Kinetics of CH₂OO reactions with SO₂, NO₂, NO, H₂O and CH₃CHO as a function of pressure

Kinetics of CH₂OO Criegee intermediate reactions with SO₂, NO₂, NO, H₂O and CH₃CHO and CH₂I radical reactions with NO₂ are reported as a function of pressure at 295 K. Measurements were made under pseudo-first-order conditions using flash photolysis of CH₂I₂–O₂–N₂ gas mixtures in the presence of excess co-reagent combined with monitoring of HCHO reaction products by laser-induced fluorescence (LIF) spectroscopy and, for the reaction with SO₂, direct detection of CH₂OO by photoionisation mass spectrometry (PIMS). Rate coefficients for CH₂OO + SO₂ and CH₂OO + NO₂ are independent of pressure in the ranges studied and are (3.42 ± 0.42) × 10‾¹¹ cm³ s‾¹ (measured between 1.5 and 450 Torr) and (1.5 ± 0.5) × 10‾¹² cm³ s‾¹ (measured between 25 and 300 Torr), respectively. The rate coefficient for CH₂OO + CH₃CHO is pressure dependent, with the yield of HCHO decreasing with increasing pressure. Upper limits of 2 × 10−13 cm³ s‾¹ and 9 × 10−17 cm³ s‾¹ are placed on the rate coefficients for CH₂OO + NO and CH₂OO + H₂O, respectively. The upper limit for the rate coefficient for CH₂OO + H₂O is significantly lower than has been reported previously, with consequences for modelling of atmospheric impacts of CH₂OO chemistry

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

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