Initial Size Distributions and Hygroscopicity of Indoor Combustion Aerosol Particles

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1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: Initial Size Distributions and Hygroscopicity of Indoor Combustion Aerosol Particles Wei Li & P. K. Hopke To cite this article: Wei Li & P. K. Hopke (1993) Initial Size Distributions and Hygroscopicity of Indoor Combustion Aerosol Particles, Aerosol Science and Technology, 19:3, , DOI: / To link to this article: Published online: 12 Jun Submit your article to this journal Article views: 407 View related articles Citing articles: 59 View citing articles Full Terms & Conditions of access and use can be found at

2 Initial Size Distributions and Hygroscopicity of Indoor Combustion Aerosol Particles Wei Li and P. K. Hopke* Department of Chemistry, Clarkson University, Potsdam, IVY Cigarette smoke, incense smoke, natural gas flames, propane fuel flames, and candle flames are contributors of indoor aerosol particles. To provide a quantitative basis for the modeling of inhaled aerosol deposition pattern, the hygroscopic growth of particles from these five sources as well as the source size distributions were measured. Because the experiments were performed on the bases of particles of single size, it provided not only the averaged particle's hygroscopic growth of each source, but also the detailed size change for particles of different sizes within the whole size spectrum. The source particle size distribution measurements found that cigarette smoke and incense smoke contained particles in the size range of nm, while the natural gas, propane, and candle flames generated particles between 10 and 100 nm. The hygroscopic growth experiments showed that these combustion aerosol particles could grow 10% to 120%, depending on the particle sizes and origins. INTRODUCTION Airborne particles represent a potential health threat when they deposit in the respiratory tract and thereby carry toxic or carcinogenic substances to the surface of the cells. In general, any airborne particles that contain water-soluble components can absorb water vapor and increase in size as they are inhaled and exposed to the high-humidity air in the respiratory tract. Therefore, the hygroscopic growth of aerosol particles is an important factor in the deposition of these particles in the human lungs. One of the major problems that exist in estimating the deposition pattern is the limited information that is available on the hygroscopicity of common indoor aerosol particles. Several models have been developed to account for the hygroscopic growth of particles (Ferron, 1977; Martonen, 1982; Martonen et al., 1982; Egan and Nixon, *To whom correspondence should be addressed. 1989; Ferron et al., 1985). Some experimental studies have also been made (Martonen, 1982; Tu and Knutson, 1984; Hicks and Megaw, 1985; Anselm et al., 1986). However, all of the work have been made with particles of known composition. There are only limited results (Hicks et al., 1989) available for real indoor aerosol particles that contain complex mixtures of compounds. Also, these results were derived from the changes in mass distribution, not the direct measurement of change in particle's physical size. Therefore, the direct measurement of hygroscopic growth of common indoor particles is necessary not only for the modeling of particles' respiratory deposition pattern, but also for understanding the nature of these particles. Recent developments in aerosol measurement systems permit the direct determination of the hygroscopic properties of particles produced by a variety of typical indoor particle sources. The system that provides this information is a tandem dif- Aerosol Science and Tcchnology 19: (1993) O 1993 Elsevier Scicnce Publishing Co., Inc.

3 306 W. Li and P. K. Hopke ferential mobility analyzer (TDMA). Such a system was initially developed by Liu et al. (1978) and was improved and refined by McMurry and coworkers (Rader and McMurry, 1986; Rader et al., 1987; McMurry and Stolzenburg, 1989). By combining the TDMA technique with a wetted wall reactor that can mimic the conditions in human respiratory tract, that is, relative humidity (RH) > 99% and 37"C, a system that is capable of studying the hygroscopicity of airborne particles has been constructed. The detailed descriptions of this system and test measurements with chemical compounds of known properties have been published (Li et al., 1992). By employing this system, five kinds of aerosol particles from common indoor combustion processes are studied. This paper presents the results of direct hygroscopic growth measurements of these indoor aerosols. EXPERIMENTAL The system used in the hygroscopic study is shown schematically in Figure 1. It con- Dried. Fillercd Air sists of two differential mobility analyzers whose design is essentially the same as that of TSI-3071, a wetted wall reactor (air humidifier), a particle counter (TSI- 3025), and a personal computer for system control and data collection. The detailed design of each part of the system as well as the working conditions have been presented (Li et al., 1992). The good agreement between the model predictions and measurements for the hygroscopic growth of compounds with known properties [NaCl, (NH,)HSO,, and (NH,), SO,] in previous experiments indicated that this system was capable of mimicking the conditions inside the human respiratory tract in term of the high humidity and was ready for studying real indoor aerosol particles. A slight modification was made to this system for the hygroscopic growth measurements of combustion aerosol particles. The overpressure mode was replaced by the underpressure mode so that the pump between the aerosol generation compartment and TDMA system inlet could be avoided. It is found that various pump are particle generators to some extent. For instance, even for a peristaltic pump (MasterFlex, using silicon rubber tubing) running at a moderate speed, the particle concentration generated by pump is comparable to or even higher than that of test aerosols. In many cases, it is quite difficult to avoid the interferences of background particles and obtain the desired information. With underpressure mode, the system pressure is lower than that of surrounding, and the test aerosols are drawn into the system without mechanical process involved. WETTED DMA-I WALL DMA-2 REACTOR FIGURE 1. Systematic diagram of the system used to study the aerosol's hygroscopic properties. Aerosol Generation Aerosol particles from five kinds of combustion processes were studied. They are particles from cigarette smoke, incense

4 Size Distributions and Hygroscopicity of Indoor Combustion Aerosols 307 burning, natural gas flame, propane fuel flame, and candle flame. Cigarette sidestream smoke particles were generated by a smoking machine (Heinrich Burghart, Elektro-u. Feinmechanik-Elektronik) with the standard smoking conditions (35 cm3 per puff, 2 s puff per minute) in a control volume of 0.2 m! Clean room air was supplied through a high efficiency filter to removed background particles in the control volume. The remaining background particle concentration in the control volume was < 200 particles/cm3, while the cigarette smoke particle concentration was of the order of 10"o lo6 particles/cm3. Particles from incense burning were generated in this 0.2-m3 control volume in the same manner. The cigarette mainstream smoke particles were generated by placing the burning cigarette at the inlet of a peristaltic pump (Masterflex, Cole-Parmer) that was manually controlled at the standard smoking conditions, then the cigarette smoke was induced into a 0.2-m3 dilution barrel and mixed with particle-free air. In this case, the background particle concentration in the barren was < 100 particles/ cm3 and test particle concentration was of the order of lo4 particles/cm3. The particles from natural gas, propane fuel, and candle-burning flames were generated in the 0.2-m3 barrel by placing either the Bunsen burner or candle on the bottom of the barrel. In the case of natural gas and propane fuel, 50 L/min particle-free dilution air was supplied to the barrel to remove both the background particles and the large amount of heat produced by the combustion. In the case of candle burning, only a 20 L/min flowrate of dilution air was used in order to increase the test particle concentration. After being generated in the control volume, all of the particles were dried by passing through a 1-m-long, heated copper tube and a silica gel diffusion dryer. Therefore, the maximum hygroscopic growth capacities of aerosol particles are measured. Source Particle Size Distributions The particle size distributions for each source were measured to characterize sources and help define measuring range. Figures 2-7 show the source size distributions. All of these distributions (circular symbols) were measured with the scanning electrical mobility sizing technique (Wang and Flagen, 1990) at 3L/min for sheath and excess air and 0.3 L/min for aerosol and monodisperse sample flows. It is obvious that the particles from different sources differ significantly in size. Cigarette and incense smoke contain larger particles in the range of nm, while natural gas, propane, and candle flames generate particles < 100 nm. The magnitude of the particle number concentrations in size distributions is not significant since these mainly depend on the flow rate of particle-free dilution air. The number concentration was controlled for the convenience of the experiments. The variations of number Particle diameter Dp(nm) FIGURE 2. Cigarettc mainstream smoke particle size distribution.

5 W. Li and P. K. Hopke Particle diameter Dp (nm) FIGURE 3. Cigarette sidestream smoke particle size distribution. Particle diameter, Dp (nrn) FIGURE 5. Natural gas flame particle size distribution. concentrations from run to run could be largely due to the fluctuation of the source generation processes and the environment. Therefore, the size distributions presented were the average of 10 measurements. The error bars in these figures show the standard deviations from the averages. The geometric means and geometric standard deviations of these sources are listed in Table 1. Another feature demonstrated by these size distribution curves is that all particle size distributions from single source seem to fit a lognormal distribution. The solid curves in Figures 2-8 are the fitted lognormal distribution based on the experimental data (circular symbols). The R2 in the upper corner of each figure, which is defined as Particle diameter Dp (nm) Particle diameter Dp (nrn) FIGURE 4. Incense smoke particle distribution. size FIGURE 6. Propane fuel flame particle size distribution.

6 Size Distributions and Hygroscopicity of Indoor Combustion Aerosols 309 Particle diorneter Dp (nm) FIGURE 7. Candle flame particle distribution. size shows the fitness. ye,, is the measured particle number concentration represented by circles in Figures 2-8, J is the arithmetic mean of ye,,. yfi, is the fitted particle number concentration represented by solid curve in Figures 2-8. The laboratory room air particle size distribution was also measured, and is presented in Figure 8 as a reference. The hygroscopic studies were carried out only in the major particle concentration range for each source, not only because they represent the major health effects concerned, but also because it is difficult to perform the experiments at low particle concentration. TABLE 1. Geometric Means and Geometric Standard Deviations for Indoor Combustion Aerosols Geometric Particle source mean (nm) GSD Main Stream Side Stream Candle Flame Incense Smoke Natural Gas Flame Propane Flame Particle diameter D (nm) FIGURE 8. Room air particle size distribution. Hygroscopicity Measurements Li et al. (1992) have shown both theoretically and experimentally that it is the RH and not the absolute humidity or temperature of the system that plays the dominant role in determining the hygroscopic growth of aerosol particles. Because of the relative insensitivity of growth to the absolute temperature and the greater convenience of performing experiments at room temperature, the hygroscopic studies of combustion aerosol particles were performed at room temperature. Because the RH is the most important factor to this study, it is critical that the humidity of the system is close to that in human respiratory tract, > 99%. Our chilled mirror dew point hygrometer has an accuracy of 0.4"C in measuring dew point temperature. This value corresponds to 2%RH accuracy around 100% RH. It has been found in the pure compound hygroscopic growth study (Li et al., 1992) that the measurement of hygroscopic growth of pure compounds is probably the most accurate and convenient method to measure the system humidity. In the hygroscopic growth studies of combustion aerosols, the system RH was

7 W. Li and P. K. Hopke checked to be > 99% by the hygroscopic growth of NaCl particles before and during the studies of combustion aerosols. Although the combustion aerosols were generated in relatively large isolated box and then mixed immediately with large amount of clean air, the RH of testing aerosol laden air can exceed 50%. The combustion process produces water as one of its by-products, and the temperature of aerosol generation compartment was usually higher than the ambient temperature especially for natural gas and propane fuel flames. In order to remove the ambient interferences and clearly define the meaning of hygroscopic growth data, the combustion aerosols were dried as previously described. Typically, the RH of combustion aerosol laden air was determined to be lower than 30%, which is typical of indoor air conditions. A particle's drying time may be much longer than its hygroscopic growth time, and the ambient RH at which a particle will lose water can be substantially lower than its deliquesce point. This phenomenon has been studied by Orr (1958) and Tang (1980). They reported a hysteresis loop in the hygroscopic growth and evaporation cycle for (NH,),SO, particles. Therefore, caution is needed to ensure that particles were completely dried before entering the TDMA. The detailed experimental procedure has been described elsewhere (Li et al., 1992). Basically, monodisperse aerosols were generated by DMA-1. These monodisperse aerosols were then introduced into the wetted wall reactor where they were exposed to highly humid air and would grow if they are hygroscopic (Figure 1). The final size of these grown aerosols was determined by DMA-2 coupled with a condensation nuclei counter. The coagulation of test aerosols in the test system was negligible in that the maximum number concentrations were between lo3 and lo4. It was observed that a measurable amount of aerosol particles were lost during transport and measurement. However, this problem was not of substantial concern because the objective of this study is to measure the hygroscopic growth of monodisperse particles. The fact that deposition efficiency varies with particle size will not influence the final results under this circumstances, although it will affect size distribution measurements at lowest size range. RESULTS AND DISCUSSION Under the conditions described above, the five kinds of combustion aerosol particles were studied and the results are shown in Figures 9-14 and summarized in Table 2. The horizontal axis is the diameter of initial, dry aerosol particles; the vertical axis is the growth ratio of particles diameters. The circles represent the growth measurement points for particles of selected size. It is clear that all five kinds of particles show growth. The magnitude of the size growth, ranging from 20% to > loo%, are generally less than those of water-soluble compounds particle. Like pure particles, the growth ratios of com- Initial particle diameter Do(nm) FIGURE 9. Hygroscopic growth of cigarette mainstream smoke particles.

8 Size Distributions and Hygroscopicity of Indoor Combustion Aerosols 311 t A = Run 2 1 Initial particle diameter Do(nm) FIGURE 10. Hygroscopic growth of cigarette sidestream smoke particles. 1.0 L Initial particle diameter Do (nm) FIGURE 12. Hygroscopic growth of natural gas flame particles. bustion aerosols increase with initial size. The curves are steep in small size range because of the Kelvin effect and flatten in large size range, although the curvatures are not as regular as those of pure compound particles. By comparing the growth ratio graphs, it can be concluded that both mainstream and sidestream cigarette smokes are not very different in terms of their hygroscopic growth, although their particle sizes differ significantly. However, the mainstream cigarette smoke particles may not increase in size as much as shown in Figure 9 in real situations, because they are inhaled immediately after generated 0 I n \ I RH=99 Initial particle diameter Do (nm) FIGURE 11. Hygroscopic growth of incense smoke particles. Initial particle diameter Do (nm) FIGURE 13. Hygroscopic growth of propane fuel flame particles.

9 312 W. Li and P. K. Hopke = Run 1 A = Run 2 Initial particle diameter Do (nm) FIGURE 14. Hygroscopic growth of candle flame particles. and do not have enough time to dry. In a parallel experiment where the cigarette mainstream smoke particles were induced into the TMDA system without mixing with dry air in the large barrel and passing through the diffusion dryer, it was found that the growth ratios were only between 0% and 8%. On the other, sidestream smoke particles will grow because they usually have enough time to dry in the room air after generation. Therefore, the growth data in Figure 10 represents their maximum growth capacity. Incense smoke particles are close to mainstream cigarette smoke in size range, but show only a little TABLE 2. Number Averaged Hygroscopic Growth Ratios of Indoor Combustion Aerosols Particle size range studied Average growth more hygroscopic growth than mainstream smoke particles. The similarities in Figures 9, 10, and 11 may reveal the common features of plant leaf-burning smoke particles. Natural gas and propane fuel flames are also common indoor aerosol particle sources since it was found in the house survey that the indoor particle number concentration is quite correlated with cooking time (Kamens et al., 1991). Particles from these two sources are characterized by the small sizes, nm, and are similar in both the particle size range and the moisture uptake capability, although the former has a little smaller sizes and larger growth ratios, while the later has larger sizes and smaller hygroscopic growth. Candle flame particles are also common indoor aerosol particles. Its particles size range is between the natural gas flame and cigarette or incense smoke, nm, and it showed the highest growth ratio values among the five. It may be worthwhile to define the candle-burning conditions used in our study because different burning conditions could result in quite different results. It was found in our experiment that the carbon black particles will be overwhelmingly dominant in particle number concentration if the candle wick is long and flame is large with visible black smoke. Those carbon black particles do not show significant growth and were not studied further because these burning conditions are relatively rare in real household. In our experiments, the candle wick was kept short so that no visible black smoke came from the flame. Comparisons between the white plain candles Source (nm) ratio and candles with scents were also made. Four candles with different scents were Mainstream cigarette smoke melted to make a mixed scent candle. Sidestream cigarette smoke Incense burning smoke This mixed scent candle was used to corn- Natural gas flame pare with white plain candles, supposing Propane fuel flame the mixed sample represented the average Candle flame of candles with scents. It was interesting

10 Size Distributions and Hygroscopicity of Indoor Combustion Aerosols 3 13 to find that candles with scent have smaller growth than white plain candles by about 20%. Therefore, candle particles may not differ other combustion aerosols significantly in terms of hygroscopic growth when candles with scents are used. The hygroscopic growth of laboratory room air particles is shown in Figure 15, again as a reference. To further characterize the aerosols, a hygroscopic growth model was established for these combustion aerosols by assuming that the chemical compositions are the same for all particles. This assumption is based on the fact that the hygroscopic growth curves of combustion aerosols have the same shape as those of pure salt particles. Other simplifying assumptions include the following: (1) since these particles mainly contain organic compounds, the density of the particles and droplets can be assumed to be 1.0; (2) the droplet solution is treated as ideal system because the correction data is not available; (3) room temperature is assumed to be constant at 22 C; (4) RH is between 99% and 99.5% as measured by the growth of NaCl particles; (5) surface tension, a, is inde- pendent of solutes because the correction data is not available. This model can be described as following. A particle contains both water-soluble and nonsoluble materials. Upon exposure to high humidity air, the soluble compounds will dissolve into the solution and form a liquid shell, while the nonsoluble compounds form a solid core. Since the solid core is surrounded by liquid shell, the liquid-gas interface remains the same as in a fully soluble material. By introducing a parameter, X, defined as mass ratio of insoluble materials to soluble materials, the hygroscopic growth ratio can be derived as, where d, and do are the diameter of grown and dry particle, respectively; p, and p, are the respective densities; M, is the molecular weight of water while M, is the apparent average molecular weight of soluble components; H is the relative humidity in the system; and i is the degree of dissociation of soluble components in water, which equals to the number of molecules or ions with which a dissolved molecule is dissociated in water. R, reflects the Kelvin effect, which is defined by Kelvin equation, 1.o Initial particle diameter Do (nrn) FIGURE 15. Hygroscopic growth of room air particles. where (T is surface tension of water, R is the gas content, T the absolute temperature and p is the density of the droplet liquid. All quantities in Eq. 2 except P

11 314 W. Li and P. K. Hopke are either measurable or known. Therefore, by employing the least squares method to fit the experimental data to this model, the value of P can be estimated. The curves in Figures 9-15 show the result of modeling. The calculations were performed at two humidities because it is impossible in our experiments to either measure or control the humidity as accurately as 0.1%. The humidity in the system was found to be between the 99% and 99.5% by the hygroscopic growth of NaCl particles. As defined in Eq. 3, P is a combined parameter which shows the total effect of degree of dissociation, solubility and average molecular weight of soluble components. Since these combustion aerosols are assumed to contain mostly organic compounds, and organic compounds usually are weakly dissociated in water, the i should be close to unity. For instance, the dissociation constant of formic acid in aqueous solution is 1.77 X It corresponds to the dissociation fraction of 4% and 13% for formic acid concentration of 0.1 and 0.01 M, respectively. If 10% of the molecules dissociate into ions in aqueous solution, i is only 1.1. Consequently, the value of p will mainly depend on the soluble fraction and their average molecular weight as indicated in Eq. 3. Since P is proportional to soluble fraction 1/(1 + x), and inversely proportional to M,, a large p means the particle may either contain more soluble fraction or the average molecular weight of soluble components is small, and vice versa. The P values for different sources are listed in the third column of Table 3. It is clear that smoke particles usually have larger P values while flame particles have smaller p values. This result is not surprising from the standpoint of reactants' molecular weight because smoke particles come from the combustion of plant leaves that are composed of high molecular weight molecules. Additionally, the smoking process is usually an incomplete combustion process which more likely results in high molecular weight products. In contrast, flame particles are generated either from low molecular weight gas reactants or more complete combustion process, or both, which more likely results in low molecular weight products. To further characterize the combustion aerosols, the soluble fraction of smoke particles were also determined indepen- TABLE 3. Characterization of Some Indoor Combustion Aerosol Particles RH z Soluble Source (5%) P= fraction(%) ia (1 + x)m, Main f Side i Incense Natural gas Propane Candle & aestirnated or derived from estimated values. M:

12 Size Distributions and Hygroscopicity of Indoor Combustion Aerosols 315 dently. Smoke particles were generated in the same manner as in hygroscopic studies and were collected on a filter. The filter was then placed on the surface of water in a beaker. Since the filter can be wetted by water, the soluble components would dissolve into the solution while nonsoluble components would remain on the filter. The filter was then air dried. By weighing the mass change before and after extraction, the soluble fraction were estimated and listed in fourth column of Table 3. It shows that these combustion aerosols do contain a large amount of water soluble materials as suggested by hygroscopic growth measurements. With the assumption that i = 1.0, the corresponding average molecular weights are calculated and listed in column six, which range from 166 to 382. The fact that the standard deviation of soluble fraction for incense is significantly larger than those of cigarette smoke is probably due to the color additives in incense sticks. It was found that particles from incense sticks of different colors gave quite different soluble fraction ranging from 44% to 68%. The value given in Table 3 is the overall average. Candle flame particles were also characterized in the same manner as for smoke particles. It was found that the soluble fraction is smaller than those for smoke particles. Because candle particles have relative larger hygroscopic growth, the estimated average molecular weight is only around 30. This quantity may be underestimated if the degree of dissociation of candle particles is underestimated significantly. As mentioned before, the candles used in this study was a mixed candles of four different scents. Therefore, although the measured soluble fraction was already an averaged value, the standard deviation was not as large as that for the incense smoke particles. Unfortunately, the gravimetric method was impractical for the natural gas and propane flame particles because they were too small to be collected with a significant amount of mass for microbalance analysis. For instance, natural gas flame particles have diameter around 20 nm. It took at least a week to accumulate 0.1 mg of particles on filter if the particle concentration is 100,000 particles/cm3. Therefore, the soluble fraction and estimated molecular weights are not available for flame particles in Table 3. In all of the growth ratio figures except that for incense burning, the data points for small particles are below the simulated curves while those for large particles are above the simulated curves. This phenomenon indicates that small particles are usually less water soluble than large particles. In order to provide an overall summary of the hygroscopic growth of combustion aerosol particles, the number weighted average growth ratio, which are based on the source size distributions, are also calculated according to Overall growth ratio - C dni.(growth ratio) - C dni (5) and listed in Table 2. The sources and magnitudes of errors of the hygroscopic growth measurements with our TDMA system have been discussed systematically in the previous studies for pure salt particles (Li et al., 1992). It was found that an 8% error in determining the values of hygroscopic growth ratio is possible due to the uncertainty in humidity mainly and other factors. With regard to the measurements of combustion aerosol particles, the error is expected to be higher because (1) the particle composition is much more complicated, mostly unknown, and more difficult to reproduce for combustion aerosols than for pure salt particles; and (2) the influences of environment are difficult to eliminated completely for com-

13 316 W. Li and P. K. Hopke bustion aerosols while pure salt particles are generated in a completely controlled system. In addition, unlike the pure salt particles, the magnitude of the experimental error is difficult to estimate theoretically since there are unknown factors involved. Therefore, the measurement errors were estimated by the repeatability of the experiments. Figure 9 and 13 show the typical results for cigarette side-stream smoke and candle flame as samples. The triangle symbols represent another run of the hygroscopic growth measurement which were taken on a different day with all known and controllable conditions being the same. From these repeatability experiments it was estimated that the fluctuations from run to run are < 15%. SUMMARY The hygroscopic growth of five common indoor combustion aerosols were measured over the major source output, with the number weighted average growth ratio of 1.54 for cigarette mainstream, 1.36 for cigarette sidestream, 1.67 for incense smoke, 1.63 for natural gas flame, 1.40 for propane fuel flame, and 1.85 for candle flame. These measurements show the direct evidence of the impact of humidity to the physical sizes of combustion aerosols because the quantity we measured, the particle's electrical mobility, is directly related to the particle's physical size. Therefore, these data can be used in all kinds of modeling relating to particle's deposition pattern where high humidity is considered to be an influential factor, such as the human respiratory system. This work was supported by the US Department of Energy under Grant DE FG02 90ER61029 and the Center for Indoor Air Research under Contract REFERENCES Ansclm, A,, Gebhart, J., Heyder, J., and Ferron, G. (1986). In Aerosols: Formation and Reactiuity, (G. Israel, cd.). Pergamon Press, Oxford, pp Egan, M. J., and Nixon, W. (1989). J. Aerosol Sci. 20: Ferron, G. A. (1977). J. Aerosol Sci 8: Ferron, G. A,, Hayder, B., and Kreyling, W. G. (1985). Bull. Math. Biol Hicks, J. F., and Megaw, W. J. (1985). J. Aerosol Sci. 16: Hicks, J. F., Pritchard, J. N., Black, A,, and Mcgaw, W. J. (1989). J. Aerosol Sci 20: Kamens, R., Lee, C. T., Wiener, R., and Leith, D. (1991). Atmos. Enuiron. 25: Li, W., Montassier, N., and Hopke, P. K. (1992). Aerosol Sci. Technol. 17: Liu, B. Y. H., Pui, D. Y. H., Whitby, K. T., Kittelson, D. B., Kousaka, Y., and McKensie, R. L. (1978). Atmos. Enuiron Martonen, T. B. (1982). Bull. Math. Biol. 44: Martonen, T. B., Bell, K. A,, Phalen, R. F., Wilson, A. F., and Ho, A. (1982). Ann. Occup. Hyg. 26: McMurry, P. H., and Stolzenburg, M. R. (1989). Atmos. Enuiron. 23: Orr, C., Jr., Hurd, F. K., and Corbett, W. J. (1958). J. Colloid Sci. 13: Radcr, D. J., and McMurry, P. H. (1986). J. Aerosol Sci. 17: Rader, D. J., McMurry, P. H., and Smith, S. (1987). Aerosol Sci. Technol. 6: Tang, I. N. (1980). In Generation of Aerosols and Facilities for Exposure Experiments (K. Willeke, ed.). Ann Arbor Science, Ann Arbor, MI. Tu, K. W., and Knutson, E. 0. (1984). Aerosol Sci. TechnoL 3: Wang, S. C., and Flagen, R. C. (1990). Aerosol Sci. Technol. l3: Received October 26, 1992; accepted April 21, 1993.