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Eddy Covariance Measurements of Ammonia Fluxes in a Deciduous Forest and Adjacent Grassland
Citation:
Guo, X., D. Pan, R. Daly, X. Chen, JohnT Walker, L. Tao, J. McSpiritt, AND M. Zondlo. Eddy Covariance Measurements of Ammonia Fluxes in a Deciduous Forest and Adjacent Grassland. 2020 Fall AGU Meeting, Durham, NC, December 01 - 17, 2020.
Impact/Purpose:
The Program Offices (OAQPS, OAP), western Regions (Regions 8 and 9), and States (Colorado, California, Utah) need new data to support review of the secondary NAAQS, development of critical loads for key ecosystems, and to understand the role of reduced nitrogen (NHx = ammonia gas (NH3) and ammonium aerosol (NH4+)) in PM attainment. Given the continental-scale shift in the atmospheric inorganic nitrogen budget from a predominance of oxidized to reduced forms of nitrogen and the subsequent increasing importance of NHx in reactive nitrogen deposition budgets, information on dry deposition of NH3 to natural ecosystems is particularly needed. The measurements collected under this study will be used to directly assess the contribution of NH3 dry deposition to the nitrogen deposition budget in two natural ecosystems and to examine the biogeochemical and meteorological drivers of NH3 fluxes. These data will be used to improve NH3 bidirectional exchange algorithms in CMAQ and to develop more complete and accurate deposition budgets for natural ecosystems in the southeastern U.S.
Description:
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. NH 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.
