Beyond rain: Fog as a dominant pathway for acid deposition in Japan's Montane Forests

Fog water deposition remains a poorly quantified yet important pathway for atmospheric inputs in forested mountain regions. We conducted multi-site observations at seven mountainous locations in Japan, collecting bulk deposition, throughfall, and fog water. Using an improved canopy water balance method, we estimated fog water precipitation on canopies and its associated acid deposition. Fog water precipitation in forests varied substantially across sites, with the highest value at Mt. Oyama (9.25 mm day −1 ) and the lowest at Mt. Iizuna (0.86 mm day −1 ). Sites along the Sea of Japan coast showed higher proportions of non-sea salt sulfate (nss-SO₄ 2− ), largely due to continental air masses, whereas Pacific-side sites were more influenced by nitrate from domestic vehicle emissions. Backward trajectory analysis and meteorological observations revealed that fog water deposition is significantly influenced by fog occurrence frequency, wind speed, and characteristics of the canopies and their locations in mountains. These factors together shape the spatial variability of fog water and acid inputs. Our findings highlight the substantial role of fog in the deposition of acidic substances to forest canopies, particularly in high-altitude regions with frequent fog events. This study underscores the need to incorporate fog water deposition into forest ecosystem assessments and long-term monitoring programs, especially in regions where traditional rain-based models may underestimate total atmospheric inputs. • Fog water deposition was quantified at seven montane forest sites in Japan using a canopy water balance method. • Fog water deposition on montane forest increased with its elevation. • Fog water deposition was found to dominate acid input pathways in high-elevation forests. • Acid deposition fluxes via fog differed between Japan Sea and Pacific sites due to air mass origins. • An exponential increase in fog water deposition was observed with wind speed above 1.0 m s −1 .