Anal. Chem. 1996, 68, 3638-3644
Automated System for Chemical Analysis of Airborne Particles Based on Corona-Free Electrostatic Collection Shaorong Liu and Purnendu K. Dasgupta*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
Theory and applications are described for an electrostatic annular tubular aerosol collector. The construction of the device permits the establishment of field strengths in excess of 25 kV/cm in an annular space without causing discharge. As an aerosol sample is drawn through the annular space, field charging occurs and the aerosol is collected on the surfaces of the tubular electrodes. Experiments have been conducted with monodisperse Na2SO4 aerosols in the 0.8-2.1 µm size range. With 2 kV applied between the electrodes, 90% collection is achieved at sampling rates of e0.5 L/min and 80% is collected at a sampling rate of 2 L/min. Lack of corona discharge eliminates the formation of NOx and consequently the artifact formation of NO2- and NO3-. At lower applied voltages, smaller particles are more efficiently collected than larger ones; however, at higher voltages, no significant size dependence on collection is apparent. Experiments and theory thus both suggest that the device should be effective in collecting particles across the fine particle range of interest (∼e2 µm) in the ambient aerosol. The device is coupled in an automated fashion to an ion chromatograph (IC). After the aerosol collection period, water is pumped through the annulus and simultaneously aspirated back through the central electrode and pumped through the IC preconcentrator column, followed by analysis. Instruments for aerosol investigation that rely on the electrical properties of aerosols can be divided broadly into two main categories: electrical mobility based and electrostatic precipitation based.1 A representative of the former type is the electrical aerosol analyzer, used for measuring aerosol size distribution. The best known examples of devices of the second kind are electrostatic precipitators, used industrially for controlling particle emission from stationary sources. Neither type is commonly used for the chemical analysis of aerosols. We have been interested in the automated collection and inline analysis of atmospheric aerosols.2,3 The work thus far reported centers on the use of steam condensation to make fine particles grow followed by collection in a thermoelectrically cooled maze. The latter is particularly power intensive, and alternatives would be desirable. We have recently found that the sampled aerosol can be charged in situ by a corona and then collected in a manner that allows automated periodic washing of the collection (1) Hinds, W. C. Aerosol TechnologysProperties, Behavior, and Measurement of Airborne Particles; John Wiley & Sons: New York, 1982; pp 284-314. (2) Simon, P. K.; Dasgupta, P. K. Anal. Chem. 1995, 67, 71-78. (3) Simon, P. K.; Dasgupta, P. K. Environ. Sci. Technol. 1995, 29, 1534-1541.
3638 Analytical Chemistry, Vol. 68, No. 20, October 15, 1996
surfaces and preconcentration/analysis of the wash solution by chromatography.4 Others have reported on the collection of atmospheric aerosols after using a corona to charge the particles as well.5 However, as is well-known,6 significant amounts of NOx are generated during corona discharge. Our experience is also that significant amounts of artifact nitrogen oxyanions are formed in an aerosol collection system that uses corona discharge.7 This source of artifact makes the determination of aerosol nitrate, an important component of interest in the ambient aerosol, essentially impossible. In a collection system based on corona discharge, the function of the corona is to charge the particles that are to be collected. Without such deliberate charging, ambient aerosol particles still carry an equilibrium Boltzmann charge due to random charging by omnipresent air ions.1 However, the extent of such charging is small, resulting in low collection efficiency by electrical means. For ambient particles in the 0.3-3 µm size range, our experience indicates7 that collection efficiencies of e30% are achieved at a flow rate of 0.5 L/min. We have now discovered that sufficient field charging of the sampled aerosol is possible in the electrostatic collection device itself, thereby making possible a miniaturized aerosol collection device that operates with acceptably high efficiency in the size range of interest for ambient aerosols. The principles governing aerosol collection in such a device, and a description of a complete system in which the collector is coupled to a chromatograph for automated analysis, constitute the subject of this paper. PRINCIPLES Consider the collection system shown in Figure 1: An annular tubular assembly is composed of outer tube of inner radius r1, and a concentric inner tube of outer radius r2. If the outer tube is electrically grounded and a voltage V, less than that required to initiate discharge, is applied to the center tube, the electric field strength at point P, located between the two electrodes, can be expressed as
E)
V r ln(r1/r2)
r2 < r < r1
(1)
where r is the distance between P and the central cylindrical axis. (4) Liu, S.; Dasgupta, P. K. Talanta, in press. (5) Romay, F. J.; Pui, D. Y. H.; Smith, T. J.; Ngo, N. D.; Vincent, J. H. Atmos. Environ. 1996, 30, 2607-2613. (6) Brandvold, D. K.; Martinez, P., Hipsch, R. Atmos. Environ. 1996, 30, 973-976. (7) Liu, S.; Dasgupta, P. K., unpublished studies, Texas Tech University, 19951996. S0003-2700(96)00340-X CCC: $12.00
© 1996 American Chemical Society
ds ) (dr ) υ dt )
CcneV
dt
3πηdr ln(r1/r2)
(5)
The sign of dr depends on the polarities of the voltage and the charge on the particles. If they have the same polarity, dr is positive, meaning that the particle moves outward, and vice versa. Let us first consider that q and V have the same polarity. As the aerosol particles pass through the chamber, there exists a position r′ such that for r g r′, the particles are collected by the outer electrode within their residence time t, while particles at positions r < r′ pass through the system before they reach the outer electrode. The relationship between t and r′ is readily obtained by integrating eq 5:
r′2 ) r12 -
2CcneVtr 3πηd ln(r1/r2)
r1 g r′ g r2
(6)
Similarly, when q and V have opposite polarities, there exists a position r′′ such that, for r e r′′, the particles can be electrically driven to the center electrode within their residence time tr, while particles at positions r > r′′ cannot, and
r′′2 ) r22 +
Figure 1. Annular electrostatic collector.
If a particle carrying n elementary charges is located at P, it will experience an electrostatic force, equal to
neV F ) neE ) r ln(r1/r2)
(2)
2CcqVtr 3πηd ln(r1/r2)
r1 g r′′ g r2
For simplicity, we assume that the aerosol concentration profile is uniform across the entrance of the collection device and that there is no “bounce-off”; i.e., if a particle contacts an electrode, it sticks and is not reentrained. Thence, the collection efficiency can be expressed as
( )(
) ( )( )
2 2 2 2 1 r′′ - r2 1 r1 - r′ φn ) + 2 r2-r2 2 r2-r2 1 2 1 2
2CcneVtr ) Assuming the cylinder is filled with air, on the basis of Stoke’s law and considering the Cunningham slip correction factor,8 the velocity of the moving particle under the electric field is given by
υ)
CcneV
(3)
3πηdr ln(ro/ri)
where η is the viscosity of the gas, d is the particle diameter, and Cc is the Cunningham correction coefficient. An empirical equation for this coefficient was developed by Millikan:9,10
2λ 1.1d 1.252 + 0.399 exp Cc ) 1 + d 2λ
[
(
)]
(4)
where λ is the mean free path of a gas molecule. Based on eq 3, the distance that a particle electrically moves during an infinitesimally small time period dt, can be written as (8) Mercer, T. T. Aerosol Technology in Hazard Evaluation; Academic Press: New York, 1973. (9) Millikan, R. A. Phys. Rev. 1923, 22, 1-23. (10) Gunn, R. J. Colloid Sci. 1955, 10, 107-119.
(7)
2
3πηd(r1 - r22) ln(r1/r2)
(8)
At a given flow rate, the particle residence time in the collection chamber can be calculated by
tr ) π(r12 - r22)L/Q
(9)
where L is the length of the cylindrical tube and Q is the flow rate of the aerosol. Combining eqs 8 and 9,
φn )
2CcneVL 3ηdQ ln(r1/r2)
(10)
It is possible to obtain a value of φn greater than 1 from eq 10, this simply means that the particle is collected before it traverses the entire length L of the collection device. In terms of the observed collection efficiencies, any value of φn > 1 still connotes a unity collection efficiency. Boltzmann’s law has been used to estimate the number of charges on particles for the equilibrium charge distribution of Analytical Chemistry, Vol. 68, No. 20, October 15, 1996
3639
aerosol.11,12 For a monodisperse aerosol at the equilibrium state,
Nn ) N0 exp(-n2e2/dkT)
n ) 0, 1, 2, 3, ...
(11)
where Nn and N0 are the number of particles that respectively possess n and zero elementary charge, e is the elementary charge (4.803 × 10-10 esu), d is the particle diameter (cm), k is Boltzmann’s constant (1.38 × 10-16 erg K-1), and T is the absolute temperature (K). The total number of particles is expressed as
N)
∑
Nn ) N0
n
∑
( )
exp
n
-n2e2 dkT
(12)
If a total number of N particles are aspirated through the collection device and the number of particles collected is Ncol, the latter is given by
Ncol )
∑
∑
φnNn ) N0
n
n
[ ( )] φn exp
-n2e2 dkT
(13)
The overall collection efficiency φ can then be described as
φ)
[ ( )] ∑ ( )
∑φ
Ncol
n
) N
n
exp
exp
n
-n2e2 dkT
-n2e2
(14)
dkT
In the above treatment, the velocity profile across the annulus has been assumed to be uniform for simplicity. In practice, the air flow inside the collection device is not plug flow and this assumption is unrealistic. Nevertheless, eq 14, because of its relatively simple form, is intuitively instructive in predicting the effects of the average number of elementary charges acquired by an individual particle and the particle diameter on the collection efficiency. It is possible, however, to take into account a laminar flow profile. The laminar velocity u in an annulus is given by12
u ) k [1 - (r/r1)2 + {1 - (r2/r1)2}ln(r/r1)/ln(r1/r2)] (15) where k is given by ∆Pr12/4ηL, ∆P being the pressure drop. Consider that the volumetric air flow rate Q is given by
Q)
∫ 2πru dr
(16)
integrated between the limits r2 and r1, k can be readily expressed in terms of Q. (For example, with r2 ) 0.30 cm and r1 ) 0.44 cm, as used in our experiments, k ) 364 Q cm/s, where Q is in cm3/s.) (11) Keefe, D.; Nolan, P. J.; Rich, T. A. Proc. R. Irish Acad. Sect. A 1959, 60A, 27-45. (12) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena; Wiley: New York, 1961; pp 51-53.
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Analytical Chemistry, Vol. 68, No. 20, October 15, 1996
For the fine-particle fraction of the ambient aerosol, under laminar flow conditions and for a limited residence time, the settling effect is negligible, and in the absence of an electrical field, the particles will largely follow the air streamlines. Indeed, it is well-known that particle deposition in annular denuders, for example, is negligible.13,14 Thus, within some infinitesimally small time period, dt as a particle electrically migrates to an electrode; the axial distance that the particle is carried by the airstream is readily expressed as
dl ) u dt
(17)
The corresponding radial distance dr traversed within this time due to electrical attraction has been given by eqn 5. Thus, for particles charged with the same polarity as the voltage applied, we can combine eqs 5, 15, and 17; integration then yields
l)
3πηdr ln(r1/r2) dr CcneV
∫u
(18)
Equation 18 allows us to calculate the distance that a particle moves along the axial direction as it electrically migrates to an electrode. Obviously, a given particle is never collected if l g L. Under any set of given conditions (e.g., Q, d, n, V, etc.), there exists a position R′ such that, if one integrates eq 18 from R′ to r1, l ) L. This means that all particles at r g R′ will migrate to the outer electrode, while all the particles at r < R′ will not be collected. Similarly, for particles charged oppositely to the polarity of the voltage applied, a position R′′ exists such that, for r e R′, all particles will be collected on the center electrode where they will be transmitted. The availability of R′ and R′′ then allows ready evaluation of the collection efficiency for particles if all the particles were uniformly charged:
φn )
( )(
) ( )(
)
2 2 2 2 1 R′′ - r2 1 r1 - R′ + 2 r2-r2 2 r2-r2 1 2 1 2
(19)
If the particles carry an equilibrium charge corresponding to the Boltzmann charge distribution, the overall collection efficiency is given by
Ncol φ)
)
{[( )( ) ( )( )] ( )}
N
∑ n
2 2 1 R′′ - r2
2 2 1 r1 - R′
+
2
2 r2-r2 1 2
r12 - r22
-n2e2 dkT
( )
∑ exp n
exp
-n2e2
(20)
dkT
EXPERIMENTAL SECTION System Configuration. The experimental system is schematically shown in Figure 2. It consists of three major parts: an aerosol generation system, an aerosol collection interface, and an ion chromatographic analysis system. (13) Simon, P. K.; Dasgupta, P. K. Anal. Chem. 1993, 65, 1134-1139. (14) Dasgupta, P. K. ACS Adv. Chem. Ser. 1993, 232, 41-90.
Generation of Source Aerosols. Monodisperse aerosols were used to characterize the collection device. They were generated by a Model 3450 Vibrating Orifice Aerosol Generator (TSI, St. Paul, MN) through nebulizing solutions of sodium sulfate of various concentrations. Aerosol particles, when generated by this means, possess significant amount of electrostatic charge.15 The aerosol was discharged by a Model 3054 Aerosol Neutralizer (TSI) bearing a 10 mCi Kr85 sealed source. Particles exiting the discharger can be considered approximately to be in Boltzmann charge equilibrium.16 A laser-based particle counter (Model A2212-010115-1, Met-One, Grant’s Pass, OR) was used to monitor the stability of the aerosol generation as well as to check the particle diameter generated. A pure air generator (Model 73714A, AADCO, Clearwater, FL), capable of supplying 100 L/min was used for supplying air in all the experiments. Experiments were conducted at sulfate levels typical of modest to high ambient sulfate concentrations, 0.6-6.0 µg m-3. Construction of the Aerosol Collector. Referring to Figure 2, the collector consisted of two concentric stainless steel tubes of 100 mm × 4.4 mm i.d. × 5.2 mm o.d., and 150 mm × 2.4 mm i.d. × 3.0 mm o.d. The two tubes were fixed in position through a tee joint (1/4 in., Ark-Plas Inc., Flippin, AR). The inner tube was positioned concentrically within the outer tube by means of a insulating spacer in the top arm of the tee; concentric positioning is not difficult but important for getting optimum performance. The top end of the center tube extended through the tee joint and was connected to a high-voltage power supply (Model PS350,
Stanford Research Systems, Inc., Sunnyvale, CA). The outer electrode was electrically grounded. A PTFE tube (0.64 mm i.d. × 2.4 mm o.d.) was inserted inside the center tube and was connected to the preconcentration column of the ion chromatographic system via a peristaltic pump. The PTFE tube decreases the holdup volume of the whole system for washing. Experimental Protocol. Table 1 lists the status of each equipment during a given experimental step in a measurement cycle. During the sampling process, the on/off solenoid valve (V1) was open. The aerosol sample was aspirated into the collection chamber by a vacuum pump. The sampling flow rate was controlled by a mass flow controller (Model FC280, Tylan General, Torrance, CA). A filter was put in front of the mass flow controller to prevent the aerosol from entering the flow controller. After the aerosol sample was collected for a period of time, sampling was stopped by closing V1. Water, freshly deionized through a mixed-bed ion exchange resin (Dowex MR-3, Sigma) minicolumn, was then delivered through the annular space between the electrodes to wash the collection surface. The wash effluent was simultaneously aspirated back through the center electrode by the peristaltic pump and pumped through the preconcentration column. The input and output flow rates were set nominally at the same values. All the input solution was aspirated back nevertheless because some evaporation of the solvent occurs. It is critical to have a low annular gap between the tubes; with a large gap the wash solution will simply fall by the action of gravity before it is aspirated. The outer electrode protruded slightly (∼1 mm) beyond the center electrode at the bottom end of the collection device; this formed a “buffer zone” during the washing step. The presence of this zone ensures that even if there is a transient increase in input flow relative to aspiration rate due to pump pulsation, there is no dropping off of the wash solution. Typically, ∼8 mL of deionized water was pumped through the system for complete washing and sample collection. Before the next cycle of aerosol sampling, the system was cleaned and dried by turning on both solenoid valves V1 and V2. Compressed N2 from a cylinder, regulated at 20 psi, was thus admitted into the annular space to blow away any lingering liquid and also to dry the whole system. Experimental Arrangement for the Measurement of Ambient Aerosols. In order to avoid potential interference from soluble gases, the ambient air sample was first passed through a wetted denuder before entering the collection device. Because the sampling rate was relatively low, a single tube denuder sufficed.17 The denuder was simply a glass tube (30 cm × 0.8 cm i.d. × 1 cm o.d.) with a piece of rolled up filter paper covering the inner wall and continuously wetted at the top by a 20 mM NaOH solution pumped at a flow rate of ∼0.3 mL/min via a 27gauge hypodermic needle. The capillary action of the filter paper ensures that entire inner surface of the denuder is wetted. Within the sampling flow rates used in this study, no dry spots in the denuder were noted. Preconcentration and Chemical Analysis. The chemical analysis system was based on a Dionex Model DX-100 ion chromatograph equipped with a self-regenerating suppressor and a conductivity detector (all designed for use with 4 mm bore columns). An AG4 column was placed in the sample loop position of the IC injection valve. With the injector in the “load” position,
(15) Berglund, R. N.; Liu, B. Y. H. Environ. Sci. Technol. 1973, 7, 147-153. (16) Kudo, A.; Takahashi, K. Atmos. Environ. 1972, 6, 543-549.
(17) Simon, P. K.; Dasgupta, P. K.; Vecera, Z. Anal. Chem. 1991, 65, 12371242.
Figure 2. Schematic diagram of the experimental system.
Analytical Chemistry, Vol. 68, No. 20, October 15, 1996
3641
Table 1. Protocol for Aerosol Collection and Measurement step and function
IC injection valve mode
V1
V2
sampling pump
HV
peristaltic pump
duration (min)
1. clean and dry, ongoing analysis 2. collecting aerosol, ongoing analysis 3. wash and load sample
injection injection load
on on off
on off off
off on off
off on off
off off on
5 25a 5b
a This time may vary depending on the aerosol concentration. b This time ensures that all the collected material is washed off the collection area.
the wash solution from the collector was pumped through the AG4 column for analyte preconcentration. The particle size of the AG4 packing is ∼15 µm, ensuring low-pressure drop and the ability of a peristaltic pump to pump the wash effluent through this at a flow rate of 1.0 mL/min (this flow rate was used in all experiments). An AS11 column was used for separation. A 10 mM sodium hydroxide solution was used as the eluent at a flow rate of 1.0 mL/min. Determination of the Collection Efficiencies. The input aerosol concentration was measured by sampling the aerosol through a Millipore (type HA) membrane filter at the same flow rate as that used by the electrical aerosol collection interface. The amount of sulfate in the aqueous extract of the filter was then measured by ion chromatography, thus calculating the aerosol (sulfate) concentration. The collection efficiency was taken to be the ratio of the amount of sulfate collected by the electrical collection device to the amount of sulfate aspirated through it during the experiment. RESULTS AND DISCUSSION Consideration on the Dimensions of the Collector and System Design. The goal of the present research was to develop an efficient electrical aerosol collection device that does not depend on corona discharge to charge the aerosol. It is wellknown that, for a wire-in-tube electrostatic precipitator, if one wants to obtain a stable corona without applying excessively high voltages, the center electrode should be as small in diameter as possible.18 We selected a stainless steel tube of 3.0 mm o.d. as the center electrode; this permitted a reasonably high electric field without causing discharge. This diameter is in fact close to the practical upper limit dictated by the wash and aspirate technique; with larger diameters of the inner tube, the wash solution exiting the annulus cannot be aspirated back. The annular space between the two electrodes constitutes a residence volume of ∼0.8 mL. To reduce the migration distance of the aerosol, it is also desirable that the spacing between the two electrodes be kept small, as long as the pressure drop at the desired sampling rate is maintained within a manageable limits. There is also a need to minimize the annular volume because a wash volume 3-5 times as much is necessary to ensure that all the collected material is washed off. The time necessary to do this is determined primarily by the maximum flow rate permissible through the preconcentration column, 1 mL/min in the present system. Obviously, the necessity of an overly long wash period makes a system less attractive. We arbitrarily decided on a maximum wash period of 5 min. Valve V1 serves several important functions. This valve is closed as the wash step was started. This prevents the wash (18) White, H. J. Industrial Electrostatic Precipitation; Addison-Wesley: Reading, MA, 1963; p 39.
3642 Analytical Chemistry, Vol. 68, No. 20, October 15, 1996
solution being aspirated into the vacuum conduit by any residual vacuum (even if the pump is turned off or the flow controller is set to zero). More importantly, the closure of V1 ensures that that the influent deionized water, aided by the hydrophilicity of the stainless steel, flowed smoothly through the annular space. Without V1 to provide a sealed-off enclosure, the water flows down the annulus discontinuously and some of it drops off. Polarity of the High Voltage. We found no significant difference between positive and negative high voltage on aerosol collection efficiency as long as the magnitudes were the same. Interestingly, in industrial electrostatic precipitators relying on corona discharge, a change in the polarity of the applied voltage causes significant differences in performance.16 Generation of Nitrogen Oxyanions. As previously stated, the major incentive toward designing a dischargeless collection device is to eliminate the production of NOx and thus the source of artifact aerosol nitrite and nitrate. This goal was realized. A control experiment was carried out with the pure air source and no voltage applied on the electrodes. The “blank” nitrate levels was discernible but very low (equivalent to an air concentration level well below 0.1 µg/m3). A voltage of 2 kV was then applied on the center electrode; no discernible increase in the nitrate level was detectable. Nitrite was below the limit of detection in both of the experiments. Collection Efficiency as a Function of Applied Voltage. A high electric field is very important for effective collection of particles, especially at low charge levels. The present collector design allows us to attain a high electric field. For example, an electric field of ∼28 kV/cm is obtained with a modest amount of voltage applied (2 kV). Electric fields much higher than this cannot be used anyway, due to occasional occurrences of spark discharge. Figure 3 shows the collection efficiency as a function of the voltage applied to the central electrode. The circled points with error bars are experimentally measured data. Curves a and b were calculated respectively from eqs 20 and 14. To generate curve a, R′ and R′′ in eq 20 were obtained by numerically solving eq 18. As can be seen, both curves show the same trend but predict significantly lower collection efficiencies than those experimentally observed. There are potentially two reasons for this behavior. First, some degree of field charging must occur as particles enter a high electric field.1 Second, full development of laminar flow does not occur until a significant distance inside the tubular assembly, and until such point, the collection will be slightly more efficient than in the fully developed laminar flow regime.14 However, the lesson from annular denuders suggests that this effect may not be as important as that due to field charging.14 The field charging process occurs throughout the entire collection device, and it is not easy to accurately simulate this. Nevertheless, if we assume a Gaussian charge distribution
Figure 3. Collection efficiency as a function of the voltage applied to the center electrode. Curves a, b, and c are respectively calculated from eqs 20, 14, and 22, and the circled points are experimental data: sampling flow rate, 300 mL/min, sampling time, 25 min; 1.15 µm diameter monodisperse aerosol. These conditions apply to this and all subsequent figures, except as stated.
Figure 4. Comparison of the experimentally measured data and the theoretical predictions.
for the aerosol population, i.e.,
(
)
-n2e2 dkT
Nn ) N0 exp a*
n ) 0, 1, 2, 3, ...
(21)
where 1 . a. Equation 20 takes the modified form
[ ( )] ∑ ( )
∑
-n2e2
φn exp a*
n
φ)
exp a*
n
dkT
-n2e2
(22) Figure 5. Collection efficiency as a function of sampling flow rate.
dkT
The best fit of the experimental data with eq 22 was obtained for a ) 0.13 and is shown as curve c. The agreement would appear to be very good. Figure 4 shows the comparison of the calculated efficiencies (based on eq 22, a ) 0.13) and the experimental data for aerosols of three different sizes. The good match indicates that the eq 21 with a ) 0.12 reasonably represents the charge distribution of the particles. From both theoretical and experimental data in this figure, it is also interesting to notice that, at higher efficiencies, there is very little collection bias toward particles of any particular size. In our experimental system, it is impractical to conduct experiments with particles much smaller than those that have been investigated here. The number concentration remains approximately the same; the mass concentration thus decreases with the cube of the particle diameter. At very small particle sizes, it not only requires an inordinately long time to collect enough aerosol on the filter to measure the mass concentration but blank corrections become significant enough to compromise the accuracy in determining the collection efficiency. However, both theory and experiment indicate that, for the present device, collection efficiency should continue to improve with decreasing particle size. We therefore feel confident that the device will work
with at least the same efficiency as that observed here for particles down to the lower limit of ambient particle size where compositional analysis is still important. Collection Efficiency as a Function of Sampling Flow Rate. Figure 5 shows the collection efficiency as a function of sampling flow rate. The theoretical prediction matches the experimental data reasonably well. In the theoretical treatment, only the residence time of the particles as a function of flow rate was considered. A change in flow rate of course has many other effects on particle collection, such as collection by impaction or particle reentrainment. In any case, it is notable that collection efficiencies of ∼90% were achieved at flow rates of e0.5 L/min-1. Collection of the Particulate Sulfate as a Function of Sampling Time. The collection of the particulate sulfate increased linearly with the sampling time for 1.15 µm diameter particles (linear r2 0.9990). This is useful since measurement sensitivity can be improved by increasing the sampling time. Measurement of Ambient Sulfate and Nitrate. Some measurements of ambient aerosol sulfate and nitrate were made and the results are shown in Figure 6. With natural gas used as the source for electrical power, and no other major combustion/ smelting sources, SO2 levels in Lubbock, TX, are very low. It has been previously observed2 that, in the fine particle fraction, Analytical Chemistry, Vol. 68, No. 20, October 15, 1996
3643
collection device, generation of NOx was avoided. This makes it possible to measure all the soluble constituents contained in the atmospheric aerosol. It was particularly interesting that, at high collection efficiencies (∼90%), there is little or no bias towards the collection of particles of any specific size. This makes the present approach particularly viable even in the absence of quantitative collection. Although only particles in the 0.8-2.1 µm size range have been studied here, particles of smaller size should be collected with at least as great a collection efficiency, because the electric mobility of particles increases as their size decreases.1 Therefore, this approach should be effectively used for the collection and analysis of ambient aerosols in the fine-particle fraction (e2 µm), the size class of importance in regards to both atmospheric visibility and human health effects.
Figure 6. Measurement of ambient sulfate and nitrate.
nitrates are generally present at much higher concentrations than sulfates. The data in Figure 6 are quite comparable to previously measured values at this location;2 the peak in nitrate concentration appears to correspond to the insolation maximum on a winter day. CONCLUSIONS We have investigated here a simple electrostatic aerosol collection device that is easily coupled to liquid phase analytical systems. Because there is no discharge associated with this
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Analytical Chemistry, Vol. 68, No. 20, October 15, 1996
ACKNOWLEDGMENT This article was prepared with the support of the U.S. Department of Energy, Cooperative Agreement DE-FC0495AL85832. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of DOE. This work was conducted through the Amarillo National Resource Center for Plutonium. Received for review April 10, 1996. Accepted July 31, 1996.X AC960340W X
Abstract published in Advance ACS Abstracts, September 1, 1996.
