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) × 10^-5 for CH₂OO, 26 (±10) × 10^-5 for CH₃CHOO derived from cis-but-2-ene, 33 (±10) × 10^-5 for CH₃CHOO derived from trans-but-2-ene, and 8.7 (±2.5) × 10^-5 for (CH₃)₂COO derived from 2,3-dimethyl-but-2-ene. The relative rate constants for k(SCI decomposition)/k(SCI + SO₂) are -2.3 (±3.5) × 10¹¹ cm^-3 for CH₂OO, 13 (±43) × 10¹¹ cm^-3 for CH³CHOO derived from cis-but-2-ene, -14 (±31) × 10¹¹ cm^-3 for CH₃CHOO derived from trans-but-2-ene and 63 (±14) × 10¹¹ cm^-3 for (CH₃)₂COO. Uncertainties are ±2σ 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) × 10^-4 for k(SCI + H₂O)/k(SCI + SO₂) of anti-CH₃CHOO and 1.2 (±1.1) × 10¹³ 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) × 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.