Seven-year trend and the time and seasonal dependence of fog water collected near an industrialized area in Japan

Fog chemistry was studied for 7 years (April 1997 to March 2004) at Mt. Rokko (altitude 931 m) in Kobe, Japan. Sample collection was carried out at a mountainous site close to a highly industrialized area. Almost no increasing trend was observed in the liquid water content (LWC) and in the concentrations of ion species over the study period. The frequency of fog occurrence was high in summer, with fog occurring more frequently at night. The LWC was large at night and a time dependence was observed in the ion species concentrations: NH4+ and non-sea-salt-(nss-)Ca2+ concentrations were higher during the day than at night, although only a small time dependence was observed for NO3− and nss-SO42− concentrations. The differences in the time dependence of NO3−, nss-SO42−, NH4+ and nss-Ca2+ concentrations were attributed to the time dependence of the H+ concentration in that the H+ concentration during the day was lower than that at night. A significant difference in the concentrations of ion species between ambient air and fog water was observed in components composed of fine particles, while no significant difference was seen in components composed of coarse particles.