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Environ. Sci. Technol. 2008, 42, 6469–6475

Size Distribution of Particle-Phase Molecular Markers during a Severe Winter Pollution Episode

reflects the fact that these compounds exist in particles emitted from different sources. The results of the current study will prove useful for size-resolved source apportionment exercises.

1. Introduction M I C H A E L J . K L E E M A N , * ,† SARAH G. RIDDLE,‡ AND CHRIS A. JAKOBER§ Department of Civil and Environmental Engineering, Department of Chemistry, and Agriculture and Environmental Chemistry Graduate Group, University of California, Davis, 1 Shields Avenue, Davis, California 95616

Received February 4, 2008. Revised manuscript received May 19, 2008. Accepted June 12, 2008.

Airborne particulate matter was collected using filter samplers and cascade impactors in six size fractions below 1.8 µm during a severe winter air pollution event at three sites in the Central Valley of California. The smallest size fraction analyzed was 0.056 < Dp < 0.1 µm particle diameter, which accounts for the majority of the mass in the ultrafine (PM0.1) size range. Separate samples were collected during the daytime (10 a.m. to 6 p.m. PST) and nighttime (8 p.m. to 8 a.m. PST) to characterize diurnal patterns. Each sample was extracted with organic solvents and analyzed using gas chromatography mass spectrometry for molecular markers that can be used for size-resolved source apportionment calculations. Colocated impactor and filter measurements were highly correlated (R2 > 0.8) for retene, benzo[ghi]flouranthene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[e]pyrene, benzo[a]pyrene, perylene, indeno[1,2,3-cd]pyrene, benzo[ghi]perylene, coronene, MW302 polycyclic aromatic hydrocarbon (PAHs), 17β(H)-21R(H)-30-norhopane, 17R(H)-21β(H)-hopane, Rββ-20R-C29-ethylcholestane, levoglucosan, and cholesterol. Of these compounds, levoglucosan was present in the highest concentration (60-2080 ng m-3) followed by cholesterol (6-35 ng m-3), PAHs (2-38 ng m-3), and hopanes and steranes (0-2 ng m-3). Nighttime concentrations were higher than daytime concentrations in all cases. Organic compound size distributions were generally similar to the total carbon size distributions during the nighttime but showed greater variability during the daytime. This may reflect the dominance of fresh emission in the stagnant surface layer during the evening hours and the presence of aged organic aerosol at the surface during the daytime when the atmosphere is better mixed. All of the measured organic compound particle size distributions had a single mode that peaked somewhere between 0.18 and 0.56 µm, but the width of each distribution varied by compound. Cholesterol generally had the broadest particle size distribution, while benzo[ghi]perylene and 17R(H)-21β(H)-29-norhopane generally had sharper peaks. The difference between the size distributions of the various particle-phase organic compounds * Corresponding author phone: (530) 752-8386; fax: (530) 7527872; e-mail: [email protected]. † Department of Civil and Environmental Engineering. ‡ Department of Chemistry. § Agriculture and Environmental Chemistry Graduate Group. 10.1021/es800346k CCC: $40.75

Published on Web 07/23/2008

 2008 American Chemical Society

The integrated mass of airborne particles with an aerodynamic diameter smaller than 2.5 µm (fine PM; PM2.5) is statistically associated with excess human mortality (see, for example, refs 1 and 2). Understanding the exact mechanism of injury associated with PM2.5 will likely require further understanding of the particle size distribution shape below 2.5 µm, since changes in aerodynamic diameter in this range strongly influence deposition patterns in the respiratory system (3). Numerous investigators are currently searching for the source(s) of airborne particles whose size and composition cause adverse health effects, so that emissions from these sources can be controlled to protect public health. Molecular markers are widely used in particulate matter source apportionment studies because organic compounds exist that are specific to individual sources leading to enhanced resolution (4). Studies on the organic composition of particles emitted to the atmosphere have identified molecular markers for major sources such as motor vehicle exhaust (see, for example, refs 5–8), biomass combustion (see, for example, refs 9–11), food cooking (see, for example, refs 12–15), and so forth. Recent studies have examined the size distribution of molecular marker emissions below a 2.5 µm aerodynamic diameter, with the objective to build a profile library for size-resolved source apportionment calculations (16–18). Corresponding measurements of ambient molecular marker size distributions at receptor sites during air pollution events are needed to complete the data set needed for these calculations. The Central Valley of California routinely experiences some of the highest PM2.5 concentrations in the United States, rivaling those measured in Los Angeles, where the population is 5 times larger. The most severe PM2.5 episode in recent Central Valley history occurred between December 2000 and January 2001 during the California Regional PM10/PM2.5 Air Quality Study (CRPAQS) (19). Measured PM2.5 concentrations during CRPAQS exceeded 200 µg m-3 (19) (5.7 times the current National Ambient Air Quality Standard of 35 µg m-3) The purpose of the current study is to describe the size distribution of molecular markers during CRPAQS in support of future size-resolved source apportionment studies.

2. Methods Size-resolved airborne particulate matter samples were collected with micro-orifice uniform deposit impactors (MOUDIs; MSP Corp., Shoreview, MN) and reference ambient air samplers (RAASs; Andersen Instruments, Smyra, GA) during CRPAQS (20, 21). The samples discussed in the current study were collected over 7 days during the period December 15-28, 2000 at Sacramento and over 11 days during the period December 15, 2000 to January 7, 2001 at Modesto and Bakersfield. Two samples were collected at all locations on each day: 10 a.m. to 6 p.m. (daytime) and 8 p.m. to 8 a.m. (nighttime). The sample locations span a north-south transect of the San Joaquin Valley. The greater Sacramento Metropolitan Area has the largest population (∼1,700,000), followed by Bakersfield (∼250,000), and then Modesto (190,000). MOUDIs and RAASs were equipped with upstream cyclones so that only particles with an aerodynamic diameter less than 1.8 µm were collected. PM1.8 concentrations at the VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Regression Statistics between Colocated FilterBased Samplers and MOUDI PM1.8 Measurements of Organic Compounds, Potassium, Organic Carbon, and Elemental Carbona regression statistics slope

R2

0.82 0.76 0.72 0.81 0.84 0.87 0.96 0.91 0.72 0.81 0.84 0.84 0.77 0.78

0.70 0.69 0.99 0.96 0.98 0.99 0.96 0.98 0.95 0.69 0.93 0.93 0.95 0.90

Hopanes/Steranes 17β(H)-21R(H)-30-Norhopane 0.71 17R(H)-21β(H)-Hopane 0.74 Aββ-20R-C29-Ethylcholestane 0.71

0.81 0.80 0.84

Other Organic Compounds 0.98 0.72

1.00 0.99

Other Components 0.95 0.91 0.96

0.97 1.00 0.98

PAHs fluoranthene pyrene retene benzo[ghi]fluoranthene chrysene benzo[b]fluoranthene benzo[k]fluoranthene benzo[e]pyrene benzo[a]pyrene perylene indeno[1,2,3-cd]pyrene benzo[ghi]perylene coronene MW302 PAHs

levoglucosan cholesterol

potassium organic carbon elemental carbon

a Filter-based organic carbon concentrations have been corrected for positive adsorption artifacts using a QBQ.

FIGURE 1. Agreement between measurements of particle-phase species made using colocated filter-based samplers and MOUDIs. The maximum concentration used to normalize each component is shown in the key. Each datapoint corresponds to a panel of Figure 2. Regression statistics are summarized in Table 1. sampling locations exceeded 150 µg m-3 during the evening hours (21). PM0.1 concentrations at the sampling locations reached approximately 1 µg m-3 during the evening hours (21), which is comparable to concentrations measured in Los Angeles (22, 23). Two MOUDIs were used at each location to support a full range of chemical analysis for the collected particles. The first MOUDI was loaded with Teflon substrates (Teflo R2PJ47) to support elemental analysis by inductively coupled plasma mass spectrometry analysis (24). Water-soluble ions were also measured using ion chromatography. The second MOUDI was loaded with foil substrates that were then used to quantify carbon concentrations with thermal-optical carbon analysis (21). RAAS samples were collected on Teflon filters (water-soluble ion and elemental analysis) and quartz filters (carbon analysis). A set of backup quartz filters (QBQ) was used to quantify the gas-phase adsorption artifact on the front quartz filter. All collection media used for carbon analysis were prebaked at 500 °C for 48 h to remove any carbon contamination. Samples collected on aluminum foil impaction substrates and quartz fiber filters for trace organics analysis were spiked with isotopically labeled cholesterol, an isotopically labeled sterane (RRR-20R-cholestane-d4), and two isotopically labeled polycyclic aromatic hydrocarbons (PAHs) (chrysened12 and dibenz[ah]anthracene-d14) then allowed to dry. Each sample was then sonicated in ∼15 mL of dichloromethane 6470

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and reduced in volume by evaporation under nitrogen. Samples were analyzed on a Varian gas chromatograph (GC model 3400) ion trap mass spectrometer (ITMS model 2000). The GC was equipped with an Agilent J&W DB-XLBMSD capillary column (30 m × 0.25 mm i.d. × 0.25 µm film thickness). The mass spectrometer was operated in electron impact ionization mode and selected ion monitoring mode. Cholesterol and levoglucosan were derivatized with bis (trimethylsilyl) trifluoroacetamide (BSTFA) and quantified using the largest fragment ion of the derivatized compound. PAHs were detected using the mass of parent ions. Hopanes and steranes were detected as their predominant fragment ions. Each species was identified by comparison to authentic standards or by previously published values of retention times and mass spectra. Further details of the procedures used for chemical analysis of organic compounds are provided by Riddle et al. (17, 18).

3. Results Quality Assurance Checks. Samples were stored in Petri dishes sealed with Teflon tape at -18 °C before and after collection. Approximately 10% of the sampling media was retained as field blanks to characterize background levels. External standards were used during analytical procedures to verify accuracy, and duplicate measurements were used to verify precision. All of the measurements used in the current manuscript were made using extensive quality assurance protocols, and the data were subjected to rigorous quality control checks to ensure precision and accuracy (21). Cold storage of organic carbon samples on the original collection media is generally acknowledged to be the best method for the preservation of molecular markers in airborne particulate matter. Nevertheless, the total time between

FIGURE 2. Size and composition distribution of airborne particulate matter collected at Sacramento, Modesto, and Bakersfield between December 15, 2000 and January 7, 2001. Day average samples were collected between 10 and 18 PST. Night average samples were collected between 20 and 8 PST. sample collection and sample extraction plus analysis in the current study was six years, raising the possibility that some degradation of the organic markers could have occurred during storage. Degradation rates could depend on the collection media, organic carbon loading, and the other components contained in the airborne particulate matter samples. Significant degradation during storage would likely result in poor agreement between colocated MOUDI samples collected on aluminum foil substrates (30 L min-1) and RAAS samples collected on quartz fiber filters (10 L min-1). Figure 1 illustrates agreement between colocated RAAS and MOUDI samples for the species measured in the current study. Each data point in the subpanels of Figure 1 represents a sampling period (daytime or nighttime) at one of the three sampling locations. Good agreement is observed between the colocated samples in all cases, suggesting that sample degradation was not a significant issue. Statistics for a linear regression analysis between the colocated filter-based and MOUDI samples are shown in Table 1. Correlation coefficients (R2) were > 0.9 for PAHs (excluding a few of the lightest semivolatile compounds), > 0.8 for hopanes and steranes, and > 0.99 for levoglucosan and cholesterol. The slope of the regression

line was greater than 0.7 in all cases, which is typical for comparisons between filter-based and MOUDI samplers (21). The high level of correlation between colocated samples builds confidence that the collection and analysis techniques employed in the current study are suitable for size-resolved trace organic compounds. Particle Size Distributions. Figure 2 shows the average size and composition distribution of airborne particulate matter collected at Sacramento, Modesto, and Bakersfield during the period December 2000 to January 2001 (11 sample days). Panels a, c, and e show the daytime average (10 a.m. to 6 p.m.), while panels b, d, and f show the nighttime average (8 p.m. to 8 a.m.). The accumulation mode in the distribution has strong contributions from ammonium nitrate with a peak diameter below 1 µm. Daytime ammonium nitrate concentrations are generally larger than nighttime concentrations. The chemical composition of particles with an aerodynamic diameter smaller than ∼0.18 µm is dominated by elemental carbon and organic carbon. The higher nighttime concentration of these carbon particles reflects the dominance of primary emissions (20). The carbonaceous fraction of the nighttime PM0.1 and PM0.18 mass is approximately 96% and VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Concentration of Organic Compounds, Potassium, Organic Carbon, and Elemental Carbon Measured in the PM0.1, PM0.18, and PM1.8 Size Fractions at Sacramento between December 15 and 28, 2000 Sacramento day PM0.1

PM0.18

fluoranthene pyrene retene benzo[ghi]fluoranthene chrysene benzo[b]fluoranthene benzo[k]fluoranthene benzo[e]pyrene benzo[a]pyrene perylene indeno[1,2,3-cd]pyrene benzo[ghi]perylene coronene MW302 PAHs

0.0 ( 0.24 0.0 ( 0.13 0.0 ( 0.18 0.0 ( 0.1 0.0 ( 0.02 12 ( 0.29 0.0 ( 0.23 6.9 ( 0.04 0.0 ( 0.12 0.0 ( 0.2 0.0 ( 4.97 9.9 ( 4.1 0.0 ( 1.1 0.0 ( 0.38

17B(H)-21A(H)-30-norhopane 17A(H)-21B(H)-hopane ABB-20R-C29-ethylcholestane

0.0 ( 0.03 0.0 ( 0.05 0.0 ( 0.01

PM0.1

PM0.18

PM1.8

96 ( 0.2 78 ( 0.37 282 ( 1.4 69.3 ( 0.34 55.1 ( 0.59 144 ( 0.27 20.8 ( 0.62 72.5 ( 0.79 71.2 ( 0.33 0.0 ( 0.34 370 ( 16 390 ( 16 371 ( 4.6 0.0 ( 1.5

0.0 ( 22 0.0 ( 12 42 ( 17 12 ( 9.2 27 ( 2.9 93 ( 14 19 ( 11 50 ( 2.4 55 ( 5.9 9.9 ( 9.5 91 ( 230 96 ( 190 51 ( 51 130 ( 18

17 ( 36 12 ( 14 115 ( 40 59 ( 12 96 ( 16 250 ( 16 19 ( 11 130 ( 8.2 119 ( 9.1 9.9 ( 10 355 ( 355 375 ( 279 331 ( 94 541 ( 39

670 ( 118 906 ( 76 3340 ( 588 1380 ( 112 1690 ( 145 3340 ( 134 680 ( 21 1670 ( 72 3120 ( 37 0.0 ( 15 5750 ( 1020 4060 ( 776 3040 ( 300 8850 ( 204

Hopanes/Steranes (pg/m3) 0.0 ( 0.03 0.0 ( 0.04 0.0 ( 0.07 0.0 ( 0.15 0.0 ( 0.03 0.0 ( 0.09

0.0 ( 2.5 0.0 ( 4.9 0.0 ( 0.99

63 ( 3.6 49 ( 5.5 24.8 ( 1.7

922 ( 11 980 ( 14 443 ( 9.2

354 ( 0 1.6 ( 0

2050 ( 0 20 ( 0.01

PAHs 0.0 ( 0.3 0.0 ( 0.18 5.5 ( 0.39 0.0 ( 0.13 4.0 ( 0.18 30 ( 0.3 0.0 ( 0.25 24 ( 0.06 0.0 ( 0.23 0.0 ( 0.23 102 ( 6.1 64 ( 6.1 64 ( 2.0 0.0 ( 0.72

16 ( 0 0.13 ( 0

potassium organic carbon elemental carbon

12 ( 10 310 ( 110 71 ( 110

Other Components (ng/m3) 21 ( 14 93 ( 20 800 ( 160 6100 ( 330 220 ( 160 1430 ( 330

83%, respectively. The carbonaceous fraction of the daytime PM0.1 and PM0.18 mass is approximately 91% and 81%, respectively. The remainder of the PM0.1 and PM0.18 mass is composed of nitrate, ammonium ion, sulfate, and other material. A thorough discussion of the dominant mechanisms that produce the average profiles illustrated in Figure 2 is provided by Herner et al. (20). Tables 2–4 show the measured concentrations of organic compounds, potassium, organic carbon, and elemental carbon in the PM0.1, PM0.18, and PM1.8 size fractions at Sacramento, Modesto, and Bakersfield, respectively. Daytime concentrations and nighttime concentrations are reported separately at each location. All nighttime concentrations were higher than daytime concentrations, reflecting the enhancement of primary emissions in the stagnant nighttime atmosphere and the increased prevalence of wood smoke emissions during the evening hours (20). Elemental carbon concentrations in the ultrafine size fraction (PM0.1) ranged from 29 to 71 ng m-3 during the day and 93 to 183 ng m-3 during the night. Organic carbon concentrations in the ultrafine size fraction ranged from 168 to 321 ng m-3 during the day and 393 to 825 ng m-3 during the night. The most abundant organic compound detected was levoglucosan, with ultrafine concentrations ranging from 1 to 271 ng m-3. The next most abundant species was cholesterol, with ultrafine concentrations ranging from 0.1 to 0.6 ng m-3. PAHs were measured in the ultrafine size fraction at concentrations ranging from 0 to 36 pg m-3 during the day and 0 to 227 pg m-3 at night. Hopane and sterane concentrations in the ultrafne size fraction ranged from 0 to 6.5 pg m-3. Potassium concentrations shown in Tables 2–4 may originate from wood combustion (4-18 ng K/µg total carbon), food cooking operations (1.6-9 ng K/µg total carbon), or the exhaust of non-catalyst-equipped gasoline-powered motor vehicles (0.4 ng K/µg total carbon). The highest PM1.8 concentrations of all PAHs, hopanes, and steranes were measured in Sacramento during the 9

(pg/m3)

Other Organic Compounds (ng/m3) 35 ( 0 61 ( 0 103 ( 0 0.21 ( 0 6.5 ( 0 0.39 ( 0

levoglucosan cholesterol

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Sacramento night PM1.8

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19 ( 10 770 ( 70 180 ( 70

68 ( 14 2500 ( 99 596 ( 99

345 ( 10 18700 ( 220 4450 ( 220

evening hours. This trend reflects the fact that samples were not collected adjacent to roadways, and so the measured traffic markers are dominated by the urban background traffic signal. Sacramento has greater traffic volume than either Modesto or Bakersfield. In contrast, the highest PM1.8 concentrations of levoglucosan and cholesterol were measured at Modesto during the evening hours. The Modesto sampling site was located between residential neighborhoods and light commercial areas (including restaurants), and so the measured signals may reflect emissions from localized sources. Figure 3 illustrates the normalized size distribution of total carbon, benzo[ghi]perylene, 17R(H)-21β(H)-29-norhopane, levoglucosan, and cholesterol at Sacramento, Modesto, and Bakersfield. Concentrations in each size fraction were normalized by dividing with the measured PM1.8 concentration to illustrate the relative size distribution of each species. Organic carbon and elemental carbon size distributions were highly correlated (R2 > 0.9), and so only the total carbon size distribution is shown. Total carbon size distributions have a larger mean diameter during the daytime than nighttime at Sacramento and Bakersfield. Nighttime groundbased inversions trap fresh emissions close to the surface. Mixing depths are larger during the daytime than during the nighttime, allowing carbonaceous particles from higher in the atmosphere to reach the ground-level monitor. Nighttime total carbon profiles are therefore dominated by fresh combustion emissions, while daytime total carbon profiles reflect more aged aerosols (20). Each of the organic compounds illustrated in Figure 3 is characteristic of a single class of sources. Cholesterol is emitted by meat-cooking operations, and levoglucosan is emitted mainly from biomass combustion (16). 17R(H)21β(H)-29-Norhopane is contained in lubricating oil (25), and the size distribution of this compound is highly correlated with other trace compounds contained in lubricating oil, including 17R(H)-21β(H)-hopane and Rββ-20R-C29-ethyl-

TABLE 3. Concentration of Organic Compounds, Potassium, Organic Carbon, and Elemental Carbon Measured in the PM0.1, PM0.18, and PM1.8 Size Fractions at Modesto between December 15, 2000 and January 7, 2001 Modesto day PM0.1 fluoranthene pyrene retene benzo[ghi]fluoranthene chrysene benzo[b]fluoranthene benzo[k]fluoranthene benzo[e]pyrene benzo[a]pyrene perylene indeno[1,2,3-cd]pyrene benzo[ghi]perylene coronene MW302 PAHs

0.0 ( 1.5 3.6 ( 0.83 5.9 ( 1.2 4.0 ( 0.63 7.2 ( 0.15 24 ( 1.9 4.3 ( 1.4 12 ( 0.24 10 ( 0.77 2.8 ( 1.3 23 ( 32 36 ( 26 29 ( 6.9 0.0 ( 2.4

17B(H)-21A(H)-30-norhopane 17A(H)-21B(H)-hopane ABB-20R-C29-ethylcholestane

6.5 ( 0.19 0.0 ( 0.33 2.9 ( 0.08

levoglucosan cholesterol potassium organic carbon elemental carbon

1.7 ( 0 0.063 ( 0

Modesto night

PM0.18

PM1.8

PM0.1

PM0.18

PM1.8

161 ( 1.6 154 ( 2.2 181 ( 7.1 185 ( 2.3 212 ( 3.5 646 ( 5.3 97.4 ( 3.3 316 ( 4.8 182 ( 2.2 0.0 ( 1.7 1200 ( 83 1040 ( 80 813 ( 24 1140 ( 10

7.7 ( 12 11 ( 6.8 120 ( 12 17 ( 5.2 35 ( 2.7 103 ( 8.3 23 ( 5.9 55 ( 2.1 81 ( 4.1 14 ( 5.3 140 ( 129 140 ( 105 140 ( 29 227 ( 12

31 ( 20 48 ( 8.2 155 ( 23 114 ( 9.2 176 ( 13 368 ( 13 64.8 ( 6.1 185 ( 6.3 207 ( 7.0 32.4 ( 5.8 586 ( 198 488 ( 156 333 ( 53 643 ( 25

449 ( 59 577 ( 47 1020 ( 150 1130 ( 90 1590 ( 130 2960 ( 120 679 ( 15 1390 ( 56 2120 ( 43 331 ( 9.0 4030 ( 460 2700 ( 350 2540 ( 135 3730 ( 77

0.0 ( 1.4 0.0 ( 2.7 0.0 ( 0.55

41.6 ( 2.2 51 ( 3.9 28.4 ( 1.3

361 ( 15 515 ( 22 183 ( 7.9

(pg/m3)

PAHs 4.0 ( 1.9 7.9 ( 1.2 14 ( 2.5 13 ( 0.81 20. ( 1.1 76 ( 1.9 9.2 ( 1.6 43 ( 0.46 39 ( 1.5 2.8 ( 1.5 162 ( 39 152 ( 39 135 ( 12 148 ( 4.6

Hopanes/Steranes (pg/m3) 15 ( 0.21 250 ( 1.1 13 ( 0.43 330 ( 1.6 6.8 ( 0.17 60.9 ( 0.53

Other Organic Compounds (ng/m3) 3.6 ( 0 63.4 ( 0 271 ( 0 0.19 ( 0 7.1 ( 0 0.50 ( 0

2.5 ( 10 320 ( 110 66 ( 110

Other Species 44 ( 14 1230 ( 156 285 ( 156

(ng/m3) 47.2 ( 20 9210 ( 330 1690 ( 330

1.9 ( 10 825 ( 70 170 ( 70

465 ( 0 5.25 ( 0

2080 ( 0 35.2 ( 0

46.6 ( 14 3980 ( 99 822 ( 99

261 ( 10 22700 ( 220 4550 ( 220

TABLE 4. Concentration of Organic Compounds, Potassium, Organic Carbon, and Elemental Carbon Measured in the PM0.1, PM0.18, and PM1.8 Size Fractions at Bakersfield between December 15, 2000 and January 7, 2001 Bakersfield day PM0.1 fluoranthene pyrene retene benzo[ghi]fluoranthene chrysene benzo[b]fluoranthene benzo[k]fluoranthene benzo[e]pyrene benzo[a]pyrene perylene indeno[1,2,3-cd]pyrene benzo[ghi]perylene coronene MW302 PAHs 17B(H)-21A(H)-30-norhopane 17A(H)-21B(H)-hopane ABB-20R-C29-ethylcholestane

PM0.18

Bakersfield night PM1.8

0.0 ( 1.5 0.0 ( 0.83 3.7 ( 1.2 0.72 ( 0.63 0.95 ( 0.14 6.5 ( 1.9 1.4 ( 1.4 3.5 ( 0.23 2.7 ( 0.76 0.0 ( 1.3 0.0 ( 32 3.4 ( 26 0.0 ( 6.9 0.0 ( 2.4

PAHs (pg/m3) 2.8 ( 1.9 189 ( 1.8 0.0 ( 1.2 138 ( 2.2 7.1 ( 2.5 279 ( 7.3 5.1 ( 0.81 195 ( 2.3 7.4 ( 1.1 178 ( 3.3 37 ( 1.9 439 ( 3.7 5.4 ( 1.6 90.9 ( 3.3 21 ( 0.42 207 ( 4.4 18 ( 1.5 163 ( 2.1 0.0 ( 1.5 0.0 ( 1.7 81 ( 38 1120 ( 83 74 ( 39 821 ( 80 67 ( 12 587 ( 24 166 ( 4.6 1590 ( 12

4.7 ( 0.17 0.0 ( 0.33 4.2 ( 0.07

Hopanes/Steranes (pg/m3) 9.3 ( 0.19 220 ( 0.95 5.8 ( 0.43 208 ( 1.2 7.6 ( 0.17 161 ( 0.46

PM0.1

PM0.18

PM1.8

2.3 ( 11 0.0 ( 6.1 30 ( 8.8 5.4 ( 4.6 11.6 ( 1.4 43 ( 7.0 8.2 ( 5.3 22.7 ( 1.1 25 ( 2.9 5.6 ( 4.8 37 ( 116 36 ( 94 25 ( 25 92 ( 9.3

25 ( 18 31 ( 7.3 101 ( 20 81 ( 7.7 129 ( 11 306 ( 12 58 ( 5.6 155 ( 5.9 64 ( 4.6 27 ( 5.2 478 ( 178 317 ( 140 194 ( 47 320 ( 20

338 ( 50 439 ( 36 1130 ( 130 618 ( 50 902 ( 75 2120 ( 84 348 ( 14 1210 ( 49 1580 ( 63 240 ( 11 4540 ( 438 2450 ( 335 1690 ( 165 2930 ( 172

0.0 ( 1.3 0.0 ( 2.5 6.3 ( 0.55

97 ( 3.7 39 ( 3.0 39.5 ( 1.4

332 ( 14 344 ( 15 180 ( 7.7

132 ( 0 1.1 ( 0

659 ( 0 4.04 ( 0

38 ( 14 2960 ( 99 670 ( 99

314 ( 10 17500 ( 220 3600 ( 220

Other Organic Compounds (ng/m3) 29.9 ( 0 233 ( 0 39.3 ( 0 1.56 ( 0 6.7 ( 0 0.564 ( 0

levoglucosan cholesterol

12 ( 0 0.63 ( 0

potassium organic carbon elemental carbon

1.5 ( 10 170 ( 110 29 ( 110

Other Species (ng/m3) 5.2 ( 14 186 ( 20 864 ( 156 9610 ( 330 162 ( 156 1480 ( 330

cholestane. Benzo[ghi]perylene is expected to be dominated by gasoline emissions (25), and the size distribution of this compound is highly correlated with other gasoline tracers, including coronene. Only one example of each compound

6.1 ( 10 393 ( 70 93 ( 70

class is shown in Figure 3, to more clearly illustrate the common patterns between classes. Benzo[ghi]perylene has a size distribution which peaks between a 0.18 and 0.32 µm particle diameter at all locations VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Normalized size distribution of particle-phase total carbon, benzo[ghi]perylene, 17r(H)-21β(H)-29-norhopane, levoglucosan, and cholesterol. Concentrations in each size fraction were normalized by the total PM1.8 concentration. in both the daytime and nighttime. Size distributions for other compounds show more variation with the peak diameter sometimes occurring between a 0.18 and 0.32 µm particle diameter and sometimes at larger particle sizes. The geometric standard deviation of each size distribution also varies widely. Cholesterol generally has the broadest particle size distribution, while benzo[ghi]perylene and 17R(H)-21β(H)-29-norhopane generally have sharper peaks. All trace organic compound size distributions are generally correlated better with total carbon during the nighttime versus poorer correlation during the daytime. The differences between the size distributions of the various particle-phase organic compounds reflect the fact that these compounds exist in particles emitted from different sources. Each source releases particles to the atmosphere with a characteristic size distribution (see, for example, refs 17, 18, 26–30). Coagulation gradually transforms the aerosol into an internal mixture, but this process is not instantaneous, and it does not necessarily result in uniform contributions from all sources to all size fractions of the particle distribution. The results of the 6474

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current measurements should prove useful for sizeresolved source apportionment studies.

Acknowledgments This research was supported by the California Air Resources Board, Research Division under contract #01-306; the San Joaquin Valleywide Study Agency under contract #200005PM; and the United States Environmental Protection Agency under grant#RD-83241401-0. The research described in the article has not been subject to the Environmental Protection Agency’s required peer and policy review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. The authors thank Dan Chang and Jorn Herner for help with sample collection and Nehzat Motallebi for help with project coordination.

Literature Cited (1) Air Quality Criteria for Particulate Matter (October 2004); U.S. Environmental Protection Agency: Washington, DC, 2004.

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