Kinetic Studies of the Gas Phase Reactions of Criegee Intermediates Relevant to Atmospheric Chemistry

Air quality and climate change are influenced by atmospheric composition, which is controlled by both the emissions and chemistry of trace species. Criegee intermediates (CIs) are reactive zwitterionic species with the general formula R1R2COO that are produced in the atmosphere following the ozonolysis of unsaturated volatile organic compounds (VOCs). Bimolecular reactions of stabilised Criegee intermediates (SCIs) with NO2, water vapour and SO2 are of particular interest as they have the potential to impact atmospheric budgets of NOx (NOx = NO + NO2), H2SO4, and secondary organic aerosols (SOA).
In this work, the reactions of CH2OO with SO2, NO2 and water vapour, and the reactions of CH3CHOO conformers with SO2 have been investigated across a range of temperatures and pressures relevant to the troposphere using laser flash photolysis coupled to time-resolved broadband ultraviolet absorption spectroscopy.
At 298 K, results from this work indicate that the reactions of CH2OO have rate coefficients of k = (3.56 ± 0.11) × 10-11 cm3 molecule-1 s-1 for the reaction with SO2, k = (1.24 ± 0.16) × 10- 12 cm3 molecule-1 s-1 for the reaction with NO2, k = (9.8 ± 5.9) × 10- 16 cm3 molecule-1 s-1 for the reaction with water monomers, and k = (9.52 ± 2.49) × 10-12 cm3 molecule-1 s-1 for the reaction with water dimers. No significant pressure dependence was observed for any of the CH2OO reactions investigated. The reactions of CH2OO with SO2, NO2, and water dimers exhibited negative temperature dependence, while the reaction of CH2OO with water monomers exhibited a positive temperature dependence. Atmospheric modelling shows the reaction with water dimers dominates the atmospheric loss of CH2OO under all conditions relevant to the troposphere, with sinks other than water representing less than 1 % of the overall loss of CH2OO.
For the reaction of CH3CHOO conformers with SO2, results indicate a rate coefficient of k = (4.80 ± 0.46) × 10-11 cm3 molecule-1 s-1 for the reaction of syn-CH3CHOO with SO2 at 298 K and 760 Torr. Significant collisional stabilisation of the secondary ozonide formed in the reaction is expected under atmospheric conditions. Kinetics of anti-CH3CHOO + SO2 display no significant dependence on temperature or pressure over the ranges investigated, with a mean rate coefficient of k = (1.18 ± 0.21) × 10-10 cm3 molecule-1 s- 1 over all conditions. Results indicate that the reaction of syn-CH3CHOO with SO2 is competitive with unimolecular decomposition and reaction with water vapour in areas of high SO2 concentration and low humidity, particularly at lower temperatures. The atmospheric importance of the reaction between anti-CH3CHOO and SO2 remains uncertain due to the lack of literature regarding the reactions between anti-CH3CHOO and water vapour, which should be the subject of future experimental studies into Criegee intermediates.