Tall Tower Ammonia Observations and Emission Estimates in the U.S. Midwest

Atmospheric ammonia (NH₃) has increased dramatically as a consequence of the production of synthetic nitrogen (N) fertilizer and proliferation of intensive livestock systems. It is a chemical of environmental concern as it readily reacts with atmospheric acids to produce fine particulate matter and indirectly contributes to nitrous oxide (N₂O) emissions. Here, we present the first tall tower observations of NH₃ within the U.S. Corn Belt for the period April 2017 through December 2018. Hourly average NH₃ mixing ratios were measured at 100 and 56 m above the ground surface and fluxes were estimated using a modified gradient approach. The highest NH₃ mixing ratios (>30 nmol mol⁻¹) occurred during early spring and late fall, coinciding with the timing of fertilizer application within the region and the occurrence of warm air temperatures. Net ecosystem NH₃ exchange was greatest in spring and fall with peak emissions of about +50 nmol m⁻² s^−l. Annual NH₃ emissions estimated using state-of-the-art inventories ranged from 0.6 to 1.4 × the mean annual gross tall tower fluxes (+2.1 nmol m⁻² s⁻¹). If the tall tower observations are representative of the Upper Midwest and broader U.S. Corn Belt regions, the annual gross emissions were +720 Gg NH₃-N y⁻¹ and +1,340 Gg NH₃-N y⁻¹, respectively. Finally, considering the N₂O budget over the same region, we estimated total reactive N emissions (i.e., N₂O + NH₃) of approximately 1,790 Gg N y⁻¹ from the U.S. Corn Belt, representing ~23% of the current annual new N input.