Quantitative Constraints on the Condensed-Phase Reaction Rate Constants of Criegee Intermediates with H ₂ O ₂ and H ₂ O

Although Criegee intermediates (CIs) govern key environmental oxidation pathways, their condensed-phase reaction rates with soluble gases like H 2 O 2 and H 2 O remain somewhat uncertain. Here, we use the multiphase ozonolysis of aerosolized squalene to examine competitive CI reactions with H 2 O 2 /H 2 O and aldehydes (forming secondary ozonides, SOZs). Synchrotron vacuum ultraviolet photoionization aerosol mass spectrometry reveals a shift in CI branching ratios where elevated aldehyde concentrations and reduced SOZ formation confirm that H 2 O 2 /H 2 O act as increasingly significant sinks for CIs. Using the CI + aldehyde reference reaction in the relative rate method, we quantify the condensed-phase rate constants for CIs with H 2 O/H 2 O 2 . With measured Henry’s law constants for H 2 O ( H cp = 0.12 M·atm –1 ) and H 2 O 2 ( H cp = 0.51 M·atm –1 ) in a predictive kinetic model, the condensed-phase rate constants for CI reactions with H 2 O 2 (1 × 10 –14 cm 3 ·molecules –1 ·s –1 ) and H 2 O (9 × 10 –17 cm 3 ·molecules –1 ·s –1 ) are determined. Critically, these values are consistent with gas-phase analogues, which contrast with the slowed condensed-phase reaction rates of CIs with aldehydes. This divergence suggests that solvent effects or steric hindrance may suppress CI + aldehydes reactivity while exerting negligible influence on CI reactions with small molecules such as H 2 O and H 2 O 2 .