Comparison of calculated sulfate scattering efficiencies as estimated from size-resolved particle measurements at three national locations

Size distributions and resulting optical properties of sulfur aerosols were investigated at three national parks by a Davis Rotating-drum Universal-size-cut Monitoring (DRUM) impactor. Sulfur size distribution measurements for 88, 177, and 315 consecutive time periods were made at Grand Canyon National Park during January and February 1988, Meadview, AZ during July, August, and September 1992, and at Shenandoah National Park during summer, 1990, respectively. The DRUM impactor is designed to collect aerosols with an aerodynamic diameter between 0.07 and 15.0 μm in eight size ranges. Focused beam particle-induced X-ray emission (PIXE) analysis of the aerosol deposits produces a time history of size-resolved elemental composition of varied temporal resolution. As part of the quality assurance protocol, an interagency monitoring of protected visual environments (IMPROVE) channel A sampler collecting 0–2.5 μm diameter particles was operated simultaneously alongside the DRUM sampler. During these sampling periods, the average sulfur mass, interpreted as ammonium sulfate, is 0.49, 2.30, and 10.36 μg m−3 at Grand Canyon, Meadview, and Shenandoah, respectively. The five drum stages were “inverted” using the Twomey (1975) scheme to give 486 size distributions, each made up of 72 discreet pairs of dC/dlog(D) and diameter (D). From these distributions mass mean diameters (Dg), geometric standard deviations (σg), and mass scattering efficiencies (em)) were calculated. The geometric mass mean diameters in ascending order were 0.21 μm at Meadview, 0.32 μm at Grand Canyon, and 0.42 μm at Shenandoah corresponding σg were 2.1, 2.3, and 1.9. Mie theory mass scattering efficiencies calculated from dC/dlog(D) distributions for the three locations were 2.05, 2.59, and 3.81 m2 g−1, respectively. At Shenandoah, mass scattering efficiencies approached five but only when the mass median diameters were approximately 0.4 μm and σg were about 1.5. σg near 1.5 were frequently measured at Shenandoah, rarely at Grand Canyon, and never during the summer at Meadview.