Transient Absorption Spectroscopy of C1 and C2 Criegee Intermediates: UV Spectrum and Reaction Kinetics

Abstract:

Atmospheric production and removal rates of Criegee intermediates produced in alkene ozonolysis must be understood to constrain the importance of these species in VOC oxidation and other processes. To estimate these rates, reliable detection methods and laboratory measurements of the UV absorption spectra and reaction kinetics of Criegee intermediates are needed. Here, transient absorption spectroscopy was used to directly measure the UV spectrum of the C2 Criegee intermediate CH₃CHOO in a flow reactor at 295 K. The UV spectrum was scaled to the absolute absorption cross section at 308 nm determined by laser depletion measurements in a molecular beam, resulting in a peak UV cross section of (1.27±0.11) × 10⁻¹⁷ cm² molecule⁻¹ at 328 nm. This spectrum represents the absorption of the syn and anti conformers of CH₃CHOO under near-atmospheric conditions, both of which contribute to CH₃CHOO atmospheric removal due to UV photolysis. Transient UV absorption was also used to measure the kinetics of the reaction of the C1 Criegee intermediate CH₂OO with water vapor at temperatures from 283 to 324 K. The observed CH₂OO decay is quadratic with respect to the H₂O concentration, indicating that reaction with water dimer is the primary process affecting CH₂OO loss. The rate coefficient for the reaction of CH₂OO with water dimer exhibits a strong negative temperature dependence with an Arrhenius activation energy of −8.1±0.6 kcal mol^–1. The temperature dependence increases the effective loss rate for CH₂OO (relative to 298 K) by a factor of ∼2.5 at 278 K and 70% relative humidity, and decreases the loss rate by a factor of ∼2 at 313 K and 30% humidity, which demonstrates that variations in reaction rate due to temperature differences should be included in estimates of Criegee intermediate removal via reactions with water dimer in the atmosphere.