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Seasonal and spatial variation in dithiothreitol (DTT)activity of quasi-ultrafine particles in the Los AngelesBasin and its association with chemical speciesArian Saffari a , Nancy Daher a , Martin M. Shafer b , James J. Schauer b & ConstantinosSioutas aa University of Southern California, Department of Civil and Environmental Engineering , LosAngeles , California , USAb University of Wisconsin-Madison, Environmental Chemistry and Technology Program ,Madison , Wisconsin , USAPublished online: 17 Dec 2013.
To cite this article: Arian Saffari , Nancy Daher , Martin M. Shafer , James J. Schauer & Constantinos Sioutas (2014) Seasonaland spatial variation in dithiothreitol (DTT) activity of quasi-ultrafine particles in the Los Angeles Basin and its associationwith chemical species, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and EnvironmentalEngineering, 49:4, 441-451, DOI: 10.1080/10934529.2014.854677
To link to this article: http://dx.doi.org/10.1080/10934529.2014.854677
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Journal of Environmental Science and Health, Part A (2014) 49, 441–451Copyright C© Taylor & Francis Group, LLCISSN: 1093-4529 (Print); 1532-4117 (Online)DOI: 10.1080/10934529.2014.854677
Seasonal and spatial variation in dithiothreitol (DTT)activity of quasi-ultrafine particles in the Los Angeles Basinand its association with chemical species
ARIAN SAFFARI1, NANCY DAHER1, MARTIN M. SHAFER2, JAMES J. SCHAUER2
and CONSTANTINOS SIOUTAS1
1University of Southern California, Department of Civil and Environmental Engineering, Los Angeles, California, USA2University of Wisconsin-Madison, Environmental Chemistry and Technology Program, Madison, Wisconsin, USA
A year-long sampling campaign of quasi-ultrafine particles (dp < 0.25 µm) was conducted at 10 distinct sites representing source,urban and/or near-freeway, rural receptor and desert locations across the Los Angeles air basin. Redox activity of the PM samples wasmeasured by means of the Dithiothreitol (DTT) assay and detailed chemical analysis was performed to measure the concentrationsof chemical species. DTT activity per unit air volume and unit PM mass (expressed in nmol min−1 m−3 and nmol/min/µg PM,respectively) showed similar trends across sites and seasons. DTT activity was generally higher during cold seasons (winter andfall) compared to warm seasons (summer and spring). Noticeable peaks were observed at urban near-freeway locations representing“source” sites impacted by fresh traffic emissions. Regression analysis indicated strong association (R > 0.7) between the DTT activityand the concentrations of carbonaceous species (OC, EC, WSOC and WIOC) across all seasons and strong winter-time correlationswith organic tracers of primary vehicular emissions including polycyclic aromatic hydrocarbons (PAHs), alkanes, hopanes andsteranes. Strong correlations were also observed, particularly during winter, between DTT activity and transition metals (e.g., Cr,Mn, V, Fe, Cu, Cd and Zn), which share similar vehicular sources with primary organics. A multivariate linear regression analysisindicated that the variability in DTT activity is best explained by the variability in concentrations of WSOC, WIOC, EC andhopanes. Combined contributions from these species explained 88% of the DTT activity. The appearance of WSOC as a typicaltracer of secondary organic aerosol, along with EC, WIOC and hopanes, all markers of emissions from primary combustion sources,emphasizes the contributions of both primary and secondary sources to the overall oxidative potential of quasi-ultrafine particles.
Supplemental materials are available for this article. Go to the publisher’s online edition of the Journal of Environmental Science andHealth, Part A, to view the supplemental file.
Keywords: Quasi-ultrafine, DTT, dithiothreitol, Los Angeles basin, redox activity.
Introduction
Exposure to particulate matter (PM) has been linked to sev-eral adverse health effects such as respiratory conditions,[1]
cardiovascular diseases [2] and neurological disorders.[3] Oneof the emerging hypotheses explaining the negative healthendpoints of exposure to PM is the oxidative stress derivedfrom the interaction of PM with cells.[4,5] Based on previousstudies, particle size is one of the most important factorsaffecting the health effects of PM.[6,7] Compared to largerparticles, smaller particles have higher pulmonary deposi-
Address correspondence to Constantinos Sioutas, Universityof Southern California, Department of Civil and Environmen-tal Engineering, Los Angeles, CA 90089-2531, USA; E-mail:[email protected] July 16, 2013.
tion efficiency [8] and greater surface area,[9] which increasestheir capability to carry redox active species.[10]
Ultrafine particles (UFP) are conventionally definedas particles with aerodynamic diameter smaller than0.1 µm.[11] The physical structure of these particlesand the collection method used to sample them could,however, affect the classification of UFP.[11] For instance,previous studies have shown that employing an inertialseparator for collecting UFP would categorize a specificportion of soot particles in the ultrafine fraction becauseof their agglomerate-like structure and low density. Thesame particles would, on the other hand, be classified inthe accumulation fraction using a mobility based classi-fier due to their large surface area.[12,13] The size range ofinterest in this study is particles with aerodynamic diam-eter smaller than 0.25 µm, which is referred to herein asquasi-UFP. This size range is of utmost significance from a
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public health viewpoint. A number of epidemiologicalstudies recently conducted in the Los Angeles basin(LAB), found greater associations between quasi-UFP andbiomarkers of adverse health effects (such as inflammationand platelet activation) compared to larger size ranges.[14–17]
A number of biological and chemical assays have beenformulated with the aim of measuring the toxicity of theairborne particles based on their oxidative potential. Thedithiothreitol (DTT) assay measures the capability of PMto catalyze the transfer of electrons from DTT to oxy-gen.[18,19] The rate of DTT consumption is relevant to thecapacity of PM to generate superoxide radicals and is there-fore an indicator of oxidative potential.[18,20,21] Previousresearch efforts reported that carbonaceous species (e.g.,organic and elemental carbon, OC and EC) are highly cor-related with the DTT activity.[18] Organic compounds al-together constitute a significant fraction (above 50%) ofquasi-UFP mass.[22]
Based on previous studies conducted in the LAB, the ul-trafine size range has a considerably higher intrinsic DTTactivity compared to larger PM modes.[7,18] Nonetheless,patterns of seasonal and spatial variations of the DTT ac-tivity associated with UFP have not been well establishedso far since earlier investigations were based on limitednumber of sampling sites and/or sampling time periods. Inthis study, quasi-UF samples were collected and analyzedat 10 distinct locations in the Los Angeles basin (LAB) fora period of 1 year. DTT assay was employed in order toinvestigate the seasonal and spatial variability of PM0.25-associated oxidative potential. Moreover, univariate andmultivariate regression analyses have been employed to de-termine the association of chemical species with the DTTactivity.
Materials and methods
Sampling locations
Quasi-UFP samples were collected at 10 distinct locationsacross the LAB, as shown in Figure S1 (see online sup-plementary information). Each sampling site is designatedwith a three-letter code, namely HUD, GRD, LDS, CCL,USC, HMS, FRE, VBR, GRA and LAN (in order of in-creasing distance from the coast). HUD site, located inthe Long Beach area, is considered as a “source” site forPM0.25 emissions, given its proximity to the industrial andport activities in that region along with its vicinity to Ter-minal island freeway and I-710 freeway which are greatlyimpacted by emissions from diesel trucks.[23]
GRD, LDS, CCL, USC, HMS and FRE are located inthe Los Angeles (LA) area. These sites span west LA (GRD,LDS), central LA (CCL, USC) and east LA (HMS, FRE)and are considered as typical urban sites impacted primar-ily by emissions from nearby freeways (LDS, USC, HMS,FRE) and surface streets (GRD and CCL). VBR and GRAare located in the Riverside County east of LA metropoli-
tan area and represent rural/semi-rural receptor locationsdownwind of source and urban sites.[24] LAN is located ata remote desert-like region to the north of the basin and isconsidered to be far from direct emission sources.
Description of these sampling sites is summarized inTable S1 (see online supplementary material). Prevailingwind direction in the LAB was from southwest to north-east (coast to inland) throughout the sampling campaignand highest wind speeds were observed during spring andsummer. Meteorological conditions were within the sea-sonal norms, with highest temperature and photochemicalactivity during summer and lowest temperature and photo-chemical activity in winter (as indicated by temperature andozone concentration data, presented in Table S2; see onlinesupplementary material). Further information regardinggeographical details of the sampling sites and meteorolog-ical conditions are available in previous publications.[24,25]
Sample collection method
Quasi-UFP samples were collected on a time-integratedbasis for 24 h, once a week during a weekday, starting at12:00 A.M PST and ending at 11:59 P.M PST, from April2008 to March 2009. The samples collected during that spe-cific weekday were then considered as representative of theentire week in our analysis, which may be one of the limi-tations of the study. Two parallel Sioutas personal cascadeimpactor samplers (Sioutas PCIS, SKC, Inc., Eighty Four,PA, USA [26,27]) were used, one loaded with 37-mm Teflonfilters (Pall Life Sciences, Ann Arbor, MI, USA) and theother one with 37-mm quartz filters (Whatman Interna-tional Ltd, Maidstone, UK), each operating at a flow rateof 9 LPM.
As part of the sampling campaign, PM samples were con-currently collected in other size fractions as well (PM10–2.5and PM2.5–0.25). This study, however, focuses on PM0.25only. Coarse PM size range has already been discussed ingreat detail in previous publications.[24,25,28,29] Teflon filterswere pre-weighed prior to the sampling campaign. After thesample collection, Teflon filters were stabilized with labo-ratory air under the controlled temperature and humidityconditions (21◦C ± 2◦C and 30% ± 5%, respectively) andpost-weighed using a microbalance (Model MT5, Mettler-Toledo, Inc., Highstown, NJ, USA).
Chemical analysis
The collected quartz and Teflon filters were sectioned intoportions in order to perform the chemical analyses. ECand OC measurements were performed on individual 1.5-cm2 quartz filter punches using the NIOSH thermal opti-cal transmission method.[30] The reported EC values cor-respond to thermally measured EC, defined as carbonevolved from the quartz samples after the laser reflectancereturns to the initial sample values. Evolved carbon wasevaluated after oxidization of the carbonaceous content ofthe quartz filter punch to CO2 in a pyrolysis oven (900◦C),
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followed by reduction to methane and detection using aflame ionization detector (FID).[30] Concentration of watersoluble organic carbon (WSOC) was determined from themonthly composited quartz filters, using a Sievers 900 to-tal organic carbon analyzer (Boulder, CO, USA) followingwater-extraction of the samples.[31]
The concentration of inorganic ions was measured byion chromatography (IC), performed on monthly compos-ites of Teflon filters. In order to quantify the concentra-tion of water-soluble metals and trace elements, monthly-composited Teflon filters were digested in an acid mixture(consisting of HNO3, HF and HCl), inside a Teflon di-gestion bomb equipped with microwave-assisted digestionsystem (Milestone ETHOS+). The digestates were ana-lyzed by a high resolution inductively coupled plasma sec-tor field mass spectrometry (ICP-SFMS, Thermo-FinniganElement 2; Thermo Fisher Scientific Inc., Waltham, MA,USA). Further details about these analytical methods canbe found in other publications.[30,32,33]
To measure the concentration of organic species,seasonally-composited quartz filters (composited asspring (March–May), summer (June–August), fall(September–November) and winter (December–February)were extracted in a solution of 50% dichloromethaneand 50% acetone using Soxhelets. The extracts werethen analyzed by gas chromatography mass spectrometry(GC-6980, quadrupole MS-5973, Agilent Technologies,Santa Clara, CA, USA).[34] Organic species in this studywere categorized into 7 groups, namely polycyclic aromatichydrocarbons (PAHs), hopanes, steranes, alkanes, organicacids, levoglucosan and biogenic secondary organicaerosols (SOA) tracers. PAHs were further divided intothree sub-groups including low molecular weight (MW)PAHs (MW ≤ 228), medium MW PAHs (MW = 252)and high MW PAHs (MW ≥ 276). Table S3 (see onlinesupplementary information) summarizes the species thatwere included in each of the aforementioned categories.
The consumption rate of DTT in a cell-free system con-taining filtered PM slurry (seasonal composites of Teflonfilters extracted in methanol) was measured in order toevaluate the oxidative potential. The rate at which DTT isconsumed in this process (which is proportional to the con-centration of redox active compounds in the PM sample) ismonitored under standardized conditions and the associ-ated redox activity is reported as the amount of DTT con-sumed per unit of time (i.e., nmol min−1). Details about thischemical assay can be found elsewhere.[19] Moreover, a tablesummarizing the analytical methods and filter compositesused for each analysis is available in the supplementarymaterial (Table S4; see online supplementary material).
Results and discussion
Quasi-ultrafine mass overview
Table 1 shows the mass concentration of quasi-UFP indifferent seasons, averaged across the ten sampling sites.
The percent contribution of each chemical constituent toPM is also presented in this table. The chemical speciesare grouped into water soluble organic matter (WSOM),water insoluble organic matter (WIOM), elemental carbon(EC), crustal material (CM) and trace elements (TE), andinorganic ions. Comprehensive discussion of the tempo-ral and spatial variations of these chemical groups is re-ported elsewhere[22] and only a brief summary is presentedhere. Organic matter (OM) and WSOM are estimated as1.8 times organic carbon (OC) and water soluble organiccarbon (WSOC), respectively.[35] WIOM is estimated afterdeduction of WSOM from OM. Crustal material is calcu-lated by summing the oxides of Al, K, Fe, Ca, Mg, Ti andSi, according to the following equation:[36]
CM = 1.89 Al + 1.21 K + 1.43 Fe + 1.4 Ca
+1.66 Mg + 1.67 Ti + 2.14 Si
Since Si was not measured in this study, it was estimatedas 3.41 × Al.[37] TE is obtained as the summation of allother trace elements measured in the chemical analysis.Lastly, inorganic ions are calculated as the sum of majorions (sulfate, nitrate and ammonium) and trace ions (in-cluding phosphate, chloride, Na+ and K+).
As shown in the table, organic matter (i.e., WIOM andWSOM) is by far the most abundant PM0.25 constituent,contributing to as much as 60%, 47%, 53% and 57% ofthe total quasi-UFP mass during spring, summer, fall andwinter, respectively. Contribution of inorganic ions is low-est in winter (8%) and highest in summer (22%), due to theincreased photochemical activity in summer.[22] The contri-bution of EC is relatively consistent throughout the year,ranging from a minimum of 4% in spring to a maximumof 6% in fall. CM and TE are less significant contribu-tors to UFP, accounting for 10%, 8%, 13% and 13% of thequasi-UFP mass during spring, summer, fall and winter,respectively.
Seasonal and spatial variation of DTT activity
The mass-based DTT activity of quasi-ultrafine particles(which is defined as the DTT activity normalized by PMmass, expressed in nmol DTT/min/µg PM), is depicted inFigs. 1a–d for different sites and seasons. This mass-basedDTT activity, which reflects the intrinsic oxidative poten-tial of the PM samples, is generally higher at urban and/ornear freeway sites compared to rural/semi-rural receptorlocations, which suggests the dominant contribution of ve-hicular emissions to the DTT activity.
Highest mass-based DTT activity during winter is ob-served at CCL (0.094 nmol DTT/min/µg PM), followedby FRE and HUD (0.089 and 0.088 nmol DTT/min/µgPM, respectively), possibly related to increased primaryvehicular emissions originating from the nearby surfacestreets and freeways.[38] In summer, urban USC has thehighest DTT activity (0.087 nmol DTT/min/µg PM)
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Table 1. Seasonal mass concentration of quasi-UFP (µg/m3) and percent contribution of chemical constituents to the PM mass,averaged over the 10 sampling sites.
Spring Summer Fall Winter
Mass Concentration(µg/m3)
7.7 ± 0.4 9.6 ± 0.5 11.9 ± 0.5 9.9 ± 0.5
WSOM (%) 29.6 ± 1.6 27.1 ± 1.4 25.4 ± 1.1 27.1 ± 1.2WIOM (%) 29.5 ± 1.3 20.1 ± 1.7 27.1 ± 1.3 30.1 ± 2.4EC (%) 3.7 ± 0.3 4.6 ± 0.4 6.0 ± 0.4 4.9 ± 0.5CM+TE (%) 9.6 ± 1.0 7.7 ± 0.7 12.8 ± 1.1 12.5 ± 0.9Inorganic ions (%) 14.6 ± 1.3 21.5 ± 1.2 13.3 ± 1 7.8 ± 0.7
Values represent arithmetic average ± standard error.
followed by HMS, CCL and FRE (0.077, 0.071 and 0.071nmol DTT/min/µg PM, respectively).
This could be related to the increased contribution fromsecondary organic aerosol particles, enhanced by photo-chemical activities during the transport of the particlesfrom source site HUD to downwind areas in the easternparts of the basin.[38] Except for USC, the DTT activity atall of the sampling sites is higher during winter compared tosummer. The elevated DTT level in winter may be due to theincreased partitioning of semi-volatile organic compounds,which have been shown to be species majorly contributingto the overall DTT activity of PM in earlier studies, [7,39] tothe particulate phase, as well as their higher concentrationdue to the lower mixing height of the atmosphere duringthe cold periods. Similar studies in the LAB have reported adecreased PM-induced DTT activity at high temperaturesdue to significant evaporative losses of volatile and semi-volatile organic compounds (most notably PAHs).[39,40]
Seasonal and spatial variations of volume-based oxida-tive potential (i.e., normalized by the volume of air sam-pled, expressed in nmol DTT/min/m3 of air), is shown inFigs. 2a–d. A comparison between Figs. 1 and 2 indicatesthat volume-based DTT activity, which reflects the poten-tial toxicity associated with exposure to inhaled PM, showsvery similar trends to mass-based DTT activity. DTT activ-ity was generally higher in winter or fall indicating increasedexposure to redox active PM in these periods.
Correlations between DTT activity and chemical species
To investigate the association of DTT activity with PM0.25chemical composition, regression analysis was carried outbetween the volume-based DTT activity and the concen-tration of chemical species. Due to the limited number ofdata points, the sampling sites were combined together andthe correlation was performed for each season, using the10 data points corresponding to the 10 sampling sites. Weshould also note that the regression analysis was also con-ducted between mass-normalized DTT activity and chem-ical composition (as presented in Table S5; see online sup-plementary material), and yielded generally similar resultsand led to the same major conclusions. As the first step,
univariate regression was performed in order to assess howindividual species correlate with the DTT activity and alsoto have a better insight about important species to be in-cluded in the multivariate regression analysis, which will beexplained subsequently.
The regression was performed for each season separatelyto distinguish the association of chemical species fromprimary and/or secondary sources with the DTT activ-ity. Table 2 shows Pearson correlation coefficients betweenvolume-based DTT activity and concentration of chemi-cal species (including OC, EC, WSOC, hopanes, steranes,alkanes, PAHs, organic acids, levoglucosan, biogenic SOAtracers, inorganic ions and water soluble metals). Ten datapoints corresponding to 10 sampling sites were includedin the analysis for each season. Most notable correlations(R > 0.7) are observed between DTT activity and OC,WSOC and WIOC. DTT activity also shows significant cor-relations (R > 0.7) with other groups of organic compoundsincluding PAHs, hopanes, steranes and alkanes. Earlierstudies have reported organic species as major drivers ofthe DTT activity.[18,19]
The positive association between organic compounds,most notably OC, WSOC, WIOC and PAHs, with DTTactivity has also been reported in several previous stud-ies investigating DTT activity of PM emissions from ve-hicular exhaust [40,41] and ambient air in the LAB [42] aswell as other urban locations such as Rotterdam, Nether-lands.[19] EC is also showing strong correlations with theDTT activity in fall and winter (R > 0.7), although itis not mechanistically active in DTT consumption.[18] Itcan, however, indirectly affect the DTT activity by pro-viding the available surface area for reactions leading tosuperoxide formation.[43] Moreover, it is one of the im-portant markers of primary vehicular emissions [30] andit therefore shares the same source with OC and several or-ganic compounds, which are intrinsically active in the DTTassay.
The correlation between EC and DTT activity has beenobserved and linked to its source origin (vehicular exhaust)in several previous publications as well.[18,44,45] DTT ac-tivity generally displays higher correlations with speciatedorganic groups in winter compared to summer, which can
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Fig. 1. Variation in mass-based dithiothreitol (DTT) activity(nmol/min/µg PM) at different sampling sites during: (a) spring,(b) summer, (c) fall, and (d) winter.
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Table 2. Pearson correlation coefficients (R) between dithiothre-itol (DTT) activity data (nmol/min/m3 air) and mass concentra-tion (µg/m3) of chemical species (organic compounds, carbona-ceous material, inorganic ions and water soluble metals) at the10 sampling locations.
Spring Summer Fall Winter
Low MW PAH 0.11 0.30 0.13 0.63Med MW PAH 0.27 0.40 0.47 0.83High MW PAH 0.49 0.58 0.33 0.77Hopanes 0.22 0.78 0.40 0.96Steranes 0.54 0.65 0.43 0.76Alkanes 0.57 0.57 0.78 0.84Organic Acids 0.25 0.19 0.53 0.51Levoglucosan −0.53 0.28 −0.24 −0.44Biogenic SOA Tracers 0.29 0.73 0.22 0.55OC 0.72 0.86 0.76 0.86WSOC 0.70 0.79 0.84 0.76WIOC 0.80 0.85 0.53 0.98EC 0.62 0.65 0.73 0.85NO3
− −0.37 0.20 −0.52 −0.33SO4
2- 0.21 −0.68 −0.65 0.28NH4
+ 0.15 −0.54 −0.59 −0.08V 0.03 −0.15 0.05 0.85Cr 0.69 0.74 0.78 0.88Mn 0.26 0.53 0.41 0.83Fe 0.42 0.80 0.49 0.13Co 0.51 0.14 0.38 0.84Ni 0.16 0.10 0.29 0.63Cu 0.55 0.70 0.81 0.23Zn 0.67 0.60 0.93 0.79As 0.40 0.30 0.64 0.50Cd 0.28 0.60 0.82 0.47Pb 0.68 0.47 0.61 0.19
Bold numbers indicate values with R > 0.7 and P < 0.05.
be attributed to the increased partitioning of primary semi-volatile organics in the particulate phase in cold seasons, fa-vored by lower temperatures and lower mixing height.[46,47]
Moreover, during summer, DTT activity particularly showshigh correlations with WSOC and tracers of biogenicSOA (R = 0.79 and 0.73, respectively), which under-scores the significance of atmospheric processing and agingin addition to primary sources on PM-induced oxidativepotential.
Some correlations are also observed between DTT ac-tivity and transition metals such as Cr, Fe, Cu, Zn and Cd,specifically in winter time. A number of previous studieshave also reported correlations between transition metalsand DTT activity.[7,42,48] The correlations observed betweenmetals and DTT activity in our study suggests the impor-tant role of their sources in the DTT activity. These metalswere highly co-variant with organic compounds, most no-tably OC, as displayed in Fig. S2 (see online supplementaryinformation), as a result of their common sources. Theseintrinsically redox active transition metals (e.g., Cr, Cu,V and Fe and Zn) in the quasi-UF size range originate
mainly from vehicular emissions, [49] and therefore sharethe same sources with primary organic compounds. Noassociation is observed between inorganic ions (sulfate, ni-trate and ammonium) and DTT activity. Although theseions do not actively participate in the DTT consumption,[18] their acidic properties may contribute to the overall PMtoxicity.[44]
Multivariate linear regression was employed betweenDTT activity and chemical species in order to obtain fur-ther insight on the groups that are more dominantly drivingthe DTT activity in the basin. When all sites and seasons arecombined, the best fitted regression model includes WIOC,WSOC, EC and hopanes. This model is associated with avery high coefficient of determination (R2 = 0.88) as dis-played in Table 3, suggesting that 88% of the variance in theDTT activity can be explained by these species. The pre-dicted DTT activity using the regression models as a func-tion of measured DTT activity is also presented in Fig. 3.The model, including WIOC, WSOC, EC and hopanes asindependent variables, leads to a linear fit with a slope of0.88 and an intercept of 0.07. It is noteworthy that all ofthese chemical species (particularly WSOC, WIOC and EC)also show high individual correlations with DTT activity,as indicated in Table 2.
Based on previous studies, WSOC is a marker of SOAformation,[50,51] while WIOC is mostly originating from pri-mary combustion emissions.[52,53] EC is not considered tobe redox active based on DTT consumption mechanism.[18]
It is, however, considered as a typical marker of diesel ex-haust.[30,40,45] Hopanes are also tracers of combustion emis-sions [54] that mostly originate from gasoline and diesel ve-hicular exhaust in the LAB.[38] These primary vehicularemissions are rich in several organic compounds, particu-larly PAHs and OC, which potentially play a significant rolein PM-induced oxidative stress measured by the DTT as-say.[18,55] The appearance of WSOC, along with WIOC, ECand hopanes in the multivariate linear model implies thesignificance of organic species from both primary and sec-ondary sources in the DTT activity. Comparing the partialR values for WIOC (0.64) and WSOC (0.28), however, sug-gests that primary organics might have a potentially moreimportant role in the PM-induced DTT activity across thebasin.
The P values for WSOC and hopanes in the regressionmodel (0.14 and 0.14, respectively) are slightly above therecommended levels. Additional regression analysis, how-ever, indicated that excluding WSOC and hopanes does notaffect the statistical adequacy of this linear model. The out-put of the regression model including WIOC and EC, andWIOC alone is displayed in Table S6 and Fig. S3 (see on-line supplementary material). From the table and the plot,it is evident that after excluding WSOC and hopanes fromthe analysis (due to their slightly high P value), we can stillobtain a high R2 (0.78) with WIOC and EC (which areboth tracers of primary vehicular emissions) as indepen-dent variables.
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Table 3. Output of the multivariate regression analysis with WIOC, WSOC, EC and hopanes as independent variables (all sites andseasons combined).
SpeciesUnstandardized
coefficientsStandardizedcoefficients Units P Values Partial R Overall R2
(Constant) −0.041 (nmol/min)/m3
EC 0.238 0.222 (nmol/min)/µgEC
0.043 0.386
Hopanes 0.098 0.135 (nmol/min)/ngHopane
0.141 0.285 0.88
WSOC 0.122 0.159 (nmol/min)/µgWSOC
0.144 0.283
WIOC 0.221 0.548 (nmol/min)/µgWIOC
0.000 0.641
Comparison of DTT activity values with other studies
Table 4 provides a comparison of the mass-based DTT ac-tivity of PM collected from different sources and urbanareas. The reported studies employed the same DTT assayto assess the oxidative potential of PM. It is important tonote that Steenhof et al.[19] and Ntziachristos et al.[7] mea-sured the DTT activity of ambient PM directly collected inaqueous media, while all other investigators measured theDTT activity of water-extracts of PM. The DTT activityfor PM0.18 at urban locations of Rotterdam, Netherlands,characterized as “truck traffic area” and “stop and go traffic
area,” was 0.070 and 0.171 nmol/min/µg PM, respectively.These sites have similar characteristics to our urban LosAngeles/Long Beach sites (including: HUD, GRD, LDS,CCL, USC, HMS and FRE), which are adjacent to majorfreeways/surface streets and are affected by traffic-relatedemissions from both heavy duty and light duty vehicles.[38]
The DTT activity at these LAB sites during the sameseasons (i.e., summer, fall and winter) is ranging from 0.03to 0.11 nmol/min/µg PM (Fig. 1), which is within thesame range as the Rotterdam levels. However, while Rot-terdam particle samples were directly collected in liquidsuspensions, which potentially reflects the combined redox
Fig. 3. Association between measured dithiothreitol (DTT) activity (nmol/min/m3) and predicted DTT activity (nmol/min/m3) withWIOC (µg/m3), EC (µg/m3), WSOC (µg/m3), and hopanes (ng/m3) as independent variables (color figure available online).
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Table 4. Summary of previous studies that employed the dithiothreitol (DTT) assay to examine the oxidative potential of PM.
Study Source Sample analyzed Location Sampling period Size range
DTT activity(nmol/min/ugPM)
Verma et al.[61] Ambient air Filtered water-extract of filter
Urban area,Atlanta, USA
Jan 2012–Feb2012
PM2.5 0.022 (± 0.006)
Steenhof et al.[19] Ambient air Aqueoussuspension
Rural area,Rotterdam,Netherlands
June 2007–Feb2008
PM0.18 0.025
Steenhof et al.[19] Ambient air Aqueoussuspension
Stop and gotraffic area,Rotterdam,Netherlands
June 2007–Feb2008
PM0.18 0.171
Steenhof et al.[19] Ambient air Aqueoussuspension
Truck trafficarea,Rotterdam,Netherlands
June 2007–Feb2008
PM0.18 0.070
Verma et al.[48] Ambient air(duringwildfire event)
Water- extract offilter
USC; urbannear-freeway,Los AngelesBasin, USA
24 October 2007 PM2.5 0.024 (± 0.005)
Verma et al.[42] Ambient air Water-extract offilter
USC; urbannear-freeway,Los AngelesBasin, USA
August 2009(Diurnalaverage)
PM0.18 0.068 (± 0.027)
Ntziachristoset al.[7]
Ambient air Aqueoussuspension
Riverside, ruralarea east ofLos AngelesBasin, USA
July 2003 PM 0.15 0.052
Ntziachristoset al.[7]
Ambient air Aqueoussuspension
Riverside, ruralarea east ofLos AngelesBasin, USA
July 2003 PM2.5 0.027
Geller et al.[45] Diesel engineexhaust
Water-extract offilter
Dynamometertesting facility
— total PM 0.039 (± 0.05)
Geller et al.[45] Gasoline engineexhaust
Water-extract offilter
Dynamometertesting facility
— total PM 0.025 (± 0.03)
activity of both water-soluble and water-insolublespecies,[56] the PM-induced DTT activity at rural locationsaround Rotterdam was 2–3 times lower than levels mea-sured in the water-extracted samples in our study. Theaverage DTT activity of PM0.18 between June 2008 andFebruary 2009 was 0.025 nmol/min/µg PM at rural loca-tions of Rotterdam, while PM0.25 DTT activity at the ruralsites in our study (i.e. VBR and GRA, located in Riverside),is ranging approximately from 0.04 to 0.06 nmol/min/µgPM during the same seasons.
This underscores the effect of photochemistry and sec-ondary aerosol formation which is more pronounced in LosAngeles than Netherlands, given the different meteorologyand prolonged summer period in Los Angeles. Further-more, Geller et al.[45] measured the DTT activity of totalPM emissions from diesel engine and gasoline engine vehi-cles. The levels reported are 0.039 and 0.025 nmol/min/µgPM for diesel and gasoline engines, respectively. As seen in
Figures 1(a-d), these values are generally about 2–3 timeslower than PM0.25-associated DTT activity of ambient airat near freeway sites in our study, which further suggeststhat atmospheric processing (i.e., the effect of WSOC) inPM0.25 plays a potentially important role in inducing redoxactivity.
This is also consistent with the results of several smogchamber studies conducted in recent years, which indicatedthe significant effect of aging on PM-induced DTT activity.McWhinney et al.,[57,58] Rattanavaraha et al.[59] and Liet al.[60] compared the redox cycling of freshly emitted PMfrom gasoline and diesel exhaust with that of aged (andphoto-chemically processed) PM using the same DTTassay.
Their results demonstrated that atmospheric aging ofthe fresh particles (simulated by different chamber oxi-dation methods such as ozonation and low-NOx photo-oxidation) causes a significant increase in the PM-induced
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oxidative potential. The diurnal average DTT activity(0.068 nmol/min/µg PM) reported by Verma et al.[42] forambient PM0.18 during August at USC site is quite close tothe summer-time DTT activity of PM0.25 at the same site inour study (0.087 nmol/min/µg PM). On the other hand,while the DTT activity of PM2.5 during the 2007 wildfireevent (0.024 nmol/min/µg PM)[48] exceeded that of post-fire samples by 2 times, the PM2.5-associated DTT activityduring the wildfire is 3 times lower than that of PM0.25 col-lected at the same site (USC) in our study during fall, asshown in Fig. 1.
This trend suggests that PM0.25 is potentially moreredox-active than larger-size PM2.5. This notion is furthersupported by a comparison to findings of Ntziachristoset al.,[7] which reported a PM2.5-associated DTT activityof 0.027 nmol/min/µg PM at Riverside, which is almost2 times lower than DTT activity associated with smallerPM0.25 fractions in the current study. PM0.25-associatedDTT activity at Riverside sites in this study is also compara-ble to that of PM0.15 at Riverside in July 2003.[7] Moreover,the DTT activity of PM2.5 in winter time at an urbanarea of Atlanta, Georgia, was 0.022 nmol/min/µg PM asreported by Verma et al.,[61] which is in the same range ofPM2.5-associated DTT activity in the LAB, reported byNtziachristos et al.,[7] but still 2–3 times lower than the DTTactivity of PM0.25 at urban sites of the LAB in our study.
Conclusions
Our study provides valuable insight on the seasonal andspatial variation of PM0.25 DTT activity in the LAB. Theobserved trends indicated increased redox activity of quasiultrafine PM during the cold seasons (i.e., winter and fall)compared to warmer periods (i.e., summer and spring).Moreover, the DTT activity was generally highest at urbanand/or near freeway LA sites at each season, likely due tothe abundance of vehicular combustion sources originatingfrom the nearby freeways and surface streets in the LA ur-ban area. Moreover, the regression analysis indicated thatamong all chemical species, carbonaceous PM (i.e., OC,EC, WSOC and WIOC) has the highest association withDTT activity. These species were correlated with DTT ac-tivity in all four seasons.
Other chemical compounds such as hopanes, steranesand PAHs were correlated with the DTT activity mostlyduring winter, likely due to the increased partitioning ofsemi-volatile species in the particulate phase during coldperiods. Correlations were also observed between DTT ac-tivity and transition metals (such as Cr, Fe, Cu, Zn and Cd),which underscores the importance of emission sources ofthese metals (specifically vehicular sources) to the DTTactivity of PM0.25. The best-fit multivariate linear modelincluded WIOC, WSOC, EC and hopanes as independentvariables, indicating the contribution of both primary andsecondary sources to the total DTT activity in the basin.
Thus, 88% of the total measured DTT activity could beexplained by this model.
Acknowledgments
This study was funded by the South Coast Air QualityManagement District (SCAQMD) (award #11527). Wealso would like to thank the staff at the Wisconsin StateLaboratory of Hygiene and National Institute for PublicHealth and the Environment (RIVM) for their assistancewith the chemical analyses. We also wish to acknowledgethe support of USC Provost’s and Viterbi’s Ph.D. fellow-ships.
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