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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  29 Jan 2020

29 Jan 2020

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This preprint is currently under review for the journal ACP.

The Role of Coarse Aerosol Particles as a Sink of HNO3 in Wintertime Pollution Events in the Salt Lake Valley

Amy Hrdina1,a, Jennifer G. Murphy1, Anna Gannet Hallar2, John C. Lin2, Alexander Moravek1,b, Ryan Bares2, Ross C. Petersen2,c, Alessandro Franchin3,4, Ann M. Middlebrook3, Lexie Goldberger5,d, Ben H. Lee4, Munkh Baasandorj2, and Steven S. Brown3 Amy Hrdina et al.
  • 1Department of Chemistry, University of Toronto, Toronto, ON, M5S 0A6, Canada
  • 2Department of Atmospheric Sciences, University of Utah, Salt Lake City, UT, 84112, USA
  • 3NOAA Earth System Research Laboratory (ESRL) Chemical Sciences Division, Boulder, CO, 80305, USA
  • 4Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, 80309, USA
  • 5University of Washington, Department of Atmospheric Sciences, Seattle, WA, 98195, USA
  • anow at: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA
  • bnow at: Department of Chemistry, York University, Toronto, ON, M3J 1P3, Canada
  • cnow at: Department of Physical Geography and Ecosystem Science, Lund University, Sweden
  • dnow at: Pacific Northwest National Laboratory

Abstract. Wintertime ammonium nitrate (NH4NO3) pollution events burden urban mountain basins around the globe. In the Salt Lake Valley of Utah in the United States, such pollution events are often driven by the formation of persistent cold air pools (PCAP) that trap emissions near the surface for several consecutive days. As a result, secondary pollutants including fine particulate matter less than 2.5 μm in diameter (PM2.5), largely in the form of NH4NO3, build up during these events and lead to severe haze. As part of an extensive measurement campaign to understand the chemical processes underlying PM2.5 formation, the 2017 Utah Winter Fine Particulate Study, water-soluble trace gases and PM2.5 constituents were continuously monitored using the Ambient Ion Monitoring Ion Chromatograph system (AIM-IC) at the University of Utah campus. Gas phase NH3, HNO3, HCl and SO2 along with particulate NH4+, Na+, K+, Mg2+, Ca2+, NO3, Cl, and SO42− were measured from January 21 to February 21, 2017. During the two PCAP events captured, the fine particulate matter was dominated by secondary NH4NO3. The comparison of total nitrate (HNO3 + PM2.5 NO3) and total NHx (NH3 + PM2.5 NH4+) showed NHx was in excess during both pollution events. However, chemical composition analysis of the snowpack during the first PCAP event revealed that the total concentration of deposited NO3 was nearly three times greater than that of deposited NH4+. Daily snow composition measurements showed a strong correlation between NO3 and Ca2+ in the snowpack. The presence of non-volatile salts (Na+, Ca2+, and Mg2+), which are frequently associated with coarse mode dust, was also detected in PM2.5 by the AIM-IC during the two PCAP events, accounting for roughly 5 % of total mass loading. The presence of a significant particle mass and surface area in the coarse mode during the first PCAP event was indicated by size-resolved particle measurements from an Aerodynamic Particle Sizer. Taken together, these observations imply that atmospheric measurements of the gas phase and fine mode particle nitrate may not represent the total burden of nitrate in the atmosphere, implying a potentially significant role for uptake by coarse mode dust. Using the NO3 : NH4+ ratio observed in the snowpack to estimate the proportion of atmospheric nitrate present in the coarse mode, we estimate that the amount of secondary NH4NO3 could double in the absence of the coarse mode sink. The underestimation of total nitrate indicates an incomplete account of the total oxidant production during PCAP events. The ability of coarse particles to permanently remove HNO3 and influence PM2.5 formation is discussed using information about particle composition and size distribution.

Amy Hrdina et al.

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Amy Hrdina et al.

Amy Hrdina et al.


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Short summary
Wintertime air pollution in Salt Lake Valley is primarily composed of ammonium nitrate, which is formed when gas-phase ammonia and nitric acid react. The major point in this paper is that the chemical composition of snow tells a very different story than what we measured in the atmosphere. With the dust/sea salt cations observed in PM2.5 and particle sizing data, we can estimate how much nitric acid may be lost to dust/sea salt that is not accounted for and how much more PM2.5 this could form.
Wintertime air pollution in Salt Lake Valley is primarily composed of ammonium nitrate, which is...