The Role of Hydrates, Competing Chemical Constituents, and Aerosol Surface Composition on ClNO2 Formation from Dusts

Abstract:

Reactions between dinitrogen pentoxide (N₂O₅) and mineral dust have traditionally been thought to decrease the oxidizing capacity of the atmosphere through the hydrolysis reaction, which removes nitrogen oxides from the atmosphere and thereby diminishes the production of tropospheric ozone. However, recent research shows that heterogeneous reactions of N₂O₅ on Cl-containing mineral dust from saline lakebeds (playas) can produce nitryl chloride (ClNO₂), which is a common source of the potent chemical oxidizer atomic chlorine (Cl*) that increases the oxidizing capacity of the atmosphere. At present, the physical and chemical factors associated with ClNO₂ production from this mechanism are poorly understood, which makes it difficult to predict how playa dusts can affect atmospheric oxidizing capacity. To elucidate these controlling factors, we utilized single particle and bulk analytical techniques to determine the chemical composition and mineralogy of playa sediment and dust samples from the southwest United States. Single particle analysis suggests the presence of highly hygroscopic Cl-containing minerals which can deliquesce at low relative humidity (RH), thereby facilitating ClNO₂ formation at low RH. Bulk mineralogical and surface chemical analysis also revealed the presence of competing chemical constituents that affect N₂O₅ uptake and ClNO₂ production. 10Å clay minerals (a mineral category including illite and muscovite) were found to outcompete chloride (Cl^-) for N₂O₅ and diminish ClNO₂ yields while playa sediment organic matter was found to inhibit N₂O₅ uptake. Finally, comparisons of ClNO₂ yields and Cl^- content from bulk and surface analytical techniques indicate that ClNO₂ formation occurs on the surface rather than in the bulk aerosol. These findings improve our understanding of how mineral dust emitted from dried lakebeds can increase the oxidative capacity of the atmosphere through ClNO₂ production. This is an increasingly important mechanism to understand as land use changes and climate change increase the spatial extent of playas.