Particuology 9 (2011) 606 610 Contents lists available at SciVerse ScienceDirect Particuology j o ur nal homep ag e: www.elsevier.com/locate/partic Evaluation of particle growth systems for sampling and analysis of atmospheric fine particles Dae Seong Kim a, Sang Bum Hong b, Jung-Taek Kwon c, Kihong Park a, a Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology, Buk-gu, Gwangju 500-712, Republic of Korea b Korea Polar Research Institute, Yeonsu-gu, Incheon 406-840, Republic of Korea c Laboratory of Toxicology, College of Veterinary Medicine, Seoul National University, Seoul 151-741, Republic of Korea a r t i c l e i n f o Article history: Received 3 January 2011 Received in revised form 15 April 2011 Accepted 15 April 2011 Keywords: Particle growth chamber Condensational growth Particle sampler Hygroscopic particles a b s t r a c t Three types of water-based condensational growth systems, which can enable particles to grow in size to facilitate sampling and subsequent chemical analysis, were evaluated. The first one is a mixing type growth system where aerosols are mixed with saturated water vapor, the second one is a thermal diffusive growth system where warm flow enters cold-walled tube, and the third one is a laminar flow type where cold flow enters a warm wet-wall tube. Hygroscopic sodium chloride (NaCl), ammonium sulfate ((NH 4 ) 2 SO 4 ) and ammonium nitrate (NH 4 NO 3 ), and non-hygroscopic polystyrene latex (PSL) particles, in the size range of 50 400 nm, were used to determine their growth factors through the growth systems. Our data showed that the third-type growth system could enable particles to grow most efficiently regardless of their hygroscopic property. Collection efficiency of particles in the size range of 0.05 2.5 m, in a continuous aerosol sampler after they passed through the third-type growth system was about 100%, suggesting that the third-type growth system would be the most useful among the tested growth systems for sampling and subsequent chemical analysis of fine and ultrafine particles. 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. 1. Introduction Atmospheric particles, depending on their size and chemical composition, play not only an important role such as visibility degradation, heterogeneous chemistry, cloud formation and radiation balance of the atmosphere, but also lead to deposition in human alveolar region during inhalation. The effect of fine particles on human health depends not only on their chemical composition but also on the site in the human respiratory system where they are deposited (Hinds, 1982; Poschl, 2005; Ramana et al., 2010; Zhang et al., 2008). It is thus essential to determine both the size and chemical composition of atmospheric fine particles. A variety of continuous monitoring systems using ion chromatographic detectors have been developed to determine the chemical composition of atmospheric aerosols, typically consisting of a particle growth chamber, a collector, and a detector (Khlystov, Wyers, & Slanina, 1995; Simon & Dasgupta, 1995; Weber et al., 2001). Such continuous technique can provide short-term temporal variation of ionic species. Weber et al. (2003) reported the ionic concen- Corresponding author. Tel.: +82 62 970 3279; fax: +82 62 970 2434. E-mail address: kpark@gist.ac.kr (K. Park). trations of PM 2.5 (fine particles with aerodynamic diameter less than 2.5 m) using a continuous ion monitoring system in urban Atlanta, GA. Hong, Kim, Ryu, Kim, and Lee (2008) measured soluble inorganic species, including Cl, NO 3, SO 4 2, K + and NH 4 + in PM 2.5 with a continuous measurement system. For measurements of water soluble organic species, Sullivan et al. (2004) and Lu, Rashinkar, and Dasgupta (2010) developed a semi-continuous measurement system to determine their temporal variation. In these methods, efficient collection of small particles using a particle growth system is an important factor for continuous ion monitoring. The grown particles are collected typically by using impactors or impingers. It is well known that it is difficult to collect or sample aerosol particles with sizes ranging from 0.1 to 1.0 m in diameter (Shaw, Kuhlman, Lee, & Gieseke, 2003). In general, the collection-efficiency curve of typical aerosol samplers is U-shaped, with minimum collection efficiency for 0.3 m particles (Hinds, 1982). In this study, we designed three types of growth systems, and evaluated their performance by measuring particle growth factors of hygroscopic sodium chloride (NaCl), ammonium sulfate ((NH 4 ) 2 SO 4 ), and ammonium nitrate (NH 4 NO 3 ), and nonhygroscopic polystyrene latex (PSL) particles in the size range of 50 400 nm. Also, we added a continuous particle sampler to 1674-2001/$ see front matter 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.partic.2011.04.007
D.S. Kim et al. / Particuology 9 (2011) 606 610 607 Fig. 1. Schematics of particle growth systems tested in this study: (a) Type A, mixing type growth system where aerosols are mixed with saturated water vapor, (b) Type B, thermal diffusive growth system where warm flow enters a cold walled tube, and (c) Type C, laminar flow type where cold flow enters a warm wet-walled tube. determine collection efficiency of particles after passing through the particle growth system. 2. Experimental Three types of particle growth chambers were designed and fabricated to increase particle size by means of water-based condensational growth. Fig. 1 shows the schematic diagrams of the growth systems, (a) type A, (b) type B, and (c) type C with a continuous particle sampler, which are modified from the original designs of Hong, Kim, Ryu, and Lee, et al. (2008), Suh, Han, Kim, and Choi (2005), and Hering, Stolzenburg, Quant, Oberreit, and Keady (2005), respectively. All these particle growth systems used water vapor as the condensing species. Type A, which consists of a water vapor generation system, a mixing part, and a particle growth tube, which mixes, with maximal efficiency, aerosols with saturated water vapor (Hong, Kim, Ryu, & Lee, et al., 2008) so as to enlarge the size of aerosol particles by water vapor condensation. The mixing tube (15 mm ID and 120 mm in length) was made of Pyrex glass with a volume of 20 cm 3. The relative humidity, measured at the mixing tube with a RH sensor, Fig. 2. Experimental setup for evaluation of particle growth systems including optical particle counter (OPC), a differential mobility analyzer (DMA), and a condensation particle counter (CPC).
608 D.S. Kim et al. / Particuology 9 (2011) 606 610 Fig. 3. GFs of various laboratory-generated particles of different sizes at 90% RH in Type A growth system. was fixed at 90% RH by controlling the aerosol and wet air flow rates. It was difficult to obtain a stable supersaturated condition (RH > 100%) in this growth system due to water accumulation on the mixing tube wall. Type B is a thermal diffusive growth system where warm aerosol flows into a cold walled tube, consisting of a saturator (hot steam generation system) and a cooler. Particles were mixed with saturated water vapor and passed through a Peltier (thermoelectric) cooler, built to enlarge the size of aerosol particles (Park & Lee, 2000; Park, Lee, Shimada, & Okuyama, 2001), with a volume of 10 cm 3 (8 mm ID and 200 mm in length). The temperature of the saturator was 100 C and the temperature of the cooler was maintained at 30 C to create supersaturation. Type C is a warm wet-walled tube where the cold flow enters in laminar flow, into a cold conditioner and then a warm growth tube. Design of Type C was based on the growth system of Hering et al. (2005), in which, the conditioner serves to normalize the temperature (15 C) and relative humidity of the entering airflow. Particle activation and growth occur in the growth tube, which is heated to 50 C under a stable supersaturated condition. A single wick, composed of a hydrophilic porous media (8 mm ID and 200 mm in length), lines the inner walls of the conditioner and growth tube. It is constantly wetted by capillary action (Hering et al., 2005) by means of a small pump that injects water into the wick. A particle sampler, consisting of a wet scrubber and a gas-liquid separator, was integrated into the Type C growth system. The grown particles after the growth system was accelerated through a nozzle and impacted on to the flowing distilled water in the wet scrubber. As shown in Fig. 1(c), the liquid was then drained from the bottom of the air liquid separator, while air and water vapor went into the top of the air liquid separator. Polystyrene latex (PSL), sodium chloride (NaCl), ammonium sulfate ((NH 4 ) 2 SO 4 ), and ammonium nitrate (NH 4 NO 3 ) aerosols were used to evaluate the growth systems and the particle sampler. Fig. 4. Particle size distributions of (a) sodium chloride, (b) ammonium sulfate, (c) ammonium nitrate, and (d) PSL particles before and after the Type B growth system under supersaturated condition.
D.S. Kim et al. / Particuology 9 (2011) 606 610 609 PSL particles are considered standard non-hygroscopic particles in terms of size. Other particles, mostly hygroscopic, abound in ambient atmosphere. The size of the test aerosols ranged from 0.05 to 2.5 m in diameter. Fig. 2 shows an experimental setup for measuring the size distribution of particles before and after the particle growth system. The aerosols produced by an atomizer passed through a diffusion dryer, Po-210 charge neutralizer, and dilution chamber. These aerosols were then introduced into the particle growth system or bypassed the growth system, and were then sampled for measuring their size distribution. The volume of the reference chamber was similar to that of the particle growth chamber. The Dust Monitor (GRIMM Aerosol Technik, Model 1.109) which uses optical particle counter (OPC), and the scanning mobility particle sizer (SMPS, TSI Inc.) consisting of a differential mobility analyzer (DMA) and a condensation particle counter (CPC) were used to determine particle size distribution and to measure the collection efficiency of particles through the sampler. 3. Results and discussion The growth factor (GF), which is the ratio of size of particles at increased relative humidity (RH) to that at dry condition (RH 10%), was measured with the Type A growth system, and GFs of various laboratory-generated particles of different sizes at 90% RH were summarized in Fig. 3 with comparison with previously reported data. Park, Kim, and Park (2009) measured the hygroscopic GF of various ultrafine particles using the tandem differential mobility analyzer (TDMA) technique, showing that the GFs remained constant provided particle sizes were larger than 20 nm. In present study, the GFs of sodium chloride, ammonium sulfate and ammonium nitrate particles were about 2.0, 1.5 and 1.4, respectively, all similar to the results of Park, Kim, and Park (2009), and close to their values, theoretical and experimental, within 4 5%. In the case of PSL particles, however, the growth factor was about 1.0. The size of non-hygroscopic PSL particles did not increase under the given RH condition. We found it difficult to obtain a stable supersaturated condition (RH > 100%) in this growth system due to water accumulation on the mixing tube wall. Thus, Type A growth system is useful only for collecting hygroscopic particles. Fig. 4 shows the particle size distributions of sodium chloride, ammonium sulfate, ammonium nitrate, and PSL particles before and after the Type B growth system where a stable supersaturated condition was accomplished. Mode diameters (the diameter at highest concentration) of sodium chloride particles increased from 0.1 2.1 m to 0.4 2.4 m, as shown in Fig. 4(a). Fig. 4(b) and (c) shows that the mode diameters of ammonium sulfate and ammonium nitrate particles increased from 0.1 to 1.9 m and from 0.4 to 2.2 m, respectively. As shown in Fig. 4(d), the size distributions of the PSL particles before and after the Type B growth system remained the same. Apparently, the non-hygroscopic PSL particles require higher stable supersaturation than the current condition to make them grow, which was hard to obtain in the Type B growth system. Fig. 5 shows size distributions of sodium chloride, ammonium sulfate, and PSL particles before and after the Type C growth system under supersaturated condition. The diameter of the sodium chloride and ammonium sulfate particles increased, respectively, from 0.1 to 4.0 5.0 m and from 0.4 to 4.0 5.0 m. And, the diameters of PSL particles increased, respectively, from 0.1 to 2.5 3.0 m and from 0.4 to 2.5 3.0 m. As shown in Fig. 5, both non-hygroscopic and hygroscopic aerosols grew efficiently. In addition, the particle growth did not depend on particle size. Our data showed that the Type C among the tested growth systems under specified Fig. 5. Particle size distributions of (a) sodium chloride, (b) ammonium sulfate, and (c) PSL particles before and after the Type C growth system under supersaturated condition. conditions enabled particles to grow most efficiently regardless of hygroscopic property of the particles. We also evaluated particle collection efficiency by adding the continuous sampler at the exit of the Type C growth system (see Fig. 1(C)). Fig. 6 shows the measured particle collection efficiency (determined by measuring concentrations before and after the continuous sampler) as a function of particle size with and without the particle growth system. A part of particles, about 2 5%, was lost in the growth system. The current collection efficiency is based on comparison of concentrations before and after the continuous
610 D.S. Kim et al. / Particuology 9 (2011) 606 610 approach 100% for particle size in the range of 0.05 2.5 m. Type C, among the tested growth systems, is thus considered the most useful for sampling fine and ultrafine particles independent of their hygroscopicity, and for subsequent chemical analysis of the particles. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MEST) (NRF-2011-0015548) and by the Basic Research Project through a grant provided by GIST. References Fig. 6. Collection efficiency of particles as a function of particle size with and without the particle growth system. sampler downstream of the growth chamber or reference chamber (no growth). Sodium chloride, ammonium sulfate, and PSL particles were used as test aerosols for this experiment. Without the particle growth system, the collection efficiency looks like a U curve as a function of particle size, which is the common shape of collection-efficiency curves for most aerosol particles. The dominant particle collection mechanism depends on Brownian diffusion for ultrafine particles (<100 nm) and inertial impaction for large particles (>1 m) in wet scrubbers (Kim, Park, Song, Kim, & Lee, 2003). The minimum collection efficiency was observed around a particle size of 0.3 m where both mechanisms are weak. As shown in Fig. 6, with the particle growth system, the collection efficiency remains more or less constant, suggesting that further grown particles in the particle growth system are much easier to collect in the sampler due to their increased inertia. The collection efficiency was found to approach 100% through the whole particle size range, implying that the particle sampler used in the current study with a growth system (Type C) could be used efficiently for subsequent sampling and chemical analysis. 4. Conclusions Three types of growth systems (Type A: a mixing type growth system where aerosols are mixed with saturated water vapor, Type B: a thermal diffusive growth system where warm flow enters a cold-wall tube, and Type C: a laminar flow type where cold flow enters a warm wet-wall tube) were designed to increase particle size by means of water-based condensational growth, and were evaluated by measuring particle size distributions of hygroscopic sodium chloride (NaCl), ammonium sulfate ((NH 4 ) 2 SO 4 ), and ammonium nitrate (NH 4 NO 3 ), and non-hygroscopic polystyrene latex (PSL) particles in the size range of 50 400 nm by sampling before and after each growth system. 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