Atmospheric Ammonia Fluxes in the USA Corn Belt: Insights from Aircraft Measurements and WRF-CHEM Modeling of a Spring Peak Emission Period

Ammonia (NH3) emissions from agriculture significantly influence regional nitrogen budgets and air quality. The U.S. Corn Belt is a global NH3 emissions hotspot driven by intense fertilizer application and livestock husbandry. In May 2021, a month during which peak emissions have historically occurred, we conducted flights over this region to measure ambient NH3, CO2, and CH4 with multiple Picarro instruments. The measurements from this campaign were used to investigate the magnitude of NH3 fluxes from different land use types in the U.S. Corn Belt, including livestock, row-crop, and non-agricultural regions. Using upwind-downwind mass balance techniques, these flights reveal the net emissions and deposition dynamics in the targeted regions. Flux and concentration measurements from this campaign were compared to WRF-CHEM results using the 2017 National Emission Inventory to assess the ability of the inventory and model to accurately reproduce the observed concentrations and fluxes. Results from this work provide insights into the relative importance of NH3 source regions, relative priority of targets for emissions mitigation, and the ability of state-of-the-art models, inventories, and fast response instrumentation to accurately represent ammonia dynamics.