Continuous Collection of Soluble Atmospheric Particles with a

To determine the extent of breakthrough through the PC, a backup ...... Online measurements of water-soluble organic acids in the gas and aerosol phas...
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Anal. Chem. 2005, 77, 8031-8040

Continuous Collection of Soluble Atmospheric Particles with a Wetted Hydrophilic Filter Masaki Takeuchi, S. M. Rahmat Ullah, and Purnendu K. Dasgupta*

Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061 Donald R. Collins

Department of Atmospheric Science, Texas A&M University, TAMU 3250, College Station, Texas 77843-3150 Allen Williams

Illinois State Water Survey, 2204 Griffith Drive, Champaign, Illinois 61820-7495

Approximately one-third of the area (14-mm diameter of a 25-mm diameter) of a 5-µm uniform pore size polycarbonate filter is continuously wetted by a 0.25 mL/min water mist. The water forms a continuous thin film on the filter and percolates through it. The flowing water substantially reduces the effective pore size of the filter. At the operational air sampling flow rate of 1.5 standard liters per minute, such a particle collector (PC) efficiently captures particles down to very small size. As determined by fluorescein-tagged NaCl aerosol generated by a vibrating orifice aerosol generator, the capture efficiency was 97.7+% for particle aerodynamic diameters ranging from 0.28 to 3.88 µm. Further, 55.3 and 80.3% of 25- and 100-nm (NH4)2SO4 particles generated by size classification with a differential mobility analyzer were respectively collected by the device. The PC is integrally coupled with a liquid collection reservoir. The liquid effluent from the wetted filter collector, bearing the soluble components of the aerosol, can be continuously collected or periodically withdrawn. The latter strategy permits the use of a robust syringe pump for the purpose. Coupled with a PM2.5 cyclone inlet and a membrane-based parallel plate denuder at the front end and an ion chromatograph at the back end, the PC readily operated for at least 4-week periods without filter replacement or any other maintenance. Automated near-continuous measurement of the composition of atmospheric aerosol has been an area of active research interest for the past decade. At least one recent review is available.1 Nevertheless, filter-based collection and off-line extraction and analysis still constitute the dominant practice.2 Sampling on a filter over an extended period results in positive or negative artifacts * To whom correspondence should be addressed. E-mail: [email protected]. (1) Dasgupta, P. K.; Poruthoor, S. K. Automated Measurement of Atmospheric Particle Composition. In Sampling and Sample Preparation for Field and Laboratory; Pawliszyn, J., Ed.; Wilson and Wilson’s Comprehensive Analytical Chemistry Series XXXVII; Elsevier: Boston, MA, 2002; pp 161-276. (2) McMurry P. H. Atmos. Environ. 2000, 34, 1959-1999. 10.1021/ac051539o CCC: $30.25 Published on Web 11/03/2005

© 2005 American Chemical Society

due to gas-particle and particle-particle interactions.3-7 The artifacts attributable to gases can be prevented by incorporating a suitable diffusion denuder prior to the filter for removal of the gaseous components. However, it is difficult, if not impossible, to eliminate negative artifacts due to the volatilization of particles from the filter. For automated analysis, the major components of interest are generally soluble ions, and although mass spectrometry and other nonwet techniques are being extensively investigated at the present time, ion chromatography (IC) has become the analytical technique of choice for on-line analysis in much the same fashion that it has been the favored technique for off-line analysis for some time. The development of IC-coupled automated particle analysis is briefly traced below. The quantitative collection of submicrometer-size particles is difficult by impaction techniques. Having failed to solve this problem in a direct fashion, we adopted an approach of growing the particles by steam condensation prior to impaction in a cooled rectangular maze. The first actual application was to measure nitrogen (NH3/NH4+, HNO3/NO3-) deposition in an agricultural field in Switzerland,8 with detailed characterization of the approach published subsequently.9,10 Also about the same time, Khlystov (3) Tsai, C.-J.; Perng, S.-N. Atmos. Environ. 1998, 32, 1605-1613. (4) Eatough, D. J.; Obeidi, F.; Pang, Y.; Ding, Y.; Eatough, N. L.; Wilson, W. E. Atmos. Environ. 1999, 33, 2835-2844. (5) Weber, R.; Orsini, D.; Duan, Y.; Baumann, K.; Kiang, C. S.; Chameides, W.; Lee, Y. N.; Brechtel, F.; Klotz, P.; Jongejan, P.; ten Brink, H.; Boring, C. B.; Genfa, Z.; Dasgupta, P. K.; Hering, S.; Stolzenburg, M.; Dutcher, D. D.; Edgerton, E.; Hartsell, B.; Solomon, P.; Tanner, R. J. Geophys. Res. 2003, 108D, 8421. (6) Hitzenberger, R.; Berner, A.; Galambos, Z.; Maenhaut, W.; Cafmeyer, J.; Schwarz, J.; Mu ¨ ller, K.; Spindler, G.; Wieprecht, W.; Acker, K.; Hillamo, R.; Ma¨kela¨, T. Atmos. Environ. 2004, 38, 6467-6476. (7) Schaap, M.; Spindler, G.; Schulz, M.; Acker, K.; Maenhaut, W.; Berner, A.; Wieprecht, W.; Streit, N.; Mu ¨ ller, K.; Bru ¨ ggemann, E.; Chi, X.; Putaud, J.P.; Hitzenberger, R.; Puxbaum, H.; Baltensperger, U.; ten Brink, H. Atmos. Environ. 2004, 38, 6487-6496. (8) Blatter, A.; Neftel, A.; Dasgupta, P. K.; Simon, P. K. In Physico-Chemical Behavior of Atmospheric Pollutants; Angletti, G., Restelli, G., Eds.; Proc. 6th European Symposium, Report EUR 15609/2 EN, Luxembourg, 1994; pp 767-772. (9) Simon, P. K.; Dasgupta, P. K. Anal. Chem. 1995, 67, 71-78. (10) Simon, P. K.; Dasgupta, P. K. Environ. Sci. Technol. 1995, 29, 1534-1541.

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et al. described an approach to grow particles with steam, followed by collection using two serial cyclones,11 and Buhr et al. described a continuously wetted glass frit system for the collection and extraction of particles; in the latter case, however, maximum attainable sampling rate was limited and the average collection efficiency of 0.3-1-µm particles was 79 ( 8%.12 Direct impaction of particles on a liquid surface is the simplest approach for the wet analysis of particles. Karlsson et al. reported an IC-coupled single-stage impactor with a flowing liquid film as the impaction surface for continuous particle analysis. Unfortunately, even with a sampling flow rate of 10 L/min, the device could not achieve a 50% cutoff below 0.4 µm.13 Similarly, a packed-bed impactor proved not to be particularly efficient whether wetted or dry.14 Electrostatic collection with or without corona charging have also been attempted and found to be only partially effective.15,16 The original steam introduction approaches have meanwhile been adopted, modified, endowed with significantly faster response time capabilities, and coupled to inertial preconcentrators, both by the original authors and by others who have made significant new innovations.17-26 Nevertheless, we have continued to look for alternatives in this laboratory because steam generation is energy intensive and cumbersome overall. Moreover, it appears to promote artifact nitrate and especially nitrite formation by reaction of NOx, not removed by the denuders, with hot steam. It is possible that a mode of condensing water vapor on particles, different from the use of hot steam, can be attractive;27 whether it will be practical for collection of particles for analysis remains to be demonstrated. We have worked meanwhile on a glass-fiber filter-based instrument in which one filter is sampled while the other is washed and dried. The washings are subjected to preconcentration and chromatography. This was described for the automated collection and colorimetric measurement of aerosol Cr(VI).28 Subsequently, (11) Khlystov, A.; Wyers, G. P.; Slanina, J. Atmos. Environ. 1995, 29, 22292234. (12) Buhr, S. M.; Buhr, M. P.; Fehsenfeld, F. C.; Holloway, J. S.; Karst, U.; Norton, R. B.; Parrish, D. D.; Sievers, R. E. Atmos. Environ. 1995, 29, 2609-2624. (13) Karlsson, A.; Irgum, K.; Hansson, H.-C. J. Aerosol Sci. 1997, 28, 15391551. (14) Liu, S.; Dasgupta, P. K. Microchem. J. 1999, 62, 50-57. (15) Liu, S.; Dasgupta, P. K. Talanta 1996, 43, 1681-1688. (16) Liu, S.; Dasgupta, P. K. Anal. Chem. 1996, 68, 3638-3644. (17) Poruthoor, S. K.; Dasgupta, P. K.; Genfa, Z. Environ. Sci. Technol. 1998, 32, 1147-1152. (18) Poruthoor, S. K.; Dasgupta, P. K. Anal. Chim. Acta 1998, 361, 151-159. (19) Ito, K.; Chasteen, C. C.; Chung, H.-K.; Poruthoor, S. K.; Genfa, Z.; Dasgupta, P. K. Anal. Chem. 1998, 70, 2839-2847. (20) Slanina, J.; ten Brink H. M.; Otjes, R. P.; Even, A.; Jongejan, P.; Khlystov, A.; Waijers-Ijpelaan, A.; Hu, M.; Lu, Y. Atmos. Environ. 2001, 35, 23192330. (21) Zellweger, C.; Ammann, M.; Hofer, P.; Baltensperger, U. Atmos. Environ. 1999, 33, 1131-1140. (22) Lo ¨flund, M.; Kasper-Giebl, A.; Tscherwenka, W.; Schmid, M.; Giebl, H.; Hitzenberger, R.; Reischl, G.; Puxbaum, H. Atmos. Environ. 2001, 35, 28612869. (23) Weber, R. J.; Orsini, D.; Daun, Y.; Lee, Y.-N.; Klotz, P. J.; Brechtel, F. Aerosol Sci. Technol. 2001, 35, 718-727. (24) Kidwell, C. B.; Ondov, J. M. Aerosol Sci. Technol. 2001, 35, 596-601. (25) Sieru, B.; Stratmann, F.; Pelzing, M.; Neusu ¨ aˆ, C.; Hofmann, D.; Wilck, M. Aerosol Sci. 2003, 34, 225-242. (26) Orsini, D. A.; Sullivan, A.; Sierau, B.; Baumann, K.; Weber, R. J. Atmos. Environ. 2003, 37, 1243-1259. (27) Hering, S. V.; Stolzenburg, M. R.; Quant, F. R.; Keady, P. B.; Oberreit, D. A Fast-response, nanoparticle water-based condensation counter. AAAR Specialty conference, Atlanta, GA. February 2005; Abstract 8C-2. (28) Samanta, G.; Boring, C. B.; Dasgupta, P. K. Anal. Chem. 2001, 73, 20342040.

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we used the same approach for automated IC analysis of the filter extract.29 This system is virtually an exact analogue of manual denuder filter collection but one that is fully automated and provides good time resolution. There are no conceptual problems with this method. In practice, we found that the only filters which will withstand repeated washing and drying and will continue to collect small particles efficiently over a long period are glass- and quartz-fiber filters. One interesting positive attribute is that insoluble black carbon (BC) is readily retained by the filters, and it is easily feasible to perform incremental aethalometric measurements30,31 on the filters to measure BC. The significant negative attribute is that, even with one or more backup membrane or paper filters, fibers eventually leach from the fiber filters and penetrate downstream. This is not a problem with open tubular analysis systems as used in the colorimetric determination of Cr(VI) but creates a built-in maintenance issue with chromatographic systems due to frit or bed blockage and consequent pressure buildup. The requirement of hot dry purified air for the filter drying cycle can also be demanding in some field situations. Borrowing heavily on the original efforts of Cofer and Edahl,32,33 who used a similar device for gas collection, Al-Horr et al.34 more recently described a hydrophobic filter reflux/mist chamber PC that did not require the use of steam. It was initially assumed the particles are captured by the mist as well as grow in the supersaturated environment, just as the soluble gases were captured by the fine water mist in the original mist chamber. However, it was later realized that capture of the aerosol at the filter surface, possibly by some aspect of the water droplets/film that forms thereon, must be the dominant mechanism. Such a collector, mated to a parallel plate wet denuder (PPWD) sampling at 5 standard liters per minute (SLPM), shows quantitative collection of small particles and typically can be operated for 1-2 weeks before filter replacement is needed. The need for filter replacement arises from one of two reasons: (a) insoluble particles, accumulated on/in the pores of the filter, cause increased pressure drop; (b) sampling of (naturally present biogenic) surfactants lead to gradual loss of filter hydrophobicity. Both lead to liquid water breakthrough through the filter over time. Across the spectrum of analytical instrumentation, efforts are presently underway to make them smaller, while maintaining at least the same performance characteristics. Recently, we reported a membrane-based parallel plate denuder (MPPD), much smaller relative to the 5 L/min PPWD, which can effectively operate up to a sampling rate of 1.7 L/min.35 Because the denuder liquid is contained behind a wettable membrane, it can remain stationary in the denuder during a sampling cycle and only withdrawn periodically (thereby supplying fresh absorber) for analysis. The (29) Boring, C. B.; Al-Horr, R.; Genfa, Z.; Dasgupta, P. K. Anal. Chem. 2002, 74, 1256-1268. (30) Hansen, A. D. A.; Rosen, H.; Novakov, T. Appl. Opt. 1982, 21, 3060-3062. (31) Hansen, A. D. A.; Rosen, H.; Novakov, T. Sci. Total. Environ. 1984, 36, 191-196. (32) Cofer, W. R.; Collins, V. G.; Talbot, R. W. Environ. Sci. Technol. 1985, 19, 557-560. (33) Cofer, W. R.; Edahl, R. A. Environ. Sci. Technol. 1986, 20, 979-984. (34) Al.-Horr, R.; Samanta, G.; Dasgupta, P. K. Environ. Sci. Technol. 2003, 37, 5711-5720. (35) Takeuchi, M.; Li, J.; Morris, K. J.; Dasgupta, P. K. Anal. Chem. 2004, 76, 1204-1210.

Figure 1. Particle collector based on hydrophilic filters, schematically shown.

necessary liquid handling can now be carried out by a (multitasking) syringe pump. We report here a hydrophilic filter-based PC that operates over a long period without replacement and also ideally matches the MPPD. Configurationally, the geometry is exactly the reverse of the hydrophobic filter PC;34 the PC sits below the denuder, the water mist percolates through a hydrophilic filter with a nominal pore size much larger than the particles it collects. The PC is so designed that the percolated particle extract need not necessarily be continuously pumped out: rather, it accumulates in a reservoir from where it can be withdrawn periodically for analysis, again permitting the use of a (multitasking) syringe pump. EXPERIMENTAL SECTION Construction and Assembly of the Particle Collector. More than 10 designs of hydrophilic filter-based PCs were evaluated. The final design, which we fully characterized, is shown schematically in Figure 1. The PC consists of an air/liquid nozzle, a mist chamber, and an air/liquid separator, which are all machined from poly(methyl methacrylate) (Plexiglas) cylinders. Sample airflow passes through a stainless steel inlet tube (6.4-mm o.d., 4.3-mm i.d., 60 mm long), which is directly connected to the end of the denuder, terminates in a tapered (75°) nozzle (0.76-mm terminal orifice), and enters the mist chamber. Immediately on the outer edge of the nozzle is a stainless steel tube with a fine capillary tip

that conforms to the outer taper of the nozzle. Water can be made to flow under pneumatic pressure, gravity, or actively pumped peristaltically through this capillary tip to form a fine mist, aided by the accelerated airflow from the nozzle. To avoid excessive pressure drop in the water delivery tip, it is composed of two sections; the very tip is composed of a short section (∼3 mm) of a 200 µm × 410 µm (i.d. × o.d., R-HTX-27) that is inserted and cemented at the upstream end in to a larger tube 510 µm × 810 µm (i.d. × o.d., R-HTX-21, type 304 stainless steel, Small Parts Inc., Miami Lakes, FL). The mist is directed to wet the combination of a paper filter (top, 180-µm thickness, Whatman type 1) and a hydrophilic membrane filter (bottom, Isopore polycarbonate filter, 5-µm pores, 5-20% porosity, 10-µm thickness, Millipore Corp.), both 25 mm in diameter. The mist chamber and the air/ liquid separator below it are so machined so that the filter pair is retained taut between them in the form of a shallow-rimmed dish with a 14-mm exposed diameter (154-mm2 active area). This active area is continuously washed with the fine mist. Any filter support is deliberately not used to minimize liquid holdup time. A pair of O-rings (11.1-mm i.d., 14.2-mm o.d., ORB-013, Small Parts Inc.) sits in machined grooves on the air/liquid nozzle exterior and seals it to the mist chamber. The O-ring seals permit facile vertical adjustment of the distance (22-37 mm) between the nozzle and the filter pair that effectively defines the size of the mist chamber. It is important to optimize this distance to (a) keep all of the filter area wet without excessive wall wetting (which would increase response time) and (b) allow sufficient residence time in the mist chamber to allow the aerosol to grow in the supersaturated environment. In the chamber, aerosol particles grow and are captured by the wetted filter pair, and the soluble extract is immediately aspirated through the filter pair. The presence of the paper filter helps uniform wetting and water distribution on the membrane filter. In the air/liquid separator, the water drops are inertially separated from the airflow and accumulate in the sample storage reservoir (capacity of the present design is 15 mL but can of course be designed to have any desired capacity) until the liquid is pumped out. Particle Generation. Experiments were conducted with two separate techniques to generate particles in two different size ranges. In the first system, a constant output atomizer (model 3076, TSI Inc., Shoreview, MN) utilized 11 mM (NH4)2SO4 as the nebulizer solution to generate aerosol that was used with a classifier to generate particles e100 nm in size. The atomizer was operated with an airflow rate of 3.0 L/min at 35 psi resulting in a liquid consumption rate of ∼0.1 mL/min. A Nafion dryer (PD070-18T-24SS, Permapure LLC, Toms River, NJ) was used to adjust the relative humidity to 35%. The (NH4)2SO4 aerosol from the atomizer was classified by a high-flow differential mobility analyzer (Aerosol Dynamics, Inc.)36 In the second system, monodisperse aerosols in the 0.283.88-µm aerodynamic diameter range were generated with a vibrating orifice aerosol generator (VOAG, model 3450, TSI Inc.) using 0.5 mg/L fluorescein solution in ultrapure water as the feed without and with 0.2, 2, 5, and 10 mM NaCl added to the feed. The VOAG was operated with a 10-µm-diameter orifice, 80 µL/ min liquid flow rate, 160-kHz drive frequency, 1.5 SLPM primary (36) Stolzenburg, M.; Kreisberg, N.; Hering, S. Aerosol Sci. Technol. 1998, 29, 402-418.

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airflow, and 35.0 SLPM dilution airflow. Aerosol-free “zero” air was generated by a pure air generator (model 737-14, AADCO, Clearwater, FL). The generated primary aerosol was diluted and dried with the zero air. The aerosol then passed through a Kr-85 neutralizer (model 3054, TSI Inc.) to allow the aerosol to attain equilibrium Boltzmann charge and passed through an acrylic column (12-cm i.d., 170 cm tall) for drying to be completed. The RH of this stream (22 °C) remained consistently at 97.5% when using a sampling flow rate of 1 SLPM with a liquid flow rate of 0.25 mL/ min. In fact, the combination filter pair performed so well that this essentially quantitative collection efficiency for the same particles was also observed with 5-µm pore size membrane filters and much smaller nozzle-filter distances. In actual ambient sampling, the 5-µm filters lasted much longer than the 2-µm filters (before pressure drop increased to a point where filter replacement was needed). We chose therefore the combination of a paper filter with the 5-µm pore size hydrophilic membrane filter for field use. Table 1 shows the particle collection efficiency and various losses for the PC depicted in Figure 1 operated at a sampling rate of 1.5

Table 1. Particle Collection Efficiency and Lossa (n ) 3) spherical equiv diam, µm

aerodynamic diameter, µm

Se, %

0.20 ( 0.01 0.42 ( 0.01 1.32 ( 0.05 2.06 ( 0.12 2.90 ( 0.10

0.025 0.10 0.28 ( 0.01 0.59 ( 0.01 1.81 ( 0.07 2.79 ( 0.17 3.88 ( 0.13

55.3 ( 5.3 80.3 ( 3.5 97.7 ( 0.2 98.1 ( 0.8 98.1 ( 0.7 98.3 ( 0.3 98.5 ( 0.4

0.22 ( 0.01

0.31 ( 0.02

After 1-Week Operation of PC 99.0 ( 0.3 99.4 ( 0.3

PCe, %

S L, %

DL, %

N L, %

99.1 ( 0.6 99.2 ( 0.1 98.9 ( 0.1 99.0 ( 0.2 98.9 ( 0.4

21.0 ( 11.9 6.9 ( 3.2 1.4 ( 0.4 1.1 ( 0.7 0.8 ( 0.6 0.6 ( 0.4 0.4 ( 0.0

0.2 ( 0.1 0.1 ( 0.2 0.1 ( 0.1 0.2 ( 0.0 0.1 ( 0.0

1.2 ( 0.4 1.0 ( 0.6 0.7 ( 0.6 0.5 ( 0.4 0.2 ( 0.0

0.3 ( 0.1

0.2 ( 0.1

0.1 ( 0.1

a PC with the filter paper and 5-µm hydrophilic filter operates at the sampling flow rate of 1.5 SLPM with the liquid flow rate of 0.25 mL/min. Se, particle collection efficiency of the overall system; PCe, particle collection efficiency of the PC itself; SL () DL + NL) system particle loss; DL, particle loss in the denuder liquid; NL, particle loss on the nozzle of PC and inlet tube of denuder.

SLPM and a liquid flow rate of 0.25 mL/min. The system particle collection efficiency, Se, was g97.7% for particle aerodynamic diameters that ranged from 0.28 to 3.88 µm and actually improved slightly after 1 week of sampling of ambient air. The collection efficiency of the PC itself, PCe, was g98.9% and no major difference was observed between particle sizes. The system particle loss, SL, increased with decreasing particle size, which was mainly caused by the nozzle/tube loss, NL, likely due to the greater diffusivity of small particles. The denuder loss, DL, was e0.2%, which means the particles do not affect the measurement of gases concentration with the denuder. For the very small particles (e100 nm), generally the measurement uncertainty was higher and depended on the constancy of the output stability of the aerosol generator. As may be expected, the collection efficiency (as measured with the particle number-counting technique), did significantly decrease to 55.3 ( 5.3 and 80.3 ( 3.5% for 25- and 100-nm particles, respectively. However, considering that these are efficiencies represented by a 5-µm pore size filter, this is still quite remarkable. Effective Reduction of Pore Size. It is of course well known that a uniform pore size membrane filter will not substantially capture particles that are significantly smaller than the filter pores. The use of a 5-µm pore size filter in the present case to capture submicrometer particles with good efficiency is possible for several reasons: (a) the particles grow rapidly in the supersaturated environment, and equally importantly, (b) the effective pore size of the filter decreases because water flows through the pores. We measured the pressure drop across the individual components (nozzle, paper filter, membrane filter, etc.) under dry and wetted conditions so that the pressure drop across the membrane filter could be individually assessed. We first used ∆P measured for the membrane filter with dry air and d ) 5 × 10-4 cm to calculate β via eq 12 to be 0.099, in agreement with the manufacturer’s stated range of 0.05-0.20.42 As eq 12 shows, at a given value of ∆P, d has a relatively weak dependence on β, being proportional to β-1/2. In the following, we have therefore calculated pore diameters using β ) 0.125, which is at the center of the manufacturer’s stated range of β. The values of ν and F used were for water-saturated air at the pressure that is the best estimate for the air as it transits through the filter. This corresponds to the laboratory ambient pressure minus the pressure drop across the nozzle and the paper filter minus half the pressure drop across the membrane filter. For example, for 1.5 SLPM airflow and a

Figure 5. Effective pore size of a 5-µm pore size hydrophilic filter calculated from the pressure drop during wetted operation as a function of the water flow rate. Airflow rate 1.5 SLPM.

water flow rate of 0.25 mL/min (∆Pmembrane ) 262.2 mmHg; ν and F, 2.10 × 10-5 m2/s and 0.827 kg/m3 at 533 mmHg absolute pressure and 22 °C; l, 10-5 m;42 Uin, 0.201 m/s at 664 mmHg absolute pressure), the effective pore size of the filter was computed to be 0.51 µm. It is to be noted that water flow through the filter itself causes a negligible pressure drop. A water flow rate of 0.25 mL/min across an active area 14 mm in diameter corresponds to a flow rate of 0.16 mL cm-2 min-1, whereas the manufacturer specifies that up to 2000 mL cm-2 min-1 water can be aspirated through this filter. We measured the pressure drop across the filter (14-mm active diameter) at low water flow rates in the absence of airflow. In the range of 0.0-4.0 mL/min, the following equation was obeyed:

pressure drop (mmHg) ) 0.69 × flow rate (mL/min) + 0.03,

r2 ) 0.9987 (14)

From this, for a flow of 0.25 mL/min water, the pressure drop is calculated to be ∼0.2 mmHg, negligible compared to the observed pressure drop. The change in the calculated effective pore size with increasing water flow rate is shown in Figure 5. Much of the pore area is thus actually occupied by water, and the effective pore size is much smaller than the nominal pore size in actual operation. Analytical Chemistry, Vol. 77, No. 24, December 15, 2005

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Figure 6. Chromatogram of blank air after operating (a) 2, (b) 5, and (c) 10 h. The chromatograms are vertically offset for clarity. Chromatogram d is that of an aqueous standard equivalent to 0.22 µg/m3 each of (1) chloride, (2) nitrite, (4) nitrate, and (5) sulfate. Peak 3 is due to carbon dioxide/carbonate, not deliberately added.

Analyte Response Time. The 10-90% rise time and 90-10% fall time of fluorescence signal were 3.0 ( 1.0 and 4.7 ( 0.7 min (n ) 4 each); these times are acceptable for the present application and are less than the corresponding values reported for the hydrophobic filter/mist chamber based PC (5.5 and 6.8 min, respectively).34 The holdup time on the chamber walls did not appear to contribute significantly to the observed response time, the value calculated from eq 13 was 1.1 ( 0.9 min. Accordingly, unlike what was found for the substantially larger hydrophobic filter/mist chamber based PC, treating the chamber walls with a hydrophobic agent (Rain-X, Sopus Products, Houston, TX) seemed to have no significant effect on the response time. Field Maintenance. Generally commercially available filters contain significant levels of ions, especially chloride and sulfate. It is necessary to reduce those levels to background/negligible values before reliable data can be obtained. Filters can be prewashed before placement in the PC, but typically, it is difficult to keep them from getting recontaminated if prewashed filters are stored. A significant wash period for the filters in the instrument may be unavoidable. For these reasons, it is desirable to reduce filter replacement frequency and the period needed to wash the filters before reliable data can be obtained to minimize dead time for real data. The blanks of both membrane and paper filters were evaluated. A new unwashed filter pair was placed in the PC. House air, purified through silica gel, activated charcoal, and soda lime, was sampled through the PC at 1.5 SLPM and house deionized water, freshly passed through a mixed-bed deionizer minicolumn flowed into the PC at 0.25 mL/min. The PC effluent was continuously pumped into a preconcentration column by a peristaltic pump, and each anion concentration was detected with an IC system at every 15 min. Figure 6 shows the chromatogram of a standard solution equivalent to that contained in 15 min of sampled air that contained 200 ng/m3 chloride, nitrite, nitrate, and sulfate and the blank air sample response after 2, 5, and 10 h of continuously washing the filters while continuously operating the PC. Note that peak 3 arises from dissolved CO2/ 8038 Analytical Chemistry, Vol. 77, No. 24, December 15, 2005

Figure 7. Temporal variations of nitrate and sulfate concentrations in PM2.5 at a rural field site in Bondville, IL.

carbonate; this was not deliberately added to the standard and is of course present in any air sample (it is not completely removed by soda lime columns)sthe CO2 response can be removed by a carbon dioxide removal device43 incorporated before the detector. (Such devices are now commercially available but were not used in the present work as the CO2 peak did not interfere in the quantitation of nitrate.) Note also that the first two (unnumbered and unidentified) peaks in chromatograms a-c are known to arise from leachates from the peristaltic pump tubing. These results show that the filter blanks become acceptable (contributes