Evaluation of chemical composition in fog water near the summit of a high mountain in Japan

Inorganic ionic species contained in fog water and in atmospheric aerosols were analyzed to study the acidification processes of fog droplets in summer near the summit of Mt. Norikura in Japan. Liquid water content (LWC) was also measured during the fog event. Fog water was found to be highly acidic. Nucleation scavenging was less contributive to the acidification of fog water, because NH4+ was contained in aerosols in a sufficient concentration to neutralize the particulate S42− which was the predominant anion in the aerosols. Comparison of the ion concentrations in fog water with those in aerosols showed that the acidification progressed rapidly at the initial stage of fog formation, primarily due to S42− enhancement and secondarily due to NO3− formation. It was supposed that about 60% of S42− in fog water was formed by SO2 gas dissolution followed by liquid phase reaction with H2O2 in the droplets. The results of the observation also suggested that NO3− in fog water was increased by HNO3 gas scavenging. During the mature stage of fog event, LWC could not always explain the temporal variation in the concentrations. This fact also suggests that gas scavenging contributes to S42− and NO3− concentrations in fog water.