Research Direct Identification of Trace Metals in Fine and Ultrafine Particles in the Detroit Urban Atmosphere SATOSHI UTSUNOMIYA, § KELD A. JENSEN, † GERALD J. KEELER, ‡ AND RODNEY C. EWING* ,§ Departm ent of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063, National Institute of Occupational Health, Lersø Parkalle ´ 105, DK-2100 Copenhagen, Denmark, and Department of Environmental Health Sciences, School of Public Health, University of Michigan, 109 Observatory Street, 2518 SPH I, Ann Arbor, Michigan 48109-2029 Exposure to airborne particulates containing low con- centrations of heavy metals, such as Pb, As, and Se, may have serious health effects. However, little is known about the speciation and particle size of these airborne metals. Fine- and ultrafine particles with heavy metals in aerosol samples from the Detroit urban area, Michigan, were examined in detail to investigate metal concentrations and speciation. The characterization of individual particles was completed using high-angle annular dark-field scanning transmission electron microscopy (HAADF- STEM) combined with conventional high-resolution TEM techniques. The trace elements, Pb, As, La, Ce, Sr, Zn, Cr, Se, Sn, Y, Zr, Au, and Ag, were detected, and the elemental distributions were mapped in situ at the nanoscale. The crystal structures of the particles containing Pb, Sr, Zn, and Au were determined from their electron diffraction patterns. Based on the characterization of the representative trace element particles, the potential health effects are discussed. Most of the trace element particles detected in this study were within a range of 0.01-1.0 μm in size, which has the longest atmospheric residence time (∼100 days). Increased chemical reactivity owing to the size of nanoparticles may be expected for most of the trace metal particles observed. Introduction Epidemiological studies have reported an increase in res- piratory symptoms, hospitalization, and cardiovascular diseases in humans with an increase in particulate atmo- spheric air pollution (1-4). Long-term exposure to certain trace metals (“air toxics”)can also result in pathogenic effects rangingfrom learningdisabilities,development ofrespiratory inflammation to cancer, and damage of vital organs (5-10). Several particle-related parameters (e.g., particle size, chemical composition, and solubility) have been found to play an important role in the development adverse health effects from air pollution. For example, in vivo experiments showed a ∼10-fold increase in inflammation after inhalation ofthe same mass-dose ofultrafine (e100 nm) carbon black, TiO2 and latex particles as compared to the effect from particles larger than 200nm (11, 12).The increase in biological response induced by these insoluble compounds may be linked to specific surface parameters (13). Several studies have also shown that the composition of the particles and their solubility plays an important, but still unclarified, role on the induction of inflammatory proteins and cytoxicity (14-16). Specifically, the effects of soluble polyvalent transi- tion metals have received much attention, because through redoxreactionsand formation offree radicals,theycan induce oxidative stress at the subcellular level(17-20).However,an in vitro study using nontoxic and partially soluble particles (e.g., Mg-, Ca-, and Ba-sulfates) suggests that the presence of particles alone plays an important role, because these phasesinduce a systematicinflammatoryresponse beginning at concentrations around their solubility-limit in the cell media (16). Owingto their toxicityor carcinogenecity,adverse health effects are also associated with the exposure to certain trace and heavy metals in air pollution (9, 10, 21-23). Currently, 10 metalcompounds are listed among the 188 hazardous air pollutant substances (“HAPS” or “air toxics”) defined under the Clean Air Act Amendments of 1990. These metal substances include compounds of As (0.03 μg/ m 3 ), Be (0.02 μg/ m 3 ), Cd (0.02 μg/ m 3 ), Cr 6+ (0.008-0.1 μg/ m 3 ), Co (0.1 μg/ m 3 ), Pb (1.5 μg/ m 3 ), Mn (0.05 μg/ m 3 ), Hg (0.09 μg/ m 3 ) Ni (0.1-0.2 μg/ m 3 ), and Se (0.08-20.0 μg/ m 3 ) along with the chemically complex diesel engine emissions (5 μg/ m 3 ). Th e concentrations in parentheses are Inhalation reference Concentrations (RfC) compiled by the U.S. EPAOffice of Air Quality Planning and Standards (24). Emission ofheavyand trace elementsinto the atmosphere can also affect the environment; either directly by their ecological toxicity or indirectly through bio- or geochemical accumulation that potentially can result in oral exposure through the food web (25, 26).However,in contrast to human health effects from inhaled particles and air toxics, only soluble compoundscan be ecotoxic,because onlythe soluble fraction readily enters to the hydrobiochemical cycle (9, 25, 27, 28). Hence,to assess their potentialimpact on human health, as well as their ecotoxicology and dispersion into the environment, it is important to identify the speciation of air toxic compounds and their specific physicochemical pa- rameters. If toxic trace elements are homogeneously dis- persed as impurities in insoluble larger-size particles, risks to the health and environment are less than if they occur as major constituents in individual, trace-metal, nanoscale particles. However, if the heavy metals occur in ultrafine particles, the size will influence their reactivity, toxicity, and their fate in the ambient environment (29, 30). Ultrafine particles are known to have increased solubility,as compared to larger-size particles of the same composition because of the increased surface-to-volume ratio for smaller particle sizes (30). Previous analyses have shown that trace elements in airborneparticulatesbymassaremostabundantin the0.5-1 μm-size fraction (31). However, the bulk composition of particles in urban areas varies depending on the local emission sources. Ultrafine particles collected in Southern *Correspondingauthor phone: (734)647-8529;fax: (734)647-8531; e-mail: rodewing@umich.edu. § Geological Sciences, University of Michigan. † National Institute of Occupational Health. ‡ Department of Environmental Health Sciences, University of Michigan. 10.1021/es035010p CCC: $27.50  2004 American Chemical Society VOL. 38, NO. 8, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2289 Published on Web 03/17/2004