Experimental and Theoretical Studies of Isoprene Reaction with NO3

Absolute determination of chemical kinetic rate constants by optical tracking the reaction on the second timescale using cavity-enhanced absorption spectroscopy

We report a new spectroscopic platform coupled to an atmospheric simulation chamber for the direct determination of chemical rate constants with high accuracy at a second time-scale resolution. These developed analytical instruments consist of an incoherent broadband cavity enhanced absorption spectrometer using a red light emitting diode (LED) emitting at ∼662 nm (LED-IBBCEAS) associated with a multipass cell direct absorption spectrometer (MPC-DAS) coupled to an external cavity quantum cascade laser (EC-QCL) operating in the mid-infrared region at approximately 8 μm (EC-QCL-MPC-DAS). Spectrometers were employed to investigate the NO₃-initiated oxidation of four selected volatile organic compounds (VOCs) for the determination of the corresponding rate constants with a dynamic range of 5 orders of magnitude (from 10^-11 to 10^-16 cm³ molecule^-1 s^-1). Rate constants of (6.5 ± 0.5) × 10^-15, (7.0 ± 0.4) × 10^-13, and (5.8 ± 0.5) × 10^-16 cm³ molecule^-1 s^-1 for propanal, isoprene and formaldehyde, respectively, were directly determined by fitting the measured concentration-time profiles of NO₃ and VOCs (measured using a proton transfer reaction time-of-flight mass spectrometer, PTR-ToF-MS) to chemical models based on the FACSIMILE simulation software (version 4.2.50) at 760 torr and 293 ± 2 K. The obtained rate constants are in good agreement with the most recent recommendations of the IUPAC (International Union of Pure and Applied Chemistry). In addition, a rate constant of (2.60 ± 0.30) × 10^-11 cm³ molecule^-1 s^-1 for the oxidation of 2-methoxyphenol by NO₃ radicals was first determined using the absolute kinetic method. Compared to the mostly used indirect relative rate method, the rate constant uncertainty is reduced from ∼20% to ∼12%. The results demonstrated the high potential of using modern spectroscopic techniques to directly determine the chemical reaction rate constants.

Explosive Secondary Aerosol Formation during Severe Haze in the North China Plain

Severe haze events with exceedingly high-levels of fine aerosols occur frequently over the past decades in the North China Plain (NCP), exerting profound impacts on human health, weather, and climate. The development of effective mitigation policies requires a comprehensive understanding of the haze formation mechanisms, including identification and quantification of the sources, formation, and transformation of the aerosol species. Haze evolution in this region exhibits distinct physical and chemical characteristics from clean to polluted periods, as evident from increasing stagnation and relative humidity, but decreasing solar radiation as well as explosive secondary aerosol formation. The latter is attributed to highly elevated concentrations of aerosol precursor gases and is reflected by rapid increases in the particle number and mass concentrations, both corresponding to nonequilibrium chemical processes. Considerable new knowledge has been acquired to understand the processes regulating haze formation, particularly in light of the progress in elucidating the aerosol formation mechanisms. This review synthesizes recent advances in understanding secondary aerosol formation, by highlighting several critical chemical/physical processes, that is, new particle formation and aerosol growth driven by photochemistry and aqueous chemistry as well as the interaction between aerosols and atmospheric stability. Current challenges and future research priorities are also discussed.

Theoretical and experimental study of peroxy and alkoxy radicals in the NO3-initiated oxidation of isoprene

Under atmospheric conditions, nitrate-RO₂ radicals are equilibrated and react predominantly with HO₂, RO₂ and NO. The nitrate-RO chemistry is affected strongly by ring closure to epoxy radicals, impeding formation of MVK/MACR.

Kinetic and product studies of the reactions of NO3 with a series of unsaturated organic compounds

Rate coefficients for the reaction of NO₃ radicals with 6 unsaturated volatile organic compounds (VOCs) in a 7300 L simulation chamber at ambient temperature and pressure have been determined by the relative rate method. The resulting rate coefficients were determined for isoprene, 2-carene, 3-carene, methyl vinyl ketone (MVK), methacrolein (MACR) and crotonaldehyde (CA), as (6.6 ± 0.8) × 10^-13, (1.8 ± 0.6) × 10^-11, (8.7 ± 0.5) × 10^-12, (1.24 ± 1.04) × 10^-16, (3.3 ± 0.9) × 10^-15 and (5.7 ± 1.2) × 10^-15 cm³/(molecule•sec), respectively. The experiments indicate that NO₃ radical reactions with all the studied unsaturated VOCs proceed through addition to the olefinic bond, however, it indicates that the introduction of a carbonyl group into unsaturated VOCs can deactivate the neighboring olefinic bond towards reaction with the NO₃ radical, which is to be expected since the presence of these electron-withdrawing substituents will reduce the electron density in the π orbitals of the alkenes, and will therefore reduce the rate coefficient of these electrophilic addition reactions. In addition, we investigated the product formation from the reactions of 2-carene and 3-carene with the NO₃ radical. Qualitative identification of an epoxide (C₁₀H₁₆OH⁺), caronaldehyde (C₁₀H₁₆O₂H⁺) and nitrooxy-ketone (C₁₀H₁₆O₄NH⁺) was achieved using a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) and a reaction mechanism is proposed.

Carbenium ion-mediated oligomerization of methylglyoxal for secondary organic aerosol formation

Secondary organic aerosol (SOA) from photochemical oxidation of volatile organic compounds represents one of the most dominant constituents of fine particulate matter in the troposphere, with profound implications for air quality and climate. However, the fundamental chemical mechanisms leading to SOA formation remain highly uncertain. Here, we show oligomer formation from methylglyoxal with the carbenium ions as the key intermediate using quantum chemical calculations. This cationic oligomerization is demonstrated to proceed via barrierless pathways and occurs at fast rates on weakly acidic aqueous aerosols and/or cloud droplets under typical tropospheric conditions. In contrast to a previously proposed hydration mechanism, out results reveal that the carbenium ion-mediated oligomerization of methylglyoxal provides a major SOA source from anthropogenic and biogenic emissions.

Secondary organic aerosol (SOA) represents a major constituent of tropospheric fine particulate matter, with profound implications for human health and climate. However, the chemical mechanisms leading to SOA formation remain uncertain, and atmospheric models consistently underpredict the global SOA budget. Small α-dicarbonyls, such as methylglyoxal, are ubiquitous in the atmosphere because of their significant production from photooxidation of aromatic hydrocarbons from traffic and industrial sources as well as from biogenic isoprene. Current experimental and theoretical results on the roles of methylglyoxal in SOA formation are conflicting. Using quantum chemical calculations, we show cationic oligomerization of methylglyoxal in aqueous media. Initial protonation and hydration of methylglyoxal lead to formation of diols/tetrol, and subsequent protonation and dehydration of diols/tetrol yield carbenium ions, which represent the key intermediates for formation and propagation of oligomerization. On the other hand, our results reveal that the previously proposed oligomerization via hydration for methylglyoxal is kinetically and thermodynamically implausible. The carbenium ion-mediated mechanism occurs barrierlessly on weakly acidic aerosols and cloud/fog droplets and likely provides a key pathway for SOA formation from biogenic and anthropogenic emissions.

Gas-Phase Reactions of Isoprene and Its Major Oxidation Products

Isoprene carries approximately half of the flux of non-methane volatile organic carbon emitted to the atmosphere by the biosphere. Accurate representation of its oxidation rate and products is essential for quantifying its influence on the abundance of the hydroxyl radical (OH), nitrogen oxide free radicals (NO _x), ozone (O₃), and, via the formation of highly oxygenated compounds, aerosol. We present a review of recent laboratory and theoretical studies of the oxidation pathways of isoprene initiated by addition of OH, O₃, the nitrate radical (NO₃), and the chlorine atom. From this review, a recommendation for a nearly complete gas-phase oxidation mechanism of isoprene and its major products is developed. The mechanism is compiled with the aims of providing an accurate representation of the flow of carbon while allowing quantification of the impact of isoprene emissions on HO _x and NO _x free radical concentrations and of the yields of products known to be involved in condensed-phase processes. Finally, a simplified (reduced) mechanism is developed for use in chemical transport models that retains the essential chemistry required to accurately simulate isoprene oxidation under conditions where it occurs in the atmosphere-above forested regions remote from large NO _x emissions.

Reassessing the atmospheric oxidation mechanism of toluene

Photochemical oxidation of aromatic hydrocarbons leads to tropospheric ozone and secondary organic aerosol (SOA) formation, with profound implications for air quality, human health, and climate. Toluene is the most abundant aromatic compound under urban environments, but its detailed chemical oxidation mechanism remains uncertain. From combined laboratory experiments and quantum chemical calculations, we show a toluene oxidation mechanism that is different from the one adopted in current atmospheric models. Our experimental work indicates a larger-than-expected branching ratio for cresols, but a negligible formation of ring-opening products (e.g., methylglyoxal). Quantum chemical calculations also demonstrate that cresols are much more stable than their corresponding peroxy radicals, and, for the most favorable OH (ortho) addition, the pathway of H extraction by O₂ to form the cresol proceeds with a smaller barrier than O₂ addition to form the peroxy radical. Our results reveal that phenolic (rather than peroxy radical) formation represents the dominant pathway for toluene oxidation, highlighting the necessity to reassess its role in ozone and SOA formation in the atmosphere.

Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO₃) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO₃-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO₃-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO₃ radical, the difficulty of characterizing the spatial distributions of BVOC and NO₃ within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry-climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO₃-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO₃-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

A QSAR for the prediction of rate constants for the reaction of VOCs with nitrate radicals

A QSAR for the prediction of rate constants for the degradation of volatile organic compounds by nitrate radicals is developed using the Partial Least Squares technique. The QSAR is based on experimental data published in the literature for 260 compounds. They are modeled by a set of calculated descriptors from standard descriptor generation tools and from quantum chemistry. Out of several diversity-based partitionings of the data set a diverse set of 99 compounds turned out to be the optimum choice with regard to simplicity and performance. The final QSAR model is characterized by r(2) = 0.831 (fit) and q(2) = 0.823 (prediction), and by an r(2)pred = 0.862 for the n = 155 external validation set. The QSAR needs 3 latent variables. The most important descriptors for the QSAR are the ionization potential, obtained from density functional theory, and the energy of the highest occupied molecular orbital, which are modulated by fingerprints indicating the presence of specific molecular fragments like functional groups or ring systems. The applicability domain of the new QSAR was studied for some compound classes which are important for the crop protection industry, including (di)hydroxbenzenes and heterocyclic compounds.

Isoprene NO3 Oxidation Products from the RO2 + HO2 Pathway

We describe the products of the reaction of the hydroperoxy radical (HO(2)) with the alkylperoxy radical formed following addition of the nitrate radical (NO(3)) and O(2) to isoprene. NO(3) adds preferentially to the C(1) position of isoprene (>6 times more favorably than addition to C(4)), followed by the addition of O(2) to produce a suite of nitrooxy alkylperoxy radicals (RO(2)). At an RO(2) lifetime of ∼30 s, δ-nitrooxy and β-nitrooxy alkylperoxy radicals are present in similar amounts. Gas-phase product yields from the RO(2) + HO(2) pathway are identified as 0.75-0.78 isoprene nitrooxy hydroperoxide (INP), 0.22 methyl vinyl ketone (MVK) + formaldehyde (CH(2)O) + hydroxyl radical (OH) + nitrogen dioxide (NO(2)), and 0-0.03 methacrolein (MACR) + CH(2)O + OH + NO(2). We further examined the photochemistry of INP and identified propanone nitrate (PROPNN) and isoprene nitrooxy hydroxyepoxide (INHE) as the main products. INHE undergoes similar heterogeneous chemistry as isoprene dihydroxy epoxide (IEPOX), likely contributing to atmospheric secondary organic aerosol formation.

Reaction of stabilized criegee intermediates from ozonolysis of limonene with water: ab initio and DFT study

The mechanism of the chemical reaction of H2O with three stabilized Criegee intermediates (stabCI-OO, stabCI-CH3-OO and stabCIx-OO) produced via the limonene ozonolysis reaction has been investigated using ab initio and DFT (Density Functional Theory) methods. It has been shown that the formation of the hydrogen-bonded complexes is followed by two different reaction pathways, leading to the formation of either OH radicals via water-catalyzed H migration or of α-hydroxy hydroperoxide. Both pathways were found to be essential sources of atmospheric OH radical and H2O2 making a significant contribution to the formation of secondary aerosols in the Earth's atmosphere. The activation energies at the CCSD(T)/6-31G(d) + CF level of theory were found to be in the range of 14.70-21.98 kcal mol-1. The formation of α-hydroxy hydroperoxide for the reaction of stabCIx-OO and H2O with the activation energy of 14.70 kcal mol-1 is identified as the most favorable pathway.

Role of OH-initiated oxidation of isoprene in aging of combustion soot

We have investigated the contribution of OH-initiated oxidation of isoprene to the atmospheric aging of combustion soot. The experiments were conducted in a fluoropolymer chamber on size-classified soot aerosols in the presence of isoprene, photolytically generated OH, and nitrogen oxides. The evolution in the mixing state of soot was monitored from simultaneous measurements of the particle size and mass, which were used to calculate the particle effective density, dynamic shape factor, mass fractal dimension, and coating thickness. When soot particles age, the increase in mass is accompanied by a decrease in particle mobility diameter and an increase in effective density. Coating material not only fills in void spaces, but also causes partial restructuring of fractal soot aggregates. For thinly coated aggregates, the single scattering albedo increases weakly because of the decreased light absorption and practically unchanged scattering. Upon humidification, coated particles absorb water, leading to an additional compaction. Aging transforms initially hydrophobic soot particles into efficient cloud condensation nuclei at a rate that increases in the presence of nitrogen oxides. Our results suggest that ubiquitous biogenic isoprene plays an important role in aging of anthropogenic soot, shortening its atmospheric lifetime and considerably altering its impacts on air quality and climate.

Physiochemical properties of alkylaminium sulfates: hygroscopicity, thermostability, and density

Although heterogeneous interaction of amines has been recently shown to play an important role in the formation and growth of atmospheric aerosols, little information is available on the physicochemical properties of aminium sulfates. In this study, the hygroscopicity, thermostability, and density of alkylaminium sulfates (AASs) have been measured by an integrated aerosol analytical system including a tandem differential mobility analyzer and an aerosol particle mass analyzer. AAS aerosols exhibit monotonic size growth at increasing RH without a well-defined deliquescence point. Mixing of ammonium sulfate (AS) with AASs lowers the deliquescence point corresponding to AS. Particles with AASs show comparable or higher thermostability than that of AS. The density of AASs is determined to be 1.2-1.5 g cm(-3), and an empirical model is developed to predict the density of AASs on the basis of the mole ratio of alkyl carbons to total sulfate. Our results reveal that the heterogeneous uptake of amines on sulfate particles may considerably alter the aerosol properties. In particular, the displacement reaction of alkylamines with ammonium sulfate aerosols leads to a transition from the crystalline to an amorphorous phase and an improved water uptake, considerably enhancing their direct and indirect climate forcing.

Theoretical study on the reaction mechanism of ozone addition to the double bonds of keto-limonene

The reaction mechanism of ozone (O3) addition to the double bonds of gas phase keto-limonene was investigated using ab initio methods. Two different possibilities for O3 addition to the double bond were considered and two corresponding van der Waals complexes (Complex 1 and Complex 2) were found for 1-endo and 2-endo. The rate constants were calculated using the transition state theory at the CCSD(T)/6-31G(d) + CF//B3LYP/6-31G(d,p) level. The high-pressure limit of the total rate constant at 298 K was 3.51 x 10(-16) cm3/(molecule x sec), which was in a good agreement with the experimental data.

Heterogeneous reactions of alkylamines with ammonium sulfate and ammonium bisulfate

The heterogeneous reactions between alkylamines and ammonium salts (ammonium sulfate and ammonium bisulfate) have been studied using a low-pressure fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS) at 293 ± 2 K. The uptake of three alkylamines, i.e., monomethylamine, dimethylamine, and trimethylamine, on ammonium sulfate shows a displacement reaction of ammonium by aminium, evidenced by the release of ammonia monitored using protonated acetone dimer as the reagent ion. For the three alkylamines, the initial uptake coefficients (γ(0)) range from 2.6 × 10(-2) to 3.4 × 10(-2) and the steady-state uptake coefficients (γ(ss)) range from 6.0 × 10(-3) to 2.3 × 10(-4) and decrease as the number of methyl groups on the alkylamine increases. A different reaction mechanism is observed for the uptake of the three alkylamines on ammonium bisulfate, which is featured by an acid-base reaction (neutralization) with irreversible alkylamine loss and no ammonia generation and occurs at a rate limited by diffusion of gaseous alkylamines to the ammonium bisulfate surface. Our results reveal that the reactions between alkylamines and ammonium salts contribute to particle growth and alter the composition of ammonium sulfate and bisulfate aerosols in the atmosphere.

Heterogeneous reaction of NO(2) on fresh and coated soot surfaces

The heterogeneous reaction of nitrogen dioxide (NO(2)) on fresh and coated soot surfaces has been investigated to assess its role in night-time formation of nitrous acid (HONO) in the atmosphere. Soot surfaces were prepared by incomplete combustion of propane and kerosene fuels under lean and rich flame conditions and then processed by heating to evaporate semivolatile species or by coating with pyrene, sulfuric acid, or glutaric acid. Uptake kinetics and HONO yield measurements were performed in a low-pressure fast-flow reactor coupled to a chemical ionization mass spectrometer (CIMS), using atmospheric-level NO(2) concentrations. The uptake coefficient and the HONO yield upon interaction of NO(2) with nascent soot depend on the type of fuel and combustion regime and are the highest for samples prepared using fuel rich flame. Heating the nascent soot samples before exposure to NO(2) removes the organic material from the soot backbone, leading to a significant increase in NO(2) uptake coefficient and HONO yield. Continuous exposure to NO(2) reduces the reactivity of soot because of irreversible deactivation of the surface sites. Our results support the oxidation-reduction mechanism involving adsorptive and reactive centers on soot surface where NO(2) is converted to HONO and other products. Coating of the soot surface by different materials to simulate atmospheric aging has a strong impact on its reactivity toward NO(2) and the resulting HONO production. Coating of pyrene has little effect on either reaction rate or HONO yield. Sulfuric acid coating does not alter the uptake coefficient, but significantly reduces the amount of HONO formed. Coating of glutaric acid significantly increases NO(2) uptake coefficient and HONO yield. The results of our study indicate that the reactivity and HONO generating capacity of internally mixed soot aerosol will depend on the chemical composition of the coating material and hence will vary considerably in different polluted environments.

Heterogeneous chemistry of alkylamines with sulfuric acid: implications for atmospheric formation of alkylaminium sulfates

The heterogeneous interaction of alkylamines with sulfuric acid has been investigated to assess the role of amines in aerosol growth through the formation of alkylaminium sulfates. The kinetic experiments were conducted in a low-pressure fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS). The measurements of heterogeneous uptake of methylamine, dimethylamine, and trimethylamine were performed in the acidity range of 59-82 wt % H(2)SO(4) and between 243 and 283 K. Irreversible reactive uptakes were observed for all three alkylamines, with comparable uptake coefficients (gamma) in the range of 2.0 x 10(-2) to 4.4 x 10(-2). The measured gamma value was slightly higher in more concentrated sulfuric acid and at lower temperatures. The results imply that the heterogeneous reactions of alkylamines contribute effectively to the growth of atmospheric acidic particles and, hence, secondary organic aerosol formation.

Formation of nanoparticles of blue haze enhanced by anthropogenic pollution

The molecular processes leading to formation of nanoparticles of blue haze over forested areas are highly complex and not fully understood. We show that the interaction between biogenic organic acids and sulfuric acid enhances nucleation and initial growth of those nanoparticles. With one cis-pinonic acid and three to five sulfuric acid molecules in the critical nucleus, the hydrophobic organic acid part enhances the stability and growth on the hydrophilic sulfuric acid counterpart. Dimers or heterodimers of biogenic organic acids alone are unfavorable for new particle formation and growth because of their hydrophobicity. Condensation of low-volatility organic acids is hindered on nano-sized particles, whereas ammonia contributes negligibly to particle growth in the size range of 3-30 nm. The results suggest that initial growth from the critical nucleus to the detectable size of 2-3 nm most likely occurs by condensation of sulfuric acid and water, implying that anthropogenic sulfur emissions (mainly from power plants) strongly influence formation of terrestrial biogenic particles and exert larger direct and indirect climate forcing than previously recognized.

Theoretical investigation of the NO₃radical addition to double bonds of limonene

The addition reactions of NO(3) to limonene have been investigated using ab initio methods. Six different possibilities for NO(3) addition to the double bonds, which correspond to the two C-C double bonds (endocyclic or exocyclic) have been considered. The negative activation energies for the addition of NO(3) to limonene are calculated and the energies of NO(3)-limonene radical adducts are found to be 14.55 to 20.17 kcal mol-1 more stable than the separated NO(3) and limonene at the CCSD(T)/6-31G(d) + CF level. The results also indicate that the endocyclic addition reaction is more energetically favorable than the exocyclic one.

Kinetic and Mechanistic Study of the Gas-Phase Reactions of a Series of Vinyl Ethers with the Nitrate Radical

NO(3) oxidation of methyl, ethyl, propyl, and butyl vinyl ethers has been studied under tropospheric conditions (atmospheric pressure and T = 293 +/- 3 K) in the LISA indoor simulation chamber. NO(3) was produced inside the reactor by thermal decomposition of N(2)O(5) previously added to the air-VOC mixture, and concentrations were monitored using FTIR spectrometry. All the kinetic experiments were carried out by relative rate technique using isoprene as reference compound, leading to the rate constants k(1) = (7.2 +/- 1.5) x 10(-13), k(2) = (13.1 +/- 2.7) x 10(-13), k(3) = (13.3 +/- 3.0) x 10(-13), and k(4) = (17.0 +/- 3.7) x 10(-13) cm(3) molecule(-1) s(-1) for methyl, ethyl, propyl, and butyl vinyl ethers, respectively. Main oxidation products have been identified like being formaldehyde and respectively methyl, ethyl, propyl, and butyl formates. Production yields of oxidation products were close to 50%. Oxygenated nitrates and peroxynitrates were also detected.

Quantification of Hydroxycarbonyls from OH−Isoprene Reactions

Hydroxycarbonyls arising from OH-initiated reactions of isoprene have been quantified by the technique of a flow reactor coupled to proton-transfer reaction mass spectrometry (PTR-MS) detection. The yields of C5- and C4-hydroxycarbonyls are (19.3 +/- 6.1)% and (3.3 +/- 1.6)%, respectively, measured at a flow tube pressure of about 100 Torr and at a temperature of 298 +/- 2 K. A yield of (8.4 +/- 2.4)% is obtained for the unsaturated carbonyl C5H8O, confirming that internal OH addition represents the minor channel in the initial OH-isoprene reaction. The results show that those carbonyl compounds account for the most previously unquantified carbon, enabling the isoprene carbon closure. The study also reveals novel aspects of the delta-hydroxyalkoxy radical degradation mechanism, which is essential for modeling tropospheric O3 formation. In addition, this work demonstrates the application of PTR-MS for quantification of products of hydrocarbon reactions, which should have profound impacts on elucidation of the chemistry of atmospheric anthropogenic and biogenic hydrocarbons.

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.