Distributions and sources of water-soluble organic acids in fog water from mountain site (Lake Mashu) of Hokkaido, Japan

Abstract

To better understand the distributions of low molecular weight (LMW) organic acids and their sources as well as their contribution to fog acidity that may decline a Japanese birch in mountain site (somma of Lake Mashu) of northeastern Hokkaido, Japan, we collected fog water samples at Lake Mashu site in May and July, 2015. The samples were analyzed for various organic species such as LMW monocarboxylic acids, dicarboxylic acids, oxocarboxylic acids, biogenic secondary organic aerosol (SOA)-tracers (isoprene- and α-pinene-oxidation products), and tracers of primary biological aerosol particles (PBAPs, e.g., saccharides), together with inorganic ions. Formic, acetic, and oxalic acids were detected as dominant carboxylic acids. Their concentrations in the fog samples were significantly lower than those from urban area in North America, Asia, and Europe and from forest area in Taiwan, except for oxalic acid in May. Concentrations of oxalic acid in May were higher than those in July, being consistent with higher values of SO₄²⁻ and NO₃⁻. Based on the air mass back trajectories, the Asian continent is suggested to contribute higher levels of dicarboxylic acids in fog water in spring. Concentrations of formic and acetic acids together with biogenic tracers in the present study were higher in July than in May. In July samples, concentrations of formic plus acetic acids showed positive correlations with biogenic SOA tracers and PBAPs. Oxalic acid also correlated with biogenic SOA tracers and PBAPs. Primary emission from biogenic sources and secondary formation are important factors to control the levels of LMW mono- and di-carboxylic acids in fog from Lake Mashu in July. pH of fog water ranged from 3.8 to 5.9. Total cation equivalents (Na⁺, NH₄⁺, K⁺, Ca²⁺, Mg⁺, and H⁺) were comparable to total anion equivalents (SO₄²⁻, NO₃⁻, Cl⁻, and detected organic anions). Contributions of organic acid equivalents to organic plus inorganic acid equivalents were low (range: 4–17%). Major ions in fog water of Lake Mashu were inorganic during the sampling periods. This study demonstrates that levels of organic and inorganic acids are not high enough to cause a damage on the tree ecosystem in the surroundings of Lake Mashu.