Aerosol Science and Technology, 40:545–556, 2006 c American Association for Aerosol Research Copyright ISSN: 0278-6826 print / 1521-7388 online DOI: 10.1080/02786820600714353
Laboratory Study of Simulated Atmospheric Transformations of Chromium in Ultrafine Combustion Aerosol Particles Michelle Werner,1 Peter Nico,2 Bing Guo,3 Ian Kennedy,3 and Cort Anastasio1 1
Department of Land, Air & Water Resources, University of California, Davis, California, USA Earth Sciences Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California, USA 3 Department of Mechanical and Aeronautical Engineering, University of California, Davis, California, USA 2
While atmospheric particles can have adverse health effects, the reasons for this toxicity are largely unclear. One possible reason is that the particles can contain toxic metals such as chromium. Chromium exists in the environment in two major oxidation states: III, which is an essential nutrient, and VI, which is highly toxic and carcinogenic. Currently little is known about the speciation of chromium in airborne particles or how this speciation is altered by atmospheric reactions. To investigate the potential impacts of atmospheric aging on the speciation and toxicity of chromiumcontaining particles, we collected chromium and chromium-iron combustion ultrafine particles on Teflon filters and exposed the particles to a combination of light, ozone, water vapor, and, in
some cases, basic or acidic conditions. After the aging process, the aged and not-aged samples were analyzed for Cr oxidation state using X-ray Absorption Near Edge Spectroscopy (XANES). We found that the aging process reduced Cr(VI) by as much as 20% in chromium particles that had high initial Cr(VI)/Cr(total) ratios. This reduction of Cr(VI) to Cr(III) appears to be due to reactions primarily with light and hydroperoxyl radical (HO2 ) in the chamber. Particles that had low initial Cr(VI)/Cr(total) ratios experienced no significant change in Cr oxidation states after aging. Compared to particles containing only Cr, the addition of Fe to the flame decreased the Cr(VI)/Cr(total) ratio in fresh Cr-Fe particles by ∼60%. Aging of these Cr-Fe particles had no additional effects on the Cr(VI)/Cr(total) ratio.
Received 2 September 2005; accepted 23 March 2006. We thank Dr. Matthew Newville of the GSECARS at the Advanced Photon Source (APS), Argonne National Laboratory, for his assistance. The majority to the X-ray data presented above were collected at GeoSoilEnviroCARS (Sector 13), which is supported by the National Science Foundation—Earth Sciences (EAR-0217473), Department of Energy—Geosciences (DE-FG02-94ER14466) and the State of Illinois. Use of the APS was supported by the U.S. Department of Energy under Contract No. W-31-109-Eng-38. We would also like to thank Dr. Matthew Marcus of beamline 10.3.2 of the Advanced Light Source (Lawrence Berkeley National Laboratory), which is supported by the US Department of Energy (DE-AC02-05CH11231). Portions of this research were also carried out at the Stanford Synchrotron Radiation Laboratory, operated by Stanford University on behalf of the U.S. Department of Energy. Finally, we thank David Paige (Paige Instruments) for constructing the ozone generator, John Newberg and Mike JimenezCruz for building the ozone dilution and delivery system, and Michelle Gras of the UC Davis Interdisciplinary Center for Plasma Mass Spectrometry for conducting the ICP-MS analyses. This work was supported by grant number 5 P42 ES04699 from the National Institute of Environmental Health Sciences (NIEHS) of the NIH, and by a fellowship from the UC Davis NEAT-IGERT program to Michelle Werner (NSF IGERT Grant # 9972741). Partial support was also provided by the U.S. Department of Energy under contract number DE-AC02-05CH11231. The contents of this paper are solely the responsibility of the authors and do not necessarily represent the official views of the NIEHS, NIH. Address correspondence to Cort Anastasio, One Shields Avenue, Department of Land, Air & Water Resources, University of California, Davis, CA 95616. E-mail:
[email protected]
INTRODUCTION Understanding the concentrations and composition of particulate matter (PM) in the atmosphere has become a focus for research because fine particles can be inhaled deeply into the lungs and cause a variety of negative health affects, such as bronchial irritation, reduced lung function, respiratory disease, cancer, and mortality (Pope et al. 1995; Schlesinger 2000). On a mass basis, ultrafine PM (particles with aerodynamic equivalent diameters ≤0.1 µm) can have more adverse health effects than larger particles of the same composition (Ferin et al. 1990; Oberdorster et al. 1992). Ferin et al. propose that this is because the smaller particles can penetrate the interstitial space of the lungs and because the large number of particles being deposited can overwhelm lung defenses, such as removal by alveolar macrophages. While ultrafine particles can be present at very high number concentrations, they make up only a small portion of the total particle mass in the atmosphere (Anastasio and Martin 2001). Though their high number concentrations might influence the toxicity of ultrafine particles, their composition, specifically their transition metal content, can also influence toxicity. For example, it has been observed that the transition metal component of particles is associated with acute lung injury (Dreher et al. 1997). The initial composition of “fresh” ultrafine particles is determined by the PM sources, which include fossil fuel 545
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combustion, incineration, and other human activities (Anastasio and Martin 2001). In addition, the composition of ultrafine PM will be altered by atmospheric aging involving oxidationreduction reactions as well as the condensation of secondary species such as organics, nitrate, and sulfate (Finlayson-Pitts and Pitts 2000). Because ultrafine particles can have relatively long atmospheric lifetimes (between 1–10 days for particles between 10–100 nm (Brasseur et al. 1999); they can be transported over long distances and undergo extensive atmospheric reactions. Chromium is a transition metal of particular interest in PM because its toxicity varies based on oxidation state. Chromium exists in two major oxidation states: +3, which is an essential nutrient, and +6, which is highly toxic and carcinogenic (Cieslak-Golonka 1996). It has been proposed that Cr(VI) is toxic because it can penetrate cells, be reduced to Cr(III), and then generate reactive oxygen species, such as hydroxyl radical, that can cause DNA damage (Cohen et al. 1993). The highest exposures to Cr(VI)-containing particulate matter likely occurs to workers during welding, chrome plating, spray painting, and chrome pigment productions (Cohen et al. 1993). However, a study in Los Angeles showed that ambient fine particles can also contain significant amounts of chromium and that the amount of Cr (relative to the total mass of metals) is greater in the ultrafine particles than that in the fine particles (Hughes et al. 1998). In that study chromium accounted for up to ∼10% of the transition metal mass in ultrafine particles (diameters
