The Influence of Industrial Halogens on Air Quality in Salt Lake City, Utah

Northern Utah experiences some of the worst air quality in the United States in winter. During periodic cold air pool episodes, emitted pollutants are trapped by the mountainous terrain and react for many days to form high concentrations of ammonium nitrate aerosol. In 2017, the Utah Winter Fine Particulate Study made air quality measurements onboard the NOAA Twin Otter aircraft. During repeated transects of the plume from a magnesium refinery on the west side of the Great Salt Lake, we found extremely high levels of the halogen species Cl₂, Br₂, BrCl, and HCl and complete ozone depletion within the plume. Using a plume-crossing technique, we estimated the emissions of all halogens and found Cl₂ and HCl to be in good agreement with facility-reported emissions. Bromine species do not exist in the inventories, but our observations imply a large source of these potent oxidizers. Using a zero-dimensional box model (F0AM) and a three-dimensional chemical transport model (CAM-Chem), we demonstrated that photochemically-produced halogen radicals deplete ozone entirely in the immediate vicinity, and then increase the production of PM_2.5 in the populated downwind areas of Salt Lake City. The CAM-Chem model estimated a 10 – 25% increase in surface PM_2.5 during a pollution episode from this single industrial source. Following publication of these results, the Utah State Legislature passed House Bill 220 which mandated the state to compile an inventory of industrial halogens. Additionally, an upcoming study in Utah during the summer of 2024 will aim to elucidate the role of these halogens during summer ozone episodes.