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Indoor/Outdoor Relationships and Anthropogenic Elemental Signatures in Airborne PM2.5 at a High School: Impacts of Petroleum Refining Emissions on Lanthanoid Enrichment Ayse Bozlaker, Jordan Peccia, and Shankararaman Chellam Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06252 • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017

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Environmental Science & Technology

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Indoor/Outdoor Relationships and Anthropogenic Elemental Signatures in Airborne PM2.5 at a High School:

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Impacts of Petroleum Refining Emissions on Lanthanoid Enrichment

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Ayse Bozlaker1, Jordan Peccia2, and Shankararaman Chellam1, 3, *

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1

Department of Civil Engineering, Texas A&M University, College Station, TX 77843-3136

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2

Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06520

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Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122

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*

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Department of Civil Engineering, Texas A&M University, College Station, TX 77843-3136. Phone: (979) 458 5914;

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email: [email protected]

J. Walter “Deak” Porter ‘22 & James W. “Bud” Porter ’51 Professor and corresponding author, Zachry

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ABSTRACT: Outdoor emissions of primary fine particles and their contributions to indoor air quality

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deterioration were examined by collecting PM2.5 inside and outside a mechanically ventilated high school in the

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ultra-industrialized ship channel region of Houston, TX over a 2-month period. By characterizing 47 elements

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including lanthanoids (rare earth elements), using inductively coupled plasma-mass spectrometry, we captured

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indoor signatures of outdoor episodic emissions arising from non-routine operations of petroleum refinery

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fluidized-bed catalytic cracking units. Average indoor-to-outdoor (I/O) abundance ratios for the majority of

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elements were close to unity providing evidence that indoor metal-bearing PM2.5 had predominantly outdoor

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origins. Only Co had an I/O abundance ratio > 1 but its indoor sources could not be explicitly identified. La and

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17 other elements (Na, K, V, Ni, Co, Cu, Zn, Ga, As, Se, Mo, Cd, Sn, Sb, Ba, W, and Pb), including air toxics were

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enriched relative to the local soil both in indoor and outdoor PM2.5 demonstrating their anthropogenic origins.

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Several lines of evidence including receptor modeling, lanthanoid ratios, and La-Ce-Sm ternary diagrams pointed

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to petroleum refineries as being largely responsible for enhanced La and total lanthanoid concentrations in the

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majority of paired indoor and outdoor PM2.5.

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TOC/Abstract Art

24 25 26

outdoor PM from petroleum refining causing indoor La enrichment

crustal and road dust PM

outdoor PM from motor vehicles

school indoor air quality

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outdoor PM from biomass burning

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INTRODUCTION

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Airborne fine particulate matter (PM2.5) is an important concern in schools since children can potentially

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develop respiratory diseases following exposure in the indoor environment.1-3 Metal-enrichment in PM2.5 can

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further increase the risk of adverse cardiovascular, brain, and respiratory health effects.4-6 This is of particular

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importance for schools located in industrialized regions since anthropogenic metal emissions to the ambient

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outdoor atmosphere can transport indoors and negatively impact children’s health.7-9

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One interesting urban test-bed to evaluate transport of metal-enriched outdoor aerosols into schools is

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Houston, Texas, which is the 5th largest metropolitan region of the United States with a population of over 6

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million. This area is home to a ship channel that serves United States’ largest petrochemical complex, the second

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busiest port by cargo volume, and myriad other concatenating industries. Epidemiological research and risk

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analysis has implicated Houstonians’ increased cardiopulmonary risk and mortality to exposure to PM2.5 and some

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of its elemental components including Cu, As, Ni, and V10-12 that have been traced to anthropogenic sources near

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the ship channel.13-16 The city of Galena Park is located in the proximity of the ship channel and is predominantly

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populated by Hispanics (81.4%) with lesser numbers of African Americans and Native Americans (7.6%) according

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to the 2010 census. The 2010 median per capita income of its residents was only $13,650 with 20.4% of them

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living below the poverty line.17 High concentrations of outdoor PM2.5 and metals have been reported historically in

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this region13-16 suggesting a disproportionate exposure of particulate contaminants to the low-income minority

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population raising environmental justice concerns. Although concentrations, sources, and composition of ambient

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outdoor aerosols near the Houston Ship Channel have been investigated,13-15, 18-21 no measurements of indoor

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aerosols in this region have been reported.

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To date, very few measurements of the trace elemental composition of indoor and outdoor PM2.5 have been

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performed for United States’ schools,22-26 although more data are available for homes and residential buildings.27-

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indoor enrichment of several elements including hazardous air pollutants such as Sb and Pb.25 In contrast, a

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unique aspect of Houston area aerosols is their lanthanoid (i.e. rare earths) enrichment emanating from fluid

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catalytic cracking (FCC) units.13, 14, 18, 20 Airborne rare earth elements are readily absorbed by humans30, 31 leading

These studies have largely focused on representative and transition elements, with one study showing the

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to a variety of toxicological symptoms.32 Additionally, children exposed to refinery emissions exhibit increased

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wheezing, bronchial inflammation, reduced lung function and other symptoms33, 34 necessitating this research.

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The principal objective of this manuscript is to evaluate the extent of infiltration and/or ventilation of outdoor

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PM2.5 into a school located in a highly industrialized region. We are particularly interested to determine whether

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petroleum refining emission events14, 18, 35 that release substantial amounts of aerosols enriched in rare earths and

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other metals14, 18, 20 influence indoor PM2.5 concentrations. To accomplish this, paired indoor and outdoor PM2.5

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was collected from a high school in the Houston Ship Channel region over a two-month span and comprehensively

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analyzed for lanthanoids, transition metals and representative elements. To our knowledge, this is the first work

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to comprehensively measure the concentrations of all 14 lanthanoids in PM2.5 inside a United States school and

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relate it to outdoor anthropogenic contamination. Impacts of outdoor emissions on indoor PM2.5 composition

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were qualitatively investigated using indoor/outdoor (I/O) ratios, elemental abundance patterns, enrichment

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factors, tracer element ratios, and ternary diagrams. Contributions of primary outdoor sources to indoor and

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outdoor PM2.5 mass were quantified by chemical mass balance (CMB) modeling,36, 37 which further allowed us to

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isolate sources that enriched lanthanum and total lanthanoids in indoor and outdoor PM2.5.

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MATERIALS AND METHODS

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Sampling. Samples were collected at a high school (latitude: 29.7875, longitude: -95.1828) in Galena Park, TX

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that is attended by approximately 4,500 students and surrounded by Interstate and State Highways (Supporting

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Information, SI-Figure S1 and SI-Figure S2). The windows of the two-storied, 55-year old brick building were

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always kept closed because of local weather conditions and its close proximity to the densely industrialized ship

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channel region (including FCC refineries, ships, and barges) and heavily trafficked roads. During the sampling

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period, winds were predominantly blowing from the South with an average speed of 6.8±3.0 miles per hour (SI-

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Figure S3). The HVAC system was operated continuously and consisted of chilled water system with two 600-ton

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York Chillers (Model AP-215), air handling units, rooftop units and variable air volume units in each room and

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hallways operated with 85% return air (15% fresh air). The minimum efficiency reporting value (MERV) rating of

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air filters was 9, which is expected to remove only 30-35% of PM2.5 mass.38 Hence, potential routes of entrance of

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outdoor PM2.5 includes infiltration through the building envelope and through active outdoor air ventilation ACS Paragon Plus Environment Page 4

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coupled with passage through filters. The air exchange rate was not explicitly measured, since it would only

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provide limited insight into the I/O ratio due to the other complex processes (filtration, deposition, and

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resuspension) that also must be characterized to understand the building-based contributions to the I/O ratio.39

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Indoor and outdoor PM2.5 were collected using Partisol-Plus 2025 sequential air samplers (Rupprecht &

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Patashnick) operated at a flow rate of 1m3/h. The indoor sampler was located on a 1m high countertop in an

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unoccupied stockroom adjacent to a teacher’s office on the second floor, which had two air conditioning vents.

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Since its doors were typically kept shut, our measurements possibly represent a lower bound on the students’ PM

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exposure. Outdoor PM2.5 was collected from the second floor roof on the West side of the building overlooking

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the baseball field from a height of ~8m aboveground (SI-Figure S2). Thirteen paired indoor and outdoor PM2.5

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samples (26 total) were collected on Whatman 46.2mm diameter PTFE membrane filters over 2-6 consecutive

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days between March 11 and May 11, 2011 (SI-Table S1).

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Sample Preparation and Analysis. Blank filters and indoor and outdoor PM2.5 samples were acid-digested

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(HNO3, HF, H3BO3) in two-stages using Teflon vessels and a microwave oven (HP-500, MARS 5, CEM Corp.).20, 40

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Sixteen representative elements (Na, Mg, Al, Si, K, Ca, Ga, As, Se, Rb, Sr, Sn, Sb, Cs, Ba, Pb), 15 transition metals

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(Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Cd, Hf, W), 14 lanthanoids (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,

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Tm, Yb, Lu) and 2 actinides (Th, U) were analyzed by a quadrupole inductively coupled plasma-mass spectrometer

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equipped with a reaction cell (DRC-q-ICP-MS; ELAN DRC II, PerkinElmer SCIEX). Ammonia was used as the cell gas

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to reduce mass interferences for Al, Ca, V, Cr, Mn, Fe, Ni, Co, Cu, Zn, As, Se, and Cd. Multi-element calibration

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standards (n=6) and mixed internal standard of 115In and 209Bi (20µg/L) were freshly prepared by dilution of single-

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element stock solutions (10µg/mL, High-Purity Standards) with 0.4M HNO3. The high accuracy and precision of our

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microwave digestion and ICP-MS protocols were demonstrated by (i) quantitatively recovering all certified and

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uncertified elements (86-119%) from Standard Reference Materials (SRMs) SRM1648a (urban PM) and SRM1633b

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(coal fly ash) as shown in SI-Figure S4, (ii) analyzing four laboratory and field blanks, (iii) completely recovering

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pre-digestion Ru spikes (102-109%), and (iv) accurately analyzing the mid-point calibration standard during sample

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measurements (91-119%). More quality control/assurance details are provided in the Supporting Information

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section and SI-Table S2. ACS Paragon Plus Environment Page 5


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RESULTS AND DISCUSSION

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Indoor and Outdoor PM2.5 Mass Concentrations. Figure 1 depicts a time series of indoor and outdoor PM2.5

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mass concentrations (see also SI-Table S1). As observed, indoor and outdoor levels were consistently in-phase,

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exhibiting collocated peaks and valleys, and consequently significantly positively correlated (R2 = 0.73, p < 0.01)

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expected of rooms with no indoor sources of particles. Indoor mass concentrations varied from 2.3 to 4.1µg/m3,

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which is in the lower range of reported values in United States schools (2.4-37.0µg/m3).22, 26, 41, 42 Importantly,

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indoor levels (3.0±0.52µg/m3) were 3.8-5.5 times lower than corresponding outdoor values (13.4±1.4µg/m3)

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leading to very low average I/O mass concentration ratio of 0.22±0.02. The low I/O ratio and reduced coefficient

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of variation (only 11%) suggests limited penetration of outdoor PM2.5 though the building envelope, the

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perpetually closed windows, and filters employed in the mechanical ventilation system.43 The strong correlation

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between indoor and outdoor PM2.5 mass values as well as indoor PM2.5 mass and corresponding I/O ratios (R2 =

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0.68, p < 0.01) suggest dominance of outdoor sources for indoor PM2.5.44 18

-3

Mass concentration (µg m )

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16 Outdoor PM2.5

14 12 10 4

Indoor PM2.5

May 8

May 2

Apr 27

Apr 21

Apr 15

Apr 9

Apr 3

Mar 28

Mar 22

Mar 20

Mar 18

Mar 14

117 118

Mar 12

2

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Figure 1. Time series of indoor and outdoor mass concentrations. Note the break in the y-axis between 4-10µ µg/m .

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Airborne Elemental Concentrations and Their Indoor and Outdoor Relationships. Associated with PM mass,

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indoor elemental concentrations were also substantially lower than the corresponding outdoor values (SI-Figure

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S5 and SI-Figure S6). The sum of the airborne concentrations of 47 target elements in PM2.5 was 0.31±0.16µg/m3

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for indoor and 1.4±0.50µg/m3 for outdoor, contributing 6-23% (11% on average) to both indoor and outdoor PM2.5

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mass. Crustal and sea salt components including Na, Mg, Al, Si, Ca, Ti, Mn, and Fe dominated total elemental


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concentrations in I/O PM2.5 accounting for 97% on average suggesting outdoor origins of indoor PM.

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numbered heaviest lanthanoids, i.e. Tm and Lu (and the actinide Th) were detected in < 20% of indoor samples

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and therefore excluded from further analysis. This is attributed to the lower abundances of heavy lanthanoids

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compared with their lighter counterparts in the Earth’s crust45 and in other lanthanoid sources,13, 46-48 and very low

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indoor PM2.5 mass concentrations. Importantly, individual lanthanoids (excluding Tm and Lu) were well-correlated

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with PM2.5 mass (r=0.70±0.05) and with each other (r=0.82±0.20) in both indoor and outdoor PM2.5. These results

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coupled with I/O concentration ratios < 1 for individual lanthanoids (0.11±0.09 for La to 0.47±0.32 for Ho) and

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Σ12Ln (sum of the concentrations of 12 lanthanoids; 0.14±0.10) reflect their tendency to be primarily released in

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unison. Hence, these elements were grouped separately from other representative elements and transition

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metals in further discussions. Data were also expressed as µg element per g PM2.5 in Figure 2 (i.e. elemental

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abundance) to account for significantly different indoor and outdoor PM2.5 mass concentrations. As seen, average

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I/O abundance ratios were close to unity for the majority of elements providing additional evidence for infiltration

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and/or ventilation of outdoor aerosols. 40

-1

-1

Average indoor abundances (µg g )

Further, although all targeted elements were consistently detected in outdoor samples, the two odd-

Average indoor abundances (µg g )

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Ce

10

La

Nd

1

Pr Gd

Sm

Dy

Er

Eu

Yb 0.1 Tb

(a)

Ho

0.1

1

10

40 -1

Average outdoor abundances (µg g )

10

5

Si

10

4

10

3

10

2

10

1

10

0

Al

Ca

K Fe Zn SnV Ti Ni Sb Cu Ba Cr Mn Pb As Zr Sr Se Mo Ga Rb

Co

Cd

Mg Na

W Y Cs Hf

(b)

U

10

-1

10

-1

10

0

10

1

10

2

10

3

10

4

-1

10

5

Average outdoor abundances (µg g )

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Figure 2. Comparison of average elemental abundances in indoor and outdoor PM2.5. The solid diagonal line represents a 1:1 relationship. Lanthanoid elements are shown on the left panel (a) and all the other elements are on the right panel (b).

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Exceptions to this general rule included Na, Mg, Ga, Ba, and La (depicted in red color in Figure 2), which

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exhibited low average I/O abundance ratios of 0.08±0.04, 0.20±0.08, 0.32±0.19, 0.17±0.06, and 0.40±0.22

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respectively. As expected, Na, Mg, Ga, Ba, and La had 6-30, 3-9, 2-10, 3-12, and 2-8 fold elevated abundances in



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outdoor PM2.5, respectively. Only Co (depicted in blue color in Figure 2b) had an average I/O abundance ratio > 1,

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arising from episodic indoor releases of Co-bearing PM2.5, causing 10-330 fold elevated abundances in 5 samples

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inside the school. The average cobalt I/O abundance ratio for the remaining 8 samples approached unity

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(1.1±0.36) indicating that infiltration and/or ventilation of outdoor PM2.5 was the major indoor source even for

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this element.

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Anomalies in Indoor and Outdoor PM2.5 Elemental Compositions with Respect to the Local Soil. PM2.5

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elemental abundances were normalized to corresponding local soil values49 to identify emission signatures of

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metal-bearing PM2.5 sources. Figure 3 depicts results for two representative paired indoor and outdoor samples,

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where again lanthanoids and non-lanthanoids are shown separately for clarity’s sake. In all cases, patterns in

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Figure 3a through Figure 3d were irregular (i.e. not horizontal), representing anomalies of numerous elements

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compared to the local soil. Additionally, normalized indoor and outdoor profiles for all elements with the

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exception of Na, Mg, Co, Ga, and Ba (the same elements identified in Figure 2) were in-phase in Figure 3c and

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Figure 3d, providing further evidence that indoor levels were predominantly controlled by infiltration and/or

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ventilation of metal-bearing outdoor PM2.5. Again, only Co was more abundant inside the school as a consequence

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of its indoor generation.


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Indoor PM2.5 Outdoor PM2.5

0.1

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

3

10

(c)

April 6 - 12 Indoor PM2.5

2

10

Outdoor PM2.5

1

10

0

10

-1

10

Indoor PM2.5 Outdoor PM2.5

0.1

La

Yb

Na Mg Al Si K Ca Ti V Cr Mn Fe Ni Co Cu Zn Ga As Se Rb Sr Y Zr Mo Cd Sn Sb Cs Ba Hf W Pb U

Sample abundance / Local soil abundance

160

April 12-18

(b) 1

10

2

Ce

Pr

(d)

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Yb

April 12 - 18 Indoor PM2.5 Outdoor PM2.5

10

1

10

0

-1

10

Na Mg Al Si K Ca Ti V Cr Mn Fe Ni Co Cu Zn Ga As Se Rb Sr Y Zr Mo Cd Sn Sb Cs Ba Hf W Pb U

April 6 - 12


(a)

1



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161 162 163 164 165

Figure 3. Comparison of elemental distribution patterns for two representative paired indoor and outdoor PM2.5 samples after 49 normalization to corresponding local soil values. Note that the time period April 6-12 corresponds to “background” and April 12-18 corresponds to refinery emission events as will be discussed later. Lanthanoids for the two time periods are shown in the top panels (a) and (b) and non-lanthanoids are shown in the bottom panels (c) and (d).

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Lanthanum exhibited dichotomous behavior after normalization50 with distinct positive anomalies (La >> Ce)

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in 10 paired samples (shown in Figure 3b for one representative sample) whereas La ≈ Ce for the remaining 3

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paired samples (shown in Figure 3a for one representative sample). Local soil-normalized lanthanoid patterns in

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indoor and outdoor PM2.5 were compared with various lanthanoid-bearing sources including FCC catalysts,13 #6

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grade fuel oils,47 steel plant dust,46 coal-fired power plant emissions,48 vehicular emissions,51 and local road dust49

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(SI-Figure S7). Distinct La enrichment over Ce in the event samples corresponds to the lanthanoid-pattern in FCC

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catalysts, fuel oils, and steel plant emissions (Figure 3b and SI-Figure S7) whereas similar normalized La and Ce

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values signifies largely crustal origins for non-event samples (Figure 3a). An identical pattern of La enrichment

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over Ce and other heavier lanthanoid members in PM2.5 in Houston, TX and other locations has been primarily



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attributed to emissions from FCC units in petroleum refineries with lesser contributions from oil combustion and

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shipping activities.13, 14, 18, 20, 47, 52-54

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These observations of the degree of anthropogenic modifications to elemental abundances in PM2.5 compared

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to the local soil in Figure 3 were confirmed by examining enrichment factors using Ti in local soil as the crustal

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reference (SI-Figure S8). Elevated La enrichment factors were calculated for nearly all indoor and outdoor samples

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that exhibited positive La anomalies (corresponding to Figure 3b). Additionally, La was slightly more enriched in

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outdoor PM2.5 (3.4-24.3) than indoor (3.7-8.2) consistent with the decrease in La abundance in indoor PM2.5

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(Figure 2a). This is attributed to preferential La-enrichment in the coarser fraction of FCC stack emissions (0.7-14

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µm)55, which infiltrates and/or ventilates to a lesser extent than the fine fraction.56 Further, enrichment factors for

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light lanthanoids (La, Ce, Pr, Sm, and Gd) in indoor and outdoor PM2.5 with respect to Nd in FCC catalysts were ≈ 1

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(1.3±0.9) verifying that their origin can be predominantly traced to petroleum refining FCC units.13 The only

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exception was Eu that exhibited high enrichment factors (19.4±8.5) in both indoor and outdoor PM2.5 with respect

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to the FCC catalysts and showed positive anomaly in their local soil-normalized lanthanoid patterns (Figure 3a and

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Figure 3b) as in other environmental samples50 also reflecting possible contributions from vehicular emissions and

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road dust (SI-Figure S7).

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Distinct positive anomalies for non-lanthanoids including Na, K, V, Ni, Co, Cu, Zn, Ga, As, Se, Mo, Cd, Sn, Sb,

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Ba, W, and Pb were also observed for two representative indoor and outdoor samples (Figure 3c and Figure 3d).

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These elements have been commonly reported to be emitted by anthropogenic sources in the Houston Ship

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Channel area.14-16, 18, 20 Average enrichment factors of these elements in indoor PM2.5 were between 3 (Zn, Ga,

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and Ba) and 307 (Sb) with respect to Ti in the local soil demonstrating their non-crustal origins. Importantly, all

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these elements enriched inside the school were also enriched in outdoor PM2.5 (SI-Figure S8). Greater abundances

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and correspondingly higher enrichment factors of Ga and Ba in outdoor PM2.5 (Figure 2b, Figure 3c, Figure 3d, and

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SI-Figure S8) are traced to road dust.49,

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attributed to sea salt.57, 58 Another sea salt component, Na displayed weak excursions in local soil-normalized

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indoor PM2.5 whereas it showed distinct positive anomalies and correspondingly higher enrichment factors in

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outdoor PM2.5 (Figure 2b, Figure 3c, Figure 3d, and SI-Figure S8). Similar results have been also been reported for

51

Mg exhibited positive anomalies in outdoor PM only57 and was


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Houston homes where mechanically generated aerosol particles including road dust, soil, and sea salt had very

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low infiltration factors resulting in their low abundances in the indoor environment.56

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Cobalt was highly enriched in five indoor PM2.5 samples (average enrichment factor = 245 with respect to the

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local soil) suggesting its episodic indoor emissions59 even though it was only slightly enriched in outdoor PM2.5

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(average outdoor enrichment factor = 6). Although its indoor source could not be definitively identified, cobalt has

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also been previously reported to be enriched in indoor dust and aerosols,60, 61 and probably arose from paints,

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lacquers, and/or building materials. All other elements showed crustal origins since average enrichment factors

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were close to unity. Indoor and outdoor enrichment factors for all other non-crustal elements were comparable

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(SI-Figure S8) due to their prevailing origins in infiltration and/or ventilation.

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Source Contributions to Indoor and Outdoor PM2.5 Mass with Emphasis on Lanthanoids. Details of the CMB

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procedure, source profiles, and input data statistics are given in the SI (SI-Table S2). Since the dominant source of

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indoor PM2.5 was infiltration and/or ventilation, outdoor sources of primary particles and their individual

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contributions to indoor and outdoor PM2.5 mass were computed by CMB. Robust model runs were achieved as

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evidenced by high R2 (0.81-0.97; average 0.91) and low χ2 (0.2-2.4; average 0.69).37 CMB apportioned 46-95%

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(average 62%) of indoor and 54-95% (average 76%) of outdoor PM2.5 mass to 10 outdoor sources. SI-Figure S9 and

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SI-Figure S10 summarize average and daily contributions of individual sources to indoor and outdoor PM2.5 mass

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over the entire sampling period, respectively. As seen, vehicular emissions contributed most to PM2.5 mass

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consistent with the school’s proximity to several heavily trafficked highways as shown in the site map (SI-Figure

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S1) and the importance of gasoline-driven vehicles to ambient PM in Houston, TX.49, 51, 62 Similar results were also

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reported for indoor and outdoor PM2.5 in a California school that was located near busy roadways.24 This was

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followed by vegetative burning, soil and road dust, petroleum refineries, Ca-rich material, oil combustion, and

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coal combustion. Sea salt contributed about 5% to outdoor PM2.5 but negligibly to indoor PM2.5 consistent with

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Figure 3 leading to its resolution only for outdoor PM2.5 similar to an earlier investigation of Houston-area

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residences.56 Incineration and steel plants played negligible roles for both indoor and outdoor PM2.5. Importantly,

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contributions of individual outdoor sources to indoor and outdoor PM2.5 were very similar, with slightly higher

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values estimated for outdoor PM2.5, consistent with infiltration and/or ventilation being the dominant source of ACS Paragon Plus Environment Page 11


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indoor PM2.5. The only exception was for soil/road dust (10% for indoor versus 5.5% for outdoor on average).

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CMB modeling also allowed us to isolate outdoor sources of lanthanoid-rich particles infiltrating and/or

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ventilating indoors and resulting enrichment in La and total lanthanoid content of indoor PM2.5. Since only FCC

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units and oil combustion were calculated to be significant sources of La-rich particles, a time series of their

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estimated contributions to each indoor and outdoor sample are illustrated in Figure 4 together with the

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corresponding La fraction of total lanthanoids, ∑12Ln. As seen, over the entire study duration, refinery FCC units’

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contributions to total PM2.5 mass were low, averaging only 0.14±0.09 and 0.85±0.62µg/m3 for indoor and outdoor

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PM2.5 respectively. Lowest FCC catalyst contributions were calculated for samples collected on March 17-19,

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March 21-23, and April 6-12, averaging < 1.5% of both indoor and outdoor PM2.5 mass. Corresponding La fraction

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in total lanthanoids (Σ12Ln) were 19%-24% (21±3%) for indoor PM2.5 and 24-34% (29±5%) for outdoor PM2.5, which

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are in the range (21-31%) associated with crustal and light-duty vehicular emissions45, 49, 51 as discussed in Figure

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3a. Hence, indoor and outdoor PM2.5 during these days correspond to routine operation of FCC units.13, 14, 18, 19 FCC unit

Oil combustion/shipping

-3

Source contribution estimates (µg m )

Outdoor

(a)

80

2.8

60

2.1 1.4

40

0.7

20

0.0

0

0.4

100

(b)

Indoor

80

0.3 60 0.2 40 0.1

20

May 5-11

Apr 30-May 5

Apr 24-30

Apr 18-24

Apr 12-18

Apr 06-12

Mar 31-Apr 6

Mar 25-31

Mar 21-23

Mar 19-21

Mar 17-19

Mar 13-15

0 Mar 11-13

0.0

La contribution to total lanthanoids (%)

100 3.5

239 240 241 242 243

Figure 4. Contributions of the two major La-rich PM emission sources (refinery fluid-cracking catalysts and oil combustion/shipping activities) estimated by CMB modeling and shown in the left y-axis as a stacked-bar graph. Percent contributions of La to ∑12Ln content of indoor and outdoor PM2.5 are shown in the right y-axis in brown color as a time-series line plot. Results for outdoor air are shown in the top panel (a) and for indoor air are shown in the bottom panel (b).

244

In contrast to these three paired samples, the remaining 10 paired indoor and outdoor PM2.5 displayed ACS Paragon Plus Environment Page 12

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245

significant La enrichment. Their La fractions in Σ12Ln were between 40-59% (49±7%) for indoor and between 40-

246

83% (61±14%) for outdoor PM2.5, closely mirroring the range reported for FCC catalysts (40-79%)13 and #6 grade

247

fuel oil (30-54%).47 Average contributions of FCC catalyst emissions to these 10 samples increased to 5.0±2.3%

248

(2.0-8.1%) for indoor PM2.5 mass and 7.9±4.6% (3.0-18.2%) for outdoor PM2.5 mass (SI-Figure S9 and SI-Figure S10)

249

in accordance with earlier reports of oil-cracking catalysts emission events in Houston, TX.14, 18, 19 Strong direct

250

correlations between the La fraction in Σ12Ln and estimated FCC source contributions in indoor and outdoor PM2.5

251

(also evidenced by co-located peaks and valleys in Figure 4 and Pearson r values of 0.61 for indoor PM2.5 and 0.89

252

for outdoor PM2.5) demonstrated that La enrichment was largely determined by episodic emissions from proximal

253

petroleum refineries.14,

254

encompassed a short-lived episodic release along with a longer period of routine FCC operations. Measured La

255

enrichment was further verified using lanthanoids as elemental tracers as explained in the next section.

256 257

18, 20, 52, 53, 63

Since PM was collected over 2-6 days, the 10 La-enriched samples likely

The only other significant source of La-rich PM2.5, i.e. oil combustion/shipping activities, contributed far less; averaging only 0.05±0.01µg/m3 to indoor PM2.5 mass and 0.35±0.24µg/m3 to outdoor PM2.5 mass (Figure 4).

258

Tracking Episodic Catalyst Emissions from Petroleum Refineries Using Lanthanoids. Shifts from crustal to

259

anthropogenic signatures of lanthanoids were used to identify PM influenced by non-routine FCC unit

260

operations.14, 18, 20, 52 To this end, Table 1 summarizes concentration ratios of La to other light lanthanoids (Ce, Pr,

261

Nd, and Sm) and V calculated for indoor and outdoor PM2.5 along with those previously reported for ambient

262

PM2.5 in Houston14, 19 and for local soil.49 The same ratios reported for average UCC45 and other potential outdoor

263

sources of lanthanoid-bearing PM13, 46, 48, 49, 51 are provided in SI-Table S3. As seen, all ratios for the 3 paired indoor

264

and outdoor samples identified in the previous section as corresponding to routine FCC operation were coincident

265

with complementary local soil/road dust and earlier reports of ambient PM2.5 collected during routine FCC

266

operation. This confirms that lanthanoids in indoor and outdoor PM2.5 collected on March 17-19, March 21-23,

267

and April 6-12 predominantly originated from resuspension of crustal material.



268 269 270

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Table 1. Tracer element ratios (as a range of min – max (mean) values) for paired indoor and outdoor PM2.5 samples that were collected at the school during routine and non-routine operations of FCC units in petroleum refineries along with those previously reported values 14, 19 49 for ambient PM2.5 from the Houston Ship Channel area and for local soil. Indoor PM2.5 (This study)

Ambient PM2.5 from the Houston Ship Channel area

Outdoor PM2.5 (This study)

Ratios Event (n=10)

Non-Event (n=3)

Event (n=10)

Non-event (n=3)

La/Ce

1.0 - 3.9 (1.7)

La/Pr

10.8 - 25.9 (17.3)

0.33 - 0.48 (0.42)

1.1 - 7.6 (3.3)

4.1 - 4.8 (4.5)

13.7 - 103. 6 (41.1)

La/Nd

2.8 - 11.9 (6.4)

1.3 - 1.6 (1.4)

La/Sm

14.6 - 58.5 (38.5)

La/V

0.01 - 0.04 (0.02)

Local soil

Event

Non-event

0.55 -0.87 (0.72)

1.0 - 6.8 (2.6)

0.51 - 0.89 (0.70)

0.63

5.9 - 10.3 (8.2)

6.9 - 40.2 (18.2)

4.5 - 8.0 (6.4)

5.8

3.8 - 29.6 (11.9)

1.7 - 2.8 (2.3)

2.4 - 22.7 (7.2)

1.2 - 2.2 (1.8)

1.5

4.8 - 14.6 (9.2)

20.9 - 120.5 (46.0)

5.0 - 11.8 (8.0)

12.1 - 76.8 (36.6)

6.5 - 10.1 (8.7)

7.1

0.003 - 0.02 (0.01)

0.02 - 0.07 (0.05)

0.01 - 0.02 (0.01)

0.01 - 1.8 (0.17)

0.02 - 0.23 (0.09)

1

271

Consistent with modeling results, lanthanoid patterns in the other 10 paired indoor and outdoor PM2.5 were

272

distorted from crustal material, exhibiting elevated La concentrations over other lanthanoids (e.g. La/Ce = 1.0-7.6;

273

Table 1). La to Ce, Pr, Nd, and Sm ratios in all 10 paired indoor and outdoor PM2.5 samples influenced by FCC

274

events overlapped with those previously reported for oil-cracking catalysts13, 48 and oil-fired plants47, 48 as well as

275

those for Houston aerosols exposed to episodic catalysts releases from refinery FCC units14, 19 (Table 1 and SI-

276

Table S3). Outdoor Σ12Ln concentrations doubled under the influence of episodic FCC emissions

277

(551.1±241.7pg/m3) from the background (273.1±89.9pg/m3) similar to earlier reports.14, 20 Corresponding average

278

Σ12Ln concentrations in paired indoor samples also increased from their background (44.5±26.7pg/m3) to

279

60.3±34.5pg/m3 during FCC events.


35

-3

-3

Lanthanoid concentrations in PM2.5 (pg m )

Environmental Science & Technology Lanthanoid concentrations in PM2.5 (pg m )

Page 15 of 21

Indoor PM2.5 30 25 20

FCC events (n = 10)

La

La/Ce = 1.7 La/Pr = 17.3 La/Nd = 6.4 La/Sm = 38.5

Ce

15 2

R = 0.94, p < 0.01

10 5

Nd Eu Er Sm Pr 0 Yb DyGd TbHo 0

(a)

2000

4000

6000

400 350 300 250

2

50

Eu Nd Yb Gd Pr Er Sm 0 Ho TbDy

(b)

2000

4000

6000

8000 -1

Lanthanoid concentrations in FCC catalysts (µg g ) Lanthanoid concentrations in PM2.5 (pg m )

-3

-3

Lanthanoid concentrations in PM2.5 (pg m )

Ce

Routine days (n = 3) La/Ce = 0.42 La/Pr = 4.5 La/Nd = 1.4 La/Sm = 9.2 La

10 2

R = 0.97, p < 0.01 Nd 5

0

Ce

0

Indoor PM2.5

15

R = 0.97, p < 0.01

100

-1

20


150

8000

25

La

FCC events (n = 10)

200

Lanthanoid concentrations in FCC catalysts (µg g )

280

Outdoor PM2.5

Pr Gd Dy Eu Sm Ho YbEr 0

5

(c) 10

15

20

25

30

35

40 -1

Lanthanoid concentrations in the local soil (µg g )

120

Outdoor PM2.5

Ce

Routine days (n = 3) 100 80


La

60 2

R = 0.99, p < 0.01 40 20 0

Nd Pr Dy Sm Eu Gd Er Ho Tb Yb 0

5

(d) 10

15

20

25

30

35

40 -1

Lanthanoid concentrations in the local soil (µg g )

281 282 283 284 285 286

Figure 5. Positive and significant correlations between average lanthanoid concentrations in FCC catalysts and those in indoor and outdoor PM2.5 monitored during non-routine refinery FCC unit operations (top panels (a) and (b)). Episodic indoor and outdoor samples and cracking catalysts showed similar rare earth distribution patterns demonstrating that FCC units were the source of lanthanoidbearing PM2.5 for both indoor and outdoor environments. Indoor and outdoor PM2.5 samples collected during routine refinery 49 operations were strongly correlated with the local soil showing their crustal origins (bottom panels (c) and (d)).

287

It is emphasized that La-enrichment in samples need not overlap with episodic events because impacts at

288

receptor sites are also influenced by meteorology and atmospheric dispersion.63 The prevailing wind direction

289

during sampling was predominantly from the South (S, SSE, and SE; SI-Figure S1 and SI-Figure S3) transporting

290

emissions from the ship channel and possibly Galveston area to the receptor location. In any case, variations in

291

dispersion conditions during sampling could also influence the extent of La-enrichment in individual samples.

13

292

Excellent correlations were obtained between average lanthanoid concentrations in FCC catalysts and those in

293

indoor and outdoor PM2.5 under the influence of non-routine FCC operations (Figure 5a and Figure 5b).

294

Correlations between average lanthanoid concentrations in indoor and outdoor PM2.5 during routine refinery

295

operations and those in the local soil were also extremely strong (Figure 5c and Figure 5d). This confirms CMB ACS Paragon Plus Environment Page 15


Page 16 of 21

296

modeling results that lanthanoid enrichment was largely attributable to FCC catalysts during episodic events

297

whereas during routine FCC operation lanthanoid patterns were consistent with crustal resuspension.

298

Relative Compositions of Source and Receptor Particles. Additional clues to the relative influence of sources

299

rich in light lanthanoids on indoor and outdoor PM2.5 were obtained using La-Ce-Sm (Figure 6a) and La-Ce-V

300

(Figure 6b) ternary diagrams adjusted so that the local soil abundances appear at the centroid. As expected from

301

similar La to Ce, Pr, Nd, and Sm ratios (Table 1 and SI-Table S3), local soil and UCC were located near each other

302

(Figure 6a). Vehicular emissions were slightly enriched in Ce and depleted in La and Sm with respect to local soil.

303

Major local La-rich sources such as FCC catalysts and oil combustion/shipping migrate towards the La-apex

304

overlapping with the 10 paired indoor and outdoor PM2.5 collected during FCC episodes. In contrast, the 3 paired

305

indoor and outdoor samples collected during routine FCC operations (identified in earlier sections) clustered

306

around the local soil and vehicular emissions. 0.0

(a)

1.0

0.2

Sa

m na diu

Routine (This study)

0.4

0.8

Va

ma ri

m

Indoor and outdoor PM2.5

0.0 0.2

0.4

0.6

0.8

1.0

0.6

0.6

0.4

0.8

0.2

1.0

0.4

.62 x0

riu

um

FCC event (This study)

0.8

m riu

0.6

1.0

0.2

Indoor and outdoor PM2.5

0.4

Ce

x1

(b)

Ce

1. 2

4

0.8

0.6

0.0

0.0

Local soil UCC Local road dust Car emissions FCC catalysts Oil combustion / Shipping

0.2

1.0 0.0

0.0 0.2

0.4

0.6

0.8

1.0

Lanthanum x 0.99

Lanthanum x 1.58

307 308 309 310

Figure 6. Three component La-Ce-Sm (left panel, (a)) and La-Ce-V (right panel, (b)) plots showing PM2.5 influenced by FCC units and other 13, 47-49, 51, 64, 65 49 possible outdoor sources of lanthanoid-bearing particles. All values were adjusted so that the local soil appears at the center.

311

Simultaneous variations in La-Ce-V and the La/V ratio were used to differentiate FCC emissions from oil

312

combustion/shipping activities. Shipping, oil and pet-coke combustion cause enormous V enrichment relative to

313

La47, 52, 54, 64, 65 leading to characteristically low La/V ratios between 0.0001–0.11 depending on the type of fuel,

314

pushing these sources towards the V-apex in Figure 6b. In contrast, FCC catalysts13 and stack emissions from FCC

315

units have markedly higher La/V ratios of 131±70 and 13 respectively (SI-Table S3) aligning near the La apex in

316

Figure 6b. The La-Ce-V ternary diagram illustrates that all indoor and outdoor PM2.5 collected over the duration of


Page 17 of 21


317

this investigation were substantially enriched in V; also reflected in their very low indoor and outdoor La/V ratios

318

(0.003-0.07) corresponding to oil combustion/shipping activities (SI-Table S3).

319

Results reported herein demonstrate that most of the indoor PM2.5 mass and its metallic components in a

320

mechanically ventilated school located in a highly industrialized area arose via transport from the outside. Co was

321

the sole exception exhibiting enrichment only in indoor PM2.5. Quantifying lanthanoids that serve as unique

322

tracers for primary emissions from catalytic cracking units along with a wide suite of elements in paired indoor

323

and outdoor PM2.5 facilitated robust estimation of outdoor source contributions to indoor PM2.5 mass. Since

324

samples were collected from a largely unoccupied room inside the school, our measurements potentially reflect a

325

lower-bound in indoor air concentrations. Of the sources identified via CMB modeling, automotive emissions

326

contributed most to indoor and outdoor PM2.5 mass. Importantly, airborne fine particles originating during non-

327

routine operations of refinery FCC units were shown to penetrate the building envelope leading to La-enrichment

328

(with respect to local soil) in indoor PM2.5. Hazardous air pollutants such as Ni, As, Se, Cd, Sb, and Pb, enriched in

329

outdoor air with respect to local soil were detected inside the school environment, albeit at substantially lower

330

concentrations than outdoor values (e.g. 0.13ng/m3 in indoor and 0.61ng/m3 in outdoor for Ni). Finally, since

331

proximity to emission sources, source strength, and meteorological conditions govern outdoor air quality at

332

receptor locations,66 additional work is needed to investigate potential spatial and temporal differences in

333

children’s exposure to PM2.5 via infiltration and/or ventilation in schools.

334

Supporting Information. Additional details on sampling, laboratory work (including QA/QC), CMB modeling

335

procedures and estimated source contributions, signature elemental ratios, sampling site maps, wind roses,

336

elemental concentrations and I/O ratios, source profiles normalized to local soil levels, and enrichment factors are

337

provided. This material is available free of charge via the internet at http://pubs.acs.org.

338

Acknowledgments. The present study has been supported by NPRP award number NPRP 7-649-2-241 from the

339

Qatar National Research Fund (a member of The Qatar Foundation). Sima Mathew-Tanner, Will Payne, Steve

340

Paciotti, Nick Spada, and Suresh Danadurai assisted with sampling.

341 342 ACS Paragon Plus Environment Page 17


343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393

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