Dramatic Slowdown of Bimolecular Reactions of Criegee Intermediates in Organic Aerosols

Although Criegee intermediates (CIs) play a central role in driving the transformation of organic molecules, their condensed-phase reaction kinetics remains poorly understood. While CIs undergo unimolecular decomposition to produce OH radicals, stabilized CIs participate in a host of other bimolecular reactions forming larger molecular weight products, including secondary ozonides (k_SOZ), alkoxyalkyl hydroperoxides (k_AlAHP), and acyloxyalkyl hydroperoxides (k_AcAHP). Here, we demonstrate, using prior literature and new measurements of heterogeneous organic aerosol reactions using O₃ or OH radicals, that these condensed-phase bimolecular reaction rate coefficients k_SOZ (10^-20 cm³·molecule^-1·s^-1), k_AlAHP, and k_AcAHP (10^-19 cm³·molecule^-1·s^-1) are 7-10 orders of magnitude slower than analogous gas-phase reactions (∼ 10^-10-10^-13 cm³·molecule^-1·s^-1). This contrasts with current evidence that the rate coefficients for unimolecular steps in the condensed phase are similar in magnitude to those in the gas phase. Kinetic models using gas-phase-like bimolecular coefficients consistently overestimate the importance of bimolecular relative to unimolecular pathways while underpredicting the aerosol reactivity by up to 30-fold. The slowing of bimolecular reactivity in aerosols is likely due to solvation effects and the increased steric hindrance of larger molecular weight CIs and their scavengers in the condensed phase. These results challenge the conventional view that the high reactant density of an organic aerosol should favor bimolecular over unimolecular reactions, underscoring the need for more accurate CI condensed-phase rate constants for reliable modeling of oxidative processes across a diverse set of atmospheric and environmental systems.