TO: Texas Air Research Center FROM: Shankar Chellam, University of Houston (currently at Texas A&M) SUBJECT: Final Report PROJECT NUMBER: 413UHH0143A PROJECT TITLE: Detailed elemental characterization of Saharan dust to quantify its contributions to PM2.5 and PM10 during episodic intrusions in Houston PROJECT PERIOD: September 1, 2013 – July 15, 2015 DATE: November 7, 2015 BACKGROUND The arid regions of North Africa are the world’s largest contributor of dust, estimated to emit about 800 Tgy-1 of soil dust annually. This North African particulate matter (PM) is transported across the Atlantic Ocean to the southern continental United States increasing the ambient PM10 and PM2.5 burden in Texas typically during the summer and early fall seasons. It is emphasized that isolating air pollutant contributions is difficult in complex urban environments. Also, the ability of detailed chemical speciation to differentiate sources is challenged because of the existence of myriad local sources. Another important source in Texas is primary emissions from motor vehicles. According to the United States Department of Transportation’s Federal Highway Administration, the State of Texas ranks second only to California in terms of vehicle miles traveled, with nearly 19 billion miles driven in November 2011 alone. Realistic estimates of tailpipe and non-tailpipe particulate emissions under actual on-road conditions incorporating several real-world sources of variability including maintenance histories, vehicle age, engine size and type, driving habits, and season can be obtained from measurements in roadway tunnels. We are interested in developing methods to quantify long-range transported PM as well as those locally emitted from motor vehicles. This report briefly summarizes our findings and the reader is referred to our peer- reviewed publications listed at the end for more detailed information. OBJECTIVES The objectives of the research completed during this period were to: Accurately measure major and trace elements in different mineral dust sources and in ambient PM in receptor locations to quantitatively estimate contributions of long-range dust and locally entrained crustal material and local anthropogenic sources. Comprehensively characterize wide suite of elements in PM2.5 and PM10 emitted from light duty vehicles (LDVs) traversing a tunnel with emphasis on Rh, Pd, and Pt and develop a novel LDV source profile by including platinum group elements. METHODOLOGY For accurate source apportionment, we developed a representative source profile of aerosols originating in North Africa by collecting samples in a remote location in the Caribbean (Barbados) during dust episodes before they entered the continental United States. Daily PM2.5 and PM10 were also collected at Clinton Drive and Channelview using R&P 2025 samplers. African dust passing over Barbados were collected at Ragged Point, which is a field station on the east coast of Barbados. Two sets of PM2.5 and PM10 samples were also collected from inside the Washburn Tunnel. PM samples were acid-digested in two steps in Teflon-lined vessels (HP-500 Plus) using a microwave (MARS 5, CEM Corp.). Fifteen main group elements (Na, Mg, Al, Si, K, Ca, Ga, As, Se, Rb, Sr, Sn, Sb, Ba, Pb), 14 transition metals (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, Cd, W) and 14 lanthanoids (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were quantified by dynamic reaction cell – quadrupole – ICP-MS (ELAN DRCII, PerkinElmer). For platinum group metals, aqua regia was used first before the above-described method was implemented. RESULTS AND DISCUSSION Figure 1. Back trajectories demonstrating the North African origin of dust during July 25, 26, and 27 in Houston. Several lines of evidence pointed that the air mass originated in North Africa and moved across the tropical Atlantic and entered the Western Gulf of Mexico, impacting air quality over the Houston area on 25-27 July, 2008. Figure 1 is one such example that depicts NOAA HYSPLIT back trajectories. Electron micrographs and energy dispersive X-ray spectroscopy of aerosol samples from Barbados collected during Saharan dust outbreaks revealed similar morphology, size distribution, and elemental characteristics to long-range transported North African dust collected in Europe. Figure 2. Representative energy dispersive X‐ray spectrum of Saharan dust. In this project, we demonstrated that chemical mass balancing can successfully isolate, differentiate and quantify the relative contributions from local and global mineral dust sources through detailed measurements of a wide suite of elements in ambient PM. We identified a major dust storm originating in Northwest Africa in mid-July 2008 which eventually impacted air quality in Houston during July 25, 26, and 27, 2008. Daily PM2.5 and PM10 samples were collected at two sites in Houston over a 2-week period encompassing the Saharan dust episode to quantify the transported mineral dust concentrations during this peak event. Average PM concentrations more than doubled during the Saharan intrusion compared with non-Saharan. Relative concentrations of several elements often associated with anthropogenic sources were significantly diluted by crustal minerals coincident with the large-scale Saharan dust intrusion. During non- Saharan days, local mineral dust sources including cement manufacturing, and soil and road dust contributed in total 26% to PM2.5 mass and 50% to PM10 mass; during the three-day Saharan episode the total dust contribution increased to 64% for PM2.5 and 85% for PM10. Importantly, this approach was also able to determine that local emissions of crustal minerals dominated the period immediately following the Saharan dust episode: simple quantification of bulk crustal materials may have misappropriated this elevated PM to trans-Atlantic transport of Saharan dust. During this time period, we also completed publications and presentations related to measurement of platinum group metals in tunnel PM to develop a unique source profile for on-road light duty vehicles. Average source profile abundances were derived by subtracting ventilation air concentrations from those measured in the tunnel and dividing by the elevation in PM mass concentrations. Total tailpipe and non-tailpipe LDV PM emissions were dominated by crustal species such as Ca, Si, Fe, Al, Mg, K, and Ti (0.23 for Ti to 15.3% for Ca in PM10 and 0.26 for Ti to 8.6% for Ca in PM2.5). These elements exhibited a strong preference to the coarse size mode and had enrichment factors close to unity, indicating that they largely originated from the crustal mineral component of road dust. However, their abundances in tunnel PM differed from upper continental crust values reflecting small but significant contributions from light duty vehicles, which was nevertheless overshadowed by crustal material resuspension in the tunnel. Of all these crustal elements, only Fe had a higher contribution to PM2.5 (5.8%) compared with PM10 (3.4%) indicative of non-mineral sources including engine and brake wear. Anthropogenic elements such as Ba, Zn, Zr, Cu, Sn, Sb, Ga, and Pb that are well- known to be emitted by mobile sources were also prominent in the derived light duty vehicles source profiles of PM2.5 derived from the tunnel (0.01 to 0.56%) and PM10 (0.01 to 0.24%). These elements were 3.2 (Zr in PM10) to 2,025 (Sb in PM2.5) times more abundant than their UCC values corresponding to their high enrichment factors. Zn and Pb in PM2.5 accounted for only 15 and 19% of their respective PM10 levels, indicating that their major source is mechanical resuspension of enriched road dust within the tunnel. Cu, Ga, Zr, Mo, Cd, Sn, Sb, Ba, and W in PM2.5 constituted a higher fraction of PM10 (34% for Zr to 68% for Mo) indicating a greater influence from direct LDV emissions (tailpipe exhaust and brake/tire wear). PGE abundances were 493 (Pt in PM10) to 24,141 (Rh in PM2.5) times their UCC values leading to extremely high enrichment factors (890-36,575) as reported above. Their PM2.5 abundances constituted 36-60% of PM10 levels suggesting substantial tailpipe emissions of PGEs confirming their applicability as unique LDV tracers. LIST OF PUBLICATIONS AND PRESENTATIONS 1. Bozlaker, A., J.M. Prospero, M.P. Fraser, and S. Chellam (2013). Quantifying the Contribution of Long-Range Saharan Dust Transport on Particulate Matter Concentrations in Houston, Texas, using Detailed Elemental Analysis. Environmental Science and Technology, 47 10179-10187. 2. Bozlaker, A., N.J. Spada, M.P. Fraser, and S. Chellam (2014). 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 and Technology, 48 (1) 54-62. This paper is the most downloaded article from ES&T in the January – June 2014 period and the 3rd most downloaded article over the entire year of 2014. 3. Chellam, S., N.J. Spada, and A. Bozlaker “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,” Association of Environmental Engineering and Science Professors’ Education and Research Conference, New Haven, CT, June 13-16, 2015. 4. Chellam, S. A. Bozlaker, N.J. Spada, and M.P. Fraser “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,” Symposium on Trace Materials in Air, Soil, and Water, Division of Environmental Chemistry, 249th ACS National meeting, Denver, Colorado, March 22-26, 2015. .