Thermal and photochemical oxidation of self-assembled monolayers on alumina particles exposed to nitrogen dioxide

Alumina is an important component of airborne dust particles as well as of building materials and soils found in the tropospheric boundary layer. While the uptake and reactions of oxides of nitrogen and their photochemistry on alumina have been reported in the past, little is known about the chemistry when organics are also present. Fourier transform infrared (FTIR) spectroscopy at ～23 °C was used to study reactions of NO(2) on γ-Al(2)O(3) particles that had been derivatized using 7-octenyltrichlorosilane to form a self-assembled monolayer (SAM). For comparison, the reactions with untreated γ-Al(2)O(3) were also studied. In both cases, the particles were exposed to water vapor prior to NO(2) to provide adsorbed water for reaction. As expected, surface-bound HONO, NO(2)(-), and NO(3)(-) were formed. Surprisingly, oxidation of the organic by surface-bound nitrogen oxides was observed in the dark, forming organo-nitrogen products identified as nitronates (R(2)C[double bond, length as m-dash]NO(2)(-)). Oxidation was more rapid under irradiation (λ > 290 nm) and formed organic nitrates and carbonyl compounds and/or peroxy nitrates in addition to the products observed in the dark. Mass spectrometry of the gas phase during irradiation revealed the production of NO, CO(2), and CO. These studies provide evidence for oxidation of organic compounds on particles and boundary layer surfaces that are exposed to air containing oxides of nitrogen, as well as new pathways for the formation of nitrogen-containing compounds on these surfaces.     

Kinetics, products, and mechanisms of secondary organic aerosol formation

Secondary organic aerosol (SOA) is formed in the atmosphere when volatile organic compounds (VOCs) emitted from anthropogenic and biogenic sources are oxidized by reactions with OH radicals, O(3), NO(3) radicals, or Cl atoms to form less volatile products that subsequently partition into aerosol particles. Once in particles, these organic compounds can undergo heterogenous/multiphase reactions to form more highly oxidized or oligomeric products. SOA comprises a large fraction of atmospheric aerosol mass and can have significant effects on atmospheric chemistry, visibility, human health, and climate. Previous articles have reviewed the kinetics, products, and mechanisms of atmospheric VOC reactions and the general chemistry and physics involved in SOA formation. In this article we present a detailed review of VOC and heterogeneous/multiphase chemistry as they apply to SOA formation, with a focus on the effects of VOC molecular structure on the kinetics of initial reactions with the major atmospheric oxidants, the subsequent reactions of alkyl, alkyl peroxy, and alkoxy radical intermediates, and the composition of the resulting products. Structural features of reactants and products discussed include compound carbon number; linear, branched, and cyclic configurations; the presence of C[double bond, length as m-dash]C bonds and aromatic rings; and functional groups such as carbonyl, hydroxyl, ester, hydroxperoxy, carboxyl, peroxycarboxyl, nitrate, and peroxynitrate. The intention of this review is to provide atmospheric chemists with sufficient information to understand the dominant pathways by which the major classes of atmospheric VOCs react to form SOA products, and the further reactions of these products in particles. This will allow reasonable predictions to be made, based on molecular structure, about the kinetics, products, and mechanisms of VOC and heterogeneous/multiphase reactions, including the effects of important variables such as VOC, oxidant, and NO(x) concentrations as well as temperature, humidity, and particle acidity. Such knowledge should be useful for interpreting the results of laboratory and field studies and for developing atmospheric chemistry models. A number of recommendations for future research are also presented.     

Products and mechanism of secondary organic aerosol formation from reactions of linear alkenes with NO3 radicals

Secondary organic aerosol (SOA) formation from reactions of linear alkenes with NO(3) radicals was investigated in an environmental chamber using a thermal desorption particle beam mass spectrometer for particle analysis. A general chemical mechanism was developed to explain the formation of the observed SOA products. The major first-generation SOA products were hydroxynitrates, carbonylnitrates, nitrooxy peroxynitrates, dihydroxynitrates, and dihydroxy peroxynitrates. The major second-generation SOA products were hydroxy and oxo dinitrooxytetrahydrofurans, which have not been observed previously. The latter compounds were formed by a series of reactions in which delta-hydroxycarbonyls isomerize to cyclic hemiacetals, which then dehydrate to form substituted dihydrofurans (unsaturated compounds) that rapidly react with NO(3) radicals to form very low volatility products. For the approximately 1 ppmv alkene concentrations used here, aerosol formed only for alkenes C(7) or larger. SOA formed from C(7)-C(9) alkenes consisted only of second-generation products, whereas for larger alkenes first-generation products were also present and contributions increased with increasing carbon number apparently due to the formation of lower volatility products. The estimated mass fractions of first- and second-generation products were approximately 50:50, 30:70, 10:90, and 0:100, for 1-tetradecene, 1-dodecene, 1-decene, and 1-octene SOA, respectively. This study shows that delta-hydroxycarbonyls play a key role in the formation of SOA in alkene-NO(3) reactions and are likely to be important in other systems because delta-hydroxycarbonyls can also be formed from reactions of OH radicals and O(3) with hydrocarbons.     

Multifunctional organic nitrates as constituents in European and US urban photo-smog

Air samples of the atmospheric ground layer in the cities of Ulm in Germany, Las Vegas, Nevada, and Salt Lake City, Utah, in the US were analyzed for organic nitrates. The air samples were taken around noon in summer at sunny weather conditions. 43 (C6-C13) alkyl mononitrates, 24 (C3-C6) alkyl dinitrates, and 19 (C2-C6) hydroxy alkyl nitrates have been identified. The analytical protocol included high-volume-sampling, NP-HPLC group separation, high resolution capillary gas chromatography, and detection by the highly selective mass spectrometer detector (SIM mode, m/z = 46). The levels of the sum of 15 (C6-C10) alkyl mononitrates ranged from 2.9 to 11.0 parts per trillion (ppt(v)). The levels of the sum of 21 (C3-C6) alkyl dinitrates ranged from 2.3 to 10.5 ppt(v), and the levels of the sum of 7 (C2-C4) hydroxy alkyl nitrates ranged from 7.3 to 28 ppt(v), respectively, in the urban air samples. These results emphazise the contribution of the alkyl dinitrates and hydroxy alkyl nitrates besides the alkyl mononitrates to the budget of NO(Y) compounds. No major differences in levels and pattern of the organic nitrates are present in air of the German and the US cities.     

15N CIDNP investigations of the peroxynitric acid nitration of L-tyrosine and of related compounds

Peroxynitric acid (O2NOOH) nitrates L-tyrosine and related compounds at pH 2-5. During reaction with O2(15)NOOH in the probe of a 15N NMR spectrometer, the NMR signals of the nitration products of L-tyrosine, N-acetyl-L-tyrosine, 4-fluorophenol and 4-methoxyphenylacetic acid appear in emission indicating a nitration via free radicals. Nuclear polarizations are built up in radical pairs [15NO2* , PhO*]F or [15NO2* , ArH*+]F formed by diffusive encounters of 15NO2 with phenoxyl-type radicals PhO or with aromatic radical cations ArH*+. Quantitative 15N CIDNP investigations with N-acetyl-L-tyrosine and 4-fluorophenol show that the radical-dependent nitration is the only reaction pathway. During the nitration reaction, the 15N NMR signal of 15NO3- also appears in emission. This is explained by singlet-triplet transitions in radical pairs [15NO2* , 15NO3*]S generated by electron transfer between O2(15)NOOH and H15NO2 formed as a reaction intermediate. During reaction of peroxynitric acid with ascorbic acid, 15N CIDNP is again observed in the 15N NMR signal of 15NO3- showing that ascorbic acid is oxidized by free radicals. In contrast to this, O2(15)NOOH reacts with glutathione and cysteine without the appearance of 15N CIDNP, indicating a direct oxidation without participation of free radicals.     

Nighttime radical observations and chemistry

The nitrate radical, NO(3), is photochemically unstable but is one of the most chemically important species in the nocturnal atmosphere. It is accompanied by the presence of dinitrogen pentoxide, N(2)O(5), with which it is in rapid thermal equilibrium at lower tropospheric temperatures. These two nitrogen oxides participate in numerous atmospheric chemical systems. NO(3) reactions with VOCs and organic sulphur species are important, or in some cases even dominant, oxidation pathways, impacting the budgets of these species and their degradation products. These oxidative reactions, together with the ozonolysis of alkenes, are also responsible for the nighttime production and cycling of OH and peroxy (HO(2) + RO(2)) radicals. In addition, reactions of NO(3) with biogenic hydrocarbons are particularly efficient and are responsible for the production of organic nitrates and secondary organic aerosol. Heterogeneous chemistry of N(2)O(5) is one of the major processes responsible for the atmospheric removal of nitrogen oxides as well as the cycling of halogen species though the production of nitryl chloride, ClNO(2). The chemistry of NO(3) and N(2)O(5) is also important to the regulation of both tropospheric and stratospheric ozone. Here we review the essential features of this atmospheric chemistry, along with field observations of NO(3), N(2)O(5), nighttime peroxy and OH radicals, and related compounds. This review builds on existing reviews of this chemistry, and encompasses field, laboratory and modelling work spanning more than three decades.     

Atmospheric chemistry of C3-C6 cycloalkanecarbaldehydes

The rate coefficients for the gas phase reaction of NO3 and OH radicals with a series of cycloalkanecarbaldehydes have been measured in purified air at 298 +/- 2 K and 760 +/- 10 Torr by the relative rate method using a static reactor equipped with long-path Fourier transform infrared (FT-IR) detection. The values obtained for the OH radical reactions (in units of 10(-11) cm3 molecule(-1) s(-1)) were the following: cyclopropanecarbaldehyde, 2.13 +/- 0.05; cyclobutanecarbaldehyde, 2.66 +/- 0.06; cyclopentanecarbaldehyde, 3.27 +/- 0.07; cyclohexanecarbaldehyde, 3.75 +/- 0.05. The values obtained for the NO3 radical reactions (in units of 10(-14) cm3 molecule(-1) s(-1)) were the following: cyclopropanecarbaldehyde, 0.61 +/- 0.04; cyclobutanecarbaldehyde, 1.99 +/- 0.06; cyclopentanecarbaldehyde, 2.55 +/- 0.10; cyclohexanecarbaldehyde, 3.19 +/- 0.12. Furthermore, the reaction products with OH radicals have been investigated using long-path FT-IR spectroscopy and proton-transfer-reaction mass spectrometry (PTR-MS). The measured carbon balances were in the range 89-97%, and the identified products cover a wide spectrum of compounds including nitroperoxycarbonyl cycloalkanes, cycloketones, cycloalkyl nitrates, multifunctional compounds containing carbonyl, hydroxy, and nitrooxy functional groups, HCOOH, HCHO, CO, and CO2.     

Atmospheric fate of methacrolein. 1. Peroxy radical isomerization following addition of OH and O2

Peroxy radicals formed by addition of OH and O(2) to the olefinic carbon atoms in methacrolein react with NO to form methacrolein hydroxy nitrate and hydroxyacetone. We observe that the ratio of these two compounds, however, unexpectedly decreases as the lifetime of the peroxy radical increases. We propose that this results from an isomerization involving the 1,4-H-shift of the aldehydic hydrogen atom to the peroxy group. The inferred rate (0.5 ± 0.3 s(-1) at T = 296 K) is consistent with estimates obtained from the potential energy surface determined by high level quantum calculations. The product, a hydroxy hydroperoxy carbonyl radical, decomposes rapidly, producing hydroxyacetone and re-forming OH. Simulations using a global chemical transport model suggest that most of the methacrolein hydroxy peroxy radicals formed in the atmosphere undergo isomerization and decomposition.     

Bifunctional alkyl nitrates – trace constituents of the atmosphere

Mono- and multifunctional esters of nitric acid (alkyl nitrates or organonitrates) form very complex mixtures of organic trace constituents in air. An analytical method was developed which combines selectivity in separation and detection in order to simplify this complexity in analytical terms. Mononitrates, dinitrates, keto nitrates, hydroxy nitrates of alkanes and alkenes, respecitvely, and bifunctional terpene nitrates were synthesized as reference substances. A specially developed new HPLC stationary phase (organonitrate phase) allows a group separation of mono-, di-, and hydroxy nitrates. After the HPLC preseparation the single components were finally separated by capillary HRGC-ECD and HRGC-MSD on polar and non-polar stationary phases. Mass spectrometric detection in the selected-ion-mode using the highly selective NO₂ ⁺ fragment (m/z = 46 amu) led to very good selectivities for the nitric acid ester moiety. The analysis of a 100 m³ ambient air sample using this new analytical protocol allowed the identification of seven hydroxy nitrates and 24 dinitrates ranging from C2 to C7, 22 of them for the first time ever.     

Multifunctional organic nitrates as constituents in European and US urban photo-smog

Air samples of the atmospheric ground layer in the cities of Ulm in Germany, Las Vegas, Nevada, and Salt Lake City, Utah, in the US were analyzed for organic nitrates. The air samples were taken around noon in summer at sunny weather conditions. 43 (C₆–C₁₃) alkyl mononitrates, 24 (C₃–C₆) alkyl dinitrates, and 19 (C₂–C₆) hydroxy alkyl nitrates have been identified. The analytical protocol included high-volume-sampling, NP-HPLC group separation, high resolution capillary gas chromatography, and detection by the highly selective mass spectrometer detector (SIM mode, m/z = 46). The levels of the sum of 15 (C₆–C₁₀) alkyl mononitrates ranged from 2.9 to 11.0 parts per trillion (ppt(v)). The levels of the sum of 21 (C₃–C₆) alkyl dinitrates ranged from 2.3 to 10.5 ppt(v), and the levels of the sum of 7 (C₂–C₄) hydroxy alkyl nitrates ranged from 7.3 to 28 ppt(v), respectively, in the urban air samples. These results emphazise the contribution of the alkyl dinitrates and hydroxy alkyl nitrates besides the alkyl mononitrates to the budget of NO_Y compounds. No major differences in levels and pattern of the organic nitrates are present in air of the German and the US cities.     

Chemical Composition and Reaction Mechanisms for Secondary Organic Aerosol from Photooxidation of Toluene

A laboratory study was carried out to investigate the secondary organic aerosol (SOA) products from photooxidation of the aromatic hydrocarbon toluene. The experiments were conducted by irradiating toluene/CH₃ONO/NO/air mixtures in a home‐made smog chamber. The aerosol time‐of‐flight mass spectrometer (ATOFMS) was used to measure the size and the chemical composition of individual secondary organic aerosol particles in real‐time. According to a large number of single aerosol diameters and mass spectra, we obtained the size distribution and chemical composition of SOA statistically. Expeperimental results showed that aerosol created by toluene photooxidation is predominantly in the form of fine particles, which have diameters less than 2.5 μm (i.e. PM2.5), and the predominant components of aerosol are furane, methyl glyoxylic acid, phenol, benzaldehyde, benzyl alcohol, cresol, 3‐hydroxy‐2,4‐dioxo‐pentanal, methyl nitrophenol, and 5‐hydroxy‐4,6‐dioxo‐2‐heptenal. The possible reaction mechanisms leading to these products were also discussed.     

Cholesterol ozonolysis: kinetics, mechanism, and oligomer products

Fine particles of cholesterol were reacted with ozone under pseudo-first-order conditions in an aerosol bag reactor. Gas-phase ozone was monitored using an ozone meter. Particle size distribution functions were determined using a scanning mobility particle sizer, which selected particle sizes for introduction into a photoionization aerosol mass spectrometer (PIAMS). PIAMS was used to determine the concentration of cholesterol in the aerosol as a function of reaction time. Dilution corrected rate coefficients were used to calculate the reactive uptake coefficient for ozone onto cholesterol particles as (2.8 +/- 0.4) x 10(-6). Uptake was found to be independent of particle diameter for the sizes studied (100 and 200 nm), suggesting that the uptake is surface mediated. The reaction products were also collected on filters and analyzed by electrospray ionization (ESI) mass spectrometry with both direct infusion and liquid chromatography sample introduction. The main primary reaction products contained one, two, or three oxygens added to the cholesterol moiety. Secondary oligomeric products were also observed, consisting of covalently bound dimers and trimers. Tandem mass spectrometry was used to confirm the expected structures of these compounds. The dimers appear to be acyl hydroperoxides, consistent with a previously reported mechanism for the reaction in a nonparticipating solvent. Finally, the magnitude of the uptake coefficient confirms that cholesterol is suitable as a local source tracer for source apportionment of ambient organic aerosol.     

High-resolution mass spectrometric analysis of secondary organic aerosol produced by ozonation of limonene

Chemical composition of secondary organic aerosol (SOA) formed from the ozone-initiated oxidation of limonene is characterized by high-resolution electrospray ionization mass spectrometry in both positive and negative ion modes. The mass spectra reveal a large number of both monomeric (m/z < 300) and oligomeric (m/z > 300) condensed products of oxidation. A combination of high resolving power (m/Deltam approximately 60,000) and Kendrick mass defect analysis makes it possible to unambiguously determine the molecular composition of hundreds of individual compounds in SOA samples. Van Krevelen analysis shows that the SOA compounds are heavily oxidized, with average O : C ratios of 0.43 and 0.50 determined from the positive and negative ion mode spectra, respectively. A possible reaction mechanism for the formation of the first generation SOA molecular components is considered. The discussed mechanism includes known isomerization and addition reactions of the carbonyl oxide intermediates generated during the ozonation of limonene. In addition, it includes isomerization and decomposition pathways for alkoxy radicals resulting from unimolecular decomposition of carbonyl oxides that have been disregarded by previous studies. The isomerization reactions yield numerous products with a progressively increasing number of alcohol and carbonyl groups, whereas C-C bond scission reactions in alkoxy radicals shorten the carbon chain. Together these reactions yield a large number of isomeric products with broadly distributed masses. A qualitative agreement is found between the number and degree of oxidation of the predicted and measured reaction products in the monomer product range.     

Real-time detection of individual secondary organic aerosol particle from photooxidation of toluene using aerosol time of flight mass spectrometer

Aromatic hydrocarbons are air pollutants in urban atmosphere and mainly from anthropogenic sources, i.e., emission from automotive exhaust and productive process of industry, and they constitute an important fraction of total volatile organic compounds (VOCs). Photochemical ozone and secondary organic aerosols (SOA) could be formed from the photochemical proc- esses of aromatic hydrocarbons. SOA may impact visibility of air, formation of clouds, change of the climate, and human health serio…     

Surface-catalyzed chlorine and nitrogen activation: mechanisms for the heterogeneous formation of ClNO, NO, NO2, HONO, and N2O from HNO3 and HCl on aluminum oxide particle surfaces

It is well-known that chlorine active species (e.g., Cl(2), ClONO(2), ClONO) can form from heterogeneous reactions between nitrogen oxides and hydrogen chloride on aerosol particle surfaces in the stratosphere. However, less is known about these reactions in the troposphere. In this study, a potential new heterogeneous pathway involving reaction of gaseous HCl and HNO(3) on aluminum oxide particle surfaces, a proxy for mineral dust in the troposphere, is proposed. We combine transmission Fourier transform infrared spectroscopy with X-ray photoelectron spectroscopy to investigate changes in the composition of both gas-phase and surface-bound species during the reaction under different environmental conditions of relative humidity and simulated solar radiation. Exposure of surface nitrate-coated aluminum oxide particles, from prereaction with nitric acid, to gaseous HCl yields several gas-phase products, including ClNO, NO(2), and HNO(3), under dry (RH < 1%) conditions. Under humid more conditions (RH > 20%), NO and N(2)O are the only gas products observed. The experimental data suggest that, in the presence of adsorbed water, ClNO is hydrolyzed on the particle surface to yield NO and NO(2), potentially via a HONO intermediate. NO(2) undergoes further hydrolysis via a surface-mediated process, resulting in N(2)O as an additional nitrogen-containing product. In the presence of broad-band irradiation (λ > 300 nm) gas-phase products can undergo photochemistry, e.g., ClNO photodissociates to NO and chlorine atoms. The gas-phase product distribution also depends on particle mineralogy (Al(2)O(3) vs CaCO(3)) and the presence of other coadsorbed gases (e.g., NH(3)). These newly identified reaction pathways discussed here involve continuous production of active ozone-depleting chlorine and nitrogen species from stable sinks such as gas-phase HCl and HNO(3) as a result of heterogeneous surface reactions. Given that aluminosilicates represent a major fraction of mineral dust aerosol, aluminum oxide can be used as a model system to begin to understand various aspects of possible reactions on mineral dust aerosol surfaces.     

C4-C14-alkyl nitrates as organic trace compounds in air

The long chain alkyl nitrates (C _n >5) form a complex spectrum of natural and anthropogenic organic trace compounds in air. HRGC/ECD and HRGC/MSD using 56 amu as the signal reveal a standard pattern of isomeric n-alkyl nitrates in semi-rural air. This is regulated by the input of the corresponding alkanes, their rate constants for the reaction with OH, the rate constant of the alkylperoxy radicals for the reaction to alkyl nitrates, the atmospheric concentrations of NO/NO₂ and by the rate constants of the alkyl nitrates for the reaction with OH radicals as the major removal reaction. The complex pattern of signals given by the ECD in the retention index range between 700 and 2000 has been observed before but this is the first time that it has been assigned to a defined group of chemical compounds.The environmental impact of the occurrence of the different groups of alkyl nitrates has yet to be evaluated. Their general property as organic stabilizers for NO/NO₂ and therefore as precursors of NO ₃ ^- ions in rain and their biological potentials are also known. The long chain alkyl nitrates act as lipophilic carriers for nitric acid.     

Separation, detection and occurrence of (C2–C8)-alkyl- and phenyl-alkyl nitrates as trace compounds in clean and polluted air

Nitric acid, HNO₃, and nitrous acid, HNO₂, are forming stable esters with alcohols, the alkyl nitrates and alkyl nitrites. Both groups of compounds are used as fuel additives, explosives and pharmaceuticals. Alkyl nitrates are also formed as complex mixtures during incomplete combustion and the abiotic transformation of alkanes, alkenes and aldehydes in air. Organic nitrates can be assigned to anthropogenic and natural sources alike. Here the synthesis of reference mixtures of alkyl nitrates is described starting with alcohols, alkyl bromides, alkyl iodides or alkanes, respectively, sampling techniques in air analysis, and the separation of alkyl nitrates and alkyl dinitrates by high resolution capillary gas chromatography using various stationary phases and electron capture (HRGC/ECD) as well as mass spectrometric detection (HRGC/MSD). A highly selective detection mode for alkyl nitrates and alkyl di- and trinitrates — in general in the presence of other organic trace compounds — is the single ion monitoring of 46 amu in GC/MS. The separation and occurrence of alkyl- and phenyl-alkyl nitrates in polluted air is reported using capillary gas chromatography with electron capture and ion trap MS detection. The reaction of alkanes with nitric acid at room temperature simulates in a good approximation the pattern of alkyl nitrates formed in air chemistry.     

Atmospheric reaction of OH radicals with 1,3-butadiene and 4-hydroxy-2-butenal

The gas-phase reaction of OH radicals with 1,3-butadiene and 4-hydroxy-2-butenal in the presence of NO has been studied in a flow tube operated at 295 +/- 2 K and pressures of 950 mbar of synthetic air or 100 mbar of an O(2)/He mixture. OH radicals were generated using three different experimental approaches, namely, ozonolysis of tetramethylethylene (dark reaction), photolysis of methyl nitrite, or via the reaction of HO(2) with NO (HO(2) from the reaction of H-atoms with O(2)). Products of the reaction of OH radicals with 1,3-butadiene were HCHO (0.64 +/- 0.08), acrolein (0.59 +/- 0.06), 4-hydroxy-2-butenal (0.23 +/- 0.10), furan (0.046 +/- 0.014), and organic nitrates (0.06 +/- 0.02) accounting for more than 90% of the reacted carbon. There was no significant dependence of product yields on experimental conditions which were varied in a wide range. The formation of the 1,4-addition product 4-hydroxy-2-butenal was confirmed unambiguously for the first time. The rate coefficient k(OH + 4-hydroxy-2-butenal) = (5.1 +/- 0.8) x 10(-11) cm(3) molecule(-1) s(-1) was determined using a relative rate technique (p = 100 mbar, T = 295 +/- 2 K). Products of the reaction of OH radicals with 4-hydroxy-2-butenal were glycolaldehyde (0.40 +/- 0.06), glyoxal (0.17 +/- 0.04), trans-butenedial (0.093 +/- 0.033), and organic nitrates (0.043 +/- 0.015) as well as further carbonylic substances remaining unidentified so far. Corresponding reaction mechanisms describing the formation of the detected products are proposed, and the relevance of these data for atmospheric conditions is discussed.     

Oxidation mechanism of aromatic peroxy and bicyclic radicals from OH-toluene reactions

Theoretical calculations have been performed to investigate mechanistic features of OH-initiated oxidation reactions of toluene. Aromatic peroxy radicals arising from initial OH and subsequent O(2) additions to the toluene ring are shown to cyclize to form bicyclic radicals rather than undergoing reaction with NO under atmospheric conditions. Isomerization of bicyclic radicals to more stable epoxide radicals possesses significantly higher barriers and, hence, has slower rates than O(2) addition to form bicyclic peroxy radicals. At each OH attachment site, only one isomeric pathway via the bicyclic peroxy radical is accessible to lead to ring cleavage. The study provides thermochemical and kinetic data for quantitative assessment of the photochemical production potential of ozone and formation of toxic products and secondary organic aerosol from toluene oxidation.     

Heterogeneous reaction of suspended phosmet particles with NO3 radicals

As an organophosphorous insecticide, phosmet is widely used for plant protection as well as against the ectoparasites on productive livestock. It emits into the atmosphere in both gas and particulate phases via spray drift from treatments and postapplication emissions, but its degradation in the atmosphere is not well-known up to now. In this study, the heterogeneous reaction of phosmet absorbed on the azelaic acid particles with NO(3) radicals is investigated in real-time using a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS). Two primary products and their nitro-substituted derivates are observed. The apparent reaction rate of phosmet particles with NO(3) radicals is also obtained by a relative rate method with isoprene and 1,4-cyclohexadiene as the reference compounds. The concentrations of the reference substance and phosmet particles are synchronously detected with an atmospheric gas analysis mass spectrometer and the VUV-ATOFMS. Under the experimental conditions, the average apparent reaction rate obtained is (1.92 ± 0.09) × 10(-12) cm(3) molecule(-1) s(-1), and the corresponding atmospheric lifetime toward NO(3) radicals is roughly estimated to be ~0.3 h.