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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 21 May 2019

Submitted as: research article | 21 May 2019

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

Wintertime Spatial Distribution of Ammonia and its Emission Sources in the Great Salt Lake Region

Alexander Moravek1, Jennifer G. Murphy1, Amy Hrdina1, John C. Lin2, Christopher Pennell3, Alessandro Franchin4,5, Ann M. Middlebrook5, Dorothy L. Fibiger4,5,a, Caroline C. Womack4,5, Erin E. McDuffie4,5,6,b, Randal Martin7, Kori Moore7,c, Munkhbayar Baasandorj2,3, and Steven S. Brown5,6 Alexander Moravek et al.
  • 1Department of Chemistry, University of Toronto, Toronto, ON, M5S 3H6, Canada
  • 2Department of Atmospheric Sciences, University of Utah, Salt Lake City, UT, 84112, USA
  • 3Division of Air Quality, Utah Department of Environmental Quality, Salt Lake City, UT, 84114, USA
  • 4Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, 80309, USA
  • 5NOAA Earth System Research Laboratory (ESRL) Chemical Sciences Division, Boulder, CO, 80305, USA
  • 6Department of Chemistry, University of Colorado, Boulder, CO, 80309, USA
  • 7Department of Civil and Environmental Engineering, Utah State University, Logan, UT, 84322, USA
  • anow at: California Air Resources Board, Sacramento, CA, 95814, USA
  • bnow at: Department of Physics and Atmospheric Science, Dalhousie University,Halifax, NS, B2H 4R2, Canada
  • cnow at: Space Dynamics Laboratory, Logan, UT, 84341, USA

Abstract. Ammonium-containing aerosols are a major component of winter time air pollution in many densely populated regions around the world. Especially in mountain basins, the formation of persistent cold air pool (PCAP) periods can enhance particulate matter with diameters less than 2.5 μm (PM2.5) to levels above air quality standards. Under these conditions, PM2.5 in the Great Salt Lake Region of northern Utah has been shown to be primarily composed of ammonium nitrate, however, its formation processes and sources of its precursors are not fully understood. Hence, it is key to understand the emission sources of its gas-phase precursor, ammonia (NH3). To investigate the formation of ammonium nitrate, a suite of trace gases and aerosol composition were sampled from the NOAA Twin Otter aircraft during the Utah Winter Fine Particulate Study (UWFPS) in January and February 2017. NH3 was measured using a Quantum Cascade Tunable Infrared Laser Differential Absorption Spectrometer (QC-TILDAS), while aerosol composition, including particulate ammonium (pNH4), was measured with an aerosol mass spectrometer (AMS). The origin of the sampled air masses was investigated using the Stochastic Time-Inverted Lagrangian Transport (STILT) model and combined with an NH3 emission inventory to obtain model-predicted NHx (= NH3 + pNH4) enhancements. Comparison of these NHx enhancements with measured NHx from the Twin Otter shows that modelled values are a factor of 1.6 to 4.4 lower for the three major valleys in the region. Among these, the underestimation is largest for Cache Valley, an area with intensive agricultural activities. We find that one explanation for the underestimation of wintertime emissions may be the seasonality factors applied to NH3 emissions from livestock. An investigation of inter-valley exchange revealed that transport of NH3 between major valleys was limited and PM2.5 in Salt Lake Valley (the most densely populated area in Utah) was not significantly impacted by NH3 from the agricultural areas in Cache Valley. We found that in Salt Lake Valley around two thirds of NHx originated within the valley, while about 30 % originated from mobile source and 60 % from area source emissions in the region. For Cache Valley, a large fraction of NOx potentially leading to PM2.5 formation may not be locally emitted but mixed in from other counties.

Alexander Moravek et al.
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Alexander Moravek et al.
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Short summary
Ammonium nitrate is a major component of fine particulate matter of wintertime air pollution in the Great Salt Lake Region (UT, USA). We investigate the sources of ammonia in the region by using aircraft observations and comparing them to modeled ammonia mixing ratios based on emission inventory estimates. The results suggest that ammonia emissions are underestimated, specifically in regions with high agricultural activity, while ammonia in Salt Lake City is mainly of local origin.
Ammonium nitrate is a major component of fine particulate matter of wintertime air pollution in...