China's coastal seas, located at the interface between the East Asian continent and the northwestern Pacific Ocean, are strongly influenced by continental emissions and marine air masses. Owing to the combined effects of the East Asian monsoon and land-sea breeze circulations, aerosol chemistry in this region is highly complex. However, the formation of secondary inorganic aerosols (SIA), aerosol acidity, and their environmental effects remain poorly understood. This study conducted a series of field campaigns over China's coastal seas to investigate the formation mechanisms of SIA, aerosol optical properties, and the drivers of aerosol acidity. The results show that photochemical oxidation dominated SIA formation in the marine boundary layer over the South China Sea, while land-sea breeze circulations substantially regulated this process. Internal mixing between sea salt and anthropogenic aerosols buffered aerosol acidity, with sea-salt-related particles contributing up to 28% of the aerosol mass during a typical sea-breeze event. Long-term observations over the East China Sea revealed pronounced seasonal variations in aerosol optical properties, with higher aerosol optical depth (AOD) in winter and spring than in summer and autumn. Sulfate, nitrate, ammonium (SNA), OC, EC, and K⁺ were identified as the major contributors to fine-mode AOD. Moreover, the ratio of AOD to aerosol light extinction coefficient effectively distinguished continental anthropogenic influences from marine air masses. Using the NH_X phase-partitioning equilibrium method coupled with a thermodynamic model, this study quantitatively identified the major drivers of aerosol acidity over China's coastal seas. Ammonia dominated aerosol pH variability over the southern and eastern China seas, whereas relative humidity was the primary controlling factor in the northern China sea. This study also presents the first characterization of the diurnal variation of aerosol pH over China's coastal seas. These findings advance the understanding of aerosol transport, chemical transformation, and acidity at the land-ocean interface and provide scientific support for improving atmospheric chemistry models in coastal regions.