Kinetic studies of C 1 and C 2 Criegee intermediates with SO 2 using laser flash photolysis coupled with photoionization mass spectrometry and time resolved UV absorption spectroscopy

• Published in: Physical chemistry chemical physics : PCCP Vol. 20; no. 34; pp. 22218 - 22227
• Main Authors: Howes, N U M, Mir, Z S, Blitz, M A, Hardman, S, Lewis, T R, Stone, D, Seakins, P W
• Format: Journal Article
• Language: English
• Published: England 17.08.2018
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/C8CP03115K

Recent, direct studies have shown that several reactions of stabilized Criegee intermediates (SCI) are significantly faster than indicated by earlier indirect measurements. The reaction of SCI with SO2 may contribute to atmospheric sulfate production, but there are uncertainties in the mechanism of the reaction of the C1 Criegee intermediate, CH2OO, with SO2. The reactions of C1, CH2OO, and C2, CH3CHOO, Criegee intermediates with SO2 have been studied by generating stabilized Criegee intermediates by laser flash photolysis (LFP) of RI2/O2 (R = CH2 or CH3CH) mixtures with the reactions being followed by photoionization mass spectrometry (PIMS). PIMS has been used to determine the rate coefficient for the reaction of CH3CHI with O2, k = (8.6 ± 2.2) × 10-12 cm3 molecule-1 s-1 at 295 K and 2 Torr (He). The yield of the C2 Criegee intermediate under these conditions is 0.86 ± 0.11. All errors in the abstract are a combination of statistical at the 1σ level and an estimated systematic contribution. For the CH2OO + SO2 reaction, additional LFP experiments were performed monitoring CH2OO by time-resolved broadband UV absorption spectroscopy (TRUVAS). The following rate coefficients have been determined at room temperature ((295 ± 2) K):CH2OO + SO2: k = (3.74 ± 0.43) × 10-11 cm3 molecule-1 s-1 (LFP/PIMS),k = (3.87 ± 0.45) × 10-11 cm3 molecule-1 s-1 (LFP/TRUVAS)CH3CHOO + SO2: k = (1.7 ± 0.3) × 10-11 cm3 molecule-1 s-1 (LFP/PIMS)LFP/PIMS also allows for the direction observation of CH3CHO production from the reaction of CH3CHOO with SO2, suggesting that SO3 is the co-product. For the reaction of CH2OO with SO2 there is no evidence of any variation in reaction mechanism with [SO2] as had been suggested in an earlier publication (Chhantyal-Pun et al., Phys. Chem. Chem. Phys., 2015, 17, 3617). A mean value of k = (3.76 ± 0.14) × 10-11 cm3 molecule-1 s-1 for the CH2OO + SO2 reaction is recommended from this and previous studies. The atmospheric implications of the results are briefly discussed.