Sorption of a Diverse Set of Organic Vapors To Diesel Soot and Road

Jul 21, 2005 - In a traffic-dominated environment sorption of organic pollutants to exhaust aerosols can strongly determine their further fate. The so...
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Environ. Sci. Technol. 2005, 39, 6632-6637

Sorption of a Diverse Set of Organic Vapors To Diesel Soot and Road Tunnel Aerosols C H R I S T I N E M . R O T H , * ,† KAI-UWE GOSS,* AND R E N EÄ P . S C H W A R Z E N B A C H Swiss Federal Institute for Environmental Science and Technology (EAWAG), Ueberlandstrasse 133, Postfach 611, CH-8600 Duebendorf, Switzerland, and Swiss Federal Institute of Technology (ETH), Zurich, Switzerland

In a traffic-dominated environment sorption of organic pollutants to exhaust aerosols can strongly determine their further fate. The sorption properties of two aerosol samples representing different exhaust sources have been determined for a large set of diverse organic vapors. For pure diesel soot we could identify adsorption to elemental carbon (EC) as the dominant sorption process. We used our experimental equilibrium adsorption coefficients to derive a predictive model for adsorption on soot in line with adsorption models for other surfaces published earlier. On road tunnel aerosols, both adsorption to EC and absorption in organic matter (OM) governed the observed sorption and the data could not be further evaluated in terms of a specific sorption mechanism.

Introduction The transport and fate of organic pollutants in the atmosphere is strongly dependent on sorption to aerosols. In the atmosphere, organic vapors can partition on or in a variety of surfaces and bulk phases, e.g., water droplets, surface of water droplets, snow and ice crystals, minerals, and carbonaceous and salt particles. Some of the inorganic surfaces and bulk phases have already been characterized with respect to their sorption properties (1-5 and references therein). In contrast, carbonaceous atmospheric particles are diverse and not well-defined. Recently, considerable efforts have been undertaken to evaluate the composition of carbonaceous aerosols and their sorption characteristics (e.g., 6-8), but a comprehensive understanding is still missing. In an urban environment, possible sorption processes are adsorption to inorganic surfaces, such as minerals or salts, adsorption to elemental carbon (EC), or absorption into organic matter (OM). A widely used approach to describe sorption on and in aerosols is a one-parameter linear free energy relationship (LFER), using either the saturated vapor pressure (p/L) or the octanol-air partitioning coefficient (Ki oa) as compound descriptor. It has been shown that these one-parameter LFERs are not suitable to describe adsorption or absorption of a diverse set of organic compounds (1-3, 9). Furthermore, over the past few years it has become a common approach to estimate gas/particle partitioning by assuming that * Address correspondence to either author. Fax: ++41-1-82352-10. E-mail: [email protected] (K.-U.G.), [email protected] (C.M.R.). † Present address: Harvard University, Boston, Massachusetts. 6632

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sorption only occurs as absorption into an organic, octanollike fraction of the aerosols and that this fraction is fully available for absorption (10-14). However, Dachs and Eisenreich, and Lohmann and Lammel have suggested that adsorption to EC may contribute significantly to the sorption capacity of ambient aerosols (15, 16). Obviously, this would have important consequences: the sorption capacity of the aerosols would depend on the EC surface area rather than the OM mass of the aerosols and therefore absorption in octanol could not be used as a general model for the sorption process in such aerosols. In a recent paper, Mader and Pankow attempted to distinguish the relative importance of adsorption to EC and absorption in OM (measured as organic carbon, OC) for nonpolar compounds by normalizing data from a controlled field experiment to the EC and OM content of the particles (6). Overall, this study did not give a clear answer although the data appeared to be more consistent with an absorptive mechanism. The objectives of this work and the work presented in a companion paper (17) are to examine sorption to various aerosol samples in order to shed some more light on the above questions. Diesel soot and road tunnel aerosols samples were chosen representing aerosols of different composition (EC and OC content) and of different origin. The soot used in this study was exhaust soot sampled at the outlet of a diesel engine (18). During combustion of fuels, incomplete burning leads to the formation of small carbonaceous particles similar to graphite. This EC agglomerates in larger particles, while organic vapors adsorb on this EC during exhaust and cooling (8). Soot therefore is a mixture of EC and OM, but represents a rather EC-dominated aerosol type (see compilation in ref 8, p 704). The composition of the road tunnel aerosols can be expected to be similar, but not identical, to the diesel soot. The road tunnel aerosols come from a variety of vehicles instead of a single diesel engine and had been exposed to the atmosphere for a short time, whereas the diesel soot was collected directly at the exhaust. The sorption data are interpreted with a poly-parameter model (1, 2, 9), that takes into account all relevant intermolecular interactions, i.e., van der Waals interactions, electron-donor/-acceptor interactions, and cavity formation (only in case of absorption). This model allows evaluation of the influence of the properties of the sorbent (surface or bulk phase) and of the properties of the compound on the sorption coefficient. In addition, it provides a valuable tool to predict the sorption behavior for compounds not investigated experimentally and a way to distinguish adsorption from absorption.

Materials and Methods Gas-Chromatographic Analysis. Gas/particle partitioning has been measured with an inverse gas chromatographic (IGC) system (see refs 1-4, 19, 20 for detailed information on this method). In IGC, dynamic sorption experiments with the sorbent as stationary phase provide retention volumes that are a measure of the sorption intensity on and/or in the sorbent. The net retention volume (Vi net) is determined using the following relation:

Vi net ) ti rQcor - Vtracer

(1)

where ti r is the retention time of the compound, Qcor is the volumetric gas flow rate corrected for the pressure drop across the column, and Vtracer is the elution volume of an inert tracer 10.1021/es049204w CCC: $30.25

 2005 American Chemical Society Published on Web 07/21/2005

(i.e., methane). The retention time is determined by the first statistical moment of the peak (20, pp 69). For thermodynamic interpretation of sorption data it is pivotal to normalize adsorption data to the surface area and absorption data to the volume of the sorbent. Therefore, adsorption coefficients Ki surface/air [(mol/msurface2)/(mol/mair3) ) m] are related to Vi net and the total surface area (Atot) of the adsorbent by the following:

Ki surface/air ) Vi net/Atot

(2)

where Atot ) maerosol × SSAaerosol, which is simply the product of the adsorbent mass (maerosol) and the specific surface area of the adsorbent, i.e., the aerosols (SSAaerosol). Absorption coefficients, Ki bulk/air [(mol/gbulk)/(mol/mair3) ) mair3/gbulk], are related to Vi net and the total bulk mass (m, e.g., maerosol) of the absorbent by the following:

Ki bulk/air ) Vi net/maerosol

(3)

A method to assign experimentally determined sorption data to one of the two sorption processes is explained in Results and Discussion and in the Appendix. Note that eqs 2 and 3 assume linear sorption isotherms and sorption equilibrium. Materials and Experiments. Diesel soot was purchased from the National Institute of Standards & Technology (NIST, Gaithersburg, MD) sold as Standard Reference Material 2975, Diesel Particulate Matter (Industrial Forklift). The material was collected with a filtering system by the Donaldson Company around 1990 (21), and was homogenized by NIST (18). The road tunnel aerosols were purchased from the Institute for Reference Materials and Measurements (IRMM, Geel, Belgium) sold as CRM 605. They were collected in a road tunnel in the city center of Birmingham, U.K. in September 1993, sieved, ground, sieved again, and freezedried (22). Both materials were used as received. The road tunnel aerosols were used as pure column packing in IGC. The diesel soot was diluted in a ratio of 3:100 (3% w/w, calculated from weighing in a homogeneous mixture) with methyl-silanized glass beads (DMCS-treated, Alltech, 125150 µm, used as received), because the strong sorption on pure soot packing resulted in retention times that were too high. The samples were packed in steel columns (3 mm i.d., 1 cm length, with steel frits of 1-µm porosity at the column inlet and outlet). The packed columns were placed in a water bath (15.0 ( 0.1 °C) and connected by stainless steel capillary tubings (i.d. 0.5 mm) to the injector and the FID of the GC (HRGC 5160 Mega Series with EL 580 Electrometer, Carlo Erba Instruments). To adjust the packed columns to a chosen RH, the carrier gas (N2) was saturated with water vapor in a gas wash bottle at a controlled temperature before entering the GC. The temperature of the gas wash bottle was set lower than that of the bath in which the columns were immersed, to reach the chosen RHs, which were 50 and 70%. Gas flow rates were controlled by a needle valve and measured with a mass flow meter (F-111C-HAO-11-V, Bronkhorst). They ranged between 2 and 40 mL/min for diesel soot experiments, and between 2 and 18 mL/min for the road tunnel experiments. The pressure drop in the system was measured with a pressure transducer (26PCCFB6G, Honeywell, see Supporting Information). The temperature of the water saturator and the measured gas flow rate were corrected accordingly. The columns were equilibrated overnight at the selected RH. Retention times of 78 (on diesel soot) and 63 (on road tunnel aerosols) organic compounds were measured by injecting 1-3000 µL of the headspace above the pure compounds (used as received, purity at least 96%, purchased

from Fluka and Merck) with a gastight syringe. An overview of compounds used is given in Tables S2 and S3 in the Supporting Information (SI). The specific surface area (SSAaerosol) of the diesel soot was reported by NIST to be 91 m2/g. The SSAaerosol of the road tunnel aerosols was determined by the Brunauer-EmmetTeller nitrogen adsorption method and was 7.4 m2/g. EC and OC measurements have been conducted by Sunset Laboratories (Tigard, OR) using a thermal-optical method (NIOSH 5040, 5% accuracy). The bulk samples were spread on quartz fiber filters for this analysis. Diesel soot contained 75.0 ( 3.7% EC and 5.7 ( 0.3% OC; the road tunnel aerosols contained 4.1 ( 0.2% EC and 8.8 ( 0.4% OC (w/w). Note that there is no agreement in the literature on the exact type of measurement and definition of EC and OC values, and different methods yield different results (8, 23, 24). Today however, the thermal-optical method used is the most accepted and most accurate one available commercially. We will use the term EC for elemental or black carbon in reference to the nonorganic, noncarbonate, solid fraction of carbon.

Results and Discussion Evaluation of the Chromatographic System. None of the compounds were retarded in the IGC system with an empty column. The net retention volume Vi net showed repeatability within 10% for both aerosol samples. For the evaluation of retention volumes with eqs 2 and 3, it had to be examined whether sorption was kinetically limited or occurring in the nonlinear range of the isotherm, as both could interfere with an interpretation of the results based on equilibrium, homogeneous sorption sites, and no sorbate-sorbate interactions. This was done by varying the carrier gas velocity and by varying the vapor concentration. Kinetic Limitation. A compound has to diffuse into small pores or into a bulk phase of the sorbent to access those sorption sites that have no direct contact with the carrier gas. In general, soot can contain diffusion-limiting micropores (e.g., 25, 26). The diesel soot however has a SSAaerosol, which is much lower than activated carbon or similar materials, that are clearly dominated by micropores (e.g., 25-27). NIST also reports pore diameters between 4 and 35 nm, which corresponds to mesopores. The time available for these diffusion processes depends on the carrier gas velocity and has to be long enough to allow diffusion into the whole pore or bulk phase. Note, however, that even nonequilibrium conditions might still allow an evaluation of the peaks, because the first moment of the peak is not necessarily changed (28). For our experiments we found that variation of the flow rate by factors of 20 (diesel soot) and 9 (road tunnel aerosols) for a range of nonpolar and polar compounds did not show any significant effect on the observed retention. Thus, we conclude that kinetic limitations did not influence the retention of our sorbates on these aerosol samples at the flow rates used. Nonlinearity. If sorption occurs in the nonlinear range of the sorption isotherm, then the retention volume of a given sorbate will vary with varying vapor concentrations. On the road tunnel aerosols (50% RH), 4 nonpolar and 12 polar compounds were measured using concentrations varying up to an order of magnitude, showing no dependency of retention on concentration. Furthermore, the maximal surface coverage in our experiments of 0.2% to 22% also indicates that nonlinearity caused by sorbate-sorbate interactions was unlikely to occur. On diesel soot (50% RH), retention did show a concentration effect. The effect occurred for some nonpolar and all polar compounds tested, resulting in a decrease of the retention volume by 10-55% (30% on average) when the concentration was increased by a factor of 10. This indicates that sorption took place in the nonlinear regime of the isotherm. However, the observed concentration VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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effect was rather small, so that the assumption of a linear sorption isotherm in the concentration range tested here still holds. The surface concentrations in our experiments were 10-8 to 10-6 mol/m2, which corresponds to 0.4% to 40% surface coverage. The range of concentrations used in the final data sets corresponds to 0.4% to 18% surface coverage, within which no change of retention was observed. Gas concentrations were 5 × 10-6 to 4 × 10-1 mol/m3. Sorption in the nonlinear range of the isotherm implies that the reported sorption coefficients are valid only in the concentration range used in our experiments and cannot readily be extrapolated to other concentrations. Further Evaluation of Experiments. Adsorption on diesel soot was measured on a mixture of glass beads and soot; hence the contribution of the glass bead surface to the observed retention volume had to be examined. For 49 of the compounds used here, the adsorption coefficients on the pure glass beads have been determined (SI). For those compounds, the maximum retention volume on the surface of the glass beads present in the mixture could be calculated directly, assuming that no glass bead surface was covered by soot but all was accessible for adsorption. For the other compounds, the glass beads/air-adsorption coefficient was calculated by a LFER derived from the experimental data set on pure glass beads (SI). For all compounds the net retention volume on glass beads was found to be less than 2% of the total observed retention. Therefore influence of the glass beads in our experiments could safely be neglected. For some polar compounds a slight decrease of the retention volume on diesel soot could be seen over the course of the experiments (during 11 days), indicating that the polar properties of the soot slightly changed. A possible explanation is that some polar organic compounds initially present in the soot could have been blown out during the course of the experiment. Any chemical reactions can be excluded considering the age of the sample (∼13 years). However, the decrease was 10 to 20%, which is still in the range of the repeatability. Therefore, this effect was not specifically taken into account. During the course of the experiments, the retention volume of selected compounds on road tunnel aerosols did not show any change, indicating constant aerosol properties. A variation of relative humidity between 50 and 70% had no substantial effect on the observed sorption on diesel soot (see data in Table S2). The influence of the relative humidity was therefore not further investigated. Adsorption and Absorption Coefficients. The adsorption coefficients and the corresponding errors for 78 compounds on diesel soot and the absorption coefficients of 69 compounds on road tunnel aerosols are listed in Tables S2 and S3 in the Supporting Information. See below for the distinction of adsorption and absorption for the two samples. The sorption coefficients on diesel soot are normalized to the specific surface area, and those on road tunnel aerosols are normalized to the measured OC mass. The compounds exhibit a wide range of physicochemical properties and cover more than 3 orders of magnitude in Ki diesel soot/air and Ki road tunnel/air. The relative errors in Ki aerosol/air were about 10%. The methodology of error calculation is described in SI. Diesel Soot: Prevailing Sorption Mechanisms Based on EC and OC Content. If we assume that absorption into the aerosol OM (5.7% OC of total weight) can be modeled by any one of three very different organic solvents (hexadecane, octanol, and methanol) we can use the respective solvent/ air partition constants to estimate the maximal sorption capacity that OM may have contributed to the total sorption capacity of the aerosols. On average, only 10% of the observed sorption could be explained by OM assuming nonpolar or slightly polar sorption properties (hexadecane or octanol), while 28% on average could be explained with very polar sorption properties (methanol). Adsorption to typical inor6634

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FIGURE 1. Comparison of calculated values (eq 5) with experimental values for diesel soot (EC), 15 °C, 50% RH (R2 ) 0.94, n ) 74). ganic aerosol surfaces (with a specific surface area of 2.5 m2/g (2, 3, 29, 30)) also could not explain the observed sorption for any of the compounds (for selected compounds see Table S4 in SI). This comparison strongly implies that the observed sorption on diesel soot can neither be explained by absorption in OM nor by adsorption to noncarbonaceous surfaces. Rather, adsorption to EC must have been the dominating mechanism for the studied diesel soot. Hence, all experimental diesel soot data were normalized to the SSAdiesel soot, and are considered as adsorption coefficients to “pure” EC from a diesel source. Adsorption Model for EC. We have previously introduced a poly-parameter LFER for adsorption to surfaces based on intermolecular interactions, namely the van der Waals interactions (vdW) and electron-donor/-acceptor interactions (ED/EA, i.e., hydrogen bonds) between the organic vapor and the surface (1 and references therein). The following model is valid at 15 °C:

log Ki surface/air ) 0.135((0.003) × (γsurfvdW)0.5 × log Ki hexadecane/air + 5.11((0.15) × EAsurf × Σ βi + 3.60((0.28) × EDsurf × Σ Ri - 8.47 (4) where log Ki hexadecane/air (m3/m3) is the hexadecane/air partition coefficient at 25 °C (see ref 1 for the temperature of the parameter) (31, 32), which is taken as a measure of the vdW property of i, ΣRi is the electron-acceptor (H-donor) and Σβi is the electron-donor (H-acceptor) property of the compound i (5, 33). (γsurfvdW)0.5 is the square root of the van der Waals component of the surface free energy which serves as the van der Waals surface property (19, 34), EAsurf is the electron-acceptor property of the surface, and EDsurf is the electron-donor property of the surface relative to a water surface. The scaling factors in eq 4 were calibrated with experimental data for adsorption to a water surface (1). The constant -8.47 is derived from the standard state of adsorption (for details see ref 1). Our experimental adsorption coefficients for EC (Table S2 in SI) were fitted with eq 4 to derive the surface parameters for EC. The model for adsorption to EC then takes the following form (Ki EC/air in m) and describes the reported data set well (n ) 74, R2 ) 0.94, see Figure 1):

TABLE 1. Comparison of EC Surface Properties from This Work with Selected Surface Properties Reported in Earlier Works, All at 15 °Ca surface

RH (%)

0.5 (γvdW surf ) (mJ/m2)0.5

EAsurf (dimensionless)

EDsurf (dimensionless)

elemental carbon (EC) salts: KNO3, NH4Cl, (NH4)2SO4, NaCl minerals: bentonite, kaolinite, quartz, CaCO3, Al2O3 bulk water surface

50 20-60 40-90 100

8.08 ( 0.07 6.19-7.05 4.84-6.79 4.69

0.48 ( 0.02 0.58-0.85 0.80-1.06 1

0.75 ( 0.06 0.81-1.04 0.75-1.13 1

a

From refs 1-3.

log Ki EC/air ) 0.135((0.003) × 8.08((0.07) × log Ki hexadecane/air + 5.11((0.15) × 0.48((0.02) × Σ βi + 3.60((0.28) × 0.75((0.06) × Σ Ri - 8.47 (5) The EC surface parameters resulting from eq 5 can be compared to the parameters of other surfaces derived with the same model (refs 1-3; Table 1). The van der Waals parameter (γsurfvdW)0.5 can also be compared to literature values of the surface free energy obtained by other methods ( e.g., 35, 36). It is evident from Table 1 that the van der Waals surface parameter of the diesel soot/EC lies between those of a water surface, minerals, or salts at any RH, and the value for graphite which is around 11.0 (in (mJ/m2)0.5) (35, 37). The electron-acceptor and the electron-donor parameters of diesel soot are significantly smaller than those for a water surface or other hydrophilic surfaces, i.e., the sample is rather nonpolar compared to the surfaces given in Table 1 (2, 3). This is consistent with our finding that RH has only a minor effect on sorption to diesel soot. These two findings are not too surprising, as one would assume an EC-dominated exhaust sample to exhibit surface properties similar to those of graphite, but more oxidized than a pure graphite sample. We therefore suggest that the adsorption equation presented here may serve as a generic model for air/EC adsorption at 15 °C (eq 5), however limited to rather high surface concentrations of the adsorbate. This model can be extrapolated to compounds not tested here. To the best of our knowledge, this is the first time that experimental data as well as a predictive tool for adsorption from air to EC from a combustion process are presented. Others have reported water/soot sorption coefficients before (e.g., 38-40). However, it is thermodynamically not correct to calculate air/EC adsorption coefficients from experimental water/EC adsorption coefficient using Henry’s Law coefficients (41). Note that our diesel soot may not be representative for particles from other diesel engines because it had a smaller OC content than those reported in the literature for other diesel soots (8, 42-44 and refs therein). Absorption in OM would probably be more important or even dominate sorption in those diesel soots. In addition, EC surfaces might not be completely available due to internal mixing (surfaces covered with bulk material, see below) (45). Road Tunnel Aerosols: Prevailing Sorption Mechanisms Based on EC and OC Content. Similar considerations as made for the diesel soot can be conducted for the road tunnel aerosols. With the measured OC content (8.8% ( 0.4%) and with Ki solvent/air values for various solvents from the literature, we find that absorption into OM could fully explain the sorption on road tunnel aerosols, or even overestimate it (selected compounds in Table S4 in SI). This overestimation indicates that the aerosol OM may not have been fully available for sorption. By applying the adsorption model for EC derived from the diesel soot data (eq 5), we can also estimate how much the EC content of the road tunnel aerosols may have contributed to the overall observed sorption. Especially for

the polar compounds we found that a large part of the observed sorption could be explained by adsorption to EC (Table S4). In fact, for some compounds EC adsorption even overestimates the sorption measured. This indicates that only part of the EC surface in the tunnel aerosols was available for adsorption. Surfaces can be unavailable due to internal mixing of the aerosol components, as reported in the literature (45, 46). Unavailability of such surfaces in internally mixed aerosols would result in a contribution to the total sorption that is lower than that calculated in Table S4. Finally, also the large fraction of noncarbonaceous material (87% ( 4.4%) that was assumed to consist of minerals, salts, and metal oxides could explain (and even overestimate) the measured sorption of polar, but not of nonpolar compounds (Table S4). In this case we could not, however, readily conclude that such surfaces were not accessible because the real content of such materials is unknown. In summary, it appears that all aerosol components (OM, EC, and minerals/salts) have contributed significantly to the observed sorption. It also appears that neither aerosol OM nor EC were completely available for sorption. As an important consequence we conclude that the sorption capacity of an internally mixed aerosol cannot be treated as the sum of the sorption capacity of all its constituents when they are measured in their pure form. Distinction of Adsorption and Absorption Using Intermolecular Interaction Models. Evaluation of the experimental data with interaction models is an alternative tool to tackle the question of dominant sorption mechanism. If true adsorption coefficients, log Ki surface/air, normalized to the surface area of the adsorbent, are regressed by eq 4 with the constant as a fitting parameter, then this fitted constant should yield the value of -8.47 which was theoretically derived in earlier work (1). Indeed, we have always obtained this value for solid surfaces where only adsorption can occur (within our statistical standard error) (2, 3). If sorption data for a mixed sorbent are erroneously treated as adsorption coefficients (log Ki surface/air in m), although absorption in OM of the sorbent is the dominating process, then the fit will derive a constant differing significantly from the value of -8.47 for true adsorption (see Appendix). For the diesel soot, the fits of the surface-area-normalized sorption coefficients gave a constant of -8.54 ( 0.14 (R2 ) 0.94). Thus, as expected diesel soot is in agreement with our adsorption model (for details see Appendix) and the considerations stated above. Note that this agreement also strongly supports our earlier conclusion that we did not encounter kinetic problems in the sorption process. If the equilibrium time provided in the experiments had not been sufficient, so that the molecules had not seen all the available surface area, then the value of the fitted constant would have been smaller than the theoretical constant. The fitted surface properties of the soot would not have been able to adjust for such a kinetic problem as one can easily show by calculating a hypothetical example. For the road tunnel aerosol sample, Table S4 indicates that polar and nonpolar compounds might sorb by a different mechanism. Therefore, the data sets were evaluated in VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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separate subsets (polar and nonpolar compounds). The fitted constants (see Appendix) for the road tunnel aerosols deviate clearly from the theoretical adsorption constant. We can also calculate from sample characteristics what value this fitted constant should obtain, if absorption dominates the observed sorption (see Appendix). But for both subsets on road tunnel aerosols, the fitted value deviated also from the one calculated for absorption. In fact, the constant fitted from the data set was between the two theoretical values, slightly closer to the value for absorption. This suggests that both sorption mechanisms may have played a role for the road tunnel aerosols. Therefore, it seems likely that adsorption to a surface (EC, mineral, or other) and absorption into OM both contributed to the overall observed sorption. The importance of both mechanisms would vary for polar and nonpolar compounds. Comparison with EC and OC contents of traffic-dominated aerosols in the literature reveals that both tend to be higher than those in the road tunnel aerosols used here (8, 42-44). Our results for road tunnel aerosols suggest that absorption into OM might be relevant already at an EC/OC ratio of 0.5. At lower EC/OC ratios absorption into OM is even more likely to play a significant role for the overall sorption. Note that nondiesel vehicle exhaust appears to have a lower EC/OC ratio than diesel exhaust (8, 42). We can conclude that OM played a more important role for the road tunnel aerosols than for the diesel soot sample in our experiments. This indicates a trend from adsorption-dominated exhaust aerosols (diesel soot sampled at the source) to aerosols (road tunnel aerosols) where both adsorption and absorption played a role. The domination of adsorption to EC may therefore be of quite limited importance for atmospheric aerosols in general.

Appendix Discussion of Constants in Sorption Models. When evaluating sorption data on aerosols, one does not know a priori whether adsorption or absorption is the dominating process. However, with a large and diverse set of experimental data there is a chance to distinguish between the two processes, if one of them dominates. The constant -8.47 (m) in our adsorption model (eq 4) is derived on theoretical grounds from the standard state of adsorption (1, 47). It describes the surface concentration of a compound relative to its gas-phase concentration at standard conditions, if there were no interactions between the surface and the compound molecules. This term does not depend on the type of surface considered. In our previous work with solid surfaces (4 salts and 7 minerals (2, 3)), on which only adsorption could occur, we validated this theoretical constant by fitting our experimental adsorption coefficients with an equation in which the constant was treated as a fitting parameter. In this case the fitted constants constfitted ads agreed well with the theoretical value of consttheor ) -8.47 (m) within the standard error ads ((0.3 log units). If an experimental data set of sorption coefficients normalized to the surface area, is fitted to the same adsorption equation, but if sorption is actually dominated by absorption into a bulk phase instead of adsorption to the surface, then the constant (constfitted ads ) will in most cases deviate significantly from -8.47 (m). In fact, the expected value for this constant, if absorption data are erroneously treated as adsorption data, can be calculated (constexpected , denoted with “ads” because it refers to coefads ficients normalized to the surface area and used in the adsorption equation, eq 4):

constexpected ) constbulk - log(SSAsorbent(m2/g) × ads FOM(g/m3) × fOM(g/g)) (6) 6636

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It depends on the OM fraction of the aerosols, SSAsorbent of the aerosols, and the constant, constbulk. This constant, constbulk, is obtained if the absorption coefficients (correctly normalized to the volume of the sorbent and the gas phase) are fitted with van der Waals and electron-donor/-acceptor parameters. Here, FOM is the specific density of OM which can be estimated as 0.9 to 1.2 × 106 g/m3 (we used 0.9 × 106 here). Constbulk is known to be close to 0.0 ( 0.3 for all kinds of organic absorbents if their volume is completely accessible and if sorption coefficients are dimensionless ((mol/Volbulk)/ (mol/Volair)) (48, 49). Using the data for the diesel soot (SSAdiesel soot ) 91 m2/g and fOM ) 0.08 g/g, estimated from fOC ) 0.057 g/g with an OM/OC ratio of 1.4) we expect the constant constexpected to be ads in the range of -9.01 ( 0.3 if all sorption occurred in the form of absorption in OM. The actual fit of the surface-areanormalized sorption coefficients (in m) gave a constfitted ads ) -8.54 ( 0.14 (R2 ) 0.94) for diesel soot. This is in good agreement with the theoretical constant for true adsorption (-8.47 ( 0.3) while it deviates significantly from the expected constant for absorption (i.e., -9.01 ( 0.3). The data sets of the road tunnel aerosols were analyzed with the above calculation for the polar and nonpolar subsets of our compounds (for details see section on Road Tunnel Aerosols). For the road tunnel aerosols (SSAroad tunnel ) 7.5 m2/g, and fOM ) 0.123 g/g) the calculation with eq 6 yields a constexpected ) -7.74 ( 0.3. The fitted constants of the ads 2 nonpolar subset (constfitted ads ) -7.94 ( 0.18, R ) 0.95) and of fitted the polar subset (constads ) -7.80 ( 0.27, R2 ) 0.91) both rather agree with the expected value for dominating absorption (i.e., -7.74 ( 0.3) but deviate from the theoretical value of adsorption (-8.47 ( 0.3). The differences though are so small that it is likely that both sorption processes contributed significantly to the observed total sorption on the road tunnel aerosols.

Acknowledgments We thank Sandra Endres for carrying out the experiments on glass beads; Hermann Mo¨nch and Johanna Buschmann for help with the BET-measurements; Craig Corrigan (University of California, San Diego) for helpful discussions; and Hans Peter Arp, Kathrin Fenner, and Zach Schreiber for critical comments on an earlier version of the manuscript.

Supporting Information Available Additional experimental details and data. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review May 28, 2004. Revised manuscript received May 10, 2005. Accepted May 18, 2005. ES049204W

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