Unveiling the Formation Mechanisms of Secondary Organic Aerosols in Humid Winter Haze: The Role of Multiphase Partitioning and Aqueous-Phase Processing

Aqueous-phase secondary organic aerosol (aqSOA) is a major contributor to SOA in severe winter haze. However, incomplete representation of aqSOA in regional models limits our understanding of the mechanisms driving enhanced SOA formation under humid haze conditions. Here, a multiphase partitioning scheme of semivolatile organic compounds was incorporated into the Community Multiscale Air Quality model, enabling a process-resolved investigation of SOA formation during a winter haze episode in the Beijing–Tianjin–Hebei region. The updated model substantially enhanced aqSOA, whereas total SOA increased only modestly due to competing gas–organic and gas–aqueous partitioning of semivolatile products. Multiphase partitioning contributed 77.9–82.3% of total SOA, mainly attributable to gas–organic partitioning of intermediate-volatility organic compound oxidation products. Gas–aqueous partitioning, dominated by aromatic oxidation products, contributed 34.2% of aqSOA under clean conditions, which increased to 48.1% during haze periods, becoming a major driver of SOA enhancement as pollution intensified. This amplified aqSOA influence was driven by pronounced aerosol hygroscopic growth associated with prominent sulfate formation under high relative humidity. The findings elucidate the mechanisms driving enhanced SOA formation during humid winter haze and demonstrate that incorporating multiphase partitioning is essential for accurately representing SOA evolution in air quality models.