Modeling the sources and transport processes during extreme ammonia episodes in the U.S. Corn Belt

Authors: HU, CHENG - University Of Minnesota; GRIFFIS, TIMOTHY - University Of Minnesota; Baker, John; WOOD, JEFFREY - University Of Missouri; MILLET, DYLAN - University Of Minnesota; YU, ZHONGJIE - University Of Minnesota; LEE, XUHUI - Yale University

Submitted to: Journal of Geophysical Research Atmospheres
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/19/2019
Publication Date: 12/22/2019
Citation: Hu, C., Griffis, T.J., Baker, J.M., Wood, J.D., Millet, D.B., Yu, Z., Lee, X. 2019. Modeling the sources and transport processes during extreme ammonia episodes in the U.S. Corn Belt. Journal of Geophysical Research Atmospheres. 125(2):e2019JD031207. https://doi.org/10.1029/2019JD031207.
DOI: https://doi.org/10.1029/2019JD031207

Interpretive Summary: Ammonia (NH3) is a major atmospheric pollutant that leads to the production of aerosols, which have negative impacts on human health. Agriculture is the primary source of ammonia emissions, and the Midwestern U. S. is particularly critical, due to nitrogen fertilizer inputs and the presence of major animal production facilities. To this point there have been significant uncertainties in estimating the sources and distribution of ammonia, due to the lack of data. Here we have combined continuous measurements of NH3 from a tall (100 m) radio tower with a widely used atmospheric chemistry model to study episodes of extreme (both high and low) NH3 emission episodes to gain a better understanding of the processes affecting NH3. We studied the period from February to November in both 2017 and 2018. We found a) that a period of peak emissions in November 2017 was associated with much above normal air temperatures; b) NH3 emissions were primarily associated with agriculture, with 41% due to fertilizer and 26% due to livestock; c) after accounting for NH3 deposition, the estimated annual mean net emission of NH3 for agricultural land in the region was 1.60 ± 0.06 nmol m-2s-1. These results will be useful in constructing global NH3 budgets and in searching for wats to reduce NH3 emissions.

Technical Abstract: Atmospheric ammonia (NH3) is the primary form of reactive nitrogen (Nr) and a precursor of ammonium (NH4+) aerosols. Ammonia has been linked to adverse impacts on human health and biodiversity loss in ecosystems and plays a key role in aerosol radiative forcing. The Midwestern United States is the major NH3 source in North America as a result of densely populated livestock operations and large synthetic fertilizer inputs. Here, we combine tall tower (100 m) observations in Minnesota and WRF-CHEM modeling to investigate high and low NH3 emission episodes within the United States Corn Belt to improve our understanding of the distribution of emissions and transport processes. We examined observations and performed model simulations for cases in February to November of 2017 and 2018. The results indicated that: 1) peak emissions (i.e. in November 2017) were enhanced by above normal air temperatures yielding a Q10 for emissions of 2.5; 2) NH3 emissions for the inner-most study domain were dominated by agricultural sources (i.e. fertilizer and livestock emissions from Minnesota and Iowa) accounting for 41.2% and 26.6% of total emissions, respectively. 3) Intensive livestock emissions from northern Iowa accounted for 17.5% of the variation in tall tower NH3 mixing ratios; 4) Simulated NH3 dry deposition was 0.13 ± 0.01 nmol m-2 s-1 for forests and 0.94 ± 0.02 nmol m-2 s-1 for agricultural lands. After accounting for NH3 deposition we estimated a mean annual net ecosystem NH3 exchange of 1.60 ± 0.06 nmol m-2 s-1 for agricultural lands in 2017.