Divergent Drivers of Aerosol Acidity: Evidence for Shifting Regulatory Regimes in a Coastal Region

Abstract. Aerosol acidity plays a crucial role in multiphase atmospheric chemistry, influencing aerosol composition, gas-particle partitioning, and the oxidative capacity of atmosphere. However, the mechanisms governing aerosol acidity in coastal area under extreme weather remains challenging due to its complexity of atmospheric transport. Here, we investigate aerosol pH in Shenzhen, a coastal megacity in China, by integrating field observations with multiphase buffer theory and ISORROPIA simulations. Our observations captured both a typhoon episode and typical non-typhoon periods with two contrasting regimes: during non-typhoon periods, aerosols were consistently buffered by the NH₄⁺/NH₃ pair, with relative humidity serving as the primary driver of pH variability, enabling reliable predictions using multiphase buffer theory. In contrast, during a typhoon episode, sea salt derived nonvolatile cations emerged as the dominant drivers, violating the charge balance for NH₄⁺/NH₃ buffering and leading to poor performance of buffer theory. Under these conditions, ISORROPIA simulations with constant aerosol water content reproduced the observed pH more reliably, highlighting a compositional rather than meteorological control. Our results provide the direct field-based evidence for regime shifts in aerosol acidity regulation in coastal area, and underscore the need for chemical transport models to account for composition-meteorology interactions to improve acidity predictions under extreme weather events.