Method and Instrument for Continuous Monitoring of Aerosols

Sulfur gases, and particles are common air pollutents. The measurement of air-borne particulate sulfur provides important data in air pollution monitoring and control. The common methods for measuring air-borne sulfur particulates are based on collections of sulfur particles on filters or impactors followed by chemical analysis of the resulting samples. Analysis techniques include: gravimetric analysis based on conversion to BaSO.sub.4, Ba(C10.sub.4) titrimetry, infrared attenuated total reflectance spectroscopy, ion chromatography, X-ray fluorescence spectroscopy, and X-ray photoelectron spectroscopy. All of these techniques depend on collections of samples followed by subsequent laboratory analysis. These methods utilize considerable periods of time and do not provide for continuous measurements of air-borne particles.

A semi-continuous sulfuric acid aerosol monitor has been developed. This monitor uses the same instrument to sample and make an analysis of the sampling. The monitor operates by collecting samples on Teflon filters and then volatilizing them into a dry N.sub.2 stream flowing to a flame photometric detector. The sampling times are in the 15 to 30 minute range. The sulfur particles samples are collected on surfaces which are generally exposed to ambient sulfur gases. The possibility of conversion of some of the gaseous sulfur to particle form on these surfaces will produce erroneous results. Any collection surface subjected to atmospheric aerosols would become contaminated, and its collection and subsequent characteristics will change with time.

Methods which do not depend on collecting of particles on surfaces have been developed. A humidified nephelometer has been used to identify sulfuric acid aerosols. A temperature programmed heater at the inlet to a nephelometer has also been used to identify sulfuric acid aerosols. Both of these methods have analysis times of a few minutes.

Another technique to continuously monitor the sulfur aerosol uses a flame photometric monitor. A sulfur gas scrubber at the inlet to the monitor allows only sulfur particles to enter the flame. The particle concentration is directly measured. The output signal level of this instrument is barely above the flame background noise. In order to improve the signal to noise ratio, the sulfur particle must be integrated for several minutes. A filter is switched to the line for several minutes to obtain an integrated average background signal. The difference between the average non-filtered signal and the signal with the filter in line gives the signal component due to the sulfur particles. The total analysis time is approximately five minutes.

The invention is directed to a method and instrument operable to continuously detect aerosols without interference from gaseous pollutants. The instrument has an electrostatic precipitator powered by a pulsed high voltage power supply. The power supply is triggered to turn on and off by a reference signal from a lock-in amplifier. During the application of power to the precipitator the aerosol is collected in the precipitator. The pulsed modulated aerosol flows from the precipitator into a detector, as a flame photometric detector. The output from the detector has two components, a small modulated component and a large steady component. The modulated component is fed to the lock-in amplifier. The steady component relates to the gaseous sulfur in the sample. The amplifier is tuned to the frequency and phase of the signal output of the detector. The signal output from the lock-in amplifier can be read directly from a meter or can be connected to a recording device.

An object of the invention is to provide a method and instrument used to make a reliable and continuous measurements of aerosols, as sulfur, phosphorous, sodium and other metals, hydrocarbons, and nitrogen compounds. Another object of the invention is to provide an instrument for continuous measurement of sulfur aerosols having applications in air pollution monitoring, environmental chemistry, and laboratory studies of sulfur aerosol. A further object of the invention is to provide a method and apparatus to continuously monitor particulate sulfur levels in real time under field conditions ranging from ambient levels of about 1 microgram per cubic meter of sulfate to source emission levels which may be as high as 50 micrograms per cubic meter. An additional object of the invention is to provide an electrostatic precipitator that has high collection efficiency, is easy to clean, and has short turn-on and turn-off times. These and other objects and advantages of the invention are embodied in the method and apparatus disclosed herein.

FIG. 1 is a block diagram of the pulsed electrostatic precipitation instrument of the invention;

FIG. 2 is a longitudinal cross sectional view of the electrostatic precipitator of FIG. 1;

FIG. 3 is a sectional view taken along line 3--3 of FIG. 2 and;

FIG. 4 is a sectional view taken along line 4--4 of FIG. 2.

The instrument is diagramatically illustrated in FIG. 1. The instrument, indicated generally at 10, has an electrostatic precipitator 11 receiving aerosol sample 12. Electrostatic precipitator 11 is sequentially energized or turned on and off by a high voltage square wave 13 emanating from a square wave high voltage source 14. When precipitator 11 is on, it charges and precipitates the particles removing them from the aerosol sample stream moving through the precipitator. When precipitator 11 is off, all the aerosol flows through the precipitator 11 and exits therefrom. A modulated aerosol 16 flows from precipitator 11. The modulated aerosol 16 flows into a flame photometric detector 17. Detector 17 can be a commercial Meloy SA 285 flame photometric detector. The modulated aerosol in the detector 17 results in a modulated output from the detector because the detector is sensitive to the aerosol. The modulated portion of the detector output signal is a function of the aerosol concentration in the sample.

Flame photometric output 18 is fed to lock-in amplifier 19. Amplifier 19 selectively amplifies the modulated a.c. component of the detector output signal at the frequency of the square wave high voltage source 14 applied to precipitator 11. Since precipitator 11 does not modulate any gas phase materials, the a.c. component depends primarily on the aerosol concentration of the sample. Gas phase sulfur content, for example, SO.sub.2, is indicated by the d.c. component of the detector output. Thus, the instrument simultaneously measures gaseous and particle sulfur. The output signal 21 from amplifier 19 can be read directly from a meter (not shown) on the front of the amplifier or can be connected to a recording device. The output signal is a continuous measure of the sulfur particle concentration entering the instrument.

The instrument 10 has three main components: electrostatic precipitator 11, flame photometric detector 17, and lock-in amplifier 19. Aerosol sample 12 enters the instrument 10 through the electrostatic precipitator 11 and then flows into flame photometric detector 17. When precipitator 11 is off, both gaseous sulfur compounds and the sulfur particles pass through the precipitator 11 to the flame photometric detector 17 and produce an output response. When precipitator 11 is turned on, Only gaseous sulfur compounds pass through the precipitator 11 and are detected by flame photometric detector 17. Precipitator 11 is turned on and off at regular intervals whereby the amplitude of the pulsed component of the signal can be used as an indication of the sulfur gas concentration. Lock-in amplifier 19 is used to measure the amplitude of the pulsed component of flame photometric detector output signal 18. Amplifier 19 is synchronized to the signal 18 by both frequency and phase so that much of the random flame noise from detector 17 is rejected. A small a.c. signal is extracted from a fairly large and noisey d.c. signal.

Electrostatic precipitator 11, identified as a chopper, is characterized as having a high collection efficiency and short turn-on and turn-off times. Several types of precipitators can be used. The precipitator can have a common charging and collection region or separate charging and collecting regions. One type of precipitator is shown as 22 in FIGS. 2, 3, and 4.

Electrostatic precipitator 22 has an elongated cylindrical body or casing 23 having a longitudinal chamber 24. A tubular inlet member 26 is mounted on the top of body 23. The lower portion of member 26 fits into the upper end of chamber 24 and is held in assembled relation with body 23 with a cap 27. The plurality of screws 28 secure cap 27 to the top of body 23. An elongated metal cylinder, tube or sleeve 29 is concentrically positioned within tubular member 26. Cylinder 29 has an inside cylindrical surface 31 providing a particle collection surface. A plurality of holding bolts 32 threaded into body 23 engage the lower portion of cylinder 29 to mount cylinder 29 on body 23. Cylinder 29 has an inlet or mouth 33 leading to an elongated passage or charging and collection region 34. Passage 34 has a uniform diameter throughout its length and is surrounded by the cylindrical particle collecting surface 31.

An elongated wire 36 is located along the longitudinal central axis of metal cylinder 29. Wire 36 is a fine, 25 .mu.m diameter tungsten wire. The upper or inlet end of wire 36 is attached to an insulator wire 37. Insulator wire 37 is mounted on a bridge or support 38, positioned in tubular member 26 adjacent mouth 33. A high voltage connector 39 is mounted on the mid portion of body 23. The lower end of wire 36 is secured to the end of connector 39 extended into chamber 24. In use a high negative voltage of about -2200V is applied to wire 39 to produce a corona discharge. The aerosol particles flowing through passage 34 are charged by the negative ions produced and are precipitated onto cylindrical surface 31 by the high electric field existing between wire 36 and the grounded metal cylinder 29.

When the voltage is turned off, the aerosol particles flow or penetrate through the passage 34 without being precipitated.

A center line means is used to extract a portion of the modulated aerosol in chamber 24. About 10% of the aerosol is collected by the center line means. The center line means comprises an isokinetic tubular probe 42 extended upwardly into chamber 24. Probe 42 has a passage 43 for carrying the modulated aerosol from chamber 24 to flame photometric detector 17. A plate 44 secured to probe 42 is used to mount the probe on the bottom of body 23. A plurality of screws 46 secure plate 44 to body 23. The remaining 90% of the aerosol flow in chamber 24 is not used. It is discharged to the atmosphere via a side bore 47 in body 23 and a tubular hose connector 48 mounted on body 23. Bore 47 is open to the bottom of chamber 24. Probe 42 extends into chamber 24 along the central longitudinal axis of the chamber. Probe 42 exit ends beyond the second passage 47 locating the inlet end upstream from the inlet of passage 47. A filter (not shown) can be used to remove the aerosols from the excess gases and aerosols.

Tests of the efficiency of electrostatic precipitator 22 in the 0.01 to 1 .mu.m particle diameter range show that it has nearly 100% collection efficiency at an applied voltage of -2200V on corona wire 36.

The operation of a specific embodiment of instrument 10 is described as follows. The specific embodiment includes the electrostatic precipitator 22, a Meloy SA 285 flame photometric detector 17, and a Princeton Applied Research lock-in amplifier 19. The aerosol sample flows into the tubular member 26 into passage 34 of metal cylinder 29. The power from the square wave high voltage source 14 is applied to connector 39 thereby supplying a high voltage to wire 36. Preferably, a high voltage of -2200V is applied to wire 36 to produce a corona discharge. The square wave high voltage source 14 is operated at approximately 2.2 kV. The aerosols moving through passage 34 are charged with negative ions produced by the corona discharge. These charged particles are precipitated onto the cylindrical inside surface 31 of the metal cylinder 29. The gaseous components are not charged and flow through the passage 34 into chamber 24. The square wave high voltage source 14 is sequentially triggered by the reference signal from the lock-in amplifier 19 to intermitently supply power to precipitator 22. When the voltage is turned off, the aerosol particles in passage 34 can flow through the passage 34 without being precipitated on the metal cylinder 29. The result is that there is a modulated aerosol sample in chamber 24. A portion of the aerosol sample in chamber 24 moves through passage 43 of probe 42 to the flame phtotmetric detector 17. Preferably, about 10% of the sample in chamber 24 is suppled to detector 17. The remaining 90% exits to the atmosphere via the side bore 47. A filter (not shown) can be used before the aerosol is discharged back into the atmosphere.

The modulated or pulsed aerosol 16 flows from precipitator 22 into the flame photometric detector 11. The flame photometric detector 17 operates to detect both gases and particulate sulfur. The detector output 18 has a data signal as to the total sulfur content of the aerosol sample into the detector. The modulated aerosol 16 moving into the detector 17 causes the detector output signal 18 to fluctuate since it is related directly to the sulfur content of the aerosol. The fluctuating component of detector 17 is directly related to the sulfur particle concentration. This component is amplified by the frequency and phase sensitive lock-in amplifier 19. Amplifier 19 is tuned to the precipitator modulation frequency with the result that the amplifier output signal 21 is a continuous measure of sulfur particle concentration entering instrument 10.

Amplifier 19 is a commercially available lock-in amplifier. The frequency rate is 0.2 HZ. This frequency is used because it is compatible with the frequency response of flame photometric detector 17. Higher frequencies can be used with detectors having faster responses. The faster detectors will allow higher chopping frequencies which result in better signal to noise ratios and overall instrument response.

Instrument 10 herein described is operable to detect sulfur aerosols. Other types of detectors can be used in lieu of detector 17 to measure other types of aerosols, for example, the following aerosols can be measured: phosphorous by flame photometry, sodium and other metals by flame photometry, hydrocarbons by flame ionization and nitrogen compounds by flame ionization. The main detector characteristic is that it should have a fairly quick response to particles containing the material to be monitored. Detector time constants of less than about 10 seconds are desireable.

While there has been shown and described a prefered embodiment of the instrument and method of continuously monitoring atmospheric aerosols it is understood that modifications, changes in the precipitator, detector, and amplifier can be made by those skilled in the art without departing from the invention.