I. Barnes

OH-initiated oxidation of benzene

The present work represents a continuation of part I of this series of papers, in which we investigated the phenol yields in the OH-initiated oxidation of benzene under conditions of low to moderate concentrations of NOx, to elevated NOx levels. The products of the OH-initiated oxidation of benzene in 700–760 Torr of N2/O2 diluent at 297 ± 4 K were investigated in 3 different photochemical reaction chambers. In situ spectroscopic techniques were employed for the detection of products, and the initial concentrations of benzene, NOx, and O2 were widely varied (by factors of 6300, 1500, and 13, respectively). In contrast to results from previous studies, a pronounced dependence of the product distribution on the NOx concentration was observed. The phenol yield decreases from approximately 50–60% in the presence of low concentrations ( 10 000 ppb) NOx concentrations. In the presence of high concentrations of NOx, the phenol yield increases with increasing O2 partial pressure. The rate constant of the reaction of hydroxycyclohexadienyl peroxyl radicals with NO was determined to be (1.7 ± 0.6) × 10−11 cm3 molecule−1 s−1. This reaction leads to the formation of E,E-2,4-hexadienedial as the main identifiable product (29 ± 16%). The reaction of the hydroxycyclohexadienyl radical with NO2 gave phenol (5.9 ± 3.4%) and E,E-2,4-hexadienedial (3.4 ± 1.9%), no other products could be identified. The residual FTIR product spectra indicate the formation of unknown nitrates or other nitrogen-containing species in high yield. The results from the present work also show that experimental studies aimed at establishing/verifying chemical mechanisms for aromatic hydrocarbons must be performed using NOx levels which are representative of those found in the atmosphere.

Reactivity studies of organic substances towards hydroxyl radicals under atmospheric conditions

Abstract Relative reactivities of OH radicals towards organic compounds of tropospheric importance have been measured at room temperature and atmospheric pressure in a 420l reaction vessel using a new simple experimental technique. The method involves the use of pernitric acid (PNA) HO2NO2 as a “dark” source of HO2 radicals which are inert towards the hydrocarbons used in the study but in the presence of NO react rapidly to give OH radicals. The relative rate constants of OH reactions initiated by adding an excess of NO to the PNA containing system were obtained by comparing the overall concentration changes of the hydrocarbons relative to that of a standard substance, i.e. propene. Results obtained from conventional measurements of the time decay of hydrocarbons in a NO2/propene photo-oxidation system are given for comparison. Both results are compared with direct rate constant measurements taken from the literature.

FTIR product study of the OH initiated oxidation of dimethyl sulphide: Observation of carbonyl sulphide and dimethyl sulphoxide

Abstract The OH initiated oxidation of dimethyl sulphide (DMS: CH3SCH3) has been performed in a large photoreactor under NOx-free conditions using the 254 run photolysis of OH radicals and long path in situ FTIR to monitor reactants and products. Sulphur dioxide (SO2) and dimethyl sulphoxide (DMSO: CH3SOCH3) were the major sulphur containing products. The formation yield of SO2 was 80 ± 10% S and was sensitive to the prevailing reactor temperature. The yield of DMSO could not be accurately determined due to its rapid further oxidation with OH, however, yields of ∼ 20% S were observed in synthetic air in the initial stages of the reaction. The results support that the O2 dependent pathway in the oxidation of DMS will result primarily in the formation of DMSO. Carbonyl sulphide was also observed with a yield of 0.7 ± 0.2% S. From the experimental evidence it is concluded that OCS is being formed via the oxidation of thioformaldehyde (H2CS) formed in a minor channel involving the reaction of CH3S radicals with O2. Although the yield is small a consideration of the source strengths of DMS reported in the literature shows that the oxidation of DMS could contribute between 0.10 and 0.28 Tg (OCS) yr−1 to the global OCS budget.

Chemiluminescence of SO(1Δg,1Σ+g) sensitized by O2(1Δg)

Abstract In a fast flow system mixtures of SOCl 2 with helium were passed through a muwave discharge and added downstream to a flow of metastable O 2 ( 1 Δ g ) molecules. Chemiluminescence of the SO( 1 Δ g - 3 Σ − g ) 10.0) band was observed at 1.705 μm in addition to the already known (0.0), (0.1), (1.1), and (2.2) bands of the SO( 1 Σ + g -3Σ − g ) transition. The results indicate that a complex series of energy transfer and energy pooling processes are operative in the system.

Atmospheric Photooxidation of Fluoroacetates as a Source of Fluorocarboxylic Acids

A 1080 L environmental chamber with in situ FTIR spectroscopy detection was used to study the product distribution and the mechanism of the Cl-initiated photooxidation of a series of fluoroacetates. The gas-phase reactions of Cl atoms with ethyl trifluoroacetate (CF(3)C(O)OCH(2)CH(3)), methyl trifluoroacetate (CF(3)C(O)OCH(3)), and methyl difluoroacetate (CF(2)HC(O)OCH(3)) were investigated at 296 +/- 2 K and atmospheric pressure (approximately 760 Torr) of synthetic air. The fate of the fluoroalkoxy radicals formed in the reaction with Cl atoms mainly occurs through (i) an H-atom abstraction by reaction with O(2,) to produce the corresponding fluoroanhydride and (ii) an alpha-ester rearrangement via a five-membered ring intermediate to give the corresponding fluoroacetic acid. The yields of fluoroacids (CF(2)XC(O)OH, with X = H, F) obtained were as follows: 78 +/- 5, 23 +/- 2, and 30 +/- 5% for CF(3)C(O)OCH(2)CH(3), CF(3)C(O)OCH(3), and CF(2)HC(O)OCH(3,), respectively. Yields of <or=20, <or= 80, and <or=55% have been estimated for the anhydride formation from CF(3)C(O)OCH(2)CH(3), CF(3)C(O)OCH(3), and CF(2)HC(O)OCH(3), respectively. Formation of CF(2)O, with yield of 13 +/- 2% has been observed for the reaction of Cl with CF(2)HC(O)OCH(3). The measured yields are rationalized in terms of mechanisms consisting of competitive reaction channels for the radicals formed in the oxidation, that is, reaction with O(2), alpha-ester rearrangement and a decomposition pathway. The stability of the five-membered transition state of the alpha-ester rearrangement is correlated with the acid yields observed for the different fluoroacetates. Atmospheric implications, especially with regard to the fluorocarboxylic acid formation, are discussed.

Rate coefficients for the gas‐phase reaction of OH radicals with selected dihydroxybenzenes and benzoquinones

Rate coefficients have been determined for the gas-phase reaction of the hydroxyl (OH) radical with the aromatic dihydroxy compounds 1,2-dihydroxybenzene, 1,2-dihydroxy-3-methylbenzene and 1,2-dihydroxy-4-methylbenzene as well as the two benzoquinone derivatives 1,4-benzoquinone and methyl-1,4-benzoquinone. The measurements were performed in a large-volume photoreactor at (300 ± 5) K in 760 Torr of synthetic air using the relative kinetic technique. The rate coefficients obtained using isoprene, 1,3-butadiene, and E-2-butene as reference hydrocarbons are kOH(1,2-dihydroxybenzene) = (1.04 ± 0.21) × 10−10 cm3 s−1, kOH(1,2-dihydroxy-3-methylbenzene) = (2.05 ± 0.43) × 10−10 cm3 s−1, kOH(1,2-dihydroxy-4-methylbenzene) = (1.56 ± 0.33) × 10−10 cm3 s−1, kOH(1,4-benzoquinone) = (4.6 ± 0.9) × 10−12 cm3 s−1, kOH(methyl-1,4-benzoquinone) = (2.35 ± 0.47) × 10−11 cm3 s−1. This study represents the first determination of OH radical reaction-rate coefficients for these compounds.

FTIR spectroscopic studies of the CH3S + NO2 reaction under atmospheric conditions

Abstract Product studies using FTIR absorption spectrometry have been performed in a 420 xxx reaction chamber on the 254 nm photolysis of mixtures containing CH 3 SSCH 3 and NO 2 at ppm concentrations in 760 Torr of O 2 /N 2 diluent. The results indicate that the CH 3 S radicals formed by photolysis of CH 3 SSCH 3 react primarily with NO 2 , forming CH 3 SO and NO. In the presence of O 2 an unstable intermediate whose IR absorption spectrum resembles a peroxynitrate compound is observed. The intermediate has been tentatively assigned to methyl sulfinyl peroxynitrate, CH 3 S(O)OONO 2 . Other products include SO 2 , CH 3 SNO, CH 3 SNO 2 , CH 3 NO 3 , CH 3 SO 3 H, and HCHO.

Near-infrared emissions from the 1Δg and 1Σ+g states of S2

Abstract In a discharge flow system O2(1Δg) sensitized chemituminescence has been observed from the two low lying1Δg and1Σ+g states of S2 in the wavelength region 1.0 to 1.8 μm. The hitherto unknown energies of the two states were found to be 5730 ± 20 cm−1 and 7961 ± 15 cm−1, respectively. Due to the near energy resonance (ΔE ≈ 79 cm−1) between the1Δg state of O2 and the1Σ+g state of S2 effective energy exchange between both molecules is found to occur.

FTIR spectroscopic study of the gas-phase reaction of HO2 with H2CO

The reaction of formaldehyde with HO2 radicals in the presence of O2 and NO2 has been studied in a 420 l reaction chamber at 0° C in 533 mbar of synthetic air. Reactants and products were measured by FTIR absorption spectrometry-Additional evidence is presented for the formation of the HOCH2OO radical as the primary reaction product, by the IR spectroscopic identification of its NO2 recombination product HOCH2OONO2. By computer simulation of the concentration-time profiles of HO2NO2, H2CO and HOCH2OONO2, the rate constants (0°C, 533 mbar, M = air) k1 = (1.1 ± 0.4) × 10-13 cm3 s-1 and k-1 = 20-10+20 s-i have been derived for the reactions (1, -1) HO2 + H2CO ⇌ HOCH2OO.

Stability and infrared spectra of mono-, di-, and trichloromethanol

Abstract CH 2 ClOH, CHCl 2 OH, and CCl 3 OH were prepared by UV irradiation of CH 3 OH/Cl 2 /N 2 gas mixtures. Absorption cross-sections (base e) of σ (CH 2 ClOH) at 1093 cm −1 =6.16×10 −19 , σ (CHCl 2 OH) at 1109 cm −1 =1.22×10 −18 , and σ (CCl 3 OH) at 1119 cm −1 =8.96×10 −19 cm 2 molecule −1 were determined. The chlorinated methanols decayed with first-order kinetics to HCl and the corresponding carbonyl compound. The decay rates increased with increased contact of the chloromethanols with the reactor walls, indicating that decomposition is heterogeneous. An upper limit of 1.05×10 −2 s −1 was established confirm this slow rate of homogeneous decomposition.