Kinetic Studies of the Gas Phase CH2OO Criegee Intermediate Relevant to Atmospheric Chemistry Using Time-Resolved UV and IR Absorption Spectroscopy

The chemistry of Criegee intermediates, produced in the atmosphere via oxidation of unsaturated volatile organic compounds by ozone, has potentially important impacts on atmospheric composition and thus on air quality and climate. In recent years, there have been significant advances in our understanding of the properties and chemistry of Criegee intermediates following the advent of photolytic sources for use in laboratory experiments. Since the discovery of this photolytic production method of Criegee intermediates, various methods have been employed for their detection, which have yielded vast information on Criegee spectra and the kinetics of Criegee reactions with other atmospheric species. However, discrepancies persist in Criegee intermediate spectra, rate coefficients of Criegee intermediate reactions and also in product yields. In this work, the UV absorption cross sections of the simplest Criegee intermediate CH2OO, and kinetics of the CH2OO self reaction and the reaction of CH2OO with I are reported as a function of pressure. Measurements were made at 298 K using 248 nm pulsed laser flash photolysis of CH2I2/O2/N2 gas mixtures coupled with time resolved broadband UV absorption spectroscopy at pressures between 6 and 300 Torr. Results give a peak absorption cross¬ section of (1.37 ± 0.29) × 10-17 cm2 at ~340 nm and a rate coefficient for the CH2OO self reaction of (8.0 ± 1.1) × 10-11 cm3 s-1, with no significant pressure dependence of the absorption cross sections or the self reaction kinetics over the range investigated. On the contrary, the rate coefficient for the reaction between CH2OO and I demonstrates pressure dependence over the range investigated, with a Lindemann fit giving k0 = (4.4 ± 1.0) × 10-29 cm6 s-1 and k∞ = (6.7 ± 0.6) × 10-11 cm3 s-1. The origins of IO in the system have been investigated, the implications of which are discussed. Additionally, the CH2OO + SO2 reaction at room temperature was selected to develop and characterise a robust and economical instrument that can be applied to a wide range of problems in atmospheric chemistry and beyond. The development, characterisation and initial results from the experiment using 266 nm pulsed laser flash photolysis coupled with time resolved mid IR quantum cascade laser (QCL) absorption spectroscopy are reported. The IR absorption spectrum of CH2OO and rate coefficient of the CH2OO + SO2 reaction with respect CH2OO loss and SO3 production are reported at 298 K and pressures in the range 20-100 Torr. Results indicate the CH2OO spectrum to be in good agreement with that of a previously reported measurement in terms of relative peak heights and positions in the wavenumber region 1285.5917-1286.0605 cm-1, and a rate coefficient for the CH2OO + SO2 reaction of (3.8 ± 0.5) × 10-11 cm3 s-1 from both CH2OO and SO3 measurements, with no significant pressure dependence over the pressure range investigated. The product yield of SO3 from the reaction of CH2OO + SO2 is determined to be independent of pressure over the range investigated, with an SO3 absorption cross section of (5.5 ± 2.3) × 10-19 cm2 at ~1388.7 cm-1.