Article pubs.acs.org/est
Organic Aerosols Associated with the Generation of Reactive Oxygen Species (ROS) by Water-Soluble PM2.5 Vishal Verma,† Ting Fang,† Lu Xu,‡ Richard E. Peltier,§ Armistead G. Russell,∥ Nga Lee Ng,‡ and Rodney J. Weber*,† †
School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332, United States School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § Department of Environmental Health Sciences, University of Massachusetts Amherst, Amherst, Massachusetts 01003, United States ∥ School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡
S Supporting Information *
ABSTRACT: We compare the relative toxicity of various organic aerosol (OA) components identified by an aerosol mass spectrometer (AMS) based on their ability to generate reactive oxygen species (ROS). Ambient fine aerosols were collected from urban (three in Atlanta, GA and one in Birmingham, AL) and rural (Yorkville, GA and Centerville, AL) sites in the Southeastern United States. The ROS generating capability of the water-soluble fraction of the particles was measured by the dithiothreitol (DTT) assay. Watersoluble PM extracts were further separated into the hydrophobic and hydrophilic fractions using a C-18 column, and both fractions were analyzed for DTT activity and water-soluble metals. Organic aerosol composition was measured at selected sites using a high-resolution time-of-flight AMS. Positive matrix factorization of the AMS spectra resolved the organic aerosol into isoprene-derived OA (Isop_OA), hydrocarbon-like OA (HOA), less-oxidized oxygenated OA, (LO-OOA), more-oxidized OOA (MO-OOA), cooking OA (COA), and biomass burning OA (BBOA). The association of the DTT activity of water-soluble PM2.5 (WS_DTT) with these factors was investigated by linear regression techniques. BBOA and MO-OOA were most consistently linked with WS_DTT, with intrinsic water-soluble activities of 151 ± 20 and 36 ± 22 pmol/min/μg, respectively. Although less toxic, MO-OOA was most widespread, contributing to WS_DTT activity at all sites and during all seasons. WS_DTT activity was least associated with biogenic secondary organic aerosol. The OA components contributing to WS_DTT were humic-like substances (HULIS), which are abundantly emitted in biomass burning (BBOA) and include highly oxidized OA from multiple sources (MO-OOA). Overall, OA contributed approximately 60% to the WS_DTT activity, with the remaining probably from water-soluble metals, which were mostly associated with the hydrophilic WS_DTT fraction.
1. INTRODUCTION Ambient aerosols may lead to oxidative stress by transporting oxidants on particle into the respiratory system, or introducing aerosol components that are capable of catalytically generating reactive oxygen species (ROS) both in vivo and in vitro.1−11 The latter form of ROS is the focus of this work and is potentially an important route since it can lead to significant ROS generation in vivo, and an imbalance between oxidants and antioxidants. The resulting oxidative stress is hypothesized as one of the mechanisms responsible for a myriad health effects associated with particulate matter (PM) pollution.8,12−15 However, despite numerous studies, the exact mechanisms and the ambient particulate components that create this imbalance are largely unknown. A few chemical components of ambient PM, such as quinones or quinone-type compounds and transition metals (e.g., Fe, Mn, Cu, V, and Ni), have been identified to catalyze redox reactions in biological systems.16−19 © 2015 American Chemical Society
Recently, other parameters, such as the surface of soot, have also been shown to promote many of these reactions.20,21 There are several routes of PM ROS generation and the subsequent toxicity expressions. A few of them have been mimicked in various chemical and biological assays, such as antioxidants depletion assays,11,22 Fenton and Fenton-type reactions,23−26 antioxidants inactivation assays,26,27 stress proteins expressions,28 and other cellular responses.29 The dithiothreitol (DTT) assay, which simulates PM-catalyzed electron transfer from cellular antioxidants (e.g., NADPH) to O222 is one of the commonly utilized chemical assays. The response of this assay has been found to correlate with several Received: Revised: Accepted: Published: 4646
November 14, 2014 March 1, 2015 March 6, 2015 March 6, 2015 DOI: 10.1021/es505577w Environ. Sci. Technol. 2015, 49, 4646−4656
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Sampling sites included four urban (Jefferson Street, Georgia Tech, Roadside−all in Atlanta, and Birmingham, AL) and two rural (Yorkville, GA and Centerville, AL) sites. The Jefferson Street and Georgia Tech sites are located near the city center, approximately 3 km apart, and are representative of the urban Atlanta region. The “Roadside” site is less than 1 km from the Georgia Tech site, but is adjacent to an interstate freeway (I85/75) and thus represents fresh aerosols emitted from vehicular traffic. Yorkville is a rural site located 75 km west of Atlanta and has minimal primary vehicular emissions. Birmingham and Centerville are the urban-rural pair of SEARCH (Southeastern Aerosol Research and Characterization Network) sites in Alabama. The SCAPE sampling plan and the sites have been described in more detail in previous publications.32,37 These sites are referred to as JST (Jefferson Street), GT (Georgia Tech), RS (Roadside), YRK (Yorkville), CTR (Centerville), and BHM (Birmingham). An Aerodyne HR-ToF-AMS was installed in parallel to the HiVol sampler rotating between sites to characterize the chemical composition of nonrefractory PM1. The working principle of HR-ToF-AMS has been extensively described in the literature.38,39 Briefly, ambient aerosol is sampled through an aerodynamic lens, which focused particles nominally ranging from 35 nm to 1.5 μm into a narrow beam. The beam of particles impacts on a hot surface (600 °C), where the nonrefractory species are flashed vaporized. The vapor is ionized by 70 eV electron ionization and then extracted into the time-of-flight mass spectrometer. HR-ToF-AMS recorded data with a 2 min interval. A nafion-dryer was placed upstream of HR-ToF-AMS (relative humidity (RH < 20%)) to eliminate the effect of RH on particle collection efficiency of the vaporizer. HR-ToF-AMS data analysis was performed using the standard AMS Analysis toolkits (SQUIRREL v1.53 and PIKA v1.12) in Igor Pro 6.34 (WaveMetrics Inc.). The dates of filter collection using HiVols at all sites and the AMS deployment at specific sites are provided in the Supporting Information (SI) (Table S1). Combining data from all sites, AMS sampling overlapped for roughly 182 days with the HiVol sampling. 2.2. Filter Storage, Extraction and Hydrophobic/ Hydrophilic Separation. The samples after collection from various sites were wrapped in prebaked aluminum foil and immediately stored in a freezer (−18 °C). These samples were taken out from the freezer only prior to the chemical and ROS generation analysis, which started in March 2013 and continued until January 2014. Filter extraction protocols for the DTT assay and water-soluble metals analysis are provided in Verma et al.37 Briefly, circular punches (1 in. diameter) were taken from the filters and extracted in deionized water (Milli-Q; >18 MΩ). Three punches for DTT assay and four punches for metals were extracted separately, each in 15 mL of water. These extracts were then filtered using PTFE 0.45 μm pore syringe filters (Fisher brand). A fraction (about half) of each filtered extract was passed through a C-18 solid phase extraction (SPE) column (octadecyl carbon chain bonded silica; 60A, 40−75 μm, 100g, 20% carbon load) to separate the PM components into hydrophobic (retained on the column) and hydrophilic fractions (passed through the column). The hydrophobic organic compounds separated in this manner from the aerosol extracts are commonly called water-soluble HULIS due to their apparent similarity in the chemical composition with humic substances from soil and aquatic system.40−42 The hydrophilic fraction was collected and the column partially regenerated by rinsing with methanol. The column was then rinsed with
biological markers, both at the cellular (e.g., hemeoxygenase (HO-1) expression28 and MTT (3-(4,5-dimethylthiazol-2-yl)2,5-diphenyltetrazolium Bromidean indicator of cellular metabolic activity) reduction30) and individual organism level (e.g., exhaled nitric oxide (NO) fraction in test human subjects31). The DTT activity of a large number of aerosol filter samples (N > 450) collected in the Southeastern United States was determined as part of the Southeastern Center for Air Pollution and Epidemiology (SCAPE), using an automated instrument for conducting the assay on aerosols extracts.32 The data set was used to identify major emission sources associated with PM-catalyzed ROS generation by employing a variety of source apportionment techniques (e.g., positive matrix factorization and chemical mass balance).33 Three major sources−biomass burning, secondary (or atmospherically processed) organic aerosol (SOA, the specific type could not be determined), and vehicular emissions were found to significantly contribute to the DTT activity. Although limited information could be inferred on the specific components in these sources, both organic aerosol (OA) and transition metals appeared to be the major factors driving ROS generation. Unlike metals that can be individually measured, OA contains myriad organic compounds, of which only a very small fraction is typically speciated. Thus, it is currently not possible to comprehensively identify most of the specific redox-active organic components and understand their ROS generation mechanism. An alternative approach is to identify major types of OA, or group OA by sources, based on unique mass spectral signatures obtained from a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) using factor analysis. The standard approach for analyzing data from this instrument is to use positive matrix factorization (PMF34) to identify various so-called factors, which includes hydrocarbon-like OA, oxygenated OA (OOA), and biomass burning OA. Here we combine identified OA factors with filter-based measurements of water-soluble DTT activity of ambient PM2.5 (WS_DTT) to quantify OA toxicity, providing a comparison in toxicity between different OA factors and identifying the classes of organic components contributing most to WS_DTT activity. Since humic-like substances (HULIS) have been shown to generate ROS in past studies,35,36 their contribution to WS_DTT was also measured by separating the water-soluble PM components into hydrophobic and hydrophilic fractions using a solid phase extraction column technique. Combining the WS_DTT activities of the overall fine aerosol, and hydrophilic/hydrophobic subfractions, with measurements of water-soluble metals and AMS OA factors provides new insights on the sources and relative contributions of various OA components and metals on the ability of water-soluble aerosol components to generate ROS.
2. EXPERIMENTAL METHODS 2.1. Sampling Plan and Sites. Intermittent sampling was conducted for roughly one year, from June 08, 2012 to July 16, 2013 (total 220 days). Ambient PM2.5 were collected using two high-volume samplers (HiVol, Thermo Anderson, nondenuded, nominal flow rate 1.13 m3/min, PM2.5 impactor) in pairs−one always at a fixed sampling site (Jefferson Street), with the other rotating between different sites in each month. Particles were collected on prebaked 8 × 10″ quartz filters (Pallflex Tissuquartz, Pall Life Sciences) from 12 noon −11 am (23-h integrated samples). 4647
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Figure 1. Composition of organic aerosol (OA) indicated by mass fractions measured by the high-resolution time-of-flight aerosol mass spectrophotometer (HR-ToF-AMS) at selected sampling sites during different seasons. BBOA: biomass burning OA, COA: cooking OA, HOA: hydrocarbon-like OA, LO-OOA: less-oxidized oxygenated OA, MO-OOA: more-oxidized oxygenated OA, Isop_OA: isoprene-derived OA.
deionized water before passing the next PM sample. Thus, the same column was used for at least four PM samples before it was discarded. An illustration of the protocol for the filter extraction and SPE separation for metals and DTT analysis is shown in SI Figure S1. In contrast to the conventional HULIS extraction protocol, samples in our study were not acidified before passing through the C-18 column. Unlike previous studies,40−43 which were more focused on measuring the HULIS chemical composition, our analysis was aimed at measuring the redox characteristics of PM, which could be altered by acidifying the samples. For example, the capability of transition metals to form complexes with organic compounds has been shown to be sensitive to changes in pH.44,45 Since complexation with organic compounds can be an important factor in determining the metals availability for carrying out redox reactions and also partitioning on the C-18 column, acidifying the samples might convolute the assessment of metals contribution in the overall ROS generating potential of PM. Further, only a marginal effect ( 25 (range = 25−38), except JST-November (N = 14). Error bars denote standard deviation (1σ) of the monthly averages.
2.4. AMS Data Analysis. PMF was performed on the highresolution organic mass spectra for organic aerosol source apportionment. Detailed discussion on the PMF procedure and results can be found in Xu et al.,47 and are also included in the SI (Figure S2). In short, data pretreatment and PMF operation follows the procedure described by Ulbrich et al.34 Multiple solutions (1−10 factors) were generated with different initial conditions and FPEAK values to explore the possibility of multiple local minima and rotational ambiguity. For each site, the most satisfactory solution was obtained after carefully examining the scaled residuals, factors correlation with tracer species, factors diurnal trend, and characteristic signatures in mass spectrum, following the procedure described in Zhang et al.48
Note, we use the same terminology for the AMS PMF factors, that is, isoprene-derived OA (Isop_OA), less-oxidized oxygenated OA (LO-OOA), more-oxidized oxygenated OA (MOOOA), hydrocarbon-like OA (HOA), cooking OA (COA), and biomass-burning OA (BBOA), as reported by Setyan et al.49 and Xu et al.47 Our analyses47 indicated that Isop_OA is mainly linked to isoprene SOA from the uptake of epoxydiols (IEPOX),50−53 whereas LO-OOA mass is mostly from the reactions between terpenes and nitrate radicals. Thus, these two factors are linked to emissions of biogenic volatile organic compounds. The BBOA factor is identified based on the prominent signals of m/z 60, and m/z 73, also correlated with levoglucosan. The exact source of MO-OOA is not clear, but it is chemically aged (oxygenated), which may have more mixed precursors (both biogenic and anthropogenic),54 and possibly some contribution from biomass burning. For example, Bougiatioti et al.55 found that the OOA factor identified by the mass spectra contains the oxidized fractions from biomass burning aerosols. In the present study, we found that MO-OOA is consistently and highly correlated (R > 0.70) with potassium (K)a rough marker of biomass burning,56 particularly in winter (Table S2 in the SI). Therefore, it is possible that the BBOA factor at these sites represents fresh biomass burning (BB) emissions (i.e., primary BBOA), while aged BB aerosols are included in MO-OOA, especially in winter when biomass burning is most prevalent.57 Despite relatively uniform spatial distributions for OA (or OC, organic carbon) and WSOC,37 there is significant variability in the actual chemical composition of OA among these sites (Figure 1). In summer, Isop_OA, MO-OOA and LO-OOA dominated OA at the rural site (YRK and CTR), while the urban site (GT) had additional contributions from primary OA (POA), in the form of HOA and COA. The HOA fraction is highest (15%) at RS, but SOA (Isop_OA+MO-OOA
3. RESULTS AND DISCUSSION 3.1. Aerosol Composition. Aerosol chemical composition data measured on the HiVol filter samples collected in SCAPE is discussed elsewhere.37 In general, PM2.5 in the southeast are dominated by organic compounds (OA > 50%), followed by ammonium sulfate ((NH4)2SO4 = 20−35%). Elemental carbon (EC) and nitrate (NO3−) constitute a small fraction of the PM mass (55% for Mn, Fe, and Cu) in the hydrophobic fraction (note the exceptional GT data in Figure 3), while DTT is mainly hydrophilic (67%). Metals concentrations were particularly high at the RS site (see Verma et al.37), but showed no significant correlation with WS_DTT. Note that we did not measure the concentrations of metals in specific oxidation states, which determine their redox capability. For example, Charrier and Anastasio58 noted that Fe (II) is three times more efficient in oxidizing DTT than Fe (III). If the ambient metals at these sites are present in multiple oxidation states, then a consistent correlation of WS_DTT with the water-soluble metal concentrations may not necessarily be expected. None of the components, except BBOA, was strongly correlated with WS_DTT in the merged data set, indicating that biomass burning is a major source of WS_DTT activity. 3.3.1. Intrinsic Water-Soluble DTT Activity of Organic Aerosols (Or Relative Toxicity of AMS OA Factors). Various AMS-identified organic aerosol components and selected water-soluble metals were correlated with WS_DTT (R ≥ 0.60; p < 0.05; in Table 1) and were used as the independent variables (in concentration units) to form a multiple linear 4651
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same order of magnitude as obtained from Charrier and Anastasio;58 the differences are reasonable considering that our data is from ambient measurements and our model does not account for the nonlinear behavior of metals with DTT activity. Furthermore, we did not measure the specific oxidation states of these metals, that is. Cu (II) and Mn (II) as in Charrier and Anastasio.58 Among the organic aerosol factors, BBOA yielded the most consistent intrinsic water-soluble activity, that is, least % variation of its predicted coefficient among the various sites. It is noteworthy that the intrinsic water-soluble DTT activity of BBOA, 151 ± 20 pmol/min/μg, (average of four sites) is the highest among all organic aerosols, and is 2−4 times the typical intrinsic total activity measured by McWhinney et al.20 for diesel exhaust particles (DEPs; Table 2, that is, total DTT activity per mass of DEPs), which has been associated with several detrimental health effects.59−63 These results imply that biomass burning emissions might pose a potentially serious health hazard, as they tend to be widespread, and likely to rise in the future. Regression analysis conducted on the merged data set (last row of Table 1) also yielded BBOA as the only significant variable in the model driving WS_DTT, with a coefficient very close to the average value obtained from the individual sites. COA has the second-highest intrinsic water-soluble DTT activity (90 ± 51 pmol/min/μg) among organic aerosols, however, its coefficient was obtained from the regression analysis only at one site in winter (RS-February) and therefore has lower confidence (standard error = 57% of the coefficient). The intrinsic water-soluble DTT activity of MO-OOA is also significant (36 ± 22 pmol/min/μg) and likely important as it was obtained from multiple (N = 4) sites. Isop_OA also showed very low intrinsic water-soluble activity (8.8 ± 21 pmol/min/μg), and only at the YRK site. An outdoor chamber
Table 2. Average Intrinsic Water-Soluble DTT Activity for Different PM Components (OAs and Metals) As Estimated from the Linear Regression Model intrinsic water-soluble DTT activity, pmol/min/μg aerosols
this study
Isop_OA MO-OOA BBOA COA watersoluble Mn watersoluble Cu DEP ambient PM
8.8 ± 21a 36 ± 22 151 ± 20 90 ± 51a 85754 ± 23671a 11945 ± 1931
10−70
other studies
references
2000070000b
Charrier and Anastasio58
1000030000b
Charrier and Anastasio58
20−60c 5−60
McWhinney et al.20 Charrier and Anastasio,58 Cho et al.,22 Verma et al.67,68
a
Since the activity for Isop_OA, COA and water-soluble Mn was obtained based on the data set from a single site, the standard error in the coefficient was used to represent uncertainties. For other components, uncertainty represents the standard deviation (1 σ) of the average calculated across all sites (N (number of sites) = 4 for MO-OOA, 4 for BBOA and 3 for water- soluble Cu)). bThe intrinsic activities are for Cu (II) and Mn (II) which were obtained from the equations derived by Charrier and Anastasio,58 for typical ambient concentrations of water-soluble Mn (3- 1 ng/m3) and water-soluble Cu (20−5 ng/m3). Note, the activity decreases with increasing ambient concentrations. cIntrinsic total DTT activity of DEP, (watersoluble+insoluble).
activity was calculated from the empirical equations provided by Charrier and Anastasio.58 Overall, the intrinsic water-soluble DTT activity of these metals estimated in our study are of the
Figure 4. Contribution of various PM components (organic aerosols and water-soluble metals (i.e., WS_Mn and WS_Cu)) to the water-soluble DTT activity, determined from the product of average intrinsic water-soluble activity (as predicted by the multiple linear regression model; Table 2) and average ambient concentration of the PM components. The estimated water-soluble DTT activity from these components (e.g., organic aerosols (hatched region) and metals) is compared to the measured total WS_DTT and its hydrophobic fraction (hydrophobic WS_DTT). 4652
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Environmental Science & Technology study by Rattanavaraha et al.64 tested the effect of photochemical aging of different particles on ROS generation and showed significant DTT activity associated with Isop_OA, even higher than aged DEPs. However, a direct comparison of the intrinsic activity of Isop_OA between these two studies is not possible, as the filters in that study were extracted in methanol and the chamber conditions (i.e., VOC/NOx ratios) are not consistent to the ambient conditions in our study. LO-OOA (terpene SOA) and HOA (or POA) did not reveal any watersoluble activity in this analysis. 3.3.2. Contribution of Different Components to the overall DTT Activity of PM. The average intrinsic water-soluble DTT activity of different components (i.e., organic aerosols and metals) as calculated in Table 2, were multiplied by their respective average ambient concentrations at each site to reconstruct the total WS_DTT associated with those components (Figure 4). Considering the limitation of the stepwise method for selecting only strong variables in the multivariate regression model, here we include all components that were significantly correlated with WS_DTT in the univariate regression, where significant is arbitrarily set at R ≥ 0.60; p < 0.05. The measured total DTT activity (average as circles and 1 σ as error bars) of the water-soluble ambient PM (WS_DTT) is also shown in the same figure for assessing the degree to which the measured activity could be explained by that predicted from the sum of different components. In addition, the contribution of OA alone to WS_DTT, predicted by the model, is compared with the measured hydrophobic water-soluble DTT activity (recall Mn and Cu were almost exclusively associated with the hydrophilic fraction in Figure 3). First, considering the contributions of various organic aerosols to WS_DTT; despite a lower intrinsic water-soluble activity than BBOA or COA, the high concentrations and prevalence of MO-OOA results in it substantially contributing to WS_DTT at all sites. Highest contributions from MO-OOA are in summer, indicating some component of the SOA or oxidation processes as a dominant source of ROS generation associated with organic aerosols. Some of the MO-OOA DTT activity, especially in winter, is possibly due to atmospherically processed (i.e., oxidized) biomass burning emissions. Note that even the fresh biomass burning aerosols (BBOA) makes a substantial contribution to WS_DTT at all sites in winter and at CTR in summer. The relative contribution of BBOA to WS_DTT is higher at rural sites (YRK 60%, CTR 39%) compared to urban sites (JST 34% and RS 31% of total OA contribution) due to a wider range of emission sources (e.g., vehicles) in the urban environment. COA also makes a significant contribution in winter, but only at urban sites (JST (38%) and RS (39% of total OA contribution to WS_DTT)). Agreement between measured WS_DTT and the summed activity from different components is reasonable at some sites (YRK-June, GT-August, JST-November; within ±25%), but not at others. At RS (both fall and winter) and rural sites (YRKDecember and CTR-June), summed activity is substantially lower from the measured WS_DTT. A possible cause for underestimation is the uncertainty in the calculated intrinsic water-soluble DTT activity for some components (e.g., COA and Mn). The ability of Mn and Cu to oxidize DTT varies nonlinearly with metals concentration,58 making it difficult to estimate their intrinsic activity (i.e., activity depends on metals concentration). This does not appear a concern for Cu, at least in our results, as indicated from the consistent estimate of its intrinsic water-soluble DTT activity obtained from three sites
(GT-August, RS-September and JST-November; % CV = 16), however for Mn, whose coefficient was obtained from a single site (YRK-December) and has relatively large standard error (28%), this could be a factor. Assessing the contribution of metals is further complicated by the dependence of their reactivity on oxidation states. Note, the sites having the largest discrepancies in WS_DTT, that is, RS-February and CTR-June, did not include any contribution from the metals as they were not correlated with WS_DTT at those sites (Table 1). The contribution of various components of ambient PM to WS_DTT at different sites in this study is consistent with the WS_DTT-source apportionment results discussed in our previous publication. 33 While the BBOA fraction of WS_DTT in the present analyses exclusively reflects the contribution from biomass burning, the MO-OOA fraction is mostly due to SOA (or atmospherically aged) components mainly from vehicular emissions and biomass burning (recall biogenic SOA, that is, Isop_OA and LO-OOA were not significantly associated with WS_DTT activities). As for the metals, despite the complexity of assessing their exact contribution, they can most likely be attributed to vehicular sources (both direct emissions and/or vehicle-induced dust resuspension), and to some extent, biomass burning (see Verma et al.33). An important observation from Figure 4 is that the summed activity from different organic aerosol components generally agrees well (within the error bars, except for RS-September) with the DTT activity associated with hydrophobic fraction of the water-soluble PM (compare the top hatched bar to open symbols in Figure 4). Since there are practically no metals in the hydrophobic fraction, the agreement between the summed activities of organic components as estimated from the model, with the measured DTT activity of the hydrophobic watersoluble PM, implies that HULIS is the major group of components driving the ROS generation mechanism in the water-soluble fraction of OA. Previous measurements have shown that aromatic compounds, which are known to participate in electron transfer reactions, are exclusively associated with HULIS,43,65,66 making them likely components responsible for the DTT oxidation. Another implication of the closure between DTT activity of the hydrophobic fraction with the summed activities of organic aerosols, is that the water-soluble DTT activity of the hydrophilic PM fraction should then be driven mainly by metals, consistent with Mn and Cu being almost exclusively in the hydrophilic fraction (Figure 3). Thus, our results indicate that both HULIS (hydrophobic) and metals (hydrophilic) contribute significantly to WS_DTT, although their relative fractions vary spatially and across seasons due to variability in their emissions sources. This is in contrast to previous studies, which have generally attributed DTT activity of PM to a single group of species, that is, either organic compounds,67−70 or metals58 based on a smaller sample size collected from a limited number of sites. Based on the hydrophobic/hydrophilic separation of WS_DTT, on average (±1σ), organic aerosol components contributed 60 ± 14% and water-soluble metals 40 ± 14% to the overall WS_DTT activity in this study. The ability to roughly separate metals versus organic aerosol contributions to overall WS_DTT activity by hydrophobicity represents a potentially useful tool when measurements of metals and OA speciation are not available. 4653
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1, Figure S2, Table S1, and S2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone 404.894.1750; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This publication was made possible by U.S. EPA grant R834799. This publication’s contents are solely the responsibility of the grantee and do not necessarily represent the official views of the U.S. EPA. Further, U.S. EPA does not endorse the purchase of any commercial products or services mentioned in the publication. Measurements in Alabama were supported by an NSF grant 1242258. We thank R. Erik Weber who helped in sample collection and sample preparation for DTT and metals analysis, Janessa R. Rowland for assistance with filter extractions, and the SEARCH personnel for their many contributions supporting the field deployments.
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