Accurate Kinetic Studies of OH + HO₂ Radical–Radical Reaction through Direct Measurement of Precursor and Radical Concentrations with High-Resolution Time-Resolved Dual-Comb Spectroscopy

Abstract

The radical−radical reaction between OH and HO 2 has been considered for a long time as an important reaction in tropospheric photochemistry and combustion chemistry. However, a significant discrepancy of an order of magnitude for rate coefficients of this reaction is found between two recent experiments. Herein, we investigate the reaction OH + HO 2 via direct spectral quantification of both the precursor (H 2 O 2 ) and free radicals (OH and HO 2 ) upon the 248 nm photolysis of H 2 O 2 using infrared two-color timeresolved dual-comb spectroscopy. With quantitative and kinetic analysis of concentration profiles of both OH and HO 2 at varied conditions, the rate coefficient k OH+HOd 2 is determined to be (1.10 ± 0.12) × 10 −10 cm 3 molecule −1 s −1 at 296 K. Moreover, we explore the kinetics of this reaction under conditions in the presence of water, but no enhancement in the k OH+HOd 2 can be observed. This work as an independent experiment plays a crucial role in revisiting this prototypical radical−radical reaction.

New Instrument for Time-Resolved OH and HO2 Quantification in High-Pressure Laboratory Kinetics Studies

“…We note that in 2023 Speak et al reported a value of k 9 about 11 times lower than the current IUPAC recommendation of 1.1 × 10 –10 cm 3 ·molec –1 ·s –1 . However, a 2024 study by Chen et al confirmed the previous consensus, and we view k 9 = 1.1 × 10 –10 cm 3 ·molec –1 ·s –1 as reliable. Our modeled OH decays are thus sensitive only to the initial radical concentrations, providing an opportunity to confirm these expected concentrations by kinetic fits.…”