Kinetics of the reactions of OH with CO, NO, NO₂ and of HO₂ with NO₂ in air at 1 atm pressure, room temperature and tropospheric water vapour concentrations

Abstract. The termolecular reactions of hydroxyl radicals (OH) with carbon monoxide (CO), nitric oxide (NO) and nitrogen dioxides (NO₂) and the termolecular reaction of hydroperoxyl radicals (HO₂) with NO₂ greatly impact the atmospheric oxidation efficiency. Few studies have directly measured the pressure dependent rate coefficients in air at 1 atm pressure and water vapour as third collision partners. In this work, rate coefficients were measured with a high accuracy (<5 %) at 1 atm pressure, room temperature and in humidified air using laser flash photolysis and detection of the radical decay by laser-induced fluorescence. The rate coefficients derived in dry air are: (2.39±0.11) · 10^-13 cm³ s^-1 for the OH reaction with CO, (7.3±0.4) · 10^-12 cm³ s^-1 for the OH reaction with NO, (1.23±0.04) · 10^-11 cm³ s^-1 for the OH reaction with NO₂, and (1.56±0.05) · 10^-12 cm³ s^-1 for the HO₂ reaction with NO₂. For the OH reactions with CO and NO, no dependence on water vapour was observed for the range of water partial pressures tested (3 to 22 hPa), and for NO₂, only a weak increase of 3 % was measured in agreement with the study by Amedro et al. (2020). For the rate coefficient of HO₂ with NO₂ an enhancement of up to 25 % was observed. This can be explained by a faster rate coefficient of the reaction of the HO₂-water complex with NO₂ having a value of (3.4±1.1) · 10^-12 cm³ s^-1.