K. H. Becker

Product study and mechanisms of the reactions of α-pinene and of pinonaldehyde with OH radicals

The reactions of α-pinene and of its main oxidation product, pinonaldehyde (3-acetyl-2,2-dimethyl-cyclobutyl-ethanal), with OH radicals have been studied in the laboratory using Fourier transform infrared spectroscopy for real-time monitoring of the gas-phase chemical species and a Scanning Mobility Particle Sizer system (3071 A, TSI) for the study of the secondary aerosol formation. All gas-phase molar yields were quantified using calibrated reference of the pure compound, except for the nitrates products. The results were: for the α-pinene experiments in the presence of NO x , pinonaldehyde, (87 ± 20)%; total nitrates (18 ± 9)%; formaldehyde, (23 ± 9)%; acetone (9 ± 6)%; for the α-pinene experiments in the absence of NO x : pinonaldehyde, (37 ± 7)%; formaldehyde, (8 ± 1)%; acetone, (7 ± 2)%; for the pinonaldehyde experiments in the presence of NO, formaldehyde (152 ± 56)% and acetone (15 ± 7)%. The aerosol measurements showed that the condensed products accounted for the missing carbon in the gas-phase balance. The partitioning of the products into the condensed phase was found to be potentially significant under experimental conditions but less than 10% for initial α-pinene concentrations lower than 10 13 molecule cm -3 and hence negligible under atmospheric conditions in the absence of aerosol seeds. On the basis of these results a comprehensive mechanism for the gas-phase reaction of α-pinene with OH in the presence of NO x has been proposed, including quantitative values for all the involved branching ratios.

Nitrous Oxide (N2O) Emissions from Vehicles

N2O is an important greenhouse gas and accurate emission data are required to assess its impact on global climate. It is well established that automobiles, particularly those equipped with 3-way catalysts, emit N2O. However, the vehicle contribution to the global N2O budget is uncertain. We report results of N2O emission measurements performed in a road tunnel in Germany and using a chassis dynamometer system in the USA. We estimate that the global vehicle fleet emits (0.12±0.06) Tg yr−1 of N2O. From the emission factor (g N2O/g CO2) determined an annual N2O emission of (0.12±0.06) Tg yr−1 of N2O (0.08±0.04 Tg N yr−1) for the global vehicle fleet has been estimated which represents 1–4% of the atmospheric growth rate of this species.

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.

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.

A kinetic study of the atmospheric photolysis of ?-dicarbonyls

Photolysis frequencies of biacetyl, methyl glyoxal, and glyoxal have been determined relative to the photolysis frequency of NO2. The values were measured under natural sunlight conditions in a large-volume outdoor reaction chamber. The experimental results obtained are compared to photolysis frequencies used in current models of tropospheric photo-oxidant formation. For biacetyl, a ratio of J(biacetyl)/J(NO2) = (0.0364 ± 0.0026) was found, in good agreement with previous measurements and models. For methyl glyoxal, however, the experimental photolysis frequency ratios were significantly larger than those calculated from models. This could only partly be explained by reaction of methyl glyoxal with HO2. Due to this inconsistency, it is preferred not to cite a ratio for J(methyl glyoxal)/J(NO2). The agreement between calculated and experimental photolysis frequency ratios of glyoxal was reasonably good, a ratio of J(glyoxal)/J(NO2) = 0.0109 has been found, with estimated overall error margins of about 30%.

NEW RESULTS ON THE ATMOSPHERIC PHOTOOXIDATION OF SIMPLE ALKYLBENZENES

Abstract A study of the photolysis of two postulated intermediates in the OH initiated photooxidation of aromatic hydrocarbons, benzene oxide/oxepin and toluene-1,2-oxide/2-methyloxepin, has been conducted under the real light conditions of a large volume outdoor reaction chamber. The main focus of the study was on the formation of the hydroxylated aromatic hydrocarbons, i.e., phenol from benzene oxide/oxepin and the cresol isomers from toluene-1,2-oxide/2-methyloxepin. Photolysis frequencies were measured relative to that of NO2, which was determined with a filter radiometer. A linear dependency was found within the experimental errors, the ratios of the photolysis frequencies of the arene oxides to that of NO2 were found to be (4.41±0.44)·10−2 and (3.99±0.48)·10−2 for benzene oxide/oxepin and toluene-1,2-oxide/2-methyloxepin, respectively. The yield of phenol in the photolysis of benzene oxide/oxepin was found to be (43.2±4.5)%, independent of the light intensity. For toluene-1,2-oxide/2-methyloxepin, an o-cresol yield of (2.7±2.2)% was determined, the formation of other cresol isomers was not observed. The results obtained are in agreement with arene oxides being intermediates in the atmospheric photooxidation of benzene and toluene, they are discussed with regard to their implications for the formation of hydroxylated aromatics in the OH initiated photooxidation of benzene and toluene.

Investigation of N2O formation in the NCO+NO reaction by Fourier-transform infrared spectroscopy

Abstract The NCO+NO reaction was investigated at room temperature in a static photoreactor using time-resolved Fourier-transform infrared spectroscopy. NCO radicals were generated by photolysing CINCO diluted in argon at 254 nm. From the concentration-time profiles of the products CO, CO 2 and N 2 O, the following branching ratios were determined: 0.35 ± 0.06 for the product channel N 2 O + CO, and 0.65 ± 0.06 for the product channel N 2 + CO 2 .