15B.3 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)

Thursday, 16 January 2020: 4:00 PM
207 (Boston Convention and Exhibition Center)
Cassandra J. Gaston, RSMAS, MIAMI, FL; and H. M. Royer, D. Mitroo, P. Blackwelder, S. Hayes, S. Haas, K. A. Pratt, and T. E. Gill

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 (γN2O5) ranged from ~10-3 to 10-1 and ClNO2 yields (ΦClNO2) 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.
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