Dramatic Slowdown of Gas Phase 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 remain poorly understood. While CIs rapidly decompose unimolecularly to produce OH radicals, stabilized CIs participate in a host of other bimolecular reactions forming larger molecular weight secondary ozonides (kSOZ), alkoxyalkyl hydroperoxides (kAlAHP), and acyloxyalkyl hydroperoxides (kAcAHP). Here we demonstrate, using prior literature and new measurements of heterogeneous reactions of organic aerosols exposed to O3 or OH radicals, that these condensed-phase bimolecular reaction rate coefficients kSOZ (10^-20 cm3molecule-1s-1), kAlAHP, and kAcAHP (10^-19 cm3molecule-1s-1) are 7-10 orders of magnitude slower than analogous gas-phase reactions (~ 10^-10 - 10^-13 cm3molecule-1s-1). This contrasts with current evidence that the rate coefficients for unimolecular steps in the condensed phase are similar in magnitude to those reported 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. These results challenge the conventional view of that the high reactant density of an organic aerosol should generally favor bimolecular over unimolecular reactions; underscoring the need for more accurate CI condensed-phase rate constants for reliable modeling of oxidative processes across diverse set of atmospheric and environmental systems.