Surprisingly long lifetime of methacrolein oxide, an isoprene derived Criegee intermediate, under humid conditions
Yen‐Hsiu Lin, Cangtao Yin, Kaito Takahashi, Jim J. Lin
Abstract
Abstract Ozonolysis of isoprene, the most abundant alkene, produces three distinct Criegee intermediates (CIs): CH 2 OO, methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO). The oxidation of SO 2 by CIs is a potential source of H 2 SO 4 , an important precursor of aerosols. Here we investigated the UV-visible spectroscopy and reaction kinetics of thermalized MACRO. An extremely fast reaction of anti -MACRO with SO 2 has been found, k SO2 = (1.5 ± 0.4) × 10 −10 cm 3 s −1 (±1 σ , σ is the standard deviation of the data) at 298 K (150 − 500 Torr), which is ca. 4 times the value for syn -MVKO. However, the reaction of anti -MACRO with water vapor has been observed to be quite slow with an effective rate coefficient of (9 ± 5) × 10 −17 cm 3 s −1 (±1 σ ) at 298 K (300 to 500 Torr), which is smaller than current literature values by 1 or 2 orders of magnitude. Our results indicate that anti -MACRO has an atmospheric lifetime (best estimate ca. 18 ms at 298 K and RH = 70%) much longer than previously thought (ca. 0.3 or 3 ms), resulting in a much higher steady-state concentration. Owing to larger reaction rate coefficient, the impact of anti -MACRO on the oxidation of atmospheric SO 2 would be substantial, even more than that of syn -MVKO.