Winter Nitrate Formation at Mammoth Cave National Park - Regional Precursor Transport and Thermodynamic Partitioning
Jihee Ban1, Taehyun Park1, † , Amy Sullivan1, Andrey Marsavin1, Bret A. Schichtel2,3, Anthony J. Prenni2,3, Johnathon Jernigan4, Jim Renfro5, and Jeffrey L. Collett, Jr.1
1 Department of Atmospheric Science, Colorado State University, Fort Collins, CO, 2 Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, CO, 3 National Park Service Air Resource Division, Lakewood, CO, 4 National Park Service, Mammoth Cave National Park, KY, 5 National Park Service, Great Smoky Mountains National Park, Gatlinburg, TN, † Now at Department of Chemistry, Colorado State University, Fort Collins, CO
Wintertime nitrate remains an important component of fine particulate matter (PM2.5) in many parts of the United States, despite long-term declines in NOx emissions. Here, we investigate the formation of particulate nitrate at Mammoth Cave National Park (MACA) using intensive field observations of PM2.5 mass and composition and concentrations of gaseous ammonia and nitric acid collected from January 9 to February 10. 2025. Nitrate was the dominant PM2.5 constituent during the study period and contributed most strongly during high-concentration episodes. Back-trajectory and residence time analyses showed that nitrate enhancements were associated primarily with transport from the north-central and upper Midwest regions, while sulfate-dominated conditions were linked to transport from the southeast.
Thermodynamic simulations with ISORROPIA-II, constrained by in situ observations, reproduced the observed aerosol composition well. Sensitivity analyses showed that PM2.5 responded strongly to changes in total nitrate (TNO3 = HNO3 + NO3-), but only weakly to changes in total ammonia (TNHX = NH3 + NH4+). With nitric acid availability acting as a frequent limiting factor, continued reductions in NOx emissions should be effective for mitigating wintertime PM2.5 in the region.