Comparisons Between Size-Segregated Resuspended Soil Samples and Ambient Aerosols in the Western United States

Abstract

Soil derived particles generally form the major fraction of aerosol mass measured in remote, arid sites in the western United States, based upon results from the 40 station EPA/UCD Western Fine Particle monitoring network. Details of the mechanisms that cause the observed soil mass, size profiles, and chemistry are complicated by the strong dependence of composition on soil particle size. Changes of an order of magnitude or more can be found in the elemental ratios of gross constituents as a function of size at a single site. Site to site differences can be equally large. In order to aid in the interpretation of aerosol data, and especially distinguish between soils and fly ash, over 100 soil samples were taken at or near aerosol collection sites in 8 mountain states, chosen so as to represent likely sources for wind blown soil near the samplers. Each sample was seived into 5 fractions under dry conditions. The finest fraction was then resuspended in an air stream and injected into a 5 stage Lundgren rotating drum impactor with cut points at 25,8,3, and 0.8 microns equivalent aerodynamic diameter. Each stage was analyzed by particle induced x-ray emission (PIXE) for elements sodium and heavier. Coarse fractions were poorly correlated with the local aerosol mass, and bulk analyses were widely different from the aerosols and the fine particle modes in the resuspended soils. Relatively minor chemical shifts were seen in the 2.5 to 15 micron size range, but finer aerosols and soils differ strongly from crustal averages.