Due to frequent interactions between continental pollution and marine atmosphere along with land-sea breezes, the coastal fog chemistry and atmospheric effects are more complex. Laboratory simulations suggest that liquid droplets play the significant role in the formation of secondary inorganic and organic aerosols via aqueous or interfacial heterogeneous reactions by acting as efficient media, however, field measurements providing direct evidence are lacking. Here we present field observations of sea fog chemistry at two coastal sites in northern (Qingdao, Shandong) and southern China (TMS, Hong Kong) and investigate their atmospheric effects. Although with substantially different chemical compositions, we found consistently strong acidity of coastal sea fog in both sites, with an average pH value of 3.88. Recent observation of coastal fog in Qingdao demonstrated that the smaller fog droplets had stronger acidity, likely due to more enrichment of acidic ions in smaller droplets. More sulfate than nitrate was found in TMS, but nitrate exceeded sulfate in Qingdao, suggesting the different role of secondary ions in sea fog acidification under various anthropogenic pollution conditions. The contributions of acidification and neutralization factors were estimated, and the role of enriched chloride ions was analyzed. In contrast to the large depletion of chloride and bromide in aerosol particles, they were highly enriched in sea fogs at TMS. Once droplets evaporate, reactive bromine and chlorine species are likely formed under acidic conditions and released into the gas phase. Upon irradiation after sunrise, atomic bromine can be generated quickly, initiating the oxidation of atmospheric mercury. Following a second step of oxidation by high concentrations of anthropogenic pollutants such as NO₂, elemental mercury is oxidized to highly soluble divalent mercury, which is consequently absorbed by sea salt aerosols and scavenged from the gas phase. Further efforts are deserved to look deep into the effects of land-sea interactions on coastal atmospheric chemistry and corresponding ecosystem responses.