Abstract
Abstract. Diurnal variations of ClO, HO2, and HOCl were simultaneously observed by the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) between 12 October 2009 and 21 April 2010. These were the first global observations of the diurnal variation of HOCl in the upper atmosphere. A major reaction for the production of HOCl is ClO + HO2 → HOCl + O2 (Reaction (R1)) in extra-polar regions. A model study suggested that in the mesosphere, this is the only reaction influencing the amount of HOCl during the night. The evaluation of the pure reaction period, when only Reaction (R1) occurred in the Cly chemical system, was performed by checking the consistency of the HOCl production rate with the ClO loss rate from SMILES observation data. It turned out that the SMILES data at the pressure level of 0.28 hPa (about 58 km) in the autumn mid-latitude region (20–40°, February–April 2010) during night (between modified local time 18:30 and 04:00) were suitable for the estimation of the rate constant, k1. The rate constant obtained from SMILES observations was k1(245 K) = (7.75 ± 0.25) × 10−12 cm3 molecule−1 s−1. This result is consistent with results from a laboratory experiment and ab initio calculations for similar low-pressure conditions.
Highlights
The Reaction (R1) is the rate-limiting step of a catalytic ozone depletion cycle that causes about 7 % and 10 % of the ozone loss in the extra-tropical lower stratosphere and in the Arctic stratospheric vortex, respectively (Lee et al, 2002; Chipperfield et al, 1994)
Several laboratory studies on the rate constant of the Reaction (R1), k1, have been reported (Stimpfle et al, 1979; Knight et al, 2000; Nickolaisen et al, 2000; Hickson et al, 2007). k1 has relatively large uncertainties compared with the rate constants of other major reactions in the atmospheric chemistry
We evaluate the “purity” of Reaction (R1) using both of the rate of hypochlorous acid (HOCl) production and the rate of chlorine monoxide (ClO) loss
Summary
The Reaction (R1) converts active chlorine monoxide (ClO) into hypochlorous acid (HOCl) as a short-lived reservoir in the atmosphere: ClO + HO2 → HOCl + O2. Several laboratory studies on the rate constant of the Reaction (R1), k1, have been reported (Stimpfle et al, 1979; Knight et al, 2000; Nickolaisen et al, 2000; Hickson et al, 2007). The validity of k1 values from laboratory studies have been discussed using atmospheric observations and model calculations of HOCl. Several atmospheric observations of HOCl in the lower/middle stratosphere have been reported. “purity” means that only the Reaction (R1) modifies the concentration of ClO and HOCl, and the effect of competitive reactions does not appear in the observation. This “purity” condition is essential for the accurate estimation of k1. The photolysis of HOCl occurs during daytime, and ClO is consumed by the reaction ClO + NO2 + M →ClONO2 + M during nighttime
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