Eddy Covariance Measurements of Ammonia Fluxes in a Deciduous Forest and Adjacent Grassland

The deposition of reactive nitrogen (Nr) to natural ecosystems poses various environmental concerns, including soil acidification and water eutrophication. Ammonia (NH₃) has become an increasingly important component of Nr deposition across the United States in recent years. Modeling the exchange of NH₃ between land and atmosphere is non-trivial due to its bi-directional nature, and many underlying processes are still not well understood, especially the exchange mechanisms in forests. NH₃ is also a challenging species to measure accurately, making it difficult to validate the models. Here we present in-depth analysis of NH₃ fluxes above a deciduous forest and an adjacent grassland in North Carolina, measured over summer and fall 2017 by open-path sensors and calculated using the eddy covariance method. The sensors were deployed on a 44-m tower (forest) and a 2-m tripod (grassland) and demonstrated 10-Hz precisions of 0.17 and 0.30 ppbv. Net deposition (-8.3 ng NH₃ m^-2 s^-1) and net emission (1.9 ng NH₃ m^-2 s^-1) were observed over the forest and the grassland, respectively. NH₃ fluxes at both sites showed clear diurnal patterns, with the largest fluctuations occurring at midday and smaller ones in late afternoon. Concurrent biogeochemistry measurements suggest higher emission potentials of NH₃ from the grassland, consistent with the observed diurnal flux patterns. Conditional bivariate probability function (CBPF) shows that areal sources dominate in this region. 24-hour back trajectories of air flow generated using HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) showed northeast as the prevailing direction of incoming air during periods of higher deposition to the forest. Potential sources of NH₃ in this region will be discussed using satellite imagery and livestock inventories. Our results highlight the importance of differentiating between natural ecosystems when modeling NH₃ fluxes. Our work also shows that longer-term datasets are needed to characterize the seasonal and interannual variability of NH₃ air-surface exchange in natural ecosystems.