Mie Theory Evaluation of Species Contributions to 1990 Wintertime Visibility Reduction in the Grand Canyon

Abstract

Contributions of the major fine particle species to light scattering were evaluated using Mie theory from measurements of sizeresolved chemistry and particle hygroscopicity obtained during the 1990 NGS Visibility Study at Hopi Point, Grand Canyon, from January 9, 1990 through March 31, 1990. It was found that scattering efficiencies of participate carbon mass ranged from 1.5 to 8 m2 per gram of carbon particle mass (assumed equal to 1.4 times carbon mass), with an average value of 5.4 ± 1.5 m2/g. Sulfur size distributions, which are available for the entire 80-day study, show that sulfate scattering efficiencies depend on both relative humidity and on median particle size, and that “dry” (RH <60 percent) sulfur scattering efficiencies range from about 1.5 to 4.5 m2 per gram of ammonium sulfate. This range reflects the wide variability in median sulfur particle size (0.07 to 0.66 urn) that was observed during the study. For the 20-day period during which size distribution data for the major fine particle species are available, the contributions of soil dust, carbon, sulfates, and nitrates to scattering are 0.38, 5.0, 4.8, and 0.7 Mm−1 respectively, and average daily percentage contributions are 4.2, 50.1, 38.5, and 6.9 percent. Sulfur mass median diameters >0.3 μm were only found during periods of high relative humidity, but not all high humidity periods had large sulfur mass median diameters. Sulfate scattering efficiencies for >60 percent RH were smaller for instances of direct transport of <36 hr from Navajo Generating Station to Hopi Point than for the regional background. Scattering efficiencies for longer transport times resembled the regional background.