Water vapor effect on the HNO₃ yield in the HO₂ + NO reaction: Experimental and theoretical evidence

International audienceThe influence of water vapor on the production of nitric acid in the gas-phase HO2 + NO reaction was determined at 298 K and 200 Torr using a high-pressure turbulent flow reactor coupled with a chemical ionization mass spectrometer. The yield of HNO3 was found to increase linearly with the increase of water concentration reaching an enhancement factor of about 8 at [H2O] = 4 × 1017 molecules cm−3 (~50% relative humidity). A rate constant value k1bw = 6 × 10−13 cm3 molecule−1 s−1 was derived for the reaction involving the HO2*H2O complex: HO2*H2O + NO → HNO3 (1bw), assuming that the water enhancement is due to this reaction. k1bw is approximately 40 times higher than the rate constant of the reaction HO2 + NO → HNO3 (1b), at the same temperature and pressure. The experimental findings are corroborated by density functional theory (DFT) calculations performed on the H2O/HO2/NO system. The significance of this result for atmospheric chemistry and chemical amplifier instruments is briefly discussed. An appendix containing a detailed consideration of the possible contribution from the surface reactions in our previous studies of the title reaction and in the present one is included