Water Vapor Effect on the HNO3 Yield in the HO2 + NO Reaction: Experimental and Theoretical Evidence

The 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 x 10(17) molecules cm(-1) (similar to 50% relative humidity). A rate constant value k(1bw) = 6 x 10(-13) cm(-1) molecule(-1) s(-1) was derived for the reaction involving the HO2 center dot H2O complex: HO2 center dot H2O + NO -> HNO3 (1bw), assuming that the water enhancement is due to this reaction. k(1bw) 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.