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Environmental Measurements Methods
Polyurethane Foam (PUF) Disk Samplers for Measuring Trace Metals in Ambient Air Eftade Emine Gaga, Tom Harner, Ewa Dabek-Zlotorzynska, Valbona Celo, Greg J. Evans, Cheol-Heon Jeong, Sabina Halappanavar, Narumol Jariyasopit, and Yushan Su Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.9b00420 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019
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Environmental Science & Technology Letters
Polyurethane Foam (PUF) Disk Samplers for Measuring Trace Metals in Ambient Air
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Eftade O.Gaga†∥*, Tom Harner†*, Ewa Dabek-Zlotorzynska♣, Valbona Celo♣, Greg Evans, Cheol-Heon Jeong, Sabina Halappanavar§, Narumol Jariyasopit†ψ, Yushan Su
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†Air Quality Processes Research Section, Environment and Climate Change Canada, Toronto, Canada M3H 5T4
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♣ Air Quality Research Division, Environment and Climate Change Canada, Ottawa, ON, Canada K1V 1H2
*
[email protected],
[email protected] 10
∥ Environmental Engineering Department, Eskişehir Technical University, Eskişehir, Turkey, 26555
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Department of Chemical Engineering & Applied Chemistry, University of Toronto, Canada
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§ Mechanistic Studies Division, ERHSD, HECSB, Health Canada, Ottawa, Canada K1A 0K9
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ψSiriraj Metabolomics and Phenomics Center, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok,
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Ontario Ministry of the Environment, Conservation and Parks, Toronto, Ontario, Canada M9P 3V6
M5S3E5
Thailand, 10700
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Abstract
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A new method is presented for measuring atmospheric concentrations of trace metals in
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airborne particulate matter using polyurethane foam (PUF) disk passive air samplers (PUF-
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PAS) and passive dry deposition air samplers (PAS-DD), which until now have mainly been
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applied to organic pollutants in air. A field calibration study was conducted at one of the sites
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where measurements of trace metals were available using conventional methods. Uptake
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profiles of PUF-PAS and PAS-DD samplers were linear over the full 56 days that the samplers
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were deployed. The results confirm the ability of both passive sampler types to provide time-
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integrated measurements of airborne trace metals. For the PUF-PAS sampler, the derived
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sampling rates (R) were generally in the range of default values derived for organic pollutants
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(i.e. 4 ± 2 m3/day). For the PAS-DD sampler, the collection of the larger depositing particles
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resulted in elevated effective sampling rates, which were up to ~4 times higher than for PUF-
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PAS. Sampling rates for PAS-DD were more variable compared to PUF-PAS, probably due to
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variability of the association of various trace metals with larger particles. Results from the PAS-
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DD sampler were also converted to effective dry deposition fluxes and were as high as 6700 1 ACS Paragon Plus Environment
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µg/m2/day for iron (Fe). This study provides a proof concept and methodology for the
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application of PUF disk-based samplers as a versatile and cost-effective tool for studying trace
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metals, in addition to organics, in ambient air. The method was used to assess concentrations
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of 25 trace metals in ambient using PUF-PAS samples deployed across 6 urban sites in the
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Greater Toronto Area, impacted by different emission sources to air. Highest trace metal
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concentrations were measured at sites impacted by traffic.
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Keywords: Polyurethane Foam (PUF) disk sampler, trace metals, passive sampling, urban air
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pollution
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Introduction
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Due to the toxicity and health effects associated with exposure and inhalation of atmospheric
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particulate matter (PM) there is an incentive for developing simple passive sampling tools that
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measure PM and its related constituents in ambient air. The atmospheric deposition of PM is
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also important as a delivery pathway to terrestrial surfaces (e.g. soils, vegetation etc) and
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ecosystem exposure. Particle deposition is a function of particle size with larger particles
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deposited closer to the source. 1,2,3,4
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Passive samplers have been used for monitoring organic and inorganic pollutants in various
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indoor and outdoor environments.5–10 Compared to active samplers, passive samplers are less
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expensive, require less maintenance and infrastructure/electricity to operate and are easy to
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use, which make them ideally-suited for spatial investigations of air pollutants.11,12 PUF disk
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passive samplers have been shown to capture both gas-phase and particle-phase persistent
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organic pollutants (POPs) in ambient air with an equivalent air sampling rate of about 4
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m3/day. The porous nature of PUF helps to ensure that particles are entrained in the PUF
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matrix once deposited and therefore less susceptible to resuspension to air. 7,13,14,15,16,17
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We hypothesized that PM bound trace metals, which are important components of PM, can
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also be effectively collected by PUF passive samplers. Polyurethane foam disk passive air
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samplers (PUF-PAS) have been widely used to measure air concentrations of POPs and other
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emerging organic chemicals at the local, regional and even global scale. 5,7,18,6 Recently, a new
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program was initiated in Toronto, Canada called ATOUSSA (Assessing Toxicity of Organics in
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Urban Source Sectors for Air) which targets polycyclic aromatic compounds (PACs) and other 2 ACS Paragon Plus Environment
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POPs. The overarching objective of the ATOUSSA project is to link levels of these chemical
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constituents in ambient air to potential adverse human health impacts. As part of a pilot study
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under the ATOUSSA project PUF-PAS samplers were co-deployed at sampling locations over
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one sampling period to assess trace metals in air.
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At the same time, a calibration study was carried out at one of the ATOUSSA sites where trace
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metals were being measured using conventional methods. The field uptake study evaluated
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the performance of the PUF-PAS sampler as well as a second sampler type – the PAS-DD
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(Passive Dry Deposition). Whereas the PUF=PAS sampler provides a measure of contaminants
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present in ambient air, including those that can be inhaled, the PAS-DD sampler targets the
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components that deposit from air to terrestrial surfaces, where they can be taken up by
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ecosystems. In summary, the objectives of this study include,
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(i)
polyurethane foam disks.
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(ii)
To characterize and compare the uptake profiles of PUF-PAS and PAS-DD. samplers for measuring trace metals and deposition from ambient air.
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To develop sampling methods for measuring trace metals collected by
(iii)
To employ this method to assess the spatial distribution of trace metals in ambient air.
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Materials and Methods
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Sampling
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For the uptake study, two different samplers, PUF-PAS and PAS-DD were co-deployed in
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duplicate at the Downsview site (DV, 43o46’48.03”N, 79o28’3.81”W) from March 29 to May
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24, 2017 as shown in Figure 1 and Figure S1. Samplers were deployed approximately 2 m
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above the ground. PUF disks (14 cm diameter × 1.35 cm thick; surface area 365 cm2, mass 4.40
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g, volume 207 cm2, Tisch Environmental, Cleves, OH, USA) were precleaned prior to
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deployment. Parallel samplers were deployed for 1, 2, 4, 6 and 8 weeks. During field sampling,
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PUF disks were housed horizontally inside precleaned stainless steel chambers (PUF-PAS
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sampler, Tisch Environmental, TE-200-PAS) and between the parallel plates of the PAS-DD
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sampler (Tisch Environmental, TE-PAS-DD) (Figure 1). Field blanks (n=4) for both PUF-PAS and
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PAS-DD were collected by exposing the sampling media for a few seconds to ambient air and
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then storing and treating the same way as samples. 3 ACS Paragon Plus Environment
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Double-bowl PUF disk samplers for trace metal determination were deployed during winter,
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February to March 2017 (∼60 days), across 6 locations in Greater Toronto Area, at sampling
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sites targeting organic pollutants as part of the ATOUSSA Project (Figure 2). The sampling
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locations have different source characteristics as described in Table S1. All samples were
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stored in pre-cleaned, solvent-rinsed glass jars following the GAPS network protocols.19
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Sample Preparation, Analysis and QA/QC
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As a first step, the homogeneity of the trace metals deposited onto the PUF disks was tested.
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Subsamples of three PUF disks which were deployed for 2 months at the Downsview field site
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were used for this purpose. The three disks were each cut into eight equal wedges using a
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stainless steel blade that were used to obtain even smaller wedges (~0.1 g) for analysis.
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Wedges from each disk were selected randomly for analysis including 3 wedges from disks 1
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and 2 (SD1 and SD2 in Table S2) and 6 wedges from disk 3 (SD3). For the majority of the trace
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metals (~80 %) the % RSD values were below 25 %. The higher RSD values were not associated
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with detection limit issues and the data were not excluded from the analysis.
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To assess blanks, experiments were first carried out on PUF disks that were precleaned using
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long-standing methods that were developed for POPs analysis5. This revealed high blank
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levels of trace metals and indicated that an additional cleaning step was required. Several
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methods of precleaning the PUF disks were evaluated. The best method, which was applied in
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this study, involved rinsing the PUF disk 3 times with fresh deionized (DI) water, followed by
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ultrasonication in 1% HNO3 for 1.5 hours. The disks were then further rinsed several times
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with DI water to remove acid residues, and dried under N2 gas. The success of the the acid
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rinsing step on reducing blank levels is shown in Table S3. Sample to field blank ratios (S/B)
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for the uptake study samples were calculated for each deployment step for both samplers and
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results are shown in Table S4. The S/B ratios improved (increased) over time and were also
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higher for PAS-DD samplers compared to PUF-PAS, due to higher loading rates for PAS-DD
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compared to PUF-PAS. At the end of 8-weeks of deployment, S/B values were greater than 5
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for most trace metals and greater than 100 for several of them. Low S/B values (i.e. close to
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1), indicating blank contamination issues, were observed for Ni, Zn, Sr, and Mo.
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Samples were analyzed for both water-soluble and acid-soluble trace metals. For acid-soluble 4 ACS Paragon Plus Environment
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metals, PUF disk sub-samples (wedges) were processed by microwave assisted acid digestion
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in preparation for analysis by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for
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determination of total trace element concentrations. Briefly, 100 mg of PUF material was
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accurately weighed directly into the 55-mL MARS Xpress digestion vials (CEM Corporation,
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Matthews, NC) and pre-digested overnight at room temperature with a mixture of 7.5 mL of
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40% (v/v) HNO3 and 2.5 mL of H2O2. Then, all samples were digested for 20 min at 200 C in
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closed pressurized vessels in the microwave oven. The same digestion procedure was used
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during the preliminary experiments for reducing blank trace element concentrations of PUF
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disks. The extracted solutions were diluted to a final concentration of 4% (v/v) HNO3 prior to
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ICP-MS analysis. For the water-soluble fraction, samples (0.25 g of PUF material) were
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extracted with DI water (18 MΩ) in an ultrasonic bath for 30 min and then filtered using 0.22
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µm syringe filters to remove insoluble materials. Samples were transferred to Nalgene bottles
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and acidified with 1% (v/v) HNO3 prior to ICP-MS analysis of the water soluble fraction.
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Analysis details and QA/QC parameters are given in Supporting Information (Text, S1,Table S5
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to S7).
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Results and Discussion
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Uptake profiles for PUF-PAS and PAS-DD samplers
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The uptake profiles of trace metals over time are shown in Figure S2 for both PAS-DD and PUF-
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PAS samplers, which were deployed as duplicates, with raw data presented in Table S8.
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Variability in duplicates is expressed as a ratio of sampler 1 to sampler 2 for each trace metal.
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In the vast majority of samples, the variability was within a factor of 2 (i.e. ratio between 0.5
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to 2) with only a few exceptions. This is consistent with variability reported for passive air
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samplers measuring POPs in ambient air.18 Figure S2 indicates that most of the trace metals
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exhibit near-linear uptake profiles with the amount of chemical collected on the samplers
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proportional to the deployment time. This is consistent with previous calibration studies of
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the samplers investigating particle-associated POPs. 20,21,22
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The PAS-DD, which incorporates a PUF disk as the collection substrate positioned between
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two open parallel flat plates that are shielded above, has a more “open” design compared to
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PUF-PAS sampler (Figure 1). This allows larger particles (>10m) that are typically settling
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through ambient air to flow between the parallel plates and be deposited onto the PUF disks 5 ACS Paragon Plus Environment
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as they would onto other terrestrial surfaces 23; whereas the PUF-PAS, collects particles in a
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similar size range (i.e.
