Effects of Condensed-Phase Oxidants on Secondary Organic Aerosol Formation.

In this study we investigate the hypothesis that oxidants present within atmospheric particles can promote the formation of highly oxidized organic aerosol (OA) via oxidation reactions in the condensed phase. Secondary organic aerosol (SOA) was generated from the ozonolysis of α-pinene and isoprene in an environmental chamber, with seed particles systematically varied in order to assess the effects of condensed-phase oxidant levels on SOA loading and composition. The effects of particle phase (aqueous vs dry), condensed-phase oxidant source (none vs H2O2 vs Fenton chemistry), and irradiation (none vs UV) were all examined. For experiments conducted with aqueous particles but without any added oxidants, UV irradiation resulted in a small but measurable enhancement in the oxygen-to-carbon ratio (O/C). OA formed in the presence of aqueous oxidants was substantially more oxidized, with the highest oxidant concentrations leading to OA with an O/C as high as 1.4 for α-pinene and 2.0 for isoprene, strongly suggesting the formation of oxalate. High aqueous oxidant levels also resulted in increased loss of carbon from the condensed phase. This OA was more oxidized than in any other ozonolysis experiment reported to date, indicating that, when present, aqueous oxidants can have a dramatic effect on SOA formation. However, oxidant concentrations within atmospheric aqueous particles remain poorly constrained, making it difficult to assess the impacts of aqueous-phase oxidation on the loadings and oxidation state of atmospheric OA.

Secondary organic aerosol formation from acyclic, monocyclic, and polycyclic alkanes.

A large number of organic species emitted into the atmosphere contain cycloalkyl groups. While cyclic species are believed to be important secondary organic aerosol (SOA) precursors, the specific role of cyclic moieties (particularly for species with multiple or fused rings) remains uncertain. Here we examine the yields and composition of SOA formed from the reaction of OH with a series of C10 (cyclo)alkanes, with 0-3 rings, in order to better understand the role of multiple cyclic moieties on aerosol formation pathways. A chamber oxidation technique using high, sustained OH radical concentrations was used to simulate long reaction times in the atmosphere. This aging technique leads to higher yields than in previously reported chamber experiments. Yields were highest for cyclic and polycyclic precursors, though yield exhibited little dependence on number of rings. However, the oxygen-to-carbon ratio of the SOA was highest for the polycyclic precursors. These trends are consistent with aerosol formation requiring two generations of oxidation and 3-4 oxygen-containing functional groups in order to condense. Cyclic, unbranched structures are protected from fragmentation during the first oxidation step, with C-C bond scission instead leading to ring opening, efficient functionalization, and high SOA yields. Fragmentation may occur during subsequent oxidation steps, limiting yields by forming volatile products. Polycyclic structures can undergo multiple ring opening reactions, but do not have markedly higher yields, likely due to enhanced fragmentation in the second oxidation step. By contrast, C-C bond scission for the linear and branched structures leads to fragmentation prior to condensation, resulting in low SOA yields. The results highlight the key roles of multigenerational chemistry and susceptibility to fragmentation in the formation and evolution of SOA.

Secondary organic aerosol formation via the isolation of individual reactive intermediates: role of alkoxy radical structure.

The study of the chemistry underlying secondary organic aerosol (SOA) formation is complicated by the large number of reaction pathways and oxidation generations available to a given precursor species. Here we simplify such complexity to that of a single alkoxy radical (RO), by forming SOA via the direct photolysis of alkyl nitrite (RONO) isomers. Chamber experiments were conducted with 11 C10 RONO isomers to determine how the position of the radical center and branching of the carbon skeleton influences SOA formation. SOA yields served as a probe of RO reactivity, with lower yields indicating that fragmentation reactions dominate and higher yields suggesting the predominance of RO isomerization. The largest yields were from straight-chain isomers, particularly those with radical centers located toward the terminus of the molecule. Trends in SOA yields can be explained in terms of two major effects: (1) the relative importance of isomerization and fragmentation reactions, which control the distribution of products, and (2) differences in volatility among the various isomeric products formed. Yields from branched isomers, which were low but variable, provide insight into the degree of fragmentation of the alkoxy radicals; in the case of the two ß-substituted alkoxy radicals, fragmentation appears to occur to a greater extent than predicted by structure-activity relationships. Our results highlight how subtle differences in alkoxy radical structure can have major impacts on product yields and SOA formation.

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 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.

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.

Radical Reactivity in the Condensed Phase: Intermolecular versus Intramolecular Reactions of Alkoxy Radicals.

Condensed-phase alkoxy (RO) radicals can undergo unimolecular (e.g., intramolecular H atom abstraction) reactions as well as bimolecular (intermolecular H atom abstraction) reactions, though the competition between these two channels is not well constrained. Here, we examine this branching by generating RO radicals from the photolysis of a large alkyl nitrite (C20H41ONO) in hexanes and nebulizing the mixture into an aerosol mass spectrometer for analysis. Product ions associated with unimolecular (isomerization) reactions were observed to increase upon photolysis. However, no formation of the C20 alcohol (C20H41OH, the expected product from RO + RH reactions) was observed, suggesting that bimolecular reactions are at most a minor channel for this condensed-phase system (involving saturated hydrocarbons). This result, combined with previous studies of liquid-phase RO radicals carried out at higher concentrations, suggests that when 1,5-H atom abstraction reactions are facile (i.e., in which a 1,5-H atom shift from a secondary or tertiary carbon can occur), this channel will dominate over bimolecular reactions.

Average chemical properties and potential formation pathways of highly oxidized organic aerosol.

Measurements of ambient organic aerosol indicate that a substantial fraction is highly oxidized and low in volatility, but this fraction is generally not reproduced well in either laboratory studies or models. Here we describe a new approach for constraining the viable precursors and formation pathways of highly oxidized organic aerosol, by starting with the oxidized product and considering the possible reverse reactions, using a set of simple chemical rules. The focus of this work is low-volatility oxidized organic aerosol (LV-OOA), determined from factor analysis of aerosol mass spectrometer data. The elemental composition and volatility of the aerosol enable the determination of its position in a three-dimensional chemical space (defined by H/C, O/C, and carbon number) and thus its average chemical formula. Consideration of possible back-reactions then defines the movement taken through this chemical space, constraining potential reaction pathways and precursors. This approach is taken for two highly oxidized aerosol types, an average of LV-OOA factors from ten field campaigns (average formula C10.5H13.4O7.3), and extremely oxidized LV-OOA (from Mexico City, average formula C10H12.1O8.4). Results suggest that potential formation pathways include functionalization reactions that add multiple functional groups per oxidation step, oligomerization of highly oxidized precursors, and, in some cases, fragmentation reactions that involve the loss of small, reduced fragments.

Photochemical induced changes of in vitro estrogenic activity of steroid hormones.

Steroid estrogens are endocrine disrupting contaminants frequently detected in natural waters. Because these estrogens can elicit significant biological responses in aquatic organisms, it is important to study their rates and pathways of degradation in natural waters and to identify whether the transformation products retain biological activity. Photochemical kinetics experiments were conducted under simulated solar light for the hormones 17ß-estradiol (E2), 17α-ethinylestradiol (EE2), estrone (E1), equilin (EQ), and equilenin (EQN) under direct and indirect photolysis conditions. All of these hormones were susceptible to direct photodegradation, with half-lives ranging from 40 min for E1 to about 8 h for E2 and EE2. Indirect photolysis experiments with added Suwannee River fulvic acid (SRFA) lead to faster degradation rates for E2, EE2, and EQ. Added SRFA caused slower photodegradation rates for E1 and EQN, indicating that it acts primarily as an inner filter for these analytes. The well-established yeast estrogen screen (YES) was used to measure the estrogenicity of the analytes and their photoproducts. Results of YES assay experiments show that only the direct photolysis of E1 gave estrogenic products. Lumiestrone, the major E1 direct photolysis product, was isolated and characterized. It formed in 53% yield and exhibited moderate estrogenic activity. When photolysed in the presence of perinaphthenone, a potent synthetic sensitizer, E1 degraded via an indirect photolysis pathway and did not produce lumiestrone or any other active products. These results suggest that under typical natural water conditions photochemical reactions of E2, EE2, EQ, and EQN are expected to produce inactive products while E1 will give the estrogenic product lumiestrone in moderate yield.