Kinetic studies of C1 and C2 Criegee intermediates with SO2 using laser flash photolysis coupled with photoionization mass spectrometry and time resolved UV absorption spectroscopy

Abstract

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.