AbstractThe Nitrogen, Aerosol Composition, and Halogens on a Tall Tower campaign (February–March 2011) near Boulder, Colorado, investigated nighttime ClNO2 production and processing. Virtually all particulate Cl was in the form of ionic Cl−. The size distributions of Cl− and Na+ were similar, with most of the mass in the supermicrometer size fraction, suggesting primary sources for both. Median Cl− concentrations were about half those of Na+ and Ca2+ for particle diameters centered at 1.4 and 2.5 µm. To investigate potential sources of Na+ and Cl−, four cases were studied that featured the prevalence of Na+ and Cl− and different transport pathways based on FLEXible PARTicle dispersion model (FLEXPART) retroplumes. Estimates of supermicrometer Na+ particle lifetime against deposition indicate that long‐range transport of marine aerosols could account for the observed Na+. However, measured molar ratios of Ca2+ to Na+ (0.143–0.588) compared to seawater (0.022) indicate significant contributions from crustal sources to the supermicrometer aerosol composition during these four case studies. Further, low molar ratios of Mg2+ to Na+ (0.007–0.098) relative to seawater (0.114) suggest that some of the Na+, and presumably associated Cl−, originated from non‐sea‐salt sources. The heterogeneous chemical composition of saline soils throughout the western U.S., along with the nonlinearity of wind‐driven soil deflation as a function of various surface soil properties, precludes a quantitative apportionment of soil, marine, and anthropogenic sources to the observed coarse‐fraction aerosol. Nonetheless, results suggest that deflation of saline soils was a potentially important source of particulate Cl− that sustained atmospheric ClNO2 production and associated impacts on oxidation processes over northern Colorado.
