Atmosphere-forest Exchange of Ammoniacal Nitrogen in a Subalpine Deciduous Forest in Central Japan during a Summer Week

Abstract

The present study aimed to investigate the diurnal variations in air concentrations and exchange fluxes of ammoniacal nitrogen (NHx: ammonia (NH3) and particulate ammonium) in a subalpine deciduous forest in central Japan during a week in summer. The NH3 concentrations (0.50 μg N m-3; on average) showed a clear circadian variation, i.e., high and low in the daytime and nighttime, respectively. The concentration of particulate ammonium in the coarse fractions was extremely low, whereas that for the PM2.5 fraction was relatively high (0.55 μg N m-3; on average). The main inorganic ion components of PM2.5 at the study site were ammonium and sulfate. The exchange fluxes of NHx were bidirectional. Both the maximum and minimum values occurred in the daytime, i.e., 0.39 mg N m-2 hr-1 of downward flux and 0.11 mg N m-2 hr-1 of upward flux for NH3 and 0.25 mg N m-2 hr-1 of downward flux and 0.13 mg N m-2 hr-1 of upward flux for PM2.5 ammonium. The exchange fluxes of NHx at night could be considered as zero. The mean deposition velocity during the research period was almost zero for both NH3 and PM2.5 ammonium. The atmosphere-forest exchange of NHx in the forest during the study period was balanced. The remarkably large deposition of NHx was attributable to meteorological events such as showers the night before that thoroughly washed the forest canopy and subsequent clear skies in the morning, which enhanced convection. The cleaning effect of rainfall and the rapid change in convection in the early morning should be monitored to evaluate and generalize the gas and particle exchange in a forest.

Keywords

• ammonia
• atmosphere-forest exchange
• dry deposition
• emission
• particulate ammonium