphology and evaporation is dependent on whether SOA from limonene is formed before or during the formation of SOA from ß-pinene. This difference in particle morphology is present even at high relative humidity (∼70%).

Photochemical Aging of α-pinene and ß-pinene Secondary Organic Aerosol formed from Nitrate Radical Oxidation.

The nitrate radical (NO3) is the dominant nighttime oxidant in most urban and rural environments and reacts rapidly with biogenic volatile organic compounds to form secondary organic aerosol (SOA) and organic nitrates (ON). Here, we study the formation of SOA and ON from the NO3 oxidation of two monoterpenes (α-pinene and ß-pinene) and investigate how they evolve during photochemical aging. High SOA mass loadings are produced in the NO3+ß-pinene reaction, during which we detected 41 highly oxygenated gas- and particle-phase ON possessing 4 to 9 oxygen atoms. The fraction of particle-phase ON in the ß-pinene SOA remains fairly constant during photochemical aging. In contrast to the NO3+ß-pinene reaction, low SOA mass loadings are produced during the NO3+α-pinene reaction, during which only 5 highly oxygenated gas- and particle-phase ON are detected. The majority of the particle-phase ON evaporates from the α-pinene SOA during photochemical aging, thus exhibiting a drastically different behavior from that of ß-pinene SOA. Our results indicate that nighttime ON formed by NO3+monoterpene chemistry can serve as either permanent or temporary NOx sinks depending on the monoterpene precursor.

Fundamental Time Scales Governing Organic Aerosol Multiphase Partitioning and Oxidative Aging.

Traditional descriptions of gas-particle partitioning of organic aerosols (OA) rely solely on thermodynamic properties (e.g., volatility). Under realistic conditions where phase partitioning is dynamic rather than static, the transformation of OA involves the interplay of multiphase partitioning with oxidative aging. A key challenge remains in quantifying the fundamental time scales for evaporation and oxidation of semivolatile OA. In this paper, we use isomer-resolved product measurements of a series of normal-alkanes (C18, C20, C22, and C24) to distinguish between gas-phase and heterogeneous oxidation products formed by reaction with hydroxyl radicals (OH). The product isomer distributions when combined with kinetics measurements of evaporation and oxidation enable a quantitative description of the multiphase time scales to be simulated using a single-particle kinetic model. Multiphase partitioning and oxidative transformation of semivolatile normal-alkanes under laboratory conditions is largely controlled by the particle phase state, since the time scales of heterogeneous oxidation and evaporation are found to occur on competing time scales (on the order of 10(-1) h). This is in contrast to atmospheric conditions where heterogeneous oxidation time scales are expected to be much longer (on the order of 10(2) h), with gas-phase oxidation being the dominant process regardless of the evaporation kinetics. Our results demonstrate the dynamic nature of OA multiphase partitioning and oxidative aging and reveal that the fundamental time scales of these processes are crucial for reliably extending laboratory measurements of OA phase partitioning and aging to the atmosphere.

Isomeric product detection in the heterogeneous reaction of hydroxyl radicals with aerosol composed of branched and linear unsaturated organic molecules.

The influence of molecular structure (branched vs linear) on product formation in the heterogeneous oxidation of unsaturated organic aerosol is investigated. Particle phase product isomers formed from the reaction of squalene (C30H50, a branched alkene with six C═C double bonds) and linolenic acid (C18H30O2, a linear carboxylic acid with three C═C double bonds) with OH radicals are identified and quantified using two-dimensional gas chromatography-mass spectrometry. The reactions are measured at low and high [O2] (∼1% vs 10% [O2]) to understand the roles of hydroxyalkyl and hydroxyperoxy radical intermediates in product formation. A key reaction step is OH addition to a C═C double bond to form a hydroxyalkyl radical. In addition, allylic alkyl radicals, formed from H atom abstraction reactions by hydroxyalkyl or OH radicals play important roles in the chemistry of product formation. Functionalization products dominate the squalene reaction at ∼1% [O2], with the total abundance of observed functionalization products being approximately equal to the fragmentation products at 10% [O2]. The large abundance of squalene fragmentation products at 10% [O2] is attributed to the formation and dissociation of tertiary hydroxyalkoxy radical intermediates. For linolenic acid aerosol, the formation of functionalization products dominates the reaction at both ∼1% and 10% [O2], suggesting that the formation and dissociation of secondary hydroxyalkoxy radicals are minor reaction channels for linear molecules. The distribution of linolenic acid functionalization products depends upon [O2], indicating that O2 controls the reaction pathways of the secondary hydroxyalkyl radical. For both reactions, alcohols are formed in favor of carbonyl functional groups, suggesting that there are some key differences between heterogeneous reactions involving allylic radical intermediates and those reactions of OH radicals with simple saturated hydrocarbons.

Influence of molecular structure and chemical functionality on the heterogeneous OH-initiated oxidation of unsaturated organic particles.

The kinetics and products of the heterogeneous OH-initiated oxidation of squalene (C30H50, a branched alkene with 6 C═C double bonds) particles are measured. These results are compared to previous measurements of the OH-initiated oxidation of linoleic acid (C18H32O2, a linear carboxylic acid with 2 C═C double bonds) particles to understand how molecular structure and chemical functionality influence reaction rates and mechanisms. In a 10% mixture of O2 in N2 in the flow reactor, the effective uptake coefficients (γeff) for squalene and linoleic acid are larger than unity, providing clear evidence for particle-phase secondary chain chemistry. γeff for squalene is 2.34 ± 0.07, which is smaller than γeff for linoleic acid (3.75 ± 0.18) despite the larger number of C═C double bonds in squalene. γeff for squalene increases with [O2] in the reactor, whereas γeff for linoleic acid decreases with increasing [O2]. This suggests that the chain cycling mechanism in these two systems is different since O2 promotes chain propagation in the OH + squalene reaction but promotes chain termination in the OH + linoleic acid reaction. Elemental analysis of squalene aerosol shows that an average of 1.06 ± 0.12 O atoms are added per reactive loss of squalene prior to the onset of particle volatilization. O2 promotes particle volatilization in the OH + squalene reaction, suggesting that fragmentation reactions are important when O2 is present in the OH oxidation of branched unsaturated organic aerosol. In contrast, O2 does not influence the rate of particle volatilization in the OH + linoleic acid reaction. This indicates that O2 does not alter the relative importance of fragmentation reactions in the OH oxidation of linear unsaturated organic aerosol.

OH-initiated heterogeneous oxidation of cholestane: a model system for understanding the photochemical aging of cyclic alkane aerosols.

Aerosols containing aliphatic hydrocarbons play a substantial role in the urban atmosphere. Cyclic alkanes constitute a large fraction of aliphatic hydrocarbon emissions originating from incomplete combustion of diesel fuel and motor oil. In the present study, cholestane (C27H48) is used as a model system to examine the OH-initiated heterogeneous oxidation pathways of cyclic alkanes in a photochemical flow tube reactor. Oxidation products are collected on filters and analyzed by a novel soft ionization two-dimensional gas chromatography/mass spectrometry technique. The analysis reveals that the first-generation functionalization products (cholestanones, cholestanals, and cholestanols) are the dominant reaction products that account for up to 70% by mass of the total speciated compounds. The ratio of first-generation carbonyls to alcohols is near unity at every oxidation level. Among the cholestanones/cholestanals, 55% are found to have the carbonyl group on the rings of the androstane skeleton, while 74% of cholestanols have the hydroxyl group on the rings. Particle-phase oxidation products with carbon numbers less than 27 (i.e., "fragmentation products") and higher-generation functionalization products are much less abundant. Carbon bond cleavage was found to occur only on the side chain. Tertiary-carbon alkoxy radicals are suggested to play an important role in governing both the distribution of functionalization products (via alkoxy radical isomerization and reaction with oxygen) and the fragmentation products (via alkoxy radical decomposition). These results provide new insights into the oxidation mechanism of cyclic alkanes.

OH-initiated oxidation of sub-micron unsaturated fatty acid particles.

The heterogeneous reaction of OH radicals with sub-micron unsaturated fatty acid particles in the presence of H2O2 and O2 is studied to explore how surface OH addition reactions initiate chain reactions that rapidly transform the chemical composition of an organic particle. In the presence of 20.7 ppm [H2O2] in a 10% mixture of O2 in N2, the effective uptake coefficients of oleic acid, linoleic acid and linolenic acid are found to be 1.72 ± 0.08, 3.75 ± 0.18 and 5.73 ± 0.14, respectively. These effective uptake coefficients are larger than unity, providing clear evidence for particle-phase secondary chain chemistry. The effective uptake coefficient increases linearly with the number of C=C double bonds in the unsaturated fatty acid molecule. Elemental composition analysis reveals that there is an addition of, on average, 0.57 ± 0.02, 0.61 ± 0.01 and 0.73 ± 0.04 O atoms per reactive loss of oleic acid, linoleic acid and linolenic acid, respectively, which suggests that OH addition to the C=C double bond is not the sole reaction pathway that consumes the molecular species. These results suggest the potential presence of secondary reactions that consume the unsaturated fatty acid molecular species without increasing the particulate oxygen content. As the unsaturated fatty acid particles become more oxygenated, volatilization also becomes significant. The magnitudes of the effective uptake coefficients are found to be dependent on the concentrations of OH, O2 and H2O2 in the flow reactor. A plausible reaction mechanism is presented to show how surface OH addition reactions initiate chain reactions that rapidly transform an unsaturated organic particle's physicochemical properties.

Real time in situ chemical characterization of sub-micron organic aerosols using Direct Analysis in Real Time mass spectrometry (DART-MS): the effect of aerosol size and volatility.

Direct Analysis in Real Time (DART) mass spectrometry is an atmospheric pressure ionization technique suitable for in situ chemical analysis of organic aerosols. Here, mass spectra are obtained by introducing a stream of nanometer-sized aerosols into the ionization region, which is an open space between the ion source and the atmospheric inlet of mass spectrometer. Model single component aerosols are used to show how the aerosol size and volatility influence the measured ion signals at different DART gas temperatures. The results show that for equivalent aerosol mass concentrations, the ion signal scales with particle surface area, with smaller diameter oleic acid aerosols yielding higher ion signals relative to larger diameter aerosols. For the aerosols of the same size, but different vapor pressures, the ion signal is larger for more volatile succinic acid aerosols than less volatile adipic and suberic acid particles. From the measured changes in aerosol size, produced by the DART source, the radial probing depth for these model aerosols range from 1 to 10 nm, the magnitude of which depends upon the physiochemical properties of the aerosols and DART gas temperature. An aerosol evaporation model reveals that the ion signal is correlated with changes in aerosol size and depends upon the total quantity of evaporated aerosol mass, consistent with a mechanism in which gas-phase molecules are first desorbed from the aerosol surface prior to ionization. The results of this work serve as a basis for future investigations of the mass spectra, ionization pathways, and probing depth of the aerosols using DART.

The influence of molecular structure and aerosol phase on the heterogeneous oxidation of normal and branched alkanes by OH.

Insights into the influence of molecular structure and thermodynamic phase on the chemical mechanisms of hydroxyl radical-initiated heterogeneous oxidation are obtained by identifying reaction products of submicrometer particles composed of either n-octacosane (C28H58, a linear alkane) or squalane (C30H62, a highly branched alkane) and OH. A common pattern is observed in the positional isomers of octacosanone and octacosanol, with functionalization enhanced toward the end of the molecule. This suggests that relatively large linear alkanes are structured in submicrometer particles such that their ends are oriented toward the surface. For squalane, positional isomers of first-generation ketones and alcohols also form in distinct patterns. Ketones are favored on carbons adjacent to tertiary carbons, while hydroxyl groups are primarily found on tertiary carbons but also tend to form toward the end of the molecule. Some first-generation products, viz., hydroxycarbonyls and diols, contain two oxygen atoms. These results suggest that alkoxy radicals are important intermediates and undergo both intramolecular (isomerization) and intermolecular (chain propagation) hydrogen abstraction reactions. Oxidation products with carbon number less than the parent alkane's are observed to a much greater extent for squalane than for n-octacosane oxidation and can be explained by the preferential cleavage of bonds involving tertiary carbons.

Real time in situ chemical characterization of submicrometer organic particles using direct analysis in real time-mass spectrometry.

Direct analysis in real time mass spectrometry (DART-MS) is used to analyze the surface chemical composition of nanometer-sized organic aerosol particles in real time at atmospheric pressure. By introducing a stream of particles in between the DART ionization source and the atmospheric pressure inlet of the mass spectrometer, the aerosol is exposed to a thermal flow of helium or nitrogen gas containing some fraction of metastable helium atoms or nitrogen molecules. In this configuration, the molecular constituents of organic particles are desorbed, ionized, and detected with reduced molecular ion fragmentation, allowing for compositional identification. Aerosol particles detected include alkanes, alkenes, acids, esters, alcohols, aldehydes, and amino acids. The ion signal produced by DART-MS scales with the aerosol surface area rather than volume, suggesting that DART-MS is a viable technique to measure the chemical composition of the particle interface. For oleic acid, particle size measurements of the aerosol stream exiting the ionization region suggest that the probing depth depends upon the desorption temperature, and the probing depth is estimated to be on the order of 5 nm for a 185 nm diameter particle at a DART heater temperature of 500 °C with nitrogen as the DART gas. The reaction of ozone with submicrometer oleic acid particles is measured to demonstrate the ability of this technique to identify products and quantify reaction rates in a heterogeneous reaction.

Observation of rates and products in the reaction of NO3 with submicron squalane and squalene aerosol.

The reactive uptake coefficients γ, for nitrate radical, NO(3), on ∼100 nm diameter squalane and squalene aerosol were measured (1 atm pressure of N(2) and 293 K). For squalane, a branched alkane, γ(NO(3)) of 2.8 × 10(-3) was estimated. For squalene which contains 6 double bonds, γ(NO(3)) was found to be a function of degree of oxidation with an initial value of 0.18 ± 0.03 on fresh particles increasing to 0.82 ± 0.11 on average of over 3 NO(3) reactions per squalene molecule in the aerosol. Synchrotron VUV-ionization aerosol mass spectrometry was used to detect the particle phase oxidation products that include as many as 3 NO(3) subunits added to the squalene backbone. The fraction of squalene remaining in the aerosol follows first order kinetics under oxidation, even at very high oxidation equivalents, which suggests that the matrix remains a liquid upon oxidation. Our calculation indicates a much shorter chemical lifetime for squalene-like particle with respect to NO(3) than its atmospheric lifetime to deposition or wet removal.

Heterogeneous OH oxidation of motor oil particles causes selective depletion of branched and less cyclic hydrocarbons.

Motor oil serves as a useful model system for atmospheric oxidation of hydrocarbon mixtures typical of anthropogenic atmospheric particulate matter, but its complexity often prevents comprehensive chemical speciation. In this work we fully characterize this formerly "unresolved complex mixture" at the molecular level using recently developed soft ionization gas chromatography techniques. Nucleated motor oil particles are oxidized in a flow tube reactor to investigate the relative reaction rates of observed hydrocarbon classes: alkanes, cycloalkanes, bicycloalkanes, tricycloalkanes, and steranes. Oxidation of hydrocarbons in a complex aerosol is found to be efficient, with approximately three-quarters (0.72 ± 0.06) of OH collisions yielding a reaction. Reaction rates of individual hydrocarbons are structurally dependent: compared to normal alkanes, reaction rates increased by 20-50% with branching, while rates decreased ∼20% per nonaromatic ring present. These differences in rates are expected to alter particle composition as a function of oxidation, with depletion of branched and enrichment of cyclic hydrocarbons. Due to this expected shift toward ring-opening reactions heterogeneous oxidation of the unreacted hydrocarbon mixture is less likely to proceed through fragmentation pathways in more oxidized particles. Based on the observed oxidation-induced changes in composition, isomer-resolved analysis has potential utility for determining the photochemical age of atmospheric particulate matter with respect to heterogeneous oxidation.

OH-initiated heterogeneous aging of highly oxidized organic aerosol.

The oxidative evolution ("aging") of organic species in the atmosphere is thought to have a major influence on the composition and properties of organic particulate matter but remains poorly understood, particularly for the most oxidized fraction of the aerosol. Here we measure the kinetics and products of the heterogeneous oxidation of highly oxidized organic aerosol, with an aim of better constraining such atmospheric aging processes. Submicrometer particles composed of model oxidized organics-1,2,3,4-butanetetracarboxylic acid (C(8)H(10)O(8)), citric acid (C(6)H(8)O(7)), tartaric acid (C(4)H(6)O(6)), and Suwannee River fulvic acid-were oxidized by gas-phase OH in a flow reactor, and the masses and elemental composition of the particles were monitored as a function of OH exposure. In contrast to our previous studies of less-oxidized model systems (squalane, erythritol, and levoglucosan), particle mass did not decrease significantly with heterogeneous oxidation. Carbon content of the aerosol always decreased somewhat, but this mass loss was approximately balanced by an increase in oxygen content. The estimated reactive uptake coefficients of the reactions range from 0.37 to 0.51 and indicate that such transformations occur at rates corresponding to 1-2 weeks in the atmosphere, suggesting their importance in the atmospheric lifecycle of organic particulate matter.