Populated mountain basins, including the Salt Lake Valley (SLV) in Utah, suffer from wintertime stagnation events that trap emissions near the surface and cause fine particulate matter (PM_2.5) concentrations to reach unhealthy levels. Previously limited by a lack of nighttime measurements, this study uses 2017 UWFPS aircraft campaign data, in combination with a box model, to show that nitrogen chemistry above the surface at night is a major source of PM_2.5 during a wintertime event in the SLV.