The Role of Hydrates, Competing Chemical Constituents, and Surface Composition on ClNO Formation.

Atomic chlorine (Cl) affects air quality and atmospheric oxidizing capacity. Nitryl chloride (ClNO) - a common Cl source-forms when chloride-containing aerosols react with dinitrogen pentoxide (NO). A recent study showed that saline lakebed (playa) dust is an inland source of particulate chloride (Cl) that generates high ClNO. However, the underlying physiochemical factors responsible for observed yields are poorly understood. To elucidate these controlling factors, we utilized single particle and bulk techniques to determine the chemical composition and mineralogy of playa sediment and dust samples from the southwest United States. Single particle analysis shows trace highly hygroscopic magnesium and calcium Cl-containing minerals are present and likely facilitate ClNO formation at low humidity. Single particle and mineralogical analysis detected playa sediment organic matter that hinders NO uptake as well as 10 Å-clay minerals (e.g., Illite) that compete with water and chloride for NO. Finally, we show that the composition of the aerosol surface, rather than the bulk, is critical in ClNO formation. These findings underscore the importance of mixing state, competing reactions, and surface chemistry on NO uptake and ClNO yield for playa dusts and, likely, other aerosol systems. Therefore, consideration of particle surface composition is necessary to improve ClNO and air quality modeling.