Performance Evaluation of an Andersen Cascade Impactor with an Additional Stage for Nanoparticle Sampling

This study describes the design and performance of an ambient air sampler consisting of an Andersen cascade impactor using inertial filter technology (ANIF) as a supplemental stage to separate nano-particles smaller than 70 nm. The design of the inertial filter resulted in an aerodynamic cutoff size of dp50 ~70 nm, a satisfactory sharpness in classification and a separation behavior comparable to that of a previously reported nanosampler (NS). The pressure drop at the backup filter in the sampler was ~30 kPa at a flow rate of 28.3 L/min. The ANIF has a number of advantages over currently available samplers, such as LPI and nano-MOUDI, such as reducing the loss of semi-volatile components in ultrafine particles by evaporation at reduced pressures, as well as having a smaller initial cost for the equipment for nanoparticle collection. Furthermore, the size distribution of the ambient particles measured with the ANIF compared favorably with those measured by conventional instruments that are currently available on the market.


INTRODUCTION
Fine particles with diameters of less than 2-3 μm, or PM 2.5 , in ambient air also frequently contain high levels of hazardous chemicals.This is particularly true for particles with diameters of less than 1 μm (Spurny, 1999;Mayland and Pui, 2007).Consequently, evaluating the chemical characteristics of these ultrafine nanoparticles is particularly important for an understanding of the impact to general health of airborne particulates that are small enough to enter the human respiratory system.Because a large proportion of nanoparticles penetrate the lung periphery, i.e., the alveolar region, particles deposited in the alveoli are readily transferred to the blood and are then quickly dispersed throughout the human body, including infants (Hinds, 1999;Bolch et al., 2001;Semmler-Behnke et al., 2012).Furthermore, at least one report on the effect that oral exposure to nanoparticles has on the human body, has determined that all kind of nanoparticle behavior in the environment could be issues that should be investigated (Mahler et al., 2012).
For an accurate evaluation of the health effect of airborne particulates, it is necessary to determine the chemical composition of particles with respect to particle size.This is because the deposition site of inhaled particles changes with particle size, and the clearance time of the deposited particles varies depending on the deposition sites, leading to differences in toxicity, even for the same composition of particles.This information is particularly important for nanoparticles (smaller than 100 nm in diameter) but reliable data is very scarce.
In order to conduct various quantitative chemical analyses of atmospheric particles, a relatively large mass of particles, possibly on the order of a mg, must be collected from atmospheric air by filtration.Although particles smaller than 0.1 μm, i.e., nanoparticles, account for a large proportion of the total population, their mass is very small.Therefore, all of the following nanoparticle samplers are designed to have a long sampling time required to collect a sufficient mass of atmospheric nanoparticles.
A number of samplers are available including lowpressure impactors (LPI) (Hering et al., 1978a, b;Kauppinen and Hillamo et al., 1989) and a nano-multi orifice uniform deposit impactor (nano-MOUDI II) (Fang et al., 1991;MSP, 2012).The latest impactor technology is a noble novel active personal nanoparticle sampler (PENS) with cutoff size of 100 nm at a flow rate of 2 L/min within a pressure drop of 14.1 kPa (Tsai et al., 2012).A differential mobility analyzer (DMA) (Knutdon and Whitby, 1975) may be used as a classifier followed by the collection of classified particles with a filter (Ono-Ogasawara et al., 2009).However, all of these devices have drawbacks, which include a small sampling rate, low charging efficiency for nanoparticles, the production of artifacts, and the loss of unstable chemicals by evaporation due to the large pressure drop (Hata et al., 2009a).
The authors (Otani et al., 2007;Eryu et al., 2009;Furuuchi et al., 2010a) developed an inertial filter to overcome these difficulties.This filter has significant advantages, such as a nanometer-size cutoff (d p50 ) diameter at a moderate pressure drop (< 20-30 kPa), as well as a sufficiently high sampling flow rate that permits the rapid collection of particles.Based on the results of lab and field tests by the authors (Furuuchi et al., 2009;Hata et al., 2009b), a sampler, which consisted of impactor stages of PM 10 /PM 2.5 /PM 1 /PM 0.5 followed by an inertial filter stage was developed (Furuuchi et al., 2010a) andcommercialized (KANOMAX, 2012).The authors also developed a personal sampler to evaluate exposure to nano-particles based on the inertial technology (Furuuchi et al., 2010b).The inertial filter technology may have possibilities beyond its application to the original Nanosampler (NS), such as devices that could be used to supplement existing samplers such as the Andersen cascade impactor and the high-volume air sampler.Although there might be a need for such supplemental devices, investigation into it has not been done.
In the present study, as a possible application of the inertial filter, an inertial filter stage with a cutoff size of 70 nm was designed to supplement the Andersen cascade impactor as the last stage before the backup filter.The collection efficiency curve and pressure drop were compared with those for the Nanosampler (Furuuchi et al., 2010a;KANOMAX, 2012) and the low pressure impactor (LPI).The Andersen cascade impactor using an inertial filter with a cutoff size of d p50 = 70 nm was used for the collection of ambient particles, and the obtained size distribution was compared with those from NSs and LPI.Sulfate and nitrate ions as well as retrieved particle-bound polycyclic aromatic hydrocarbons in nano-particles were also compared to discuss the influence of evaporation losses.

STRUCTURE OF THE DEVELOPPED SAMPLER
Fig. 1 shows a schematic drawing and picture of the Andersen cascade impactor using inertial filter technology (ANIF).The sampler consists of an 8-stage impactor from the Andersen cascade impactor (Tokyo dyrec AN-200), and a nozzle section that will smoothly correct the flow from the final impactor stage with a 0.43 μm cutoff size and then introduce the flow to the inertial filter stage.All stages are fixed by existing springs with extension plates.In order to uniformly collect particles on the backup filter located downstream from the inertial filter, the separation between the nozzle exit to the filter surface was adjusted to 48 mm.The sampler was designed to operate at a flow rate of 28.3 L/min, which is the same as the normal operating condition for the Andersen cascade impactor.The inertial filter was designed so that the webbed stainless steel fibers (Nippon Seisen Co. Ltd., felt type, SUS-304, 5.6 μm diameter (σ g = 1.1)) are packed on a support of crossed 200 μm stainless steel wires in a plastic holder (polyoxymethylene, POM) with a diameter of 4 mm and a length of 9 mm (Fig. 2).The holder was placed downstream from the nozzle section.The adoption of a filter holder helped facilitate the handling of samples and it can be easily replaced with a new holder onsite without directly touching the fibers.The specifications of the inertial filters are summarized in Table 1.

MEASURING THE PERFORMANCE OF THE INERTIAL FILTER
Fig. 3 shows the experimental setup used to test the performance of the inertial filters, which is the same as that used in a previous paper (Furuuchi et al., 2010a).Zinc chloride (ZnCl 2 ) particles, generated using an evaporationcondensation type aerosol generator were used as the test aerosol.Zinc chloride was heated by an infrared ray image furnace (ULVAC, RHL-E25P) at a temperature between 260-320°C, and was then cooled to produce particles.The monodispersed ZnCl 2 particles were classified using a DMA (TSI Inc., MODEL 3071), then diluted with clean air and introduced to the inertial filter at a flow rate of 28.3 L/min.The collection efficiency based on particle number was determined using an aerosol electrometer (TSI, Model 3068).The pressure drop through the inertial filter was measured by a digital manometer (Sokken, Model PE-33-A1).The mobility equivalent diameter measured by the DMA was converted to an aerodynamic diameter using the density of generated ZnCl 2 particles, which was determined using an Aerosol Particle Mass Analyzer (KANOMAX, APM 3600), or, by using a 1,870 kg/m 3 average for the sizes ranging from 50-100 nm.The number of particles larger than 0.3 μm was measured using an optical particle counter (Rion, KC-01).The pressure drop through the inertial filter was measured during the separation performance test.The total pressure drop from the ambient pressure to the sampler exit was also measured.

SAMPLING OF AMBIENT PARTICLES
In order to evaluate the performance of the ANIF that was developed for ambient particles, ambient aerosol particles were sampled using the ANIF simultaneously with two NSs with different inertial filters cutoff size and a lowpressure impactor (LPI, Tokyo Dylec, LP-20).The sampling conditions are summarized in Table 2. Ambient particles were collected on quartz fiber filters (Pallflex, 2500QAT-UP) conditioned at 20°C and 50% RH in a weighing chamber (Tokyo dyrec PWS-PM2.5) for 48 hours before and after the sampling.
The average blank value of the filters was 8.5 ± 5 pg/cm 2 for 2-3 ring PAHs and 5.5 ± 5 pg/cm 2 for 4-6 ring PAHs.These blank values were significantly less than the concentrations of each compound in all samples analyzed.

Separation Performance of the Sampler
In Fig. 4, the collection efficiency curves for the inertial filter are compared with NSs with inertial filters of d p50 showing 70 (Furuuchi et al., 2010a) and 100 nm (KANOMAX, 2011), impactor of MOUDI (MSP) and PENS (Tsai et al., 2012) respectively.The sharpness of the classification curve was evaluated, as shown in Fig. 4  (1) where d p84 and d p16 is the aerodynamic diameter at collection efficiencies of 84 and 16%, respectively.The designed cutoff size of the inertial filter for the ANIF was d p50 ~70 nm.The collection efficiency curve of the inertial filter for the ANIF was very similar to that of the NS (d p50 ~70 nm, σ = 1.6).The sharpness of the inertial filter was better than that of the commercial NS of KANOMAX (d p50 ~95 nm, σ = 1.9) and worse than that of either the PENS (d p50 ~100 nm, σ = 1.3) or the MOUDI (d p50 ~100 nm, σ = 1.2).Fig. 5 shows the collection efficiency curves for all stages of the ANIF, where the efficiency curves for impactor stages are from a literature (Vaughan, 2003).The pressure drop through the inertial filter for the ANIF was 22-25 kPa at a flow rate of 28.3 L/min, corresponding to the filtration velocity of 30 m/s.This is almost equal to that of the NS with a d p50 = 70 nm (Furuuchi et al., 2010a, ~25 kPa).The total pressure drop of ~30 kPa through the impactor stages, the nozzle stage, and through the inertial filter and a backup filter was much less than with either an LPI (70-80 kPa) or a nano-MOUDI (~60 kPa) with a 60-70 nm cutoff size.However, the pressure drop was even less with nanoparticle separators, which have a larger cutoff size, such as with a personal sampling device with a two-stage inertial filter (Furuuchi et al., 2010b, d p50 ~140 nm with 5.7 kPa) or with a PENS (Tsai et al., 2012, d p50 ~100 nm with 14.1 kPa.Although the performance of the normal pump for the Andersen cascade impactor was insufficient to operate the ANIF with a d p50 = 70 nm, it can be applied to one with the inertial filters with a cutoff size of d p50 = 100 nm at a total pressure drop of less than 25 kPa.Hence, users need not replace any facilities for nano-particle collection.

Ambient Particles
The cumulative size distributions on a mass basis obtained by the ANIF, the LPI and the NSs are compared in Fig. 6.In general, a good agreement existed in the cumulative fraction between the different samplers, but there was a slight difference in the mass fractions of the nanoparticles with sizes of less than 0.1 μm.This can be attributed to the loss of semi-volatile components, such as nitrates, by evaporation, due to the large pressure drop in the case of LPI.
The concentrations of nano-particles, the mass fractions of nano-particles to the total particle mass, and the concentrations of 4-6 ring PAHs and ions in the nanoparticles are compared in Fig. 7.The concentration and mass fractions of chemicals evaluated for samples from the ANIF and the NS were rather similar while those from the LPI were much less than the others.This difference was too large to be described only by the difference in the particle diameters.Hence, this may be attributed to the large pressure drop in the LPI, although evaporation loss of ions cannot describe the total difference in the particle mass.These results show that there is a clear advantage of the ANIF and NS for the evaluation of semi-volatile chemicals.

CONCLUSIONS
This paper describes the design and performance of an ambient air sampler that consists of an Andersen cascade impactor that uses inertial filter technology in a supplemental stage to separate nano-particles smaller than 70 nm.
The designed inertial filter had an aerodynamic cutoff size of d p50 ~70 nm with a classification sharpness σ = 1.6, which was similar to the reported inertial filter (Furuuchi et al., 2010a).The total pressure drop of the developed device (the ANIF) and its inertial filter with a cutoff size of d p50 ~70 nm from the sampler inlet to the backup filter was ~30 kPa at a flow rate of 28.3 L/min.This pressure drop allows the use of conventional pumps that are currently available on the market.A smaller total pressure drop of ~20 kPa of the ANIF may be available for a d p50 = 100 nm of the inertial filter, which would also allow the use of a normal pump for the Andersen sampler.As a result, the ANIF has advantages over currently available samplers such as the LPI and the nano-MOUDI -less loss of semi-volatile components in ultrafine particles by evaporation at reduced pressure and a reduced cost to prepare a sampler capable of testing nano-particles.
The size distributions of ambient particles measured with the ANIF compared favorably with those measured using conventional instruments that are currently available on the market, as well as with the performance of the NS and the LPI.When compared with LPI, the ANIF showed much less evaporation of semi-volatile components, which obviously allows for a more accurate evaluation of them.

Fig. 1 .
Fig. 1.Schematic diagram and picture of the Andersen sampler with an inertial filter (ANIF).

Table 1 .
Specifications of tested inertial filters.Experiment setups for the performance testing of the inertial filter.

Table 2 .
Period and condition for ambient particle sampling.