A181-0005
Water catalysis is the major contributor to the reaction of OH and HO2 under atmospheric conditions.

Tuesday, 15 December 2020
Poster
Thomas Speak1, Mark A Blitz2,3, Paul W Seakins3,4 and Diogo de Jesus Medeiros3,5, (1)University of Leeds, School of Chemistry, Leeds, LS2, United Kingdom, (2)National Centre for Atmospheric Science, Leeds, United Kingdom, (3)University of Leeds, School of Chemistry, Leeds, United Kingdom, (4)National Centre for Atmospheric Science, University of Leeds, Leeds, United Kingdom, (5)USP University of Sao Paulo, São Paulo, Brazil
Abstract:
The reaction of the hydroxyl (OH) and the hydroperoxyl (HO2) radicals (reaction 1) has often been considered important to understanding radical radical chemistry. (1) The reaction is important to the understanding of atmospheric chemistry, astrochemistry, and combustion systems. As a result this reaction has been the subject of a vast array of experimental studies from which the IUPAC recommended value is k1= (1.1 ± 0.1) × 10-10 cm3 s-1.(2) Much work has also been expended attempting to link the low temperature (300 K) rate coefficient with the temperature dependence observed at elevated temperatures (> 800 K).

New experimental work carried out here, in both a conventional slow flow pulsed laser photolysis (PLP) cell (300 Torr), and a novel high-pressure PLP system (1800 Torr) (3) that monitored both OH and HO2, measured a much lower value of k1= (1.0 ± 0.5) × 10-11 cm3 s-1. In addition, there was a significant water enhancement to the rate coefficient when water was varied between 0 – 3.2 × 1017 cm-3, with k1= (7.8 ± 0.1) × 10-11 cm3 s-1 with 3.2 × 1017 cm-3 H2O. The observation of a much lower observed rate coefficient for the water free reaction, and of significant water enhancement was supported by ab-initio calculations and MESMER simulations. With the implication of these results being that under atmospheric conditions the reaction of OH and HO2 will proceed via the water catalysed route (>85 % for [H2O] = 2.6 × 1017 cm-3 at 298 K).

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  • SPEAK, T.H., M.A. BLITZ, D. STONE and P.W. SEAKINS. A New Instrument for Time Resolved Measurement of HO2 Radicals. Meas. Tech. Discuss., 2019, 2019, pp.1-30.
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