Thermal Stability of Peroxynitrates

PAN [CH₃C(O)O₂NO₂] has been measured ubiquitously in the troposphere, and in the stratosphere HO₂NO₂ has also been analysed. The thermal lives of both compounds as a function of pressure and temperature have been established reasonably well by laboratory studies, in particular for PAN. Other peroxynitrates including the halogenated derivatives are less important as NO_x carriers or temporary RO₂ reservoirs in the atmosphere at least at ambient temperature. Other loss processes of peroxynitrates, besides thermal decomposition, are the reaction with OH radicals and photolysis. A few experimental results also seem to support heterogeneous decomposition of RO₂NO₂ on surfaces. The relative importance of thermal decomposition rate, photolysis rate and rate of the OH reaction depends on temperature, pressure, spectral distribution of solar light intensity and the OH concentration. In principle, lower temperature and lower pressure at higher altitudes increase the importance of photolysis and OH reactions as sinks of RO₂NO₂, because thermal decomposition rates become much slower. The RO₂NO₂ are produced by the addition of NO₂ to RO₂ radicals. The RO₂-NO₂ bond energy is relatively weak and lies in the range of 85–120 kJ/mol according to recent measurements. Because these bond energies determine the thermal lifetimes, it can be estimated that at 1 bar total pressure and room temperature, the lifetime ranges from 10−1 to 104 s. In the present work, the thermal lifetimes of a variety of RO₂NO₂ species were studied as a function of temperature and pressure. The observed pressure dependencies of the thermal decomposition rates were fitted by calculations based on the Troe-treatment of unimolecular reactions. The data obtained within the present work allow predictions about the thermal decomposition rates of RO₂NO₂ species not yet studied.