Factors Controlling Ammonium Nitrate Formation in Cold Polluted Environments (Invited Presentation)

High loadings of fine particulate matter (PM) are a problem in many regions of the western U.S. during the winter season. During the Utah Winter Fine Particle Study (UWFPS) in January-February of 2017, measurements of the chemical composition of fine particles and their gas phase precursors were made from a Twin Otter aircraft and at multiple ground sites in Northern Utah. During persistent cold air pool events, fine particles built up to high mass loadings (> 30 μg m^-3) and were dominated by ammonium nitrate. Analysis of the total ammonium and total nitrate budgets shows that the sensitivity of PM to its precursors varies spatially across the region. Measured ammonia enhancements observed at the ground and from the aircraft were much larger than enhancements predicted using the Utah-adjusted 2014 National Emissions Inventory (NEI) in combination with footprints from the Stochastic Time-Inverted Lagrangian Transport (STILT) model driven by High Resolution Rapid Refresh (HRRR) meteorological fields. The inventory suggests that the emissions in the region are dominated by agriculture and contributions from livestock are assigned a large amount of seasonal variability. Examination of long-term measurements of gas phase ammonia and wet-deposited ammonium in the region suggests that the amplitude of the seasonality imposed on the ammonia emissions is too large. Emissions of ammonia from mobile sources also appear to be significantly underestimated. During polluted periods, measurements of significant non-volatile cations (sodium and calcium) in the fine mode appear to be associated with episodes of high primary particle loadings. Based on the relationship between nitrate and various cations in the snowpack, along with calculated size-resolved aerosol surface area, we estimate that coarse mode particles may play a role in sequestering gas phase nitric acid in the region, with consequences for fine particle loadings.