Mechanistic and Kinetic Studies of Selected Oxidation Steps of Tropospheric Interest

Summary

Reaction mechanisms and the kinetics of selected steps in the tropospheric oxidation of simple radical intermediates and of unsaturated organic compounds have been studied. As oxidants the nitrate radical NO₃, ozone O₃ and the peroxy radical CH₃O₂ were employed.

The primary interaction of NO₃ with radical species X at ambient temperatures yields rovibrationally excited products XONO₂ which in the gas phase immediately decompose to yield XO and NO₂. This way X = H, CH₃, HO, NO, HO₂, CH₃O₂, Cl and ClO are oxidised by NO₃ with high efficiency. This mechanism has been verified for X = H, HO and CH₃ by ab initio calculations on MP2-level of theory. Interaction of NO₃ with unsaturated organics starts through an electrophilic radical attack at the double bond forming a rovibrationally excited adduct radical with free rotation at the attacked bond. The adduct radical either spontaneously eliminates NO₂ to form a stable epoxide or becomes stabilised in air by formation of a nitroalkyl-peroxy radical.

O₃ alkene systems under atmospheric conditions were found to be a source for OH radicals. From experiments with an excess of CO added, source strengths of 0.50 ±0.1, 0.37 and 0.24 ë 0.2 were obtained for TME, trans-2-butene and isoprene, respectively. Elucidation of the mechanism for OH generation is attempted by following intrinsic reaction pathways of the intermediates by model calculations.

A very sensitive and specific method of CH₃O₂ detection has been developed in the project. It is based on REMPI-MS techniques employing laser irradiation at two wavelengths. In EI-MS studies radical reaction of CH₃O₂ with other radicals were investigated including the species NO₃, halogen atoms, ClO and BrO.