Kinetics and mechanism of the acetylperoxy plus HO2 reaction

The reaction of HO2 with CH3C(O)O-2 is examined using flash photolysis and FTIR smog chamber techniques. Time-resolved UV spectroscopy is used to follow the transient peroxy species. It yields reasonable concentration versus time profiles for CH3C(O)O-2 and HO2, but indicates anomalously high levels of secondary CH3O2 radicals. Transient IR diode laser absorption confirms the HO2 decay rates; however, the anticipated reaction model substantially underestimates the observed decay. The model is augmented by assuming that, in analogy with formaldehyde, there exists a reaction between HO2 and acetaldehyde (the precursor for CH3C(O)O-2). Consistent with this, the fitted rate for the hypothesized reaction increases with increasing initial acetaldehyde level. Relative rate measurements reveal that chlorine atoms remove more CH3CHO relative to CH3OH in air as compared to nitrogen diluent. This supports the hypothesis since, in the presence of oxygen, HO2 is formed and presents an additional acetaldehyde removal pathway. Employing the augmented model, analyses of HO2 decay traces yield a CH3C(O)O-2 + HO2 rate constant of k(1) = (3.9(-2.3)(+5.0)) x 10(-13)e((1350+/-250)/r) cm(3) s(-1). Reasons are discussed for why the present rate constants are 2-3 times larger than previously reported. FTIR-smog chamber studies reveal the reaction to proceed via two channels to (a) peracetic acid and O-2 and to (b) acetic acid and O-3, with a branching fraction at 295 K that is less than half of the literature value. Time-resolved UV absorption measurements support this smaller fraction; averaged together the two methods give k(1b)/k(1) = 0.12 +/- 0.04. As part of this work, relative rate techniques are used to measure k(Cl + CH3C-(O)OH) = (2.5 +/- 0.3) x 10(-14) cm(3) s(-1) and k(Cl + CH3C(O)OOH) = (4.5 +/- 1.0) x 10(-15) cm(3) s(-1) at 295 K.