South China Sea (SCS) and Western Pacific (WP) are important regions for studying air-sea interactions and their impacts on clouds and climate. However, the immersion freezing potential of marine atmospheric aerosols to form ice crystals in these regions remain poorly constrained. Here, aerosol particles were collected aboard the R/V Tan Kah Kee over the SCS and WP. Individual particle was characterized using computer-controlled scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, and immersion freezing experiments were conducted to determine their ice nucleation activity. Aerosols over the SCS in winter and spring showed higher ice nucleation activity than those collected in summer and autumn, while WP summer aerosols exhibited activity comparable to that of SCS summer aerosols. Individual particles were classified into five major types: fresh sea salt, aged sea salt, sulfate, organic/sulfur containing, and magnesium–calcium rich particles. Among these, magnesium-calcium rich particles were identified as the most efficient ice nucleating components, whereas aged sea salt and CNOS particles exhibited weak immersion freezing activity, and fresh sea salt and sulfate particles showed negligible activity. Enhanced elemental diversity and terrestrial air mass influence were associated with increased ice nucleation activity. Ice nucleating particle concentrations were highest over the SCS in spring and winter and lowest over the WP in summer. Seasonal and regional differences were most pronounced at relatively warm freezing temperatures. Furthermore, ice nucleation parameterizations were developed providing new constraints for representing immersion freezing in marine atmosphere for cloud and climate models.

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