Halogen Production from Saline Playa Dust Emitted from the Great Salt Lake: Implications of the Shrinking Great Salt Lake on Regional Oxidant Budgets

Atomic chlorine (Cl•) is a powerful atmospheric oxidant that can affect air quality and climate by facilitating the production of tropospheric ozone and secondary particulate matter as well as alter the atmospheric lifetime of methane. One important source of Cl• is the photolysis of nitryl chloride (ClNO₂), formed from the reaction of dinitrogen pentoxide (N₂O₅) and chloride-containing aerosol. However, sources of chloride-containing aerosol in inland regions, and their subsequent reaction kinetics, remain poorly constrained. To better understand inland sources of Cl•, 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 of N₂O₅ (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 non-reactive 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 as an oxidant source 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.