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Environ. Sci. Technol. 2000, 34, 3690-3697

Adsorption onto Aerosol Soot Carbon Dominates Gas-Particle Partitioning of Polycyclic Aromatic Hydrocarbons JORDI DACHS* AND STEVEN J. EISENREICH Department of Environmental Sciences, Rutgers University, 14 College Farm Road, New Brunswick, New Jersey 08901-8551

KP )

CP NSaTSPTe(Q1-Qv)/RT ) CGTSP 1600po

where CG and CP are the gas and particle phase concentrations (ng m-3), NS is the number of adsorption sites (sites cm-2), aTSP is the surface area of the TSP (m2 g-1), T is the temperature (K), R is the universal gas constant (8.3 10-3 kJ K-1 mol-1), Q1 and QV are the enthalpies of desorption and volatilization (kJ/mol), respectively, and poL is the compounds’ subcooled liquid vapor pressure (Torr). Conversely, when partitioning is dominated by absorption into the organic matter, KP may be given by (7)

KP )

Gas-particle partitioning has an important influence on the fate of atmospheric polycyclic aromatic hydrocarbons (PAHs) and other semivolatile organic compounds (SOCs). In the present paper, gas- and aerosol-phase PAH concentrations and organic and elemental carbon concentrations in the aerosols measured in the Baltimore atmosphere and over the adjacent Chesapeake Bay in July 1997 were used to assess the mechanisms driving gasparticle partitioning of PAHs. The relative importance of adsorption onto the soot carbon and absorption into aerosol organic matter is evaluated by means of estimated soot/ air (KSA) and octanol/air (KOA) partition coefficients, respectively. The results show that absorption into organic carbon may account for less than 10% of the total PAHs in the particulate phase. Adsorption onto the soot phase predicts accurately the total suspended particulate matter normalized partition coefficients (KP) for PAHs. For example, KSA predicts KP values for phenanthrene over the Chesapeake Bay within a factor of 3. KP predictions at the Baltimore atmosphere are within a factor of 5 to 10 of measured KP values. This is consistent with a lack of equilibrium between the gas and aerosol soot phase.

Introduction Gas/particle partitioning is an important mechanism affecting the fate and transport of semivolatile organic compounds (SOCs) such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) (1, 2). During the past decade, an intense research effort has been underway to parametrize the gas/particle partitioning of SOCs in order to predict atmospheric concentration of SOCs accurately. A series of pioneering papers by Pankow and co-workers developed the modeling framework for the sorption processes on which subsequent studies have been based (3-9). Adsorption onto the aerosol surface and absorption into the aerosol organic matter are the two mechanisms describing the gas/particle partitioning of SOCs. When the partitioning is dominated by adsorption, the total suspended matter (TSP, µg m-3) normalized partition coefficient (Kp, m3 µg-1) is given by (3) * Corresponding author phone: 34-93-400-6100; fax: 34-93-2045904; e-mail: [email protected]. Present address: Department of Environmental Chemistry, IIQAB-CSIC, Jordi Girona 18-26, Barcelona 08034, Catalunya, Spain. 3690

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(1)

L

CP fOM760RT ) CGTSP MW ζ po106

(2)

OM OM L

where fOM is the fraction of organic matter phase on the TSP, MWOM is the mean molecular weight of the organic matter phase (g mol-1), and ζOM is the activity coefficient of the sorbate in the organic matter phase. Both equations predict KP to be inversely proportional to poL. Experimentally, it is usually difficult to discern the predominant mechanism in the environment based only on the dependence of KP on poL. Furthermore, the prediction of the actual KP values from eqs 1 and 2 is problematic since many of the parameters (e.g., NS, aTSP, MWOM, ζOM) are unknown for atmospheric aerosols. Predictions of KP values for a wide range of SOCs such as PCBs and polychlorinated naphthalenes (PCNs) on the basis of the octanol/air partition coefficient apply the following equation (10, 11)

KP ) fOM

ζOCT MWOCT K ζOM MW F 1012 OA

(3)

OM OCT

where ζOCT is the activity coefficient of the sorbate in octanol, MWOCT is the molecular weight of octanol and FOCT is the density of octanol (kg/L). The fact that KP values of PCBs, PCNs and other SOCs are faithfully predicted from KOA suggests that gas-particle partitioning is consistent with absorption into the organic matter for these compounds (11). However, eq 3 under-predicts KP values of PAHs, often by 10 to 50 times (11). One explanation is that the ratio ζOCT/ζOM is higher for PAHs than for PCB; however, this statement remains unproven. A second explanation for the high KP values for PAHs is the occurrence of a nonexchangeable fraction in soot particles and slow reequilibration of PAHs in combustion aerosols as they are diluted in ambient air (11-13). Higher concentrations of PAHs than that predicted from the OC concentrations in aquatic sediment have also been reported (14, 15). This has been interpreted as indicating the presence of a soot phase to which PAHs are strongly sorbed (15). Gustafsson et al. proposed a method to quantify the PAH concentrations in the sedimentary soot phase. They measured the fractional soot content in sediments fSC (g of soot C/g sed) and estimated the soot-carbon normalized partition coefficient KSW (L kg-1) from values obtained from PAH adsorption onto activated carbon (15, 16). Thus, the partition coefficient (Kd, L kg-1) between water and sediment may be estimated by

Kd ) fOCKOC + fSCKSW

(4)

where fOC is the fraction of organic carbon in the sediment 10.1021/es991201+ CCC: $19.00

 2000 American Chemical Society Published on Web 08/02/2000

FIGURE 1. Map showing the location of the sampling sites in Baltimore, MD and over the Chesapeake Bay. and KOC (L kg-1) is the organic carbon normalized partition coefficient between water and sediment. Equation 4 predicted quantitatively the Kd values for PAH in well-characterized sediments (15). The occurrence of soot in sediments is a consequence of atmospheric depositional loadings (17-19). This suggests that the phase association between PAHs and soot carbon is present in atmospheric aerosols before their introduction into the aquatic environment. Soot particles are byproducts of the combustion of liquid and gaseous fuels and their production depends strongly on the ratio of carbon to oxygen during combustion (20). PAHs are formed concurrently with soot particles and also play an important role in soot formation and particle growth (20). Furthermore, PAHs have a high affinity for carbonaceous materials as discerned from adsorption experiments with carbon black and activated carbon and theoretical predictions (16, 21, 22). This is consistent with an observed correlation between PAH aerosol concentrations and soot content (23). Therefore, adsorption of PAHs onto the soot fraction of atmospheric aerosols, or primary aerosol carbon with which it is highly correlated, may be an important mechanism affecting the gas-particle partitioning of PAHs. The objectives of this paper are to extend the partitioning model between water and sedimentary soot carbon suggested by Gustafsson et al. (15) to air-soot partitioning and to assess the potential role of adsorption onto soot carbon and absorption into the organic matter as mechanisms driving the gas-particle partitioning of PAHs.

Experimental Section Sampling Sites and Strategy. Air samples (gas + particulate) were taken at two sampling sites representative of the urban Baltimore area and adjacent Chesapeake Bay. The first site was located on the roof (ca. 25 m height) of the Maryland Science Center (39°16.45′N, 76°36.40′W) at the inner harbor of Baltimore city. The second site was over the Chesapeake Bay (39°10.56′N, 76°19.67′W), about 20 km east of Baltimore (Figure 1). The over water samples were taken on the bow of EPA R/V Anderson, with samplers maintained upwind of ship exhaust. At each site, consecutive 4-hour samples were collected from July 22 to July 25, and 12-hour samples were taken from July 25 to July 28. Atmospheric particulate and gas-phase samples were obtained with modified high volume air samplers (calibrated flow rate of ∼0.3 to 0.5 m3 min-1) using quartz fiber filters (QFFs, Whatman) and polyurethane foam (PUF). PUFs were pre-cleaned in a Soxhlet apparatus for two periods of 24 h with acetone and petroleum ether, respectively. QFFs were preweighed in a temperature and humidity controlled laboratory after being baked at 450 °C for 4 h. At each sampling site, two side by side samples were taken. One of the samples was used for organic carbon (OC) and elemental carbon (EC) analysis, and the second sample was used for PAH analysis. Analytical Procedure for PAHs. Before extraction, PUFs and QFFs were spiked with d10-fluoranthene and d10-benzo[a]pyrene which were used as surrogate standards. PUFs and QFFs were extracted for 24 h in a Soxhlet apparatus with VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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petroleum ether and dichloromethane, respectively. All extracts were first concentrated by rota-evaporation to 1-2 mL, while further concentration to ∼0.5 mL was performed under a gentle stream of purified N2. Samples were then fractionated on a 3% H2O-deactivated alumina (4 g) column. The first nonpolar fraction containing nonaromatic hydrocarbons was obtained by eluting with 13 mL of hexane, while the second fraction containing the PAHs was eluted with 15 mL of hexane-dichloromethane (2:1). The second fraction was then concentrated to ca. 200 µL under a gentle stream of purified N2. The internal standards d10-phenanthrene and d10-pyrene were added to the vials prior to injection into the gas chromatograph. PAHs were quantified by gas chromatography (HP 6890) coupled with mass spectrometry-electron impact (HP 5973 MSD) operating in selective ion mode (ions: 178, 184, 192, 198, 202, 206, 216, 226, 228, 252, 276, 278 and 300). A 30 m × 0.25 mm i.d. column DB-5 (J&W Scientific) capillary column with a film thickness of 0.25 m was used. The temperature program began at 50 °C for 1 min, increased to 125 °C at 25 °C/min, followed by a 8 °C/ min ramp up to 300 °C. Quantification was performed by the internal standard procedure. PAHs from fluorene to chrysene were quantified relative to d10-phenanthrene and corrected by d10-fluoranthene surrogate recoveries. High molecular weight PAHs, benzofluoranthenes to coronene, were quantified relative to d12-benzo[a]pyrene and corrected by d10pyrene recoveries. Surrogate standard recoveries ranged from 72 to 98%. Matrix spikes were processed together with the samples and their recoveries ranged from 77 to 96%. Laboratory and field blanks were also processed together with the samples. PAH masses recovered from blanks were always below 5% of real sample values. A total of 34 PAHs were quantified, and the detailed description of their occurrence and temporal trends is reported elsewhere (24). In the present paper, the gas-particle partitioning is assessed for phenanthrene, pyrene, fluoranthene and chrysene. These are the PAHs that were detected in both the gas and aerosol phases for all the sampling events and whose physical-chemical properties such as Henry’s law constants (25) and octanol-air partition coefficients (26) are known. Analytical Procedure for OC and EC. The determination of OC and EC was performed by thermal-optical reflectance as described elsewhere (27) at Sunset Laboratory Inc. (Forest Grove, Oregon). The quantification limit for both OC and EC by this method was ca. 0.2 g m-3. The observed levels were above the quantification limit for all the samples from the Baltimore site and 15 of 24 of the EC measurements from the 4-hour samples taken on board the R/V Anderson during July 22-24. Inorganic carbon content was below the detection limit (0.2 g m-3) for all the sampling periods. Meteorological Data. Temperature, wind direction and speed and relative humidity for the Chesapeake Bay and the Baltimore sampling sites were obtained from the measurements made on board the R/V Anderson and from a meteorological tower located at the Maryland Science Center in Baltimore, respectively. The data were averaged for each sampling period. Suitability of the Data Set for a Gas-Particle Partitioning Study. Since there are a number of potential sampling artifacts that can lead to over- and under-estimations of the gas and aerosol phase concentrations of SOCs (28), examination of the suitability of the data set for detailed analyses of gas-particle partitioning is necessary. A potential sampling artifact occurs when gaseous PAHs sorb to filter and particle surfaces, thus leading to an over-estimation of particle-phase PAHs. Using the same type of air-sampler and filters, Simcik et al. found that less than 5% of mass recovered from the primary filter was sorbed on a secondary filter (29). Our sampling strategy consisted of mostly 4- and several 12 h samples. This sampling artifact could be especially important for the 4-hour samples and for PAHs with higher MW and 3692

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TABLE 1. Total Suspended Particulate Matter (TSP), Organic Carbon (OC) and Elemental Carbon (EC) Concentrations at the Baltimore and Chesapeake Bay Atmospheres in July 1997 m-3)

TSP (µg OC (µg m-3) EC (µg m-3) fOC fEC

Chesapeake Bay (n ) 24)

Baltimore (n ) 24)

28.5 (4.5-83.8) 4.1 (0.25-9.1) 1.5 (0.31-7.3) 0.15 (0.055-0.22) 0.051 (0.012-0.19)

50.6 (23.1-94.8) 5.9 (3.1-10) 1.3 (0.21-2.55) 0.12 (0.061-0.19) 0.027(0.01-0.073)

sampling volumes of ∼100 m3. It is not possible to assess in a direct way the relative importance of this potential artifact due to the lack of back-up filters. However, comparison of the results obtained with the 4-hour (∼120 m3 air) and the 12-hour samples (∼500 m3 air) shows no significant difference on the KP values. Furthermore, the overestimation of the particulate phase would be more important at low TSP concentrations, i.e., at the Chesapeake Bay site. However, significantly higher KP values were not observed over the Chesapeake Bay in comparison with the Baltimore site. All these observations are consistent with a lack of a significant sampling artifact due to sorption to the filter. Breakthrough of volatile PAHs from the PUF can lead to under-estimated concentrations of gas-phase PAHs. Breakthrough was assessed with split PUFs that showed that phenanthrene recoveries from the back-up PUF were always lower than 20% of the mass recovered with the front PUF. This percentage was insignificant for fluoranthene, pyrene and chrysene. Therefore, breakthrough did not influence the measured gas-phase concentrations. The use of 4-hour samples minimized other potential sampling artifacts such as variability in wind direction and speed, relative humidity and atmospheric PAH concentrations. Finally, several studies have shown that there are not significant differences when comparing results obtained with Hi-Vol samplers, diffusion separator, denuders and impactors (30, 31). Therefore, the present data set are suitable for detailed examination of gasparticle partitioning of PAHs.

Results and Discussion Concentrations of TSP, OC and EC. Concentrations of TSP, OC and EC are given in Table 1. TSP concentrations ranged from 23 to 95 µg m-3 in the Baltimore atmosphere and are similar to those reported in other urban areas (20). TSP concentrations observed over Chesapeake Bay were similar (40-84 µg m-3) to those at the urban site when wind directions were from the urban area. Conversely, TSP concentrations were significantly lower (4.5-40 µg m-3) for other wind vectors. This demonstrates the important influence of the urban/industrial Baltimore area on the adjacent Chesapeake Bay atmosphere. The average OC concentration measured in the Baltimore atmosphere (5.9 µg m-3) is similar to the value reported for other urban atmospheres such as Philadelphia (4.5 µg m-3) and Chicago (5.4 µg m-3) but lower than those measured in the Los Angeles atmosphere (8.3 µg m-3) (32). Aerosol OC accounts for 6 to 19% of the atmospheric particulate matter at the Baltimore site. The average value of EC in the Baltimore atmosphere (1.3 µg m-3) is comparable to those described for other urban areas such as Chicago (1.3 µg m-3) and Philadelphia (0.8 µg m-3). EC levels for 9 out of 24 of the 4-hour samples at the Chesapeake Bay site were below quantification limit. For the remaining 15 events the average EC concentration (1.5 µg m-3) was similar to that at the urban site. However, this average is influenced by two sampling periods for which concentrations of EC were unusually high (7.3 and 4.3 µg m-3 observed in July 25th and 26th). Omitting the two highest concentrations, the average EC concentration over the Chesapeake Bay was 0.7 µg m-3, which is 40 to 60% lower than in the Baltimore atmosphere.

TABLE 2. Average and Range of Gas- and Aerosol-Phase PAH Concentrations at Each Sampling Site Chesapeake Bay (n ) 23) m-3)

phenanthrene (ng fluoranthene (ng m-3) pyrene (ng m-3) chrysene (ng m-3)

Baltimore (n ) 24)

gas phase

aerosol phase

gas phase

aerosol phase

5.57(1.51-18.5) 0.848(0.225-1.71) 0.548(0.122-1.78) 0.007(0.002-0.02)

0.051(0.014-0.291) 0.057(0.014-0.281) 0.067(0.011-0.672) 0.085(0.018-0.215)

12.5(1.26-43.9) 3.43(0.597-14.8) 2.14(0.478-9.26) 0.023(n.d.-0.011)

0.089(0.007-0.189) 0.162(0.046-0.289) 0.14(0.014-0.265) 0.063(0.026-0.141)

TABLE 3. Physical-Chemical Parameters and Partition Coefficients at 298 K for Phenanthrene, Fluoranthene, Pyrene and Chrysene

phenanthrene fluoranthene pyrene chrysene f

Ha (Pa m3 mol-1)

∆Ha (kJ mol-1)

∆ Sa (kJ mol-1 K-1)

log KOWb

Ac

Bc

log KOA

log KSWe (L kg-1)

log KSAf (L kg-1)

4.29 1.96 1.71 0.53

47.3 38.7 42.9 100.9

0.106 0.07 0.084 0.268

4.57 5.22 5.18 5.86

-5.62 -4.56 -5.94 n.a.g

3942 3985 3985 n.a.g

7.57c 8.80c 8.88c 9.38d

7.1 7.8 7.7 8.5

9.8 10.9 10.8 12.2

a Bamford et al. (35). b Mackay et al. (49). c Harner and Bidleman (26). Estimated from the ratio of KSW and H′. g n.a. means not available.

Atmospheric Occurrence of PAHs. Atmospheric concentrations of phenanthrene, fluoranthene, pyrene and chrysene are listed in Table 2. Concentrations of PAHs were ∼2 to 4 times higher at the Baltimore site than over Chesapeake Bay. For example, average gas-phase concentrations of phenanthrene and fluoranthene were 5.6 and 0.85 ng m-3 over Chesapeake Bay, while the average gas-phase concentration at the urban site were 12.5 and 3.4 ng m-3 for phenanthrene and fluoranthene, respectively. The PAH concentrations over the Chesapeake Bay are within a factor of 2 of measurements made at other locations near/over the Chesapeake Bay (33-35). Even though the range and average PAH concentrations are similar to those reported previously, there is a 10-fold variability in the aerosol and gas-phase concentrations at both sites. This variability is often related to wind direction, where higher concentrations are obtained for winds from the urban/industrial area (24). However, the variability of aerosol-phase PAH concentrations is also related to EC concentrations in the aerosols. For example, for the two samples with high EC concentrations at the Chesapeake Bay site, PAH concentrations were 10-fold higher than for the other sampling periods (24). This suggest that EC, which accounts for a significant fraction of soot carbon (20), exerts a strong influence on the PAH concentrations in the aerosol phase and thus may be an important sorption phase for controlling the gas-particle partitioning of PAHs. Soot/Air and Octanol/Air Partition Coefficients. To quantify the PAH content in the soot phase, a soot/air partition coefficient is needed. As suggested by Gustafsson et al. (15), the soot/water partition coefficient KSW is assumed to equal the activated carbon/water partitioning coefficients reported in the literature (16). Soot carbon is not exactly structurally the same as activated carbon and may have different sorptive properties. However, they do have similar specific surface areas. Indeed, the surface area of the activated carbon used by Walters and Luthy (Filtersorb 400, aAC ) 1000 m2 g-1) is within a factor of 3 of the specific surface are reported for soot carbon (370 m2 g-1) (21). In the present paper, we assume that EC accounts for most of soot carbon, and there is no distinct difference between soot and elemental carbon. In reality, soot carbon is also composed by a fraction of organic carbon that may be coating the elemental carbon. This organic matter phase is accounted as organic carbon by the thermal-optical reflectance analytical procedure used for OC and EC determination. Henry’s law constant is required to estimate KSA. The dimensionless Henry’s law constant (H′) gives the ratio between gaseous and dissolved concentrations in equilibrium

d

Estimated from the ratio of KOW and H′. e Walters and Luthy (16).

at a given temperature (T). H′ can be estimated (25) using the heat (∆H, kJ mol-1) and entropy (∆S, kJ mol-1 K-1) of volatilization listed in Table 3 by

ln H′ ) -

∆H ∆S + RT R

(5)

Therefore, the soot/air partition coefficient (KSA, L kg-1) at the temperature T is given by

KSA )

KSW H′

(6)

The values of KSW obtained from adsorption experiments on activated carbon (16) and the respective values of KSA for phenanthrene, fluoranthene, pyrene and chrysene are listed in Table 3 at 298 K. Both KSW and KSA are 2 orders of magnitude higher than KOW and KOA, respectively. Therefore, PAHs have a graeter tendency to sorb onto the soot phase than into the organic matter per mass of sorbing phase. Assuming that EC is a surrogate for the soot phase, the overall gas-particle partition coefficient that accounts for both the organic matter and the soot phases is given by

KP )

fOMMWOCTζOCT FOCTMWOMζOM1012

KOA + fEC

aEC aAC1012

KSA

(7)

where fEC is the fraction of elemental carbon in the aerosol, aAC is the surface area of the activated carbon and aEC is the specific surface are of the elemental carbon. Temperature exerts a strong influence on the values of KOA and KSA. The temperature dependence of KOA values can be obtained by

log KOA ) A +

B T

(8)

where A and B were estimated by Harner and Bidleman (26) by measurements of KOA at different temperatures (Table 3). The influence of temperature on KSA is taken into account in part by the temperature corrected H′ values used in eq 6. However, KSW may also be temperature-dependent, but this dependence is impossible to estimate with the available data; therefore, the value reported at 298 K is used. Predictions of Gas-Particle Partitioning of PAHs. The KP values for each sampling period were estimated for phenanthrene, fluoranthene, pyrene and chrysene at the urban and Chesapeake Bay sites. Figures 2 and 3 show the predicted KP values from KOA (eq 3) and by taking into account both phases: organic matter and soot C (eq 7). The VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Measured and predicted values of KP (ug m-3) over the Chesapeake Bay for phenanthrene, fluoranthene, pyrene and chrysene. Circles (b) are the measured KP values. Squares (9) are the predicted KP values from KOA (eq 3). Triangles (2) are the predicted KP values from KSA and KOA (eq 7). assumptions made when applying eqs 3 and 7 were that the organic matter fraction fOM is 1.5 times the organic carbon fraction (fOC) and that the ratios MWOCTζOCT/MWOMζOM and aAC/aEC used in eqs 3 and 7, respectively, are equal to one. Using these assumptions, Harner and Bidleman were able to predict the KP values for PCBs and PCNs in an urban atmosphere using eq 3 (11). Furthermore, it has been argued that the value of the ratio ζOCT/ζOM may be similar for different compounds (36). This suggests that the above assumptions may be realistic for PAHs. Thus the assumptions are that octanol and elemental carbon are good surrogates of organic matter and soot, respectively. KOA and KSA were corrected by the average temperature during the sampling period using eqs 5 and 8. However, the parameters A and B used in eq 8 are not available for chrysene; therefore, KOA for this compound was estimated as the ratio of KOW and H′ (Table 3). The measured KP values at each site are also depicted in Figures 2 and 3 for comparative purposes. Over the Chesapeake Bay (Figure 2), the predictions of KP from KOA are significantly lower (p < 0.001) than the measured values. For example, KOA under-predicts KP values of phenanthrene and chrysene by a factor of 8 to 266 and from 20 to 1059, respectively. Conversely, values predicted using KSA (eq 7) are statistically equal (p < 0.05) to the measured KP for all the PAHs. For example, KP for phenanthrene is always predicted within a factor of 3. Even temporal trends in KP, such as the decrease of KP values for the last 3 days of 3694

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sampling, is also predicted by taking into account soot phase PAHs. Temporal trends are especially well predicted for chrysene (Figure 2). At the urban site, KOA also significantly under-predicts (p < 0.05) the measured KP values for all the PAHs studied. For example, it under-predicts KP values by factors ranging from 2 to 142 for phenanthrene and from 8 to 236 for chrysene. Conversely, a paired t-test of predicted values using KSA (eq 7) and KP measurements exhibits no significant difference (p < 0.05). However, KP measurements exhibit a higher variability at the urban site than over the Chesapeake Bay (Figures 2 and 3), and soot phase PAHs do not fully account for the variability depicted by the temporal trends of KP. Figure 4 shows the measured KP values against the predicted KP values from KSA using eq 7. The results obtained by a linear regression of measured versus predicted KP values are also shown. At both sites the slope and intersection are not statistically different from one and zero, respectively. KP values exhibit more scatter at the Baltimore site than over the Chesapeake Bay resulting in a poorer prediction of the temporal trends of KP at the urban site. Measured KP values in the Baltimore atmosphere are under-predicted by as much as a factor of 6 or are over-predicted by as much as a factor of 10. Nevertheless, predictions obtained by taking into account the soot-phase PAHs are significantly better than those obtained by accounting for organic matter and KOA.

FIGURE 3. Measured and predicted values of KP (ug m-3) at the Baltimore sampling site for phenanthrene, fluoranthene, pyrene and chrysene. Circles (b) are the measured KP values. Squares (9) are the predicted KP values from KOA (eq 3). Triangles (2) are the predicted KP values from KSA and KOA (eq 7). The urban site is located at the inner harbor of Baltimore, with multiple sources of PAHs in all directions such as vehicles, industry, oil-fired power plants and volatilization from the harbor and Chesapeake Bay (24, 34, 35, 37). Thus, the dynamics of PAHs is complicated and an important fraction of PAHs may have been only recently introduced in the atmosphere. Adsorption and desorption of SOCs to combustion aerosols may take hours to reach equilibrium (38, 39) and may be due to the occurrence of a liquidlike organic phase coating the elemental carbon. Diffusion from this liquid phase to the inner particle, elemental carbon in our case, may be the limiting mass transfer rate for the gasparticle partitioning (38). Therefore, the discrepancy between measured and predicted KP values at the urban site is consistent with slow reequilibration between the gas and particulate phases. Conversely, over the Chesapeake Bay, air masses were not derived from the urban/industrial Baltimore area except for 4 sampling periods out of 23. Thus, particulate matter may be older and PAHs may have time to reequilibrate between the gas and particulate phases, resulting in better predictions for gas-particle partitioning (eq 7). Mechanisms Governing Gas-Particle Partitioning of PAHs. The accurate predictions of KP by accounting for the soot phase give important insights of the prevalent mechanisms of gas-particle partitioning. First, it provides evidence that PAHs have a much higher affinity for the soot or EC phase than to the organic matter in environmental aerosols. Second, absorption into organic matter accounts for less than

10% of the total PAHs sorbed on the aerosols; thus adsorption is the prevalent mechanism for gas-particle partitioning of PAHs at both the urban site and over the Chesapeake Bay. To extrapolate these conclusions to other atmospheric environments, the relative contribution to KP of each term on the right side of eq 7 is needed. This will permit elucidation of the prevalent mechanisms for gas-particle partitioning of PAHs depending on aerosol chemical composition. With the assumptions explained above for molecular mass of organic matter and activity coefficients in octanol and organic matter, three cases or scenarios may be considered.

Case I:

fECKSAfOCT >5 fOMKOA fECKSAfOCT > 0.2 fOMKOA

Case II:

5>

Case III:

fECKSAfOCT < 0.2 fOMKOA

Case I occurs when the KSA/KOA . 1 leading to gas-particle partitioning dominated by adsorption to the soot phase even though the content of elemental or soot carbon may be lower than the content of organic matter (fEC < fOM). The gas-particle partitioning in the Baltimore urban and Chesapeake Bay VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Measured versus predicted KP (ug m-3) values at each site. The regression line is for all the available data from phenanthrene, fluoranthene, pyrene and chrysene. The intersection was statistically equal to zero. coastal atmospheres correspond to this scenario, as discussed above. Conversely, Case II occurs when both absorption into the organic matter and adsorption onto the soot carbon is important. Finally, Case III corresponds to the situation where absorption into the organic matter is the prevalent sorption mechanism. Since KSA are 2 orders of magnitude higher than KOA, cases II and III can only be achieved when fOM is much higher than fEC. Potential candidates for Cases II and III are the gas-particle partitioning of PAHs to secondary organic aerosol or to vegetation-derived aerosols, where the content of organic matter is much higher than the soot content (20). This is consistent with several studies that have suggested that absorption into the organic matter is the dominant mechanism for gas-particle partitioning of PAHs on secondary organic aerosols (40, 41). However, the more intriguing implication come from the exchangeable character of soot phase PAHs. The parametrization of soot phase PAHs shown in eq 7 corresponds to equilibrium conditions between the gas and particulate phase. Still, it is possible that there is a fraction of nonexchangeable PAHs. However, soot phase PAHs estimated using KSA are subject to adsorptive-desorptive interactions with the gas phase even though they are associated with the soot carbon. Therefore, most soot phase PAHs are exchangeable with the gas phase. The nonexchangeable fraction may 3696

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account for some of the under-predictions of KP for some of the sampling periods (Figures 2-4). It is also possible that part of the nonexchangeable fraction is not extracted with the analytical procedure used. Implications for the Environmental Fate and Biogeochemical Cycles of PAHs. The strong association of PAHs to soot in aerosols and marine sediments as evidenced in this study and in a previous work (15) suggests that soot carbon plays a major role on the biogeochemical cycle of PAHs. Even though soot carbon concentrations are usually lower than organic carbon concentrations in aerosols and other environmental matrixes (15, 20), the higher KSW and KSA partition coefficients in comparison to KOW and KOA make soot carbon a major pool for PAHs in the environment. The organic matter pool will dominate as a sorptive phase only when the content of organic matter is 100 fold higher than the content of soot carbon. In the atmosphere, this may happen in secondary organic aerosols. Conversely, in the marine environment, high concentrations of organic carbon are found in the photic zone of the water column (42, 43). Thus sorption to organic matter may be the predominant mechanism in the water column of the marine environment. This is consistent with vertical profiles of particle-associated PAHs in the marine environment, where there is a concurrent maximum of PAHs and organic matter and/or phytoplankton biomass near the surface (44, 45). However, most organic matter is recycled in the water column before it accumulates in the sediment (42, 43), while soot carbon is refractory and reaches the sediment with little recycling (18). Soot carbon accounts for an important fraction (12-31%) of the sedimentary organic carbon (46), and thus soot particles may dominate the final sink and accumulation of PAHs in sediments in aquatic environments. Since, soot carbon is refractory, the strong association of PAHs to soot would inhibit recycling back to the water column and in-situ biological transformation. Furthermore, the fact that most soot in recent sediments is derived from fossil fuel combustion and subsequent atmospheric transport and deposition (17, 19) is consistent with the observation of similar PAH patterns in aerosols and sediments (47, 48). In the present paper, adsorption of PAHs onto the organic matter has not been considered, but it could also play an important role. However, a better knowledge of the parameters used in eq 1 or partition coefficients for adsorption onto organic matter are needed in order to take into account this mechanism. Further research should lead to the acquisition of a more mechanistic picture of the processes governing gas-particle partitioning of PAHs and other semivolatile organic compounds. This should be done by a better chemical and physical characterization of aerosols together with an effort to develop and apply mechanistic models to observational measurements.

Acknowledgments T. R. Glenn, R. Pelleriti, C. L. Gigliotti, P. Brunciak, E. D. Nelson, T. P. Franz, J. E. Baker, J. Scudlark, and T. Church are kindly acknowledged for their help in the field, the laboratory or logistics. The crew of the US EPA R/V Anderson is acknowledged for their help during sampling. J. Dachs acknowledges a postdoctoral fellowship from the Spanish Ministry of Education and Culture. This work was funded in part by grants from the U.S. Environmental Protection Agency (Project EPA/CR 822046-01; A. Hoffman, Project Officer), the NJ Sea Grant College Program (NOAA) from the U.S. EPA (M. Weinstein, Project Officer), and the New Jersey Agricultural Experiment Station.

Supporting Information Available Concentrations of phenanthrene, fluoranthene, pyrene and chrysene in the particulate and gas phases for each sampling

period and TSP, OC, EC concentrations and the average temperature for each sampling period. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review October 20, 1999. Revised manuscript received June 12, 2000. Accepted June 12, 2000. ES991201+

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