Temperature Dependence Study of the Kinetics and Product Yields of the HO_2 + CH_3C(O)O_2 Reaction by Direct Detection of OH and HO_2 Radicals Using 2f-IR Wavelength Modulation Spectroscopy

Abstract

The HO_2 + CH_3C(O)O_2 reaction consists of three product channels: CH_3C(O)OOH + O_2 (R1a), CH_3C(O)OH + O_3 (R1b), and OH + CH_3C(O)O + O_2 (R1c). The overall rate constant (k_1) and product yields (α_(1a), α_(1b), and α_(1c)) were determined over the atmospherically-relevant temperature range of 230 - 294 K at 100 Torr in N_2. Time resolved kinetics measurements were performed in a pulsed laser photolysis experiment in a slow flow cell employing simultaneous infrared (IR) and ultraviolet (UV) absorption spectroscopy. HO_2 and CH_3C(O)O_2 were formed by Cl-atom reactions with CH_3OH and CH_3CHO, respectively. Heterodyne near- and mid-infrared (NIR and MIR) wavelength modulation spectroscopy (WMS) was employed to selectively detect HO_2 and OH radicals. Ultraviolet absorption at 225 nm and 250 nm was used to detect various peroxy radicals as well as ozone (O_3). These experimental techniques enabled direct measurements of α_(1c) and α_(1b) via time-resolved spectroscopic detection in the MIR and the UV, respectively. At each temperature, experiments were performed at various ratios of initial HO_2 and CH_3C(O)O_2 concentrations to quantify the secondary chemistry. The Arrhenius expression was found to be k1(T) = 1.38^(+1.17)_(-0.63)×10^(-12)exp[(730±170)/T] cm^(-3) molecule^(-1) s^(-1). Α_(1a) was temperature-independent while α_(1b) and α_(1c) increased and decreased, respectively, with increasing temperatures. These trends are consistent with the current recommendation by the IUPAC data evaluation. Hydrogen-bonded adducts of HO_2 with the precursors, HO_2.CH_3OH and HO_2.CH_3CHO, played a role at lower temperatures; as part of this work, kinetics of the adducts were also measured