Pressure and temperature dependence of the unimolecular decomposition of HO2NO2

Abstract

The pressure and temperature dependence of the unimolecular decomposition of pernitric acid HO2NO2→NO2+HO2 have been studied in the presence of excess nitric oxide by Fourier transform infrared spectroscopy in a 5800 liter, Teflon-lined chamber over the temperature range 261 to 295°K with N2 or O2 pressures between 1 and 760 torr. The rate constant in the second-order kinetic regime was determined to be 5.2×10−6 exp[−19 900±500/RT] cm3 molecule−1 sec−1 for N2 pressures up to ∼7 torr. The pressure dependence of the reaction between the second-order region and 1 atm pressure was determined for N2 and O2 at 278°K and for N2 at 261°K and fitted to an empirical rate constant expression for use in atmospheric modeling calculations. Extrapolation of the data at 278°K gave a limiting first-order high pressure rate constant of ∼0.018 sec−1. The relative efficiency of O2 as an M gas relative to N2 (1.0) between 1 and 760 torr was 0.83±0.12. The implications of these results for the reaction of HO2 with NO2 and for HO2NO2 kinetics in the stratosphere are discussed.