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Eddy Covariance-derived Ammonia Emission and Deposition in Adjacent Natural Ecosystems
Citation:
Guo, X., D. Pan, J. McSpiritt, L. Tao, Johnt Walker, R. Daly, AND M. Zondlo. Eddy Covariance-derived Ammonia Emission and Deposition in Adjacent Natural Ecosystems. American Geophysical Union, Washington,DC, December 10 - 14, 2018.
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:
Ammonia (NH3) is the most abundant alkaline gas in the atmosphere and a precursor to ammonium (NH4+) aerosols. NH3 and NH4+ also deposit to ecosystems, degrading the environment through eutrophication of surface waters and soil acidification. Agricultural activities dominate anthropogenic NH3 emissions, but significant uncertainties exist on the fate of these emissions further downwind. NH3 deposition is poorly constrained by observations, in part due to the measurement challenges of gas phase NH3 itself. To this end, we developed and deployed two laser-based open-path NH3 sensors over a natural grassland and adjacent deciduous forest canopy at Duke Forest, North Carolina from August to November 2017 to examine NH3 fluxes in adjacent natural ecosystems. While the site location is relatively clean with respect to local NH3 emissions, southeasterly winds place it downwind of the intense agricultural emissions in eastern North Carolina. The sensors were mounted on a 44-m tall tower over the forest canopy and on a 2-m tower over the grassland. A MARGA gradient system and NHx denuders were also available at the site for intercomparison. The forest and grassland sensors demonstrated 10 Hz precision of 0.17 and 0.30 ppbv, respectively. NH3 fluxes were calculated from the 10 Hz concentration and three-dimensional wind velocity using the eddy covariance method. Preliminary results suggest net deposition of NH3 onto the forest canopy and weak net emission from the grassland. Mean NH3 fluxes of -9.2 ng/m2/s and 1.4 ng/m2/s were measured by the forest and the grassland systems over the entire campaign. Diurnal profiles of NH3 at the forest and the grassland showed both highest deposition and highest emission during the midday. Smaller peaks of fluxes were also observed in the late afternoon. Ongoing analyses examine the potential biogeochemical and micrometeorological drivers of differences in fluxes between the two ecosystems and relationships between fluxes and trajectories of air passing over the upwind agricultural areas. Our results point to variability of NH3 fluxes among natural ecosystems as an important sub-grid process and source of uncertainty in chemical transport models used for deposition assessments.