Modeling the Sources and Transport Processes During Extreme Ammonia Episodes in the U.S. Corn Belt

Atmospheric ammonia (NH₃) is the primary form of reactive nitrogen (N_r) and a precursor of ammonium (NH₄ ⁺) aerosols. Ammonia has been linked to adverse impacts on human health, the loss of ecosystem biodiversity, and plays a key role in aerosol radiative forcing. The midwestern United States is the major NH₃ source in North America because of dense livestock operations and the high use of synthetic nitrogen fertilizers. Here, we combine tall-tower (100 m) observations in Minnesota and Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) modeling to investigate high and low NH₃ emission episodes within the U.S. Corn Belt to improve our understanding of the distribution of emission sources and transport processes. We examined observations and performed model simulations for cases in February through November of 2017 and 2018. The results showed the following: (1) Peak emissions in November 2017 were enhanced by above-normal air temperatures, implying a Q₁₀ (i.e., the change in NH₃ emissions for a temperature increase of 10°C) of 2.5 for emissions. (2) The intensive livestock emissions rom northern Iowa, approximately 400 km away from the tall tower, accounted for 17.6% of the abundance in tall-tower NH₃ mixing ratios. (3) Ammonia mixing ratios in the innermost domain 3 frequently (i.e., 336 hr, 48% of November 2017) exceeded 5.3 ppb, an important air quality health standard. (4) In November 2017, simulated NH₃ net ecosystem exchange (the difference between NH₃ emissions and dry deposition) accounted for 60–65% of gross NH₃ emissions for agricultural areas and was 2.8–3.1 times the emissions of forested areas. (5) We estimated a mean annual NH₃ net ecosystem exchange of 1.60 ± 0.06 nmol · m⁻² · s⁻¹ for agricultural lands and −0.07 ± 0.02 nmol · m⁻² · s⁻¹ for forested lands. These results imply that future warmer fall temperatures will enhance agricultural NH₃ emissions, increase the frequency of dangerous NH₃ episodes, and enhance dry NH₃ deposition in adjacent forested lands.