A XANES study of chromium and iron speciation in laboratory-generated and ambient particulate matter 2.5

The overall goal of this study was to use X-ray Absorption Near-Edge Structure (XANES) in order to speciate Cr and Fe in laboratory generated combustion particles and in the ambient fine particles. Our initial goal was to investigate the potential impacts of atmospheric aging on the speciation of chromium-containing particles. 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. 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.; In the second part of the study, we wanted to determine the speciation of Cr in ambient fine particles. Using micro-focused XANES, we analyzed the chromium speciation in fine particles (diameters ≤2.5 mum) collected at three sites in northern California: Placerville, Davis, and Sacramento. At all of the sites the main Cr species were Cr(III), with Cr(OH) 3 or a chromium-iron, chromite-like phase, termed Cr-Fe here, being the dominant species. Cr(VI)-containing particles were found only in Sacramento. All three sites contained some reduced Cr species, either Cr(0) or Cr 3C2, although these were minor components.; Finally, in the last part of this study we explored the effects of flame temperature and composition on the Cr speciation of the particles and determine whether the Fe speciation is affected by the aging process. In fresh particles we typically saw 2 to 3 times less Cr(VI) in samples that contained Cr and Fe compared to samples that only contained Cr. Particles that contained both Cr and Fe made in the coolest flame still had detectable Cr(VI) while the sample without Fe had no detectable Cr(VI). We also found the Cr-Fe species was the dominant chromium species at all flame temperatures when iron was added to the flame. Between 4 and 24 hours there was a 20% decrease in the maghemite composition of the particles, indicating Fe(III) reduction.