TAKEAWAY
Gas-phase ammonia is an important precursor to northern Utah’s PM2.5. As such, it is critical to understand the strengths and distribution of its wintertime sources.
Northern Utah’s gas-phase ammonia concentrations are consistently among the highest in the nation.
During winter inversion episodes, ammonium nitrate makes up about half of the Wasatch Front’s PM2.5 mass and up to 90 percent of Cache Valley’s PM2.5 mass. Ammonium nitrate is formed by reactions involving nitric acid, which is created from volatile organic compounds, oxides of nitrogen, and gas-phase ammonia. The effectiveness of potential solutions is dependent on understanding which of those compounds (nitric acid or ammonia) are less available.
Research has found that ammonium nitrate PM2.5 formation along the Wasatch Front is limited by oxides of nitrogen early in inversion episodes, but transforms to ammonia-limited formation later in an inversion, when PM2.5 concentrations increase.
This pattern, however, doesn’t hold in Cache Valley; the airshed there has two to six times more ammonia than nitric acid. In that case, the abundance of ammonia suggests that trying to reduce it would not result in a decrease in PM2.5 mass until at least half of the ammonia is removed.
Northern Utah’s gas-phase ammonia concentrations are consistently among the highest in the nation. Cache Valley recorded a 2019-2021 average of 17.4 micrograms per cubic meter, highest in the nation, and the Salt Lake City area recorded an average of 4.3 micrograms per cubic meter, the 13th highest in the nation.
Studies have been completed and are ongoing to identify ammonia concentrations across time, locating potential “hot spots” of unanticipated elevated emissions, and finding under-counted sources, such as direct emissions from diesel and gasoline-fueled vehicles, landfills, and from agriculture.
