ARTICLE pubs.acs.org/est
Chemical Speciation of Vanadium in Particulate Matter Emitted from Diesel Vehicles and Urban Atmospheric Aerosols Martin M. Shafer,*,† Brandy M. Toner,‡ Joel T. Overdier,† James J. Schauer,† Sirine C. Fakra,§ Shaohua Hu,|| Jorn D. Herner,|| and Alberto Ayala|| †
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Environmental Chemistry and Technology Program, University of Wisconsin—Madison, 660 North Park Street, Madison, Wisconsin 53706, United States ‡ Department of Soil, Water, and Climate, University of Minnesota Twin Cities, 1991 Upper Buford Circle, St. Paul, Minnesota 55108, United States § Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States Research Division, California Air Resources Board, P.O. Box 2815, Sacramento, California 95812, United States
bS Supporting Information ABSTRACT: We report on the development and application of an integrated set of analytical tools that enable accurate measurement of total, extractable, and, importantly, the oxidation state of vanadium in sub-milligram masses of environmental aerosols and solids. Through rigorous control of blanks, application of magnetic-sector-ICPMS, and miniaturization of the extraction/separation methods we have substantially improved upon published quantification limits. The study focused on the application of these methods to particulate matter (PM) emissions from diesel vehicles, both in baseline configuration without after-treatment and also equipped with advanced PM and NOx emission controls. Particle size-resolved vanadium speciation data were obtained from dynamometer samples containing total vanadium pools of only 0.2 2 ng and provide some of the first measurements of the oxidation state of vanadium in diesel vehicle PM emissions. The emission rates and the measured fraction of V(V) in PM from diesel engines running without exhaust after-treatment were both low (2 3 ng/mile and 13 16%, respectively). The V(IV) species was measured as the dominant vanadium species in diesel PM emissions. A significantly greater fraction of V(V) (76%) was measured in PM from the engine fitted with a prototype vanadium-based selective catalytic reductors (V-SCR) retrofit. The emission rate of V(V) determined for the V-SCR equipped vehicle (103 ng/mile) was 40-fold greater than that from the baseline vehicle. A clear contrast between the PM size-distributions of V(V) and V(IV) emissions was apparent, with the V(V) distribution characterized by a major single mode in the ultrafine (2.5 μm (coarse mode), 1.0 2.5 μm, 0.5 1.0 μm, and 0.25 0.5 μm (accumulation modes, or fine), of the PCIS were loaded with 25 mm, 2 μm, Teflon filters (Pall Science), while 37 mm, 2 μm filters (Pall Science) were employed in the pseudo-ultrafine stage (95% (n = 24), and mass balances were, within error, quantitative. For the engine emission PM extracts, mass balances ranged from 92% to 126% (mean = 106%, n = 34), and column retention (i.e., V(V) + V(IV)) from 71% to 98%. Precision of the Chelex oxidation state speciation method was assessed from measurements of replicate standards, and replicate samples were used to evaluate the overall (sample extraction and Chelex processing) method precision. For defined standards (1 30 ng total V) precision was typically in the 5 15% RSD range (Figure S2) and in the range 2 20% RSD for environmental samples and reference materials (0.2 11 ng V; Figure S4), though the uncertainty of trace components can be much larger. These precision metrics are comparable to that reported by Wang and Sa~nudo-Wilhelmy,21 though in this study we are working at concentrations an order of magnitude lower. The high sensitivity of the method and performance with sub-nanogram (
