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 foll ow 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 acet aldehyde (the precursor for CH3C(O)O-2). Consistent with this, the fitted r ate for the hypothesized reaction increases with increasing initial acetald ehyde level. Relative rate measurements reveal that chlorine atoms remove m ore CH3CHO relative to CH3OH in air as compared to nitrogen diluent. This s upports the hypothesis since, in the presence of oxygen, HO2 is formed and presents an additional acetaldehyde removal pathway. Employing the augmente d model, analyses of HO2 decay traces yield a CH3C(O)O-2 + HO2 rate constan t of k(1) = (3.9(-2.3)(+5.0)) x 10(-13)e((1350+/-250)/r) cm(3) s(-1). Reaso ns are discussed for why the present rate constants are 2-3 times larger th an previously reported. FTIR-smog chamber studies reveal the reaction to pr oceed 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 sma ller 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)O OH) = (4.5 +/- 1.0) x 10(-15) cm(3) s(-1) at 295 K.