Electrical Detection of Airborne Particulates Using Surface Ionization Techniques Richard L. Myers and Wade L. Fite" Department of Physics, University of Pittsburgh, Pittsburgh, Pa. 15260
A variety of individual aerosol particles can be detected by causing them to impinge onto a heated metal surface where they pyrolyze and transfer surface-ionizable constituents to the surface within times of 10-6-10-2 sec, depending on the filament temperature. These surfaceionizable constituents then become ionized and are released from the surface as a burst of ions. For particles of similar composition, the number of ions per pulse is related to the particle size. This technique has been studied and has been applied to the continuous monitoring of submicron urban particulate matter.
Sodium and potassium are the sixth and seventh most abundant elements in the earth's crust and are common impurities in virtually all substances. The atoms of these metals are converted to ions upon striking a heated surface having a sufficiently high work function (1). Detection of these ions is commonly used in molecular beam research. When an aerosol particle less than 10 I.( in diameter strikes a heated surface, the time required to heat up to the surface temperature is calculated to be of the order of 0.1-10 psec, depending on the composition of the particle and its contact with the surface. If in the process of heating, the particle melts, sublimes, decomposes through the breaking of chemical bonds or otherwise pyrolyzes, and if the particle contains sodium, potassium, or other surfaceionizable atoms or molecules, either as a prime constituent or as an impurity, these atoms are released, come into contact with the surface and become surface ionized. The effect is to release a burst of ions that, upon detection, indicates the arrival of a particle a t the surface. This report summarizes some results of investigations about the basic process of pyrolysis plus surface ionization for particulate detection and describes some results of using the technique for particulate detection in urban air.
Fundamental Studies The apparatus used to study the basic phenomena, shown in Figure 1, consists of a differentially pumped high-vacuum system into which the aerosol to be tested is admitted through a pinhole. Once inside the vacuum system, the particles travel in a straight line to a heated filament. Any ions formed at the filament are focused into a quadrupole mass filter, the output of which is sent into an ion multiplier. The amplified signals are then analyzed electronically for pulse height, shape, and count rate. In each case, the integrating time of the pulse-counting circuitry was kept longer than the duration of the real ion pulses coming from the filament. Under this condition, the measured pulse height is proportional to the total charge delivered by the real ion pulse, while the measured rise time is equal to the duration of the ion pulse. In the following discussions, the "pulse duration" refers to the duration of the real ion pulse, as measured by the rise time of the electronic pulse. Under the experimental conditions, background ion currents consisting primarily of K' (80%) and Na+ (10%) 334
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with a large number of hydrocarbon and metal ions making up the rest of the spectrum, were observed. These ions are formed a t the surface by surface ionization of impurities in the surface material and in the residual gas in the vacuum. In the present application of surface ionization, it is not the currents which are troublesome, but rather the fluctuations in the currents which appear to the circuitry as pulses similar to those generated by particulates. Using a filament producing a steady current of 106 ions/ sec, and a circuit time constant of 20 psec, the authors found that to limit these background pulses to a count rate of l/sec requires setting the pulse height discriminator to 700 ions per pulse. Setting the discrimination level to 1000 ions per pulse virtually eliminated the background pulses. Background pulses of 1000 ions eliminated by discrimination in the circuitry are to be contrasted with pulses from real particles of interest. For example, carbonyl iron powder particles with diameters from 2-4 p (passed through a vertical elutriator to eliminate clumps) upon striking a heated (1400°C) oxidized tungsten surface, produced pulses of up to 3 X 106 ions per pulse. The ions observed were about 70% K-,20% N a t , with cesium and lithium constituting the remaining 10%. Coal dust, after passing through a 6-stage Anderson impactor specified to remove over 90% of particles larger than 1 p produced high count rates of pulses in excess of 6 X 106 ions composed about equally of Naf and K + , Cigarette smoke particles, after having been filtered through a 0.2-p (pore size) millipore filter, produced pulses of mostly K+ of up to lo5 ions. Rubidium was conspiciously absent from the major components of the spectrum. It was observed that as the temperature was increased from 900-1400°C the pulse durations decreased from about to about sec. Also there is a filament temperature between about 1000" and 1200°C where the pulse height is maximum. Whether the decrease of pulse height a t higher temperatures is caused by some change in the work function of the Shutter
Particles
Filament
'[-i--I(
0
+
I/ /
Ouodrupole Mass F i l t e r
Ion Multiplier
Figure 1. Schematic of basic apparatus for detecting particles using technique of pyrolysis followed by surface ionization
surface or by some specific pyrolysis problem, such as the tendency for a particle to be driven from a very hot surface before pyrolysis is completed, is not known. That such “reflection” of particles, particularly those with diameters in excess of a few microns, does occur is suggested by the observation of the ratios of pulse height to particle size. The relationship which holds for particles less than 1 p in diameter weakens as the size is increased and the pulse heights become almost independent of particle size above about 10 p . Thus, although we have found no upper limit on particle size for detectability, the method does seem to be limited with regard to sizing of particles on the basis of pulse height to a range below a few microns in diameter. Test particles believed to be less than 0.1 p in diameter were generated by spraying dilute solutions of alkali and alkali earth salts into dry air and allowing the droplets to evaporate. The principal conclusions reached were that (1) the optimum temperature for the development of a pulse depends on the ionization potential and vapor pressure of the element being ionized, with the required temperature being higher for Na+ than for K + , and (2) the efficiency with which alkali atoms within a particle are ionized is of the order of 10% of the efficiency with which the same atoms from an atomic beam are surface ionized. Evidently, for sufficiently small particles, the surface ionizable impurities are fairly efficiently transferred to the hot surface during the pyrolysis process. These observations are in general agreement with the independent research carried out by other investigators (2). Coal dust, iron powder, cigarette smoke and certain other particles were also detectable by the formation of negative ion pulses a t the heated surface. In this case the surface used was a thoria-coated iridium filament with the low (2.7 eV) work function conducive to the formation of negative ions. Particles containing appreciable amounts of halogen atoms, NOa, or CN radical were found to produce pulses of the corresponding ions. In the case of all the naturally occurring particles detected by means of negative ions, tine predominant ion formed has been C1- . The pulses formed displayed shorter pulse durations and considerably smaller amplitudes than corresponding alkali metal pulses. For example, when the aerosol used was a suspension of CsCl crystals (formed by spraying an aqeuous solution of CsCl into dry air), the pulse duration for the C1- pulse was about 3 psec, as compared to 10 psec for the C:i+ pulse, and the amplitude of the Cs+ pulse was about 10 times greater than the amplitude of the C1- pulse. On the other hand, the background signal of negative ions is considerably smaller than the positive
Copillory
m
Tube\
ion signals, so that the use of negative ion detection is not without merit. To increase detectability and extend the range of the method into centimicron particle detection, the introduction of specific impurities onto the particle’s surface has been used. A rubidium oven shown in Figure 1 was inserted into the vacuum chamber in such a way that, prior to striking the filament, the particles were made to traverse a crossed beam of rubidium atoms that adhere to the surface of each particle and render it detectable. Rubidium was chosen for this technique because it is absent from the background species, it has the second lowest ionization potential among the elements, and its vapor pressure makes it possible both to produce an intense beam a t easily obtained temperatures and to trap that beam once it has crossed the interaction area. Filtered ( < 0 . 3 p ) cigarette smoke particles, which were detectable by means of their K t ion pulses, but which contain little or no natural Rb, easily produced measurable pulses of Rb+ upon being subjected to the Rb crossed beam. The pulse heights and ,noise levels indicate that particles down to a few hundred Angstroms may be detectable.
Applications When the apparatus was opened to ordinary laboratory air, as opposed to filtered air, a significant increase in the number of positive pulses/sec was noted, indicating that the device shown in Figure 1 is capable of monitoring the level of alkali-containing particles present in ordinary urban air. A simplified apparatus shown in Figure 2 was constructed, in which the aerosol is drawn through a capillary tube into a region maintained a t a vacuum of 1-10 torr. It can be shown that under these conditions particles with diameters larger than about 50 A will impact with nearly’ 100% efficiency onto any obstruction. The filaments used here were made of platinum, iridium, and rhodium, and were maintained a t temperatures near 1100°C. The ions were collected onto an electrode which was biased several hundred volts negative with respect to the filament. The collector electrode was ac coupled to a pulse-analyzing circuit which rejected background noise and recorded the average number per second of pulses above the noise level. The geometry (the capillary-to-filament distance and the filament thickness and length) was chosen such as to give a pulse count rate of about 100 counts/sec from unfiltered air a t a pollution level of about 100 pg/m3 as reported by the Allegheny County Air Pollution Control authorities-a moderately clear day in Pittsburgh. When such air was examined it was found that the
Biosinq ClfCUlt
Anolyzing Circuit
I o n Collector
Heated
Surfoce
Figure 2. Simplified apparatus for continuous air monitoring
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count rate, upon passage of the air through a Nuclepore membrane filter with a pore radius of 1 p was 30% of the rate using unfiltered air. Calculated transmission for this filter exceeds 30% only for particles in the 0.02-0.1 p radius range with the maximum size transmitted being about 1 p . The largest pulses observed had charges of about 4 x 10-12 C, or about 2.5 x 107 ions. This is about 10% of the product of the surface ionization efficiency of free alkali atoms times the number of such atoms a t a concentration of 1% (i.e., comparable to the alkali impurity level in coal, soil, and common minerals) in a 1.0-p particle with a mass density of 1g/cm3. The device described above has been used for 24-hr air sampling in the Oakland section of Pittsburgh. In this case, the particles being sampled were submicron (the air was passed through a 1-p Nuclepore filter) and, to be detected, were required to contain an appreciable amount of alkali metals (the level of the pulse height discriminator was set high). Under these circumstances, large increases in nighttime particulate level were detected nearly every night, including weekends. The contribution from rush hour traffic was found to be small, perhaps because the sampling device was located in a light traffic area upwind from the main streets, and the percentage of Na and K in particulate auto exhaust is small compared to the percentage in fuel oil emission particulates ( 3 ) and in coal ash ( 4 ) . The second of these considerations has been somewhat substantiated by the observation that when the level of the pulse-height discriminator is set sufficiently high that the instrument does not respond to auto exhaust (1973 Capri, regular gasoline), the instrument is still highly responsive to submicron (filtered by a 1-p pore size Nuclepore filter) particles in the smoke from a burning bed of coal. While gasoline tends to have a low content of alkali
metals, it does contain a high content of halides, mostly in the form of additives. By reversing the polarity of the potentials applied to the apparatus, the instrument was made to detect gasoline combustion particulates by negative surface ionization. Further details of the instrumentation used in these studies are being prepared for publication elsewhere.
Summary The process of pyrolysis plus surface ionization has been used with many aerosols and seems to respond to every type of particle, provides immediate response to the presence of aerosols, and seems to work best for particles in the submicron range. For sufficiently small particles, pulse height appears to be related to the particle size in a sufficiently simple manner to permit the technique’s being used for particle sizing. As a means of detecting particles separated by size by other means, the technique appears quite general. The detection of both positive and negative ions concurrently apparently provides information relative to the source of particulate matter in the atmosphere, and does so in real time. Literature Cited (1) Lew, H., “Methods of Experimental Physics.” Vol. 4, part A, V. Hughes, H . Schultz, Eds. p 393, 1968. (2) Davis, W . D., talk presented at the American Society for Mass Spectrometry, 21st Annual Conference on Mass Spectrometry and Allied Topics, May 20-25, 1973, San Francisco, Calif. (3) Miller, M. S., Friedlander, S. K., Hidy, G. M.. “Aerosols and Atmospheric Chemistry,” G. M . Hidy, Ed., pp 301-12, Academic Press, 1972. (4) Lowry, H. H., “Chemistry of Coal Utilization,” Vol. 1, pp 23, 490, Wiley & Sons, 1945.
Received March 14, 1974. Accepted November 22, 1974.
Volume Resistivity-Fly Ash Composition Relationship Roy E. Bickelhaupt Southern Research Institute, 2000 Ninth Ave. South, Birmingham, Ala. 35205
H This research was undertaken to establish a relation-
ship between electrical resistivity and chemical composition for fly ash. Commercially produced ashes having a wide variation in chemical composition were used. The ashes were characterized and made into sintered-disk specimens for resistivity and chemical transference experiments. Characterization revealed that the ashes were principally spherically shaped, glassy particles. The results confirmed that volume conduction was controlled by an ionic mechanism in which the alkali metal ions, mainly sodium, served as charge carriers. It was observed that the iron concentration of the ash affected the magnitude of resistivity in an inverse manner. Chemical transference and other ancillary experiments suggested that the presence of iron influenced the number of alkali metal ions capable of migration. Relationships were established between resistivity and specimen porosity, temperature, lithium-sodium concentration, and iron concentration. These relationships were combined to give an expression with which one can predict volume resistivity.
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Environmental Science & Technology
Approximately 30 million tons of fly ash are produced annually in the United States by burning fossil fuels. Electrical resistivity is one of the critical parameters influencing the design and successful operation of electrostatic precipitation devices used to collect the ash. Therefore, a study of this ash property was undertaken. Although the study involves the entire normal resistivity-temperature range (100-400”C), this paper is restricted to the region in which volume conduction is of primary > -225°C. Volume conduction denotes importance-i.e., the transport of electrical charge through the individual fly ash particulates and the multiplicity of contacts. Above 225”C, resistivity is principally governed by the following factors: the amount and chemical constitution of the various microconstituents making up the fly ash, the degree of continuity of the ash layer, the temperature, and the voltage gradient across the ash layer. It has been previously concluded ( I ) that the volume conduction mechanism is ionic in a manner analogous t o that of glass and that the alkali metals, principally sodium, are the charge carriers. This conclusion was reached from the evidence of the proportionality between mass
