used by a wide array of organizations, including universities and research centers, government (for example, the Chemical Speciation Network), and industry. All methods used are state-of-the-art methods and have adequate sensitivity for the types of samples collected.

Gravimetry

The mass on the filters was measured with gravimetry. Using standard methods (40 CFR 50 [2010]), the blank filters were equilibrated at 23ºC and 35% relative humidity for 24 hours and then weighed before they were sent to the field. After sample collection, the filters were sent back to the laboratory for a second equilibration and reweighing. Airborne-mass concentration was calculated as the difference between filter weights divided by the volume of air sampled.

Although there were limitations in the sampling methods, as described in Chapter 2, there are no questions about these laboratory measurements because the weighing can be highly precise and accurate. However, there is concern about the potential loss of PM from the filters during shipping. The measurements showed PM concentrations high enough to suggest sampler overload, as discussed in Chapter 2.

X-Ray Fluorescence

EDXRF was used to measure 40 chemical elements nominally: sodium, magnesium, aluminum, silicon, phosphorus, sulfur, chlorine, potassium, calcium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, arsenic, selenium, bromine, rubidium, strontium, yttrium, zirconium, molybdenum, palladium, silver, cadmium, indium, tin, antimony, barium, gold, mercury, thallium, lead, lanthanum, and uranium. For samples of airborne particles, x-ray fluorescence (XRF) was operated by using the thin-sample approach in which the sample is assumed to be thin relative to the range of the excitation or emission x rays in matter. For low-atomic-number elements or heavy elements in which outer-shell x rays are used for analysis, there can be attenuation of the emitted x rays, and particle-size-correction factors are often applied to compensate for absorption of the x rays in the sample. In the analysis of the EPMSP samples, however, no particle-size corrections were applied, and low-atomic-number elements are probably substantially underestimated.

The mass concentrations observed in the EPMSP are much higher than those currently measured in the United States. However, they are comparable with values observed 30-40 years ago when studies of airborne-particle composition began to use XRF as an analytic tool (Jaklevic et al. 1981). The MiniVol sampler uses a 47-mm filter that permits a reasonable mass loading while providing a uniform, thin film sample that is suitable for analysis. However, when



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