Environ. Sci. Technol. 2008, 42, 8033–8038
A New Real-Time Method for Determining Particles’ Sphericity and Density: Application to Secondary Organic Aerosol Formed by Ozonolysis of r-Pinene A L L A Z E L E N Y U K , * ,† J U A N Y A N G , † CHEN SONG,† RAHUL A. ZAVERI,† AND DAN IMRE‡ Pacific Northwest National Laboratory, Richland, Washington 99354, and Imre Consulting, Richland, Washington 99352
Received May 16, 2008. Revised manuscript received August 10, 2008. Accepted August 13, 2008.
Particle volumes are most often obtained by measuring particle mobility size distributions and assuming that the particles are spherical. Particle volumes are then converted to mass loads by using particle densities that are commonly estimated from measured mobility and vacuum aerodynamic diameters, assuming that the particles are spherical. For aspherical particles, these assumptions can introduce significant errors. We present in this work a new method that can be applied to any particle system to determine in real time whether the particles are spherical or not. We use our second-generation single particle mass spectrometer (SPLAT II) to measure with extremely high precision the vacuum aerodynamic size distributions of particles that are classified by differential mobility analyzer and demonstrate that the line shape of these vacuum aerodynamic size distributions provide a way to unambiguously distinguish between spherical and aspherical particles. Moreover, the very same experimental system is used to obtain the size, density, composition, and dynamic shape factors of individual particles. We present an application of this method to secondary organic aerosols that are formed as a result of ozonolysis of R-pinene in the presence and absence of an OH scavenger and find these particles to be spherical with densities of 1.198 ( 0.004 and 1.213 ( 0.003 g cm-3, respectively.
Introduction Density is an important physical property of aerosol particles. It is used to convert aerosol size distributions into mass loads, to model particle aerodynamic properties, and is often correlated with particle optical properties (1). Monitoring changes in particle densities can also be used to follow aerosol chemical reactions (2) and provide the means to quantitatively calculate the composition of internally mixed individual particles (3), but traditionally, particle composition has been used to deduce the aerosol density (4, 5). A number of methods of measuring aerosol density have been developed and applied in both laboratory and in field studies (2, 6-18). One of the more common approaches is to calculate the density on the basis of measurements of particle mobility * Corresponding author e-mail:
[email protected]. † Pacific Northwest National Laboratory. ‡ Imre Consulting. 10.1021/es8013562 CCC: $40.75
Published on Web 10/01/2008
2008 American Chemical Society
and aerodynamic (11-16) or vacuum aerodynamic diameters (2, 17-21). This approach is applicable to spherical particles and can be used to yield very precise particle densities, particularly when the differential mobility analyzer (DMA) is used to classify particles with narrow distribution of mobility diameters and another instrument is used to measure the aerodynamic diameters of the same particles (3, 18, 20, 22, 23). Most often, however, mobility and aerodynamic diameters are measured by two separate instruments, in parallel, on the same air mass, but not the very same particles. It is important to take into account the fact that the two instruments have different size-dependent detection efficiencies (19, 24). Moreover, the interpretation of measurements for aerosol samples that contain mixtures of particle types, each with its own unique size distribution, is impossible, because it is not known how to relate the independentlymeasuredmobilityandaerodynamicdiameters. For aspherical particles, the same measurements yield particle effective density (11, 12, 17, 23, 25), which is a function of particle density and particle shape. The detailed discussion of the relationships between particle mobility diameter (dm), vacuum aerodynamic diameter (dva), volume equivalent diameter (dve), particle density (Fp), effective density (Feff), and DSF is provided elsewhere (17, 18, 23). A few of the salient equations are given in the Supporting Information for the convenience of the reader. According to eqs 3 and 4 of the Supporting Information, the mobility diameters of aspherical particles are larger than their volume equivalent diameters and the vacuum aerodynamic diameters are smaller than the volume equivalent diameters by factors that are approximately equal to their dynamic shape factor (DSF). As a result, the effective density of aspherical particles, which is defined as the ratio of the vacuum aerodynamic and mobility diameters, is nearly inversely proportional to the square of the particle DSF. To set the scale that an approximation of particle sphericity introduces, we will use as an example aspherical particle with a DSF of 1.14, which is identical to the DSF of a doublet of uniform spheres. A particle with this DSF and volume equivalent diameter of 300 nm will have mobility diameter of 330 nm and vacuum aerodynamic diameter of Fp263/F0 nm (where Fp and F0 are the particle density and unit density, respectively). As a result, this particle’s effective density will be 20% lower than its particle density. Note that the relationship between particle DSF and mobility diameters means that particle volumes are overestimated by a factor that is approximately equal to the cube of the particle DSF. Returning to our example of a particle with a DSF of 1.14, we find that if it was assumed to be spherical and its volume was calculated on the basis of its measured mobility diameter, the calculated particle volume would be ∼33% higher than the true particle volume. These types of inaccuracies can play an important role in many aspects of aerosol science. An important example, which we will discuss in this paper, relates to the precise yield with which secondary organic aerosols (SOA) form. Organic aerosols and internally mixed aerosols containing a large fraction of organic species represent a significant fraction of the atmospheric aerosol composition, accounting for 20 to 90% of the total atmospheric dry aerosol mass (26, 27). Moreover, a significant fraction of the organic mass is produced through the oxidation of volatile organic compounds from both anthropogenic and biogenic sources, with the oxidized products either nucleating to form new particles or condensing on pre-existing atmospheric particles. At present field measurements find that VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the amount of SOA is higher than what is predicted by approximately a factor of 10 (28). This finding provides an important driving force aimed at developing better understanding of the processes that control SOA formation (28-30). To develop quantitatively reliable models of these processes, there is a need to properly characterize the properties of SOA and to quantify the yields with which they are formed in the oxidizing atmosphere. Currently, these yields are most commonly calculated on the basis of the measured mobility size distributions and effective densities (5, 19, 21, 24) with the assumption that these particles are spherical. It would be advantageous to have a method that can in real time determine whether the particles are spherical or not. Aside from the new method we present here, there has been only one other approach used to determine particle sphericity in real time. This approach relies on the fact that the divergence of particle beams, which are formed by a commonly used aerodynamic lens inlet, is larger when the particles are aspherical (2, 31-35). By quantifying the resulting beam broadening due to asphericity, it is possible to distinguish between spherical and aspherical particles (36, 37). In this paper, we present a new approach to distinguish in real time between spherical and aspherical particles. We use our second-generation single particle mass spectrometer (SPLAT II) to measure the line shape of the vacuum aerodynamic size distribution (dva) of DMA classified particles and demonstrate that it can be used to unambiguously determine whether particles are spherical or not. Moreover, as we will show, the very same data are used to determine, with high precision, particle densities, effective densities, DSF, and their compositions. The paper presents three sets of measurements. In the first set, we use spherical uniform polystyrene latex (PSL) beads, which are commonly used as NIST certified size standards, to characterize the line shape of dva distributions that are obtained by SPLAT II and proceed to illustrate the effect that particle shape has on the dva line shape using doublets of the same PSL beads. Next, we show that the combined SPLAT II/DMA system can be used to perform the very same measurements on polydisperse particles. Here we determine the density, DSF, and composition of NaCl particles and NaCl particles that are coated with a liquid organic and with a solid organic. In the last section, we apply the SPLAT II/DMA system to identify particle sphericity and precisely measure the density of SOA particles formed by the ozonolysis of R-pinene.
Experimental Section SPLAT II/DMA. SPLAT II is our second generation of single particle mass spectrometer, the specific features of which will be described in detail in separate publications. Here we will provide only some of the relevant features. Similar to the original SPLAT, SPLAT II uses an aerodynamic lens to transport the particles from the ambient, high pressure atmosphere into the vacuum system with extremely high efficiencies (34, 38). The aerodynamic lens forms a particle beam with very low divergence (31-35, 39) and imparts on each particle a velocity that is a narrow function of the particles vacuum aerodynamic diameter (34). This relationship makes it possible to use the measured velocities to determine the particle vacuum aerodynamic diameters with precision that is better than 0.5% (34). Each particle is detected by light scattering at two optical detection stages that are spaced 10 cm apart. The time it takes the particle to travel between the two stages is used to determine particle velocity, which is a function of its vacuum aerodynamic diameter. Particle detection and its measured velocity are then used to generate a trigger to fire the UV excimer laser pulse at a time that coincides with the particle’s 8034
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arrival in the ionization region of a reflectron time-of-flight mass spectrometer (TOF-MS). The UV pulse ablates the particle and generates ions, the masses of which are analyzed with the TOF-MS. An excimer laser (GAM Lasers Inc., Model EX10) operated at 193 nm wavelength is softly focused into a spot that is ∼550 µm by 700 µm in size to produce laser fluences in the range of 0.1 to 1 J/cm2. Individual particle mass spectra are acquired using an angular reflectron TOFMS (R. M. Jordan, Inc., Model D-850) and digitized by an A/D converter (Gage Applied Technologies, Inc., Model CompuScope 14200). In the experiments described here, a DMA (TSI Inc., model 3081) is used to select particles with narrow size distributions of known mobility diameters, which are then characterized by SPLAT II. In many cases, because of the presence of multiply charged particles with the same electrical mobility, the measurements are simultaneously conducted on particles with a number of well-defined volume equivalent diameters. Since these particles have different vacuum aerodynamic diameters, they are easily resolved by SPLAT II and additional information can be obtained. The width of the mobility size distribution of the classified particles depends on the specific DMA operating conditions. Typically, the ratio of the sheath to sample flow rates is 10, which translates to a mobility distribution with a full width at half-maximum (fwhm) of 5%, defined as the ratio of the line width to the average mobility diameter. The larger the sheath to sample flow ratio, the narrower the line width. We have demonstrated that SPLAT II has the capability to measure particle aerodynamic diameters with better than 0.5% accuracy (34) and showed the capability to obtain particle densities with better than 0.5% accuracy (18). The figures presented in the results and discussions section below were constructed by averaging measurements conducted on more than 1000 particles. Generation of 302 nm PSL Particles. PSL beads (Duke Scientific) with a diameter of 302 ( 5 nm, with size uniformity of e3% were suspended in water/methanol solutions, aerosolized using an atomizer (TSI Inc., model 3076) and dried with two diffusion dryers (TSI Inc., model 3062) connected in series. PSL singlets and doublets were identified in DMA scans and selected for sampling by SPLAT II. In some experiments particles were simultaneously sampled by SPLAT II and collected onto copper transmission electron microscopy (TEM) grids using the time resolved aerosol collector (TRAC) to obtain scanning electron microscope (SEM) images (40). Generation and Coatings of NaCl Particles by Dioctyl Phthalate and Pyrene. NaCl particles were generated by aerosolizing a NaCl/water solution using an the atomizer, dried with two diffusion dryers and classified at 146 nm with the DMA. The dried particles were passed over a supersaturated vapor of either dioctyl phthalate (DOP) or pyrene. The temperature of these organics was varied (40-70 °C) and the mobility diameters of the coated particles were measured by a scanning mobility particle sizer (SMPS, TSI Inc., Model 3936). When the desired coating was achieved, the temperature was stabilized. Prior to measurements by SPLAT II the coated particles were classified again using a second DMA. SOA by Ozonolysis of r-Pinene. The experiments were performed in a ∼100 L Teflon bag at low relative humidity (RH < 1%). R-Pinene (Aldrich, 98% purity) was warmed and volatilized before its introduction into the Teflon bag at a concentration of 0.3 ppmv. In some experiments Cyclohexane (Aldrich, 99% purity) was introduced into the Teflon bag at a concentration of ∼212 ppmv as an OH radical scavenger, while in other experiments no scavenger was used. The initial concentration of O3 was twice as high as that of R-pinene. Particles were formed by homogeneous nucleation and their size distributions determined using the SMPS. SPLAT II was
FIGURE 1. Line shapes of vacuum aerodynamic size distributions of 302 nm PSL beads (dashed line and sold circles) and their doublets (sold line and open circles). The insert shows micrographs of the corresponding particles. used to measure the vacuum aerodynamic diameters and size distributions of DMA classified particles, and to record individual particle mass spectra to determine simultaneously particle DSF, density, and composition. For this set of experiments, the instrument calibration relied on DMA classified DOP particles to generate a SPLAT II/DMA calibration curve. This approach eliminates the need to calibrate each instrument separately and generates a calibration curve under conditions that are identical to those used to measure the unknown. To improve the precision, we increased the ratio of sheath to sample flow rates from 10 to 15. The measured time-of-flight distributions of DOP particles that are classified at 100 nm by the DMA show particles with as many as seven charges. The time-of-flight versus particle vacuum aerodynamic diameter data are fitted with the power function. The line shapes of the vacuum aerodynamic size distributions of DOP particles show fwhm of 3.0 ( 0.3%.
Results and Discussion PSL Particles: Line Shapes of Vacuum Aerodynamic Size Distributions of Spherical and Aspherical Particles. To provide an easy to quantify measure of the SPLAT II performance capabilities and to establish a clear relationship between particle shape and the width of the dva size distributions, as measured by SPLAT II, we start with PSL beads. Because these particles have well-defined narrow distribution of sizes, they serve as an excellent model case. In Figure 1, we illustrate the line shape of the measured vacuum aerodynamic size distribution of PSL spheres with a diameter of 302 ( 5 nm and size uniformity of e3%. To emphasize the line shape and to ease comparison, we plot the normalized fraction of the number of particles as a 0 )/d0 in percent. Here, d0 and d function of (dva - dva va va va are the peak of the vacuum aerodynamic size distribution and the measured vacuum aerodynamic diameters, respectively. Figure 1 shows a narrow line shape with a fwhm of 2.7%, which is in agreement with the manufacturer specified dispersion in particle diameters. Figure 1 also shows the line shape of the vacuum aerodynamic size distribution of PSL doublets. This size distribution exhibits a line shape with fwhm of 10%, which
FIGURE 2. Line shapes of vacuum aerodynamic size distributions of DMA classified: cubic NaCl particles with a fwhm of 10%, the same NaCl particles coated with solid pyrene with a fwhm of 13%, and liquid DOP with a fwhm of 5%. is significantly broader than the line width of 3.8% that is predicted for the doublet purely on the basis of the original spread in particle diameters. Combining the doublet’s known volume equivalent diameter of 380.5 nm and the measured vacuum aerodynamic diameter of 352 nm, we calculate a DSF of 1.14 for this particle. This value is in excellent agreement with previously reported values for the PSL doublets in a random orientation (23). The increase in line width represents the fact that the particle DSF depends on the particle orientation. In this case, it is known that the DSF of the doublet in the parallel orientation is 1.02, whereas it is 1.37 in the perpendicular one (41). Because the Reynolds number in the aerodynamic lens inlet is smaller than ∼0.03 (42), no particle alignment is expected (43) and the measured DSFs should be dominated by values for random orientation in free molecular regime. The absence of alignment results in broadening of the vacuum aerodynamic size distribution reflecting the particle’s orientation-dependent DSF. NaCl Particles and NaCl particles Coated with DOP and Pyrene. To perform the same type of analysis as was presented above for other particle types requires the use of a DMA to select a narrow distribution of mobility diameters and transport these particles to be characterized by SPLAT II. Figure 2 shows three superimposed plots that illustrate the line shapes of the vacuum aerodynamic size distributions of the three particle types that are schematically illustrated in the inset. An examination of Figure 2 shows that despite the fact that all three particle types were classified by the DMA under identical operating conditions, the line shapes of the vacuum aerodynamic size distributions are significantly different. Pure NaCl particles exhibit asymmetric line shape with a fwhm of 10%. The measured effective density of these particles is 1.85 g cm-3 that should be compared to the particle density of 2.165 g cm-3. These values can be used to calculate a DSF of 1.08 for NaCl particles, which is in agreement with previously reported values (23, 43, 44). Note that for NaCl particles there is a difference of 7 nm between the observed mobility diameter of 146 nm and the calculated volume equivalent diameter of 139 nm, which could amount to an overestimation by 16% when translated to volume, if the particles were assumed to be spherical. When the very same NaCl particles with dm ) 146 nm are coated with DOP, a liquid organic substance, the line shape VOL. 42, NO. 21, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. (a) Vacuum aerodynamic size distributions of DMA classified SOA particles. The figure shows the results of measurements with and without an OH scavenger (dashed and solid lines, respectively). The mobility diameters are indicated above each peak of 234 nm that were formed in the presence of an OH scavenger; (b) an example of the line shape of the dva distribution of SOA particles; and (c) dependence of dva on dm and its linear fit for the set of measurements that are summarized in. narrows down to 5%. This line width is consistent with a value observed for a wide range of spherical particles, when classified by a DMA that is operated with a sheath to sample flow ratio of 10, as is the case here. This observation provides unequivocal evidence that the DOP coated NaCl particles are spherical. The mobility diameter of the 257 nm of the spherical DOP coated NaCl particles is equal to their volume equivalent diameter, and the known volume equivalent diameter of the NaCl core particles is 139 nm. Together these numbers can be used to calculate a NaCl volume fraction of 15.9% for these DOP coated NaCl particles. Furthermore, using the known densities of NaCl (2.165 g cm-3) and DOP (0.986 g cm-3), the calculated volume fractions, and the assumption of volume additivity, a particle density of 1.17 gcm-3 can be calculated. This value is in very good agreement with the measured density for the same particles of 1.16 ( 0.01 gcm-3. The excellent agreement between the calculated and measured densities provides further support for our conclusion that these particles are spherical. In contrast with the DOP-coated NaCl particles, the line shape of the pyrene-coated NaCl particles exhibits a line width of 13%, slightly larger than that of the NaCl core particles. This increase in line width clearly indicates that the pyrene-coated particles are not spherical. Given that pyrene is a solid, this finding is not surprising. The mobility diameter of the pyrene-coated particles of 241 nm and their vacuum aerodynamic diameter of 272 nm yield an effective density of 1.13 g cm-3. The fact that the effective density of these particles is smaller than the density of either of the two particle constituents (1.271 g cm-3 for pyrene and 2.165 g cm-3 for NaCl) provides additional, unambiguous support for the conclusion that these particles are aspherical. Using the measured effective density, the calculated volume equivalent diameter of the NaCl cores, and the known densities of NaCl and pyrene, we calculated the DSF of the pyrene coated NaCl particles to be 1.155, yielding a volume equivalent diameter of 218.6 nm with dm ) 241 nm, which translates to a pyrene weight fraction of 63.0%. Note that in this case the assumption of particle sphericity would result in an overestimation of the particle volume by 36%. The simple examples that were presented above provide strong evidence for the existence of a robust relationship between particle shape and the width of the vacuum aerodynamic size distributions of particles with narrow mobility size distributions. We have tested this relationship on a significant number of systems and find it to be 8036
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reproducible (18, 23, 45). The general trend is that particle asphericity results in broadening of vacuum aerodynamic size distributions of DMA selected particles. When the DMA is operated with a sheath to sample flow rate ratio of 10, spherical particles produce symmetric line shapes with a fwhm of ∼5%, whereas for aspherical particles, the line width is typically 10% or larger and asymmetric line shapes are often exhibited. Shape and Density of SOA Particles Formed by the Ozonolysis of r-Pinene. Below we describe the results of a set of the measurements intended to determine whether SOA particles produced by the ozonolysis of R-pinene are spherical and to measure their density with high precision. In typical laboratory experiments, a known quantity of precursors are introduced into a reaction chamber and their concentrations are monitored as a function of time as they are being consumed by oxidation. In parallel, the particle size distributions are measured, most often with a SMPS. Under the assumption of particle sphericity, the mobility diameters are used to calculate SOA volume. The effective density of the SOA particles is estimated on the basis of their mobility and vacuum aerodynamic size distributions (19, 21, 24), which typically are measured in parallel. These two size distributions are then typically fitted to match the shape of the rising edge of the two size distributions by finding the “right” effective density. Assuming sphericity, the calculated particle volumes and effective densities are then used to compute a particle mass, from which the SOA formation yields can be obtained. Figure 3a shows the vacuum aerodynamic size distributions of DMA classified SOA particles produced by ozonolysis of R-pinene with and without scavenger, which are marked by dashed and solid lines, respectively. The corresponding particle mobility diameters are marked above each peak. Figure 3b shows the line shape of SOA particles with mobility diameter 233.5 nm that were formed in the presence of an OH scavenger. On the basis of the observed line shapes of all the dva distributions, we conclude that these, homogeneously formed SOA particles are spherical. In Figure 3c, we show a plot of the measured vacuum aerodynamic diameters the mobility diameters of the SOA particles that were formed in the absence of scavenger and a linear fit to the data, which shows that the particle density is size independent. A summary of the SOA data is presented in Table 1. The average line width of 3.0 ( 0.3% for all SOA runs clearly indicates that these particles are spherical. The densities of these particles are 1.198 ( 0.004 and 1.213 ( 0.003 g cm-3 for SOA particles formed with and without scavenger,
TABLE 1. Summary of the Measured Densities and Line Shapes of SOA Particles dva
density (g cm-3)
fwhm %
154.0 186 240.6 292 179.9 218 285.0 346 285.0 346 318.7 387 average 149.9 180 233.5 279 average
1.2078 1.2136 1.2118 1.2140 1.2140 1.2143 1.213 ( 0.003 1.2008 1.1949 1.198 ( 0.004
3.0 3.2 2.7 3.3 3.3 2.9 3.1 2.7 2.5 2.6
condition
no scavenger
scavenger
dm
respectively. These values are in good agreement with those previously estimated for the same system (19, 21).
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Acknowledgments This work was supported by the U.S. Department of Energy Office of Basic Energy Sciences, Chemical Sciences Division. This research was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research at Pacific Northwest National Laboratory (PNNL). Funding for C.S. and R.A.Z. was provided by the PNNL Laboratory Directed Research and Development (LDRD) program. PNNL is operated by the U.S. Department of Energy by Battelle Memorial Institute under Contract DEAC06-76RL0 1830.
Supporting Information Available
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Relationships between particle mobility diameter, vacuum aerodynamic diameter, volume equivalent diameter, density, effective density, and DSF (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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