Insights into the Production of Nitryl Chloride (ClNO2) in Inland Regions from Saline Playas and the Role of Playa Dust Mineralogy in Determining Halogen Yields (Invited Presentation)

Nitryl chloride (ClNO₂), formed when dinitrogen pentoxide (N₂O₅) reacts with chloride-containing aerosol, photolyzes to produce chlorine radicals that facilitate the formation of tropospheric ozone. ClNO₂ has been measured in both coastal and inland areas. While sea salts produce ClNO₂ in coastal regions, the sources of particulate chloride required to form ClNO₂ in inland regions remain unclear. We presented the first laboratory measurements demonstrating the efficient production of ClNO₂ from saline playas (e.g., dried lakebeds)—an inland source of particulate chloride. We also detected molecular bromine (Br₂) and nitryl bromide (BrNO₂) during our experiments, highlighting that playas also facilitate the heterogeneous production of brominated compounds. N₂O₅ reactive uptake coefficients (γ_N2O5) ranged from ~10^-3 to 10^-1 and ClNO₂ yields (Φ_ClNO2) were >50% for all playas tested except one. We found that as the evaporite (e.g., soluble mineral) content of playa dusts increases, γ(N₂O₅) decreased and Φ(ClNO₂) increased. To probe these findings, we performed detailed bulk and single-particle analysis on different playa dust samples using ion chromatography, X-ray Diffraction (XRD), single-particle mass spectrometry, and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS). The major finding of our work is that aerosol surfaces dominated by silicates react more efficiently with N₂O₅ than water or chloride and suppress the formation of ClNO₂; these competing reactions are not accounted for in current parameterizations of ClNO₂ yields. In contrast, mineral aerosols with appreciable quantities of evaporites that coat the particle surface behave like deliquesced sodium chloride generating yields of ClNO₂ near unity and values of γ(N₂O₅)~0.03. Surprisingly, we also measured high yields of ClNO₂ even at relative humidities (RHs) well-below the deliquescence point of sodium chloride and found that magnesium chloride-containing minerals can facilitate the production of ClNO₂ even at RH as low as 10%. Overall, our results underscore the important role of mineralogy, surface composition, and competing reactions between silicates and chloride with N₂O₅ for predicting the production of ClNO₂ from saline playa dusts in inland regions.