Uncertainties and Gaps in Comprehensive Kinetic Modeling of Secondary Organic Aerosol Formation from α-Pinene Ozonolysis

Secondary organic aerosol (SOA) formation from α-pinene ozonolysis has been widely studied owing to its important contribution to atmospheric organic aerosol. Many new mechanistic insights have been reported recently, diversifying potential α-pinene ozonolysis SOA formation pathways. However, our ability to integrate these mechanisms and kinetically simulate α-pinene ozonolysis SOA formation has not been fully evaluated. In this work, we updated the detailed α-pinene ozonolysis mechanism in the Master Chemical Mechanism (MCM) with processes including peroxy radical (RO2) unimolecular autoxidation and bimolecular reactions with HO2 and RO2, the fates of alkoxy radicals (RO), gas-particle partitioning, and condensed-phase dimer ester formation. A kinetic model was built to evaluate the new mechanism along with existing mechanisms in simulations of SOA mass concentrations and chemical composition. Uncertainties of key parameters such as reaction rate constants, product yields and volatilities, and gas-particle partitioning coefficients are tested to demonstrate the gaps in current understandings of the α-pinene ozonolysis mechanism. Our results suggest that uncertainties in MCM reactions, RO2 autoxidation, RO2 + RO2 dimerization kinetics, as well as the gas-particle partitioning parameters, all significantly impact α-pinene ozonolysis SOA simulations. Although the SOA mass concentrations can be well predicted by tuning the parameters within reasonable ranges, all models overestimate the SOA oxygen-to-carbon ratios and peroxide fractions, indicating unrecognized pathways. By comprehensively analyzing these uncertainties and gaps, this work provides new insights for future α-pinene ozonolysis mechanistic studies.