Kinetics of the Cross Reactions of CH3O2 and C2H5O2 Radicals with Selected Peroxy Radicals

Rate constant and branching ratio of the reaction of ethyl peroxy radicals with methyl peroxy radicals

The cross-reaction of ethyl peroxy radicals (C₂H₅O₂) with methyl peroxy radicals (CH₃O₂) (R1) has been studied using laser photolysis coupled to time resolved detection of the two different peroxy radicals by continuous wave cavity ring down spectroscopy (cw-CRDS) in their AÃ-X̃ electronic transition in the near-infrared region, C₂H₅O₂ at 7602.25 cm^-1, and CH₃O₂ at 7488.13 cm^-1. This detection scheme is not completely selective for both radicals, but it is demonstrated that it has great advantages compared to the widely used, but unselective UV absorption spectroscopy. Peroxy radicals were generated from the reaction of Cl-atoms with the appropriate hydrocarbon (CH₄ and C₂H₆) in the presence of O₂, whereby Cl-atoms were generated by 351 nm photolysis of Cl₂. For different reasons detailed in the manuscript, all experiments were carried out under excess of C₂H₅O₂ over CH₃O₂. The experimental results were best reproduced by an appropriate chemical model with a rate constant for the cross-reaction of k = (3.8 ± 1.0) × 10^-13 cm³ s^-1 and a yield for the radical channel, leading to CH₃O and C₂H₅O, of (ϕ_1a = 0.40 ± 0.20).

A neglected pathway for the accretion products formation in the atmosphere

Highly oxygenated organic molecules (HOM) formed by the autoxidation of α-pinene initiated by OH radicals play an important role in new particle formation. It is believed that the accretion products, ROOR´, formed by the self- and cross-reaction of peroxy radicals (RO₂ + R'O₂ reactions), have extremely low volatility and are more likely to participate in nucleation. However, the mechanism of ROOR´ formation has not been fully demonstrated by experiment or theoretical calculation. Herein, we propose a novel mechanism of RO₂ reacting with α-pinene (RO₂ + α-pinene reactions) that have much lower potential barriers and larger rate constants than the reaction of RO₂ with R'O₂, which explains the ROOR´ formation found in the mass spectrometry experiments. The ROOR´ resulting from the reaction of RO₂ with α-pinene can produce HOM dimers and trimers with a higher oxygen-to‑carbon (O/C) ratio through a autoxidation chain. We also demonstrated that the presence of NO_x and HO₂ radical will reduce the RO₂ concentration, but cannot completely inhibit the formation of HOM monomers and ROOR´. Even if one or both of RO₂ radicals are acyl peroxy radicals (RC(O)O₂), the potential barriers of the reactions between RC(O)O₂ and α-pinene (RC(O)O₂ + α-pinene reactions) are lower than that of RO₂ reacting with RC(O)O₂ (RO₂ + RC(O)O₂ reactions) or RC(O)O₂ self-reactions (RC(O)O₂ + RC(O)O₂ reactions). The current work revealed, for the first time, a mechanism of RO₂/RC(O)O₂ reacting with α-pinene in the atmosphere, which provides new insight into the atmospheric chemistry of accretion products as SOA precursors.

Gas-Phase Peroxyl Radical Recombination Reactions: A Computational Study of Formation and Decomposition of Tetroxides

The recombination (“dimerization”) of peroxyl radicals (RO₂•) is one of the pathways suggested in the literature for the formation of peroxides (ROOR′, often referred to as dimers or accretion products in the literature) in the atmosphere. It is generally accepted that these dimers play a major role in the first steps of the formation of submicron aerosol particles. However, the precise reaction pathways and energetics of RO₂• + R′O₂• reactions are still unknown. In this work, we have studied the formation of tetroxide intermediates (RO₄R′): their formation from two peroxyl radicals and their decomposition to triplet molecular oxygen (³O₂) and a triplet pair of alkoxyl radicals (RO•). We demonstrate this mechanism for several atmospherically relevant primary and secondary peroxyl radicals. The potential energy surface corresponds to an overall singlet state. The subsequent reaction channels of the alkoxyl radicals include, but are not limited to, their dimerization into ROOR′. Our work considers the multiconfigurational character of the tetroxides and the intermediate phases of the reaction, leading to reliable mechanistic insights for the formation and decomposition of the tetroxides. Despite substantial uncertainties in the computed energetics, our results demonstrate that the barrier heights along the reaction path are invariably small for these systems. This suggests that the reaction mechanism, previously validated at a multireference level only for methyl peroxyl radicals, is a plausible pathway for the formation of aerosol-relevant larger peroxides in the atmosphere.

Rate Constants and Branching Ratios for the Self-Reaction of Acetyl Peroxy (CH3C(O)O2•) and Its Reaction with CH3O2

The self-reaction of acetylperoxy radicals (CH3C(O)O2•) (R1) as well as their reaction with methyl peroxy radicals (CH3O2•) (R2) have been studied using laser photolysis coupled to a selective time resolved detection of three different radicals by cw-CRDS in the near-infrared range: CH3C(O)O2• was detected in the Ã-X˜ electronic transition at 6497.94 cm−1, HO2• was detected in the 2ν1 vibrational overtone at 6638.2 cm−1, and CH3O2• radicals were detected in the Ã-X˜ electronic transition at 7489.16 cm−1. Pulsed photolysis of different precursors at different wavelengths, always in the presence of O2, was used to generate CH3C(O)O2• and CH3O2• radicals: acetaldehyde (CH3CHO/Cl2 mixture or biacetyle (CH3C(O)C(O)CH3) at 351 nm, and acetone (CH3C(O)CH3) or CH3C(O)C(O)CH3 at 248 nm. From photolysis experiments using CH3C(O)C(O)CH3 or CH3C(O)CH3 as precursor, the rate constant for the self-reaction was found with k1 = (1.3 ± 0.3) × 10−11 cm3s−1, in good agreement with current recommendations, while the rate constant for the cross reaction with CH3O2• was found to be k2 = (2.0 ± 0.4) × 10−11 cm3s−1, which is nearly two times faster than current recommendations. The branching ratio of (R2) towards the radical products was found at 0.67, compared with 0.9 for the currently recommended value. Using the reaction of Cl•-atoms with CH3CHO as precursor resulted in radical profiles that were not reproducible by the model: secondary chemistry possibly involving Cl• or Cl2 might occur, but could not be identified.

Theoretical investigations on mechanisms and kinetics of CH2XO2 (X=F, Cl) with Cl reaction in the atmosphere

The reactions of the CH₂XO₂ (X=F, Cl) with chlorine radical have been firstly investigated utilizing the BMC-CCSD//B3LYP method. The comprehensive calculations indicate that the association-elimination and S_N2 displacement reaction mechanisms existed on the singlet potential energy surface (PES), and H-abstraction and S_N2 displacement reaction mechanism existed on the triplet PES for the CH₂XO₂ (X=F, Cl) + Cl reactions. On the triplet PES, the dominant reactions are production of P3_X (CHXO₂ (X=F, Cl) + HCl) by direct H-abstraction. On the singlet PES, three energy-rich adducts, IM1_X (CH₂XOOCl (X=F, Cl)), IM2_X (CH₂XOClO (X=F, Cl)), and IM3_X (CH₂(OX)OCl (X=F, Cl)), are produced. RRKM-computed reveals that IM1_X (CH₂XOOCl (X=F, Cl)) produced by collisional stabilization occupied the reaction T ≤ 500 and 400 K, respectively, while P1_X (CHXO (X=F, Cl) + HOCl) are forecasted to be the dominant products at high temperatures. The atmospheric lifetime of CH₂FO₂ and CH₂ClO₂ in Cl is around 1.18 and 2.50 weeks, respectively. Time-dependent density functional theory (TDDFT) computations imply that IM1_X (CH₂XOOCl (X=F, Cl)) will photolyze under the sunlight. The current results could guide us to well understand the mechanism of the CH₂XO₂ (X=F, Cl) + Cl reactions and may be helpful to understand Cl-combustion chemistry. Graphical Abstract Predicted rate constant of the dominant pathways and the total rate constants at 760 Torr, N2 in the temperature region of 200-3000 K for the CH₂XO₂ (X=F, Cl) + Cl reactions.

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.

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.

Natural and anthropogenic ethanol sources inNorth America and potential atmospheric impacts of ethanol fuel use

We used an ensemble of aircraft measurements with the GEOS-Chem chemical transport model to constrain present-day North American ethanol sources, and gauge potential long-range impacts of increased ethanol fuel use. We find that current ethanol emissions are underestimated by 50% in Western North America, and overestimated by a factor of 2 in the east. Our best estimate for year-2005 North American ethanol emissions is 670 GgC/y, with 440 GgC/y from the continental U.S. We apply these optimized source estimates to investigate two scenarios for increased ethanol fuel use in the U.S.: one that assumes a complete transition from gasoline to E85 fuel, and one tied to the biofuel requirements of the U.S. Energy Indepence and Security Act (EISA). For both scenarios, increased ethanol emissions lead to higher atmospheric acetaldehyde concentrations (by up to 14% during winter for the All-E85 scenario and 2% for the EISA scenario) and an associated shift in reactive nitrogen partitioning reflected by an increase in the peroxyacetyl nitrate (PAN) to NO(y) ratio. The largest relative impacts occur during fall, winter, and spring because of large natural emissions of ethanol and other organic compounds during summer. Projected changes in atmospheric PAN reflect a balance between an increased supply of peroxyacetyl radicals from acetaldehyde oxidation, and the lower NO(x) emissions for E85 relative to gasoline vehicles. The net effect is a general PAN increase in fall through spring, and a weak decrease over the U.S. Southeast and the Atlantic Ocean during summer. Predicted NO(x) concentrations decrease in surface air over North America (by as much 5% in the All-E85 scenario). Downwind of North America this effect is counteracted by higher NO(x) export efficiency driven by increased PAN production and transport. From the point of view of NO(x) export from North America, the increased PAN formation associated with E85 fuel use thus acts to offset the associated lower NO(x) emissions.

Theoretical study on the mechanism and kinetics for the self-reaction of C2H5O2 radicals

Oxygen-to-oxygen coupling, direct H-abstraction and oxygen-to-(α)carbon nucleophilic substitution processes have been investigated for both the singlet and triplet self-reaction of C(2)H(5)O(2) radicals at the CCSD(T)/cc-pVDZ//B3LYP/6-311G(2d,2p) level to evaluate the reaction mechanisms, possible products and rate constants. The calculated results show that the title reaction mainly occurs through the singlet oxygen-to-oxygen coupling mechanism with the formation of entrance tetroxide intermediates, and the most dominant product is C(2)H(5)O + HO(2) + CH(3)CHO (P5) generated in channel R5. Beginning from the radical products of P5 (C(2)H(5)O, HO(2)) and reactant (C(2)H(5)O(2)), five secondary reactions HO(2) + HO(2) (a), HO(2) + C(2)H(5)O (b), C(2)H(5)O + C(2)H(5)O (c), HO(2) + C(2)H(5)O(2) (d), and C(2)H(5)O + C(2)H(5)O(2) (e) mainly proceed on the triplet potential energy surface. Among these reactions, (a), (b), and (d) are kinetically favorable because of lower barrier heights. The calculated rate constants of channel R5 between 200 and 295 K are almost independent of the temperature, which is in agreement with the experimental report. With regard to the final products distribution, CH(3)CHO, C(2)H(5)OH, C(2)H(5)OOH, H(2)O(2), and (3)O(2) are predicted to be major, whereas C(2)H(5)OOC(2)H(5) should be in minor amount.

Proton affinity of methyl peroxynitrate

The equilibrium structures, harmonic vibrational frequencies of methyl peroxynitrate, and structures of protonated methyl peroxynitrate have been investigated using ab initio methods. The methods include the single- and double-excitation quadratic configuration (QCISD) methods and the QCISD(T) method, which incorporates a perturbational estimate of the effects of corrected triple excitation. The lowest-energy gas-phase form of protonated methyl peroxynitrate is a complex between CH3OOH and NO2+. The CH3OOH.NO2+ complex is bound by 22 +/- 2 kcal/mol. The estimated proton affinity of methyl peroxynitrate is 178.8 +/- 3 kcal/mol. A general trend for the proton affinity of ROO-NO2 (peroxynitrates) compounds is discussed.

A reinvestigation of the kinetics and the mechanism of the CH3C(O)O2 + HO2 reaction using both experimental and theoretical approaches

The kinetics and the mechanism of the reaction CH(3)C(O)O(2)+ HO(2) were reinvestigated at room temperature using two complementary approaches: one experimental, using flash photolysis/UV absorption technique and one theoretical, with quantum chemistry calculations performed using the density functional theory (DFT) method with the three-parameter hybrid functional B3LYP associated with the 6-31G(d,p) basis set. According to a recent paper reported by Hasson et al., [J. Phys. Chem., 2004, 108, 5979-5989] this reaction may proceed by three different channels: CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)OOH + O(2) (1a); CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)OH + O(3) (1b); CH(3)C(O)O(2)+ HO(2)--> CH(3)C(O)O + OH + O(2) (1c). In experiments, CH(3)C(O)O(2) and HO(2) radicals were generated using Cl-initiated oxidation of acetaldehyde and methanol, respectively, in the presence of oxygen. The addition of amounts of benzene in the system, forming hydroxycyclohexadienyl radicals in the presence of OH, allowed us to answer that channel (1c) is <10%. The rate constant k(1) of reaction (1) has been finally measured at (1.50 +/- 0.08) x 10(-11) cm(3) molecule(-1) s(-1) at 298 K, after having considered the combination of all the possible values for the branching ratios k(1a)/k(1,)k(1b)/k(1,)k(1c)/k(1) and has been compared to previous measurements. The branching ratio k(1b)/k(1), determined by measuring ozone in situ, was found to be equal to (20 +/- 1)%, a value consistent with the previous values reported in the literature. DFT calculations show that channel (1c) is also of minor importance: it was deduced unambiguously that the formation of CH(3)C(O)OOH + O(2) (X (3)Sigma(-)(g)) is the dominant product channel, followed by the second channel (1b) leading to CH(3)C(O)OH and singlet O(3) and, much less importantly, channel (1c) which corresponds to OH formation. These conclusions give a reliable explanation of the experimental observations of this work. In conclusion, the present study demonstrates that the CH(3)C(O)O(2)+ HO(2) is still predominantly a radical chain termination reaction in the tropospheric ozone chain formation processes.

Atmospheric Chemistry of Propionaldehyde: Kinetics and Mechanisms of Reactions with OH Radicals and Cl Atoms, UV Spectrum, and Self-Reaction Kinetics of CH3CH2C(O)O2 Radicals at 298 K

The kinetics and mechanism of the reactions of Cl atoms and OH radicals with CH3CH2CHO were investigated at room temperature using two complementary techniques: flash photolysis/UV absorption and continuous photolysis/FTIR smog chamber. Reaction with Cl atoms proceeds predominantly by abstraction of the aldehydic hydrogen atom to form acyl radicals. FTIR measurements indicated that the acyl forming channel accounts for (88 +/- 5)%, while UV measurements indicated that the acyl forming channel accounts for (88 +/- 3)%. Relative rate methods were used to measure: k(Cl + CH3CH2CHO) = (1.20 +/- 0.23) x 10(-10); k(OH + CH3CH2CHO) = (1.82 +/- 0.23) x 10(-11); and k(Cl + CH3CH2C(O)Cl) = (1.64 +/- 0.22) x 10(-12) cm3 molecule(-1) s(-1). The UV spectrum of CH3CH2C(O)O2, rate constant for self-reaction, and rate constant for cross-reaction with CH3CH2O2 were determined: sigma(207 nm) = (6.71 +/- 0.19) x 10(-18) cm2 molecule(-1), k(CH3CH2C(O)O2 + CH3CH2C(O)O2) = (1.68 +/- 0.08) x 10(-11), and k(CH3CH2C(O)O2 + CH3CH2O2) = (1.20 +/- 0.06) x 10(-11) cm3 molecule(-1) s(-1), where quoted uncertainties only represent 2sigma statistical errors. The infrared spectrum of C2H5C(O)O2NO2 was recorded, and products of the Cl-initiated oxidation of CH3CH2CHO in the presence of O2 with, and without, NO(x) were identified. Results are discussed with respect to the atmospheric chemistry of propionaldehyde.