Characterization of the Southwestern Desert Aerosol, Meadview, AZ.

Sulfate, organic carbon, and soil dust were the major components of the fine aerosol at Meadview, AZ, during the summer of 1992. Sulfate mass median diameters (typically 0.15-0.27 μm) were much smaller than mass median diameters for organic carbon (typically 0.43-0.83 μm). Organic carbon size distributions were broader and more varied. Intersampler comparisons show that sulfur and sulfate measurement technology provided precise and relatively accurate (within 2-22%) concentrations. However, large differences were observed between IMPROVE filter and MOUDI impactor carbon concentrations. This is indicative of the large uncertainties with which carbon concentrations are measured. The IMPROVE backup filter subtraction procedure was partially responsible for these differences. Meadview sulfate was not completely neutralized by ammonium; SO2 concentrations were comparable to sulfate concentrations; and virtually all of the nitrate was present as gas-phase nitric acid. Our estimates suggest that primary organic aerosol from urban areas accounts for no more than 24% on average of the organic aerosol found at Meadview, AZ. The remainder is most likely secondary and biogenic OC, as well as OC from local and regional anthropogenic sources.