Long-term observation of fog chemistry and estimation of fog water and nitrogen input via fog water deposition at a mountainous site in Hokkaido, Japan

Abstract

To evaluate water and nitrogen input via fog deposition, the fog water chemistry and deposition around a crater lake (Lake Mashu) in northern Japan were investigated in the growing seasons of trees (summer to autumn) in 2006–2012. The fog samples were collected using an active fog collector and droplet size distribution was measured with a droplet size spectrometer (FM-100). The visibility (VIS)–liquid water content of fog (LWC) relationship differed between summer and autumn. Large fog droplets decreased in autumn and the calculated LWC from FM-100 measurements (LWCobs) were lower than those in summer at the same VIS. Two empirical curves between LWCobs and VIS were obtained and used for better estimation of fog deposition. Fog deposition was calculated from LWC empirically derived from past VIS data and deposition velocity (Vd) estimated using wind speed and vegetation parameters. The mean pH of fog water was 4.6 and the percentage of samples of pH4.0 or more was 93%. Compared to previous literature of exposure experiments of acid mist on plants, fog acidity in this study did not seem to injure plant leaves. The water input via fog deposition accumulated for each growing season was estimated at 107–140mm, corresponding to 18–23% of the precipitation in the same period. Nitrogen deposition via fog deposition in a plant growing season was estimated at 26–30meqm−2 (3.6–4.2kg Nha−1). As a long-term trend, NO3− concentration in fog water decreased significantly. However, the long-term trends of fog and nitrogen depositions were not clear.