A study of nighttime nitrogen oxide oxidation in a large reaction chamber—the fate of NO 2, N 2O 5, HNO 3, and O 3 at different humidities

Abstract

Inorganic reactions important for the nighttime chemistry of nitrogen oxides in surface air were studied. The experiments were performed in a new, large reaction chamber with a volume of 260 m 3 and a surface/volume ratio better than 1 m −1. The inner surface of the chamber is Teflon FEP. The formation of N 2O 5 and HNO 3 in ambient air with an initial content of ≈ 1.3 ppm NO 2 and ≈ 1.3 ppm O 3 was monitored at 8, 20, and 70% relative humidity for periods of up to five days. The mixing ratios of NO 2, N 2O 5, and HNO 3 were measured simultaneously by in-chamber FTIR absorption spectroscopy. O 3 and NO were measured by UV absorption and chemiluminescence. Model calculations for the nitrogen oxide/ozone system were performed. By comparison of the model calculations with the experimental data, the rate coefficients of two slow reactions, the unimolecular decomposition of NO 3 and the gas-phase formation of HNO 3 from N 2O 5 and water were determined. An upper limit for the rate coefficient for the unimolecular decomposition of NO 3 of ⩽ 1.4 × 10 −4s −1 was obtained, which corresponds to a lifetime of 120 min. The experiments provide evidence that the conversion of N 2O 5 with gaseous water to gas-phase HNO 3 is a superposition of two slow processes: a second-order reaction, N 2O 5 + H 2O, with a rate coefficient of 2.6( ± 0.1) × 10 −22cm 3 molecule −1s −1, and a third-order reaction, first order in N 2O 5 and second order in H 2O, with a rate coefficient of 2( ± 0.05) × 10 −39 cm 6 molecule −2s −1. The third-order process could be due to a reaction of N 2O 5 with water on the chamber walls or alternatively to a gas-phase reaction, possibly even with water dimers. The implications of both alternatives for the atmospheric lifetime of N 2O 5 with respect to its gas-phase conversion to HNO 3 are discussed.