Relative Humidity Constrains the Saturated Oxidation Capacity of Sulfate Formation

Despite substantial emission reductions in recent years, severe winter haze events in the North China Plain (NCP) still occur. Long-term observations in Beijing (2013–2018) show that once pollution accumulation begins the final amount of sulfate formed remains comparable to that of the historical pollution era, even though total sulfur emissions have sharply declined. This suggests that key factors controlling wintertime sulfate formation remain insufficiently understood. To address this gap, we developed a mechanistic constraint model that reveals a relative-humidity (RH)-regulated “saturated oxidation capacity”, defined here as the upper limit of sulfate that an aerosol system can produce. In acidic aqueous aerosol, increasing RH enhances SO2 dissolution but more strongly dilutes aqueous reactants and ionic strength, suppressing effective oxidation rates. Consequently, sulfate formation approaches an upper limit under high-RH conditions. This mechanism explains the typical pattern in winter haze that sulfate usually accumulates during decreasing moderate RH but finally stabilizes or saturates at high-RH periods. Online process observations further support this behavior. Sensitivity analyses show that this RH-limited capacity broadly emerges in weakly acidic aerosol regimes dominated by transition-metal-ion-driven sulfur oxidation, offering a physically interpretable pH–RH regime for sulfate formation and mechanistic constraints for haze mitigation and multiphase chemical modeling.