Kinetics of the Reactions of the Criegee Intermediate CH₂OO with Water Vapour: Experimental Measurements as a Function of Temperature and Global Atmospheric Modelling

The kinetics of reactions between the simplest Criegee intermediate, CH2OO, and water vapour have been investigated at temperatures between 262 and 353 K at a total pressure of 760 Torr using laser flash photolysis of CH2I2–O2–N2–H2O mixtures coupled with broadband time-resolved UV absorption spectroscopy. Results indicate that the reaction with water monomers represents a minor contribution to the total loss of CH2OO under the conditions employed in this work, with an estimated rate coefficient for CH2OO + H2O (R1) of (9.8 ± 5.9) × 10−17 cm3 molecule−1 s−1 at 298 K and a temperature dependence described by k1 = (3.2 ± 1.1) × 10−13 exp(−(2410 ± 270)/T) cm3 molecule−1 s−1. The reaction of CH2OO with water dimers, CH2OO + (H2O)2 (R2), dominates under the conditions employed in this work. The rate coefficient for R2 has been measured to be k2 = (9.5 ± 2.5) × 10−12 cm3 molecule−1 s−1 at 298 K, with a negative temperature dependence described by k2 = (2.85 ± 0.40) × 10−15 exp((2420 ± 340)/T) cm3 molecule−1 s−1, where rateR2 = k2[CH2OO][(H2O)2]. For use in atmospheric models, we recommend description of the kinetics for R2 in terms of the product of the rate coefficient k2 and the equilibrium constant KDeq (k2,eff = k2KDeq) for water dimer formation to allow the rate of reaction to be expressed in terms of water monomer concentration as rateR2 = k2,eff[CH2OO][H2O]2 to avoid explicit calculation of dimer concentrations and impacts of differences in values of KDeq reported in the literature. Results from this work give k2,eff = (1.96 ± 0.51) × 10−32 cm6 molecule−2 s−1 at 298 K with a temperature dependence described by k2,eff = (2.78 ± 0.28) × 10−38 exp((4010 ± 400)/T) cm6 molecule−2 s−1. No significant impacts of a reaction between CH2OO and three water molecules were observed in this work, potentially as a result of the relative humidities used in this work (up to 57% at 298 K). Atmospheric implications of the results have been investigated using the global chemistry transport model GEOS-Chem. Model simulations indicate that the reaction with water dimers dominates the loss of CH2OO in the atmosphere and limits the impacts of other reactions of CH2OO, with the reaction with water dimers representing >98% of the total loss of CH2OO in the troposphere.