Anal. Chem. 1995, 67, 71-78
Continuous Automated Measurement of the Soluble Fraction of Atmospheric Particulate Matter Poruthoor K. Simon and Pumendu K. Dasgupta* Department of Chemistty and Biochemistry, Texas Tech Universiv, Lubbock, Texas 79409-1061
A new approach is introduced for the quantitative collection of aerosol particles to submicrometer size. The design and construction of a continuously operating system is described. Particles are continuously transferred to a liquid stream utilizing condensation of supersaturated vapor to promote particle growth followed by collection that relies on impaction and the thermophoretic effect. The automated system is simple to construct and operate and provides quantitative collection of aerosol particulate matter even at a flow rate of 10 Wmin. Individual measurements of aerosol particles and soluble gases are achieved by combining the system with a wetted wall parallel plate diffusion denuder. Interfaced to an inexpensive ion chromatographfor downstream analysis, the detection limits of the overall system for particulate sulfate, nitrite, and nitrate are 2.2,0.6, and 5.1 ng/m3, respectively, for a 8 min sample. The system has been used for the field measurementof gaseous and particulate components; results are presented. Aerosols play an important role in tropospheric and stratospheric chemistry. The ambient tropospheric aerosol is a complex mixture of water, electrolytes, ionic solids, metal oxides, glasses, and carbonaceous materials.' Many believe that the intluence of aerosols on global climate may be as important as that of greenhouse gases. However, much less attention has been paid to aerosols. Considerable spatial and temporal variations in both aerosol composition and concentration make it difficult to evaluate the global role of aerosols relative to the gaseous pollutants.2 Presently there is no generally applicable automated instrumentation for the composition specific measurement of aerosol concentration. The role atmospheric aerosols play in any situation of concern depends on their concentration, chemical composition, and particle size. A complete understanding requires a knowledge of all three of the above parameters, in issues ranging from the destruction of stratospheric ozone by ice clouds, to deterioration of visibility, to adverse health effects of inhaled particulate matter. The following are among the major reasons to develop facile instrumentation for automated chemical analysis of aerosols: (1) enhance the understanding of chemical and physical phenomena underlying atmospheric pollution, (2) apportion chemical constituents of suspended particulate matter to individual sources, (3) evaluate the extent to which the chemical composition of the particulate matter affects human health, atmospheric visibility and dry deposition, and (4) determine compliance with air quality (1) Stelson, A W. Enuiron. Sci. Technol. 1990,24, 1676-1679. (2) Preiniing, 0. Atmos. Enuiron. 1 9 9 1 , 2 5 A , 2443-2444. (3) Friedlander, S. K; Lippmann, M. Enuiron. Sci. Technol. 1 9 9 4 , 28, 148A150A. 0003-2700/95/0367-0071$9.00/0 0 1994 American Chemical Society
standards, especially now that leading authorities are suggesting the necessity for a composition-specific standard for particulate matter concentration.3 Extant monographs describe available particle collection instruments, principles, techniques, and applications of such meth~dology.~ Many automated instruments are available commercially for the measurement of aerosol size. In contrast, chemical analyses of aerosols have largely been conducted in an off-line manner. Noteworthy previous research has been principally aimed toward the automated measurement of a specific aerosol constituent? e.g., HzS04 or aerosol nitrate/ammonium. Singleparticle analysis on collected particles has been carried out by various optical methods.6 Chemiluminescence detection has been used to determine nanogram quantities of nitrate after thermal decomposition of nitrate particles? however, thermal speciation may have potential interferences? One frequently used approach is to perform spectroscopic analysis of the collected aerosol. The general application of optical techniques used for the analysis of collected atmospheric particulate matter has been revie~ed.~ Aerosol particles can be collected by various techniques, including inertial classification, gravitational settling, centrifugation, filtration, and electrostatic and thermal precipitation. Of these, filtration, with or without inertial classification,is the most commonly utilized approach.10 Collection efficiency and other characteristics of various filters have been evaluated and are available in the literature." The sample is typically collected for a long period, usually many hours. A common limitation to all methods involving filter collection and subsequent analysis is that it is labor intensive and difficult to automate. Further, one is unable to follow rapid changes in aerosol composition. There are also problems particular to specific analytes and specific collection/analysis methods; e.g., sampling surfaces often absorb or react with gases and particles leading to positive or negative (4) (a) Murphy, C. M. Handbook of Particle Sampling tAnalysis Methods, Verlag
Chemie International Inc.: Deerfield Beach, FL, 1984. (b) Willeke, K, Baron, P.A, Eds. Aerosol Measurement-Principles, Techniques and Applications; Van Nostrand Reinhold: New York, 1993. (c) Watson, J. G., Chow, J. C. In Aerosol Measurement-Principles, Techniques and Applications; Willeke, K, Baron, P. A, Eds.; Van Nostrand Reinhold: New York, 1993; pp 622-639. (5) (a) h l a u f , K G.; Fellin, P.;Wiebe, H.A; H. I. Schiff; Braman, R S.; R Gilbert, R Atmos. Enuiron. 1985, 19, 325-333. (b)Lindqvist, F. Atmos. Enuiron. 1985, 19, 1671-1680. (6) Bardess, D.; Levin, Z.; Ganor, E. Atmos. Enuiron. 1992, 26A, 675-680. (7) (a) Yamamoto, M.; Kosaka,H. Anal. Chem. 1994, 66, 362-367. (b) Cox, R D. Anal. Chem. 1980, 52, 332-335. (c) Spicer, C. W.; Joseph, D. W.; Schumacher, P.M. Anal. Chem. 1985,57,2338-2341. (d) Braman, R S.; Shelly, T.J.; McClenny, W. A Anal. Chem. 1982, 54,358-364. (8) Sturges, W. T.; Harrison, R M. Enuiron. Sci. Technol. 1988, 22, 13051311. (9) Appel, B. R. InAerosol Measurement-Principles, Techniques andApp1ication.y Willeke, IC, Baron, P.A, Eds.; Van Nostrand Reinhold: New York, 1993; pp 233-259.
Analytical Chemistry, Vol. 67, No. 1, January 1, 1995 71
deviations.12 Particles collected on filters may be lost by volatilization during and after collection; reaction with other collected particles may also lead to volatilization Significant losses of nitrate particles are reported for both filter collection and impaction methods.lZe Even the ability of some commonly available filter media to collect fine particles has been disputed.13 Particle losses or errors from gas absorption have been reported in other collection techniques as we11.12c,dSome interferences are particularly well known; problems in particulate nitrate measurements from gaseous nitric acid have been reported by many researcher^.^ In sulfate aerosol measurement by collection on nylon filters, positive interference from SO2 is variable and can be as high as 70%.12a,b Continuous collection and wet analysis based instrumentation has the potential to be fast, sensitive, and affordable. The analytical signature by an aerosol constituent, once collected in an aqueous medium, is not unique relative to potentially present gas phase species (e.g., NO3- is produced both from HN03@ and aerosol nitrate). It is therefore essential that soluble gases be removed. The most expedient means to do this is to use a d h s i o n denuder.14 To provide good analytical time resolution without sacrificing sensitivity, it is further necessary to collect the sample (and remove the soluble gases) at a relatively high flow rate. This is possible with the recently introduced silica-coated continuously wetted denuders, especially of the parallel plate design.15 Because of the flowing liquid film, surface saturation effects are obviated even over long operating periods. The next problem to be overcome is to devise a means to transfer submicrometer size particles continuously in to a liquid and achieve this at a relatively high sampling rate. The present paper reports an automated continuously operating aerosol collection system that relies on aerosol growth by supersaturation and condensation of water vapor, followed by collection that utilizes both impaction and the thermophoretic effect. The system is coupled to an ion chromatograph (IC), and the overall analytical capability is demonstrated. (10) (a) Mathai, C. V.; Watson, J. G., Jr.; Rogers, C. F.; Chow, J. C.; Thombach, I.; Zwicker, J. 0.; Cahiil, T.;Feeney, P.; Eldred, R; Pitchford, M.; Mueller, P. K. Enuiron. Sd'.Technol. 1990,24,1090-1099. (b) Ligoki, M. P.; Pankow, J. F. Enuiron. Sci. Technol. 1 9 8 9 , 23, 75-83. (c) Lane, D. A; Johnson, N. D.; Barton, S. C.; Thomas, G. H. S.; Schroeder, W. H. Enuiron. Sci. Technol. 1 9 8 8 , 22, 941-947. (d) Rapsomanikis, S.; Wake, M.; Kitto, A M. N.; Harrison, R M. Enuiron. Sci. Technol. 1 9 8 8 , 22, 948-952. (e) Knapp, K. T.; Durham, J. L.: Ellestad, T. G. Environ. Sci. Technol. 1 9 8 6 , 20, 633637. (0Sturges, W. T.; Harrison, R M. Atmos. Enuiron. 1 9 8 9 , 2 3 , 19871996. (g) Walter John, W.; Wall, S. M.; Ondo, J. L. Ah" Enuiron. 1988, 22, 1627-1635. (h) Pankow, J. F. Atmos. Enuiron. 1 9 8 7 , 2 1 , 2275-2283. (i) Koutra!&, P.; Wolfson. J. M.; Spengler, J. D. Atmos. Enuiron. 1 9 8 8 , 2 2 , 157-162. (j) Sturges, W. T.; Shaw, G. E. A M " Environ. 1993,27A, 29692977. (k) Ferm, M. Atmos. Enuiron. 1 9 8 6 , 20, 1193-1201. (11) Liu, B. Y. H.; Pui, D. Y. H.; Rubow, K. L. Aerosols in fhe Mining and Industrial Work Environments; Ann Arbor Science: Ann Arbor, MI, 1983. (12) (a) Durham, J. L.; Spiller, L. L.; Ellestad, T. G. Atmos. Enuiron. 1987, 21, 589-598. @) Chan, W. H.: Orr,D. B.; Chung, D. H. S. Atmos. Enuiron. 1 9 8 6 , 2 0 , 2397-2401. (c) Fogg, T. R Atmos. En~iron.1 9 8 6 , 2 0 , 16331634. (d) Highsmith, V. R; Bond, A E.; Howes, J. E., Jr. Atmos. Enuiron. 1986,20,1413-1417. (e) Zhang, X.; McMuny, P. H.Atmos. Enuiron. 1 9 9 2 , 26A, 3305-3312. (0 Hering, S. V.; Lawson, D. R; Allegrini, I.: Febo, A; Pemno, C.; Possanzini, M.; Sickles, J. E., II; Anlauf, K. G.: Wiebe, A: Appel, B. R; et al. Atmos. Enuiron. 1 9 8 8 , 22, 1519-1539. (13) (a) Kitto, M. E.; Anderson, D. L. Atmos. Enuiron. 1988, 22, 2629-2630. (b) Lodge, J. P., Jr. Atmos. Enuiron. 1 9 8 6 , 20, 9. (14) Dasgupta, P. K. ACS Adv. Chem. Ser. 1993,No. 232, 41-90. (15) (a) Simon, P. K; Dasgupta, P. K.; Vecera, Z. Anal. Chem. 1991,63,12371242. (b) Simon, P. K: Dasgupta, P. K. Anal. Chem. 1993,65,1134-1139.
72 Analytical Chemistry, Vol. 67, No. 1, January 1, 1995
EXPERIMENTALSECTION Aerosol Generation. Sodium nitrate, sodium sulfate, sulfuric acid, and ammonium sulfate aerosols were generated by nebulizing corresponding liquid solutions with a Venturi-type nebulizer with 1 standard liter per minute L(sTp) min-* dry, particlefree "zero" air. The generated aerosol was dried by dilution with zero air at a flow rate of 14 L(STP) min-I. The dilution chamber allowed a residence time of several seconds. Relative humidity was measured at various points in the flow path. Aerosol size is dependent on the concentration of the solution nebulized as well as the relative humidity of the carrier and dilution air. In this work, the aerosol concentration was changed by varying solution concentration. The measurement of size distribution of the test aerosol is discussed in a later section. Aerosol concentration was changed by further dilution with zero air. All reagents used were of analytical grade and were procured either from Mallinckrodt or Baker. Distilled deionized water was used for making all reagents. Gas Removal System. For reliable analysis of atmospheric particles, potentially interfering soluble gases must be removed before particle collection. A continuously operating highefficiency parallel plate diffusion denuder (PPDD) was used for this purpose. This type of denuder has been shown to collect soluble acidic gases quantitatively up to relatively high flow rates without deterioration of performance over time. Design and construction of this device was reported earlier.'" The current procedure for fabricating the PPDD is as follows; other details are in the previous paper. An area of -150 cm2was chemically etched on a 6.5 cm x 50 cm piece of 3 mm thick glass plate. The etched area was in the shape of a 29 cm x 5 cm rectangle, with the bottom short edge of the rectangle being made in to a V shape with the tip of the V projecting just beyond the etched area. The bottom of the PPDD is used as the sample inlet. The etched glass was washed well and coated with a thin layer of NaOH/NazSiO, mixture and then with a layer of silica powder. It was then heated in an oven at 690 "C for -10 min and slowly cooled to room temperature. The silica coating thus produced is chemically bonded to the supporting glass plate and is highly wettable. Two such plates are joined together with two PTFE film covered Plexiglas spacers (7 mm x 3 mm x 500 mm) to form a rectangular conduit and held in place by hot-melt adhesive. Water or 0.5 mM HzOz solution injected from the outside apertures to the silicacoated surface flows down by gravity and is collected at the bottom. The liquid effluent containing the soluble gases can be analyzed for gas composition. Previous studies15bhave shown that NaN03 aerosols, 0.80 pm in mass median aerodynamic diameter (MMAD) (ug= 1.66 and thus containing a significant number of particles significantly larger than 1 pm), show negligible (-0.2%) loss in the PPD at typical sampling rates. In keeping with the data previously reported, the transmission efficiency for 0.25 pm diameter NaN03 particles was determined to be essentially quantitative, 99.8 .i.0.2%. Particle Collection System (PCS). The system is shown schematically in Figure 1. Gaseous effluent from the PPDD bearing the aerosol was introduced to a mixing chamber MC through a short segment of a gently curved PTFE tubing (6.7 mm i.d. x 120 mm). Water was pumped through a coil of nickel tubing (0.51 x 1.6 mm i.d./o.d., 1000 mm long) maintained at -300 "C. The water is thus converted to superheated steam that is introduced to the mixing chamber MC along with the aerosol.
I '-I
a
A e m l Inlet, 1.d. 8 mm
T E E
4 ERISTAL PUMP
-r;
THERMOELECTRIC COOLER
METER
AIR PUMP
ION CHROMATOGRAPH
WASTE
.
Figure 1 Schematic representation of the particle collection system.
The chamber MC was made of Pyrex glass and is shown in Figure 2a. The bottom end of the inlet was made of an 8 cm segment of an 8 mm i.d. circular tube flattened to a rectangular shape of 12 mm x 2 mm internal dimensions and twisted by 360". This design enhances the rapid mixing of the steam and the aerosol. The effluent from MC enters through a side aperture into the stainless steel maze (SSM) using a short PTFE conduit (4.2 x 150 mm). The maze is shown in Figure 2b. An 8 mm thick stainless steel block contains a 5 mm deep 3 mm wide channel that makes 90" turns 40 times within a total channel length of -70 cm. This results in a mean distance of 1.75 cm between changes in the flow direction. The maze was cooled from the bottom and maintained at -2 "C, by means of using a custom-fabricatedcooler that utilizes four Peltier elements (CP 1.471-06L, Melcor, Trenton, NJ),and ensured complete condensation of the steam introduced into liquid water. The water drops containing dissolved and insoluble particles were collected using an inertial air/liquid separator (Figure 2c) connected to the maze by a short PTFE tube (5.3 x 100 mm). The exit air was aspirated from the separator by an air pump via a mass flow controller (FG280, Tylan General, Los Angeles, CA). Analytical System. The analysis of the effluent liquid sample was performed downstream by an IC system configured for anion analysis. Standard chromatographic conditions were used;15ball ion chromatograph (IC) components were from Dionex Corp. (Sunnyvale, CA). The sample was concentrated on a AG5 preconcentrator column. Two of these were configured on an eight-port dual stack valve such that while one being loaded with the sample, the other connected to the chromatographic column. Typically, the valve was switched every 8 min. All performance data cited pertains to this time resolution except as noted. The liquid effluent flow rates from the denuder and the PCS can vary, depending on the relative humidity of the air sample. To ensure that all the sample is injected to the analytical system, the liquid effluent aspiration rate was always maintained higher than the highest liquid effluent flow rate observed. This obligatorilyresults in injection of small amounts of air in to the IC system. However, as has been previously ~ h ~ ~then chromatographic , l ~ ~ system
6 mm
b
I
I
I
Outlet
\
I
69 mm
'AIR
OUTLET
~FLIQUID OUTLET 1 mm
Figure 2. Details of the important components of the particle collection system: (a) mixing chamber, (b) stainless steel maze, and (c) gashquid separator.
tolerates the introduction of small amounts of air without deterioration in performance. Particle Size Distribution and Collection Efficiency. The size distribution of the test aerosol was determined by a laserbased optical particle counter (Model A2212-.01-115-1, Met-One, Analytical Chemistry, Vol. 67, No. 1, January 1, 1995
73
Grant's Pass, OR). The aerosol was sampled at the denuder inlet to eliminate errors arising from the growth of particles due to passage through the moist denuder.15b To avoid any size discrimination at the inlet, the sampling was done through a Y joint at a near-isokinetic sampling velocity. The resulting data were fitted to a log-normal distribution using a least-squares minimization routine (MINSQ, Salt Lake City, UT). The reported MMAD is based on this fit and the known density of the compound nebulized. For the estimation of the collection efficiency of the test aerosol, the exit air of the air/liquid separator was sampled through a 47 mm diameter glass fiber filter. Particles not collected by the system are trapped on the filter. Analysis of the filter extract compared to that of the collection system effluent can thus be used to compute the system collection efficiency.This approach, however, does not account for any inlet losses of the collection system. In a second approach, used with NaN03 and Na2S04 aerosols, sampling was conducted in parallel on a glass fiber filter and the denuder/steam chamber/maze collection system. Sampling periods of 30 -150 min were used. The total amount of particles collected by each system, normalized by the individual sampling rates, was then compared to determine the collection efficiency of the automated system. Interference from Soluble Gases. The efficiency of the PPDD to remove potentially interfering gases was evaluated with various acidic and basic gases such as SOZ, HN02, and NH3. Gases such as NO and NO2 are not expected to be significantly removed by the PPDD. Interference from these gases for the particle collection and analysis system was also evaluated. For SO2 and NOz, gas standards were made by diluting the effluent from gravimetrically calibrated gas permeation devices (VICI Metronics, Santa Clara, CA) having emission rates of 3075 and 2570 ng/min, respectively. For HONO, the standard gas was generated by a technique based on the dissociation of NHhN02, as previously described.16 The emission rate was calibrated independently by wet analysis to be 15.4 ng/min. For NH3, the standard gas was generated by a Henry's law based deviceI7and was calibrated independently by a fluorometric method.I8 Interference studies were carried out by two different strategies. In the first approach, the system was tested with the test gases in the absences of any particles. The liquid effluent from the PCS was analyzed to determine the extent to which gases are being collected by it and causing positive errors. In the second approach, the test aerosol was sampled with and without the test gases with a view to determining any increase in the analyte concentration in the liquid effluent of the PCS in the presence of the test gas. In both approaches, the liquid effluent from the PPDD was also analyzed to verify the efficiency of removal of the test gas by the denuder. Ammonium was measured by a fluorometric method,18 all the other anionogenic gases were measured by IC. RESULTS AND DISCUSSION
Stability of Aerosol Generation. Long-term stability of the aerosol generation system is important in accurately determining a number of the performance parameters. This was evaluated by generating (NHJzS04 aerosol. The system was operated in 10 min sampling/analysis cycles, and the total equivalents of (16)Vecera, 2.; Dasgupta, P. IC Enoiron. Sci. Technol. 1991,25, 255-260. (17) Dasgupta, P.IC;Dong, S. Afmos. Environ. 1986,20,565-570. (18)Genfa, Z.;Dasgupta, P. IC;Dong, S. Enuiron. Sci. Technol. 1989,23,14671474.
74
Analytical Chemistry, Vol. 67,No. 1, January 1, 1995
sulfate collected were measured. The relative standard deviation (RSD) was found to be 7.8% over a 10 h period (n = 60). This performance obligatorily includes any deviations in particle collection efficiency as well. Obviously, for any reported result in which the performance is assessed by comparison of data obtained over two or more sampling periods, the stability of the aerosol source can become the limiting factor. For this reason, all the collection efficiency data reported here were based on concurrent sampling. Particle Collection. Initial attempts were based on an electrostatic collection principle. We attempted to achieve continuous transfer of the aerosols to the liquid stream with the help of a high electric field. The instrument setup was essentiallythe same as that of a PPDD except that a dc voltage was applied between the two wetted glass plates by connecting the terminals of a variable high-voltage power supply to the liquid outlets on each of the plates. We hoped that there would be some residual charge on the particles under most conditions or that such would be induced under the electric field. The particles should thus be collected on the wet surface of the appropriately charged plate. The particles impacting on each plate would then be carried out by the liquid flowing down the plate and analysis performed in exactly the same manner as the standard PPDD effluent after appropriate grounding provisions. In practice, the maximum voltage we could apply between the plates was 2.5 kV, corresponding to a field strength of 8.3 kV/cm. At higher voltages, arcing inevitably occurred between the two plates. This is not surprising since the air between the plates is essentially saturated with water. Particle collection efficiency increased with increased applied voltage, but the maximum collection efficiency obtained at subarcing voltages was 5 50%for (NH4)2S04 aerosols of -1 pm MMAD. An arrangement to charge the particles prior to the collection is expected to improve the collection efficiency. Probably the most expedient way to achieve this is to use a corona discharge source or a radioactiveb-source. We decided, however, to abandon this route because of concerns regarding NO, production in a discharge source or potential hazards with a radioactive emitter. Further, the use of a high-voltage source in a wet environment may present safety problems, especially in a field-deployable instrument. The second approach involved the fabrication of a wetted-wall cooled impactor capable of supporting a high sampling rate. A @rex glass tube (1000 mm x 12 mm i.d.) was coated inside with silica in the same manner as the PPDD to provide a highly wettable surface. Except for 5 cm length at each end, the rest of the tube was flattened (cross section -3 x 14 mm) at high temperature. The flattened portion was then bent several times back and forth to create 22 U turns with a mean distance of -4 cm between successive turns (the conduit was now only 30 cm in total length). At each end the tube was provided with end fittings'%that allowed injection and withdrawal of liquids without disruption of the airflow. The whole tube was surrounded by a jacket through which a cooling fluid was introduced to maintain the temperature at a value close to 2 "C. Soluble gas denuded aerosol and steam were introduced through the inlet at the top. The liquid effluent collected from the bottom was sent to the IC for analysis. The results with this device were very encouraging. Even with a steam injection rate as little as 500 mg/min, (NH4)r SO4 aerosol of 0.7 pm MMAD was quantitatively collected. The thermoelectrically cooled maze-based design was a direct SUC-
cessor to this, conceived in an effort to streamline the cooling needs and make the apparatus less fragile. In a perpendicular maze, particles impact on the maze walls due to a change in momentum. It was initially hypothesized that if a small volume of water is also introduced via a fine orifice at the air inlet of the maze, the water will be aerosolized by the highvelocity air flow and the fine droplets thus formed should attach to the aerosol particles, which should then impact on the walls more easily. Experiments showed, however, that the collection efficiency for (NH4)2S04 aerosol of 0.7 pm MMAD was less than 70%. To overcome any possible problems due to an incompletely wetted maze wall, 0.1%(w/v) of a nonionic fluorocarbon surfactant (Zonyl FSN, DuPont) was used instead of water. This did not increase the collection efficiency to any measurable extent. We concluded that the low collection efficiency is because of the small particle size rather than any wettability problems. This was supported by the ability of the same device to collect a greater fraction of the inlet aerosol when the mean size of the aerosol was increased. It is well-known that very small particles are difficult to collect quantitatively by impaction. Having failed to collect the small particles directly, we thus embarked on a strategy to make the particles grow. As early as 1888, Aitkenlgused vapor saturation to make very small particles grow so that they are large enough to be optically counted. Even today, available condensation nuclei counters rely on this principle. Typically, the aerosol is equilibrated with 2-propanol vapor. The vapor-saturated aerosol is cooled, often by adiabatic expansion, to induce supersaturation. Under this condition, the particles do not necessarily grow large enough to ensure collection by impaction as they do with steam injection. In the present system with steam injection, the sample is supersaturated with water vapor almost instantly, even before external cooling is imposed. The role of the mixing chamber is to provide sufficient residence time for the steam to fully condense on the particles and to assure complete mixing. It is instructive to compute the degree to which such growth occurs. If we assume an idealized case of a monodisperse aerosol of density 2.0 g/cm3, a sampling rate of 10 L/min, an aerosol concentration of 100 pg/m3 (this is much higher than typical ambient fine particle concentration), a steam injection rate of 500 mg/min, and homogeneous growth, it is readily computed that a particle will grow in the radial dimension by 100-fold (final particle density is taken to be the same as that of liquid water); i.e., a particle with an original diameter of 10 nm will grow to 1pm. Simple considerations will also indicate that the factor by which each particle grows increases with higher steam injection rates, lower sampling rates, and lower aerosol concentrations. The process of particle growth is vital to the present collection process, and the importance of steam injection cannot be overemphasized. The chamber MC can thus be appropriately called the cloud chamber. By simile, the cooled maze in this system is a rain chamber. Agglomeration occurs by impaction and the process of vapor condensation is completed by cooling. Note that the moisture content of the sample stream exiting the PPDD is high and cooling to 2 "C can add another -100 mg/min of liquid water to the system effluent. Finally, the collection of the aerosol in the cooled maze is aided by the thermophoretic effect wherein a particle is (19) Aitken, J. h o c . R. SOC.Edinburgh 1888, 35.
Table I. Efficiency of Aerosol Collection
composition of aerosol sulfuric acid sulfuric acid ammonium sulfate ammonium sulfate ammonium nitrate ammonium nitrate sodium nitrate sodium nitrate sodium sulfate sodium sulfate
MMAD,
concn,
Pm
m/m3
~
% collected
by system
0.5475 f 0.0034 1.63 96.97* 0.7139f 0.0068 9.85f 2.06 99.95f 0.06 0.7175 f 0.0054 6.56k 0.17 99.23 f 1.08 1.0057f 0.0065 27.96f 0.53 99.21f 0.14 0.6134f .0065 6.13f 0.36 99.88 f 0.04 13.27f 0.51 99.31f 0.66 0.840f 0.01 0.6038f 0.0097 72.11f 3.5 100.27f 0.52 99.46f 0.09 0.86* 307.4f 2.2 0.4012f 0.0021 64.46f 1.4 99.61f 0.10 0.67b 302.4ic 3.6 99.44f 0.31
Uncertainty was not determined. At low levels, the h i t e filter blank for the postsystem filter is the source of a significant negative bias for the computation of collection efficiency. Not experimentally determined. The value is estimated from the known relationship between diameter and concentration of the test aerosol. (1
attracted to a cold surface.20This process is particularly efficient in the present case since the droplets entering the maze are relatively warm and the maze surface is very cold. Overall collection efficiencies for the system was excellent. A collection efficiency of 99.5%for NaN03 aerosol (-50 pg/m3) of 0.53 pm MMAD was observed with a steam injection rate of 600 mg/min and a sampling rate of 9 L(STF') min-l. Under the same condition, NaZS04 aerosol (-190 pg/m3) of MMAD 0.57 pm was collected with a 98%efficiency, which increased to 99.5%with an increase in the steam injection rate to 800 mg/min. All further experiments were conducted with a steam injection rate of 800-900 mg/min. The collection efficienciesfor various aerosols were tested; the results are tabulated in Table 1. It is apparent that the present system shows excellent collection efficiencies for all the test aerosol systems. Reproducibility of collection efficiency over a long period is also good, within 0.5%for a set of two collection experiments conducted 6 months apart. The 10-90% response time of the PCS itself, as determined by putting a filter on and off at the sampler inlet, is between one and two chromatographic cycles while the 90- 10%response time is 5 1 chromatographic cycle. The high flow rate of air through the maze results in the liquid being swept over the maze surface, and thus this self-cleaning action is quite efficient. Detection Iimits. The detection limit of the combined particle collection/IC analysis system, calculated as three standard deviations of the blank (particlefree zero air) greater than the blank are 2.2, 0.6 and 5.1 ng/m3 for sulfate, nitrite, and nitrate, respectively. The above LODs include the combined uncertainties in aerosol generation, particle collection, and IC analysis. Interferences. If a soluble gas is not fully removed by the PPDD, the gas will be collected by the cold aqueous droplets in the maze, leading to a positive error in the estimation of the concentration of different aerosol constituents. Quantitative collection of potentially interfering soluble acid or basic gases in the PPDD is therefore essential. Experiments with test gases were carried out at concentrations substantially &her than ambient values (except for HONO, where source limitations allowed us to perform testing only at typical nighttime urban concentrations) at a sampling rate of 5 L(STP) (20)Waldmann, L;Schmitt, K H.InAerosol Science; Davies, C.N., Ed.;Academic Press: New York, 1966 pp 137-162. Analytical Chemistry, Vol. 67, No. 1, January 1, 1995
75
5-
Table 2. Efficiency of SO2 Removal as a Function of PPDD Liquid Flow Rate'
denuder water flow, mL/min
anions, nequiv particle collection systemb
0.27 0.415 0.564 0.723
22.8 f 1.4 6.70 f 0.7 1.10 f 0.4 ndc
UQfrom NO only .E 4
--a from NO2 only
'0
5
r".-m 3 --
PPDD
6 from NO2 + 250 ppb NO
8 n
a
67.3 f 1.4 84.3 f 1.8 88.0 f 1.1 90.8 f 1.4
a Test gas 170 ppb SO2, sampled at 5 SLPM, and ambient pressure 680 mmHg. Steam injection rate 0.52 g/min. Not detectable.
min-'. The gas removal efficiency of the PPDD is dependent on the liquid flow rate over the surface of the PPDD. At high gas concentrations and sampling rate, it becomes particularly important to use a sufficient liquid flow rate to maintain a fully wetted surface as well as to avoid surface saturation. The removal efficiency for 170 ppb SO2 by the PPDD is shown in Table 2 as a function of the denuder liquid flow rate. The removal efficiency becomes essentially quantitative at liquid flow rates of >0.5 mL/ min. All further particle collection and interference studies were conducted with a denuder liquid flow rate of 20.6 mL/min. Most other water soluble atmospheric gases, e.g., HCI, NH3, HONO, HN03,etc., have diffusion coefficients equal to or greater than that of SO2 and display intrinsic aqueous solubilities at least as great as that of S02. It was therefore expected that these gases would also be efficientlyremoved. For 62 ppb NH3,1.5 ppb HN02 and 170 ppb SO2 tested individually, no measurable levels of the corresponding ions were found in the liquid effluent of the PCS. In all cases where the system was tested with an aerosol with and without a test gas, the difference between the amounts of the aerosol constituents measured in the PCS was less than the standard deviation of the individual measurements. In the case of NO and NO2, the Henry's law constants are 1.93 x and 1.0 x M/atm, respectively, and thus their uptake on a wetted surface merely by intrinsic solubility is very poor. Though two mechanisms are proposed for the reaction of water with NO and NO2, as shown below,2I
A
0
C
D
Figure 3. Nitrite generated in particle collection system (PCS) from NO and NO2. A-D represent four different concentration regimes tested.
the reaction kinetics are so slow at room temperature that these cannot constitute important pathways for their removal by the PPDD. This means that most of the NO and NO2 originally present in the system also goes through the PCS. In the PCS, however, briefly the system temperature is quite high due to the introduction of hot steam. This promotes the hydrolytic reactions and produces measurable quantities of nitrogen oxyanions in the liquid effluent. Over the entire test concentration range of 0-250 ppb (-310 ,ug/m3) NO and 0-80 ppb (-160 ,ug/m3) NO2, NOz-
far outweighed Nos-, typically by a factor of lO:l, for any test condition (NO alone, NO2 alone, and NO NO2 together). This suggests that mechanisms other than the reactions listed above may be involved. However, measurementswith a chemiluminescencebased NO, monitor indicated that both the NO and NO2 sources were contaminated to a small degree with the other gas. Because the overall amount of the gas that is collected by the PCS is still very small (
