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The fluorescence assay by gas expansion (FAGE) method for the measurement of the methyl peroxy radical (CH

The kinetics of the unimolecular decomposition of the stabilized Criegee intermediate

The kinetics of the reaction OH/OD + SO

The kinetics of the reaction OH/OD(v = 1,2,3) + SO

A HO2 mass accommodation coefficient of α = 0.23 ± 0.07 was measured onto submicron copper(II)-doped ammonium sulfate aerosols at a relative humidity of 60 ± 3%, at 293 ± 2 K and at an initial HO2 concentration of ∼1 × 109 molecules cm−3 by using an aerosol flow tube coupled to a sensitive fluorescence assay by gas expansion (FAGE) HO2 detection system. The effect upon the HO2 uptake coefficient γ of adding different organic species (malonic acid, citric acid, 1,2-diaminoethane, tartronic acid, ethylenediaminetetraacetic acid (EDTA), and oxalic acid) into the copper(II)-doped aerosols was investigated. The HO2 uptake coefficient decreased steadily from the mass accommodation value to γ = 0.008 ± 0.009 when EDTA was added in a one-to-one molar ratio with the copper(II) ions, and to γ = 0.003 ± 0.004 when oxalic acid was added into the aerosol in a ten-to-one molar ratio with the copper(II). EDTA binds strongly to copper(II) ions, potentially making them unavailable for catalytic destruction of HO2, and could also be acting as a surfactant or changing the viscosity of the aerosol. The addition of oxalic acid to the aerosol potentially forms low-volatility copper−oxalate complexes that reduce the uptake of HO2 either by changing the viscosity of the aerosol or by causing precipitation out of the aerosol forming a coating. It is likely that there is a high enough oxalate to copper(II) ion ratio in many types of atmospheric aerosols to decrease the HO2 uptake coefficient. No observable change in the HO2 uptake coefficient was measured when the other organic species (malonic acid, citric acid, 1,2-diaminoethane, and tartronic acid) were added in a ten-to-one molar ratio with the copper(II) ions.

The rate constant of the reaction between methanol and the hydroxyl radical has been studied in the temperature range 56-202 K by pulsed laser photolysis-laser induced fluorescence in two separate experiments using either a low temperature flow tube coupled to a time-of-flight mass spectrometer or a pulsed Laval nozzle apparatus. The two independent techniques yield rate constants that are in mutual agreement and consistent with the results reported previously below 82 K [Shannon et al. Nat. Chem. 2013, 5, 745-749] and above 210 K [Dillon et al. Phys. Chem. Chem. Phys. 2005, 7, 349-355], showing a very sharp increase with decreasing temperature with an onset around 180 K. This onset is also signaled by strong chemiluminescence tentatively assigned to formaldehyde, which is consistent with the formation of the methoxy radical at low temperature by quantum tunnelling, and its subsequent reaction with H and OH. Our results add confidence to the previous low temperature rate constant measurements and consolidate the experimental reference data set for further theoretical work required to describe quantitatively the tunnelling mechanism operating in this reaction. An additional measurement of the rate constant at 56 K yielded a value of (4.9 ± 0.8) × 10(-11) cm(3) molecule(-1) s(-1) (2σ), showing that the rate constant is increasing less rapidly at temperatures below 70 K.

The kinetics of the OH + glyoxal, (HCO)2, reaction have been studied in N2 and N2/O2 bath gas from 5-80 Torr total pressure and 212-295 K, by monitoring the OH decay via laser induced fluorescence (LIF) in excess (HCO)2. The following rate coefficients, kOH+(HCO)2 = (9.7 ± 1.2), (12.2 ± 1.6), and (15.4 ± 2.0) × 10(-12) cm(3) molecule(-1) s(-1) (where errors represent a combination of statistical errors at the 2σ level and estimates of systematic errors) were measured in nitrogen at temperatures of 295, 250, and 212 K, respectively. Rate coefficient measurements were observed to be independent of total pressure but decreased following the addition of O2 to the reaction cell, consistent with direct OH recycling. OH yields, ΦOH, for this reaction were quantified experimentally for the first time as a function of total pressure, temperature, and O2 concentration. The experimental results have been parametrized using a chemical scheme where a fraction of the HC(O)CO population promptly dissociates to HCO + CO, the remaining HC(O)CO either dissociates thermally or reacts with O2 to give CO2, CO, and regenerate OH. A maximum ΦOH of (0.38 ± 0.02) was observed at 212 K, independent of total pressure, suggesting that ∼60% of the HC(O)CO population promptly dissociates upon formation. Qualitatively similar behavior is observed at 250 K, with a maximum ΦOH of (0.31 ± 0.03); at 295 K, the maximum ΦOH decreased further to (0.29 ± 0.03). From the parametrization, an OH yield of ΦOH = 0.19 is calculated for 295 K and 1 atm of air. It is shown that the proposed mechanism is consistent with previous chamber studies. While the fits are robust, experimental evidence suggests that the system is influenced by chemical activation and cannot be fully described by thermal rate coefficients. The atmospheric implications of the measurements are briefly discussed.

Laser-induced fluorescence from the CH3I-Cl and ICH2I-Cl adducts formed in association reactions between chlorine atoms and CH3I and CH2I2 has been observed for the first time. The LIF excitation and dispersed fluorescence spectra have been measured in the range 345-375 nm and 380-480 nm, respectively, at 204 and 296 K. The excitation spectra exhibit vibrational fine structure, and a semiquantitative analysis of the spectra yields a similar binding energy for both adducts of approximately 60 kJ mol(-1). The adduct fluorescence is efficiently quenched by N2 and exhibits a zero-pressure lifetime of approximately 25-30 ns. Using LIF excited from the CH3I-Cl and ICH2I-Cl adducts, the kinetics of the reactions of atomic chlorine with methyl iodide and diiodomethane have been investigated, the results showing that both reactions proceed via two independent channels, an association reaction to form the adduct and a bimolecular abstraction reaction. At T approximately 200 K, the association reaction is predominant, and CH3I-Cl formation is irreversible, with rate coefficients for adduct formation found to be pressure-dependent and in reasonable agreement with the literature. At approximately 200 K, removal of the adduct is dominated by reaction with radical species (CH3 and ClSO) and by self-reaction, which proceed at close to the gas kinetic limit. At 296 K, CH3I-Cl formation is reversible, and the equilibrium constant, K(p) = (70.9 +/- 27.4) x 10(3) atm(-1), was determined, which is in excellent agreement with the literature, and the adduct does not significantly react with CH3I. The uncertainty is at the 95% confidence level (2sigma) and includes systematic errors. At approximately 200 K, the ICH2I-Cl adduct is again stabilized, with pressure-dependent rate coefficients reaching the high pressure limit at lower pressures than for the Cl + CH3I reaction. At room temperature, the ICH2I-Cl adduct is removed via an additional unimolecular decomposition channel, which dominates over the reversible decomposition channel to reform Cl + CH2I2. Neither adduct was observed to undergo significant reaction with molecular oxygen at approximately 200 or 296 K, with an upper limit rate coefficient determined as k < 10(-16) cm(3) molecule(-1) s(-1).

The standard gas-phase enthalpies of formation of chlorinated benzenes, phenols and dibenzo-p-dioxins have been predicted using G3X and/or G3XMP2 model chemistries coupled with isodesmic reactions and compared to the previous theoretical and experimental values. A set of values for chlorinated benzenes are first suggested based on experimental measurements and the closed agreed G3X calculations with different isodesmic reactions. The results on polychlorinated dibenzo-p-dioxins (PCDDs) show a large difference between G3XMP2 and previous experimental measurements and predictions using group additivity methods, semiempirical quantum chemistry, and DFT calculations, especially for highly chlorinated species. Using the well-balanced isodesmic reactions (IR3 and IR5), the discrepancies between G3XMP2 and DFT predictions on PCDDs can be reduced to within 16 kJ/mol. The relative stability of PCDD isomers can be rationalized by the positional interactions, and the overestimation by DFT with less balanced isodesmic reactions is due to the overestimation of the ortho-Cl-Cl repulsive interactions when comparing with G3XMP2. Our calculations suggest further experimental measurements, especially on highly chlorinated phenols and PCDDs.

Measurements of the acetyl yield from acetone photolysis have been made using laser flash photolysis/laser induced fluorescence. Phi(total)(lambda,p,T) was determined over the ranges: 266or = lambda/nmor = 327.5, 0.3or = p/Torror = 400 and 218or = T/Kor = 295. The acetyl yield was determined relative to that at 248 nm by conversion to OH by reaction with O2. Linear Stern-Volmer plots (1/[OH] vs [M]) describe the data for lambda300 nm, but for lambda300 nm, nonlinear Stern-Volmer plots were observed. This behavior is interpreted as evidence for dissociation from two excited states of acetone: S1 when the Stern-Volmer plots are linear and both S1 and T1 when Stern-Volmer plots are nonlinear. A model for acetone photolysis is proposed that can adequately describe both the present and literature data. Barriers to dissociation are invoked in order to explain the dependence of pressure quenching of the acetone photolysis yields as a function of wavelength and temperature. This pressure quenching was observed to become more efficient with increasing wavelength, but it was only above approximately 300 nm that a significant T dependence was observed, which became more pronounced at longer wavelengths. This is the first study to observe a T-dependent phi(total)(lambda,p,T). A parametrized expression for phi(total)(lambda,p,T) has been developed and is compared against the recommended literature data by running box model simulations of the atmosphere. These simulations show that acetone photolysis occurs more slowly at the top of the troposphere.