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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 Jul 2018

Research article | 09 Jul 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

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

Alessandro Franchin1,2, Dorothy L. Fibiger1,2,a, Lexie Goldberger3,b, Erin E. McDuffie1,2, Alexander Moravek4, Caroline C. Womack1,2, Erik T. Crosman5, Kenneth S. Docherty6, William P. Dube1,2, Sebastian W. Hoch5, Ben H. Lee3, Russell Long7, Jennifer G. Murphy4, Joel A. Thornton3, Steven S. Brown1, Munkhbayar Baasandorj5, and Ann M. Middlebrook1 Alessandro Franchin et al.
  • 1NOAA Earth System Research Laboratory (ESRL) Chemical Sciences Division, 325 Broadway, Boulder, CO 80305, USA
  • 2Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309, USA
  • 3University of Washington, Department of Atmospheric Sciences, Seattle, WA 98195, USA
  • 4Department of Chemistry, University of Toronto, Toronto, ON, M5S 3H6, Canada
  • 5University of Utah, Department of Atmospheric Sciences, Salt Lake City, UT 84112, USA
  • 6Jacobs Technology, Inc., RTP, NC 27709, USA
  • 7Environmental Protection Agency (EPA), Durham, NC 27709, USA
  • anow at: California Air Resources Board, Monitoring and Laboratory Division, Sacramento, CA 95811, USA
  • bnow at: Pacific Northwest National Laboratory, Richland, WA 99354, USA

Abstract. Airborne and ground-based measurements of aerosol concentrations, chemical composition and gas phase precursors were obtained in three valleys in northern Utah (U.S.A.). The measurements were part of the Utah Winter Fine Particulate Study (UWFPS) that took place in January–February, 2017. Total aerosol mass concentrations of PM1 were measured from a Twin Otter aircraft, with an Aerosol Mass Spectrometer (AMS). PM1 concentrations ranged from less than 2μgm−3 during clean periods to over 100μgm−3 during the most polluted episodes, consistent with PM2.5 total mass concentrations measured concurrently at ground sites. Across the entire region, increases in total aerosol mass above ~2μgm−3 were associated with increases in the ammonium nitrate mass fraction, clearly indicating that the highest aerosol mass loadings in the region were predominantly attributable to an increase in ammonium nitrate. The chemical composition was regionally homogenous for total aerosol mass concentrations above 17.5μgm−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 region showed variable concentrations with height in the polluted boundary layer. Higher average mass concentrations were observed within the first few hundred meters above ground level in all three valleys during pollution episodes. Gas phase measurements of nitric acid (HNO3) and ammonia (NH3) during the pollution episodes revealed that in Cache and Utah Valley, partitioning of inorganic semi-volatiles to the aerosol phase was usually limited by the amount of gas phase nitric acid, with NH3 being in excess. The inorganic species were compared with the ISORROPIA thermodynamic model. Total inorganic aerosol mass concentrations were calculated for various decreases of total nitrate and total ammonium. For pollution episodes, our simulations of a 50% decrease in total nitrate lead to a 46±3% decrease in total PM1 mass. A simulated 50% decrease in total ammonium lead to a 36±17%µgm−3 in total PM1 mass, over the entire area of the study. Despite some differences among different locations, our results also showed a higher sensitivity to decreasing nitric acid concentrations and the importance of ammonia at the lowest total nitrate conditions. In the Salt Lake Valley, both HNO3 and NH3 concentrations controlled aerosol formation.

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Short summary
We present the results of aerosol and trace gases measurements from airborne and ground-based platforms. The measurements took place in January–February, 2017 in northern Utah, as part of the Utah Winter Fine Particulate Study (UWFPS). We characterized the chemical composition of PM1 on a regional scale, probing also the vertical dimension. We used a thermodynamic model to study the effectiveness of limiting total ammonium or total nitrate as strategy to control aerosol concentrations.
We present the results of aerosol and trace gases measurements from airborne and ground-based...