A study of nighttime nitrogen oxide oxidation in a large reaction chamber—the fate of NO2, N2O5, HNO3, and O3 at different humidities

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 m3 and a surface/volume ratio better than 1 m−1. The inner surface of the chamber is Teflon FEP. The formation of N2O5 and HNO3 in ambient air with an initial content of ≈ 1.3 ppm NO2 and ≈ 1.3 ppm O3 was monitored at 8, 20, and 70% relative humidity for periods of up to five days. The mixing ratios of NO2, N2O5, and HNO3 were measured simultaneously by in-chamber FTIR absorption spectroscopy. O3 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 NO3 and the gas-phase formation of HNO3 from N2O5 and water were determined. An upper limit for the rate coefficient for the unimolecular decomposition of NO3 of 1.4 × 10−4s−1 was obtained, which corresponds to a lifetime of 120 min. The experiments provide evidence that the conversion of N2O5 with gaseous water to gas-phase HNO3 is a superposition of two slow processes: a second-order reaction, N2O5 + H2O, with a rate coefficient of 2.6( ± 0.1) × 10−22cm3 molecule−1s−1, and a third-order reaction, first order in N2O5 and second order in H2O, with a rate coefficient of 2( ± 0.05) × 10−39 cm6 molecule−2s−1. The third-order process could be due to a reaction of N2O5 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 N2O5 with respect to its gas-phase conversion to HNO3 are discussed.