A Multiyear Constraint on Ammonia Emissions and Deposition Within the US Corn Belt

The US Corn Belt is a global hotspot of atmospheric ammonia (NH₃), a gas known to adversely impact the environment and human health. We combine hourly tall tower (100 m) measurements and bi-weekly, spatially distributed, ground-based observations from the Ammonia Monitoring Network with the US National Emissions Inventory (NEI) and WRF-Chem simulations to constrain NH₃ emissions from April to September 2017–2019. We show that: (1) NH₃ emissions peaked from May to July and were 1.6–1.7 times the annual NEI average; (2) average growing season NH₃ emissions from agricultural lands were remarkably similar across years (3.27–3.64 nmol m⁻² s⁻¹), yet showed substantial episodic variability driven by meteorology and land management; (3) dry deposition was 40% of gross emissions from agricultural lands and exceeded 100% of gross emissions in natural lands. Our findings provide an important benchmark for evaluating future NH₃ emissions and mitigation efforts.