Simulation of Ammonia Concentration Profiles and Fluxes in and above a Deciduous Forest Canopy in the Southern Appalachians

Ammonia (NH₃) concentration profiles and flux measurements were made within and above a deciduous forest canopy at the U. S. Forest Service’s Coweeta Hydrologic Laboratory in southwestern North Carolina during the spring and summer of 2015 and 2016. Along with the chemical measurements, detailed micrometeorological, environmental and soil temperature and moisture measurements were obtained from and around a 37-m walk-up tower located at the site. The ~80-year-old forest surrounding the Coweeta tower has an average maximum canopy height of 35 m and consists of an overstory composed of various deciduous species (black birch, tulip poplar, white oak, red maple, blackgum, sourwood and various hickories) and a prominent understory of rhododendrons and mountain laurels. The Atmospheric Chemistry and Canopy Exchange Simulation System for Ammonia (ACCESS-NH3) has been applied to the data obtained during this field experiment to assess how NH₃ interacts with a deciduous forest canopy and how these interactions vary as a function of environmental conditions. ACCESS-NH3 is a multilayer, single column model that simulates the bi-directional transport and exchange of ammonia throughout the soil-plant-atmosphere continuum. In this presentation, results from simulations of selected periods during the field study will be presented, compared with measured profiles and fluxes and analyzed with respect to the NH₃ source/sink behavior of the forest canopy. Preliminary results indicate that the Coweeta forest acts as a small net sink for NH₃, with most of the NH₃ released from litter on the forest floor being recaptured by the canopy.