Detecting Metals in Ambient Particulate Matter: X-Ray Fluorescence Analysis of High Volume Impaction Deposits

The design, construction, and feasibility testing of X-ray fluorescence (XRF) based instrumentation capable of detecting the presence of atomic species within ambient particulate matter (PM) on a near real-time basis has been completed successfully. The XRF analysis is made feasible for hourly ambient PM samples through the implementation of novel high-volume flow impaction and post-impaction concentration techniques. Rupprecht & Patashnick Co., Inc., conducted a preliminary study of the first-generation base system’s level of detection (LOD) for various elemental species (i.e., Cr, Mn, Ni, Zn, Cd, Ba, Hg, and Pb). The first-generation base system proved capable of measuring ambient PM trace element composition at or near the level expected in urban air samples for 1-hour duration samples. Only 10 percent of possible X-ray exposure time was utilized in the measurement process. The measurement system will benefit from further reduction of element-specific LODs through increased exposure time, increased collection time, improved collection techniques, primary X-ray filtering, and/or employing the use of variable excitation energy to the X-ray generation tube when required. A study of ambient PM collected in the Albany, NY, area during the summer of 2004 showed that the first-generation system was capable of detecting Fe, Ca, and S in 1-hour duration samples collected during very low ambient PM2.5 conditions (< 100 µg/m3). Additionally, several signal-to-noise improvement techniques were investigated in Phase I. The ultimate goal of the Phase II research project is to produce a “beta” instrument capable of providing reliable, quantitative measurements of ambient PM elemental composition on an hourly basis. The system is integrated in a package whose size, weight, and power consumption compare favorably with continuous gaseous and particle instrumentation currently deployed in air quality monitoring networks. The initial commercial use of the new monitor in the United States and internationally is projected to be for time-resolved ambient PM atomic speciation measurements by air monitoring agencies in urban areas, at locations affected by significant point sources, along traffic corridors, and by scientists for epidemiological studies and receptor modeling. State and local air monitoring agencies will require detailed information on PM and its sources to determine the best means of achieving U.S. Environmental Protection Agency-mandated state implementation plans to pinpoint the sources of pollution through “fingerprinting” at hot spots. The monitor also can be used to gauge the before-and-after effect of changes in regulations such as the use of new fuels or after-treatment devices in mobile sources and new urban or industrial development.