Kinetics of stabilised Criegee intermediates derived from alkene ozonolysis: reactions with SO₂, H₂O and decomposition under boundary layer conditions

The removal of SO2 in the presence of alkene-ozone systems has been studied for ethene, cis-but-2-ene, trans-but-2-ene and 2,3-dimethyl-but-2-ene, as a function of humidity, under atmospheric boundary layer conditions. The SO2 removal displays a clear dependence on relative humidity for all four alkene-ozone systems confirming a significant reaction for stabilised Criegee intermediates (SCI) with H2O. The observed SO2 removal kinetics are consistent with relative rate constants, k(SCI + H2O)/k(SCI + SO2), of 3.3 (+/- 1.1) x 10(-5) for CH2OO, 26 (+/- 10) x 10(-5) for CH3CHOO derived from cis-but-2-ene, 33 (+/- 10) x 10(-5) for CH3CHOO derived from trans-but-2-ene, and 8.7 (+/- 2.5) x 10(-5) for (CH3)(2)COO derived from 2,3-dimethyl-but-2-ene. The relative rate constants for k(SCI decomposition)/ k(SCI + SO2) are -2.3 (+/- 3.5) x 10(11) cm(-3) for CH2OO, 13 (+/- 43) x 10(11) cm(-3) for CH3CHOO derived from cis-but-2-ene, 14 (+/- 31) x 10(11) cm(-3) for CH3CHOO derived from trans-but-2-ene and 63 (+/- 14) x 10(11) cm(-3) for (CH3)(2)COO. Uncertainties are +/- 2 sigma and represent combined systematic and precision components. These values are derived following the approximation that a single SCI is present for each system; a more comprehensive interpretation, explicitly considering the differing reactivity for syn- and anti-SCI conformers, is also presented. This yields values of 3.5 (+/- 3.1) x 10(-4) for k(SCI + H2O)/k(SCI + SO2) of anti-CH3CHOO and 1.2 (+/- 1.1) x 10(13) for k(SCI decomposition)/k(SCI + SO2) of syn-CH3CHOO. The reaction of the water dimer with CH2OO is also considered, with a derived value for k(CH2OO + (H2O)(2))/k(CH2OO + SO2) of 1.4 (+/- 1.8) x 10(-2). The observed SO2 removal rate constants, which technically represent upper limits, are consistent with decomposition being a significant, structure dependent, sink in the atmosphere for syn-SCI