HR: 0830h
AN: A21E-1021    [PDF]
TI: Atmospheric Transformations of Chromium Species on Aerosol Nanoparticles
AU: * Werner, M
EM: mlwerner@ucdavis.edu
AF: University of California, Davis Department of Land, Air, and Water Resources, 1 Shields Ave, Davis, CA 95616 United States
AU: Nico, P
EM: psnico@stanford.edu
AF: California State University, Stanislaus, 801 W. Monte Vista Ave., Turlock, CA 95382 United States
AU: Guo, B
EM: bguo@ucdavis.edu
AF: University of California, Davis Department of Mechanical and Aeronautical Engineering, 1 Shields Ave, Davis, CA 95616 United States
AU: Kennedy, I
EM: imkennedy@ucdavis.edu
AF: University of California, Davis Department of Mechanical and Aeronautical Engineering, 1 Shields Ave, Davis, CA 95616 United States
AU: Anastasio, C
EM: canastasio@ucdavis.edu
AF: University of California, Davis Department of Land, Air, and Water Resources, 1 Shields Ave, Davis, CA 95616 United States
AB: While nanoparticles can have adverse health effects, the reasons for this toxicity are unclear. One possible reason is that the particles can contain toxic metals such as chromium. Measurements of ambient aerosols in Los Angeles have shown that as particle size decreases, the concentration of chromium increases; chromium (Cr) accounts for up to 10% of the mass of the smallest diameter particles. Chromium exists in two major oxidation states: +3, which is an essential nutrient, and +6, which is highly toxic and carcinogenic. Currently little is known about what happens to the Cr(III)/Cr(VI) ratio in chromium nanoparticles during atmospheric transport. Because the atmosphere is oxidizing in nature, one might think that oxidation of Cr(III) to Cr(VI) would occur in the troposphere. However, there are many other chemical species in aerosol particles which could reduce Cr(VI) to Cr(III). Understanding whether these changes occur in the atmosphere is important because they could alter the toxicity of the particulate matter. The goal of this project is to determine how atmospheric aging of particles affects Cr speciation. To investigate this issue, we collected chromium and chromium/iron particles on Teflon filters from a combustion flame fed with hydrogen, argon, and Cr(CO)5 with and without a source of iron. The samples were cut in half and placed in a solar simulation chamber where they were exposed to sunlight, 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). In particles that had high initial Cr(VI)/Cr(total) ratios, the aging process reduced Cr(VI) by 20%. The Cr(VI)/Cr(total) ratio in fresh particles was reduced by 60% when Fe was added to the flame. Aging of these Cr/Fe particles resulted in an additional 60% reduction in the Cr(VI)/Cr(total) ratio. Particles that had low initial Cr(VI)/Cr(total) ratios experienced no significant change in Cr oxidation states after aging. Although our conditions are simplified relative to the ambient atmosphere, our results suggest that Cr(VI) in ambient nanoparticles is reduced to Cr(III) by atmospheric reactions. In any case, our experiments reveal that atmospheric aging can alter the composition, and therefore the toxicity, of metal-containing aerosol particles.
DE: 0305 Aerosols and particles (0345, 4801)
DE: 0341 Middle atmosphere--constituent transport and chemistry (3334)
DE: 0345 Pollution--urban and regional (0305)
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting