Halogen Production from Playa Dust Emitted from the Great Salt Lake: Implications of the Shrinking Great Salt Lake on Regional Air Quality

Abstract

Halogen radicals, such as atomic chlorine (Cl·), can contribute to secondary wintertime fine particulate matter in the Salt Lake Valley. One source of Cl· is the photolysis of nitryl chloride (ClNO₂), formed from the reaction of dinitrogen pentoxide (N₂O₅) with chloride-containing aerosol. However, sources of chloride-containing aerosols in the Salt Lake Valley, and their subsequent reaction kinetics, remain poorly constrained. We analyzed playa (i.e., dried saline lakebed) samples collected from dust-emitting regions along the northern and southern areas of the shrinking Great Salt Lake to investigate their mineralogy, reactivity, and ClNO₂ forming potential. The reactive uptake coefficients (γN₂O₅) for all samples ranged from 0.005 to 0.064, with the average γN₂O₅ of the northern area samples approximately double the average γN₂O₅ of the southern area samples. We attribute the increased γN₂O₅ of northern playas to increased particulate chloride and silicate, while the reduced γN₂O₅ in southern playas is due to particulate organics and high quantities of gypsum, a nonreactive mineral. The yield of ClNO₂ is > 50% for all playas tested, with one exception. Using our kinetic data during an ambient wintertime case study, we estimate playa dust contributes up to 5% of observed ClNO₂, a lower estimate which likely increases during the spring when dust emissions are higher. Our work highlights the importance of including playa dust in current air quality models, especially as reductions of anthropogenic halogen sources are implemented in the United States, and ephemeral lakes continue to shrink globally.