TRANSMISSION ELECTRON MICROSCOPE ANALYSIS OF TRACE ELEMENTS IN INDIVIDUAL AIR PARTICULATES FROM THE CENTRAL VALLEY OF CA: A PRELIMINARY ASSESSMENT OF POTENTIAL IMPACTS OF AIR PARTICULATES ON HUMAN HEALTH

The soils of Central California have high levels of trace metals, especially Cr and Ni sourced from ultramafics rocks in the foothills of the Sierra Nevada and the Coast Range mountains. Identifying the trace element sequestration in soils in the <2μm fraction is easily done for individual particles using the Analytical Transmission Electron Microscope (AEM) and the trace metals are usually associated with only a small fraction of the soils but at relatively high concentrations. With high levels of trace elements in the soils, windborne dust in the Central Valley is also likely to contain high concentrations of trace metals which could have serious health effects if respired. Quantification of trace element sequestration in air particles collected from Central California have proven to be considerably more difficult for the following reasons: 1. Metal contents on air filters are considerably lower than in the bulk soils; 2. The trace metals are sequestered in a large variety of different particle types but are generally in lower concentrations in individual particles 3. There are considerable non-geogenic particulates in the air samples and 4. There is a paucity of trace-element standards with well characterized trace element concentrations in the low ppm range that are homogenous at the nanometer scale typical of AEM analyses. Detection limits on the TEM for some individual air particles are as low as 10ppm and at such low levels of detection a wide variety of trace metals have been analyzed in the air particulates including Cr, Ni, As, Zn, and Pb.

Initial TEM analyses show that trace metals in individual particles from air samples are considerably more diverse than those in soils, and are sequestered in a host of different particulate fractions including carbonaceous combustion products, sulfates, silicates, carbonates, Fe-Ti oxides and nitrates. Mineral particulates are more common in samples from summer air filters while combustion products make up approximately 50% of the particulates collected from urban sites during the winter months. The highest concentrations of trace metals are associated with the smallest nanoparticulates. The small size (5nm-100nm) of the metal rich particulates and their subsequent chemical reactivity make them of special concern for human inhalation.