Size-separated atmospheric aerosol particles, acidic gases, and ammonia were collected during the daytime and nighttime at Murododaira (36.6 ¿N, 137.6 ¿E, 2450 m above sea level), Mount Tateyama, Japan in early winter. Non-sea-salt (nss) SO42- in fine fractions (<2.1 μm in diameter) was the most dominant ionic constituent and occupied 44% on average of the ionic mass concentration other than H+. Concentration levels of aerosol ions and water-soluble gases were higher during the day and lower at night. Concentrations of the nighttime aerosol particles and gases (except for SO2) were similar to free-tropospheric background levels at various sites of northern midlatitudes in the literature, suggesting that nighttime data at Murododaira were representative of free-tropospheric conditions. High concentration levels of SO2 at Murododaira were close to those over the northwest Pacific Rim region during the winter season. Molar ratios of nssSO42-/SO2 can be explained by the transport time from the Asian continent, suggesting that a significant source of fine nssSO42- aerosols at Murododaira in early winter could be due to the oxidation of anthropogenic SO2 derived from the Asian continent. The equivalent ratios of nssSO42- to NH4+ in fine particles were close to 0.5 under westerly wind conditions, suggesting the existence of half-neutralized sulfate on average. ¿ 2001 American Geophysical Union