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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.

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