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 rate_R2 = 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 K^D_eq (k_2,eff = k₂K^D_eq) for water dimer formation to allow the rate of reaction to be expressed in terms of water monomer concentration as rate_R2 = k_2,eff[CH₂OO][H₂O]² to avoid explicit calculation of dimer concentrations and impacts of differences in values of K^D_eq reported in the literature. Results from this work give k_2,eff = (1.96 ± 0.51) × 10⁻³² cm⁶ molecule⁻² s⁻¹ at 298 K with a temperature dependence described by k_2,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.