Carlos M. Freitas Dinis's research while affiliated with Bergische Universität Wuppertal and other places

Within the German Tropospheric Research Programme (TFS) numerous kinetic and mechanistic studies on the tropospheric reaction/degradation of the following reactants were carried out: oxygenated VOC, aromatic VOC, biogenic VOC, short-lived intermediates, such as alkoxy and alkylperoxy radicals. At the conception of the projects these selected groups were classes of VOC or intermediates for which the atmospheric oxidation mechanisms were either poorly characterised or totally unknown. The motivation for these studies was the attainment of significant improvements in our understanding of the atmospheric chemical oxidation processes of these compounds, particularly with respect to their involvement in photooxidant formation in the troposphere. In the present paper the types of experimental investigations performed and the results obtained within the various projects are briefly summarised. The major achievements are highlighted and discussed in terms of their contribution to improving our understanding of the chemical processes controlling photosmog formation in the troposphere.

Within the German Tropospheric Research Programme (TFS) numerous kinetic and mechanistic studies on the tropospheric reaction/degradation of the following reactants were carried out:• oxygenated VOC, • aromatic VOC, • biogenic VOC, •short-lived intermediates, such as alkoxy and alkylperoxy radicals.At the conception of the projects these selected groups were classes of VOC or intermediates for which the atmospheric oxidation mechanisms were either poorly characterised or totally unknown. The motivation for these studies was the attainment of significant improvements in our understanding of the atmospheric chemical oxidation processes of these compounds, particularly with respect to their involvement in photooxidant formation in the troposphere. In the present paper the types of experimental investigations performed and the results obtained within the various projects are briefly summarised. The major achievements are highlighted and discussed in terms of their contribution to improving our understanding of the chemical processes controlling photosmog formation in the troposphere.

The gas-phase reaction of OH(X2Π) radicals with 1,3-dioxolane was investigated in the temperature range 250–550 K. The reactions of OH(X 2Π) with di-isopropoxy methane (DiPM), di-n-butoxy methane (DnBM) and di-sec-butoxy methane (DsBM) were studied below room temperature down to 250 K. The experiments were carried out in argon, mostly at a total pressure of 400 Torr. A few additional measurements for the reaction OH + 1,3-dioxolane at 50 Torr exhibited that this reaction is independent of total pressure in the range investigated. OH radicals were generated by excimer laser photolysis of H2O2 and were detected by laser-induced fluorescence. For the OH + 1,3-dioxolane reaction, no temperature dependence was observed within the experimental errors. The bimolecular rate coefficient is well described by kOH+dioxolane(T) = (8.2 ± 1.3) × 10−12 cm3 s−1. The rate coefficients measured for the reactions of OH with DiPM, DnBM and DsBM are discussed together with literature data obtained for higher temperatures. The Arrhenius plots for each reaction exhibited concave upward curvatures. From these data, the following Arrhenius expressions were derived (with T in K and E0 in kJ mol−1):

Rate coefficients for reactions of the C-2 radical in its a(3)Pi(u) electronic state with H, N and O atoms and with C3O2, C4F6, H-2, NO, N2O, C2H2, C2H4, C3H4 and C6H6 were determined. This work represents the first study of reactions of C-2(a(3)Pi(u)) radicals with the atoms investigated at room temperature and 4 Torr: total pressure. The bimolecular rate constants obtained for the atom reactions were k(C2+O) = (9.8 +/- 1.0) x 10(-11), k(C2+N) = (2.8 +/- 1.0) x 10(-11) and k(C2+N) < 6 x 10(-14), in units of cm(3) s(-1). In addition, the reaction C-2 + O was found to be independent of total pressure in the range 2-60 Torr. For the reaction C-2(a(3)Pi(u)) + ethene (C2H4) a temperature and pressure independent rate constant of (9.5 +/- 1.2) x 10(-11) cm(3) s(-1) was obtained in the temperature range 298-1000 K at 100 Torr total pressure and in the pressure range 5-100 Torr at 298 K. The following rate constants were determined at room temperature and a total pressure of 4 Torr for the reactions of C-2(a(3)Pi(u)) radicals with benzene (C6H6), acetylene (C2H2) and allene (C3H4): k(C2+C6H6) = (4.9 +/- 0.1) x 10(-10), k(C2+C2H2) = (1.0 +/- 0.1) x 10(-10) and k(C2+C3H4) = (1.9 +/- 0.3) x 10(-10), in units of cm(3) s(-1). The reaction C-2 + NO was investigated at room temperature and 100 Torr total pressure, a rate constant k(C2+NO) = (6.8 +/- 0.3) x 10(-11) cm(3) s(-1) was obtained. The reaction C-2 + N2O was studied at 4 Torr total pressure in the temperature range 300-700 K for which a temperature independent rate constant k(C2+N2O) = (3.1 +/- 0.4) x 10(-14) cm(3) s(-1) was determined.

The gas-phase reactions of OH(X2Π) radicals with di-i-propoxymethane (DiPM) and di-sec-butoxymethane (DsBM) have been studied in argon in the temperature range 295–700 K at total pressures between 50 and 400 Torr. OH radicals were generated by excimer laser photolysis of H2O2 and were detected by laser-induced fluorescence. Within the investigated ranges, the reactions of OH(X2Π) radicals with DiPM and DsBM were found to be independent of total pressure. Weak dependencies of the rate coefficients on temperature were observed. Bimolecular rate coefficients for the reactions of OH(X2Π) with DiPM and DsBM at 298 K of kOH+DiPM=(3.47±0.20)×10-11 cm3 s-1 and kOH+DsBM=(4.25±0.13)×10-11 cm3 s-1, respectively, have been determined. In order to describe the kinetics of the reactions of OH radicals with DiPM and DsBM as well as analogous acetals, a structure activity relationship (SAR) technique established for other reactant classes has been modified and applied. Compared to the former SAR method, which does not yield satisfying results for oxygenated VOCs (volatile organic compounds), the present calculations lead to much better agreement with the experimental data for dialkylacetals of the type R–O–CH2–O–R.

The gas-phase reaction of OH(X2Π) radicals with di-n-butoxymethane (DBM) has been studied in the temperature range 298–710 K at total pressures between 50 and 100 Torr argon. OH radicals have been generated by excimer laser photolysis of H2O2 at 248 nm and have been detected by laser-induced fluorescence. Within the investigated ranges, the reaction has been found to be independent of temperature and total pressure. The bimolecular rate coefficient is kOH+DBM=(3.21±0.79)×10−11 cm3 s−1. The rate coefficient is compared with the results from an estimation using structure–activity relationships. Over the whole temperature range, the calculated results are somewhat higher than the experimental rate coefficients.