Tree decline has recently been reported around the somma of Lake Mashu, Hokkaido, northeastern Japan, although its cause is uncertain. We have evaluated the concentrations of ambient reactive nitrogen compounds such as gaseous ammonia (NH3), ammonium particles (NH₄⁺), and nitrate particles (NO₃⁻) both at Lake Mashu, located inland, and at Ochiishi, located on the Japanese coast. These gas and particles were measured at both sites by a multiple filter-pack method. The concentrations of NH3 and NH₄⁺ were greater during the warm season—from May to Sep.—at Lake Mashu (40.8 nmol m⁻³ and 25.3 nmol m⁻³, respectively) than at Ochiishi, (20.1 nmol m⁻³ and 16.6 nmol m⁻³, respectively), whereas the concentration of NO₃⁻ at Lake Mashu (1.0 nmol m⁻³) was lower than at Ochiishi (7.7 nmol m⁻³). The emission of NH3 and NH4+ via agricultural activity in the area is thought to have contributed to their ambient concentrations at Lake Mashu; however, NO₃⁻ particles are thought to have been removed by gravitational settling during transport. Compared to other areas in Japan and Asia, the concentrations measured in this study were relatively low; however, the amount of nitrogen deposited in mountainous areas was heterogeneous and incorporated a contribution from fog. Further comprehensive investigation is thus required in order to gain a complete understanding of the influence that nitrogen compounds have on various ecological systems.

Keywords: ammonia emission, fog chemistry, reactive nitrogen.