| Popis: |
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. |
