M7. Kinetics and Atmospheric Implications of Peroxy Radical Cross Reactions Involving the CH₃C(O)O₂ Radical

E. Villenave ^a, R. Lesclaux ^a, S. Seefeld ^b, W. R. Stockwell ^c

^a Laboratoire de Photophysique et Photochimie Moléculaire, Université Bordeaux I, Talence, France

^b EAWAG, Swiss Federal Institute for Environmental Science and Technology, Zürich, Switzerland

^c Fraunhofer Institute for Atmospheric Environmental Science and Technology, Garmisch, Germany

e-mail: eric.villenave@nist.gov

 Organic peroxy radicals (RO₂) represent an important class of intermediates formed in the atmospheric oxidation processes of hydrocarbons.^1,2 Their tropospheric chemistry involves several competing reactions, depending on the distribution of relevant trace atmospheric constituents. It is now well accepted that for low NO_x concentrations, the principal loss process for peroxy radicals in the troposphere is by reaction with HO₂.¹ However, in hydrocarbon-rich atmospheres such as in the marine boundary layer or forest areas, it has been shown that high concentrations of RO₂ radicals can build up, such that RO₂ self- and cross reactions may also have to be taken into account, provided such reactions are fast enough.³ The objectives of the present work were to investigate the kinetics of the reactions of selected peroxy radicals with CH₃C(O)O₂ which, after HO₂ and CH₃O₂, appears to be the most abundant peroxy radical in the remote troposphere.³ Acetylperoxy radicals are known to be formed as a consequence of the photooxidation of higher carbonyl compounds, and are important in many atmospheric processes, as they appear to be a significant contributor to smog formation, as precursor of PAN (peroxyacetylnitrate).⁴

The kinetics of the cross reactions of c-C₆H₁₁O₂, sec-C₁₀H₂₁O₂, sec-C₁₂H₂₅O₂ and t-C₄H₉O₂ with CH₃C(O)O₂ were investigated in this work using the flash photolysis technique coupled with UV absorption spectrometry. Radicals were generated by photolysis of either molecular chlorine or molecular bromine, in the presence of suitable hydrocarbons and oxygen. All reaction kinetics were investigated at 760 Torr total pressure and at room temperature. Values of (1.0 ± 0.1) x 10^-11, (1.1 ± 0.2) x 10^-11, (1.0 ± 0.6) x 10^-11 and (1.1 ± 0.3) x 10^-11 (units of cm³ molecule^-1 s^-1) have been obtained for the rate constants of the reactions of CH₃C(O)O₂ radicals with c-C₆H₁₁O₂, sec-C₁₀H₂₁O₂, sec-C₁₂H₂₅O₂ and t-C₄H₉O₂ radicals, respectively.

The present results, combined with the results previously reported for primary peroxy radicals CH₃O₂ and C₂H₅O₂,^5-7 (Table 1) show that all cross reactions of CH₃C(O)O₂ are fast with rate constants around 1.0 x 10^-11 cm³ molecule^-1 s^-1, independent of the RO₂ radical structure and of its rate constant for self reaction. This suggests that all acylperoxy radicals (ACO₃) present the same high reactivity as CH₃C(O)O₂ and hence, it is proposed to assign the above rate constant value to all cross reactions of ACO₃ radicals. These new rate constant values were implemented in the Regional Atmospheric Chemistry Mechanism (RACM),⁸ to estimate the importance of the cross reactions in the chemistry of acylperoxy radicals in the troposphere. In the case of a moderately polluted troposphere, under low NO_x and high VOC concentrations, the cross reactions of acylperoxy radicals with organic peroxy radicals account for more than 20 % of the acylperoxy loss reactions (Figure 1), and PAN concentrations decrease by more than 4 %, compared with the previous estimates of Kirchner and Stockwell⁹.

(1) P.D. Lightfoot, R.A. Cox, J.N. Crowley, M. Destriau, G.D. Hayman, M.E. Jenkin, G.K. Moortgat, and F. Zabel, Atmos. Environ., 1992, 26A, 1806.

(2) T.J. Wallington, P. Dagaut, and M.J. Kurylo, Chem. Rev., 1992, 92, 667.

(3) S. Madronich and J.G. Calvert, J. Geophys. Res., 1990, 95, 5697.

(4) H.B. Singh, D. O’Hara, D. Herlth, J.D. Bradshaw, S.T. Sandholm, G.L. Gregory, G.W. Sachse, D.R. Blake, P.J. Crutzen, and M.A. Kanakidou, J. Geophys. Res., 1992, D15, 16511.

(5) E. Villenave and R. Lesclaux, J. Phys. Chem., 1996, 100, 14372.

(6) C.M. Roehl, D. Bauer, and G.K. Moortgat, J. Phys. Chem., 1996, 100, 4038.

(7) M.M. Maricq and J.J. Szente, J. Phys. Chem., 1996, 100, 4507.

(8) W.R. Stockwell, F. Kirchner, M. Kuhn, and S. Seefeld, J. Geophys. Res., 1997, in press.

(9) F. Kirchner and W.R. Stockwell, J. Geophys. Res., 1996, 101 (D15), 21007.

FIGURE 1. Fraction of ACO3 reaction rates during the 5th day of the Plume case at 298 K with 100 % emissions. The dotted lines shows the NO2 photolysis frequencies and gives a measure of the day-night cycle.