A new compact aerosol concentrator for use in conjunction with low flow-rate continuous aerosol instrumentation

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1 Aerosol Science 36 (25) A new compact aerosol concentrator for use in conjunction with low flow-rate continuous aerosol instrumentation Michael D. Geller, Subhasis Biswas, Philip M. Fine, Constantinos Sioutas Department of Civil and Environmental Engineering, University of Southern California, 362 South Vermont Avenue, KAP21, Los Angeles, CA 989, USA Received 16 August 24; received in revised form 23 November 24; accepted 23 November 24 Abstract Currently available versatile aerosol concentration enrichment systems (VACES) have proven useful for providing elevated levels of ambient particulate matter to human and animal exposures, as well as for the collection of particles in aqueous solutions for in vitro studies. Previous studies have demonstrated that such systems do not significantly alter the physical or chemical properties of the particles. The current VACES configuration consumes significant electrical power for pumping and cooling, and requires attended operation by expert operators. A recent application of the VACES has been to provide a concentrated aerosol stream to continuous particle mass spectrometers in order to increase the spectrometer s hit rate or sensitivity. These instruments usually require low intake flow rates (< 1l/ min) and often sample unattended, 24 h per day. In order to better meet the requirements of these instruments, a new mini-vaces (m-vaces) system with a lower intake flow rate (3 l/min), a lower minor flow rate (1 1.5 l/min) and allowing for more automated operation was designed, built, and tested. The system is a scaled down version of the current VACES design, with many important design improvements. Humidification of the air stream is achieved with a re-designed saturator consisting of a heated, moist absorbent material surrounding the intake flow. Cooling to achieve super saturation, and thus particle growth, is accomplished using a commercially available, solid-state, thermo-electric chiller. Once grown, the aerosol is concentrated using a new, smaller virtual impactor. Particles are then dried to their original size using a diffusion dryer filled with silica gel. Results of the laboratory evaluation include close to predicted enrichment factors for laboratory-generated particles of different composition (ammonium nitrate, ammonium sulfate, adipic acid) as well as ambient aerosols. Particle size distributions measured by an SMPS before and after enrichment and drying show that particle size distributions are not altered. An APS provided data on the size distribution of particles after growth and concentration, Corresponding authors. Tel.: ; fax: addresses: mgeller@usc.edu (M.D. Geller), sioutas@usc.edu (C. Sioutas) /$ - see front matter 25 Elsevier Ltd. All rights reserved. doi:1.116/j.jaerosci

2 M.D. Geller et al. / Aerosol Science 36 (25) but before drying. Filter based and continuous field experiments in which concentrated aerosol was compared to ambient outdoor levels also showed close to predicted enrichment factors for PM 2.5 mass and black carbon, with no significant alteration of the particle size distribution. 25 Elsevier Ltd. All rights reserved. Keywords: Particle concentrator; Versatile aerosol concentration enrichment system; Particulate matter; Virtual impaction 1. Introduction Numerous epidemiological and toxicological studies have linked ambient particulate matter (PM) with adverse health effects (NRC, 24; Li et al., 23). However, it is still unknown which physical or chemical properties of PM are responsible for the observed health outcomes. In order to address this research question, fast and accurate measurements of PM characteristics are essential for both exposure measurements associated with health effects studies and for understanding the sources and atmospheric processes contributing to ambient PM and its properties. Particle concentration technologies have proven to be effective in aiding both health effects studies (Sioutas, Koutrakis, & Burton, 1995a,b; Demokritou, Gupta, Ferguson, & Koutrakis, 22; Li et al., 23) and ambient sampling efforts (Zhao et al., 24; Geller et al., 22; Khlystov et al., 24). The first particle concentrators focused on larger particles (greater than about 1 μm in diameter) since these are easily concentrated via virtual impaction (Marple & Chien, 198; Loo & Cork, 1988; Chen & Yeh, 1987). When the interest in fine particles (less than 2.5 μm in diameter) developed, fine particle concentrators were built using series of high flow-rate, large pressure drop, slit virtual impactors to concentrate ambient PM (Sioutas et al., 1995a,b, 1997). While effective, these systems are large and immobile, they require large amounts of power, and they do not effectively concentrate particles less than about 15 nm. The increasing evidence that ultrafine particles (less than about 1 nm in diameter) can illicit greater health effects (Oberdörster, Ferin, Finkelstein, Wade, & Corson, 199; Li et al., 23; Xia et al., 24) prompted the development of particle concentrators capable of concentrating ultrafine particles as well. The versatile aerosol concentrator enrichment system (VACES) achieves fine and ultrafine particle enrichment by first growing the particles to much larger sizes via super-saturation in water vapor (Kim, Jaques, Chang, Froines, & Sioutas, 21b). The larger droplets can then be concentrated by inertial virtual impaction, and then returned to the original size by diffusion drying. Several previous studies have demonstrated that the concentrated particles are enriched by close to predicted factors (up to a factor of 4) and are not altered chemically or physically by the concentration process (Kim et al., 21a; Misra, Fine, Singh, & Sioutas, 24). PM-enriched air can be used as an elevated exposure atmosphere for animal and human exposure studies (Smith, Kim, Recendez, Sioutas, & Pinkerton, 23; Smith et al., 24; Gong et al., 23, 24). The advantage of these studies is that exposure to actual ambient particles and their complex characteristics can be investigated as opposed to simpler and less realistic laboratory generated aerosols. Particle concentrators can also be used to collect PM material suspended in aqueous solution for subsequent in vitro toxicity assays (Li et al., 22, 23; Xia et al., 24). Another application of particle concentrators is to increase the sensitivity, and thus decrease the sampling time, of ambient particle sampling instrumentation. A previous study demonstrated that enriching particles by a factor of approximately 2 enabled measurements of the diurnal cycles of the

3 18 M.D. Geller et al. / Aerosol Science 36 (25) chemical composition of ultrafine particles with a Nano-MOUDI (Geller et al., 22). In addition to time-integrated measurements such as impactor sampling, the performance of recently developed particle mass spectrometers can also be improved by operating a particle concentrator upstream of the spectrometer s inlet. For example, a study by Khlystov et al. (24) showed that the Aerodyne Inc. Aerosol mass spectrometer (AMS) (Jimenez et al., 23) has shown greater sensitivities with the VACES system attached, with little or no alteration of particle properties. Another recent study using the University of California-Davis Rapid Single-particle Mass Spectrometer (RSMS-III) has also shown that the VACES concentration process does not change the particle chemistry, while increasing the particle hit rates (Zhao et al., 24). While the VACES system is portable and does not have excessive power requirements, its operation requires constant attendance by fairly highly trained personnel. This practically limits the sampling time to between 6 and 8 h. Furthermore, the current VACES systems provide enriched sample flow rates between 5 and 6 l/min, while many particle mass spectrometers have intake flows typically on the order of 1 l/min or less. Thus, the VACES has extra, unused capacity for these applications. This study describes the development and testing of a smaller mini-vaces (m-vaces) system designed for particle mass spectrometer applications. With intake flows of 3 4 l/min, and concentrated aerosol flows of 1 2 l/min, the system is smaller, lighter, more controllable, and suitable for unattended operation over several days, thus enabling the 24 h continuous sampling often conducted with particle mass spectrometers. In addition, the system is easily modified to provide aqueous particle collections for subsequent toxicity assays or chemical analysis. 2. Concentrator design The new m-vaces concentrator is a scaled-down version of the previously developed VACES system (Kim et al., 21a,b) with several new and improved features. A schematic is shown in Fig. 1. Ambient air flows through a 2.54 cm ID inlet at 3 l/min. Although no size-selective inlet was used during the laboratory testing, the system is designed such that it can operate under a small pressure drop associated with a standard PM 2.5 (or any other cutpoint size) inlet impactor or cyclone. The intake flow then passes through the saturator, consisting of a 2.54 cm ID and 45 cm long circular channel surrounded by commercially available cellulose sponge contained within an aluminum cylinder. A peristaltic pump continuously supplies circulating water to the top of the saturator to keep the sponge saturated. The saturator assembly is heated by a heating tape wrapped around the exterior of saturator controlled by a variable transformer. The voltage is set such that the air leaving the saturator is maintained between 28 and 29 C at a relative humidity greater than 9%. Temperature and relative humidity are measured immediately downstream of the saturator using a temperature/relative humidity probe (Model 3796, Cole-Parmer Instruments Co., Vernon Hills, IL). The flow then passes through an inverted stainless steel U-tube with a gravity-fed drain. The drain allows excess condensed water from the saturator, condenser, and walls to be removed from system. Both the drain collection basin and the saturator reservoir are closed systems to allow for any minor pressure drops caused by future additions of size-selective inlets. From the U-tube, the flow passes up through the condenser, consisting of a 2.54 cm inner tube surrounded by a 7.62 cm outer tube. Both tubes of the condenser are 27 cm long. The airflow passes through the inner tube as a continuous flow of chilled ethylene glycol/water (1:1 by volume) coolant fills the space between the inner

4 M.D. Geller et al. / Aerosol Science 36 (25) Inlet Minor Flow Virtual Impactor Major Flow to Pump Saturator Condenser Dryer Heating Element Cellulose Sponge Silica Gel Diffusion Dryer Peristaltic Pump Water Reservoir Drain Recirculating Chiller Concentrated Aerosol Out Fig. 1. Schematic of m-vaces. and outer tubes. The temperature of the outer wall of the condenser is maintained at 1 C with a small, commercially available recirculating chiller (Thermocube 3-1D-1-LT Solid State Cooling Systems, Pleasant Valley, NY). This temperature is substantially higher than the temperatures used in the larger VACES ( 6 8 C), thus eliminating the build up of ice on the inner walls of the condenser tube. The condenser supersaturates the air stream and particles grow by condensation to a diameter above the cut-point of the virtual impactor, which follows the condenser. The virtual impactor is a smaller version of the VACES virtual impactors, with a designed 5% cut-point of about 1.5 μm in aerodynamic diameter for a major flow of 3 l/min and minor flow of 1.5 l/min. Inertial forces concentrate the particle-containing droplets in the minor flow of the impactor, while the particle-free major flow ( 29 l/ min) is drawn away with a vacuum pump (Model: Q-G582DX, GAST, Benton Harbor, MI). The minor flow can range from.6 to 2 l/min, depending on the application and the desired enrichment. The minor flow then passes through a custom-built diffusion dryer, consisting of an inner tube of 1.1 cm in diameter and 15 cm in length, made of metal screen surrounded by freshly baked silica gel. The dryer removes the water from the droplets and returns the particles to their original size. The particle-enriched flow exits the dryer and is ready for sampling. The minor flow is generally provided by the normal intake flow of the chosen particle sampling instrumentation located downstream of the concentrator. The entire system, including pumps and chiller, weighs less than 3 kg and occupies a space less than 4 cm wide by 6 cm deep by 15 cm high.

5 11 M.D. Geller et al. / Aerosol Science 36 (25) Experimental methods The performance of the m-vaces system and its components were thoroughly tested using a variety of laboratory-generated aerosols. Several continuous and semi-continuous particle measurement instruments were used to measure aerosol characteristics before and after enrichment. Furthermore, the m-vaces system was deployed in the field for testing with ambient urban PM in conjunction with continuous and time-integrated monitors, as discussed in the following paragraphs. The virtual impactor, developed specifically for the m-vaces, was designed to operate at an intake flow of 3 l/min and minor flows from.6 to 1.5 l/min. It was tested independently of the m-vaces by generating fluorescent particles of various sizes by atomizing dilute suspensions of monodisperse fluorescent particles (Polyscience Inc., Warrington, PA) with a constant output HEART nebulizer (VORTRAN Medical Technology, Inc., Sacramento, CA) and collecting them on Teflon Filters (2 μm pore, PTFE, Pall Corp., East Hills, NY) for fluorescence analysis, similar to Sioutas, Kim, and Chang (1999). Filters on the minor flow and from upstream of the impactor were extracted in 3 ml of ethyl acetate and extracts were analyzed with a fluorescence spectrometer (Fluorescence Detector FD-5, GTI, Concord, MA) to determine the concentration efficiency as a function of particle size for a given set of intake and minor flows of the virtual impactor. The concentration enrichment was determined by comparing the amount of fluorescence on the minor flow filter of the impactor to that on the upstream filter sampled at a flow of 3 l/min. These experiments were conducted for fluorescent PSL particles ranging from.75 to 1 μm in physical diameter. The virtual impactor s intake flow rate was set at 3 l/min, while the minor flow was varied from.6 to 1.5 l/min. In addition to fluorescent particles, several other particle types were generated by atomization in order to test the m-vaces ability to concentrate particles of different chemical compositions. Mono-disperse polystyrene latex (PSL) particles (Polyscience Inc., Warrington, PA) of various sizes as well as polydisperse ammonium sulfate, ammonium nitrate, adipic acid, and glutaric acid particles were aerosolized with a nebulizer. PSL particles were selected because of their presumed monodispersity in order to demonstrate the preservation of physical aerosol properties during the concentration process. These particles are also hydrophobic and thus represent a worse case scenario for hygroscopic growth by supersaturation. Ammonium sulfate and nitrate were selected as test aerosols because they represent the two most predominant inorganic salts in PM 2.5 in almost any location of the U.S. (Malm, Schichtel, Pitchford, Ashbaugh, & Eldred, 24). Ammonium nitrate is also a labile semi-volatile aerosol species, dissociating to ammonia and nitric acid with its dissociation rate increasing exponentially with temperature (Chang, Sioutas, Kim, Gong, & Linn, 2; Mozurkewich, 1993). Adipic and glutaric acids are dicarboxylic acid found in ambient aerosol and were chosen to represent typical products of secondary aerosol formation by ozone photooxidation of organic gaseous precursors (Cruz & Pandis, 1999; Sempere & Kawamura, 1994). Particle growth in the concentrator was measured using an Aerosol Particle Sizer (APS 332, TSI Inc., Shoreview, MN) sampling indoor air between the virtual impactor and diffusion dryer (without diffusiondrying the aerosol). Although the APS flow rate of 5 l/min was higher than the operational minor flow, the APS was able to monitor the particle-containing droplet size distributions after enrichment. The droplet size distributions were measured as a function of saturator exit temperature. The concentration enrichment of laboratory-generated particles by the m-vaces was measured with several instruments sampling alternately upstream and downstream of the system. A Scanning Mobility Particle Sizer (SMPS 3936, TSI Inc.) coupled with a condensation particle counter (CPC 322A, TSI Inc.)

6 M.D. Geller et al. / Aerosol Science 36 (25) measured the particle size distributions and number concentrations of concentrated and unconcentrated laboratory aerosols in the size range of.1.3 μm in mobility diameter. A DataRAM nephelometer (DR-2, ThermoElectron Corp, Waltham, MA) measured the mass concentrations of pre- and postconcentration enriched monodisperse PSL aerosols in the size range of.1 to 1. μm, thereby extending the particle size range used in these tests beyond the maximum range of the SMPS. Following the laboratory experiments, the m-vaces was deployed inside of the Particle Instrumentation Unit (PIU) trailer of the Southern California Supersite in June of 24. The PIU is located in an urban/industrial area about 3 km south of downtown Los Angeles, California. The site is about 1 m downwind of a major freeway and represents a typical urban mix of particle sources. Similar testing using the SMPS, CPC, APS and DataRAM were performed using ambient PM as input to the m-vaces. Continuous measurements of ambient and concentrated black carbon were also performed using an Aethalometer (Model AE-21 (UV+BC), Thermo Andersen, Smyrna, GA). In addition, the PM 2.5 mass of ambient and concentrated aerosols were determined using time-integrated filter-based methods. A Teflon filter (47 mm, 2 μm pore, PTFE, Pall Corp., East Hills, NY) was placed in the minor flow of the m-vaces. A collocated Micro-Orifice Uniform Deposit Impactor (MOUDI model 11, MSP Corp, Shoreview, MN) was used as a reference sampler for ambient PM 2.5 mass. Only the after-filter stage of the MOUDI was used, preceded by the 2.5 μm impactor stage and with the remaining MOUDI stages removed. The m- VACES was connected downstream of a 2.5 μm cutpoint slit-nozzle impactor, the design and performance evaluation of which are described in detail by Kim et al. (21b). This impactor s acceleration nozzle is.2 cm wide and 5 cm long and was designed to have a 5% cutpoint of 2.5 μm at a flow rate of 11 l/min. In our experiments, part of the acceleration nozzle was masked to reduce its length to 1.36 cm, so that it matches the 3 l/min intake flow rate of the m-vaces. Particulate mass from the minor flow of the m-vaces was compared to the collocated MOUDI measurements. The 47 mm Teflon filters were weighed before and after each field test using a Mettler 5 Microbalance (MT 5, Mettler-Toledo Inc., Highstown, NJ), under controlled relative humidity (e.g. 4 45%) and temperature (e.g., C) conditions. Laboratory and field blanks were used for quality assurance. A total of six field experiments were conducted. Each run lasted for about 2 h, and enrichment factors for PM mass were calculated. 4. Results and discussion 4.1. Laboratory characterization of the virtual impactor The virtual impactor was characterized for an intake flow of 3 l/min and minor flows of 1.5, 1. and.6 l/min, respectively. Fig. 2 shows the efficiency of the virtual impactor, indicated by particle concentration enrichment as a function of aerodynamic particle diameter. Particle losses are less than 1% irrespective of particle diameter for minor flows of 1. and 1.5 l/min. When the minor flow is decreased to 2% of the total flow, particle losses near the cutpoint of the impactor increase (Marple & Chien, 198; Sioutas, Koutrakis, & Olson, 1994; Chen, Yeh, & Cheng, 1986). From Fig. 2, the 5% cutpoint of the virtual impactor is between 1.5 and 2.2 μm for minor flow ratios in the range of 2 5%. The sharpness of any impactor s collection efficiency curve can be described by its geometric standard deviation (σ g ), which is the ratio of the aerodynamic particle diameter corresponding to 84% collection efficiency to that of the 5% cutpoint (Marple & Willeke, 1976). Based on this definition, the values of σ g

7 112 M.D. Geller et al. / Aerosol Science 36 (25) Concentration Enrichment Aerodynamic Particle Diameter (µm) Minor flow = 1.5 l/min Minor flow = 1. l/min Minor flow =.6 l/min Fig. 2. Laboratory characterization of the virtual impactor using fluorescent particles. Total intake flow = 3 l/ min. for this virtual impactor are 1.8, 1.25 and 2.32 for minor flows (and minor flow ratios) of 1.5 l/min (.5), 1. l/min (.33) and.6 l/min (.2), respectively. Marple and Chien (198) found σ g to increase as minor-to-total flow ratio decreases. Our results corroborate the theoretical analysis of Marple and Chien, but they also indicate that when the minor flow decreases below a given ratio, particle losses are incurred to the point of reducing the overall concentration efficiency of the virtual impactor. For a minor flow of 1.5 l/min the concentration enrichment rises sharply from 5 to nearly 2 as aerodynamic particle diameter increases from 1 to 2.8 μm. The enrichment of particles between 2.8 and 1 μm is nearly ideal and independent of particle size. Similarly, the concentration enrichment increases from 5 to 3 as aerodynamic particle diameter increases from 1 to 2.8 μm when the minor flow was 1. l/min. Concentration enrichment was ideal for aerodynamic particles in the range of μm with this minor flow rate. With a minor flow of.6 l/min, the concentration enrichment rises from 5 to roughly 35 as aerodynamic particle diameter increases from 1 to 2.8 μm. At this very low minor-to-total flow ratio, particle losses likely prevent the virtual impactor from achieving ideal enrichment Laboratory evaluation of particle growth Figs. 3a e show various size distributions of indoor air as measured by the APS while the saturator temperature is varied. Each figure displays the size distribution after indoor aerosol passes through the saturator, condenser, and virtual impactor. When the saturator temperature is at indoor ambient levels (i.e., 21 C, Fig. 3a), the mode particle diameter is less than 1 μm and total number concentration is about 1 cm 3 (this number concentration corresponds to particle above 55 nm, as the APS cannot reliably measure smaller particles). Figs. 3b and c show the size distributions in transition as the saturator temperature rises (23 and 24 C), and the particles are activated and grown past the cutpoint of the virtual impactor. The transitional graphs show two modes, and as the smaller diameter particles grow into the second mode, the first mode begins to disappear. The number concentration is also increasing as particles

8 M.D. Geller et al. / Aerosol Science 36 (25) particles/cm particles/cm Particle/Droplet diameter (µm) Particle/Droplet diameter ( µ m) particles/cm particles/cm Particle/Droplet diameter ( µ m) Particle/Droplet diameter (µ m) particles/cm Particle/Droplet diameter (µm) Fig. 3. Temperature dependent growth of indoor aerosols in the m-vaces. (a) Saturator temperature = 21 C, (b) saturator temperature = 23 C, (c) saturator temperature=24 C, (d) saturator temperature=25 C and (e) saturator temperature=29 C. are grown and concentrated by the virtual impactor. By the time the saturator temperature is 25 C(Fig. 3d), the mode less than 1 μm has shifted to a mode around 4 μm and number counts are approximately 36 particles per cm 3. Fig. 3e demonstrates that saturator temperature can be increased to 29 C while preserving the grown aerosol s number concentration and mode. The poly-disperse distribution of final droplet sizes is the possible result of flow and temperature inhomogeneities in the condenser, causing the aerosol to be exposed to a range of supersaturation ratios resulting in a range of final sizes Laboratory evaluation of the m-vaces The m-vaces was operated with an optimum saturator temperature between 28 C and 29 C and an optimum condenser temperature of 1 C for all aerosol species tested. At a total flow of 3 l/min, the

9 114 M.D. Geller et al. / Aerosol Science 36 (25) incoming saturated aerosol was cooled by approximately 8 C to about 2 C. These temperatures were monitored over the course of each experiment. The diffusion-dried minor flow was drawn at rates of 1. and 1.5 l/min through an SMPS/CPC to measure particle size distributions. Results from these tests are shown in Figs. 4, 5 and Table 1. Figs. 4a e illustrate the concentration enrichment and the size distributions of indoor and the laboratorygenerated aerosols tested. Each graph has concentrated particle concentrations on the primary y-axis (left side) and pre-concentrated particle concentrations on the secondary y-axis (right side) plotted as a function of particle mobility diameter. The primary axis scale is larger than the secondary axis scale by a factor equal to the ideal enrichment of the mini-vaces at that flow configuration. Thus, overlap of the two resulting size distributions indicates close to predicted particle enrichment. It is important to note that the size distributions are essentially preserved after passing through the m-vaces in each case. The largest shift of mode particle diameter is roughly 1 nm. These findings are conclusive evidence that particle coagulation does not occur in the m-vaces, which would be manifested by imperfect enrichment (i.e. decrease in particle number) and a mode shift toward larger diameters. As we noted earlier, the different aerosol species used in our laboratory experiments were chosen to reflect a range of particle chemical compositions, volatility and hygroscopicity. Figs. 4a and b demonstrate the particle enrichment of indoor air and ammonium sulfate, respectively. These tests were conducted with a minor flow of 1. l/min. Indoor aerosols display a mode at 6 nm, while the size distribution for ammonium sulfate particles peaked at 45 nm. The overlapping of the ambient and concentrated size distributions in each graph indicates near-ideal concentration enrichment by a factor of out of a maximum 3. Figs. 4c e show pre- and post-concentrated size distributions for ammonium nitrate, adipic acid, and glutaric acid particles respectively. These three tests were carried out with a minor flow of 1.5 l/min and an expected maximum enrichment factor of 2. When the concentration-enriched particle concentration axis is 2 times that of the pre-concentration axis, the ambient and concentrated size distributions overlap, indicating no distortion of particle size distributions. The total number concentration enrichments for these three particle types are approaching predicted values, ranging from 19.2 to Because concentration enrichment is nearly identical for each species tested, it appears that particle volatility does not influence the degree to which labile aerosols such as ammonium nitrate are concentrated by this system, even though these aerosols are heated and saturated. This finding corroborates similar results reported previously by Sioutas et al. (1999) during their evaluation of a prototype ultrafine particle concentrator. In a more recent study, Khlystov et al. (24) investigated the effect of concentrating semi-volatile aerosols using the thermodynamically similar in design VACES and the Aerodyne Aerosol Mass Spectrometer (AMS) during measurements of ambient aerosol in Pittsburgh, PA as part of the Pittsburgh Supersite. Khlystov et al. observed small increases in the concentrations of nitrate at small sizes after passage through the VACES, corresponding to the region of the maximum aerosol surface area and fastest gas-to-particle transfer. The formation of extra material in the concentrator was attributed to redistribution of the gas-phase material to the aerosol phase, mostly under ammonia-limited conditions, while in an ammonia-rich environment it becomes negligible. The absolute increase in concentration of nitrate observed was rather small, of the order of 1 μg/m 3 or less (i.e.,.3 2.7% of the total PM 2.5 aerosol mass concentration after concentration by a factor of 1 in that study), thus negligible for many practical purposes. Together, the results of our current study as well as that by Khlystov et al. (24) indicate that labile species, such as ammonium nitrate, are preserved during the enrichment process without a substantial change.

10 M.D. Geller et al. / Aerosol Science 36 (25) Fig. 4. Size distributions of pre- and post-concentrated particles for (a) indoor air, minor flow=1.l/ min, (b) ammonium sulfate, minor flow = 1.l/ min, (c) ammonium nitrate, minor flow = 1.5l/ min, (d) adipic acid, minor flow = 1.5l/ min and (e) glutaric acid, minor flow = 1.5l/ min (total flow = 3 l/ min).

11 116 M.D. Geller et al. / Aerosol Science 36 (25) , 9 Concentrated 15, Pre-concentrated 75 Concentrated (particles/cm 3 ) 12, 9, 6, 3, Median (nm): 76.5 Mean (nm) : 87.7 Total Conc. (p/cm 3 ): 658,98 Median (nm): 79.1 Mean (nm) : 88.4 Total Conc. (p/cm 3 ): 35, Pre-concentrated (particles/cm 3 ) (a) Particle Mobility Diameter (nm) Concentrated (particles/cm 3 ) 24, 21, 18, 15, 12, 9, 6, 3, Concentrated Pre-concentrated Median (nm): 98.7 Mean (nm) : Median (nm): 13.4 Mean (nm) : Total Conc. (p/cm 3 ): 337,581 Total Conc. (p/cm 3 ): 8, Pre-concentrated (particles/cm 3 ) (b) Particle Mobility Diameter (nm) Fig. 5. Size distributions of pre- and post-concentrated PSL particles of nominal size (a) 8 nm, minor flow = 1.5l/ min and (b) 13 nm, minor flow = 1.l/ min (total flow = 3 l/ min). Table 1 Mass concentrations of PSL test aerosols before and after passing through the m-vaces using the DataRAM PSL nominal size (nm) Before m-vaces (μg/m 3 ) After m-vaces (μg/m 3 ) Enrichment factor Similar size distribution plots are presented in Figs. 5a and b for laboratory generated PSL particles with diameters of 8 and 13 nm. The 13 nm PSL particles were tested with a minor flow of 1. l/min (ideal concentration enrichment of 3), and the 8 nm PSL particles were tested with a minor flow of 1.5 l/min (ideal concentration enrichment of 2). The purpose of testing PSL particles is two-fold. The first is to

12 M.D. Geller et al. / Aerosol Science 36 (25) validate that non-hygroscopic particles can be grown via the supersaturation process to the same degree as hygroscopic ones. The second is to further demonstrate that varying the size distribution of a generated aerosol will have no effect on the concentration enrichment of the m-vaces. Generation of polydisperse aerosols by atomization tends to result in similar size distributions as described above. The generation of monodisperse PSL particles yields size distributions that vary in shape and mode diameter, and thus would more clearly indicate a shift or change in the size distribution that results from the concentration enrichment process. Fig. 5a shows the resulting size distributions from generated PSL particles labeled by the manufacturer as 8 nm in diameter. The generated aerosol had a measured mean of 88 nm and total concentration of 35,614 particles/cm 3. After passing through the m-vaces, the total particle concentration was 658,98 with no change in mean particle diameter. Thus, this aerosol was enriched by a factor of 18.5 (out of a maximum of 2). The largest diameter of PSL particles tested with the SMPS was 13 nm as labeled by the manufacturer (Fig. 5b). The minor flow rate for this test was set to 1. l/min, and the corresponding ideal enrichment of 3 was achieved. The total concentration before entering the m- VACES was 893 particles/cm 3, enriched to 337,581 particles/cm 3 upon exiting the system. The mean particle size was measured at about 118 nm for both pre- and post-concentrated aerosols. The multiple peaks in the size distributions of PSL particles, seen in Figs. 5a and b, are due to multiple charging in the SMPS. In addition to continuous number concentration data, continuous PM 2.5 mass concentrations before and after the concentrator were recorded with a DataRAM nephelometer. PSL particles of diameters ranging from 5 to 1 nm were drawn through the m-vaces system at a minor flow of 1.7 l/min. This is the lowest possible flow rate under which the DataRAM can operate, corresponding to a maximum enrichment of Table 1 presents the mass concentration enrichment of the PSL particles tested in the laboratory. The results in Table 1 illustrate that particle enrichment is fairly constant across all particle sizes tested with a possible slight increase in concentration enrichment as particle diameter increases. The DataRAM was also used to measure mass concentrations of pre- and post-concentrated ammonium sulfate, ammonium nitrate, glutaric acid and adipic acid aerosols. Fig. 6 shows that the mass based enrichment was close to predicted across all particle types (slope = 17.2, R 2 =.96) despite their smaller particle diameters (see Figs. 4b e) Field evaluation of the m-vaces Upon completion of the laboratory evaluation of the m-vaces, the system was moved to a field location near downtown Los Angeles. The ambient size distributions at this location typically display a number mode diameter smaller than 6 nm (Fine, Shen, & Sioutas, 24). Over the course of the field evaluation, over 5 ambient and concentrated size distributions were recorded and averaged in Fig. 7. A minor flow of 1 l/min and a total flow of 3 l/min were used. Both the median and mean particle diameters (52.4 and 69.9 nm, respectively) of the ambient aerosol change less than 4% after becoming concentrated by the m-vaces. The ambient number concentration is enriched from 8689 particles/cm 3 to 249,659 particles/cm 3, a factor of 28.7 out of a maximum of 3. All of these minor changes are well within the experimental error of our measurements and the variability from experiment-to-experiment in the ambient aerosol concentrations. These results further substantiate that particles are concentrated regardless of their mobility diameters, and the shape of the ambient size distribution is preserved after it is concentrated.

13 118 M.D. Geller et al. / Aerosol Science 36 (25) Enriched Mass Concentration (µg/m 3 ) y = 17.2x R 2 =.96 Amm. nitrate Amm. sulfate Adipic acid Glutaric acid Pre-concentrated Mass Concentration (µg/m 3 ) Fig. 6. Mass concentrations of laboratory-generated aerosols before and after the m-vaces as measured by the DataRAM. 75 Concentrated 25 Concentrated (particles/cm 3 ) Ambient Median (nm): 52.4 Mean (nm) : 69.9 GSD : 2.2 Total Conc. (p/cm ): 8, Ambient (particles/cm 3 ) Particle Mobility Diameter (nm) Fig. 7. Averaged ambient and concentrated outdoor aerosol size distributions at USC (total flow=3 l/ min, minor flow=1 l/ min). Fig. 8 displays the linear relationship between ambient and post-concentrated PM 2.5 mass concentrations as measured by the DataRAM (minor flow = 1.7l/ min). The regression line has a slope of 2.8 and a very small y-intercept of 4.61 μg/m 3. The data are very well correlated with an R 2 =.95. The average concentrated-to-ambient PM 2.5 mass ratio is 19.7(±3.3), thus slightly higher than the expected enrichment of 17.7, but the discrepancy may be attributed to variable ambient concentrations or possible measurement error due to non-linearity of the DataRAM response over the order of magnitude differences between ambient and concentrated aerosol. These results indicate that the m-vaces concentrates both number and mass of ambient particles. Furthermore, they show that larger particles are concentrated as efficiently as smaller particles because the former drive the mass concentration of an aerosol.

14 M.D. Geller et al. / Aerosol Science 36 (25) Concentrated PM 2.5 Concentration ( µ g/m 3 ) y = 2.82x R 2 = Ambient PM 2.5 Concentration ( µ g/m 3 ) Fig. 8. Ambient and concentrated outdoor PM mass concentrations at USC as measured by the DataRAM (total flow =3 l/ min, minor flow = 1.7l/ min). Black Carbon Concentration (µg/m 3 ) 1, concentrated 1, 1, ambient 1 12:15 12:3 12:45 13: 13:15 13:3 13:45 14: 14:15 14:3 14:45 15: 15:15 15:3 15:45 16: 16:15 16:3 16:45 17: 17:15 Time Fig. 9. Alternating ambient and concentrated PM 2.5 black carbon concentrations at USC. The concentration enrichment efficiency of the m-vaces based on black (elemental) carbon mass concentrations was also verified with an aethalometer. A representative sample of these results is presented in Fig. 9. The ambient PM 2.5 elemental carbon averaged around 1 ng/m 3 during this run.after passing through the m-vaces, the concentration averaged around 29, ng/m 3, which is very close to the expected enrichment factor of 3. The final field evaluation used filter-based time-integrated mass measurements comparing the m-vaces to a PM 2.5 MOUDI impactor sample of ambient air. The MOUDI was set up such that

15 12 M.D. Geller et al. / Aerosol Science 36 (25) Table 2 Ambient and concentrated PM 2.5 mass concentrations at USC based on Teflon filter weights Run number Ambient mass (μg/m 3 ) Concentrated mass (μg/m 3 ) Enrichment Average ± Std. Dev. 3.1 ± ± ± 3.7 Total flow = 3 l/ min, minor flow = 1.5l/ min, ideal enrichment = 2. the PM 2.5 stage was placed directly above the after filter, which allowed collection of total PM 2.5 on one 37 mm filter. Simultaneous collection of total PM 2.5 after the m-vaces was achieved by placing a 47 mm Teflon filter in line with the minor flow just after the diffusion dryer. Table 2 shows the results of these tests for gravimetrically determined mass concentrations. The enrichment factors range from 15 to 25, with an average and standard deviation of 19.1 ± 3.7 μg/m 3, very near the ideal factor of 2. The variation, and enrichment above ideal values, is most likely caused by uncertainties in the filter weights. 5. Summary and conclusions The results show that the m-vaces is capable of concentrating particles without significant alteration of their physical properties. For laboratory generated aerosols of different chemical compositions (ammonium nitrate, ammonium sulfate, adipic acid, glutaric acid, and PSL), number and mass concentrations were enriched by close to predicted factors and the shape of particle size distributions were not significantly altered. The m-vaces also was shown to concentrate both indoor and ambient urban aerosols approaching predicted enrichment while preserving the particle number size distributions. The system also concentrated black carbon by close to predicted ratios, showing that the m-vaces does not affect PM chemical composition for this important ambient aerosol component. The m-vaces represents an improvement over the pre-existing VACES system for applications requiring a lower minor flow rate. The newly designed saturator and cooling system allow for the more stable and unattended operation required by ambient PM sampling instrumentation. Future studies will further characterize the system with respect to ambient PM chemistry using both filter-based and water-based collection and analysis. Acknowledgements This work was supported by the Asthma Consortium, funded by the South Coast Air Quality Management District AQMD Contract #462. The authors would also like to thank Harish Phuleria and Manisha Singh for their valuable assistance in completing this study.

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