Impact of Recent Agricultural Ammonia Increases on Fine Particulate Matter Burden over the Midwestern United States

Abstract

Concentrations of ammonia (NH₃), a critical precursor for PM_2.5 formation, have increased in the highly agricultural Midwestern United States (MWUS) in recent years, especially since 2014 during the winter. NH₃ emissions are not regulated at the federal level, and the impacts of recent NH₃ increases on MWUS air quality have not been well quantified. The goal of this work is to determine the impact of increasing NH₃ on the formation and chemical composition of PM_2.5 throughout the MWUS from 2007 to 2019 via observational data and sensitivity simulations using the three-dimensional (3D) chemical transport model GEOS-Chem, with an emphasis on post-2014 and wintertime trends. We find significant increases in NH₃ (43%) and ammonium (NH₄⁺) wet deposition (20%) across the MWUS for all seasons, with the highest increases during wintertime (58%). Wintertime trends in observed PM_2.5 and particulate nitrate follow trends in NH₃ emissions: decreasing trends in both reverse after 2014 alongside increasing emissions. Sensitivity simulations suggest that agricultural NH₃ contributes 40% of the PM_2.5 burden in the MWUS, ∼4x higher on average than the contiguous United States (CONUS), and the sensitivity of modeled PM_2.5 to NH₃ over time remains steady in the MWUS yet decreases over the CONUS. Reducing NH₃ emissions by 18% results in a decrease in PM_2.5 concentrations by 0.4 μg m^–3 (3.7%) on average in the MWUS. These results emphasize the disproportionate role of agricultural NH₃ emissions in determining the MWUS PM_2.5 burden in recent years. This hinders improvements in air quality and provides evidence to support the regulation of NH₃.