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. Airborne and ground-based measurements of aerosol concentrations, chemicalcomposition, and gas-phase precursors were obtained in three valleys innorthern Utah (USA). The measurements were part of the Utah Winter FineParticulate Study (UWFPS) that took place in January–February 2017. Totalaerosol mass concentrations of PM 1 were measured from a Twin Otteraircraft, with an aerosol mass spectrometer (AMS). PM 1 concentrationsranged from less than 2  µ g m −3 during clean periods to over100  µ g m −3 during the most polluted episodes, consistent withPM 2.5 total mass concentrations measured concurrently at groundsites. Across the entire region, increases in total aerosol mass above ∼2 µ g m −3 were associated with increases in theammonium nitrate mass fraction, clearly indicating that the highest aerosolmass loadings in the region were predominantly attributable to an increase inammonium nitrate. The chemical composition was regionally homogenous fortotal aerosol mass concentrations above 17.5  µ g m −3 , with 74±5  % (average  ±  standard deviation) ammonium nitrate, 18±3  %organic material, 6±3  % ammonium sulfate, and 2±2  %ammonium chloride. Vertical profiles of aerosol mass and volume in the regionshowed variable concentrations with height in the polluted boundary layer.Higher average mass concentrations were observed within the first few hundredmeters above ground level in all three valleys during pollution episodes. Gas-phase measurements of nitric acid ( HNO3 ) and ammonia ( NH3 ) duringthe pollution episodes revealed that in the Cache and Utah valleys, partitioningof inorganic semi-volatiles to the aerosol phase was usually limited by theamount of gas-phase nitric acid, with NH3 being in excess. The inorganicspecies were compared with the ISORROPIA thermodynamic model. Total inorganicaerosol mass concentrations were calculated for various decreases in totalnitrate and total ammonium. For pollution episodes, our simulations of a50 % decrease in total nitrate lead to a 46±3  % decrease in totalPM 1 mass. A simulated 50 % decrease in total ammonium leads to a 36±17  %  µ g m −3 decrease in total PM 1 mass, over the entirearea of the study. Despite some differences among locations, ourresults showed a higher sensitivity to decreasing nitric acid concentrationsand the importance of ammonia at the lowest total nitrate conditions. In theSalt Lake Valley, both HNO3 and NH3 concentrations controlledaerosol formation.

atmospheric chemistry and physics

Airborne and ground-based observations of ammonium-nitrate-dominated aerosols in a shallow boundary layer during intense winter pollution episodes in northern Utah