Nuclear Instruments and Methods in Physics Research B3 (1984) 479-482 North-Holland, Amsterdam 479 zyxwvuts LONG RANGE TRANSPORT AND TRANSFORMATION OF ANTHROPOGENIC POLLUTANTS OBSERVED IN A PIXE-BASED MONITORING NETWORK Robert A. ELDRED, Thomas A. CAHILL, Lowell L. ASHBAUGH and John S. NASSTROM Cracker Nuclear Laboratory, University of California, Davis, CA 95616, USA An automated PIXE system was used to determine the elemental concentrations on particulate samples collected in a monitoring network of 40 remote sites covering 2 million square kilometers in the western United States from 1979 to 1981. PIXE’s good sensitivity for sulfur permitted the measurement of this key element in a region of low concentrations. Sulfate particles comprise the major component of the fine mass and represent the largest threat to visibility in the region. Two cases of sulfate transport into the pristine, scenic areas of northern Arizona and southern Utah from sources around 600 km away are described: copper smelters to the south and urban center to the southwest. Evidence is also presented for the transport of sulfate to the northern Great Plains from industrial centers over 1000 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA km to the east. The simultaneous measurement of a wide range of elements by PIXE has permitted estimates of the conversion rates of SO, to sulfate in a clean, dry atmosphere. The most important result was the large variation between summer and winter. Tbis indicates that SO, emissions in winter are much less serious than similar emissions in summer. 1. Introduction The Rocky Mountain region of the western United States is noted for its pristine air and excellent visibility. Because the region contains numerous National Parks and wilderness areas that depend on scenic vistas, it is essential that good visibility be maintained. Because most visibility reduction in this area is due to the scattering of light from fine particles, a 40 site network was created in 1979 to establish baseline particulate concentrations for elements from sodium through lead and to determine possible sources [l]. Sulfur is the most important elemental species because of its key role in visibility reduction. This paper considers three examples of the transport of sulfur particles to these sites. The first looks at the change in regional sulfate concentrations produced by a sharp decrease in the emissions of the major source in the region, the copper smelters. The second examines a major sulfate episode while the smelters are off. The final case deals with the transport of sulfate from the industrial centers near the Great Lakes westward to the northwestern Great Plains, approximately 1000 km away. To do this the new technique of residence time analysis is employed. 2. Sample collection and PIXE analysis The particulate concentrations were collected at 40 remote sites covering eight states in the Rocky Moun- tains and western Great Plains, located at the centers of the circles of fig. 1. This Western Fine Particle Network 0168-583X/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) Fig. 1. Mean sulfate concentrations for each summer. The diameters are proportional to mean concentrations, with a maximum of 3 ng/m3. was operated by the Air Quality Group of the Univer- sity of California at Davis for the Environmental Pro- tection Agency, with field support by the National Park Service and other agencies. Sampling began in August 1979 for nine southern sites and October 1979 for the rest, and continued through September 1981. (Measure- ments at many of the sites are continuing as part of a long-term network for the National Park Service.) Two 72 h samples were collected each week using Aerovironment SFSSOO two-staged samples. Particles between 2.5 pm and 15 pm were collected on coated 8 pm nuclepore filters, while particles smaller than 2.5 pm passed through and were collected on a fine filter. The samples were analyzed for sodium through lead using our cyclotron-based automated PIXE system. PIXE was essential in the operation of the network because it was able to analyze the large numbers of generated samples quickly, reliably and economically. The sulfur measured by PIXE may be assumed to be IV C. AEROSOL APPLICATIONS