Measurement of Bi-Directional Ammonia Exchange Above a Maize Canopy in the Midwestern United States
Abstract:Crop fertilization contributes to more than 40% of total anthropogenic emissions of ammonia (NH3) in Illinois. NH3 released into the atmospheric environment reacts readily with acidic compounds to form small diameter (<2.5µm) particulate matter (PM2.5) resulting in human health effects and deposition to surfaces causing eutrophication. Such emissions are not well characterized, and improved measurements are needed to further validate air quality models. This study seeks to provide improved understanding of the emission pathway of agricultural NH3 by measuring bi-directional flux of gaseous NH3 in an intensively managed agroecosystem.
A relaxed eddy accumulation (REA) system was deployed above a maize canopy at the University of Illinois at Urbana-Champaign (UIUC) Energy Biosciences Institute (EBI) Energy Farm. Average NH3 flux was measured throughout the growing season in four-hour periods during the morning and afternoon. The REA coefficient (β) was monitored for the duration of the season. The highest atmospheric NH3 concentration (8.11 μg/m3) was observed during the period nearest fertilization, with a mean concentration of 4.02 μg/m3 ± 2.15 μg/m3. Generally, larger upward fluxes of gaseous NH3 coincided with higher atmospheric NH3 concentration, but large variations were observed during the season. Observed NH3 flux ranged from -0.20 µg /m2s to 0.97 µg/m2s, where negative flux indicates deposition.