Cloud and Aerosol Chemistry Together Explain the High Sulfate Aerosol Content in a Heavy Haze Episode in Central Eastern China

Sulfate is a key aerosol component affecting radiative forcing, air quality and human health. Its rapid formation in haze has drawn worldwide attention, yet its formation mechanisms remain debated. Here we implemented a heterogeneous sulfate production module and optimized related parameters within the WRF-Chem model, focusing on haze-fog episodes in Hefei, China, during January 2021. The default WRF-Chem severely underestimated observed sulfate by ∼70%. Although integrating heterogeneous reactions reduced this bias on cleaner days, their impact was minimal during heavy pollution due to evolving aerosol properties (e.g., decreasing pH, ionic strength (Is), and transition metal ions (TMIs)) that suppressed heterogeneous reaction rates, highlighting the importance of these factors in regulating their effectiveness. Given large uncertainties in the Is effect on TMI-catalyzed O₂ oxidation obtained from laboratory experiments, we applied an observationally constrained Is effect, which significantly improved sulfate simulations during heavy pollution but still cannot fully capture observed peaks. Further constraining cloudwater content using satellite-based data enabled more accurate sulfate reproduction. Optimized simulation reveals that heterogeneous (∼48%) and in-cloud (∼50%) pathways contributed comparably during light pollution, while cloud chemistry contribution increases to ∼72% during heavy pollution, with O₃ oxidation as the dominant pathway. Our findings highlight the potential importance of cloud chemistry in sulfate burden during haze episodes especially when low cloud occurs, and the need to appropriately represent cloud parameters (e.g., water content) and effects of Is in models.