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.