Characterization of ambient airborne particles by energy-filtered transmission electron microscopy

This article demonstrates that energy-filtered transmission electron microscopy (EFTEM) can be a valuable tool for the identification and analysis of nanometer- sized airborne particles. Ambient airborne particulate matter (PM) samples collected at Lexington, Kentucky were investigated using EFTEM in combination with electron energy-loss spectroscopy (EELS), energy-dispersive Xray spectroscopy (EDS), selected area electron diffraction (SAED), and high- resolution transmission electron microscopy (HRTEM). EFTEM provides a fast and convenient way to map the distribution of carbonaceous aggregates, which constitute a significant fraction of the samples. Sulfur-bearing particles are common in the submicron inorganic fraction. Sulfur elemental maps were obtained from some sulfur- bearing particles, and the oxidation state of sulfur was revealed to be primarily S(VI) as sulfate from the electron energy-loss near-edge fine structures of the sulfur L-edge. Other types of inorganic particles, such as silicon-bearing particles and transition metal oxides, were also observed in the samples and their chemical composition, crystalline phase, and microstructures were studied by using the techniques listed above. The use of EFTEM for the study of airborne PM is in its infancy, but it will undoubtedly gain increased attention for the characterization of individual particles of environmental concern, especially those that contain transition metal compounds and ultrafine carbonaceous particles, which may both have adverse effects on human health.