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Study finds light-duty gasoline vehicles responsible for about half of PM2.5 mass, 14% of PM10 in road tunnel
24 December 2013
|Contributions of various primary sources to tunnel PM mass. Credit: ACS, Bozlaker et al. Click to enlarge.|
Researchers led by a team from the University of Houston have characterized the platinum group elemental composition of PM2.5 and PM10 emissions—mainly from gasoline-fueled light-duty vehicles (LDVs)—in the Washburn Tunnel in Houston Texas as a mechanism for better determining the contribution of LDVs to particulate emissions.
In a paper in the ACS journal Environmental Science & Technology, they noted that previous investigations of the metals content in airborne tunnel PM have focused on non-platinum group elements. However, because these elements are also emitted by many other natural and anthropogenic sources, isolating the vehicular sources in urban environments is complicated. By quantifying Rh, Pd, and Pt in addition to these elements, they can facilitate more accurate estimates of LDVs’ contributions to ambient PM—the presence of these elements in tailpipe emissions is attributed to the three-way catalytic converters commonly used in gasoline-driven automobile emission control systems.
The objectives of this research are to (1) comprehensively characterize wide suite of elements in PM2.5 and PM10 emitted from LDVs traversing a tunnel with emphasis on Rh, Pd, and Pt, (2) develop a novel LDV source profile by including PGEs [platinum group elements] as unique markers, and (3) quantitatively apportion tailpipe and nontailpipe LDV emissions to airborne PM mass measured in the tunnel. Samples were collected over extended durations (3−4 weeks) in the only operational vehicular tunnel in Texas, namely the Washburn Tunnel, to obtain sufficient PM mass in order to accurately quantify numerous metals including PGEs at trace-levels using our newly developed analytical method based on dynamic reaction cell-quadrupole-inductively coupled plasma-mass spectrometry (DRC-q-ICP-MS).
… We improve upon earlier U.S. studies by reporting all three Pd-group PGEs (i.e., Rh, Pd, and Pt) emanating from an on-road fleet, nearly eliminating contributions from diesel-driven vehicles, providing data for fine and coarse size fractions, and updating elemental concentrations associated with United States LDVs for the first time in ∼13- years.—Bozlaker et al.
They collected two sets of time integrated PM samples both inside the tunnel as well as from the tunnel ventilation air supply to derive accurate LDV source profiles incorporating three platinum group elements.
Among their findings:
Average Rh, Pd, and Pt concentrations from the tunnel ventilation air supply were 1.5, 11.1, and 4.5 pgm–3 in PM2.5 and 3.8, 23.1, and 15.1 pgm–3 in PM10, respectively.
Rh, Pd, and Pt levels were elevated inside the Washburn Tunnel reaching 12.5, 91.1, and 30.1 pgm–3 in PM2.5 and 36.3, 214, and 61.1 pgm–3 in PM10, respectively.
Significantly higher enrichment factors of Cu, Zr, Rh, Pd, Sb, and Pt (referenced to Ti in the upper continental crust) inside the tunnel compared with the ventilation air supply suggested that they are unique elemental tracers of PM derived from gasoline-driven LDVs.
Using the emission profile based on PGEs and ambient quantification, mass balancing revealed that approximately half the fine PM mass in the tunnel could be attributed to tailpipe emissions; approximately one-quarter to road dust; and smaller contributions from brake (7%) and tire (3%) wear.
On the other hand, PM10 mostly originated from resuspended road dust (50%), with progressively lower contributions from tailpipe emissions (14%), brake wear (9%), and tire wear (2%).
Note that our samples only covered the months of November and December. More extensive sampling may be necessary to remove any potential bias arising from seasonal variations in operational/ambient temperature. Of particular note is the suitability of the PGEs as conserved tracers of LDV tailpipe emissions. The predominant role of LDV emissions in enriching atmospheric (and terrestrial surface environmental concentrations) of Rh, Pd, and Pt was elucidated by demonstrating that simultaneous three-component PGE variations are fundamentally different from the UCC but similar to United States autocatalysts. We plan to focus on near-highway sampling in the near future to quantify LDV contributions to fine and coarse PM fractions using the newly developed source profiles to better understand anthropogenic influences on PGE cycling, human exposure, and any associated health/environmental risks.—Bozlaker et al.
Ayşe Bozlaker, Nicholas J. Spada, Matthew P. Fraser, and Shankararaman Chellam (2013) “Elemental Characterization of PM2.5 and PM10 Emitted from Light Duty Vehicles in the Washburn Tunnel of Houston, Texas: Release of Rhodium, Palladium, And Platinum.” Environmental Science & Technology doi: 10.1021/es4031003
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