Environ. Sci. Technol. 2008, 42, 5541–5547
Ambient Gas/Particle Partitioning. 1. Sorption Mechanisms of Apolar, Polar, and Ionizable Organic Compounds H A N S P E T E R H . A R P , * ,†,‡ ´ P. SCHWARZENBACH,† AND RENE K A I - U W E G O S S * ,†,§ Institute of Biogeochemistry and Pollutant Dynamics, ETH-Zurich, Universita¨tsstrasse 16, 8092 Zurich, Switzerland, and UFZ Helmholtz Center for Environmental Research, Permoserstr. 15, 04318 Leipzig, Germany
Received December 10, 2007. Revised manuscript received April 11, 2008. Accepted May 23, 2008.
There remain several ambiguities in the literature regarding the dominating sorption mechanisms involved in gas/particle partitioning, particularly for polar and ionizable compounds. The various hypothetical mechanisms would depend differently on relative humidity (RH) and the presence of various aerosol components. Thus, in order to resolve these ambiguities, here we measured the RH-dependency of gas/particle partitioning constants, Kip, for four diverse aerosol samples and a large set of chemicals covering apolar, polar, and ionizable organic compounds. In addition, we also removed the water-soluble components from two ambient particle samples to study how their presence influences sorption behavior. The measured Kip values collectively indicate that a dual-phase sorption mechanism is occurring, in which organic compounds partition into a RH-independent water-insoluble organic matter phase and additionally into a RH-dependent mixed-aqueous phase. All Kip values could be successfully fitted to a RH-dependent dualphase sorption model. The trends in Kip data further support findings that the sorption behavior of ambient aerosol samples is different from raw mineral surfaces and soot.
Introduction Understanding the dominant sorption mechanisms of gas/ particle partitioning is crucial for accurately assessing the atmospheric fate of organic pollutants, as well as the physicochemical behavior of atmospheric particles themselves. In the late 1990s, the available evidence was pointing to organic matter (OM) as being the dominating sorption phase of atmospheric particles (e.g., ref 1). Later, elemental carbon (EC) was suggested to be another important contributor (e.g., ref 2). More recently, there have been reports that mineral surfaces can be the dominating sorption phase for polar compounds (3) and that bulk OM should not be thought of as a single sorbing phase, as it can separate into * Address correspondence to either author. Phone: ++47 22 02 1988(H.P.H.A.); ++ 49 341 235 1411(K.-U.G.). E-mail:
[email protected] (H.P.H.A.);
[email protected] (K.-U.G.). † Institute of Biogeochemistry and Pollutant Dynamics. ‡ Current Address: Department of Environmental Engineering, Norwegian Geotechnical, Institute (NGI), P.O. Box 3930 Ullevål Stadion, NO-0806, Oslo, Norway. § UFZ Helmholtz Center for Environmental Research. 10.1021/es703094u CCC: $40.75
Published on Web 06/26/2008
2008 American Chemical Society
hydrophobic and hydrophilic domains with different sorption properties (4, 5). Resolving the identity of the dominating sorption phase(s) for various organic compounds is necessary to determine which aerosol properties determine the overall sorption capacity (e.g., mineral surface area, EC surface area, total OM, etc.). Further, resolving this issue would allow for insight into possible transformation pathways. For instance, if contaminants adsorb to mineral surfaces, they are likely still available for reaction with radicals or direct photolysis. They would be less likely to undergo such reactions if they absorbed into the OM or existed in a deep soot pore. A precise knowledge of how relative humidity (RH) influences gas/particle partitioning of ambient compounds can assist in clarifying the dominating sorption mechanisms, because RH influences the possible sorption mechanisms differently. For example, increasing RH would substantially lower sorption onto mineral particles (6) but not soot (7); additionally, it would increase the size of the aqueous (or hydrophilic) phase (8) but have only a minor influence on the secondary organic aerosol (SOA) phase (9). However, data that could assess the dependence of RH is lacking for ambient particles. A field study on PAHs (10) and a later field study on chlorinated pesticides (11) showed that a strong influence of RH is not observable for these compounds. However, the representativeness of these studies on the exact role of RH is somewhat in question due to sampling artifacts (e.g., filter sorption artifacts), assumptions about average ambient RH levels, a lack of compound variability, and weak statistical correlations. The majority of work on the RH dependence of gas/particle partitioning has been on aerosol surrogates, such as combustion source particles (containing both soot and primary organic material, e.g., ref 12) and SOA (e.g., refs 12 and 13). These studies collectively indicate that the sorption of apolar and some polar compounds is at best only slightly influenced by RH. However, these studies do not account for additional partitioning into the aerosol’s aqueous phase, which might be significant considering ambient particles can more than double in volume due to water uptake (e.g., ref 8). An understanding of how individual aerosol components are mixed is also needed to clarify the dominating sorption mechanism. Because of mixing, it is unlikely that the entire EC fraction is available for partitioning, as EC is to a large extent coated with salts (particularly sulfates) (14–16) and organics (15, 16). Similarly, mineral surfaces are likely coated by a thin organic film. Nevertheless, previously, we reported that, even if EC and mineral surfaces were not coated, their maximum sorption potential is insufficient to account for measured sorption coefficients, which are best accounted for by absorption into a portion of the total aerosol OM (17). Aerosol OM consists of hydrophobic and hydrophilic domains (4, 5, 18) which have different mixing properties. The hydrophobic domain consists of water-insoluble OM (WIOM) and exists as a largely nonmixed phase; on the other hand, the hydrophilic domain consists of water-soluble OM (WSOM) and would be largely mixed with water and dissolved salts, forming a mixed-aqueous phase at high RH. These two OM fractions would differ in polarity and therefore have differing sorption properties. Strongly polar and ionizable compounds would partition more favorably into the hydrophilic, mixedaqueous phase, whereas nonpolar organic compounds would partition preferably into the hydrophobic, WIOM phase (18). On the basis of this assessment, we hypothesize that partitioning into fresh, ambient particles occurs mainly by simultaneous partitioning to a RH-independent WIOM phase and a RH-dependent mixed-aqueous phase. To test this VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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hypothesis, the influence of RH on the gas/particle partitioning of apolar, polar, and ionizable compounds was measured for four diverse ambient aerosol samples: an urban sample, a rural sample, a coastal sample, and a mineral-rich suburban sample. In addition, the role of the water-soluble fraction was investigated by comparing the sorption properties of two urban samples before and after removing their water-soluble components.
Materials and Methods Chemicals. All chemicals were purchased from Fluka (Buchs, Switzerland), except for fluorotelomer compounds from Clariant (Sulzbach, Germany) and fluorochem (Derbyshire, U.K.) and are listed in the Supporting Information (SI-Part 1). Chemicals here are categorized as being either apolar, polar, small polar, or ionizable. Small polar refers to ethanol, n-propanol, isopropanol, n-propanoic acid, and 1,4-dioxane. Ionizable refers to compounds that could potentially gain or lose a proton in the aerosol’s aqueous phase, depending on the pH. The ionizable compounds looked at are (with pKa listed in parentheses from refs 19 and 20 and B-H+ indicating the protonated form of the base) 2,6-dichlorophenol (6.65), aniline (4.87 B-H+), pentanoic acid (4.83), butanoic acid (4.83), 2,6-dimethyl aniline (3.89 B-H+), 2-chloroaniline (2.66 B-H+), and 2-methyl pyrazine (1.45 B-H+). Particle Sampling. Ambient particles were sampled using a high-volume air sampler (Digitel, Switzerland) equipped with a 10 µm (i.e., PM10) cutoff on teflon glass fiber filters (Pallflex) that were silylated and placed behind an aluminum filter mask as described previously (21). In order to obtain enough particles to overcome the filter background for our inverse gas chromatography (IGC) measurements, it was necessary to obtain as many particles as possible (ideally near 1 mg/cm2 of the filter). Loadings at this concentration severely decreased the maximum flow rate and thereby lowered the efficiency of the PM10 cutoff. Five samples were collected: two urban samples, Zurich (from Zurich, Switzerland) and Berlin (from Berlin, Germany); one coastal sample, Aspvreten (from Aspvreten, Sweden); one rural sample, Roost (from Roost, near Untersiggental, Canton Aargau, Switzerland); and one suburban sample, Duebendorf-Sahara (from Duebendorf, Switzerland). The suburban sample is referred to as Duebendorf-Sahara as during sampling, on the 20th of June, a Sahara sand event occurred that loaded the sample with Sahara particles. All sampling data and aerial views of the sampling sites can be found in the Supporting Information (SI-Part 2). Weight of Water in Aerosols. Filter punches were placed in a desiccator equipped with a RH sensor (Rotronic Hygroclip S sensor, Rotronic AG, Bassersdorf, Switzerland), in which four different RHs were generated by use of a desiccant (12% RH), no desiccant (27% RH), a saturated NaCl solution (77% RH), and a saturated KNO3 solution (92% RH). After maintaining a given RH for 4-7 days, samples were removed and weighed. The particle mass (Mp) on the filter was determined by subtracting the mass of the loaded filter from the blank filter (which was found to be independent of RH). At 77 and 92% RH, loaded filter masses noticeably decreased when placed on the balance (initially ca. 0.01 µg/s); thus, masses from the exact moment when the filters were placed on the scale were reported. Due to possible inaccuracies by this weighing method, all Mp values are assumed to have a 10% error (multiple measurements of dry samples were within 6%). The weight change compared to values at 12% RH is reported using mass growth factors, GFmass (eq 1) GFmass ) Mp ⁄ Mdry
(1)
where Mdry is the dry weight of the particles (at RH 12%). Note that the loss of mass at 12% was assumed to be due to water depletion only. 5542
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Kip Measurements. The gas/particle equilibrium partitioning coefficient, Kip, is defined as Kip (m3 ⁄ g) ) cip ⁄ciair
(2)
where cip is the equilibrium concentration of compound “i” sorbed to the particle phase (mol/g) and ciair is the equilibrium concentration of i in the air phase (mol/m3). Kip values were determined using a recently developed IGC method (21), in which fiber filters loaded with particles are used as the stationary phase in a standard gas chromatograph. The column containing the fiber filters is referred to as the “fiber filter column”, FFC. The RH in the column was regulated by purging the carrier gas, synthetic air, through a water saturator in a temperature-controlled water bath upstream of the column. After installing a column or adjusting to a different RH, at least one night was allowed to pass before measuring, to allow for equilibrium with the carrier gas. The pressure drop across the column was measured with a pressure transducer (26PCCFB6G, Honeywell, Seelze, Germany) upstream of the column and was found to be negligible and was not corrected for in all cases, except for the Berlin sample at 90% RH. The retention volume of the background system (i.e., four blank filter pieces, two frits, and capillaries), VnetblankRH, was measured at each RH for the entire compound data set. Increasing the RH from 50 to 90% did not influence VnetblankRH values except for some small polar compounds (e.g., ethanol and propanol) and ionizable compounds (e.g., aniline). RH-specific Kip values normalized to the dry weight, Mdry, were then determined using eq 3: Kip ) (VnetloadedRH-VnetblankRH) ⁄ Mdry
(3)
where VnetloadedRH is the RH-specific retention volume when the loaded filters were in the FFC. After IGC analysis, the filter weights were remeasured to test for possible filter or sample deterioration in the IGC. More details on the method, measurement errors, and example chromatograms can be found in the Supporting Information (SI-Part 5) and ref 21. Removal of the Water-Soluble Fraction. The watersoluble fraction of the Zurich and Berlin samples was removed by disconnecting the FFC from the IGC (after completing the Kip data set), connecting it to an HPLC pump (Jasco PU 980), and flushing it with 30 mL of nanopure, degassed water for 24 h. Afterward, one side of the FFC was opened, and the FFC was placed in a drying oven at 80 °C for 24 h. The FFC was then resealed and reconnected to the IGC for subsequent analysis. SEM-EDS Analysis. SEM analysis was conducted by first coating the samples with a gold spray under a vacuum for 30 min and then placing them into a Philips XL-30 SEM equipped with a Lab6 Filament and an EDAX EDS (EDS ) electron dispersive spectroscopy) for elemental analysis. PP-LFERs. Poly-parameter linear free energy relationships (PP-LFERs) that describe absorptive partitioning are an appropriate way to model partitioning into the OM component (17, 21). Kip data sets at 50% RH for individual aerosol samples were fitted to the following PP-LFER (from ref 22): log Kip (m3 ⁄ g) ) sSi + aAi + bBi + lLi + vVi + c
(4)
where Si, Ai, Bi, Li, and Vi, are the compound-specific Abraham descriptors for the polarizability/dipolarizability, electron acceptor (H-bond donor) capability, electron donor (H-bondacceptor) capability, logarithm of the hexadecane/air partition coefficient, and McGowan volume, respectively. Corresponding to the Abraham descriptors are the sorbent (i.e., particle)-specific descriptors s, a, b, l, and v, along with the fitting constant, c. Sorbent descriptors can be calculated by performing a multiple-linear regression using experimental log Kip values as the dependent variable and Abraham
TABLE 1. Mass Growth Factors (eq 1)a of Particle Samples at Different RHs GFmass sample Berlin washed Berlin Duebendorf-Sahara Aspvreten Roost Zurich
RH ) 28%
RH ) 77%
RH ) 92%
1.08 (0.15 0.92 (0.13 1.00 (0.14 1.02 (0.14 0.99 (0.14 1.03 (0.15
1.43 (0.20 0.91 (0.13 1.07 (0.15 1.12 (0.16 1.17 (0.17 1.06 (0.15
2.20 (0.31 0.91 (0.13 1.16 (0.16 1.00 (0.14 1.70 (0.24 1.68 (0.24
a Mdry assumed weight changes at 12% occurred due to water loss only. The ( values indicate standard deviation based on repeat mass measurements.
descriptors as the independent variables (values for the Abraham descriptors used here are listed in the Supporting Information, SI-Part 1).
Results and Discussion SEM. All samples contained a mixture of salt particles, mineral particles, and amorphous carbonaceous particles along with some random biological debris (e.g., pollen grains). These particles were distributed homogenously for all samples, except for the Zurich sample, and no collapsed organic particles were present. Particles > 10 µm were mainly salt particles and highly porous carbonaceous conglomerates; thus, despite the high cutoff, the obtained samples likely exhibit similar sorption properties to those of PM10 (see the Supporting Information, SI-Part 4). The Duebendorf-Sahara sample was dominated by clay and silt particles, which EDS scans revealed to be cation-containing silicates (typical for feldspars). After washing the Berlin sample, no salt particles could be found after a thorough search, and what remained were mainly fine ( 0.965 and rootmean-square errors < 0.204, as shown in Table 2. It was not necessary to exclude small polar and ionizable compounds calibrating the PP-LFERs for the washed samples. The Role of the Water-Insoluble and -Soluble Fractions. Kip values of the apolar compounds were neither substantially influenced by RH (Figure 1) nor the presence of a watersoluble fraction (Figure 2); therefore, it can be concluded that apolar compounds partition into an RH-independent, water-insoluble fraction. On the other hand, some small polar and ionizable compounds exhibited Kip values that were substantially influenced by RH (Figure 1) and partitioned readily into the RH-dependent water-soluble fraction (Figure 2; thus justifying their exclusion from the PP-LFER training sets). The Kip values for most polar compounds were not influenced by the presence of the water-soluble fraction at 50-70% RH (Figure 2), but they did show increased sorption at 90% RH (Figure 1). This is best explained by these compounds simultaneously and additively partitioning to the water-insoluble fraction (dominating at 50-70% RH) and to the aqueous aerosol fraction (becoming significant at 90% RH). The water-soluble fraction therefore primarily influences Kip by increasing the aqueous fraction at high RHs, and not directly by sorbing organic compounds. Sorption to the Water-Insoluble Aerosol Fraction. The water-insoluble fraction of ambient PM10 particles typically consists of minerals (10-15%; ref 23), EC (highly various, 2-20%; e.g., refs 23 and 24), and WIOM (5-17%; refs 23 and 24, assuming half the total OM is insoluble, as in refs 25 and 26). Sorption to minerals and EC obeys an adsorptive mechanism, whereas sorption to OM typically follows an absorptive mechanism. There are two conclusive indications that adsorption to minerals is negligible: (1) Out of nine aerosol samples studied using this method (measured here and in refs 21 and 27), the mineral-rich Duebendorf-Sahara sample exhibited the lowest Kip values. (2) As calculated in ref 3 and from what is generally VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Comparison of measured log Kip (15 °C) values at various RHs for (a) Berlin particles, (b) Roost particles, (c) Aspvreten particles, and (d) Duebendorf-Sahara particles.
FIGURE 2. Log Kip values (m3/g, 15 °C) normalized to the original dry weight before and after removal of the water-soluble components for the (a) Berlin sample and (b) Zurich sample. known about adsorption to minerals, if minerals did dominate the sorption mechanism, Kip values would decrease with increasing RH. However, in all of the samples tested here and previously (17), Kip actually increases or stays constant with increasing RH, even for the mineral-rich DuebendorfSahara sample (Figure 1d). Empirical indications that EC is not a dominating sorption phase are as follows: The surface area of urban aerosols (e.g., 0.2-2.2 m2/g, ref 17; 0.82-2.4 m2/g, ref 28) and even road tunnel aerosols (∼7.4 m2/g, ref 7) are 1-2 orders of magnitude smaller than values for diesel soot (e.g., 91 m2/g, ref 7), indicating that EC is contributing little to the overall surface area. Further, atmospheric EC is to a large extent coated with salts (particularly sulfates) (14–16) and organics (15, 16). As a result, exposed EC/air interfaces become rarer with particle age. When EC is coated with particulate OM, sorbents would have to compete with the OM already present on or blocking entry to the sorption sites (29). Removing watersoluble components in the washing study likely exposed EC surfaces that were coated with salts or WSOM. This would 5544
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have increased Kip values to some extent (assuming negligible insoluble particle washout) if sorption to EC was significant. However, a substantial increase in Kip values was not observed (Figure 2). More arguments for why we conclude that EC is not a dominating sorption phase can be found in the Supporting Information (SI-Part 7). Though we cannot definitively or quantitatively rule out the contribution of EC completely, all of the evidence taken together indicates that it is of minor importance to the studied aerosol samples. Evidence indicating partitioning to WIOM best describes the data are as follows. The trends in log Kip values here are similar to those of a previous study, in which we compare measured Kip values with the maximum possible contributions of the various aerosol components of a commercial aerosol standard, assuming that the components were nonmixed (17). This study found that only partitioning into a fraction of the total OM (possibly the WIOM) can account for all of the data. There are two additional indications that absorption is occurring in the samples studied here. The first indication comes from a comparison of Kip values for
FIGURE 3. Comparison of experimental log Kip values (15 °C, 50%RH) with PP-LFER predicted values (eq 4) for the (a) Berlin and (b) Roost samples. Polar and apolar compounds were included in the calibration data set; outlying small polar and ionizable compounds were not.
TABLE 2. Aerosol Specific Sorbent Descriptors (15°C, 50%RH, eq 4) Calibrated with log Kip Data in This Study, Excluding Small Polar and Ionizable Compoundsa
Berlin Roost Duebendorf-Sahara Aspvreten Zurich Berlin Washedb Zurich Washedc
s
l
v
b
a
c
r2
rmse
n
1.01 ( 0.09 1.45 ( 0.13 1.09 ( 0.11 0.95 ( 0.09 1.09 ( 0.15 0.79 ( 0.11 0.80 ( 0.46
0.78 ( 0.03 0.60 ( 0.05 0.66 ( 0.04 0.64 ( 0.03 0.75 ( 0.06 0.74 ( 0.04 0.73 ( 0.19
0.51 ( 0.09 0.86 ( 0.15 0.71 ( 0.12 0.49 ( 0.09 0.35 ( 0.15 0.57 ( 0.11 0.15 ( 0.72
0.30 ( 0.15 0.37 ( 0.18 0.48 ( 0.15 0.55 ( 0.14 0.64 ( 0.22 0.60 ( 0.16 -0.52 ( 0.40
3.17 ( 0.12 3.12 ( 0.14 2.91 ( 0.12 2.52 ( 0.11 2.99 ( 0.14 3.24 ( 0.14 3.12 ( 0.32
-7.42 ( 0.15 -6.99 ( 0.17 -7.28 ( 0.15 -5.95 ( 0.12 -6.84 ( 0.20 -7.10 ( 0.16 -5.95 ( 0.30
0.970 0.965 0.966 0.975 0.974 0.959 0.976
0.139 0.204 0.161 0.132 0.172 0.174 0.148
53 59 55 57 38 57 21
a ( ) standard deviation, r2 ) correlation coefficient, rmse ) root mean square error, n ) number of data. RH. c Parameters contain large errors due to being calibrated with only 21 compounds.
fluorotelomer-alcohols (FTOHs) and their hydrogenated counterparts, n-alcohols. As shown in ref 30, if an absorptive mechanism is dominating, these two compound classes should show differing ∆log K increments per carbon unit, and log K values for n-alcohols should be larger than FTOHs with the same chain length. These two trends are observed for the data in this study (see the Supporting Information, SI-Part 5). The second indication is that the fitted PP-LFER coefficients (for the water-insoluble fraction) in Table 2 before and after washing are similar to those of absorbing organic polymers and liquids in general (compare, e.g., coefficients compiled in ref 22, which are clearly distinguishable from coefficients for EC and mineral surfaces 6, 7). Sorption to the Water-Soluble Aerosol Fraction. The water-soluble fraction of terrestrial PM10 include various salts (20-40%; ref 23), WSOM (5-17%, assuming half-the OM is WSOM after refs 25 and 26), and a mixed-aqueous phase (containing dissolved salts and WSOM). As sorption to the water-soluble fraction noticeably increased with increasing RH (Figure 1), it is best explained by partitioning into the mixed-aqueous phase, which increases in size with increasing RH. Salts and WSOM do not contribute to Kip values directly (as values did not decrease due to washing, Figure 2) but rather indirectly by influencing the growth of this mixed-aqueous phase. The various salts and WSOM fractions found in aerosols each sorb water with varying intensities and exhibit a range of deliquescence points (26, 31). Below the points of deliquescence, the aqueous phase is supersaturated (or near-saturated) with salts and/or WSOM. Above the point of deliquescence, there will be a steady increase of water content with time and RH, until humidity drops below the point of deliquescence again (either because of the decrease in RH or because of the dilution of the watersoluble components).
b
Data at 75%
Dual-Phase Sorption Mechanism. To simultaneously quantify partitioning into both the WIOM and mixed-aqueous phases for all polar, apolar, and ionizable compounds, a dualphase model is used: Kip ) KipWIOM + VwRH ⁄ (DiawMdry)
(5)
where KipWIOM is the sorption coefficient of WIOM normalized to Mdry (as determined from the above PP-LFERs), VwRH is the volume of water at a given RH (which is here calculated from GFmass extrapolations and assuming that the water density is unity), and Diaw is the air-water distribution coefficient, that is Diaw ) Ria(Kiaw)S
(6)
Diaw ) (1 - Ria)(Kiaw)S
(7)
for organic acids and
for organic bases, where Kiaw is the dimensionless air-water partition coefficient for pure water (m3water/m3air); S indicates an empirical factor to account for possible “salting in”, “salting out”, and cosolvency effects with WSOM (i.e., any deviation from the behavior of pure water); and Ria is the fraction of a compound in the protonated form, that is Riaw )
1 1 + 10(pH-pKia)
(8)
Kiaw and pKa values of the studied compounds can be found in the literature. The only unknowns for eq 5 are the aerosol’s pH and S values. Regarding aerosol pH, urban aerosols are in general known to be weakly to strongly acidic. For example, ref 32 reported diurnal variations in aerosol pH from 0 to 4.5 for Pittsburgh aerosols. Direct indication that the particles VOL. 42, NO. 15, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Experimental vs predicted log Kip values (15 °C) using the dual sorption model in eq 5 at 50, 70, and 90% RH for (a-c) the Berlin sample (pH ) 3), (d-f) the Roost sample (pH ) 2), and (g-i) the Duebendorf summer sample (pH ) 2). looked at here were acidic was given by injecting the base units) because they are based on compounds exhibiting a 2,6-dimethyl pyridine (pKa 6.65 B-H+) into the IGC, which diverse range of pKa values (from pKa 1.5 to 6.7) sorbed too strongly to give a measurable peak. Regarding S, The assumed S values of 1 gave good predictions for all in terrestrial acidic aerosols, the most typical salt is (NH4)2SO4 apolar, polar, and ionizable compounds at each RH (compare (33), which can cause both “salting-out” and “salting-in” Figure 4 with Figure 1). Thus, the influences of salting-out, effects with polar compounds (31). Thus, as a first assumpsalting-in, and cosolvency effects appear to cancel each other tion, S is assumed to be unity. out (within a factor of about 3), in all cases. This is quite a In Figure 4, comparisons of experimental and fitted log convenient result for environmental modeling purposes. Kip values to eq 5 are presented for the Berlin, Roost, and Though the S value of 1 may be appropriate in general for Duebendorf-Sahara samples. acidic, (NH4)2SO4-dominated, terrestrial aerosols such as those looked at here, it may not be appropriate for all aerosols. As is evident in Figure 4a-i, accounting for additional For example, marine aerosols are dominated by NaCl; thus, partitioning into the mixed-aqueous phase via the dual-phase an overall salting-out effect (or S > 1) would be anticipated sorption model gives excellent predictions for all samples (31). (especially the hygroscopic Berlin and Roost samples) and The agreement between experimental and fitted Kip values clearly explains the RH-dependent scatter of Kip values in Figure 1, as well as the outliers from the PP-LFER fits in using the dual-phase sorption model in the Figure 4a-i is Figure 3. Note that the fitted pH values (indicated in a significant find. For the first time for ambient aerosols, it the figure caption) are statistically robust (within (0.4 pH provides experimental evidence to support the hypothesis 5546
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that gas/particle partitioning is dominated by simultaneous partitioning to the WIOM and mixed-aqueous phases. More practically, eq 5 presents a basis for simultaneously modeling ambient Kip values of apolar, polar, and ionizing compounds in a mechanistically derived and verified manner.
(14) (15)
Acknowledgments Special thanks to Susanne Schlatter (UGZ, Switzerland), Christian Bogdal (EMPA, Switzerland), Michael Madliger ¨ Berlin, (ETHZ), Ireen Kamprad, Gerhard Steinbrecher (TU Germany), and Anna-Lena Egeba¨ck (ITM, Sweden) for assistance with aerosol collection; Kathrin Fenner (EAWAG, Switzerland) and Claudia Marcolli (ETHZ) for helpful discussions; and Brian Sinnet (EAWAG, Switzerland) for SEM assistance.
Supporting Information Available Measured data, quality control, SEM images, and sampling data. This material is available free of charge via the Internet at http://pubs.acs.org.
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