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In the present study, we analyzed the chemical properties and factors impacting the sea fog water during two sea fog events over the northwestern South China Sea in March 2017, and compared our results with those of other regions. The sea fog water during these two events were highly acidic and their average pH was below 3, which was related to the high initial acidifying potential and large amounts of NO3- and SO42- not involved in the neutralization reaction. The dominant cations in the sea fog water were Na+ and NH4+. The primary anions in the sea fog water over the South China Sea were Cl- and NO3-, while that over the North Pacific Ocean was mainly SO42-, and ratios of the three fog water ions near the Donghai Island were similar. Ions in the sea fog water during the two events were mainly derived from marine aerosols, while the difference was that the first low-level sea fog airflow trajectory passed over Hainan Island. Therefore, the proportion of K+ in the first sea fog was much higher than that in sea water and the second. Sulfate was the key to fog water nucleation, which made ion concentration in the sea fog water during the second event higher than that during the first. A decrease in average diameter during the first sea fog formation led to an ion concentration increase, while the average diameter of sea fog water during the second event was lower than that during the first, which corresponded with a moderate ion concentration increase. [ABSTRACT FROM AUTHOR]