Atmospheric chemistry of toluene-1,2-oxide/2-methyloxepin

Abstract

A kinetic and product study of the atmospheric chemistry of toluene-1,2-oxide/2-methyloxepin, a possible intermediate in the OH-initiated oxidation of toluene, was conducted in a 1080 l quartz glass reaction chamber. The compound was found to be unstable, with a thermal decomposition lifetime of (333±83) min. Photolysis with superactinic lamps (320 nm<λ<460 nm, λ_max=360 nm) was found to be slow, while that with low-pressure Hg lamps (λ_max=254 nm) proceeded rapidly. For clear sky conditions at noon on July 1 at a latitude of 40°N, an atmospheric photolytic lifetime of (26±4) min can be extrapolated from the experimental data using the superactinic lamps. For the reaction with OH radicals, a reaction rate constant of (2.1±0.1)×10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ was determined; the main products were E/Z-isomers of 6-oxohepta-2,4-dienal. Reaction with NO₃ radicals was observed to proceed with a rate constant of (1.27±0.10)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹; the main products were unidentified nitrogen-containing compounds. In addition, 2E,4Z-6-oxohexa-2,4-dienal was identified; other isomers are probably formed as well. The results suggest that under atmospheric conditions both photolysis and reaction with OH radicals will be important sinks for toluene-1,2-oxide/2-methyloxepin, while reaction with NO₃ radicals will become important under laboratory conditions at high concentrations of NO_x, possibly also in heavily polluted tropospheric air masses.