New kinetic and spectroscopic measurements in the CF3Ox + NOx system

CHIAPPERO, MS; MALANCA, FE; G.A. ARGÜELLO; S. NISHIDA; K. TAKAHASHI; Y. MATSUMI; M.D. HURLEY; T.J. WALLINGTON

Application to Atmospheric Chemical Processes

Springer

Año: 2006 p. 213 - 221

CF3Ox radicals are present in the atmosphere as oxidized species of a series of CFC´s, HFC´s and HCFC´s. They react with NOx species. In particular, the reaction between CF3O2 and NO which shows two reaction channels, CF3O2 + NO à CF3O + NO2     k1a                                  (1a) CF3O2 + NO + M à CF3ONO2 + M     k1b                   (1b) has been revised using FTIR smog chamber techniques to provide the first systematic study of the nitrate yield thus allowing the determination of the branching ratio for both process. In 700 torr of N2/O2 diluent at 296 K, the branching ratio is k1b/(k1a+k1b)=(1.67±0.27)x10-2 and the CF3ONO2 forming channel appears to approach the high-pressure limit for pressures above 100 torr. This finding could probably provide more conventional energy transfer parameters to account for the pressure dependence in atmospheric models. CF3ONO2 is also the main product observed in the black lamp (lmax~355nm) photolysis of pure CF3O2NO2. The thermal decomposition of the peroxide (which does not absorb significantly beyond 300 nm) leads to an equilibrium with CF3O2 and NO2 which does absorb and thus forms NO. Subsequent reactions lead to CF3O radicals, that in the presence of NO2 form the CF3ONO2, giving the overall reaction: CF3O2NO2 + hn à CF3ONO2 + ½ O2          (2) The low branching ratio in the formation of CF3ONO2, the photochemical characteristics of CF3O2NO2 just described and its probable even distribution in the free troposphere, led us to look closer at the UV features of the peroxide. The UV spectrum at room temperature was already known. We have determined the temperature dependence between 225 and 300 K. The results show an increase of the cross sections with temperature. From the data, photochemical half-lives in the atmosphere from 20 to 450 days result, depending on altitude and solar zenith angle. In terms of the process controlling CF3O2NO2 decomposition, calculations show that from the surface to around 2 km altitude, the thermal process is faster than photolysis. Beyond 2 km, photochemistry governs the decomposition for low and mid latitudes. At higher latitudes (50 degrees) the limiting process below the tropopause is thermal decay.