Acetone-h6 or -d6 + OH reaction products: evidence for heterogeneous formation of acetic acid in a simulation chamber

Simulation chamber experiments have been carried out at room temperature to investigate the products of the acetone + OH and acetone-d6 + OH reactions using two photoreactors made of Teflon or Pyrex and coupled to GC-FTIR-FID analytical techniques. In the Pyrex chamber, the results demonstrated that the channel forming acetic acid is a minor oxidation route in the atmospheric acetone-h6 + OH reaction (yield <5%), whereas a higher yield of about 20% was obtained in the case of the acetone-d6 + OH reaction, in good agreement with previous studies. Existence of a heterogeneous way of formation of acetic acid has also been identified in the Teflon photoreactor.

Experimental and Theoretical Studies of the Kinetics of the Reactions of OH Radicals with Acetic Acid, Acetic Acid-d3 and Acetic Acid-d4 at Low Pressure

The kinetics of the reactions of OH with acetic acid, acetic acid-d3 and acetic acid-d4 were studied from 2 to 5 Torr and 263-373 K using a discharge flow system with resonance fluorescence detection of the OH radical. The measured rate constants at 300 K for the reaction of OH with acetic acid and acetic acid-d4 (CD3C(O)OD) were (7.42+/-0.12)x10(-13) and (1.09+/-0.18)x10(-13) cm3 molecule-1 s-1 respectively, and the rate constant for the reaction of OH with acetic acid-d3 (CD3C(O)OH) was (7.79+/-0.16)x10(-13) cm3 molecule-1 s-1. These results suggest that the primary mechanism for this reaction involves abstraction of the acidic hydrogen. Theoretical calculations of the kinetic isotope effect as a function of temperature are in good agreement with the experimental measurements using a mechanism involving the abstraction of the acidic hydrogen through a hydrogen-bonded complex. The rate constants for the OH+acetic acid and OH+acetic acid-d4 reactions display a negative temperature dependence described by the Arrhenius equations kH(T)=(2.52+/-1.22)x10(-14) exp((1010+/-150)/T) and kD(T)=(4.62+/-1.33)x10(-16) exp((1640+/-160)/T) cm3 molecule-1 s-1 for acetic acid and acetic acid-d4, respectively, consistent with recent measurements that suggest that the lifetime of acetic acid at the low temperatures of the upper troposphere is shorter than previously believed.

Study of Benzylperoxy Radical Using Laser Photolysis: Ultraviolet Spectrum, Self-Reaction, and Reaction with HO2 Kinetics

The ultraviolet absorption spectrum of benzylperoxy radical and the kinetics of the reactions 2C(6)H(5)CH(2)O(2) --> products (I) and C(6)H(5)CH(2)O(2) + HO(2) --> products (II) are studied. Experiments are carried out using the laser photolysis technique with time-resolved UV-visible absorption spectroscopy over the temperature range 298-353 K and the pressure range 50-200 Torr. The UV spectrum is determined relative to the known cross section of the ethylperoxy radical C(2)H(5)O(2) at 250 nm. Using factor analysis, the spectrum obtained is refined and the concentrations of the main absorbing species are extracted. The kinetic parameters are determined by analyzing and simulating the temporal profiles of the species concentrations and the experimental optical densities in the spectral region 220-280 nm. These are obtained using the recent UV spectra of the absorbing species existing in our mechanism. The Arrhenius expressions for reactions I and II are (cm(3).molecule(-1).s(-1)) k(I) = 2.50 x 10(-14)e(1562/)(T) and k(II) = 5.70 x 10(-14)e(1649/)(T). Our results are discussed and compared to literature data.

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.

High-resolution IR cavity ring-down spectroscopy of jet-cooled free radicals and other species

Initial spectral results are reported from a newly constructed cavity ringdown spectrometer. The apparatus incorporates a slit-jet expansion, with or without a discharge, to produce cold sample molecules. High spectral resolution in both the near- and mid-IR is obtained by using stimulated Raman scattering of the pulsed amplified output of a cw Ti:Sa ring laser. Molecular spectra presented include the electronic near-IR transitions a (1)Delta(g)(-)<-- X (3)Sigma(g)(-) of O(2) and B (3)Pi(g)<-- A (3)Sigma(u)(+) of metastable N(2) and vibrational overtones of H(2)O (polyad 2) and the OH radical. Fundamental vibrational transitions of CH(3) (nu(3)) in the mid-IR are also observed. This apparatus has demonstrated the potential for obtaining high-resolution spectra of both reactive and non-reactive species throughout the entire IR region.

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.

Atmospheric Chemistry of CH3CHF2 (HFC-152a): Kinetics, Mechanisms, and Products of Cl Atom- and OH Radical-Initiated Oxidation in the Presence and Absence of NOx

Smog chamber/Fourier transform infrared (FTIR) and laser-induced fluorescence (LIF) spectroscopic techniques were used to study the atmospheric degradation of CH3CHF2. The kinetics and products of the Cl(2P(3/2)) (denoted Cl) atom- and the OH radical-initiated oxidation of CH3CHF2 in 700 Torr of air or N2; diluents at 295 +/- 2 K were studied using smog chamber/FTIR techniques. Relative rate methods were used to measure k(Cl + CH3CHF2) = (2.37 +/- 0.31) x 10(-13) and k(OH + CH3CHF2) = (3.08 +/- 0.62) x 10(-14) cm3 molecule(-1) s(-1). Reaction with Cl atoms gives CH3CF2 radicals in a yield of 99.2 +/- 0.1% and CH2CHF2 radicals in a yield of 0.8 +/- 0.1%. Reaction with OH radicals gives CH3CF2 radicals in a yield >75% and CH2CHF2 radicals in a yield <25%. Absolute rate data for the Cl reaction were measured using quantum-state selective LIF detection of Cl(2P(j)) atoms under pseudo-first-order conditions. The rate constant k(Cl + CH3CHF2) was determined to be (2.54 +/- 0.25) x 10(-13) cm3 molecule(-1) s(-1) by the LIF technique, in good agreement with the relative rate results. The removal rate of spin-orbit excited-state Cl(2P(1/2)) (denoted Cl) in collisions with CH3CHF2 was determined to be k(Cl + CH3CHF2) = (2.21 +/- 0.22) x 10(-10) cm3 molecule(-1) s(-1). The atmospheric photooxidation products were examined in the presence and absence of NO(x). In the absence of NO(x)(), the Cl atom-initiated oxidation of CH3CHF2 in air leads to formation of COF2 in a molar yield of 97 +/- 5%. In the presence of NO(x), the observed oxidation products include COF2 and CH3COF. As [NO] increases, the yield of COF2 decreases while the yield of CH3COF increases, reflecting a competition for CH3CF2O radicals. The simplest explanation for the observed dependence of the CH3COF yield on [NO(x)] is that the atmospheric degradation of CH3CF2H proceeds via OH radical attack to give CH3CF2 radicals which add O2 to give CH3CF2O2 radicals. Reaction of CH3CF2O2 radicals with NO gives a substantial fraction of chemically activated alkoxy radicals, [CH3CF2O]. In 1 atm of air, approximately 30% of the alkoxy radicals produced in the CH3CF2O2 + NO reaction possess sufficient internal excitation to undergo "prompt" (rate >10(10) s(-1)) decomposition to give CH3 radicals and COF2. The remaining approximately 70% become thermalized, CH3CF2O, and undergo decomposition more slowly at a rate of approximately 2 x 10(3) s(-1). At high concentrations (>50 mTorr), NO(x) is an efficient scavenger for CH3CF2O radicals leading to the formation of CH3COF and FNO.

Computations on the A-X transition of isoprene-OH-O2 peroxy radicals

Calculations are carried out on the A state of HO2, CH3O2, and CH3CH2O2 and 10 isomers and conformers of the isoprene-OH-O2 peroxy radicals derived from OH addition to isoprene (2-methyl-1,3-butadiene). In addition to calculating vertical and adiabatic excitation energies, we consider the effect of excitation on molecular structure, and examine the OO stretching frequencies, which are known to be major features in the absorption spectra of the A states of the smaller radicals. The two methods used are the configuration interaction with single excitations (CIS) method and time-dependent density functional theory (TD-DFT), both with a range of basis sets up to 6-311++G(2df,2pd). TD-DFT overestimates excitation energies considerably, while CIS tends to underestimate them slightly. TD-DFT does seem to capture the trend in excitation energy vs. size for the smaller peroxy radicals. Conformation and configuration strongly affect the excitation energies of the peroxy radicals from isoprene. CIS calculations indicate that the intramolecular OH--O hydrogen bonds, present in the ground state of some peroxy radicals from isoprene, are weakened or broken in the excited state, while TD-DFT calculations suggest they are retained.

Quantum Chemical and Master Equation Simulations of the Oxidation and Isomerization of Vinoxy Radicals

The vinoxy radical, a common intermediate in gas-phase alkene ozonolysis, reacts with O2 to form a chemically activated alpha-oxoperoxy species. We report CBS-QB3 energetics for O2 addition to the parent (*CH2CHO, 1a), 1-methylvinoxy (*CH2COCH3, 1b), and 2-methylvinoxy (CH3*CHCHO, 1c) radicals. CBS-QB3 predictions for peroxy radical formation agree with experimental data, while the G2 method systematically overestimates peroxy radical stability. RRKM/master equation simulations based on CBS-QB3 data are used to estimate the competition between prompt isomerization and thermalization for the peroxy radicals derived from 1a, 1b, and 1c. The lowest energy isomerization pathway for radicals 4a and 4c (derived from 1a and 1c, respectively) is a 1,4-shift of the acyl hydrogen requiring 19-20 kcal/mol. The resulting hydroperoxyacyl radical decomposes quantitatively to form *OH. The lowest energy isomerization pathway for radical 4b (derived from 1b) is a 1,5-shift of a methyl hydrogen requiring 26 kcal/mol. About 25% of 4a, but only approximately 5% of 4c, isomerizes promptly at 1 atm pressure. Isomerization of 4b is negligible at all pressures studied.

Theoretical and Experimental Study of the Product Branching in the Reaction of Acetic Acid with OH Radicals

The product distribution of the reaction of acetic acid, CH(3)COOH, with hydroxyl radicals, OH, was studied experimentally and theoretically. Mass-spectrometric measurements at 290 K and 2 Torr of He of the CO(2) yield versus the loss of acetic acid yielded a branching fraction of 64 +/- 14% for the abstraction of the acidic hydrogen as follows: CH(3)COOH + OH --> CH(3)COO + H(2)O --> CH(3) + CO(2) + H(2)O. A quantum chemical and theoretical kinetic analysis showed that the abstraction of the acidic hydrogen is enhanced relative to the abstraction of -CH(3) hydrogens because of the formation of a strong pre-reactive H-bonded complex, where the H-bonds are retained in the H-abstraction transition state. The potential energy surface of the reaction is explored in detail, and the reaction products of the individual channels are identified. The theoretical product branching is found to be critically dependent on the energetic and rovibrational differences between the H-abstraction transition states.

Degradation of fluorotelomer alcohols: a likely atmospheric source of perfluorinated carboxylic acids

Human and animal tissues collected in urban and remote global locations contain persistent and bioaccumulative perfluorinated carboxylic acids (PFCAs). The source of PFCAs was previously unknown. Here we present smog chamber studies that indicate fluorotelomer alcohols (FTOHs) can degrade in the atmosphere to yield a homologous series of PFCAs. Atmospheric degradation of FTOHs is likely to contribute to the widespread dissemination of PFCAs. After their bioaccumulation potential is accounted for, the pattern of PFCAs yielded from FTOHs could account for the distinct contamination profile of PFCAs observed in arctic animals. Furthermore, polar bear liver was shown to contain predominately linear isomers (>99%) of perfluorononanoic acid (PFNA), while both branched and linear isomers were observed for perfluorooctanoic acid, strongly suggesting a sole input of PFNA from "telomer"-based products. The significance of the gas-phase peroxy radical cross reactions that produce PFCAs has not been recognized previously. Such reactions are expected to occur during the atmospheric degradation of all polyfluorinated materials, necessitating a reexamination of the environmental fate and impact of this important class of industrial chemicals.

Chemical Ionization Mass Spectrometer Instrument for the Measurement of Tropospheric HO2 and RO2

Laboratory characterizations of the peroxy radical chemical ionization mass spectrometer (PerCIMS) instrument have been performed. The instrument functions by drawing ambient air through a 50-microm-diameter orifice into an inlet held at low pressure. Peroxy radicals (HO2 and RO2) within this air are detected by amplified chemical conversion into a unique ion (HSO4-) via the chemistry initiated by the addition of NO and SO2 to the inlet. HSO4- ions are then quantified by a quadrupole filter mass spectrometer. PerCIMS provides measurements of the sum of peroxy radicals, HO2 + RO2 (HOxROx mode), or the HO2 component only (HO2 mode), achieved through the control of concentration of NO and SO2 added to the instrument. The characterization and response of this instrument have been evaluated through modeling of inlet chemistry and laboratory experiments and have also been demonstrated through successful deployment during field campaigns. The performance of PerCIMS with respect to calibration pressure and relative humidity is reported, as are the sensitivities of the instrument to organic peroxy radicals with different hydrocarbon groups. These data show PerCIMS to be a practical field instrument for the fast and accurate evaluation of the concentration of peroxy radicals over a variety of atmospheric conditions. The estimated accuracy of the derived [HOxROx] concentrations is +/- 35% (at the 95% confidence interval), while [HO2] measurements have accuracies of +/- 41% (at the 95% confidence interval). Typical precision of measurements well above the detection limit is 10%, and typical detection limits are 1 x 10(7) radicals cm(-3) for 15-s averaging times.

Atmospheric chemistry of HFE-7500 [n-C3F7CF(OC2H5)CF(CF3)2]: reaction with OH radicals and Cl atoms and atmospheric fate of n-C3F7CF(OCHO*)CF(CF3)2 and n-C3F7CF(OCH2CH2O*)CF(CF3)2 radicals

Relative rate techniques were used to measure k(OH + HFE-7500) = (2.6+/-0.6) x 10(-14), k(Cl + HFE-7500) = (2.3+/-0.7) x 10(-12), k[Cl + n-C3F7CF(OC(O)H)CF(CF3)2] = (9.7+/-1.4) x 10(-15), and k[Cl + n-C3F7CF(OC(O)CH3)CF(CF3)2] < 6 x 10(-17) cm3 molecule(-1) s(-1) at 295 K [HFE-7500 = n-C3F7-CF(OC2H5)CF(CF3)2]. From the value of k(OH + HFE-7500) an estimate of 2.2 years for the atmospheric lifetime of HFE-7500 is obtained. Two competing loss mechanisms for n-C3F7-CF(OCHO.CH3)CF(CF3)2 radicals were identified in 700 Torr of N2/O2 diluent at 295 K; reaction with O2 and decomposition via C-C bond scission with kO2/k(decomp) = 0.013+/-0.006 Torr(-1). The Cl atom initiated oxidation of HFE-7500 in N2/O2 diluent gives n-C3F7CF(OC(O)CH3)CF(CF3)2 as the major product and n-C3F7CF(OC(O)H)CF(CF3)2 as a minor product. The atmospheric oxidation of HFE-7500 gives n-C3F7-CF(OC(O)CH3)CF(CF3)2 and n-C3F7CF(OC(O)H)CF(CF3)2 as oxidation products. The results are discussed with respect to the atmospheric chemistry and environmental impact of HFE-7500.