Trace Materials in Air, Soil, and Water ... - ACS Publications

Downloaded by 118.121.39.53 on March 3, 2016 | http://pubs.acs.org Publication Date (Web): December 1, 2015 | doi: 10.1021/bk-2015-1210.ch001

Chapter 1

Measurement of the Trace Element Composition of Airborne Particulate Matter and its Use in Source Apportionment: Case Studies of Lanthanoids and Platinum Group Metals from Houston, Texas Ayşe Bozlaker and Shankararaman Chellam* Zachry Department of Civil Engineering, Texas A&M University, College Station, Texas, 77843-3136 *E-mail: [email protected].

We demonstrate that the composition and abundance sequence for many elements in airborne particles in an urban/industrial environment (i.e. Houston, TX) differs substantially from their average crustal values due to the influence of local anthropogenic sources. Evidence is also provided for the long-range transport of desert dust (i.e. crustal material) and its effects on composition and mass concentrations of ambient particulate matter in Houston using detailed elemental analysis. Lanthanoid metals and platinum group elements (PGEs) are shown to be excellent markers for primary emissions from catalytic cracking operations during petroleum refining and gasoline-driven light-duty vehicles, respectively. Therefore, systematic measurement of numerous representative, transition, and rare earth elements will assist in identifying and apportioning various anthropogenic and natural aerosol sources. Underlying such research are accurate and precise analytical methods to digest airborne particulate matter and quantify trace to major levels of a wide range of elements necessary for robust source apportionment. Our sample preparation procedures and inductively coupled plasma – mass spectrometry techniques for elemental analysis including lanthanoids, rhodium, palladium, and platinum are summarized. We use elemental abundance

© 2015 American Chemical Society In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

profiles and concentration ratios of specific tracer elements, focusing on lanthanoids and PGEs, to track locally emitted and long-range transported aerosols. Finally we list possible sources and their individual contributions to ambient particulate matter (PM) derived from source apportionment studies based on elemental characterization of ambient aerosols at receptor locations.


Introduction Determining the trace element content of inhalable airborne particles is of vital importance because of their potential adverse health impacts (1) and their use in source apportionment (2–4). Metal-bearing particles are emitted by various natural and anthropogenic sources, many of which have characteristic elemental signatures. Air-toxic metals such as As, Cd, Cr, Zn, Pb, etc., are commonly monitored because they are known or suspected to cause serious health effects but can also be used for source characterization (5, 6). However, since these elements are co-emitted by numerous sources, it is difficult to accurately isolate specific human activities or natural sources that release them. For example, Cr is used to make steel and other alloys (7) as well as automobile tire tread rubber (8) and is also present in coal-fired power plant emissions (9). Hence, identifying unique tracers that can pin-point individual sources would assist in accurate source apportionment. Two such examples that our research has focused on are lanthanoids to trace primary emissions from fluidized-bed catalytic cracking (FCC) units of petroleum refineries (3, 10–13). and platinum group elements (PGEs; Rh, Pd, and Pt) emitted from automobiles equipped with three-way catalytic converters (14–16). Lanthanoids are conventionally employed as signatures of geochemical (crustal) processes (17) but can also track primary emissions of fluidized-bed cracking catalysts from petroleum refineries (11–13, 18). Cracking catalysts are strongly enriched in light lanthanoids to impart hydrothermal stability above 750 °C and increase activity, selectivity and gasoline yield (11, 12, 18). In 2010, Texas was home to 27 of the nation’s 137 operating refineries, many of which are located in the Houston Ship Channel, and collectively distilled ~27% of the nation’s crude oil. During refining, ~2,000 tons of catalyst material is estimated to be lost daily by U.S. refineries along with more than 21 tons of lanthanoids through atmospheric emission, incorporation into products, and removal for disposal at landfills. Consequently, aerosols in Houston display a non-crustal lanthanoid signature closely resembling FCC catalysts (10, 19, 20) with smaller contributions from oil combustion and shipping activities (10). These results point to the value of monitoring lanthanoid metals to track primary particulate emissions from refineries. Interestingly, particles emitted from oil-fired plants display similar lanthanoid patterns to those from FCC units since fuel oils themselves contain residues of cracking catalyst materials (18, 21). 4 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Aeolian resuspension of soil dust from the Sahara-Sahel region is estimated to add approximately 800 Tg of dust annually to the atmosphere (22) making it the largest contributor to the world’s dust burden (23, 24). A portion of this mineral material is transported over the Atlantic Ocean to the continental United States (25), increasing ambient airborne particulate matter (PM) levels in Texas occasionally during summer (23, 26). We recently followed a major dust storm originating in Northern Africa that impacted air quality in Houston, TX by more than doubling ambient PM concentrations compared with routine or non-event days. Importantly, such African dust outbreaks dilute anthropogenically emitted elements associated with PM (e.g. Ba, V, Zn, Ni, La) by increasing the mass of crustal components (26, 27). The shift from non-crustal to crustal signatures of lanthanoids in Houston air was used to distinguish particulates that are locally emitted or transported over long distances. A second group of elements, namely Rh, Pd, and Pt (i.e. PGEs) is of interest because of their rarity and high economic value (note that they are classified as “precious metals” along with Ir, Ru, Os, Au and Ag). Rh, Pd, and Pt can be released to the environment from hospitals, industries, municipal wastewater treatment plants, and from high temperature abrasion of autocatalysts (28). PGE signatures depend on their sources, the main one being three-way catalytic converters for gasoline-driven vehicles to oxidize unburnt hydrocarbons and CO and reduce Nox (29, 30). Consequently, it is generally accepted that accumulation of Rh, Pd, and Pt in many urban environments is largely due their release from autocatalysts (particularly in areas where PGEs are not mined or produced) (31). Hence, Rh, Pd, and Pt are being widely used as markers of light duty automobiles in urban environments (32–36). Monitoring these PGEs is especially important to separate mobile source emissions from other stationary sources in heavily trafficked areas such as Texas where over 20 billion vehicle miles were driven in December 2014 alone (37). Measurement of lanthanoids and PGEs along with other main-group and transition elements requires specialized laboratory techniques since they (i) are present only at trace levels and (ii) their complete dissolution from siliceous matrices requires hydrofluoric acid and/or aqua regia (12, 15, 19, 33, 38, 39). These matrices introduce mass spectral overlaps during quadrupole inductively coupled plasma – mass spectrometry (q-ICP-MS) via isobaric and polyatomic interferences, doubly charged species, etc. Therefore extensive sample preparation techniques including 2-stage digestion at high temperature and high pressure as well as matrix separation are necessary. Isobaric interferences and spectral overlaps from polyatomic species arising from the carrier gas (i.e. argon) and the digestion + sample matrix can be reduced by inducing ion-molecule reactions in a collision or reaction cell pressurized with a reactive neutral gas (e.g. NH3) before ions reach the mass spectrometer (40). We have implemented novel protocols based on dynamic reaction cell (DRC) technology in a quadrupole ICP-MS (DRC-q-ICP-MS) to reliably quantify several elements important for air quality investigations e.g. Al, V, Ni, Zn, As, Cu, and Cd (15, 19). PGE quantification requires pre-concentration (e.g. NiS fire assay, Te or Hg co-precipitation) or matrix separation (e.g. ion-exchange) methods before q-ICP-MS (33, 41). To include automobiles in source apportionment calculations, 5 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


ultra-trace levels of PGEs need to be accurately quantified alongside many other elements, necessitating a multistep sample preparation technique, including closed-vessel microwave-assisted acid digestion, repeated evaporation steps, and matrix separation using cation exchange (15). The principal objective of this chapter is to summarize our work on measuring trace element composition of aerosols to identify and apportion their mass concentrations to local and long-range transported sources. Laboratory procedures for the analysis of a wide spectrum of representative, transition, and inner-transition elements by DRC-q-ICP-MS are briefly described. Case studies using lanthanoids and PGEs as unique signatures for tracing emissions of crude oil cracking catalysts, mineral material from the Sahara-Sahel region of North Africa, and gasoline-driven vehicles in Houston, TX are discussed. Finally we identify possible sources of primary particles and calculate their contributions to mass concentrations of ambient aerosols using elemental composition.

Materials and Methods Samples Ambient PM2.5 and PM10 Samplers (Partisol-Plus model 2025, Rupprecht & Patashnick Co.) were deployed at receptor locations in Houston, TX to collect daily airborne PM2.5 (particles ≤ 2.5 µm in aerodynamic diameter) and PM10 (particles ≤ 10 µm in aerodynamic diameter) on 47 mm PTFE membrane filters with a flow rate of 1 m3 h–1. Filters were weighed on three different days pre- and post-sampling using an Orion Cahn C-35 ultra-microbalance (Thermo Electron Corp.) to accurately measure mass of PM collected. More details on sampling locations and sample collection procedures is available in our earlier publications (10, 12, 19, 20, 26, 32).

Vehicular Emissions and Road Dust PM2.5 and PM10 were sampled from inside the Washburn tunnel of Houston, TX (latitude +29.733; longitude –95.211), which runs in the north-south direction underneath the Houston Ship Channel. Approximately 25,000 vehicles traverse the tunnel daily at average speeds in the range 55 – 75 km h–1. The vast majority of these vehicles have only 2 axles and run on gasoline fuel since larger, diesel-driven vehicles are banned as a security measure. PM sampling was performed on a catwalk inside the tunnel, 122 cm above the road surface at a distance of 44 m from the North exit. Three blower fans located on top of the tunnel supply ventilation air, which was also sampled from intake area of the fan room. Paired airborne PM2.5 and PM10 were sampled on 47 mm PTFE filters from inside the tunnel and fan room over 3 – 4 week periods (32, 33). 6 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Road dust material was collected from multiple points in the Washburn tunnel and three major surface roadways in Houston, TX. Samples were dried at 105 °C, sieved through 0.71 mm and 0.106 mm mesh sieves, respectively, and then homogenized as one lot prior to analysis (15, 33).


Long-Range Transported North African Dust Mineral dust aerosols originating from arid and semi-arid regions of North Africa were sampled at Ragged Point (+13.165, –59.432) located in the East coast of Barbados. Hence, these samples represent aerosols emanating from the Sahara-Sahel region and before they enter the continental United States. Prof. Joseph Prospero from the Rosenstiel School of Marine and Atmospheric Science at the University of Miami has been operating this site for over 4 decades and kindly provided all Barbados samples. Thirteen daily samples for this work were obtained in the months of late May, June, July, August, and mid-September from the years of 2005 – 2008 that coincided with African dust episodes. Total suspended particulates (TSP) samples were collected on Whatman-41 cellulosic filters using a high volume sampler with a flow rate of 1 m3 min–1 (26).

Sample Preparation PGE quantification in environmental matrices require specific sample preparation and analysis techniques of high sensitivity, selectivity and the control of potential isobaric and polyatomic spectral interferences. A multi-step sample preparation procedure, including closed vessel microwave-assisted acid digestion, repeated evaporation steps, and matrix separation by cation-exchange chromatography was employed to extract both PGEs and non-PGEs by modifying a previously reported digestion method used for the extraction of non-PGEs. Details of the procedures are available in our earlier publications (15, 19, 33, 38, 39) and only a brief summary is given below.

Extraction of Non-PGEs Two-stage digestion is necessary for the extraction of a wide range of trace to major elements including lanthanoids. National Institute of Standards and Technology’s (NIST’s) standard reference materials (SRMs), SRM 1648a (urban particulate matter) and SRM 1633b (coal fly ash) were used to develop and validate the method. SRMs were placed in Teflon-lined vessels (HP-500 Plus, CEM Corp.) along with HNO3 and HF, and then digested in a microwave-accelerated reaction system (MARS 5, CEM Corp.) at set points of 200 °C and 300 psig for 20 minutes. HF was used at the 1st stage to completely digest silicate matrix constituents. Next stoichiometric excess H3BO3 was added to each vessel before the 2nd stage to mask any remaining HF and re-dissolve fluoride precipitates, which is particularly important for lanthanoids (39). 7 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


The method was employed on ambient aerosol samples collected on PTFE filters (10, 19, 20, 39). HF and boric acid volumes were proportionately optimized based on the sampled mass. To extract non-PGEs in airborne PM that were collected on Whatman-41 filters, a pre-digestion step was applied before the two-stage procedure to accommodate violent reactions between the concentrated acids and cellulosic substrate of filters. The Whatman filter was first placed in the vessel along with HF and HNO3 and pre-digested in the microwave oven under less aggressive conditions (150 °C, 300 psig, 26 minutes dwell time) (26). Note that only PM samples in Barbados were collected on Whatman filters and not used for PGE measurements. It is emphasized that all blank concentrations were very low compared with sample concentrations allowing facile blank-correction. For example, the ratio of average sample to average blank concentrations for rare earths varied between 31 for Ho, 76 for Dy, 92 for Tb, ~125 for Gd, Pr, and Er, and > 150 for Eu, Tm, Sm, Nd, La, Ce, Yb, and Lu.

Extraction of Both PGEs and Non-PGEs To quantitatively extract all the elements of interest (including Rh, Pd, and Pt), HNO3, HF, HCl, and H3BO3 are all necessary. The formation of nitrosyl chloride by the reactions of HNO3 and HCl is the key step that solubilizes platinum group metals (42, 43).

However, HCl usage coupled with oxygen and nitrogen (from the atmosphere and from HNO3), results in numerous mass spectral interferences, which complicates the measurement of several other marker elements (e.g. Zn, Cr, As, Ba, Cu, Al, Fe, and V). PGE anionic chloro-complexes can be eluted through cationic resins, which retain major interferents allowing sensitive Rh, Pd, and Pt measurements from environmental samples such as road dusts and airborne PM (15, 32, 33, 44, 45). Hence, optimization of the ratio of HNO3 to HCl volumes (aqua regia) is essential for the accurate and simultaneous analysis of platinum group metals and several other elements essential for source apportionment. PGEs from European road dust BCR-723 and spent autocatalyst SRM 2556 were quantitatively recovered without using HF as reported by others (45). Therefore, all metals from ambient aerosol samples were first extracted using aqua regia (200 °C, 300 psig, 20 minute dwell time). Next the digestates were separated into two equal parts for further processing based on analytes of interest. PGE sub-samples were evaporated to dryness, taken into HCl media, and PGEs were then chemically separated from the sample matrix through cation-exchange (Dowex 50WX8 200 – 400 mesh). Sub-samples for non-PGEs followed two-stage acid digestion with HF and H3BO3 (15, 32, 33).

8 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Table 1. Comparison of Measured Elemental Concentrations with Certified and Indicative Values from European Road Dust BCR-723, Spent Auto-Catalyst SRM 2556, Urban PM SRM 1648a, and Coal Fly Ash SRM 1633b BCR-723 (µg g–1)

SRM 2556 (µg g–1)

Element

Atomic Mass

Supplied value

Measured value

Supplied value

Measured value

Al

27

37,500±2,200

37,307±1,457

400,000

449,418±34,840

Ti

48

2,580±130

2,259±45

V

51

75±1.9

90±2.3

Cr

52

440±18

427±47

Mn

55

1,280±40

1,214±21

Fe

56

32,900±2,000

32,520±921

8,000

8,705±660

Ni

59

171±3.0

167±15

Co

59

30±1.6

30±0.42

Zn

65

1,660±100

1,621±80

Rb

85

75±5

70±1.8

Sr

88

254±19

241±5.5

Y

89

13±1.8

13±0.52

Zr

91

300

317±12

Mo

96

40±0.6

42±2.4

Rh

103

0.0128±0.0013

0.0130±0.0012

51±0.50

55±2.5 Continued on next page.

In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Table 1. (Continued). Comparison of Measured Elemental Concentrations with Certified and Indicative Values from European Road Dust BCR-723, Spent Auto-Catalyst SRM 2556, Urban PM SRM 1648a, and Coal Fly Ash SRM 1633b BCR-723 (µg g–1)

SRM 2556 (µg g–1)

Element

Atomic Mass

Supplied value

Measured value

Supplied value

Measured value

Pd

106

0.0061±0.0019

0.0052±0.0014

326±1.6

357±22

Cd

112

2.5±0.4

2.6±0.06

Sb

122

28±2.3

26±0.53

Ba

137

460±40

477±14

100

95±13

La

139

7,000

6,894±206

Ce

140

10,000

11,094±296

Hf

178

2.2±0.7

2.0±0.05

Pt

195

0.0813±0.0025

0.0841±0.0058

697±2.3

743±89

Pb

207

866±16

928±61

6,228±49

6,635±152

Th

232

4.8±0.5 SRM 1684a (µg

4.6±0.26 g–1)

SRM 1633b (µg g–1)

Element

Atomic Mass

Supplied value

Measured value

Supplied value

Measured value

Na

23

4,240±60

3,794±272

2,010±30

1,890±175

Mg

24

8,130±120

8,086±277

4,820±20

5,143±452

Al

27

34,300±1,300

35,257±1,810

150,500±2,700

162,183±8,745

Si

28

128,000±4,000

124,531±8,248

230,200±800

259,890±18,043


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SRM 1684a (µg g–1)

SRM 1633b (µg g–1)

Element

Atomic Mass

Supplied value

Measured value

Supplied value

Measured value

K

39

10,560±490

10,770±884

19,500±300

21,283±1,109

Ca

40

58,400±1900

57,222±3,151

15,100±600

16,107±1,587

Ti

48

4,021±86

4,019±209

7,910±140

7,995±692

V

51

127±11

127±7.4

296±3.6

306±17

Cr

52

402±13

407±20

197±4.7

210±12

Mn

55

790±44

799±27

132±1.7

146±10

Fe

56

39,200±2,100

40,059±1,750

77,800±2,300

79,037±3,175

Ni

59

81±6.8

83±4.3

121±1.8

129±11

Co

59

18±0.68

17±0.8

50

60±1.0

Cu

64

610±70

618±42

113±2.6

120±6.8

Zn

65

4,800±270

4,620±457

210

234±21

As

75

116±3.9

127±6.6

136±2.6

145±12

Se

79

28±1.1

26±2.5

10±0.17

11±1.1

Rb

85

51±1.5

50±2.9

140

143±5.1

Sr

88

215±17

224±8.1

1,041±14

1,129±45

Cd

112

74±2.3

74±3.8

0.78±0.06

0.70±0.08

Sb

122

45±1.4

47±1.2

6.0

6.2±0.47 Continued on next page.


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Table 1. (Continued). Comparison of Measured Elemental Concentrations with Certified and Indicative Values from European Road Dust BCR-723, Spent Auto-Catalyst SRM 2556, Urban PM SRM 1648a, and Coal Fly Ash SRM 1633b SRM 1684a (µg g–1)

SRM 1633b (µg g–1)

Element

Atomic Mass

Supplied value

Measured value

Supplied value

Measured value

Cs

133

3.4±0.20

3.4±0.40

11

12±0.83

Ba

137

709±27

751±77

La

139

39±3.0

39±1.0

94

94±1.6

Ce

140

55±2.2

56±1.3

190

197±3.8

Sm

150

4.3±0.30

4.3±0.13

20

19±0.64

153

4.1

4.2±0.18

Eu Tb

159

2.6

2.7±0.15

Dy

163

17

17±0.52

Ho

165

3.5

3.0±0.24

Yb

174

7.6

7.5±0.49

Lu

175

1.2

1.1±0.49

Hf

178

5.2

4.6±0.92

6.8

6.0±0.48

W

184

4.6±0.30

4.7±0.53

5.6

5.9±0.66

Pb

207

6,550±330

6,425±255

68±1.1

72±4.4


Element

Atomic Mass

Th U

Supplied value

Measured value

SRM 1633b (µg g–1) Supplied value

Measured value

232

26±1.3

25±2.0

238

8.8±0.36

8.9±0.76

Note: Values in bold font correspond to certified values and those in regular font are indicative (uncertified) values.

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SRM 1684a (µg g–1)



Analytical Procedure A DRC-q-ICP-MS (ELAN DRC II, PerkinElmer) was used to quantify trace (1 ppb to 100 ppm) levels of PGEs (Rh, Pd, Pt) and lanthanoids (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) along with numerous other trace to major (> 1 %) levels of representative elements (e.g. Li, Be, Na, Mg, Al, Si, K, Ca, Ga, As, Se, Rb, Sr, Sn, Sb, Cs, Ba, Pb), transition metals (e.g. Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Cd, Hf, W) and actinides (e.g. Th, U) elements. NH3 was used in the DRC unit to overcome spectral overlaps for the isotopes of several elements including Al, Ca, V, Cr, Mn, Fe, Ni, Co, Cu, Zn, As, Se, and Cd. Multielement calibration standards and mixed internal standard of 115In and 209Bi at 20 µg L–1 were prepared before each use by dilution of single-element stock solutions (10 µg mL–1; High-Purity Standards) with 0.4 M HNO3. All calibration blanks and calibration standards for both PGEs and non-PGEs were spiked with the acid matrix containing all reagents employed for digestion at an appropriate amount with the same the dilution factor as the analyzed samples. DRC optimization parameters including the quadrupole dynamic bandpass tuning parameter (RPq) and cell gas flow rate as well as instrumental settings and operating conditions for standard (no NH3) and DRC (using NH3) mode of operations are provided in detail elsewhere (15, 19, 33, 38, 39). Using ammonia as the cell gas suppressed signal intensities by 1 – 2.5 orders of magnitude for Al, V, Cr, Fe, Ni, Cu, and Zn in the blank solution thereby improving the precision and accuracy of their measurements in PM samples (19). More information on improved detection limits and reduced interferences can be found in the original publications (12, 15, 19, 33, 38, 39). Concentrations of principal PGEs interferences (i.e. 63Cu, 65Cu, 68Zn, 85Rb, 88Sr, 89Y, 90Zr, 179Hf, and 206Pb) were reduced by 95.3 to 99.8 % in sample matrices following cation exchange column separation (15). Manufacturer-provided concentrations of all certified and uncertified elements in SRM1648a, SRM 1633b, SRM 2556, and BCR-720 are compared with measured values (from three separate digestions and ICP analysis) in Table 1. Analytical recoveries ranging between 85 – 120 % of supplied values for both PGEs (85 – 110 %) and non-PGEs including lanthanoids (86 – 120 %), demonstrating the accuracy of our laboratory procedures.

Results and Discussion Mass and Elemental Concentrations of Ambient Aerosols in Houston Daily average mass concentrations in the greater Houston area have been reported between 4.7 – 32.1 µg m–3 for PM2.5 and between 12.1 – 125.4 µg m–3 for PM10 (particles ≤ 10 µm in aerodynamic diameter) (10, 19, 20, 26, 46, 47). These fluctuations largely arise from inherently high variability in particulate emissions from myriad natural and anthropogenic sources, proximity of the receptor site to emission sources, and meteorological conditions. Automobile emissions during morning and evening rush hours (32, 48), non-routine emissions 14 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


from local industries (10, 12, 19, 20, 47), regional transport from Mexico and Central America, or trans-Atlantic dust transport from arid/semi-arid regions in North Africa (25, 26) are mainly associated with peak concentrations in Houston. For instance, elevated hourly mass concentrations ~40 µg m–3 for PM2.5 and ~120 µg m–3 for PM10 were measured near the heavily industrialized Houston Ship Channel coinciding with episodic catalyst emissions from crude oil cracking units of local petroleum refineries (10, 19, 20). Similarly, “background” or routine PM2.5 and PM10 concentrations were measured as 13.1 and 36.4 µg m–3, respectively during the non-dust event days in Houston, TX. PM2.5 and PM10 levels more than doubled to 27.8 and 88.7 µg m–3 respectively coinciding with long-range dust transport from the Sahara-Sahel region of North Africa between the dates of July 25 – 27, 2008 (26). Origins of episodic emissions leading to elevated ambient particulate matter concentrations were tracked using concentration ratios, abundance sequences, and enrichment factors of source-specific tracer elements, ternary diagrams, and receptor modeling techniques. Elemental abundance profiles as mass fraction of tracer species including PGEs and lanthanoids along with other “floating” species in representative source material were developed for several primary aerosol sources of particular interest to Houston, TX. These included refinery oil-cracking catalysts (12), trans-Atlantic transported North African dust collected in Barbados (26), tunnel and surface road dusts (15), and vehicular emissions (32, 48). The source profile abundances and the receptor ambient concentrations with appropriate uncertainty estimates were input to the United States Environmental Protection Agency’s (EPA’s) chemical mass balance modeling software (EPA-CMB8.2) to isolate and quantify the contributions of individual pollutant sources to PM2.5 and PM10 mass. The model consists of a system of linear equations that express concentration of each species as a linear combination of source profile abundances and source contributions. Output data includes contribution estimates and their associated standard errors for each source category (49). Previous air quality campaigns in Houston have reported that geochemical and marine markers such as Na, Mg, Al, Si, K, Ca, Ti, and Fe dominates elemental composition of ambient PM2.5 and PM10. Lanthanoids, PGEs and other maingroup and transition metals were also quantified, but at trace (1 ppb to 100 ppm) to minor (0.01 to 1.0 %) levels (10, 12, 15, 19, 20, 26, 32, 47, 50, 51). Among these elements, Ca, V, Cr, Ni, Cu, Zn, Ga, As, Se, Mo, Rh, Pd, Cd, Sn, Sb, Ba, La, W, Pt, and Pb are moderately to anomalously enriched in the proximity of the ship channel indicating “contamination” by local non-crustal sources. The extent of anthropogenic contribution was qualitatively assessed by computing the enrichment factor of individual elements in airborne particles with respect to a crustal reference such as Al, Fe, or Ti (52, 53). A list of non-crustal elements and their corresponding enrichment factors from the Washburn Tunnel and several other receptor locations in Houston are summarized in Table 2.


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Table 2. Enrichment Factors for Non-Crustal Elements in Road Dusts (15) and Airborne Particles (32) from the Washburn Tunnel, Surface Roadways (15), and Ambient Aerosols (10, 26, 32) from Locations near the Houston Ship Channel Washburn Tunnel

Urban/industrial locations

Non-crustal elements

Airborne PM2.5

Airborne PM10

Road dust

Surface roadways dust

Airborne PM2.5

Airborne PM10

Ca

3.4±0.2

7.1±0.3

3.5±0.3

1.0-5.1

0.7-14.4

0.9-19.6

V

0.5±0.3

0.8±0.2

1.4±0.05

0.5-0.9

3.0-86.5

1.8-19.8

Cr

1.9±0.4

1.8±0.4

3.6±0.5

0.5-3.7

0.9-17.3

1.0-15.5

Ni

1.9±1.43

1.2±0.7

1.7±0.2

0.5-0.6

0.8-45.9

1.4-15.1

Cu

227.1±24.1

105.5±15.0

25.3±1.6

0.8-3.4

1.0-132.6

2.1-88.0

Zn

46.5±10.6

65.7±14.6

47.1±3.0

11.5-26.8

5.5-180.1

3.5-100.6

Ga

29.0±4.4

14.5±5.3

17.1±3.0

1.3-8.5

0.6-60.9

0.7-106.2

As

94.5±9.4

89.7±1.6

15.8±0.2

0.8-6.0

5.4-319.1

0.9-187.9

Se

B.D.L.

B.D.L.

B.D.L.

B.D.L.-10.1

50.9-4763

23.2-2239

Zr

9.7±4.5

5.7±1.5

3.8±0.5

1.2-2.3

B.D.L.-6.3

0.7-4.2

Mo

156.1±11.3

51.2±10.4

35.7±5.0

1.2-14.3

10.1-433.9

7.3-178.2

Rh

36575±13500

20996±4722

15700±6236

417.4-884.5

17380-28756

8590-18900

Pd

9142±1708

4250±848.4

2726±876

41.4-427.5

4190-7749

2001-3591

Cd

259.0±116.2

137.1±34.4

46.4±7.8

2.0-13.3

14.9-916.3

8.0-349.2

Sn

381.3±48.6

179.1±31.5

28.6±0.6

7.0-34.8

4.4-726.4

3.2-270.6

Sb

3070±375.3

1342±277.6

211.6±15.1

4.5-17.6

24.9-2820

16.2-1277


Urban/industrial locations

Airborne PM2.5

Airborne PM10

Road dust

Surface roadways dust

Airborne PM2.5

Airborne PM10

Ba

14.6±2.2

7.4±3.0

9.8±1.8

0.4-3.8

1.7-30.2

1.2-105.6

La

0.4±0.3

1.0±0.4

1.8±0.3

0.8-1.9

0.5-14.4

0.8-5.7

W

3.5±1.8

5.1±6.1

33.6±5.2

1.3-6.0

1.0-125.8

0.9-108.0

Pt

2549±796.6

889.8±212.3

1642±487.7

125.5-556.0

1853-2376

1119-2107

Pb

10.3±4.2

11.7±4.1

66.6±24.7

2.4-8.0

3.0-62.8

1.6-28.1

B.D.L.: Below method detection limit. Enrichment factors are given either as average ± 1 standard deviation or in a range of minimum and maximum values.

17


Washburn Tunnel

Non-crustal elements



Particulate emissions from single-axle gasoline-driven vehicles in the Washburn Tunnel were extremely enriched in Rh, Pd, and Pt (See Table 2) with respect to their low abundances in the upper continental crust (UCC; 0.018, 0.526, and 0.599 ppb, respectively) (52). Highest enrichment factors were also calculated for PGEs in dusts from surface roadways as well as ambient aerosols just outside the Washburn Tunnel. The results provide a strong evidence for release of Rh, Pd, and Pt from light-duty vehicles and emphasize the importance of their usage as automobile exhaust signatures in urban environments. It is noted that lanthanum was not enriched in the Washburn Tunnel demonstrating that this underwater tunnel is relatively isolated from the Houston atmosphere. However, lanthanum was moderately enriched in ambient PM2.5 and PM10 under the influence of non-routine emissions from nearby petroleum refineries (10, 19, 20). Source apportionment based on detailed elemental composition (including lanthanoids, PGEs, and several others) of Houston-area ambient aerosols is discussed next. Lanthanoids To Track Nonroutine Emissions from Catalytic-Cracking Units Lanthanoid Concentrations in Ambient Particulate Matter The total lanthanoid content (∑14Ln of La to Lu) of ambient aerosols measured in a residential site and three urban/industrial sites in the Houston area has been reported to be between 0.18 – 51.3 ng m–3 for PM2.5 (20, 26, 32) and 0.23 – 20.9 ng m–3 for PM10 (10, 26, 32). These data encompass both routine (i.e. background) and non-routine (i.e. episodic) releases of primary particles from FCC units in local refineries. Episodic air emissions were confirmed with coincident entries in the self-reported emission event database maintained by the Texas Commission on Environmental Quality, ratios of La to other light lanthanoids and La/V, enrichment factors, and abundance sequences. Episodic releases of lanthanoid-enriched catalysts during non-routine operations resulted in 4.4 to 21.4 fold elevation in ∑14Ln levels (7.2 – 51.3 ng m–3) in ambient PM2.5, compared with their corresponding background values (1.6 – 2.4 ng m–3). We also measured higher ∑14Ln concentrations both in ambient PM2.5 and PM10 during an African dust intrusion occurring during a 3-day period (5.0 ± 1.4 and 15.9 ± 3.6 ng m–3, respectively) compared to five-day averages immediately prior to and after the peak (0.85 ± 0.42 and 3.4 ± 2.2 ng m–3, respectively) (26). Lanthanoid-bearing aerosols associated with high mineral material content were distinguished from their anthropogenically emitted counterparts by their rare earth distribution patterns, which were nearly identical to the lithosphere. It was also revealed that ∑14Ln content of PM10 during dust intrusions was controlled primarily by the amount of mineral material.

Lanthanoid Pattern Cracking catalysts have a significantly different lanthanoid signature compared with crustal material, which enriches La but depletes Ce and other 18 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


heavier lanthanoids during episodic releases from petroleum refineries in Houston (10, 19, 20). Similar trends have also been reported by others following transient refinery pollution events (13, 54, 55). Particulate emissions from oil-combustion activities often display similar lanthanoid patterns to those of cracking catalysts since fuel oils contain residual amounts of refining catalysts (18, 21). However, contribution of La-rich particles from oil combustion was found to be at a smaller scale since lanthanoids were strongly and positively correlated with catalytic-cracking operations and crustal material, restraining their releases from oil-combustion activities (10, 12). Additionally, these two sources that are relatively identical in their lanthanoid signatures can be separated using La/V ratios (10, 13, 56).

Figure 1. (a) Lanthanoid abundances in zeolite-based fluid cracking catalysts (12), local soil (15) and UCC (53). (b) Lanthanoid abundances in FCC and local soil that were normalized to those reported values for UCC (53). Error bars represent one standard deviation of the average. 19 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

20


Table 3. Tracer Elemental Ratios in Ambient PM Collected during Episodic Releases from Refinery Units, Intrusions of North African Dust, and Non-Event (i.e. Routine or Background) Days in Houston (10, 20, 26). Average UCC (53), Cracking Catalysts (12), Refinery Stack and Oil-Fired Power Plant Emissions (3), Vehicular Emissions (32), and Road Dust (15) Are Also Provided for Comparison Ambient PM2.5

Ambient PM10

Ambient PM2.5

Ambient PM10

Ratios

Refinery event

Non-event

Refinery event

Non-event

African dust event

Non-event

African dust event

Non-event

Local road dust

La/Ce

3.2-5.0

0.67-0.89

0.98-9.9

0.42-0.92

0.51-0.74

0.67-6.8

0.49-0.66

0.56-2.8

0.42±0.08

La/Pr

14.3-20.0

6.3-9.3

4.3-58.7

4.0-12.2

4.5-7.0

6.3-40.2

4.4-6.0

5.1-26.9

6.2±0.75

La/Nd

7.6-11.8

1.6-2.6

2.0-21.2

1.0-3.1

1.2-1.9

1.6-22.7

1.2-1.6

1.4-8.0

1.6±0.22

La/Sm

41.1-70.6

8.0-14.3

9.7-108.2

4.0-18.8

6.5-9.6

8.0-60.3

6.3-8.7

7.1-39.3

7.3±1.2

La/V

0.22-1.8

0.02-0.07

0.01-1.1

0.01-1.1

0.06-0.23

0.01-0.25

0.14-0.25

0.07-0.32

0.64±0.12

UCC

FCC emissions

Oil combustion

Vehicle emissions

Ratios

FCC

PM2.5

PM2.5-10

PM2.5

PM2.5-10

PM2.5

PM10

African dust in Barbados

La/Ce

4.3±4.6

1.2

1.3

1.4±0.15

2.5±1.1

0.62±0.25

0.44±0.04

0.49±0.01

0.5

La/Pr

9.7±5.2

N.R.

N.R.

N.R.

N.R.

12.5±3.9

6.9±0.38

4.2±0.07

4.4

La/Nd

6.4±4.1

1.8

1.9

3.3±1.9

3.2±1.4

4.0±0.82

2.0±0.04

1.1±0.02

1.1

La/Sm

55.2±23.3

19.4

19.5

28.5±9.3

30.6±13.0

13.0±1.0

8.7±0.66

5.9±0.16

6.6

La/V

131.3±69.6

13.2

17.6

0.02±0.01

0.25±0.18

0.24±0.06

0.33±0.03

0.19±0.08

0.32

N.R. Not reported. The ratios are given either as average ± 1 standard deviation or in a range of minimum and maximum values.



We also determined the elemental composition of source samples (five fresh and one spent or equilibrium zeolite-based FCC catalysts). Spent catalysts are those that are deactivated following their repeated use in the FCC unit but cannot be further regenerated due to extensive poisoning possibly due to coke, vanadium, and nickel accumulation. Ultimately, the spent catalysts have to be disposed externally, after being classified as a “hazardous waste” possibly in an appropriate landfill. These results along with a FCC source profile as mass fraction of each species can be found in our earlier studies (12, 26, 38). Lanthanoid abundance patterns in catalysts (12) and in local soil (15) before and after normalization to average UCC values (53) are depicted in Figure 1a and 1b, respectively. It can be seen in Figure 1a that crustal material and local soil follow the Oddo-Harkins rule whereas catalytic-cracking catalysts exhibit a different trend (e.g. 57La > 58Ce in catalyst but 57La < 58Ce in the UCC). Therefore, the distinct positive La anomaly in cracking catalysts relative to its crustal abundance makes it a strong tracer for its anthropogenic origins in locations influenced by refinery emissions. We measured light lanthanoids i.e. La, Ce, Pr, Nd, Sm, and Gd, abundances in cracking catalysts to be 30±14 (Sm) to 267±90 (La) times greater than those in the Earth’s crust. Similarly Eu and heavy lanthanoids, i.e. Tb, Dy, Ho, Er, and Yb were also enriched but to a lesser degree (< 12-fold). In other words, cracking catalysts are highly enriched in light lanthanoids and to a lower extent in heavy lanthanoids. Importantly, cracking catalysts are depleted in Ce, Eu, and odd-numbered heavy lanthanoids (Tb, Ho, Tm and Lu) depicting a distinct negative anomaly. As shown in Figure 1a, the lanthanoid pattern in local soil was similar to the average UCC. Hence, the UCC normalized lanthanoid profile of local soil was relatively flat, only exhibiting very weak excursions of La, Gd, Dy, Er and Yb (Figure 1b). Hence, measurement of the complete rare earth signature and Coryell-Masuda diagrams are useful tools to separate natural and anthropogenic emissions of lanthanoid-bearing PM.

Using Ambient Data for Source Characterization Concentration ratios of La to other light lanthanoids (i.e. Ce, Pr, Nd, and Sm) and V in ambient PM2.5 and PM10 during air emission events from petroleum refineries and Saharan dust intrusions are summarized in Table 3. Corresponding ratios for non-event or routine days and several lanthanoid-bearing aerosols from natural and anthropogenic sources are also shown for the sake of comparison. Since both refinery and oil combustion emissions have elevated ratios of La to light lanthanoids, the La/V ratio was used to distinguish the samples affected by these two sources (11, 18, 55, 56). Particulate emissions from fuel and petcoke combustion exhibit low La/V ratio (< 0.1) (55) with respect to UCC (0.31) (53) since they are rich in vanadium. The La/V ratio reaches as high as 13 in refinery stack emissions (3) and becomes an astonishingly high 131.3 ± 69.6 in cracking catalysts (12) due to their highly elevated La content. Also as given in Table 3, La to light lanthanoid ratios for the UCC closely overlap with those measured for mineral dust aerosols that originated in arid regions of North Africa and transported 21 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


across the Atlantic Ocean to Barbados and for local road dust (La/Ce = 0.42 – 0.50). Vehicular emissions display Ce enrichment vis-à-vis La similar to crustal materials, but can exhibit slightly higher ratios (La/Ce = 0.44 – 0.69).

Figure 2. La-Ce-V (a) and La-Ce-Sm (b) ternary diagrams for ambient PM2.5 and PM10 (10, 20, 26) under the influence of routine and non-routine emissions of refinery catalytic cracking units (3, 12, 57) and oil combustion activities (3, 58) in Houston. Other natural and anthropogenic sources (15, 26) of lanthanoid and vanadium bearing particles are included for comparison. Values were adjusted so that the UCC (53) appears in the geometric center of the figure. 22 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


As shown in Table 3 aerosols collected during episodic events were isolated from those collected during routine operations of refineries based on their La-enrichment; i.e. elevated La to other light lanthanoid ratios relative to the UCC and local soil. Similarly, higher La/V ratios in PM2.5 were measured during the refinery emission events (0.22 – 1.8) compared with those collected when no episodic events were self-reported (0.02 – 0.07). Note that during peak event days, atmospheric La/V is significantly higher than the crustal value (0.31) and during routine operation days, atmospheric La/V is significant lower than the crustal value. Therefore, fine PM is predominantly influenced by anthropogenic sources, with strong La enrichment from episodic releases of cracking catalyst. On the other hand, during routine FCC unit operation, V is enriched in the atmosphere due to extensive oil combustion emissions in the heavily industrialized ship channel area, significantly decreasing La/V compared to its crustal value. In contrast, the La/V ratio for PM10 remained in the same range (0.01 – 1.1) for both event and non-event samples. This indicates that in addition to refineries and oil combustion, resuspended crustal material contributed to the lanthanoid atmospheric chemistry preferentially for the coarse size mode in Houston. La to other light lanthanoid ratios in ambient PM collected during an African dust outbreak in Houston, TX were similar to the UCC and African dust aerosols collected in Barbados (e.g. La/Ce < 1, 6.3 < La/Sm < 9.6). As expected, lanthanoid atmospheric chemistry reflected a combination of crustal and refinery sources on days where African dust did not impact Houston (e.g. 0.56 < La/Ce < 6.8, 7.1 < La/Sm < 60.3). It is emphasized that peak-days associated with North African dust increased the total aerosol mass concentration and consequently the total lanthanoid content (∑14Ln). However, it substantially reduced La/Ce (and other La to light lanthanoids ratios) demonstrating a significant change in lanthanoid composition in Houston’s atmosphere, which is otherwise dominated by refinery emissions. Influence of lanthanoid and vanadium bearing aerosol sources on ambient PM2.5 and PM10 (10, 20, 26) is further demonstrated in Figure 2 in the form of ternary diagrams. Three component La-Ce-Sm (Figure 2b) and La-Ce-V (Figure 2a) diagrams were used to distinguish oil combustion and refinery emissions to ambient aerosols. Elemental concentrations were normalized to place average UCC abundances (53) of three components at the centroid. In both diagrams, mineral materials such as North African dust collected in Barbados (26) and local soil (15) grouped around the centroid reflecting their similar La, Ce, Sm, and V composition. Elemental composition of vehicular PM2.5 and PM10 and road dust collected from inside the Washburn Tunnel (15, 32) showed similarity to the UCC with Ce enrichment relative to La. Refinery cracking catalyst (12), FCC unit stack emissions (3, 57), and oil combustion fly ash (3, 59) crowded together near the La-apex in Figure 2b, corresponding to their La-enrichment. In such a representation, PM associated with oil combustion and shipping activities (3, 59) in Figure 2a clustered around the V-apex demonstrating their strong V-enrichment. Elemental composition of ambient PM2.5 and PM10 under the influence of refinery FCC unit emission events and North African dust outbreaks in Houston (10, 26, 39) were included to identify variations in elemental composition under different source impacts. In the La-Ce-V diagram (Figure 2a), 23 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


data for airborne particulate samples enriched in V moved away from UCC, local soil, African dust aerosols, and cracking catalysts toward the V-apex, clustering around the V-rich sources of oil combustion and shipping emissions. Relative La-Ce-V data for some particulate matter samples moved between the V-apex and other sources such as local soil, African dust, and vehicular emissions, reflecting their lanthanoid and V contributions. In the La-Ce-Sm diagram (Figure 2b), these three light lanthanoid concentrations for some ambient particulate matter samples clustered around the centroid demonstrating their crustal origins. However, several others moved away from the centroid toward the La-apex indicating their origins in petroleum refining and oil combustion.

Platinum Group Elements for Tracing Light-Duty Vehicular Emissions PGEs Levels in Road/Tunnel Dust and Airborne Particulate Matter Concentration ratios and levels of Rh, Pd, and Pt in tunnel dust, tunnel gutter, surface roadways dust (15), and airborne PM2.5 and PM10 from inside the Washburn Tunnel of Houston, TX and those in ventilation (ambient) air (32) are summarized in Table 4. Elemental compositions of each samples matrices and source profile abundances as percent mass fraction of all elements quantified in road dusts, airborne PM2.5, PM10, and PM2.5-10 emissions from the Washburn Tunnel can be provided in detail from our previous studies (15, 26, 32) PGE composition of average UCC (52) and a composite recycled catalyst from multiple manufacturers and model types obtained from Engelhard Corp. (currently BASF) (15) in the U.S. are also included in Table 4. Average abundances of Rh, Pd, and Pt dust in the tunnel catwalk ranged between 0.12 – 0.21 µg g–1, 0.59 – 1.0 µg g–1, and 0.41 – 0.67 µg g–1, respectively. Particles collected from the gutter had higher Rh and Pt abundances (0.27 and 1.1 µg g–1, respectively), possibly due to their accumulation via regular washing of tunnel walls and catwalk. In contrast, lower abundance of Pd in the gutter is attributed to its higher water solubility and mobility (60). PGE abundances displayed relatively high variation in dusts collected alongside three major surface roadways. Rh, Pd, and Pt abundances in these samples were lower than those from inside the tunnel, ranging between 0.006 – 0.008 µg g–1, 0.03 – 0.09 µg g–1, and 0.09 – 0.13 µg g–1, respectively. Lower PGE abundances in surface roadways compared with the tunnel was attributed to dilution in the ambient environment due to meteorology, rainfall, mass contribution from other non-PGE sources as well differences in vehicle fleet and driving habits. Average Rh, Pd, and Pt concentrations in ambient PM2.5 and PM10 in tunnel ventilation air were 1.5, 11.1 and 4.5 pg m–3 and 3.8, 23.1, and 15.1 pg m–3, respectively. Similar to concentration differences between tunnel and surface roadways, Rh, Pd, and Pt were significantly elevated in airborne particles inside the Washburn Tunnel compared with ventilation air. Average Rh, Pd, and Pt concentrations increased 4.0- to 9.6-fold inside the tunnel, reaching 12.5, 91.1, and 30.1 pg m–3 in PM2.5 and 36.3, 214, and 61.1 pg m–3 in PM10, respectively. 24 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Table 4. PGE Levels and Tracer Element Ratios in Different Sample Matrices Abundance (ng g–1)

25


Sample type

Ratios

Dust (15)

Rh

Pd

Pt

Pt/Rh

Pd/Rh

Pt/Pd

Washburn Tunnel dust

152±52.3

770±208

529±130

3.6±0.43

5.2±0.50

0.69±0.02

Washburn Tunnel gutter

273

717

1079

4.0

2.6

1.5

Surface road dust

6.8±1.3

53.5±30.2

106±21.5

15.9±4.7

8.2±5.3

2.5±0.85

Concentration (pg m–3)

Airborne PM (32)

Ratios

Washburn Tunnel PM2.5

12.5±5.8

91.1±28.6

30.1±11.5

2.4±0.21

7.5±1.2

0.33±0.02

Washburn Tunnel PM10

36.3±21.4

214±119

61.1±32.3

1.7±0.12

6.0±0.25

0.29±0.01

Ventilation air PM2.5

1.5±0.5

11.1±4.4

4.5±0.7

3.1±0.56

7.5±0.59

0.43±0.11

Ventilation airPM10

3.8±1.2

23.1±4.1

15.1±3.2

4.0±0.44

6.2±0.89

0.65±0.02

Abundance (µg g–1)

Source

Ratios

Autocatalyst (Engelhard)

184

1148

814

4.4

6.2

0.71

UCC (52)

0.000018

0.000526

0.000599

33.3

29.2

1.14



PGEs abundances in both tunnel and ambient environments were 63 (Pd in surface roadways dust) to 18,760 (Rh in tunnel airborne PM2.5) times higher than UCC levels. Enrichment factors were also calculated for all elements quantified in road/tunnel dust, tunnel aerosols, and aerosols present in ventilation air using Ti as a crustal reference (see Table 2). The PGEs exhibited highest enrichment in all the sample matrices from the Washburn Tunnel (with enrichment factors ranging from 890 for Pt in PM10 to 36,575 for Rh in PM2.5), providing strong evidence for their release from gasoline-driven light duty vehicles.

Figure 3. Three component diagram demonstrating the separation of United States autocatalyst (15), road dust (15) and airborne particulate matter from inside and outside the Washburn Tunnel (32) from European autocatalyst (ERM-EB504), Austrian road dust (BCR-720), and the UCC (52). Houston surface roadways dusts grouped around Canadian autocatalyst, SRM-2556 (used autocatalyst). PGEs contents in various road dusts, roadside soils, and ambient particulate matter reported around the world were taken from the literature (14, 34, 61–71). Values were adjusted so that the average UCC composition52 appears at the centroid. 26 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Rh, Pd, and Pt Composition in Road Dusts and Airborne Particles Influence of light-duty vehicles’ emissions on PGE composition of airborne particles is further illustrated in Figure 3, which simultaneously captures Rh, Pd, and Pt composition in different environmental matrices from around the globe. As before, concentrations were normalized to average UCC values so that the crustal PGE composition appears at the centroid. The United States recycled mixed-lot autocatalyst, Houston tunnel dust (15) and PM2.5 and PM10 emissions from inside the Washburn Tunnel (32) were enriched in Rh with respect to average UCC as evidenced by their position near the Rh-apex. The clustering of these three samples is highlighted by encompassing them in a blue circle. Also as given in Table 4, the Pt/Rh, Pd/Rh, and Pt/Pd concentration ratios for tunnel particles overlapped with those for the U.S. autocatalyst. This provides strong evidence that the primary PGE source is emissions associated with wear and tear of the light duty vehicles’ catalytic converters. Relative Rh, Pd, and Pt concentrations of ambient PM2.5 and PM10 from Houston, TX, Boston, MA and Mexico (14, 32, 61) also grouped in the vicinity of the U.S. autocatalyst, indicating their similar origins. Road dusts from major surface roadways in Houston displayed similar PGE patterns to those from other road dust and roadside soil samples from the U.S. (34, 62, 63), gathering around an older used autocatalyst material, SRM-2556. These samples are grouped within the brown oval. Their common feature is that they were all depleted in Rh but enriched in Pt with respect to freshly emitted autocatalyst material, probably as result of weathering processes in the ambient environment (60). Interestingly, several European samples grouped together and were separated from American samples and are depicted with a red oval. The European autocatalyst (ERM-EB504) was enriched in Rh and Pt, but depleted in Pd with respect to the U.S. autocatalyst. It clustered along with European road dust (BCR-720) and ambient PM from Sweden (64, 68), suggesting their common origin. Ambient PM from Spain (65), Germany (69, 71), and Italy (64, 67), were spatially separated from the European autocatalyst and road dust demonstrating differences in their PGE composition. These variances in PGE composition of road dusts, roadside soils, and airborne particles around the world can be attributed to geographical and temporal differences in autocatalyst composition, regional differences in vehicle fleets and other driving habits, climactic differences, and changes in their environmental transport and fate. Qualitative information on particulate matter sources identified using elemental ratios, ternary plots, and abundance sequences was further probed quantitatively using source apportionment tools as discussed below.


28


Table 5. Summary of Houston Area Source Apportionment Studies Using Aerosol Elemental Composition Range of source contribution estimates (%) Source type

PM2.5

PM10

Source signatures

North African dust, non-dust event (26)

1.3-25.4

2.0-31.5

Si, Ai, Fe, Na, Ca, Mg, K, Ti

North African dust, dust event (26)

35.3-67.3

36.7-76.8

Soil and road dust (10, 12, 20, 26, 47, 50)

2.4-51.0

9.9-39.8

Si, Mn, Ca, Fe, Ti, Mg, K, Al, Cs, Ba, Pb, Zn, Ni, Cr, V, Cu, La, Sm, Dy, Er

Si-rich source (10)

N.A.

24.5

Si, Zr, Sr, Cr

Ca-rich source (26, 47)

2.4-33.5

2.6-41.4

Ca

Motor vehicle (20, 26, 47)

2.8-35.7

0.8-30.0

Cr, Mn, Fe, Cu, Zn, Rh, Pd, Pt

Petroleum refineries, non-event (10, 12, 26)

0.6-2.1

0.3-2.3

La, Ce, Pr, Nd, Sm, Gd, Yb, Al, Si

Petroleum refineries, FCC event (20)

12.0

N.A.

Oil combustion & shipping activities (10, 12, 20, 26, 47)

0.4-11.0

0.2-35.0

V, Ni, Co, Sc, Mo

Coal combustion & high temperature operations (10, 12, 20, 26)

1.6-13.5

0.6-9.6

Se, As, Cd, Sn, Sb, Pb, Zn, Si

Industrial combustion (50)

16.9-17.6

N.A.

Zn, Cu, Ni, Fe, Si, K, Ca

Sea salt (12, 20, 26, 47, 50)

0.4-19.0

0.3-14.8

Na, Mg

Vegetative burning (20, 26, 47, 50)

0.9-18.6

N.D.

K

N.A.: Not analyzed, N.D. Not detected.



Apportionment of Primary Particulate Mass to Emission Sources Several investigators have used receptor modeling techniques based on elemental composition to determine that motor vehicles, petroleum refineries, oil combustion and shipping activities, coal combustion and other high temperature operations, vegetative burning, marine aerosols, resuspension of crustal material and road dust, and long-range transported North African dust all contribute to ambient PM2.5 and PM10 mass in Houston to different degrees (10, 12, 26, 32, 39, 47, 50). The number of sources, source tracers, and source contribution estimates are given in Table 5. Local and long-range transported mineral dust sources including North African dust, resuspended soil and road dust, and Ca-rich material are important contributors to both fine and coarse particles (26) During a 3-day outbreak in the year 2008, trans-Atlantic transport of North African dust accounted for more than half the PM2.5 and PM10 mass at two sites in the vicinity of the ship channel (i.e. Clinton Drive and Channelview). During this 3-day peak dust period, crustal elements such as Si, Al, Fe, Na, Ca, Mg, K, and Ti dominated Houston’s atmosphere, significantly reducing enrichment of all anthropogenic metals including lanthanoids referenced to the UCC. Importantly, evidence was provided from chemical mass balancing that small amounts of African dust may remain for several days before and after the peak event. Numerical modeling using a wide suite of major to trace elements also identified a calcium-rich material, potentially emanating from cement plants and gypsum material used to patch parking lots as major aerosol contributors (26, 47). Mineral materials such as resuspended combined soil and road dust (with Mn, Ca, Fe, Ti, Mg, K, Al as signatures), road dust (with Ba, Pb, Zn, Ni, Cr, V, Cu, La, Cs, Dy, Er) preferentially strongly contributed to PM10 (10, 12, 26, 47, 50). A Si-rich material that was co-emitted with Zr, Sr, and Cr was isolated for PM10. Emissions from cat-cracking units can also elevate Al and Si levels (20) in PM2.5 due to the aluminosilicate (zeolite) base of catalysts. Pure siliceous material, not measured elsewhere, has also been reported in single ultrafine particles in Houston (72) demonstrating unique Si sources in the industrialized ship channel region. Vegetative burning identified using K as a tracer and marine aerosols identified with Na and Mg are also important PM2.5 and PM10 sources in Houston (12, 20, 26, 47, 50). Interestingly, biomass burning contributed largely to fine particles whereas sea salt contributed to both PM2.5 and PM10. Source abundance profiles were developed for tailpipe and non-tailpipe PM2.5 and PM10 emissions from light-duty vehicles by making measurements inside the Washburn Tunnel (32). Elements such as Ca, Si, Fe, Al, Mg, K, and Ti displayed highest contributions especially in the coarse size mode. However, it would be nearly impossible to accurately isolate mobile source emissions using these elements since they are co-emitted in copious amounts via crustal resuspension. PGEs such as Rh, Pd, and Pt and other elements such as Cu, Zn, Ga, As, Zr, Mo, Cd, Sn, Sb, Ba, W, and Pb were prominently featured in the source profile. Enrichment factors for these elements in tunnel aerosols ranged between 3.5 for W and 36,575 for Rh in PM2.5 (see Table 2) (15, 32). Vehicular contributions to PM2.5 have been calculated to lie between 2.8 – 35.7% and between 0.8 – 30.0% 29 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


for PM10 mass (20, 26, 47) in Houston. Heavy duty vehicles were calculated to contribute to only ~5% of fine PM mass compared with 17.5% from light-duty vehicles (47). Marker elements signifying oil combustion and shipping activities include V, Ni, Sc, Mo (10, 12, 47). These metals have been measured in Houston aerosols apportioning oil combustion between 0.4 – 11% to PM2.5 and 0.2 – 35.0% to PM10 (10, 12, 20, 26, 47). Products of coal combustion and high temperature operations such as incineration, oil/coal fired boilers, smelters, and metal works such as Se, As, Cd, Sn, Sb, Pb, and Zn have also been identified (10, 12), apportioning these sources to 1.6 – 13.5% of PM2.5 and 0.6 – 9.6% of PM10 (10, 12, 20, 26, 50). As explained earlier, light lanthanoids including La, Ce, Pr, Nd, and Gd with minor contributions of Yb, Sm, Al, and Si has also been used to identify petroleum refining emissions (10, 12, 20, 26). Chemical mass balancing of samples collected in conjunction with cat-cracking emission events showed 12% contribution to PM2.520 whereas background refining contribution was only 0.6 – 2.1% (10, 12, 20, 26).

Concluding Remarks We summarized methodologies for digesting particulate matter and subsequent DRC-q-ICP-MS measurement of a suite of elements at trace – major levels. Long-term monitoring revealed the elemental signatures of several local and global sources contributing to ambient particulate matter levels in Houston. Specifically, measurements of lanthanoids and PGEs at trace levels are important to track primary emissions from petroleum refineries and light-duty gasoline-driven motor vehicles, respectively. Ambient aerosols in Houston are typically enriched in lanthanum and display significant differences in the lanthanoid abundance sequence compared with crustal material. This allows us to differentiate natural versus anthropogenic aerosol origins especially during non-routine operations of refinery catalytic cracking units. However, the lanthanoid composition of airborne particles was sometimes modified to closely resemble the crustal signature and substantially diluting the La-anomaly. These elemental signatures coupled with receptor modeling revealed periodic influences of trans-Atlantic dust transport from North Africa on aerosol mass concentrations in Texas. Concentrations of the second group of elements serving as source signatures, namely PGEs, were measured in road dust and airborne particles in a tunnel environment. A high level of care is necessary during sample preparation and q-ICP-MS to accurately and precisely measure Rh, Pd, and Pt, since these siderophilic (i.e. iron-loving) elements are present only at pg m–3 levels in the atmosphere. Being siderophiles, PGEs have dissolved in iron and migrated to the earth’s core thereby depleting themselves from the UCC. Highest enrichment of PGEs was measured inside the tunnel demonstrating them to be unique tracers for gasoline-driven automobile emissions. PGE composition of several United States matrices deviated substantially from corresponding European and Asian samples, emphasizing their geographical variability. Therefore, we recommend 30 In Trace Materials in Air, Soil, and Water; Evans, Kendra R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


PGE measurement in local samples for robust quantification of automobile contributions to measured aerosol mass. In closing, aerosols in industrialized urban environments originate from numerous local, regional, and global sources. Detailed information on individual sources and their contributions is necessary to better protect public health from particulate air pollution. One approach towards this goal that we have pursued over the years is to comprehensively measure the elemental composition of aerosols and use it as inputs to receptor models. This procedure identifies potential sources and quantifies their separate contributions to observed ambient PM2.5 and PM10 mass concentrations. It is emphasized that this method addresses only “primary” particles (i.e. those that are directly emitted) and explicitly ignores “secondary formation” (i.e. particles that are formed in the atmosphere from gaseous precursors). Nevertheless, such datasets can provide the scientific basis for the improvement of policies designed to protect public health associated with particulate air pollution. They also aid in improving public policy by focusing on relevant sources and in developing regulations based on knowledge of important sources that affect local ambient air quality.

Acknowledgments Portions of this work were funded by the Texas Commission on Environmental Quality and the Texas Air Research Center. We appreciate discussions with Profs. Matthew Fraser of Arizona State University and Joe Prospero of University of Miami during the course of this research.

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