Accurate Kinetic Studies of OH + HO 2 Radical-Radical Reaction through Direct Measurement of Precursor and Radical Concentrations with High-Resolution Time-Resolved Dual-Comb Spectroscopy.

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 time-resolved 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+HO 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+HO 2 can be observed. This work as an independent experiment plays a crucial role in revisiting this prototypical radical-radical reaction.