Comparison of Aircraft Observed, Remotely Sensed, and Modeled Ammonia Concentrations in the American Midwest.

Ammonia (NH3) emissions from agriculture contribute to air pollution, degradation of water quality, and can drive ecosystem level change. Atmospheric ammonia emissions and ambient concentrations have risen over recent decades due to increased agricultural activities and warming global temperatures. Despite this, real-time measurements of ammonia remain sparse. In May 2021, airborne NH3 measurements were collected in real time using fast (1 Hz) cavity-ring-down laser spectroscopy (Picarro G2103) over the U.S. corn belt. Flights targeted regions dominated by livestock husbandry, row crop agriculture, and forest and wetlands. Observations were used to assess the ability of the Weather Research Forecasting Model with Chemistry using the 2017 National Emissions Inventory (WRF-CHEM/NEI) and of the satellite-based Crosstrack Infrared Sounder (CrIS) to capture NH3 dynamics in this region. WRF-CHEM/NEI predicted NH3­ concentrations were generally underestimated over livestock-intensive and forested regions but exhibited better performance over regions dominated by row cropping. CrIS NH3 retrievals aligned with the flight observations more closely than the WRF-CHEM/NEI predictions, revealing the utility of remote sensing for addressing the ammonia observation gap. These analyses further reveal the need to incorporate additional direct ammonia measurements into assessments of ammonia's nutrient deposition, emissions, and air quality impacts.