Year-Round Analysis of Multiphase Sulfate Production in Aerosol Particles in East Asia

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 SO₂ in aerosol particles by H₂O₂, O₃, NO₂, HCHO, and O₂, 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 H₂O₂. Multiphase oxidation of SO₂ by the H₂O₂ pathway reduces gas-phase H₂O₂ concentrations by -40% in spring, which improves model agreement with H₂O₂ airborne observations. Oxidation pathways also shift between cities, in particular the contribution from the TMI and NO₂ 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.