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 X 10(14) to 1.1 X 10(20) 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): k0,2 = 1.04 x 10(-3)(T/300)-3.5 exp(-11000/T) cm3 molecule-1 s-1, k(infinity,2) = 6.22 x 10(14)(T/300)-0.2 exp(-11000/7)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 (approximately 15% to 20%), compared to current ones, at ground level and in the upper stratosphere.