Aerosol Phase State and Its Link to Chemical Composition and Liquid Water Content in a Subtropical Coastal Megacity

Particle phase state plays a key role in gas-particle partitioning, heterogeneous and multiphase reactions, and secondary aerosol formation. In this work, the rebound fraction and chemical composition of submicron particles were simultaneously measured to investigate the particle phase state and its link to chemical composition in a subtropical coastal urban city (Shenzhen, China). Submicron particles were found to be in the liquid state for most of the measurement period in spring. During the sampling time, both high relative humidity (RH, ranged from 40% to 93%) and inorganic mass fraction in particles (62.6 ± 12.4% of dry particles, on average) resulted in abundant aerosol liquid water (43 ± 6% in the wet PM₁, on average), which may liquefy the particles. Considering the high frequency of ambient RH > 60% and large inorganic mass fraction in aerosol particles, we deduced that particles were in the liquid state throughout the year in coastal urban areas, where this study was performed. The liquid phase particles may accelerate the mass transfer of reactive trace gases and multiphase reactions, thereby enhanced secondary aerosol formation, further resulting in a rapid growth in aerosol mass. Our work suggested that in regions heavily impacted by SO₂ and NO _x emissions, especially in developing countries, the presence of inorganics could significantly impact the phase state of ambient aerosol particles, and thus the mixing state of inorganic and organic matter should be taken into account for the investigation of the aerosol phase state in urban environments.