A Continuous Monitoring System for Strong Acidity in Aerosols

May 27, 1998 - Inhalation of acid aerosols is believed to be a major source of respiratory ailments. A fully automated instrument that can measure aci...
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Anal. Chem. 1998, 70, 2839-2847

A Continuous Monitoring System for Strong Acidity in Aerosols Kazuaki Ito,† Coleman C. Chasteen, Hyung-Keun Chung,‡ Simon K. Poruthoor, Zhang Genfa, and Purnendu K. Dasgupta*

Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061

Inhalation of acid aerosols is believed to be a major source of respiratory ailments. A fully automated instrument that can measure acidity levels meaningful to real situations is described. A 5-10 L/min sampling rate is used. First, coarse particles are removed by a cyclone, followed by the removal of acid and basic gases with a parallel plate wet denuder. Aerosols are then collected with a vapor condensation aerosol collection system (VCACS). The VCACS provides an aqueous liquid effluent in which all soluble constituents are present in the dissolved form and the undissolved material remains suspended. The effluent is concentrated sequentially on a cation exchanger and an anion exchanger, which respectively constitute the injection loops of cation and anion analysis subsystems. In the first system, non-H+-cations (primarily NH4+) are conductometrically determined as the corresponding hydroxide by elution with a strong acid plug and conductivity suppression with a hydroxide-form anion exchanger. In the second system, total (strong acid) anions are conductometrically determined by elution with a carbonate/hydroxide-based eluent and continuous suppression by a Nafion-based cation exchanger fiber suppressor. Aerosol strong acidity is determined on the basis of charge balance: H+ equivalents present ) ∑anion equivalents - ∑non-H+ cation equivalents. The system is operated continuously, typically sampling for 5-8 min at 5 L/min with an 8-10-min cycle providing a limit of detection (LOD) of 7-38 nmol m-3 in that sampling cycle. For a greater sample volume, e.g., 300 L, the LOD is 0.6-3.2 nmol m3, depending on the amount of total neutral salts concurrently present. No significant interference from acidic or basic gases has been found. The instrument was successfully used to measure acidity arising from kerosenefueled indoor space heaters and the degree of neutralization of acidity in deliberate human exposure experiments by expired ammonia. The interest in acidic aerosols is now more than a decade old; health effects of acid aerosols are of specific interest to us. While † Permanent address: Department of Environmental Science, Faculty of Engineering, Institute for Wastewater Treatment, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima 724, Japan. ‡ Permanent address: Department of Environmental Science, College of Health Science, Yonsei University, 234 Maeji-li, Hungob-myon, Wonju-gun, Kangwon do 222-701, Republic of Korea.

S0003-2700(98)00135-8 CCC: $15.00 Published on Web 05/27/1998

© 1998 American Chemical Society

the upper respiratory tract (the moist walls behaving as a “wet diffusion denuder”) efficiently removes soluble gases such as SO2, inhalable particles (aerodynamic diameter ∼0.1-2 µm) can penetrate to the deep lung and be deposited in the alveolar region with high efficiency. Based on mortality data for hundreds of thousands of adults, Pope et al.1 concluded that particulate air pollution was associated with cardiopulmonary and lung cancer mortality and is related to sulfate and fine particles in the air. The results of laboratory toxicologic studies are not always conclusive. While exposure to acid aerosol alone has generally not shown deleterious effects in either laboratory animals2 or healthy/ asthmatic volunteers3,4 even at acid levels much higher than that encountered in the ambient atmosphere, some studies conducted with much lower, “high ambient” levels of acid aerosols show evidence of bronchoconstriction occurring in young asthmatics.5 The reliability of controlled animal/human exposures at low levels can also be a concern because of neutralization of acid aerosols by expired ammonia.6 Ambient acid aerosols, on the other hand, may be protected by an organic coating that will make such neutralization more difficult.7 Inhalation of acid aerosols weakens the immune system.8 Laboratory studies suggest that ozone and acid aerosols synergistically damage the lung.9 A mechanism has already been proposed for acid aerosol-mediated oxidant damage to plant tissues.10 Most recent evidence suggests that acid aerosol of very small size is particularly effective in causing such synergism.11 Epidemiological studies also indicate that human respiratory health is intimately linked to aerosol acidity.12 Other (1) Pope, C. A., III; Thun, M. J.; Namboodiri, M. M.; Dockery, D. W.; Evans, J. S.; Speizer, F. E.; Heath, C. E., Jr. Am. J. Respir. Crit. Care Med. 1995, 151, 669. (2) Kobayashi, T.; Shinozaki, Y. J. Toxicol. Environ. Health 1993, 39, 261. (3) Frampton, M. W.; Voter, K. Z.; Morrow, P. E. Am. Rev. Respir. Dis. 1992, 146. (4) Jorres, R.; Magnussen, H. J. Aerosol Med. 1992, 5, 103. (5) Avol, E. L.; Linn, W. S.; Shamoo, D. A. Am. Rev. Respir. Dis. 1990, 142, 343. (6) Clark, K. W.; Anderson, K. R.; Linn, W. S.; Gong, H., Jr. J. Air Waste Manage. Assoc. 1995, 45, 923. (7) Middlebrook, A. M.; Thomson, D. S.; Murphy, D. M. Aerosol Sci. Technol. 1997, 27, 293. (8) Schlesinger, R. B. The effects of inhaled acid aerosols on lung defenses. In Aerosols: Research, Risk, Assessment and Control Strategies; Lee, S. D., Schneider, T., Grant, L. D., Verkerk, J. V., Eds.; Lewis: Ann Arbor, MI, 1986; pp 617-635. (9) Schlesinger, R. B.; Gorczynski, J. E.; Dennison, J. Exp. Lung Res. 1992, 18, 505. (10) Hellpointner, E.; Ga¨b, S. Nature 1989, 337, 631. (11) Kimmel, T. A.; Chen, L.-C.; Nadziejko, C. Toxicol. Appl. Pharmacol. 1997, 144, 348.

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studies have suggested increased acid aerosol exposure to result in decreased pulmonary function and increased hospital admissions for asthma and other respiratory ailments.13,14 Regulatory Standards for Aerosol Acidity? A compositionspecific regulatory standard has been suggested for atmospheric aerosols, especially in view of aerosol acidity.15 Nevertheless, until extensive data pertaining to the extent to which strong acidity is present in the ambient aerosols, lengthy debates on regulatory issues are likely to continue.16,17 The process of information gathering regarding atmospheric acidity has been underway for some time.18-23 The metropolitan acid aerosol characterization study, a four-city, five-year investigation sponsored by the EPA, is a particularly ambitious example.24,25 It has been pointed out that personal exposure to aerosol acidity can be quite different between the outdoor and the indoor environment.21,22 In regard to atmospheric acidity, indoor air can be significantly affected by the use of fossil fuel-powered, open-flame space heaters. The production of gaseous HONO from natural gas-fired heaters is well known;26,27 the production of gaseous SO2 and aerosol H2SO4 from S-containing fossil fuels is obviously likely. Current Analytical Approaches. In aerosol acidity measurement, it is vital that acidic/basic gases are first removed without removing particles. This is accomplished with diffusion denuders; design of such denuders has been described in the literature.28,29 Because acidity in the ambient aerosol is essentially confined to the fine particle fraction, it is also important to remove larger particles (that tend to be crustal and alkaline) before measuring acidity;30 this is accomplished with a cyclone. The aerosol itself is collected on an inert (e.g., poly(tetrafluoroethylene), PTFE) filter and extracted with a dilute strong acid (e.g., 10-4 M HClO4 or H2SO4, to avoid contributions of CO2 or protolyzable heavy metal ions), and the pH of the extract is measured.19,21 This technique remains the mainstay of current aerosol acidity measurements. Some degree of automation in sampling has been implemented at the sampling end by incorporating a sequential sampler;31 however, this does not affect the analytical end. Also, it has been suggested that, in any measurement technique involving filter (12) Dockery, D. W.; Schwartz, J.; Spengler, J. D. Environ. Res. 1992, 59, 362. (13) Bates, D. V.; Baker-Anderson, M.; Sizto, R. Environ. Res. 1991, 51, 51. (14) Schlesinger, R. B.; Chen, L.-C. Environ. Res. 1994, 65, 69. (15) Friedlander, S. K.; Lippmann, M. Environ. Sci. Technol. 1994, 28, 149A. (16) Reichhardt, T. Environ. Sci. Technol. 1995, 29, 360A. (17) Chow, J. C. J. Air Waste Manage. Assoc. 1995, 45, 320. (18) Keeler, G. J.; Spengler, J. D.; Castillo, R. A. Atmos. Environ. 1991, 25A, 681. (19) Waldman, J. M.; Lioy, P. J.; Thurston, G. D.; Lippmann, M. Atmos. Environ. 1990, 24B, 115. (20) Waldman, J. M.; Liang, S.-K. C.; Lioy, P. J.; Thurston, G. D.; Lippmann, M. Atmos. Environ. 1991, 25A, 1327. (21) Koutrakis, P.; Wolfson, J. M.; Spengler, J. D. Atmos. Environ. 1988, 22, 157. (22) Suh, H. H.; Spengler, J. D.; Koutrakis, P. Environ. Sci. Technol. 1992, 26, 2507. (23) Brauer, M.; Koutrakis, P.; Spengler, J. D. Environ. Sci. Technol. 1989, 23, 1408-1412. (24) Suh, H. H.; Allen, G. A.; Koutrakis, P.; Burton, R. M. J. Air Waste Manage. Assoc. 1995, 45, 442. (25) Suh, H. H.; Nishioka, Y.; Allen, G. A.; Koutrakis, P.; Burton, R. M. Environ. Health Perspect. 1997, 105, 826. (26) Vecera, Z.; Dasgupta, P. K. Int. J. Environ. Anal. Chem. 1994, 4, 311. (27) Simon, P. K.; Dasgupta, P. K. Environ. Sci. Technol. 1995, 29, 1534. (28) Dasgupta, P. K. ACS Adv. Chem. Ser. 1993, 232, 41. (29) Dasgupta, P. K.; Ni, L.; Poruthoor, S. K.; Hindes, D. C. Anal. Chem. 1997, 69, 5018. (30) Tanner, R. L. ACS Adv. Chem. Ser. 1993, 232, 229.

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collection, the strong acid content of the aerosol is probably underestimated due to the co-collection of particles such as NH4NO3 or NH4Cl and the subsequent loss of HNO3 or HCl.32 Strong acidity in the aerosol fraction, notably H2SO4 neutralized to various degrees with NH3, has characteristic signatures in the infrared, and this can be exploited;33,34 however, filter collection is still needed. On balance, there are no generally applicable automated techniques for the in situ determination of aerosol strong acidity. Potential Approaches to a Continuous Monitor. If the aerosol is continuously dissolved into an aqueous solution, continuous analysis is simpler. If the majority of the anions and the cations (other than H+) are determined, e.g., by ion chromatography (IC), it becomes possible to determine H+ by difference. For acidic precipitation samples, it is well known that [H+] measured in this fashion corresponds very well with the measured pH value.35,36 For rainwater, the net acidity can be determined by measuring the conductivity of the sample as such and after cation exchange for H+.35 However, the accuracy may be compromised when a small aerosol mass is dissolved in CO2 -saturated water, as would result in scrubbing an aerosol into an aqueous stream. Attempts to purge out CO2 can lead to other complications. On the other hand, a system that will concentrate both anions and cations on the same stationary phase and allow elution without extraneous ions is not known. An extract of the fine particle fraction of the ambient aerosol is substantially less complex in composition than typical rainfall; H+, NH4+, SO42-, and, to a smaller extent, NO3- (with Na+ and Cl- also contributing to a smaller extent in coastal regions) adequately describes the qualitative composition, especially in regard to personal exposure.22,36 A vapor condensation aerosol collection system (VCACS), in which gases are preremoved by a wet denuder and particles are allowed to grow by steam condensation and collected by impaction on a cooled maze,27,37,38 provides separate aqueous effluents for the collected water-soluble gases and the aerosol solution. Cation and anion chromatography of the aerosol extract can thus measure aerosol acidity. But such a system with two ICs would be bulky and expensive. The same information can be obtained from the total anion equivalents and the total (non-H+) cation equivalents measured by suppressed conductometry39 without chromatographic separation. This is the operating principle of the present instrument. PRINCIPLES In Figure 1, the soluble gas removal and aerosol collection system (left of the dashed line) has been previously described.37 The aerosol aqueous extract is continuously pumped sequentially (31) Jaques, P. A.; Thurston, G. D.; Kinney, P. L. Appl. Occup. Environ. Hyg. 1993, 8, 313. (32) Koutrakis, P.; Thompson, K. M.; Wolfson, J. M.; Spengler, J. D.; Keeler, G. J.; Slater, J. L. Atmos. Environ. 1992, 26A, 987. (33) McClenny, W. L.; Krost, K. J.; Daughtrey, E. H.; Williams, D. D.; Allen, G. A. Appl. Spectrosc. 1994, 48, 706. (34) Krost, K. J.; McClenny, W. L. Appl. Spectrosc. 1994, 48, 702. Stumm, W.; Morgan, J. J. Aquatic Chemistry, 2nd ed.; Wiley: New York, 1981; p 169. (35) Stairs, R. A.; Semmler, J. Anal. Chem. 1985, 57, 740. (36) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques; Wiley: New York, 1986; pp 786-813. (37) Simon, P. K.; Dasgupta, P. K. Anal. Chem. 1995, 67, 71. (38) Simon, P. K.; Dasgupta, P. K. Anal. Chem. 1993, 65, 1134. (39) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 47, 1801.

Figure 1. System schematic. See text for details.

through a cation exchanger preconcentration column CPC and an anion exchanger precolumn APC, respectively constituting the loops of six-port injector valves V1 and V2. The column order may be important; some anion exchange stationary phases consist of anionic latex on surface-sulfonated substrates where both the cationic and anionic sites are accessible. In the cation analysis flow channel (top), after a desired period of preconcentration, V1 is switched, and a third injection valve (V3) is used to inject a bolus of dilute HCl in a water carrier. This elutes the collected cations (primarily NH4+). The carrier containing eluites passes through an OH-- form packed-bed suppressor, and the resulting NH4OH flows into conductivity detector CD1. Up to a concentration of 10 µM in the detector, NH4OH is virtually fully ionized, leading to a linear detector response (theoretically expected linear r2 ) 0.9954). When Na+ is present in low concentrations, no significant suppression of NH4OH ionization is therefore expected. Based on the equivalent conductance data for NH4+ (73.5) and OH- (198.6),

non-H+ cation equivalents present in sample ) K1G1/271.6 (1)

anions. The effluent passes through an externally resin-packed, filament-filled helical tubular Nafion suppressor40 to conductivity detector CD2. A strong eluent and a low-capacity APC result in plug elution. In the VCACS, air is continuously contacted with a cold (2 °C) aqueous phase. This results in a significant CO2 content in the VCACS effluent. An optimized eluent carbonate content makes the resulting CO2 peak essentially invisible but requires a continuously flowing eluent rather than an injected pulse as in the cation system. A continuously regenerated suppressor is necessary to practice this. The anions elute as the acids. Since weak acid anions such as acetate and formate are not present in the aerosol phase to any significant extent and carbonate is not visible to the detection system by the choice of the eluent, the eluite signal is essentially due to H2SO4, with smaller amounts of HNO3 (and HCl in coastal areas) being present. Given the limiting equivalent conductance values of H+ (349.9), SO42- (80.0), NO3- (71.4), and Cl- (76.35), the conductance is dominated by H+. If the aerosol is exclusively sulfate, the relevant equation will be

total anion equivalents present in sample ) K2G2/429.9 (2)

where G1 is the peak area or height, with the ordinate being plotted in units of conductance, and K1 is a calibration constant dependent on the detector cell constant and the dispersion factor between the valve and the detector (peak height used for quantitation). Note that the observed conductance is dominated by that of OH-. At low analyte concentrations, if the sample contains 90% NH4+ and 10% Na+ (equivalent conductance 50.3), eq 1 will underestimate the cation equivalents by only 0.91%. To improve response linearity at high analyte concentrations, an additional anion exchange membrane-based converter was studied to convert NH4OH into CH3COONH4 or (NH4)C2O4. The anion analysis channel (bottom, Figure 1) is a conventional IC system without a separation column. When V2 is switched, a continuously flowing NaOH-Na2CO3 solution elutes the collected

where G2 is the detector signal and K2 is a calibration constant. If the aerosol is 10% NO3-, 10% Cl-, and 80% SO42-, eq 2 will underestimate the total anion equivalents by only 0.25%. Aerosol strong acid can now be computed by difference:

strong acid equivalents ) total anion equivalents total non-H+ cation equivalents (3)

EXPERIMENTAL SECTION Aerosol Generation and Collection System. All gas flow measurements were made with mass flow controllers (MFC, (40) Gupta, S.; Dasgupta, P. K. J. Chromatogr. Sci. 1988, 26, 34.

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model FC-280, Tylan General, San Diego, CA). These were calibrated with a primary standard digital bubble flow meter (Gilian Instruments, West Caldwell, NJ). All reported data have been corrected to correspond to 760 mmHg and 0 °C. A homemade concentric nebulizer was used in this study. In most experiments, the nebulization solution was typically composed of (NH4)2SO4, mixed with various amounts of H2SO4, and was pumped by a peristaltic pump (Pharmacia) with a flow rate of 1.7 µL/min. Stock solutions of 0.28 M H2SO4 and (NH4)2SO4 were mixed in-line in ratios from 4:1 to 1:0 to prepare the nebulizer feed. The nebulization air flow rate (Pure Air Generator, model 737-12A, AADCO, Clearwater, FL) was controlled at 10 psi, to provide 0.5 standard liters per minute (SLPM). Diluent air was introduced upstream at 6 SLPM. The generated aerosol was passed through a Kr-85 discharger column (model 3054 aerosol neutralizer, TSI, St. Paul, MN; 100 cm × 30 cm o.d.) to render the aerosol Boltzmann-charged. The aerosol stream then passed through a 20-L chamber to allow drying. All components were connected with either 0.79- or 0.95-cm-i.d. Nalgene tubing or 2.5cm-i.d. corrugated PTFE tubing. Further diluent air was introduced after the drying chamber to provide a total volumetric flow rate of 12 SLPM. The stream was divided at this point into two with a Y-connector. Through one, the VCACS sampled at a rate of 5 SLPM, and through the other, a control sample was collected on a prewashed 25-mm glass fiber filter (Whatman GF/B), also at 5 SLPM. Identical vents were placed on each arm of the Y-connector for excess aerosol flow to be vented. The above filter is referred to as the side stream filter (SSF). A second identical filter was located at the exit of the VCACS to determine any breakthrough and is referred to as the breakthrough filter (BTF). In some experiments, notably to reliably measure the actual particle collection efficiency of the VCACS, aerosol was generated at a very high concentration using 0.2 M Na2SO4 solution pumped at 50 µL/min. In this case, two 20-L drying chambers were used in series to ensure drying of the aerosol. Size Distribution of the Aerosol. The aerosol size distribution was measured by a laser-based particle counter (LPC, model A2212-010115-1, Met-One, Grant’s Pass, OR); this instrument reports the number concentrations in six ranges of optical equivalent diameters, from 0.1 to >3.0 µm. The data were fitted to a log-normal distribution with a nonlinear least-squares minimization program (MINSQ, Micro Math, Salt Lake City, UT). For the high-concentration Na2SO4 experiments, a significant mass of larger particles was generated and was mostly removed by deposition in the nebulizer throat and transport conduits. The aerosol actually sampled had the following characteristics: count median diameter (CMD) 0.40 µm, mass median diameter (MMD) 0.87 µm, and mass median aerodynamic diameter (MMAD) 1.58 µm. With (NH4)2SO4 aerosol generated for performance testing of the instrument, the size characteristics were CMD 0.2 µm, MMD 0.4 µm, and MMAD 0.8 µm. With H2SO4 aerosol, moisture is retained even after drying attempts, and CMD was about 0.3 µm, with other size statistics expected to be similarly higher. Aerosols with a NH4+:H+ ratio e1 had essentially the same size as the H2SO4 aerosol. Interference Gas Studies. Potentially interfering acidic or basic gases tested included NH3, SO2, NO2, and NO. Certified permeation devices (VICI Metronics, Santa Clara, CA), emitting 2842 Analytical Chemistry, Vol. 70, No. 14, July 15, 1998

in the range of 7.5-20 µg/min at 30 °C, were used for generating the first three gases. Nitric oxide was purchased as a 10 ppm certified standard in N2 (Scott Specialty Gases, Houston, TX). Because prolonged contact between the gases and the test aerosol may actually change the aerosol composition, the test gas was introduced into the aerosol stream immediately before the sampling inlet of the instrument. Vapor Condensation Aerosol Condensation System (VCACS). The setup used for the VCACS was exactly the same as that previously described,37 with the following operational conditions: the denuder liquid used was 5 mM morpholinoethanesulfonic acid (Serva, New York, NY) and 5 mM H2O2, adjusted to pH 4.3 with NaOH, pumped down each plate at a rate of 500 µL/min, with an air flow rate of 5 L/min. Using 50 ppbv SO2 and 20 ppbv NH3 as test gases, 99.9+% removal by the denuder was verified on the basis of effluent concentration measured by previously described SO2 and NH3 measurement systems.41,42 Liquid water was pumped at a rate of 600 µL/min through an in-line deionization cartridge into the VCACS to generate steam. The cooled maze was maintained at 2-3.5 °C, and the condensate from the air-liquid separator was aspirated at a flow rate of 625 µL/min and sent to the analytical system. Analytical System. An eight-chanel peristaltic pump (Minipuls 2, Gilson) provided liquid transport through the system. Commonly used poly(vinyl chloride) pump tubes, especially when new, release an anionic impurity that is readily detectable by IC. Viton tubing was used, therefore, throughout. Water (650 µL/min) was pumped as a carrier in the cation system. All pumped water streams contained in-line deionization cartridges. Six-port injector valves (V1-V3) and the four-port switching valve (V4) were electropneumatically actuated (type 5020P, Rheodyne, and type 38754, Dionex, respectively) as programmed by two digital valve sequence programmers (DVSP-4, Valco Instruments, Houston, TX). V3 contains a 170-µL loop which is filled with 10 mM HCl (180 µL/min). The CPC and APC had internal dimensions of 20 mm × 2 mm i.d. and were machined in-house from Plexiglas. The CPC packing was prepared by surface sulfonation of BioBeads SX-1 (chloromethylated poly(styrenedivinylbenzene), 200400 mesh, Bio-Rad) for 5 min in concentrated H2SO4 at 100 °C. The color of the resin changed from white to light gray. The beads were cooled in an ice bath, filtered by suction, and washed thoroughly with deionized water. The exchange capacity of the column was ∼200 neq. The OH--form packed-column suppressors were commercially available cartridge type suppressors (Alltech Laboratories); the cartridges were packed with an anion exchange resin of intermediate basicity (Bio-Rex 5, 100-200 mesh, 2.5 mequiv/mL, Bio-Rad) in OH- form. Each cartridge has a capacity of 3.6 mequiv. To obtain the best results, new cartridges have to be washed in place for 5-6 h and then can be used for 36-96 h with an HCl plug injected every 5 min. Although the theoretical capacity is much greater, after 96 h of use, the background conductance generally becomes too high for optimum performance. For uninterrupted week-long operation, two suppressors in series were used. After 1 week, the upstream supressor was replaced by the downstream one and a fresh one put after it. (41) Lindgren, P. F.; Dasgupta, P. K. Anal. Chem. 1989, 61, 19. (42) Zhang, G.; Dasgupta, P. K.; Dong, S. Environ. Sci. Technol. 1989, 23, 1467.

In the anion measurement system, the eluent was 2 mM NaOH and 0.5 mM Na2CO3 (500 µL/min). The APC resin was AG4-X4 (100-200 mesh, 0.8 mequiv/mL, Bio-Rad, Richmond, CA). The resin was packed in the column (2.5 mm i.d. and 20 mm long) by suction. An 80-cm-long filament-filled (30 lb Nylon monofilament fishing line) Nafion fiber (∼700 µm wet i.d., 811x, Perma-Pure Products, Toms River, NJ) helix, externally resin packed in a 5-mm-i.d., 17-cm-long PTFE tube, was used as the suppressor40 with 10 mM H2SO4 as regenerant (1.65 mL/min). Two model 213A conductivity detectors (Alltech/Wescan) were used for detection; the data were acquired by integrators (model 3394A, Hewlett-Packard) or a PC-based system. Because the liquid effluent from the VCACS must be completely removed, overaspiration is applied, and some air bubbles are aspirated. Before analysis, trapped air bubbles are removed by switching V4 and pumping degassed deionized water (625 µL/ min) through the columns while the VCACS effluent is sent to waste. The operational cycle was typically 8-10 min long: 5 min for sampling, 1 min for debubbling, and 2-4 min for elution, washout, and return to baseline. The time utilization can be improved by switching V1-V4 back to original positions as soon as possible after elution from the columns is complete (∼1 min), but this was not routinely practiced. It was found possible to use the same conductivity detector and data system for both the cation and anion channels to reduce cost, if desired. In this case, V1 and V2 do not go into the injection mode simultaneously, and yet another switching valve is used to switch the detector liquid input from one channel to the other. The debubbling step is continued in the cation system after V2 goes into the injection mode. Two minutes afterward, the detector input valve is switched, and V1/V3 are put in the inject mode. Also, commercially available (Dionex) electrically operated membrane suppressors can be substituted for either one of the suppressors used in our instrument. Normalization of Aerosol Output. The LOD of the analytical system is computed on the basis of the variability of a blank or a low-level standard. In the present case, the least reproducible part of the test system is the aerosol source, and the detection capabilities of the system can be underestimated if this is not taken into account. Imagine that NH4HSO4 aerosol is being generated with a nebulization solution of the same composition with a nominal sulfate aerosol concentration of 100 nmol/m3 (and, consequently, a nominal strong acid concentration of 100 nequiv/ m3). The absolute variability is ∼10%, such that the standard deviation of the measurement in several runs is 10 nequiv/m3 H+. In conventional approaches, one cannot obtain an LOD figure (based on S/N ) 3) better than 30 nequiv/m3. However, it is readily realized that, if we consider the relative acidity, i.e., nanoequinalents of H+ per nanoequivalents of sulfate generated, the variability would have more correctly reflected that of the measurement instrument. In our work, each set of reported results represents data from several individual observations (usually 15 or more) made under the same conditions. In presenting these results, we have calculated the data first as nanoequivalents of H+/neq sulfate present, calculated the variance of this and then multiplied it by the mean value of nanoequivalents of sulfate per cubic meter present to compute the variance of H+

Figure 2. Comparison of sulfate mass concentration calculated from particle counter data vs that measured by the present instrument.

in nanoequivalents per cubic meter. The LOD was then computed on the basis of 3 times this variance. RESULTS AND DISCUSSION Collection Efficiency of VCACS. Mass Concentration Estimation from Particle Counter Data. The collection efficiency of the VCACS for a number of soluble inorganic aerosols was previously determined to be quantitative, 99.33 ( 0.90%. This was reconfirmed in the present experiments. Sulfate concentration in the BTF extract was measurable reliably above the blank filter extract only at high aerosol concentration (730-1040 nequiv/ m3 sulfate). Nine separate experiments, 42-211 min in duration, were conducted. The VCACS effluent was collected for analysis in a single lot. This and the BTF and SSF extracts were measured. The collection efficiency of the VCACS (relative to the SSF) was determined to be 98.43 ( 0.52%. After accounting for the amount collected on the BTF, losses in the denuder and other portions of the sampling system were computed to be a maximum of 0.96 ( 0.10%. The uncertainty figures include the uncertainty of the filter blanks. Given the accuracy/reproducibility of the flow measurement equipment, aerosol collection by the VCACS is quantitative. Because of the short-term variation of the aerosol output, it was of interest to determine if the optical particle counter output can be converted to an approximate mass concentration so that any malfunction in the system can be rapidly identified. The (NH4)2SO4 aerosol output was deliberately varied to obtain the data in Figure 2. The mass concentration was estimated from the LPC number counts and the known physical density of the aerosol by assigning the geometric mean diameter of a given size range to that range. Given that the VCACS data are less frequent and lag behind the particle counter output, there is reasonable agreement between the two instruments. Similar observations were made in numerous other runs. Analytical Chemistry, Vol. 70, No. 14, July 15, 1998

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Performance of Analysis System. Liquid-Phase Calibrations. The analytical system was calibrated extensively in the liquid phase using a 1 µM (NH4)2SO4 standard, prepared by inline dilution of a 25 µM standard. Calibration, peak height based, was primarily conducted in the 0-20-nequiv range. For the cation system, the response was reasonably linear (r2 ) 0.9880, n ) 25), with an S/N ) 3 LOD of 1.25 nequiv. Based on the peak width and computed dispersion at peak maximum, the estimated peak concentration at the detector for a 20-nequiv injected sample is 56 µM. The degree of observed linearity is thus in close correspondence with the theoretically expected value (r2 ) 0.9810) calculated for the conductance of 0-56 µM NH3. A quadratic fit, however, is a much better predictor (multiple r2 ) 0.999 97) and was used for quantitation henceforth. For the anion system, linearity extended to an injected amount of 80 nequiv (r2 ) 0.9995, n ) 25), and a second-order fit could be used as a good predictor (r2 ) 0.9993) up to an injected concentration of 200 nequiv. The S/N ) 3 LOD was 0.8 nequiv. The difference in peak height responses between preconcentration columns used for 8 months and a newly packed one was 3%. The above data do not directly yield the LOD of H+, however. Of interest here is the ability to measure the difference in the cation and anion equivalents present. A simulated acidic sulfate aerosol extract, containing in this case 1 µM H2SO4 and 5 µM (NH4)2SO4, was therefore measured. With 10 nequiv of sulfate loaded on the column, the variability in the differences in response between the two channels suggested that LOD for H+ is better than 0.2 nequiv. To extend the cation measurement system to higher applicable concentrations, a number of schemes were investigated to translate NH4OH into an electrolyte that will be fully ionized even at higher concentrations. Such conversion schemes involved conversion to LiOH by cation exchange or NH4X by anion exchange; both packed-column- and membrane-based converters were investigated. The best results were obtained with an oxalateform anion exchange column placed after the OH--form suppressor. With this converter, linearity extended to 180 nequiv of NH4+ injected (r2 ) 0.9999). However, the signal decreases, and the LOD actually degrades. Since our application was primarily directed at measuring very low levels, this was not further pursued. In the cation system, when calibration was conducted with low levels of H2SO4 and Na2SO4 aerosols, there was no response from H2SO4, and the response from Na+ was 8% lower than that for a corresponding amount of NH4+, in excellent agreement with theoretical predictions. In the anion system, the nature of the cation had no effect on the response. On the other hand, when calibration was conducted with NaNO3 or NH4NO3, the peak height-based calibration slope for NO3- was 5% higher than that for SO42-, rather than being 2% lower as theoretically expected. Closer examination revealed that area-based calibration slopes do behave in the expected manner. Sulfate is more strongly retained on the stationary phase and results in a somewhat broader peak. The relative amounts of sulfate and nitrate in ambient aerosols and their nearly equivalent responses give one no overwhelming reasons to choose peak area over peak height-based calibration, however. 2844 Analytical Chemistry, Vol. 70, No. 14, July 15, 1998

Table 1. Experiments with Ammonium Sulfate Aerosols with Varying Degrees of Acidity H+ LOD, ng/m3

H2SO4: (NH4)2SO4 ratio nominal, in nebulizer feed

found in aerosol

4:1 1:1 1:5 1:7.5 1:10 1:15

3.7:1 1:0.98 1:5.4 1:8.5 1:11 1:17

background no. of exptl, projected, (NH4)2SO4 data 5SLPM, 5SLPM, 60 min concn, mg/m3 points 5 min 2.35 5.1 16.5 17.4 15.1 8.25

16 19 28 16 26 12

8.3 7.0 21.6 31.8 38.2 9.7

0.7 0.6 1.8 2.7 3.2 1.6

Performance with Aerosols. The system blank was determined by two levels of blank testing: (a) filtered air sampling and steam introduction and (b) sampling of nebulized deionized water, along with steam introduction. In (a), we observed signals equivalent to 0.72 ( 0.72 nequiv/m3 NH4+ and 2.5 ( 0.25 nequiv/ m3 SO42- (n ) 25), and in (b) 2.2 ( 0.66 nequiv/m3 NH4+ and 2.40 ( 0.31 nequiv/m3 SO42- (n ) 22). The experiment was repeated a month later, with essentially the same results. This suggests that there is little or no leaching of ionic impurities from the wetted parts in the system. (NH4)2SO4 aerosol, generated in the range of 2-40 µg/m3 (30606 nequiv/m3), showed excellent correlation between the measured cation equivalents and the anion equivalents (as quantitated on the basis of liquid-phase calibration) with the following relationship (forced through the origin):

cation equivalents ) (1.0016 ( 0.0084) × anion equivalents, r2) 0.9985 (4)

These results suggest that the overall aerosol collection and determination process works well in its entirety. Measuring Aerosol Acidity. Limits of Detection with Varying Background Concentrations. The present system obtains an H+ value by difference; as such, the LOD is likely to deteriorate as the total amount of salt present increases. Although there are several well-characterized ammonium sulfates, it is convenient here to think of (NH4)2SO4 as neutral and any H2SO4 present over this composition as the net free acidity. The different experimental runs and the results obtained are shown in Table 1; the reported LODs are based on 3 times the variability of the measured H+, after normalization of the aerosol output variability. Without any correction for the latter, the LODs are worse by a factor of 2-3. The ability to determine aerosol strong acidity in the single to double digit nanoequivalents per cubic meter range with a 5-min temporal resolution in a fully automated fashion is not currently available. Since this technique is essentially cumulative, it is reasonable to project detection limits that will result from a much larger sample volume, and these are shown in Table 1 for 60-min (0.3 m3) samples. The 1:5 H2SO4:(NH4)2SO4 composition was actually tested with 60-min sample periods, and the observed LOD was 1.5 nequiv/m3, better than that estimated from the 5-min samples. As a matter of comparison, the mean aerosol acidity in summertime Washington, D.C., was measured to be 21 nequiv/m3.25 In our experimental system, aerosol source con-

Figure 3. Variation of H+ LOD as a function of (a) background neutral salt concentration and (b) NH4+:H+ ratio.

centration and composition fluctuations are the biggest source of variability on a short-term basis. Normalization to a fixed sulfate concentration helps to remove some of the variability, but a larger sample volume also integrates the temporal variations and produces results that are likely to be more meaningful in practice. Some of these results are shown graphically in Figure 3. In general, one may expect that the LOD will deteriorate with increasing “background” (NH4)2SO4 aerosol concentration or with increasing NH4+/H+ ratio. With the exception of the data at an abscissa value of ∼16.5, an exponential increase of the LOD with either abscissa parameter is observed. Bearing in mind that these experiments were not conducted with NH4+/H+ ratio held constant while the “background” (NH4)2SO4 aerosol concentration was varied or vice versa, the results are very much in keeping with the general expectations. Interference from Acidic Gases or Ammonia. In essence, if soluble acidic or basic gases are not removed prior to the collection of the aerosol, they will be collected by the VCACS, and neutralization or augmentation of the acidity will occur. Although the efficiency of the wetted parallel plate denuders has been previously verified for SO2, in the present case it was necessary to remove SO2 and NH3 simultaneously. Further, for reactive gases such as NO or NO2 (alone or in combination) that are not removed efficiently by a wet denuder, hot steam can potentially convert these gases to nitric or nitrous acids. Interference studies were conducted with (NH4)2SO4, NH4HSO4, and H2SO4 aerosols, generated at a concentration of approximately 10 µg/m3 SO42- (0.2 nequiv/m3). All data shown are normalized to this figure. For NH4HSO4 aerosol, since the nebulizer feed is produced by in-line mixing of two different solutions, there is more variation in the exact acidity level of the aerosol generated (especially since the entire span of the experiments lasted several months). In each case, the control case involves the aerosol alone, followed by introduction of the test gas. The test gases included (a) 20 ppb NH3, (b) 170 ppb SO2, (c) 220 ppb NO, (d) 220 ppb NO2, and (e) 220 ppb each of NO and NO2. These levels are

Figure 4. Effects of potentially interfering gases: bottom, (NH4)2SO4 aerosol; middle, NH4HSO4 aerosol (nominal composition); top, H2SO4 aerosol. The control situation (aerosol only) is shown as the unfilled bar, and the corresponding gas plus aerosol experiment is shown as the shaded adjacent bar. The number of observations is indicated on each bar.

generally higher than the highest levels that are likely to be encountered in the ambient atmosphere. Initially, 5 mM H2O2 was used as the wet denuder liquid.37,38 This was effective in removing all the SO2, but some penetration of NH3 was observed. Adjustment of pH to 4.3 with a nonvolatile zwitterionic Good buffer43 was found to be effective in removing both gases. The results are shown in Figure 4. Note that the ordinate scaling is different in the panels, the ordinate span being 50, 125, and 200 nequiv/m3 from the bottom to top panel. The result for the aerosol alone is shown in the unfilled bar, while the corresponding result with the test gas is shown immediately adjacent in the shaded bar. The (1 standard deviation error bar and the number of experimental observations considered are also shown. With (NH4)2SO4 aerosol, the acidity in the control case is always zero, and there is no significant difference in the results when NH3, SO2, or NO is introduced. With high levels of NO2 (with or without NO), there appears to be some small increase in the observed acidity level, although the difference is barely statistically significant. With NH4HSO4 aerosol, the presence of ammonia clearly reduces the acidity level. This is expected: as the aerosol goes through the wet denuder, it must acquire moisture, and the wet aerosol must show some uptake of ammonia whether the dry aerosol reacts with ammonia or not. It is interesting to note that the denuder effectively competes with the (43) Ferguson, W. J.; Good, N. E. Anal. Biochem. 1980, 104, 300.

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aerosol in removing ammonia, the amount of ammonia introduced is 8 times greater than what is needed to completely neutralize the aerosol. There is no significant difference in the control vs challenge case for any of the other test cases with this aerosol. If the situation with NO2 observed with the (NH4)2SO4 aerosol is real, it would appear that the formation of nitrogen oxyacids is not facilitated by an aerosol that is already acidic. The same situation is observed with H2SO4 aerosol. The introduction of NH3 was not tested in this case, since this would be pointless. Other test cases showed no statistical difference from control. Overall, a lack of significant interference from the test gases is thus indicated. Applications of the Aerosol Acidity Instrument. Thus far, it has not been possible to test the instrument in an intercomparison mode against one of the presently practiced methods. In the summer of 1996, the instrument was shipped by air freight to researchers at New York University Medical Center. The entire crate containing this instrument was lost en route at the shipper’s hub in Columbus, OH, and, despite the best efforts on our part, has not been located since. We are presently attempting to rebuild this instrument. However, several interesting applications were tested prior to this. Without a cyclone at the inlet, ambient aerosol in Lubbock, TX, was tested to be alkaline; this is expected in this region, where calcareous particle loading in the atmosphere is high. With a cyclone (50% cutpoint at 2.5 µm), the ambient aerosol here showed no statistically significant acid or base content. Indoor Experiments. Extensive experiments were conducted indoors in several homes, including, in some cases, the effects of the use of kerosene space heaters. We have previously measured ammonia levels in the ambient laboratory air and outdoor air.42 In a 6000 ft3 laboratory with an air change every hour, the background NH3 level typically increases by ∼0.5 ppb per person occupying the laboratory, in the absence of any other identifiable ammonia source. Based on the normal range of ammonium in blood (10-30 µM), a pH of 7.4, a temperature of 310 K, and an ionic strength of 0.15, alveolar blood gas equilibration alone should lead to an NH3 level of 15-40 ppb in exhaled air. We are not aware of any detailed literature data on the levels of ammonia in expired air. One measurement report indicated 80-180 ppb NH3 in the breathing zone under conditions ranging from resting to exercising.6 The contributions from such NH3 sources inside homes are not likely to lead to large accumulated concentrations, because of relatively small output volume (0.75 m3/h per person average, at rest), unless air exchange rates are very low. However, in the wake of energy conservation measures, air exchange rates are often quite low. We observed that measured ammonia levels inside homes with stay-in pets tend to be quite high, often substantially higher than the 20 ppb level tested in our experiments as a potential interferent. During these experiments, we made no attempts to change the ventilation rate the occupants were otherwise usingsin many cases, the air handling system was off or was in the automatic mode, where it turned on a very small fraction of the time. Levels of ammonia exceeding 100 ppbv was found in one household, and the instrument measured consistently an aerosol that was 10-30 nequiv/m3 on the alkaline side (even discarding the period when the pet, a large dog, decided to sniff the instrument inlet!). 2846 Analytical Chemistry, Vol. 70, No. 14, July 15, 1998

Figure 5. Kerosene heaters cause readily measurable acid aerosol emission. A “Kero-Sun” 12 300 Btu/h heater was located on the floor, and the sampling point was located 1 m vertically, 0.7 m horizontally, away from the heater. Total volume of room, 6600 ft3; ventilation on; air temperature change was