Kinetics of the termolecular reaction of OH with NO, NO2, and of HO2 with NO2 in 1 atm air at 298K and tropospheric water vapour concentrations

Pulsed laser photolysis/laser-induced fluorescence (LP/LIF) was used to study the rate coefficients of the  OH reaction with CO, NO, NO₂ and of the HO₂ reaction with NO₂ in synthetic air at different water vapour concentrations (partial pressure up to 22 hPa) and at room temperature and at atmospheric pressure. The decay of the radicals was monitored by LIF, which allowed the calculation of the bimolecular rate coefficients. The rate coefficients for the reaction of OH with NO and NO₂ agree very well with current NASA/JPL and IUPAC recommendations and have a high accuracy (< 5%). These rate coefficients were found to be independent of the presence of water vapour at 1 atm of total pressure. At high pressures and low water vapour mixing ratios, as in the experiments in this work, only a small effect of the collisional stabilisation by water molecules is expected. The measured rate constant of HO₂ with NO₂ was found to be significantly dependent on the water vapour concentration. The water dependence can be explained by an approximately two times higher rate coefficient of the reaction of NO₂ with the HO₂ complex with water.