In the atmosphere, gaseous ammonia (NH₃) reacts readily with acidic compounds to form ammonium salts that persist as small (diameter <2.5µm) particulate matter (PM_2.5). The use of nitrogen-based fertilizers is estimated to contribute >40% of total NH₃ emissions in central Illinois based on the Carnegie Melon University (CMU) model. This research seeks to address the need for enhanced understanding of agricultural ammonia emission pathways by measuring bi-directional flux of gaseous NH₃ over a corn (Zea mays) canopy.

A relaxed eddy accumulation (REA) system designed for ammonia flux measurement was deployed in a corn plot at the Energy Biosciences Institute (EBI) at the University of Illinois (UI) and average ammonia flux was measured during four-hour periods in the morning and afternoon for the duration of the 2014 growing season. The REA coefficient (β) was found to be 0.56 ± 0.053, consistent with values presented in the literature. For the duration of the field campaign, the average gaseous ammonia concentration was 4.02 μg/m3 ± 2.15 μg/m³, with the highest concentrations (up to 8.11 μg/m³) observed near the time of fertilization. The average ammonia flux was found to vary by over an order of magnitude throughout the season, ranging from -198.44 ng/m²/s to 973.80 ng/m²/s, where negative flux indicates deposition. Overall, greater upward fluxes were observed during the 14 day period following fertilization when compared to the rest of the season. This presentation will provide an overview of the experimental setup and field campaign and provide further results and analysis of ammonia flux as measured during this effort.