Surprisingly Long Lifetime of Methacrolein Oxide, an Isoprene Derived Criegee Intermediate, Under Humid Conditions

Abstract Ozonolysis of isoprene, the most abundant alkene in the atmosphere, produces three distinct Criegee intermediates (CIs): CH2OO, methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO). Here we investigated the UV-visible spectroscopy and reaction kinetics of thermalized MACRO. An extremely fast reaction of anti-MACRO with SO2 has been found with a rate coefficient of (1.5±0.4)x10−10 cm3 s−1 at 298 K (150−500 Torr), which is ca. 4 times the value for syn-MVKO. However, the reaction of anti-MACRO with H2O has been observed to be quite slow with a rate coefficient of (8±6)x10−17 cm3 s−1 at 298 K (300 to 500 Torr), which is smaller than literature values by 1 or 2 orders of magnitude. Our results indicate that anti-MACRO has an atmospheric lifetime much longer than previously thought, resulting in a much higher steady-state concentration. Owing to larger reaction rate coefficient, the impact of anti-MACRO on the oxidation of atmospheric SO2 would be substantial, even larger than that of syn-MVKO.