Abstract: Missing sulfate production pathways have been implicated as the cause of model underestimates of sulfate during haze events in East Asia. We add multiphase oxidation of SO 2 in aerosol particles by H 2 O 2 , O 3 , NO 2 , HCHO, and O 2 , catalyzed by transition metal ions (TMIs), to the GEOS-Chem model and evaluate the model with (1) year-round ground-based observations in Seoul, South Korea, (2) airborne observations from the KORUS-AQ field campaign, and (3) fall and winter ground-based observations in Beijing, China. Multiphase chemistry contributes 14% to 90% to total sulfate production depending on the location and season and increases model daily average sulfate by 2 to 3 μg m –3 , with maximum daily increases up to 12 μg m –3 . From winter to summer, oxidation pathways shift, with the largest fraction of multiphase sulfate production occurring during spring and summer due to oxidation by H 2 O 2 . Multiphase oxidation of SO 2 by the H 2 O 2 pathway reduces gas-phase H 2 O 2 concentrations by −40% in spring, which improves model agreement with H 2 O 2 airborne observations. Oxidation pathways also shift between cities, in particular the contribution from the TMI and NO 2 pathways, which are more important in Beijing than in Seoul. This is due to higher levels of transition metals, and a larger impact of an overly shallow mixed layer in Beijing compared to Seoul. The implementation of multiphase aerosol chemistry in GEOS-Chem here allows for the use of this chemistry in other models that can address boundary layer errors, including WRF-GC and CESM-GC. The analysis presented here shows that this chemistry is important to the simulation of sulfate year-round, not only during haze events, and is unique in showing coupled gas- and aerosol-phase impacts of multiphase chemistry.
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