COMPARISON OF CALCULATED SULFATE SCATTERING EFFICIENCIES AS ESTIMATEDFROM SIZE-RESOLVED PARTICLE MEASUREMENTS AT 3 NATIONAL LOCATIONS

Size distributions and resulting optical properties of sulfur aerosols were investigated at three national parks by a Davis Rotating-drum Un iversal-size-cut Monitoring (DRUM) impactor. Sulfur size distribution measurements for 88, 177, and 315 consecutive time periods were made a t Grand Canyon National Park during January and February 1988, Meadvie w, AZ during July, August, and September 1992, and at Shenandoah Natio nal Park during summer, 1990, respectively. The DRUM impactor is desig ned to collect aerosols with an aerodynamic diameter between 0.07 and 15.0 mu m in eight size ranges. Focused beam particle-induced X-ray em ission (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 collectin g 0-2.5 mu m diameter particles was operated simultaneously alongside the DRUM sampler. During these sampling periods, the average sulfur ma ss, interpreted as ammonium sulfate, is 0.49, 2.30, and 10.36 mu g m(- 3) at Grand Canyon, Meadview, and Shenandoah, respectively. The five d rum stages were ''inverted'' using the Twomey (1975) scheme to give 48 6 size distributions, each made up of 72 discreet pairs of dC/dlog(D) and diameter (D). From these distributions mass mean diameters (D-g), geometric standard deviations (sigma(g)), and mass scattering efficien cies (e(m)) were calculated. The geometric mass mean diameters in asce nding order were 0.21 mu m at Meadview, 0.32 mu m at Grand Canyon, and 0.42 mu m at Shenandoah. The corresponding sigma(g)'s were 2.1, 2.3, and 1.9. Mie theory mass scattering efficiencies calculated from dC/dl og(D) distributions for the three locations were 2.05, 2.59, and 3.81 m(2) g(-1), respectively. At Shenandoah, mass scattering efficiencies approached five but only when the mass median diameters were approxima tely 0.4 mu m and sigma(g)'s were about 1.5. sigma(g)'s near 1.5 were frequently measured at Shenandoah, rarely at Grand Canyon, and never d uring the summer at Meadview. (C) 1997 Elsevier Science Ltd.