Theoretical studies of the reaction of hydroperoxy radicals (HO2) with ethyl peroxy (CH3CH2O2), acetyl peroxy (CH3C(O)O2), and acetonyl peroxy (CH3C(O)CH2O2) radicals

Gas-Phase Oxidation of Atmospherically Relevant Unsaturated Hydrocarbons by Acyl Peroxy Radicals

This study assesses the atmospheric impact of reactions between unsaturated hydrocarbons such as isoprene and monoterpenes and peroxy radicals containing various functional groups. We find that reactions between alkenes and acyl peroxy radicals have reaction rates high enough to be feasible in the atmosphere and lead to high molar mass accretion products. Moreover, the reaction between unsaturated hydrocarbons and acyl peroxy radicals leads to an alkyl radical, to which molecular oxygen rapidly adds. This finding is confirmed by both theoretical calculations and experiments. The formed perester peroxy radical may either undergo further H-shift reactions or react bimolecularly. The multifunctional oxygenated compounds formed through acyl peroxy radical + alkene reactions are potentially important contributors to particle formation and growth. Thus, acyl peroxy radical-initiated oxidation chemistry may need to be included in atmospheric models.

Acetonyl Peroxy and Hydroperoxy Self- and Cross-Reactions: Temperature-Dependent Kinetic Parameters, Branching Fractions, and Chaperone Effects

The temperature-dependent kinetic parameters, branching fractions, and chaperone effects of the self- and cross-reactions between acetonyl peroxy (CH3C(O)CH2O2) and hydro peroxy (HO2) have been studied using pulsed laser photolysis coupled with infrared (IR) wavelength-modulation spectroscopy and ultraviolet absorption (UVA) spectroscopy. Two IR lasers simultaneously monitored HO2 and hydroxyl (OH), while UVA measurements monitored CH3C(O)CH2O2. For the CH3C(O)CH2O2 self-reaction (T = 270-330 K), the rate parameters were determined to be A = (1.5-0.3+0.4) × 10-13 and Ea/R = -996 ± 334 K and the branching fraction to the alkoxy channel, k2b/k2, showed an inverse temperature dependence following the expression, k2b/k2 = (2.27 ± 0.62) - [(6.35 ± 2.06) × 10-3] T(K). For the reaction between CH3C(O)CH2O2 and HO2 (T = 270-330 K), the rate parameters were determined to be A = (3.4-1.5+2.5) × 10-13 and Ea/R = -547 ± 415 K for the hydroperoxide product channel and A = (6.23-4.4+15.3) × 10-17 and Ea/R = -3100 ± 870 K for the OH product channel. The branching fraction for the OH channel, k1b /k1, follows the temperature-dependent expression, k1b/k1 = (3.27 ± 0.51) - [(9.6 ± 1.7) × 10-3] T(K). Determination of these parameters required an extensive reaction kinetics model which included a re-evaluation of the temperature dependence of the HO2 self-reaction chaperone enhancement parameters due to the methanol-hydroperoxy complex. The second-law thermodynamic parameters for KP,M for the formation of the complex were found to be ΔrH250K° = -38.6 ± 3.3 kJ mol-1 and ΔrS250K° = -110.5 ± 13.2 J mol-1 K-1, with the third-law analysis yielding ΔrH250K° = -37.5 ± 0.25 kJ mol-1. The HO2 self-reaction rate coefficient was determined to be k4 = (3.34-0.80+1.04) × 10-13 exp [(507 ± 76)/T]cm3 molecule-1 s-1 with the enhancement term k4,M″ = (2.7-1.7+4.7) × 10-36 exp [(4700 ± 255)/T]cm6 molecule-2 s-1, proportional to [CH3OH], over T = 220-280 K. The equivalent chaperone enhancement parameter for the acetone-hydroperoxy complex was also required and determined to be k4,A″ = (5.0 × 10-38 - 1.4 × 10-41) exp[(7396 ± 1172)/T] cm6 molecule-2 s-1, proportional to [CH3C(O)CH3], over T = 270-296 K. From these parameters, the rate coefficients for the reactions between HO2 and the respective complexes over the given temperature ranges can be estimated: for HO2·CH3OH, k12 = [(1.72 ± 0.050) × 10-11] exp [(314 ± 7.2)/T] cm3 molecule-1 s-1 and for HO2·CH3C(O)CH3, k15 = [(7.9 ± 0.72) × 10-17] exp [(3881 ± 25)/T] cm3 molecule-1 s-1. Lastly, an estimate of the rate coefficient for the HO2·CH3OH self-reaction was also determined to be k13 = (1.3 ± 0.45) × 10-10 cm3 molecule-1 s-1.

Performance and mechanisms of iron/copper-doped zirconium-based catalyst containing hydroxyl radicals for enhanced removal of gaseous benzene

In the present study, novel copper-doped zirconium-based MOF (UIO-66) and copper-doped iron-based UIO-66 catalysts were prepared by hydrothermal synthesis method to improve the removal performance of gaseous benzene. The characteristics of the catalysts were analyzed by means of XRD, SEM, XPS, BET, and EPR. The copper loading catalyst had high crystallinity and irregular globular. The three kinds of catalysts with different Cu/Fe ratios had regular cubic shape. Compared with the catalyst supported with single copper, the bimetal Cu/Fe modification had a certain adjustment effect on the morphology, which specifically reflected in the uniform size and shape of catalyst particles with better dispersibility. The factors of different metal loading, dose of H₂O₂, and reaction temperature on benzene removal have been studied. It has been observed that in heterogeneous advanced oxidation removal of benzene, 3-Cu@UIO-66 and Cu_1.5/Fe_1.5@UIO-66 achieved the highest benzene removal efficiency of 81.2% and 94.6%, respectively. EPR results showed that the increase of Cu loading and different Cu/Fe ratios promoted the yield of hydroxyl radicals, thus promoted the benzene removal efficiency. The efficiency of heterogeneous oxidation removal of benzene first increased and then decreased with the increase of temperature due to H₂O₂ instability. DFT calculations exhibited that the Fe_oct-Cu-O site was a more effective activation site than the single Fe_oct-O site. Dissociative adsorption occurred with the O-O bond of H₂O₂ cracked, and the formed hydroxyls parallel adsorbed on the benzene surface. The combination of benzene and hydroxyls was strong chemisorption with the torsion angle of benzene ring obviously turned. The work was of great importance for identifying the roles of the novel catalyst for the removal of benzene pollutant from waste gases.

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

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

Computational Study on the Reaction of β-Hydroxyethylperoxy Radical with HO2 and Effects of Water Vapor

The reaction of β-hydroxyethylperoxy radical (β-HEP) and HO₂ with and without water was studied using quantum chemistry and kinetic calculations. The main products are HOCH₂CH₂OOH and ³O₂ for the reaction with and without water, while all other reaction channels can be neglected. The rate coefficients of the reaction follow negative temperature dependence. The pseudo-second-order rate coefficients are 2-4 orders of magnitude smaller for the reaction with saturated water vapor, indicating the negligible contribution of water in this reaction. This is probably also true for other peroxy radicals (except for HO₂), indicating that a large part of previous results on the water enhancement of reaction rate coefficients might have overestimated the influence of water.

Pathways to Highly Oxidized Products in the Δ3-Carene + OH System

Oxidation of the monoterpene Δ3-carene (C₁₀H₁₆) is a potentially important and understudied source of atmospheric secondary organic aerosol (SOA). We present chamber-based measurements of speciated gas and particle phases during photochemical oxidation of Δ3-carene. We find evidence of highly oxidized organic molecules (HOMs) in the gas phase and relatively low-volatility SOA dominated by C₇-C₁₀ species. We then use computational methods to develop the first stages of a Δ3-carene photochemical oxidation mechanism and explain some of our measured compositions. We find that alkoxy bond scission of the cyclohexyl ring likely leads to efficient HOM formation, in line with previous studies. We also find a surprising role for the abstraction of primary hydrogens from methyl groups, which has been calculated to be rapid in the α-pinene system, and suggest more research is required to determine if this is more general to other systems and a feature of autoxidation. This work develops a more comprehensive view of Δ3-carene photochemical oxidation products via measurements and lays out a suggested mechanism of oxidation via computationally derived rate coefficients.

Improved Computational Modeling of the Kinetics of the Acetylperoxy + HO2 Reaction

The acetylperoxy + HO2 reaction has multiple impacts on the troposphere, with a triplet pathway leading to peracetic acid + O2 (reaction 1a) competing with singlet pathways leading to acetic...

Rapid production of highly oxidized molecules in isoprene aerosol via peroxy and alkoxy radical isomerization pathways in low and high NOx environments: Combined laboratory, computational and field studies

Recently, we identified seven novel hydroxy-carboxylic acids resulting from gas-phase reactions of isoprene in the presence of nitrogen oxides (NO_x), ozone (O₃), and/or hydroxyl radicals (OH). In the present study, we provide evidence that hydroxy-carboxylic acids, namely methyltartaric acids (MTA) are: (1) reliable isoprene tracers, (2) likely produced via rapid peroxy radical hydrogen atom (H) shift reactions (autoxidation mechanism) and analogous alkoxy radical H shifts in low and high NO_x environments respectively and (3) representative of aged ambient aerosol in the low NO_x regime. Firstly, MTA are reliable tracers of isoprene aerosol because they have been identified in numerous chamber experiments involving isoprene conducted under a wide range of conditions and are absent in the oxidation of mono- and sesquiterpenes. They are also present in numerous samples of ambient aerosol collected during the past 20 years at several locations in the U.S. and Europe. Furthermore, MTA concentrations measured during a year-long field study in Research Triangle Park (RTP), NC in 2003 show a seasonal trend consistent with isoprene emissions and photochemical activity. Secondly, an analysis of chemical ionization mass spectrometer (CIMS) data of several chamber experiments in low and high NO_x environments show that highly oxidized molecules (HOMs) derived from isoprene that lead to MTAs may be produced rapidly and considered as early generation isoprene oxidation products in the gas phase. Density functional theory calculations show that rapid intramolecular H shifts involving peroxy and alkoxy radicals possess low barriers for methyl-hydroxy-butenals (MHBs) that may represent precursors for MTA. From these results, a viable rapid H shift mechanism is proposed to occur that produces isoprene derived HOMs like MTA. Finally, an analysis of the mechanism shows that autoxidation-like pathways in low and high NO_x may produce HOMs in a few OH oxidation steps like commonly detected methyl tetrol (MT) isoprene tracers. The ratio of MTA/MT in isoprene aerosol is also shown to be significantly greater in field versus chamber samples indicating the importance of such pathways in the atmosphere even for smaller hydrocarbons like isoprene.

Acetonyl Peroxy and Hydro Peroxy Self- and Cross-Reactions: Kinetics, Mechanism, and Chaperone Enhancement from the Perspective of the Hydroxyl Radical Product

Pulsed laser photolysis coupled with infrared (IR) wavelength modulation spectroscopy and ultraviolet (UV) absorption spectroscopy was used to study the kinetics and branching fractions for the acetonyl peroxy (CH₃C(O)CH₂O₂) self-reaction and its reaction with hydro peroxy (HO₂) at a temperature of 298 K and pressure of 100 Torr. Near-IR and mid-IR lasers simultaneously monitored HO₂ and hydroxyl, OH, respectively, while UV absorption measurements monitored the CH₃C(O)CH₂O₂ concentrations. The overall rate constant for the reaction between CH₃C(O)CH₂O₂ and HO₂ was found to be (5.5 ± 0.5) × 10^-12 cm³ molecule^-1 s^-1, and the branching fraction for OH yield from this reaction was directly measured as 0.30 ± 0.04. The CH₃C(O)CH₂O₂ self-reaction rate constant was measured to be (4.8 ± 0.8) × 10^-12 cm³ molecule^-1 s^-1, and the branching fraction for alkoxy formation was inferred from secondary chemistry as 0.33 ± 0.13. An increase in the rate of the HO₂ self-reaction was also observed as a function of acetone (CH₃C(O)CH₃) concentration which is interpreted as a chaperone effect, resulting from hydrogen-bond complexation between HO₂ and CH₃C(O)CH₃. The chaperone enhancement coefficient for CH₃C(O)CH₃ was determined to be k_A^″ = (4.0 ± 0.2) × 10^-29 cm⁶ molecule^-2 s^-1, and the equilibrium constant for HO₂·CH₃C(O)CH₃ complex formation was found to be K_c(R14) = (2.0 ± 0.89) × 10^-18 cm³ molecule^-1; from these values, the rate constant for the HO₂ + HO₂·CH₃C(O)CH₃ reaction was estimated to be (2 ± 1) × 10^-11 cm³ molecule^-1 s^-1. Results from UV absorption cross-section measurements of CH₃C(O)CH₂O₂ and prompt OH radical yields arising from possible oxidation of the CH₃C(O)CH₃-derived alkyl radical are also discussed. Using theoretical methods, no likely pathways for the observed prompt OH radical formation have been found and the prompt OH radical yields thus remain unexplained.

A theoretical investigation on the atmospheric degradation of the radical: reactions with NO, NO2, and NO3

The radical is the key intermediate in the atmospheric oxidation of benzaldehyde, and its further chemistry contributes to local air pollution. The reaction mechanisms of the radical with NO, NO₂, and NO₃ were studied by quantum chemistry calculations at the CCSD(T)/CBS//M06-2X/def2-TZVP level of theory. The explicit potential energy curves were provided in order to reveal the atmospheric fate of the radical comprehensively. The main products of the reaction of with NO are predicted to be , CO₂ and NO₂. The reaction of with NO₂ is reversible, and its main product would be C₆H₅C(O)O₂NO₂ which was predicted to be more stable than PAN (peroxyacetyl nitrate) at room temperature. The decomposition of C₆H₅C(O)O₂NO₂ at different ambient temperatures would be a potential long-range transport source of NO_x in the atmosphere. The predominant products of the reaction are predicted to be C₆H₅C(O)O₂H, C₆H₅C(O)OH, O₂ and O₃, while HO˙ is of minor importance. So, the reaction of with would be an important source of ozone and carboxylic acids in the local atmosphere, and has less contribution to the regeneration of HO˙ radicals. The reaction of with NO₃ should mainly produce , CO₂, O₂ and NO₂, which might play an important role in atmospheric chemistry of peroxy radicals at night, but has less contribution to the night-time conversion of ( and RO˙) to ( and HO˙) in the local atmosphere. The results above are in good accordance with the reported experimental observations.

Reaction between Peroxy and Alkoxy Radicals Can Form Stable Adducts

Peroxy (RO2) and alkoxy (RO) radicals are prototypical intermediates in any hydrocarbon oxidation. In this work, we use computational methods to (1) study the mechanism and kinetics of the RO2 + OH reaction for previously unexplored "R" structures (R = CH(O)CH2 and R = CH3C(O)) and (2) investigate a hitherto unaccounted channel of molecular growth, R'O2 + RO. On the singlet surface, these reactions rapidly form ROOOH and R'OOOR adducts, respectively. The former decomposes to RO + HO2 and R(O)OH + O2 products, while the main decomposition channel for the latter is back to the reactant radicals. Decomposition rates of R'OOOR adducts varied between 103 and 0.015 s-1 at 298 K and 1 atm. The most long-lived R'OOOR adducts likely account for some fraction of the elemental compositions detected in the atmosphere that are commonly assigned to stable covalently bound dimers.

Temperature Dependence Study of the Kinetics and Product Yields of the HO2 + CH3C(O)O2 Reaction by Direct Detection of OH and HO2 Radicals Using 2f-IR Wavelength Modulation Spectroscopy

The HO₂ + CH₃C(O)O₂ reaction consists of three product channels: CH₃C(O)OOH + O₂ (R1a), CH₃C(O)OH + O₃ (R1b), and OH + CH₃C(O)O + O₂ (R1c). The overall rate constant ( k₁) and product yields (α_1a, α_1b, and α_1c) were determined over the atmospherically relevant temperature range of 230-294 K at 100 Torr in N₂. Time-resolved kinetics measurements were performed in a pulsed laser photolysis experiment in a slow flow cell by employing simultaneous infrared (IR) and ultraviolet (UV) absorption spectroscopy. HO₂ and CH₃C(O)O₂ were formed by Cl-atom reactions with CH₃OH and CH₃CHO, respectively. Heterodyne near- and mid-infrared (NIR and MIR) wavelength modulation spectroscopy (WMS) was employed to selectively detect HO₂ and OH radicals. Ultraviolet absorption at 225 and 250 nm was used to detect various peroxy radicals as well as ozone (O₃). These experimental techniques enabled direct measurements of α_1c and α_1b via time-resolved spectroscopic detection in the MIR and the UV, respectively. At each temperature, experiments were performed at various ratios of initial HO₂ and CH₃C(O)O₂ concentrations to quantify the secondary chemistry. The Arrhenius expression was found to be k₁( T) = 1.38_-0.63^+1.17 × 10^-12 exp[(730 ± 170)/ T] cm³ molecule^-1 s^-1. α_1a was temperature-independent while α_1b and α_1c decreased and increased, respectively, with increasing temperatures. These trends are consistent with the current recommendation by the IUPAC data evaluation. Hydrogen-bonded adducts of HO₂ with the precursors, HO₂·CH₃OH and HO₂·CH₃CHO, played a role at lower temperatures; as part of this work, rate enhancements of the HO₂ self-reaction due to reactions of the adducts with HO₂ were also measured.

Highly Oxygenated Organic Molecules (HOM) from Gas-Phase Autoxidation Involving Peroxy Radicals: A Key Contributor to Atmospheric Aerosol

Highly oxygenated organic molecules (HOM) are formed in the atmosphere via autoxidation involving peroxy radicals arising from volatile organic compounds (VOC). HOM condense on pre-existing particles and can be involved in new particle formation. HOM thus contribute to the formation of secondary organic aerosol (SOA), a significant and ubiquitous component of atmospheric aerosol known to affect the Earth’s radiation balance. HOM were discovered only very recently, but the interest in these compounds has grown rapidly. In this Review, we define HOM and describe the currently available techniques for their identification/quantification, followed by a summary of the current knowledge on their formation mechanisms and physicochemical properties. A main aim is to provide a common frame for the currently quite fragmented literature on HOM studies. Finally, we highlight the existing gaps in our understanding and suggest directions for future HOM research.

Computational Investigation of RO2 + HO2 and RO2 + RO2 Reactions of Monoterpene Derived First-Generation Peroxy Radicals Leading to Radical Recycling

The oxidation of biogenically emitted volatile organic compounds (BVOC) plays an important role in the formation of secondary organic aerosols (SOA) in the atmosphere. Peroxy radicals (RO₂) are central intermediates in the BVOC oxidation process. Under clean (low-NO _x) conditions, the main bimolecular sink reactions for RO₂ are with the hydroperoxy radical (HO₂) and with other RO₂ radicals. Especially for small RO₂, the RO₂ + HO₂ reaction mainly leads to closed-shell hydroperoxide products. However, there exist other known RO₂ + HO₂ and RO₂ + RO₂ reaction channels that can recycle radicals and oxidants in the atmosphere, potentially leading to lower-volatility products and enhancing SOA formation. In this work, we present a thermodynamic overview of two such reactions: (a) RO₂ + HO₂ → RO + OH + O₂ and (b) R'O₂ + RO₂ → R'O + RO + O₂ for selected monoterpene + oxidant derived peroxy radicals. The monoterpenes considered are α-pinene, β-pinene, limonene, trans-β-ocimene, and Δ³-carene. The oxidants considered are the hydroxyl radical (OH), the nitrate radical (NO₃), and ozone (O₃). The reaction Gibbs energies were calculated at the DLPNO-CCSD(T)/def2-QZVPP//ωB97X-D/aug-cc-pVTZ level of theory. All reactions studied here were found to be exergonic in terms of Gibbs energy. On the basis of a comparison with previous mechanistic studies, we predict that reaction a and reaction b are likely to be most important for first-generation peroxy radicals from O₃ oxidation (especially for β-pinene), while being less so for most first-generation peroxy radicals from OH and NO₃ oxidation. This is because both reactions are comparatively more exergonic for the O₃ oxidized systems than their OH and NO₃ oxidized counterparts. Our results indicate that bimolecular reactions of certain complex RO₂ may contribute to an increase in radical and oxidant recycling under high HO₂ conditions in the atmosphere, which can potentially enhance SOA formation.

NO2 Suppression of Autoxidation-Inhibition of Gas-Phase Highly Oxidized Dimer Product Formation

Atmospheric autoxidation of volatile organic compounds (VOC) leads to prompt formation of highly oxidized multifunctional compounds (HOM) that have been found crucial in forming ambient secondary organic aerosol (SOA). As a radical chain reaction mediated by oxidized peroxy (RO2) and alkoxy (RO) radical intermediates, the formation pathways can be intercepted by suitable reaction partners, preventing the production of the highest oxidized reaction products, and thus the formation of the most condensable material. Commonly, NO is expected to have a detrimental effect on RO2 chemistry, and thus on autoxidation, whereas the influence of NO2 is mostly neglected. Here it is shown by dedicated flow tube experiments, how high concentration of NO2 suppresses cyclohexene ozonolysis initiated autoxidation chain reaction. Importantly, the addition of NO2 ceases covalently bound dimer production, indicating their production involving acylperoxy radical (RC(O)OO•) intermediates. In related experiments NO was also shown to strongly suppress the highly oxidized product formation, but due to possibility for chain propagating reactions (as with RO2 and HO2 too), the suppression is not as absolute as with NO2. Furthermore, it is shown how NO x reactions with oxidized peroxy radicals lead into indistinguishable product compositions, complicating mass spectral assignments in any RO2 + NO x system. The present work was conducted with atmospheric pressure chemical ionization mass spectrometry (CIMS) as the detection method for the highly oxidized end-products and peroxy radical intermediates, under ambient conditions and at short few second reaction times. Specifically, the insight was gained by addition of a large amount of NO2 (and NO) to the oxidation system, upon which acylperoxy radicals reacted in RC(O)O2 + NO2 → RC(O)O2NO2 reaction to form peroxyacylnitrates, consequently shutting down the oxidation sequence.

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.

Study of low temperature chlorine atom initiated oxidation of methyl and ethyl butyrate using synchrotron photoionization TOF-mass spectrometry

The initial oxidation products of methyl butyrate (MB) and ethyl butyrate (EB) are studied using a time- and energy-resolved photoionization mass spectrometer.

Atmospheric fate of methyl vinyl ketone: peroxy radical reactions with NO and HO2

First generation product yields from the OH-initiated oxidation of methyl vinyl ketone (3-buten-2-one, MVK) under both low and high NO conditions are reported. In the low NO chemistry, three distinct reaction channels are identified leading to the formation of (1) OH, glycolaldehyde, and acetyl peroxy R2a , (2) a hydroperoxide R2b , and (3) an α-diketone R2c . The α-diketone likely results from HOx-neutral chemistry previously only known to occur in reactions of HO2 with halogenated peroxy radicals. Quantum chemical calculations demonstrate that all channels are kinetically accessible at 298 K. In the high NO chemistry, glycolaldehyde is produced with a yield of 74 ± 6.0%. Two alkyl nitrates are formed with a combined yield of 4.0 ± 0.6%. We revise a three-dimensional chemical transport model to assess what impact these modifications in the MVK mechanism have on simulations of atmospheric oxidative chemistry. The calculated OH mixing ratio over the Amazon increases by 6%, suggesting that the low NO chemistry makes a non-negligible contribution toward sustaining the atmospheric radical pool.

Direct kinetic study of OH and O3 formation in the reaction of CH3C(O)O2 with HO2

The reaction between HO2 and CH3C(O)O2 has three exothermic product channels, forming OH (R3a), peracetic acid (R3b), and acetic acid plus O3 (R3c). The branching ratios of the OH- and ozone-forming reaction channels were determined using a combination of laser-induced fluorescence (LIF, for time-resolved OH concentration measurement) and transient absorption spectroscopy (TAS, for time-resolved O3 concentration measurement) following pulsed laser generation of HO2 and CH3C(O)O2 from suitable precursors. TAS was also used to determine the initial concentration of the reactant peroxy radicals. The data were evaluated by numerical simulation using kinetic models of the measured concentration profiles; a Monte Carlo approach was used to estimate the uncertainties of the rate constants (k3) and branching ratios (α) thus obtained. The reaction channel forming OH (R3a) was found to be the most important with α3a = 0.61 ± 0.09 and α3c = 0.16 ± 0.08. The overall rate coefficient of the title reaction was found to be k3 = (2.1 ± 0.4) × 10(-11) cm(3) molecule(-1) s(-1) for both HO2 and DO2. Use of DO2 resulted in an increase in α3a to 0.80 ± 0.14. Comparison with former studies shows that OH formation via (R3) has been underestimated significantly to date. Possible reasons for these discrepancies and atmospheric implications are discussed.

Theoretical study of the mechanisms and kinetics of the reactions of hydroperoxy (HO2) radicals with hydroxymethylperoxy (HOCH2O2) and methoxymethylperoxy (CH3OCH2O2) radicals

The peroxy–peroxy radical reactions show spin, conformation and temperature dependence, forming formic acid and hydroxyl radicals.

Peculiarities of β-pinene autoxidation

The thermal oxidation of the renewable olefin β-pinene with molecular oxygen was experimentally and computationally investigated. Peroxyl radicals abstract weakly bonded allylic hydrogen atoms from the substrate, yielding allylic hydroperoxides (i.e., myrtenyl and pinocarvyl hydroperoxide). In addition, peroxyl radicals add to the C=C bond of the substrate to form an epoxide. It was found that a relatively high peroxyl radical concentration, together with the high rate of peroxyl cross-reactions, make radical-radical reactions surprisingly important for this particular substrate. Approximately 60 % of these peroxyl cross-reactions lead to termination (radical destruction), keeping a radical chain length of approximately 4 at 10 % conversion. Numerical simulation of the reaction-based on the proposed reaction mechanism and known or predicted rate constants-demonstrate the importance of peroxyl cross-reactions for the formation of alkoxyl radicals, which are the precursor of alcohol and ketone products.

Computational study of the reaction mechanism of the methylperoxy self-reaction

To provide insight on the reaction mechanism of the methyperoxy (CH(3)O(2)•) self-reaction, stationary points on both the spin-singlet and the spin-triplet potential energy surfaces of 2(CH(3)O(2)•) have been searched at the B3LYP/6-311++G(2df,2p) level. The relative energies, enthalpies, and free energies of these stationary points are calculated using CCSD(T)/cc-pVTZ. Our theoretical results indicate that reactions on a spin-triplet potential energy surface are kinetically unfavorable due to high free energy barriers, while they are more complicated on the spin-singlet surface. CH(3)OOCH(3) + O(2)(1) can be produced directly from 2(CH(3)O(2)•), while in other channels, three spin-singlet chain-structure intermediates are first formed and subsequently dissociated to produce different products. Besides the dominant channels producing 2CH(3)O• + O(2) and CH(3)OH + CH(2)O + O(2) as determined before, the channels leading to CH(3)OOOH + CH(2)O and CH(3)O• + CH(2)O + HO(2)• are also energetically favorable in the self-reaction of CH(3)O(2)• especially at low temperature according to our results.

Effects of the substituents on the reactivity of carbonyl oxides. A theoretical study on the reaction of substituted carbonyl oxides with water

The reactions between fifteen carbonyl oxides and water have been investigated with the aim of contributing to a better understanding of the effects of the substituents in the reactivity of carbonyl oxides. We have employed density functional theory and large scale ab initio methods (CCSD(T), CASSCF, and CASPT2), combined with transition state theory, to investigate the addition of water to carbonyl oxide and, for those carbonyl oxides having a methyl substituent in syn, the hydrogen transfer from the methyl group to the terminal oxygen of carbonyl oxide. In this case, the water acts as a catalyst and this reaction can contribute to the atmospheric formation of a hydroxyl radical. Carbonyl oxides with electron withdrawing substituents and zwitterionic character have low energy barriers and react fast, whereas carbonyl oxides with electron releasing substituents have high energy barriers and react slowly. The position of the substituents plays also an important role and carbonyl oxides having a hydrogen atom substituent in syn react faster than carbonyl oxides having a hydrogen atom substituent in anti. The differences in the reactivity of different substituted carbonyl oxides raise up to ten orders of magnitude and the branching ratios for the two different reactions investigated are also reported.

Near-infrared kinetic spectroscopy of the HO2 and C2H5O2 self-reactions and cross reactions

The self-reactions and cross reactions of the peroxy radicals C2H5O2 and HO2 were monitored using simultaneous independent spectroscopic probes to observe each radical species. Wavelength modulation (WM) near-infrared (NIR) spectroscopy was used to detect HO2, and UV absorption monitored C2H5O2. The temperature dependences of these reactions were investigated over a range of interest to tropospheric chemistry, 221-296 K. The Arrhenius expression determined for the cross reaction, k2(T) = (6.01(-1.47)(+1.95)) x 10(-13) exp((638 +/- 73)/T) cm3 molecules(-1) s(-1) is in agreement with other work from the literature. The measurements of the HO2 self-reaction agreed with previous work from this lab and were not further refined. The C2H5O2 self-reaction is complicated by secondary production of HO2. This experiment performed the first direct measurement of the self-reaction rate constant, as well as the branching fraction to the radical channel, in part by measurement of the secondary HO2. The Arrhenius expression for the self-reaction rate constant is k3(T) = (1.29(-0.27)(+0.34)) x 10(-13)exp((-23 +/- 61)/T) cm3 molecules(-1) s(-1), and the branching fraction value is alpha = 0.28 +/- 0.06, independent of temperature. These values are in disagreement with previous measurements based on end product studies of the branching fraction. The results suggest that better characterization of the products from RO2 self-reactions are required.

Thermochemistry and kinetics of acetonylperoxy radical isomerisation and decomposition: a quantum chemistry and CVT/SCT approach

CBS-QB3 calculations have been used to determine thermochemical and kinetic parameters of the isomerisation and decomposition reactions of the acetonylperoxy radical, CH3C(O)CH2OO* , which has been formed via the reaction of acetonyl radical with O2 leading to the formation of an energised peroxy adduct with a calculated well depth of near 111 kJ mol(-1). This species can undergo subsequent 1,5 and 1,3 H-shifts to give the primary and secondary radicals: C*H2C(O)CH2OOH and CH3C(O)C*HOOH, respectively, or rearrange to give a 3-methyl-1,2-dioxetan-3-yloxy radical. Rate constants for isomerisation and subsequent decomposition have been estimated using canonical variational transition state theory with small curvature tunneling cvt/sct. The variational effect for the isomerisation channels is only moderate but the tunneling correction is significant at temperatures up to 1000 K; the formation of a primary radical by a 1,5-shift is the main reaction channel and the competition with the secondary one starts only at around 1500 K. The fate of the primary acetonylhydroperoxy radical is predominantly to form oxetan-3-one while the ketene and 1-oxy-3-hydroxyacetonyl radical channels only compete with the formation of oxetan-3-one at temperatures >1200 K. In addition, consistent and reliable enthalpies of formation have been computed for the molecules acetonylhydroperoxide, 1,3-dihydroxyacetone, methylglyoxal and cyclobutanone, and for some related radicals.

Investigation of the radical product channel of the CH3C(O)O2 + HO2 reaction in the gas phase

The reaction of CH(3)C(O)O(2) with HO(2) has been investigated at 296 K and 700 Torr using long path FTIR spectroscopy, during photolysis of Cl(2)/CH(3)CHO/CH(3)OH/air mixtures. The branching ratio for the reaction channel forming CH(3)C(O)O, OH and O(2) (reaction ) has been determined from experiments in which OH radicals were scavenged by addition of benzene to the system, with subsequent formation of phenol used as the primary diagnostic for OH radical formation. The dependence of the phenol yield on benzene concentration was found to be consistent with its formation from the OH-initiated oxidation of benzene, thereby confirming the presence of OH radicals in the system. The dependence of the phenol yield on the initial peroxy radical precursor reagent concentration ratio, [CH(3)OH](0)/[CH(3)CHO](0), is consistent with OH formation resulting mainly from the reaction of CH(3)C(O)O(2) with HO(2) in the early stages of the experiments, such that the limiting yield of phenol at high benzene concentrations is well-correlated with that of CH(3)C(O)OOH, a well-established product of the CH(3)C(O)O(2) + HO(2) reaction (via channel (3a)). However, a delayed source of phenol was also identified, which is attributed mainly to an analogous OH-forming channel of the reaction of HO(2) with HOCH(2)O(2) (reaction ), formed from the reaction of HO(2) with product HCHO. This was investigated in additional series of experiments in which Cl(2)/CH(3)OH/benzene/air and Cl(2)/HCHO/benzene/air mixtures were photolysed. The various reaction systems were fully characterised by simulations using a detailed chemical mechanism. This allowed the following branching ratios to be determined: CH(3)C(O)O(2) + HO(2)--> CH(3)C(O)OOH + O(2), k(3a)/k(3) = 0.38 +/- 0.13; --> CH(3)C(O)OH + O(3), k(3b)/k(3) = 0.12 +/- 0.04; --> CH(3)C(O)O + OH + O(2), k(3c)/k(3) = 0.43 +/- 0.10: HOCH(2)O(2) + HO(2)--> HCOOH + H(2)O + O(2), k(17b)/k(17) = 0.30 +/- 0.06; --> HOCH(2)O + OH + O(2), k(17c)/k(17) = 0.20 +/- 0.05. The results therefore provide strong evidence for significant participation of the radical-forming channels of these reactions, with the branching ratio for the title reaction being in good agreement with the value reported in one previous study. As part of this work, the kinetics of the reaction of Cl atoms with phenol (reaction (14)) have also been investigated. The rate coefficient was determined relative to the rate coefficient for the reaction of Cl with CH(3)OH, during the photolysis of mixtures of Cl(2), phenol and CH(3)OH, in either N(2) or air at 296 K and 760 Torr. A value of k(14) = (1.92 +/- 0.17) x 10(-10) cm(3) molecule(-1) s(-1) was determined from the experiments in N(2), in agreement with the literature. In air, the apparent rate coefficient was about a factor of two lower, which is interpreted in terms of regeneration of phenol from the product phenoxy radical, C(6)H(5)O, possibly via its reaction with HO(2).

Kinetics and branching ratio studies of the reaction of C2H5O2 + HO2 using chemical ionisation mass spectrometry

The overall rate coefficient for the reaction of C(2)H(5)O(2) with HO(2) was determined using a turbulent flow chemical ionization mass spectrometer (TF-CIMS) system over the pressure range of 75 to 200 Torr and temperatures between 195 and 298 K. The temperature dependence of the overall rate coefficient for the reaction between C(2)H(5)O(2) and HO(2) was fitted using the following Arrhenius expression: k(T) = (2.08) x 10(-13) exp [(864 +/- 79)/T] cm(-3) molecule(-1) s(-1). The upper limits for the branching ratios for reactive channels leading to O(3) and OH production were quantified for the first time. A tropospheric model has been used to assess the impact of the experimental error of the rate coefficients determined in this study on predicted concentrations of a number of key species, including O(3), OH, HO(2), NO and NO(2). In all cases it is found that the propagated error is very small and will not in itself be a major cause of uncertainty in modelled concentrations. However, at low temperatures, where there is a wide discrepancy between existing kinetic studies, modelling using the range of kinetic data in the literature shows a small but significant variation for [C(2)H(5)O(2)], [C(2)H(5)OOH], [NO(x)] and the HO(2) : OH ratio. Furthermore, a structure-activity relationship (SAR) was developed to rationalise the reactivity of the reaction between RO(2) and HO(2).