Particulate monitoring at US National Parks using PIXE

Abstract

In order to help the United States National Park Service understand the causes of visibility impairment at various national parks throughout the United States, the Crocker Nuclear Laboratory at the University of California, Davis, has been collecting samples of coarse and fine particles at 35 sites continuously since 1982. At these remote sites most visibility impairment is due to the scattering of light by fine particles, between 0.2 and 1 μm. We are responsible for all aspects of the monitoring program, including designing and maintaining the particulate samplers, handling and analyzing the filters, and presenting and interpreting the results. A successful monitoring program requires strict adherence to carefully prepared procedures at all stages, with numerous quality assurance checks. All the samples are analyzed for mass by microbalance and for elemental composition by PIXE. PIXE is well-suited to particulate monitoring in terms of accuracy, cost, range of elements, and spatial resolution. The concentration of fine hydrogen is measured by proton elastic scattering operated simultaneously with the PIXE analysis. We also measure the optical absorption by fine soot particles using the integrating plate method. Because the sites arc remote and the sampling is almost continuous, most of the measured variables show well-defined regional patterns when averaged over the seasons of the year. The wide range of elements permits a good characterization of the fine particles. There are wide differences in the overall composition for different regions. In the eastern US the fine mass is dominated by ammonium sulfate. In some regions of the western US, fine soil and organics are important components to the aerosol. For visibility, fine sulphur is the most single important element measureable by PIXE. For most regions sulfur is largest in summer, although in some regions the maxima are in spring or fall. The average sulfur concentration varies widely over the US. with values in the east ten times those in some regions of the west. The concentrations of other variables, such as potassium, lead, zinc, copper, hydrogen, silicon are also important in understanding the composition of fine particles.