Environ. Sci. Technol. 2003, 37, 4672-4677
Simulating Secondary Organic Aerosol Activation by Condensation of Multiple Organics on Seed Particles JEREMIAH D. FILLO, CHARLES A. KOEHLER, THAO P. NGUYEN, AND DAVID O. DE HAAN* Department of Chemistry, University of San Diego, 5998 Alcala Park, San Diego, California 92110 BENJAMIN A. GILBERT AND KEVIN P. FLINN Department of Chemistry, Lyon College, P.O. Box 2317, Batesville, Arkansas 72503-2317
The conditions under which semivolatile organic gases condense were studied in a Teflon particle chamber by scanning mobility particle sizing (SMPS) of the resultant particles. Benzaldehyde, maleic and citraconic anhydrides, n-decane, trans-cinnamaldehyde, and citral were introduced in various combinations into a particle chamber containing either particle-free nitrogen or nitrogen with dry seed particles made out of sodium chloride, D-tartartic acid, ammonium sulfate, or 1,10-decanediol. No organic gas was allowed to reach its saturation point relative to the vapor pressure of its pure liquid in any experiment. In the absence of seed particles, organic aerosol particles formed by ternary nucleation when the sum of the individual organic saturation levels reached a threshold between 1.17 and 1.86. With seed particles present, particle sizes began to increase when the sum of organic saturation levels reached 1.0. This size increase corresponds to the establishment and activation of ternary organic layers on the “clean” seed particles, as predicted by absorption partitioning theory. The observed increases in particle volume depended on initial seed particle volume, indicating that either gas diffusion rates or chemical reactions were controlling the rate of uptake.
Introduction The formation of organic aerosol in the atmosphere has proven to be a complex process with far-reaching ramifications. Organic aerosol may influence climate directly by absorbing and scattering solar radiation or indirectly by serving as cloud condensation nuclei (1). Increasing regulatory pressure to limit human exposure to high concentrations of submicron particulate matter has also fueled recent interest in organic aerosol formation. In many urban regions, secondary organic aerosol (SOA) makes up a significant fraction of the submicron aerosol (2, 3). The branching reaction pathways of atmospheric oxidation produce a large variety of oxygenated and nitrated products from a single * Corresponding author phone: (619)260-6882; fax: (619)260-2211; e-mail:
[email protected]. 4672
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aromatic parent compound (4-6), and these products typically have much lower vapor pressures and higher polarities than the parent compound. Condensation under such conditions is complex because many products present in only trace amounts may be condensing simultaneously. The condensation of SOA observed in a number of smog chamber experiments (7-14) has been successfully described using an equilibrium model where organics partition between the gas phase and a liquid organic matter (om) particulate phase (15). A gas-particle partitioning coefficient for each compound i can be expressed as (13)
KGP,i )
Χi 1 ) pi ζip°L,i
(1)
where Χi is the mole fraction of compound i in the om phase, pi is its partial pressure in the gas phase, ζi is the activity coefficient of compound i in the om phase, and p°L,i is the compound’s vapor pressure as a pure liquid at a given temperature. While neglecting enhanced volatility due to surface curvature (the Kelvin effect), the model corrects for the nonideality of the organic solutions, although such corrections must often be estimated in the absence of laboratory data. Importantly, this model rationalizes how a compound may be found in the particle phase even when subsaturated in the gas phase, as observed in smog chamber experiments where particle-phase species were identified (6, 13, 14). Equation 1 indicates, as Seinfeld has noted (16), that when multiple condensable compounds are present, a stable om phase can exist as long as the compounds are supersaturated with respect to a liquid solution droplet. Since each compound mixed in the droplet depresses the vapor pressure of the others (the Raoult effect), multiple compounds can condense together even though none are saturated in the gas phase with respect to their pure liquids. Konopka (17) and Djikaev and Donaldson (18), in theoretical works on binary condensation on soluble and insoluble seeds, respectively, also predicted that the energetic barrier to the growth of small particles predicted by the Kelvin equations would disappear under these conditions. The gas-phase saturation level (Si) of compound i is defined as
Si )
pi p°L,i
(2)
Note that when an om phase is present due to prior condensation or the presence of primary organic aerosol, at equilibrium Si ) Χiζi ) ai, the activity of compound i in the om phase. If no om phase is present, Bowman et al. (8) and Griffin et al. (19) extended the gas/particle partitioning model to define a threshold when the om phase is predicted to appear. This threshold can be stated in terms of saturation levels and activity coefficients as
Si
∑ζ g 1 i
(3)
i
At this point, a stable om phase may exist with activities ai such that Si g ai for each condensable compound i included in the summation. Condensation is then expected to occur until equilibrium is reached and Si ) ai. If the activity coefficients are assumed to be 1 as a useful first approximation 10.1021/es034110s CCC: $25.00
2003 American Chemical Society Published on Web 09/13/2003
(14), the threshold for condensation will be
Stot )
∑S g 1 i
(4)
i
While some condensation threshold is routinely observed in smog chamber experiments (6, 20, 21), the validity of eqs 3 and 4 has not to our knowledge been directly demonstrated. An experimental system is required that is free of liquid primary organic aerosol and where all organic gases are identified and quantified. Here we present laboratory measurements on such a system. We are interested in whether condensation events can be triggered without any single compound reaching gas-phase saturation, and, if so, whether the threshold eqs 3 and 4 are quantitatively correct. In urban atmospheres, gasoline vapor is generally representative of anthropogenic hydrocarbons (22), and the aromatic component in gasolinestoluene, xylenes, ethyl benzenesis primarily responsible for the aerosol formation caused by the oxidation of gasoline vapor (23, 24). The compounds benzaldehyde (BZ), citraconic anhydride (CA), succinic anhydride (SA) (Sigma Aldrich Inc., St. Louis, MO), and maleic anhydride (MA) (J. T. Baker) were selected for condensation experiments because, taken together, these compounds made up over half of the total identified SOA mass in smog chamber experiments on toluene and xylene vapor (6). To explore chemical effects, trans-cinnamaldehyde (Cin), citral (Cit), and n-decane (D) (Sigma-Aldrich) were also utilized.
Experimental Section Experiments were conducted indoors with 200- and 350-L unstirred Teflon particle chambers (Alltech Associates, Inc., Deerfield, IL) at room temperature (293-296 K). Filtered and dried house air or filtered USP nitrogen was used for filling and rinsing the chamber. The chamber was rinsed at least twice before every experiment. All flows to and from the bag during an experiment were metered using rotameters (Fisher Scientific Co.), electronic flowmeters (Tylan Corp., Torrance, CA), or differential pressure measurements on calibrated inertial impactors (TSI Inc., St. Paul, MN). Liquid organics were added by evaporation with gentle heating in a glass bulb with a metered flow of air or nitrogen. Solid organics were added by sublimation from a packed copper tube in a similar way. Each organic injection occurred continuously over a period of 30 min or more. The total amount of each organic gas added was quantified by the change in mass of the bulb and inlet tubing. With the exception of succinic anhydride, all organics used in this study were volatile enough that subsaturation levels were easily measurable (∆mass . 1 mg). Particles were atomized (TSI model 9302) from dilute aqueous solutions of ammonium sulfate (Fisher), 1,10-decanediol, sodium chloride, or D-tartaric acid (Sigma-Aldrich). All solutions were made with ultrapure water (>18 MΩ conductivity). Liquid particles produced by the atomizer were dried by flowing through home-built diffusion dryers containing anhydrous calcium sulfate powder surrounding an open particle flow pathway. This technique lowers relative humidity to below 5% (25). In experiments with 1,10-decanediol, a heat gun was used on the copper line between the atomizer and the diffusion dryer to disrupt observed formation of micelles. All aerosols passed through inertial impactors (bore diameter ) 0.0710 cm) to remove larger particles. Finally, excess electrical charge was removed from the particles by a radioactive neutralizer (Nuclecel P2021, NRD, Grand Island, NY). Aerosol from the bag was sampled at 0.2-0.3 L/min and analyzed at 5-min intervals by a scanning mobility particle sizer (SMPS, TSI 3080 electrostatic classifier and TSI 3010 condensation particle counter) to give the particle size
distribution from 0.01 to 0.9 µm. Particle counts (and volumes) were corrected for dilution caused by gas addition. In some experiments, a monodisperse aerosol was created by sending the aerosol through the electrostatic classifier (at a constant voltage) on the way into the chamber. First-order wall loss rates in the Teflon chamber were observed to range from 0.0084 to 0.042 min-1 for 10 nm diameter particles and from 0.0018 to 0.012 min-1 for 100 nm diameter particles. Thus, particles had chamber half-lives in the range 0.25-6 h. After the quantity of organic gases added was determined gravimetrically, organic saturation levels were calculated using the current chamber volume and vapor pressures based on published measurements summarized by NIST (26). For most organics, an extrapolation away from the temperature range of measurement using Antoine equation parameters was necessary to reach room temperature. Because saturation levels were corrected for dilution caused by subsequent gas additions to the chamber, the assumed rate of gas addition could influence saturation levels by as much as (10%. Our assumption of constant rates of addition resulted in calculated saturation levels in the center of this uncertainty range. Organic saturation levels in the chamber during some experiments were verified by GC-MS (Hewlett-Packard 5890 gas chromatograph and model 5823 mass spectrometer) by headspace analysis versus saturated gas-phase organic vapor pressure standards. Separation was achieved on a capillary column (EC-5, 30 m × 0.25 mm i.d., 25 µm film, Alltech, temperature program 35-250 °C, injector at 250 °C, detector at 280 °C). Detection was by single ion monitoring. The accuracy of saturation levels measured in this way is not influenced by extrapolation error from published organic vapor pressures but instead by the chemical purity of the vapor pressure standards. If impurities were present in the vapor pressure standards at the time of measurement, it would result in calculated organic saturation levels that are erroneously high. All organics were stored under nitrogen to limit reactions with water or oxygen. The purity of anhydride samples was verified by NMR to be at least 98%.
Results GC-MS verification of calculated organic saturation levels during individual experiments was possible only within a factor of 2. Anhydride quantitation by GC has inherently low precision due to reactions with wet surfaces and to interconversion between acid and anhydride forms at the elevated temperatures of analysis (27). However, when the ratios of total saturation levels calculated from GC-MS measurements over those from gravimetric measurements on the gas inlet system from the entire set of GC-MS experiments were pooled, the geometric mean was 0.91 ((1σ range ) 0.791.09). This indicates that calculated saturation levels based on gravimetric measurements (and reported in this paper) are not significantly biased. GC-MS measurements also confirmed that all organic gases in every sample remained below their individual gas-phase saturation levels in the particle chamber, a key requirement of this work. The results of a run involving polydisperse tartaric acid seed particles and additions of benzaldehyde, citraconic anhydride, and maleic anhydride are shown in Figure 1. Total particle volume based on SMPS measurements is plotted before and after dilution corrections. The general decline in particle number density and volume observed throughout the experiment is due to wall losses of particles (28). As organics are added, uncorrected volume data exhibit an accelerated decline due to dilution of particles. When this dilution is corrected for, changes in volume due to particle growth can be seen superimposed against the declining background of wall losses. The total gas-phase saturation level for the organics present (Stot) exceeds one at approximately 18 000 s. A significant increase in corrected VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Time-series of SMPS number density and volume measurements of dried polydisperse aerosol in the size range of 20-880 nm, atomized from 1.0 mM D-tartaric acid in water. Number densities and corrected volumes (filled squares) are corrected for dilution by gas addition. Organic additions: BZ ) 183.9 µL of benzaldehyde; CA ) 51.9 µL of citraconic anhydride; MA ) 11.5 mg of maleic anhydride. All organics were evaporated into flowing nitrogen.
FIGURE 2. Log-normal parameters of the SMPS size distribution for the same experiment shown in Figure 1. particle volume is observed beginning at around the same time. (The leveling off seen during benzaldehyde addition may be due to a slight overcorrection for dilution since it is assumed in the correction that the contents of the bag are well-mixed at 5-min intervals. Since gases are added continuously, this assumption is not strictly valid for gases added very near the end of the 5-min interval. Gas volumes leaving the chamber therefore may not contain as many particles as calculated.) Geometric mean diameter of the particle size distribution is plotted for the same experiment in Figure 2. The initial increase in particle diameter is due to both particle coagulation to form larger particles and to wall losses, which are more rapid for smaller particles in this size regime (28). As particle number density and wall loss rates decrease during the experiment, the geometric mean diameter stabilizes. The condensation growth of the tartaric acid aerosol is again clearly seen at 18 000 s, where the geometric mean of the particle size distribution increases rapidly again. Note that there is no evidence of particle growth (above the background rate of change) during benzaldehyde addition in this experiment. 4674
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FIGURE 3. SMPS number density, volume, and geometric mean diameter measurements of dried polydisperse aerosol in the size range of 16-660 nm, atomized from 1.0 mM 1,10-decanediol in water. Organic compounds evaporated into flowing air: 350 µL of benzaldehyde; 250 µL of citraconic anhydride; 3.5 mg of maleic anhydride; SA ) 0.9 mg of succinic anhydride.
FIGURE 4. SMPS number density, volume, and geometric mean diameter measurements of dried monodisperse aerosol in the size range of 16-660 nm, atomized from 1.0 mM 1,10-decanediol in water and size-selected by passing through a differential mobility analyzer. Organic compounds evaporated into flowing nitrogen: 226.7 µL of benzaldehyde, 77.6 µL of citraconic anhydride, and 12.2 mg of maleic anhydride. Figures 3 and 4 show organic condensation on polydisperse and monodisperse 1,10-decanediol, respectively. Both figures show a simultaneous increase in particle volumes and geometric mean diameter that indicates particle growth by uptake of organic gases. This growth threshold coincides in time, in both cases, with the achievement of a total organic saturation level in excess of 1, as predicted by eq 4. In monodisperse experiments, significantly lower particle densities result in higher relative noise levels in the peak parameters. Low particle densities also slow coagulation, and since a wide variety of particle sizes are not present in a monodisperse experiment, upward shifts in geometric mean size due to size-dependent wall losses typically are not observed. Thus, any sustained increase in geometric mean size in experiments on monodisperse seed particles is a clear indicator of particle growth due to uptake of organics. Experimental results are summarized in Table 1. Stot peak is the highest total organic saturation level achieved during a given run. Particle sizes are listed only for monodisperse runs. There is a detection limit to the volume increase data
TABLE 1. Summary of Experiments Organized by Seed Particle Typea run 020627a 020703a 020717a 020628a 020704a 010703a 020604c 020619a 020618a 020604a 020530e 020603d 020611d 020612b 020613a 020617a 020718a
seed particle type
AS AS DD DD DD DD NaCl NaCl NaCl NaCl NaCl NaCl NaCl tartaric
size (nm)
71 poly poly poly poly 43 poly poly poly poly poly poly 40 poly
SA (µm2/cm3)
208 396 19 781 142 4 7370 2700 3060 2490 2240 942 46 200
BZ
CA
organics added MA SA D
X X X X X X
X X
X X
X
X
X
X X
X X
X X X X X X
X X X X X X
X X X X X X
Cit
Cinn
X X X X
X
Stot peak
dVol (µm3/cm3)
dVol/Volinit
1.86 1.17 0.55 0.25 0.54 1.47 0.00 1.24 1.16 0.00 0.00 1.39 1.51 1.25 0.99 0.85 1.09
1.53 0.04 0.03 0.50 0.80 4.44 1.15 2.02 0.10 6.00 1.40 63.20 33.80 35.20 2.20 0.45 11.40
0.15 0.06 16.74 0.17 1.03 1.97 0.02 0.03 1.88 0.70 0.50 0.05 0.06 0.85
dVol/SA (µm)
2.5E-03 2.0E-03 2.3E-01 1.5E-03 1.4E-02 2.4E-02 8.1E-04 5.2E-04 2.1E-02 1.4E-02 1.6E-02 2.3E-03 9.9E-03 5.7E-02
a AS, ammonium sulfate; DD, 1,10-decanediol; tartaric, D-tartaric acid. Diameters are listed for monodisperse seed aerosol populations. “Poly” indicates a polydisperse aerosol. D, n-decane; Cit, citral; Cinn, trans-cinnamaldehyde. Other organic symbols are as listed in Figures 1 and 3. Stot peak ) highest total organic saturation level achieved during a run. dVol ) total observed particle volume increase (corrected for dilution) observed during an experiment. dVol/Volinit ) particle volume increase normalized by dividing by seed particle volume, measured immediately prior to observed volume increase. dVol/SA ) particle volume increase normalized by seed particle surface area, measured immediately prior to observed volume increase.
that is due to noise in particle volume data (e.g., Figures 1, 3, and 4). In runs with monodisperse seed particles where total particle volumes are small, absolute noise levels are lower, and thus smaller increases in volume can be observed. Conversely, in experiments with a high density of polydisperse particles, comparatively large volume increases observed may in fact be due to random noise. Relative measures of volume increase, such as those shown in the last two columns where volume increases are normalized by seed particle volume and surface area, are therefore useful in distinguishing growth by condensation. A summary of particle volume increases observed during runs performed in the absence of seed particles is shown in Figure 5. It can be seen that organic aerosol nucleation in the absence of seed particles occurs when Stot reaches a threshold located between 1.17 and 1.86. Since no individual gas present is supersaturated with respect to its pure liquid vapor pressure, the observed nucleation must be binary or ternary. Particle nucleation requires total saturation levels greater than one because small, newly formed particles have highly curved surfaces that exhibit a pronounced Kelvin effect. Calculations using the equations given by Konopka (17) predict a nucleation threshold of Stot ) 1.07 for typical conditions in these experiments. This threshold is not inconsistent with these data, considering the 10% uncertainty in our saturation calculations. Condensation behavior observed in experiments with seed particles made of 1,10-decanediol, tartartic acid, ammonium sulfate, or sodium chloride are shown in Figure 6. The volume increase plotted is any increase in particle volume measured during or immediately after addition of an organic gas to the particle chamber, corrected for dilution. It is apparent that particle growth by uptake of organic gases is observed only in experiments with Stot g 1.0, the condensation threshold predicted by eq 4. (Because organic layers are established on preexisting particles with diameters of at least tens of nanometers, surface curvature is negligible at a molecular level and “oversaturations” to counter the Kelvin effect are not necessary.) While Figure 6 appears to indicate that more condensation occurs on NaCl seed particles than on other seed particle materials, this effect is caused by differences in the quantities of seed particles present, as shown below. What is not apparent from Figure 6 is that growth, indicated
FIGURE 5. Observed aerosol volume increases observed in experiments conducted in the absence of seed particles with benzaldehyde, trans-cinnamaldehyde, citraconic anhydride, citral, maleic anhydride, or succinic anhydride present. Particle volume increases are total observed during experiment based on SMPS data corrected for dilution. Stot peak is the highest value for Stot achieved during the experiment. Error bars represent the total range of variability in each parameter. by sustained increases in particle volume and geometric mean diameter, was observed in every run with Stot g 1.0, as seen in Table 1. However, the observed increases in particle volume were greatest in the presence of high concentrations of seed particles. If the observed volume increase (corrected for dilution) is normalized by dividing by initial seed particle volume, as seen in Figure 7, the threshold for particle growth can be seen to lie where Stot reaches 1, and the dependence on seed particle type is removed. When Stot > 1, each organic is unsaturated with respect to its pure liquid but supersaturated with respect to an ideal ternary organic mixture. The om layer is then established by ternary condensation of the VOL. 37, NO. 20, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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tions. The limitation could be due to diffusion-limited mass transfer or a chemical reaction at the particle surface or within the newly formed om layer.
Discussion These experiments demonstrate that an om layer can be established on solid inorganic and organic seed particles by “collective saturation”, without any single organic species reaching gas-phase saturation levels with respect to its pure liquid. Presumably, the initial appearance of the om layer is due a smooth transition from adsorptive partitioning of an organic monolayer at the solid surface (which is not measurable by SMPS techniques) to absorptive partitioning into multiple layers of organic molecules exhibiting the Raoult effect, as originally predicted by Pankow (29). Since particle growth was observed in every experiment where Stot reached one, we conclude that there is no significant barrier to the establishment of the latter once gas-phase concentrations have reached the point where an om layer would be thermodynamically stable. Furthermore, our data provide direct support for the accuracy of the thresholds for the establishment of an om layer predicted by Bowman et al. (8) and Griffin et al. (19). FIGURE 6. Observed aerosol volume increases observed in experiments conducted with ammonium sulfate, 1,10-decanediol, sodium chloride, or D-tartaric acid seed particles present with benzaldehyde, citraconic anhydride, decane, and maleic or succinic anhydride in the gas phase. Symbols indicate type of seed particle present: filled triangles ) NaCl; open triangles ) tartaric acid; filled squares ) ammonium sulfate; open squares ) 1,10-decanediol. Values are calculated as in Figure 5.
FIGURE 7. Observed aerosol volume increases normalized by initial seed particle volumes for the set of experiments with seed particles present shown in Figure 6. Initial seed particle volumes are corrected SMPS measurements made immediately prior to the observed increase in aerosol volume. Symbols are the same as in Figure 6. organic gases present. Condensation continues until gasphase concentrations have dropped enough to allow each gas to reach the equilibrium saturation point with the organic phase. If the organics present all have similar vapor pressures, as they do in these experiments, it might be reasonable to expect, based on absorptive partitioning theory and the absence of a prior om phase, that the total volume increase should depend only on the amount by which Stot exceeds one. Instead, the added dependence of particle growth on initial seed particle volumes suggests that observed particle growth is kinetically limited under our experimental condi4676
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The onset of organic condensation on seed particles should occur at Stot ) 1 only if the organics in an experiment have activity coefficients of unity in the om layer formed. Since condensation on seed particles was observed to begin at Stot ) 1 in this work, benzaldehyde, trans-cinnamaldehyde, and citraconic and maleic anhydrides must be chemically compatible (ζi ≈ 1). It is reasonable to assume that other partially oxidized organic compounds could also establish a single om phase by collective saturation, provided that they have similar polarities. In smog chamber simulations of SOA formation, a complex and typically unidentified mixture of semivolatile secondary organics is produced in increasing amounts during an experiment by the ongoing oxidation of hydrocarbons. In the presence of clean inorganic seed particles, secondary organic concentrations increase until eq 3 is satified, and condensation of multiple organics establishes an om layer on the particles. In the hypothetical absence of the phenomenon of collective saturation, secondary organic concentrations would have to increase further until one of the products reaches its individual gas-phase saturation point. Stot would reach a higher level during this time, which in turn would cause a greater amount of condensation (dropping Stot back to 1) once one product has begun to condense. Once Stot reaches 1, however, the amount of condensation with and without collective saturation would be the same. Thus, smog chamber yield measurements are not sensitive to the mechanism of initial formation of an om layer and do not even depend on the presence of seed particles (12). The phenomenon of collective saturation would result only in an earlier appearance of SOA in smog chambers than would otherwise be expected. Only in a smog chamber experiment where the vast majority of secondary products are identified could the timing of the appearance of SOA be used to verify the mechanism of collective saturation in SOA formation. In the atmosphere, pollutant concentrations are typically much lower than in smog chambers, but the variety of pollutants present is much larger. The activation of SOA via collective saturation is inherently more significant under such conditions. Multiple semivolatile secondary organics present in only trace amounts may, if chemically compatible (ζi ≈ 1), collectively establish an om layer on seed particles when the sum of their saturation levels reach 1. Indeed, such a pathway makes it less likely that organic-free aerosol particles would be encountered at a given time and place in the atmosphere.
Acknowledgments This research was supported by an award from Research Corporation. Support from NASA EPSCoR through the Arkansas Space Grant Consortium is gratefully acknowledged.
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(15) Pankow, J. F. Atmos. Environ. 1994, 28, 189-193. (16) Seinfeld, J. H. Atmospheric Chemistry and Physics of Air Pollution; John Wiley & Sons: New York, 1986. (17) Konopka, P. J. Aerosol Sci. 1997, 28, 1411-1424. (18) Djikaev, Y. S.; Donaldson, D. J. J. Geophys. Res. 1999, 104, 1428314292. (19) Griffin, R. J.; Cocker, D. R., III; Seinfeld, J. H. Environ. Sci. Technol. 1999, 33, 2403-2408. (20) Hurley, M. D.; Sokolov, O.; Wallington, T. J.; Takekawa, H.; Karasawa, M.; Klotz, B.; Barnes, I.; Becker, K. H. Environ. Sci. Technol. 2001, 35, 1358-1366. (21) Cocker, D. R., III; Flagan, R. C.; Seinfeld, J. H. Environ. Sci. Technol. 2001, 35, 2594-2601. (22) Odum, J. R.; Jungkamp, T. P. W.; Griffin, R. J.; Forstner, H. J. L.; Flagan, R. C.; Seinfeld, J. H. Environ. Sci. Technol. 1997, 31, 1890-1897. (23) Odum, J. R.; Jungkamp, T. P. W.; Griffin, R. J.; Flagan, R. C.; Seinfeld, J. H. Science 1997, 276, 96-99. (24) Strader, R.; Lurmann, F.; Pandis, S. N. Atmos. Environ 1999, 33, 4849-4863. (25) De Haan, D. O.; Brauers, T.; Oum, K.; Stutz, J.; Nordmeyer, T.; Finlayson-Pitts, B. J. Int. Rev. Phys. Chem. 1999, 18, 343-385. (26) National Institute of Standards and Technology. Chemistry WebBook; NIST Standard Reference Database 69; March 2003 Release; http://webbook.nist.gov/chemistry/. (27) Di Lorenzo, A. J. Chromatogr. 1971, 55, 303-308. (28) McMurry, P. H.; Rader, D. J. Aerosol Sci. Technol. 1985, 4, 249. (29) Pankow, J. F. Atmos. Environ 1994, 28, 185-188.
Received for review February 6, 2003. Revised manuscript received August 1, 2003. Accepted August 5, 2003. ES034110S
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