Reaction of HO₂ with ClO:  Flow Tube Studies of Kinetics and Product Formation between 215 and 298 K

Abstract

Rate coefficients for the reaction between HO2 and ClO radicals were obtained using the discharge-flow/mass-spectrometry technique at total pressures of 1.1−1.7 Torr of He, and between 298 and 215 K. The room-temperature rate constant, determined using seven different combinations of HO2 and ClO precursors, was found to be (7.1 ± 1.8) × 10-12 (±2σ) cm3 s-1. The temperature-dependent overall reaction rate coefficient is described by k (3) (298−215 K) = (7.1 ± 0.4) × 10-12 exp(−16 ± 17/T) cm3 s-1. The previous observation of a strong negative temperature dependence in the title reaction below 298 K was not observed, resulting in a significantly lower rate coefficient at stratospheric temperatures. HOCl was the only product of the reaction, and an upper limit of 1% for the branching ratio for the formation of O3 and HCl was obtained at the low pressures of these experiments.

Kinetics and mechanism of the reaction of OH with ClO

“…d At temperatures T ≤ 270 K, the HO 2 profiles were better fitted with the rate constant 7.1 × 10 −12 cm 3 molecule −1 s −1 (independent of temperature) as measured recently [23].…”

Direct estimation of the rate constant of the reaction ClO + HO<sub>2</sub> → HOCl + O<sub>2</sub> from SMILES atmospheric observations

“…Table 1 shows k 1 and the error (1σ ) calculated from previous laboratory studies at 225 K (which corresponds to the typical temperature of the lower stratosphere). A discrepancy of a factor of 2 between the k 1 values from Stimpfle et al (1979) and Knight et al (2000) can be noticed. There is no consistency in the previous laboratory studies.…”

“…We compared our derived k 1 , hereafter denoted "SMILES k 1 ", with previous laboratory experiments (Stimpfle et al, 1979;Knight et al, 2000;Nickolaisen et al, 2000;Hickson et al, 2007), an ab initio calculation (Xu et al, 2003), and JPL 2011 recommendation (Sander et al, 2011). Figure 7 shows the comparison of k 1 from our work and that from previous works.…”

“…Several laboratory studies on the rate constant of the Reaction (R1), k 1 , have been reported (Stimpfle et al, 1979;Knight et al, 2000;Nickolaisen et al, 2000;Hickson et al, 2007). k 1 has relatively large uncertainties compared with the rate constants of other major reactions in the atmospheric chemistry.…”

“…Laboratory measurements k 1 (225K) * 1σ * Stimpfle et al (1979) 13.80 None Nickolaisen et al (2000) 11.36 3.03 Knight et al (2000) 6.61 0.66 Hickson et al (2007) 8.98 3.20 * Units: 10 −12 cm 3 molecule −1 s −1 . (Kovalenko et al, 2007;von Clarmann et al, 2012).…”