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

Abstract

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 NH 4 + was contained in aerosols in a sufficient concentration to neutralize the particulate S 4 2− 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 S 4 2− enhancement and secondarily due to NO 3 − formation. It was supposed that about 60% of S 4 2− in fog water was formed by SO 2 gas dissolution followed by liquid phase reaction with H 2O 2 in the droplets. The results of the observation also suggested that NO 3 − in fog water was increased by HNO 3 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 S 4 2− and NO 3 − concentrations in fog water.