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(Ed.)
Atmospheric ammonia (NH3) is the primary form of reactive nitrogen (Nr) and a precursor ofammonium (NH4+) aerosols. Ammonia has been linked to adverse impacts on human health, the loss ofecosystem biodiversity, and plays a key role in aerosol radiative forcing. The midwestern United States is themajor NH3source in North America because of dense livestock operations and the high use of syntheticnitrogen fertilizers. Here, we combine tall‐tower (100 m) observations in Minnesota and Weather Researchand Forecasting model coupled with Chemistry (WRF‐Chem) modeling to investigate high and low NH3emission episodes within the U.S. Corn Belt to improve our understanding of the distribution of emissionsources and transport processes. We examined observations and performed model simulations for cases inFebruary through November of 2017 and 2018. The results showed the following: (1) Peak emissions inNovember 2017 were enhanced by above‐normal air temperatures, implying aQ10(i.e., the change in NH3emissions for a temperature increase of 10°C) of 2.5 for emissions. (2) The intensive livestock emissionsrom northern Iowa, approximately 400 km away from the tall tower, accounted for 17.6% of theabundance in tall‐tower NH3mixing ratios. (3) Ammonia mixing ratios in the innermost domain 3frequently (i.e., 336 hr, 48% of November 2017) exceeded 5.3 ppb, an important air quality health standard.(4) In November 2017, simulated NH3net ecosystem exchange (the difference between NH3emissionsand dry deposition) accounted for 60–65% of gross NH3emissions for agricultural areas and was2.8–3.1 times the emissions of forested areas. (5) We estimated a mean annual NH3net ecosystem exchangeof 1.60 ± 0.06 nmol · m−2·s−1for agricultural lands and−0.07 ± 0.02 nmol · m−2·s−1for forested lands.These results imply that future warmer fall temperatures will enhance agricultural NH3emissions, increasethe frequency of dangerous NH3episodes, and enhance dry NH3deposition in adjacent forested lands.
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