Aqueous-Phase Oxidation of cis-β-Isoprene Epoxydiol by Hydroxyl Radicals and Its Impact on Atmospheric Isoprene Processing.

Isoprene epoxydiols are well-known intermediates of the isoprene oxidation in the gas phase under pristine conditions. According to their Henry's Law constants, they are well water-soluble and will undergo a phase transfer to the tropospheric aqueous phase to be further processed there. In the present study, the aqueous-phase oxidation by hydroxyl radicals (OH) of the isomer cis-β-isoprene epoxydiol (cis-β-IEPOX) was investigated with regards to its kinetics and reaction product distribution. For the reaction of cis-β-IEPOX with hydroxyl radicals, a second-order rate constant of (1.4 ± 0.1) × 109 L mol-1 s-1 was determined at 298 K. Short chain carbonyls, such as glycolaldehyde with a maximum molar fraction of 6.9%, hydroxyacetone (7.4%), methylglyoxal (10.3%), and glyoxal (6.6%), as well as acetic acid (13.9%), were observed as major oxidation products. A total molar recovery of 75% was obtained in the oxidation experiments. The overall contribution of the aqueous-phase oxidation of cis-β-IEPOX was estimated to range from 29.7 to 21.3% to the atmospheric processing depending on the scenario.