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

Concentrations of ammonia (NH3), a critical precursor for PM2.5 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 PM2.5 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 PM2.5 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 PM2.5 burden in the MWUS, ∼4x higher on average than the contiguous United States (CONUS), and the sensitivity of modeled PM2.5 to NH3 over time remains steady in the MWUS yet decreases over the CONUS. Reducing NH3 emissions by 18% results in a decrease in PM2.5 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 PM2.5 burden in recent years. This hinders improvements in air quality and provides evidence to support the regulation of NH3.