Wasatch Front Ammonia and Chloride Observations (WaFACO)

  • Principal Investigators: Randal S. Martin (USU), Kerry Kelly (UU), Jaron Hansen (BYU)
  • Funded for: $210,000
  • Study Period: 7/1/2018 – 12/31/2019
  • DAQ Contact: Nancy Daher

The study will conduct ambient measurements centered around gaseous ammonia (NH3) and hydrochloric acid (HCl) during the winter of 2018/2019 and the summer of 2019. As discussed below, additional supporting measurements of PM2.5 distribution and composition and mobile measurements of temporally-refined NH3 and other parameters will also be conducted.

Numerous previous studies, most recently the 2017 multi-agency Utah Winter Fine Particulate Study (UWFPS)  have examined the composition and formation mechanisms of the local wintertime particulate. Consistent among all of these studies is the predominance of ammonium nitrate as the main component of the elevated PM2.5 concentrations across the Wasatch Front. Indeed, within the summary of the 2017 UWFPS, it is pointed out that although the local airsheds appear to be slightly ammonia-rich in regards to ammonium nitrate formation, there are times, especially during extended persistent wintertime inversions, in which the atmosphere switched to a more ammonia-limited regime. It should be noted that it was also found that the Cache Valley was consistently more ammonia-rich. Directly quoting the UWFPS report:

“This may suggest that the system in Utah Valley and particularly the one in the Salt Lake Valley is close to the equivalence point between nitrate limited and ammonium limited regimes, and reductions in either reagent (nitrate and/or ammonia) may be effective in leading to reductions in ammonium nitrate aerosol.”

The UWFPS report also pointed out the current NH3 emissions inventory does not adequately replicate the observed NH3 measurements, both in distribution and ambient concentration and concluded that the uncertainty in NH3 data are a significant deficiency in the current state of knowledge. Therefore, a more detailed understanding of NH3 behavior along the Wasatch Front needs to be a priority in order for appropriate remediation scenarios to be implemented.

In discussions with Utah DAQ and EPA personnel, questions have also been raised about the influence of atmospheric chloride on both particulate formation chemistry and oxidant photochemistry during both the wintertime pollution events and summertime ozone episodes. A small study  was conducted in the summer of 2015 using 15 passive samplers to assess the hydrochloric acid (HCl) distribution along the northern Wasatch Front and the southern and southwestern edge of the Great Salt Lake. A notable gradient was found with higher values from west to east, possibly owing to the large magnesium refinery on the western edge of the Great Salt Lake. These results were also observed in the 2017 UWFPS, previously referenced. As with NH3, a more complete understanding of the availability of gas-phase chloride, as represented by HCl, will aid in the development of more reliable data and models for the management of Wasatch Front air quality.