Abstract
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) × 10(11) cm(-3) for CH2OO, 13 (±43) × 10(11) cm(-3) for CH3CHOO derived from cis-but-2-ene, -14 (±31) × 10(11) cm(-3) for CH3CHOO derived from trans-but-2-ene and 63 (±14) × 10(11) 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) × 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) × 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.
Highlights
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
We propose two possible explanations for this observation: firstly that it arises from differing kinetics of the two CH3CHOO conformers formed in but-2-ene ozonolysis; secondly that the behaviour may reflect the presence of an additional oxidant being formed in the ozonolysis system that reacts with SO2 but is less sensitive to here. 3.2.2 CH2OO + (H2O)
In the case of the stabilised Criegee intermediates (SCI) formed from TME ozonolysis, (CH3)2COO, there is always a methyl group in a syn position to the carbonyl oxide moiety, the analysis presented in Section 3.3.2 for the CH3CHOO isomers does not apply
Summary
This does not mean the results are inconsistent with reaction of CH2OO with (H2O)[2]. It is seen that this value is rather insensitive to the inclusion of the (H2O)[2] term in eqn (E4) as the value is within the uncertainties of the linear fit to the data presented in Fig. 3 – see Table 2 Converting this value to an absolute value using the k2 from Welz et al.[15] gives k3 = 9.9 (Æ2.9) Â 10À16 cm[3] sÀ1. This yields a k5/k2 value of 0.10 (Æ0.01), giving an upper limit k5 value of 3.9 (Æ0.39) Â 10À12 cm[3] sÀ1
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