Measurement of rate coefficients for the unimolecular decomposition of N₂O₅

The kinetics of the decomposition of dinitrogen pentoxide (N₂O₅) in the presence of excess nitric oxide (NO) in a bath of nitrogen was studied as a function of temperature and pressure using Fourier transform infrared spectroscopy to monitor the N₂O₅ concentration. The available data encompassing temperatures from 253 to 384 K and third-body concentrations from 4.3 × 10¹⁴ to 1.1 × 10²⁰ molecule^-3 have been analyzed, although rate coefficients over the entire range of pressures have not been measured at all temperatures. The following recommendations are made to fit parameters to the Troe expression for k(N₂O₅ + M): k_0,2 = 1.04 × 10^-3(T/300)^-3.5 exp(-11000/T) cm³ molecule^-1 s^-1, k_∞,2 = 6.22 × 10¹⁴(T/300)^-0.2 exp(-11000/T)^s-1, F_c = 2.5 exp(-1950/T) + 0.9 exp(-T/430). These parameters, when combined with data for k(NO₂ + NO₃ + M), yield N₂O₅ equilibrium constant values (k(N₂O₅ + M)/k(NO₂ + NO₃ + M)) systematically higher than recent evaluations of direct measurements by 19-37% but closer to earlier measurements. With these new recommendations, calculated atmospheric decomposition lifetimes for N₂O₅ are decreased slightly ( ∼15% to 20%), compared to current ones, at ground level and in the upper stratosphere.