Surprisingly long lifetime of methacrolein oxide, an isoprene derived Criegee intermediate, under humid conditions.

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 (±1σ, 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 (±1σ) 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.