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): CH2OO, methyl vinyl ketone oxide (MVKO) and methacrolein oxide (MACRO). The oxidation of SO2 by CIs is a potential source of H2SO4, 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 SO2 has been found, k(SO2) = (1.5 +/- 0.4) x 10(-10) cm(3) s(-1) (+/- 1 sigma, sigma 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) x 10(-17) cm(3) s(-1) (+/- 1 sigma) 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 SO2 would be substantial, even more than that of syn-MVKO. Methacrolein oxide is one of the ozonolysis products of isoprene present in our atmosphere, but its relevance under humid conditions is not fully understood. Here, UV-vis spectroscopy and reaction kinetics studies show much longer lifetimes of anti-methacrolein oxide than previously thought, suggesting much higher steady-state concentrations and higher impact on the oxidation of atmospheric SO2.

Automated summary

The study reveals that anti-MACRO reacts quickly with SO2 but slowly with water vapor, leading to a longer atmospheric lifetime, higher steady-state concentration, and greater impact on the oxidation of atmospheric SO2.