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Abstract:
The reaction between HO2 and CH3C(O)O-2 has three exothermic product channels, forming OH (R3a), peracetic acid (R3b), and acetic acid plus O-3 (R3c). The branching ratios of the OH- and ozone-forming reaction channels were determined using a combination of laser-induced fluorescence (LIF, for time-resolved OH concentration measurement) and transient absorption spectroscopy (TAS, for time-resolved O-3 concentration measurement) following pulsed laser generation of HO2 and CH3C(O)O-2 from suitable precursors. TAS was also used to determine the initial concentration of the reactant peroxy radicals. The data were evaluated by numerical simulation using kinetic models of the measured concentration profiles; a Monte Carlo approach was used to estimate the uncertainties of the rate constants (k(3)) and branching ratios (alpha) thus obtained. The reaction channel forming OH (R3a) was found to be the important with alpha(3a) = 0.61 +/- 0.09 and alpha(3c) = 0.16 +/- 0.08. The overall rate coefficient of the title reaction was found to be k(3) = (2.1 +/- 0.4) X 10(-11) cm(3) molecule(-1) s(-1) for both HO2 and DO2. Use of DO2 resulted in an increase in alpha(3a) to 0.80 +/- 0.14. Comparison with former studies shows that OH formation via (R3) has been underestimated significantly to date. Possible reasons for these discrepancies and atmospheric implications are discussed.
