# Patent

## Title
Gas sensing element and a method for measuring a specific gas concentration

## Abstract

The mass of particulate matter in a particle laden gas stream is measured using a first mass detector and a second mass detector. Switching means causes the particle laden gas stream and a substantially identical but particle-free gas stream to alternately engage the first mass detector and the second mass detector respectively during successive measurement time periods. A difference between a reading provided by the first mass detector and a reading provided by the second mass detector for each of the successive measurement time periods is determined. This difference intrinsically corrects for volatilization losses occurring during the measurement time periods. A measure of the mass or concentration of particulate matter in the particulate laden gas stream is determined from this difference.

## Background
This invention relates, in general, to the measurement of particulate matter suspended in a fluid medium and, more specifically, to measurement of the mass and/or concentration of particulate matter suspended in ambient air or in other gaseous environments, e.g. in diesel exhaust, or in mines, smoke stacks, industrial facilities, etc. Particulate matter is the general term which refers to condensed solid, semi-solid, or liquid material produced as a result of natural or man-made processes, and which due to small size, is capable of being suspended in the air or other fluid medium.

The measurement of particulate matter in ambient air is important for a variety of reasons, the most important of which is related to health effects. Suspended particulate matter is known to produce a variety of deleterious health effects when inhaled. As a result, regulatory agencies around the world require monitoring of the levels of particulate matter. The levels are measured in terms of concentration, i.e. micrograms of particulate matter per cubic meter of air. Reference techniques for this measurement are presently defined in terms of a mass measurement utilizing a filter medium to capture the particulate matter and the total volume of air which has been filtered by the medium over a given period of time. There are various means available to unambiguously determine the flow rate through the filter over time (and hence the volume of air sampled), but surprisingly the mass measurement is not straightforward due to the complex nature of ambient particulate matter which results in unstable mass deposition on the filter.

This problem involving the measurement of particulate matter in ambient air is well-known. The uncertainty arises since the particulate mass used as a basis for mass concentration computations is defined as the mass captured on the filter media which is not necessarily the mass of the particles as they exist in the ambient air. Unlike measurements of major criteria gaseous pollutants, what is defined as particulate matter can change its mass as a result of loss or gain of volatile substances associated with the particulate matter and filter media. While gaseous pollutants exist as definable molecular species (SO.sub.2, O.sub.3, CO, etc.), particulate matter can be a combination of different substances with different volatilization rates, reactive, desorptive, absorptive, and adsorptive properties. In addition, the mass of particulate matter landing on the filter can be affected by the filter material itself, the particulate matter already collected on the filter, the face velocity through and pressure drop across the filter, as well as by the humidity, temperature and composition of the gas stream passing through the collection medium.

Both direct and indirect measurement techniques have been employed in an effort to quantify particulate matter mass. Each method which has been developed to date, however, has limitations in obtaining a measurement of the actual mass of particulate matter as it exists in its suspended form. Direct mass measurements as represented by weighing material captured on a substrate such as a filter are susceptible to instrument effects due, for example, to temperature or pressure changes, and to volatile component losses which are not easily quantifiable. Indirect methods such as light scattering measurements on the other hand are inherently inaccurate as there is no physical connection between other properties of particles and particle mass.

To compensate for instrument effects in direct mass measurements, a differential particulate mass measurement microbalance employing a pair of oscillating quartz crystal detectors has previously been proposed. In this earlier approach, a particle laden gas stream impacts upon the first detector and a particle free gas stream impacts the second detector. The second mass detector is used as a reference to cancel out detector instrument effects from a mass reading provided by the first detector. U.S. Pat. No. 5,571,945 discloses a similar measurement approach employing a pressure sensor to measure a pressure differential between a pair of particulate matter collectors; U.S. Pat. No. 5,349,844 discloses a similar approach for use with a filter that is caused to oscillate in a direction substantially perpendicular to a plane of the filter. However, volatilization losses are not accounted for in these earlier systems.

As a result of the above described difficulties, the current reference method in the United States is a method dependent technique which does not necessarily represent an accurate measure of particulate mass as it actually exists in its undisturbed state in the air. The reference method consists of filter equilibration under a defined range of temperature and humidity conditions, a pre-collection weighing of the filter, the installation of the filter in a manual sampler, the sampling of ambient air (for a 24-hour period), the removal of the filter from the sampling device, a post-collection conditioning under the same equilibration conditions as before, and finally a post-collection weighing. This methodology is intended to provide a consistent set of measurements between identical samples.

However, for the reasons stated above, results based on this method do not represent measurements to which an accuracy can be assigned, even loosely, i.e. to what accuracy is the particulate mass as it exists in the atmosphere measured by the mass determined from the filter? This is a serious problem, and one has to accept the fact that these measurements are only an indication of particulate levels. As a result, the current reference method represents simply a standardized procedure, and not a scientifically-based measurement standard for airborne particulate matter.

Volatile components are a confounding influence on these measurements. While the filter resides in the sampling hardware, important factors that influence the reactions taking place on the filter substrate, such as temperature and humidity, vary in an ill-defined manner. During sampling, the mass on and of the filter can increase dramatically during periods of decreasing temperature and increasing relative humidity (nighttime), and may experience substantial loss of semi-volatile materials when the temperature increases and humidity decreases (daytime). These same type of effects can be associated with air mass changes, and other meteorological events. Further, the collection filter may be exposed to widely varying hot or cold temperatures once sampling is complete and before it is removed from the sampler as well as during transportation to a laboratory for conditioning and weighing.

Not only does the mass of collected particulate matter and the filter change depending upon the conditions to which they are exposed, but the air stream through the filter creates a pressure differential across the filter which tends to strip off volatile components of the particulate matter. In effect, the interaction of the particles with the filter tends to modify the nature of the particulate matter as soon as it is collected, thereby affecting the accuracy of the desired measurement of the particulate matter as it is suspended in ambient air. As health concerns heighten, and measurement instrumentation becomes more sensitive, there is a trend towards measurement of even finer particulate matter, e.g. particles of 2.5 microns or less. With smaller particles, the impact of volatilization losses upon the mass measurement readings becomes even more pronounced.

A compelling need thus exists for a measurement instrument that can accurately measure the mass or concentration of particulate matter suspended in ambient air or other gaseous environments.

The present invention provides a method and apparatus which overcomes the problems described above and provides, for the first time, a collection-based direct mass measurement that allows the accurate quantification of the mass of ambient air or other gas borne particulate matter including volatile components thereof. The measurement approach of the present invention not only cancels out detector instrument effects but also intrinsically corrects for volatilization losses. For purposes of this disclosure, the term "volatilization losses" is used broadly to include vaporization, absorption, adsorption, desorption, reactive and other effects which influence the mass of collected particulate matter.

In accordance with the principles of the present invention, apparatus for measuring the mass of particulate matter in a particle laden gas stream includes a first mass detector, a second mass detector and first means for providing a particle free gas stream otherwise substantially identical to the particle laden gas stream. Switching means causes said particle laden gas stream and said particle free gas stream to alternately engage said first mass detector and said second mass detector respectively during successive measurement time periods. A difference between a reading provided by the first mass detector and a reading provided by the second mass detector for each of said successive measurement time periods is computed. This difference intrinsically corrects for volatilization losses occurring during the measurement time periods. A measure of the mass or concentration of particulate matter in the particle laden gas stream is determined from this difference.

Advantageously, the first means for providing a particle free gas stream comprises particle removal means for removing substantially all particulate matter from said particle laden gas stream. Optimally, said particle removal means removes the particulate matter from the particle laden gas stream without appreciably affecting gas stream temperature, pressure and flow rate. Such particle removal is preferably accomplished using an electrostatic precipitator. The precipitator preferably operates with a positive corona discharge and low current.

The first and second mass detectors may each comprise an oscillating element microbalance. Each detector may comprise a hollow element oscillating in a clamped-free mode, with a filter supported at a free end of the element. The filter serves to collect particulate matter from the particle laden gas stream when this stream engages the detector. Fluid control means can advantageously maintain a substantially identical gas stream flow at the filter of each detector during each measurement period. In the oscillating element microbalance embodiment, mass readings provided by the mass detectors are advantageously based upon detected change of frequency of oscillation of the oscillating element with respect to time.

In another aspect of the invention, the switching means of the mass measuring apparatus causes: (a) the particle laden gas stream to engage the first mass detector and the particle free gas stream to simultaneously engage the second mass detector during each of even numbered ones of the successive measurement time periods, and (b) said particle laden gas stream to engage the second mass detector and the particle free gas stream to simultaneously engage the first mass detector during each of odd numbered ones of the successive measurement time periods. A detector engaged by the particle laden gas stream measures mass gain while the detector engaged by the particle free gas stream measures mass lost due to volatilization of volatile components of the particulate matter. The measured mass lost is added to the measured mass gain to determine the measure of the mass of the particulate matter. Each successive measurement time period lasts for a short time, preferably fifteen minutes or less; about a minute or less being presently considered as most preferred.

In another aspect of the invention, the readings provided by the first and second mass detectors each comprise a mass rate reading, which limits accumulation of any calibration errors in the mass measurement.

In yet another aspect of the present invention, corrected mass concentration is computed from a corrected mass rate which combines mass rate readings from the first and second mass detectors.

The present invention also presents a significant improvement to existing differential particle mass measurement systems. In such systems, a particle laden gas stream engages a first mass detector and a particle free gas stream engages a second mass detector. The second mass detector is used as a reference to cancel out detector instrument effects from a reading provided by the first mass detector. According to the present invention, such a differential particle mass measurement system is improved by inclusion of switching means for causing the particle laden gas stream and the particle free gas stream to alternately engage the first mass detector and the second mass detector, respectively, during successive measurement time periods. In this fashion, correction is intrinsically provided for volatilization losses occurring during the successive measurement time periods.

In a further aspect of the present invention, apparatus for measuring the mass of particulate matter, including volatile components thereof, in a particle laden gas stream is provided. This apparatus includes means for forming the particle laden gas stream into a first stream and a second stream, a first mass detector and a second mass detector, means for directing the first stream to continually engage the first mass detector and for directing the second stream to continually engage the second mass detector, and first and second stream particle removal means for removing substantially all particulate matter from the first and second streams, respectively, when the corresponding particle removal means is activated. Control means alternately activates only one of the first stream particle removal means and the second stream particle removal means for successive measurement time periods. A difference is determined between a first reading provided by the first mass detector and a second reading provided by the second mass detector for each of the successive measurement time periods. This difference intrinsically corrects for volatilization losses occurring during the measurement time periods. A measure of the mass or concentration of particulate matter in the particle laden gas stream is determined from this difference.

Pursuant to a still further aspect of the present invention, a differential particle mass measurement method is improved. In the known method, a particle laden gas stream engages a first mass detector. The first mass detector collects a current particle sample from the gas stream during a current measurement time period and measures mass gain due thereto. A second mass detector is used as a reference to cancel out detector instrument effects. The present invention improves upon this method by using the second mass detector to also measure a change in particle property occurring during said current measurement time period. This change in particle property usually comprises a loss of mass due to volatilization of collected volatile particles. The mass lost due to volatilization as measured by the second mass detector is added to the mass gain measured by the first mass detector to yield a corrected particle mass measurement for the current measurement time period. The measured loss of mass occurs during the current measurement time period in an earlier collected particle sample; this earlier collected particle sample having been collected by the second mass detector in a preceding measurement time period. Preferably, the current measurement time period and the preceding measurement time period are of such short duration as to ensure substantially identical volatilization during said current measurement time period of the earlier collected sample and the current particle sample.

The present invention provides numerous significant benefits and advantages. Foremost among these is its intrinsic correction for volatilization losses occurring during measurement time periods. Using short measurement time periods ensures that the particulate mass measurement includes an accurate representation of the volatile mass associated with the collected particulate under any selected temperature including varying ambient temperature conditions. Since both mass detectors see the identical collector (e.g. filter) and instrument artifacts at all times, compensation for instrument effects is effective and complete when the readings from one detector are subtracted from those of the other. The preferred use of an electrostatic precipitator for particle removal prevents any pressure disturbance from occurring, and there is thus no upset of one mass detector with respect to the other in regard to differing volatilization. The electrostatic precipitator also facilitates switching of the particle content of the gas stream on and off electrically and instantaneously with no mechanical motion being necessary. The switching of the gas streams also effectively expands collector (e.g. filter) life by a factor of 2 compared to a single collector system. Further, if two differential measurement instruments are run side-by-side in accordance with the principles of the present invention, one at ambient temperature and the other at a significantly elevated temperature, then a division between volatile and non-volatile components of the ambient particulate matter can be obtained.

These and other aspects, features and advantages of the present invention will be more readily understood from the following detailed description of preferred embodiments when read in conjunction with the accompanying drawing figures in which:

FIG. 1 is a simplified schematic illustration of one embodiment of a differential particulate mass measuring apparatus of the present invention;

FIG. 2 is a graphical depiction useful in understanding how corrected mass concentration can be derived from mass rate readings of the detectors of the differential particulate mass measuring apparatus of the present invention;

FIG. 3 illustrates an alternate embodiment of the apparatus of the present invention; and

FIG. 4 presents a graphical comparison of actual particulate matter concentration measurements derived from a differential particulate mass measuring instrument of the present invention, with measurements from a single tapered element oscillating microbalance.

The goal of the present invention is to accurately measure the mass of particulate matter suspended in ambient air (or other gaseous environment) including volatile components thereof. This is accomplished by causing a particle laden gas stream and a substantially identical but particle free gas stream to alternately engage a first mass detector and a second mass detector during successive measurement time periods. With suitably short measurement time periods, taking a difference between readings provided by the first and second mass detectors, in effect adds to the measured mass gain of the collected particulate matter, the mass lost due to volatilization during the measurement time interval thereby providing an accurate measurement of total particulate mass.

A first embodiment of the particulate mass measurement instrument of the present invention is schematically illustrated in FIG. 1. The instrument 10 has two identical sides "A" and "B" (and reference numbers associated with each side are correspondingly labeled). Each side of instrument 10 includes an inlet 12 in pneumatic communication with a selectively activatable particle remover 14 and a mass detector 16. On each side of the instrument a particle laden gas stream 18 enters the respective inlet 12, passes through the respective, selectively activatable particle remover 14 and continually engages the respective mass detector 16. The term "engages" is used herein to broadly connote the interaction of a gas stream with a mass detector. Such interaction may take many different forms depending upon the nature of the mass detector employed.

Inlets 12 would normally include a separator for pre-separating out of the particle laden gas stream, particles larger than a predetermined "cut-off" size. Inlets with PM10 and/or PM2.5 separators, for example, are well known and commercially available.

The particle laden gas stream exiting inlet 12 is then channeled to and through selectively activatable particle remover 14. When activated, particle remover 14 removes substantially all particulate matter from the gas stream, without appreciably affecting gas stream temperature, pressure and flow rate. Such particle removal can be advantageously implemented using an electrostatic precipitator of the same general type as is commonly used in air cleaning equipment. In order to reduce ozone production, an electrostatic precipitator operating with a positive corona and very low current, e.g. on the order of tens-hundreds nanoamps, is preferred. The current should be sufficient to cause the precipitator to remove substantially all particulate matter from the gas stream.

Particle removers 14A, 14B are selectively alternately activated during successive measurement time periods to remove particulate matter from one and then the other of gas streams 18A and 18B during the successive measurement time periods.

For example, for a first measurement time period, particle remover 14A may be turned on and particle remover 14B turned off. During the next measurement time period, particle remover 14B would be turned on while particle remover 14A is turned off. This alternating activation pattern continues for successive measurement time periods. Advantageously, each time period is of relatively short duration, preferably on the order of fifteen minutes or less, more preferably on the order of five minutes or less, and most preferably on the order of one minute or less.

The gas streams 18A, 18B emerging from the corresponding particle removers 14A, 14B continually engage respective matched mass detectors 16A and 16B throughout the successive measurement time periods.

Although other direct mass or indirect mass detectors such as quartz crystal microbalances, beta absorption monitors, pressure drop monitors, etc. may also be used, the preferred implementation of mass detectors 16 is a tapered hollow element oscillating microbalance. The latter instrument is preferred because of its high mass sensitivity, real-time capability, direct inertia-based mass measurement, and high collection efficiency utilizing filtration.

High mass sensitivity is important since the amount of mass which needs to be measured is in the microgram and sub-microgram range. Real-time measurement is important since mass volatilization can occur in short time frames. Direct mass measurement is desirable to avoid ambiguity in what is measured and also allows for a mass standard traceable calibration. Filtration ensures high collection efficiency. A suitable tapered element oscillating microbalance is described in commonly assigned U.S. Pat. No. 4,391,338, with further background information provided in U.S. Pat. No. 3,926,271. The teachings of these two patents, in their entirety, are incorporated herein by reference.

In the preferred microbalance, a tapered hollow element is made to oscillate in a clamped-free mode. A filter is mounted at a free end of the element and serves to collect particulate matter from the particle laden gas stream. When this stream passes through the filter and then through the hollow element, the frequency of oscillation of the hollow element varies with mass loading of the filter and is readily convertible to a mass reading. For purposes of the present invention, changes in frequency of oscillation of the oscillating element with respect to each measurement time period is advantageously converted to a mass rate (i.e. change in mass with respect to the measured time interval) to determine a corrected mass concentration, as more fully described hereinafter.

Although a tapered hollow element oscillating microbalance is preferred, oscillating elements of other forms or configurations, e.g. non-tapered, tuning fork or U-shaped, or one operating in another mode, e.g. clamped-clamped mode, or a collector made to oscillate in a direction generally perpendicular to a plane of the collector, or an impaction plate or other particle collector, may also be employed as the mass detector.

In operation, particle remover 14B is on when particle remover 14A is off and vice versa. With particle remover 14A off, mass detector 16A will measure an effective mass, M.sub.Aeff, due to frequency changes resulting from mass changes and instrument effects, over a time period, .DELTA.t: +.beta..DELTA.T+.gamma..DELTA.P  (1)

where: material adsorption/desorption temperature change, .DELTA.T, during time interval, .DELTA.t pressure change, .DELTA.P, during time interval, .DELTA.t.

During the same time interval, .DELTA.t, side "B" has its particle remover 14B on, i.e. M.sub.p and M.sub.pv are removed and are not measured on mass detector 16B, and:

Therefore: +.beta..DELTA.T+.gamma..DELTA.P-(.alpha.M.sub.G +.beta..DELTA.T+.gamma..DELTA.P)=M.sub.p +M.sub.pv  (3)

During the next measurement time period or interval, particle remover 14A is on and particle remover 14B is off. These time intervals are relatively short in order to keep the same particulate matter history on both detectors so that, for example, the adsorption/desorption effects of gases or vapors in the gas stream remains comparable allowing an accurate subtraction.

The mass detector 16 associated with the particle remover 14 that is not activated during a particular measurement time period, provides a mass reading, i.e. mass gain, representative of the mass of particulate matter collected during that time period. A measure of the mass lost due to volatilization during that same time period is provided by the other mass detector engaged by the particle free (particle remover activated) gas stream. Subtracting the two mass readings, effectively adds the lost mass to the measured mass gain to provide a corrected and accurate measure of the particulate matter mass.

In practice, the measurements can be based on mass rates utilizing the switching time as a convenient time basis. By subtracting mass rates, as opposed to total mass measurements, any slight calibration error in the measurement systems will produce only a comparable final error in the results. If mass were strictly used, this error could accumulate to unacceptable levels as monitoring time continues and total mass on both detectors would appear to drift from each other.

FIG. 2 depicts an example of how mass might vary over four successive measurement time periods as measured by the detectors 16A, 16B of the instrument of the present invention. A corrected mass concentration can be calculated in accordance with the following relationships: /.DELTA.t).sub.n =(.DELTA.m/.DELTA.t).sub.n  (4)

and

where: laden gas stream engages the first ("A") mass detector, and for odd n's, the particle laden gas stream engages the second ("B") mass detector, detector, detector,

In equation (4) above, the indexing to (-1) effectuates a subtraction of the readings.

FIG. 3 illustrates an alternate embodiment of the differential particulate mass measurement apparatus of the present invention. In measurement instrument 40, a particulate laden gas stream 42 is drawn in through a particle size selective inlet 44, of conventional construction, and then is split into three streams 46A, 46B and 46C. Particle laden gas streams 46A and 46B flow through an optional dryer or dehumidifier 47 (e.g. a Perma Pure PD.TM.-Series gas dryer of Nafion.RTM. construction available from Perma Pure Inc. of Toms River, N.J.), then through selectively activatable particle removers (e.g. electrostatic precipitators) 48A, 48B respectively, and then engage mass detectors (e.g. tapered element oscillating microbalances) 50A and 50B, respectively. Dryer 47 advantageously serves to reduce, control, or eliminate water vapor in gas streams 46A and 46B. Stream 46C represents a by-pass flow to allow proper flow rate through inlet 44. Flow controllers 52A, 52B and 52C serve to maintain the desired constant flow rates in streams 46A, 46B and 46C, respectively, and equal flow rate at each of mass detectors 50A, 50B. For example, if the flow rate out of inlet 44 is 16.7 l/min, flow controllers 52A and 52B can maintain a flow rate of 2.0 l/m in streams 46A and 46B respectively, while flow controller 52C maintains a flow rate of 12.7 l/min in stream 46C. A common vacuum pump 54 working in conjunction with the flow controllers establishes the desired flow. A particle remover switch 56 alternately activates one of particle removers 48A, 48B for successive measurement time periods as directed by controller 58. Controller 58 which can readily be implemented by a microcomputer or other known processor, also controls a frequency counter data analyzer 60 which receives frequency readings from mass detectors 50A, 50B and, in known fashion, transforms such frequency readings into mass readings. From these mass readings, a measure of the mass and/or concentration of particulate matter in the particle laden gas stream 42 is determined, as earlier described, by analyzer 60, and then provided to output device 62. As will be readily apparent to those skilled in this art, switch 56, controller 58, data analyzer 60, output device 62 and the other components of this apparatus can take many different forms.

The operation of instrument 40 is identical to that described earlier for instrument 10. Instrument 40 provides a simplification in that a single inlet and control system can be used to derive the corrected mass concentration data.

FIG. 4 depicts actual test results of a differential particulate mass monitor of the present invention and a single tapered element oscillating microbalance operating in the same vicinity during the same time period. Plot 64 depicts concentration at ambient temperature as measured by the differential particulate mass monitor; plot 66 represents concentration readings from a single tapered element oscillating microbalance maintained at 50.degree. C. As expected, the differential mass monitor of the present invention which intrinsically corrects for volatilization losses, shows higher concentration levels.

Although preferred embodiments have been described and depicted herein, it will be readily apparent to those skilled in the art that various modifications, substitutions, additions and the like can be made without departing from the spirit of the invention. For example, instead of using two particle removers, a single particle remover can be employed to create the desired particle free gas stream. Switching of the resulting particle free gas stream and the particle laden gas stream with respect to the first and second mass detectors can be accomplished by mechanically switching the single particle remover between alternate gas stream paths, or mechanically switching, e.g. with a valve, the output of the single particle remover between the two mass detectors, or mechanically switching the location of the two detectors. Rather than an electrostatic precipitator, a filter, e.g. a low pressure drop electrolet filter or other particle remover can be employed. Depending upon the intended application and operating conditions, a denuder to reduce gaseous component(s), temperature and/or humidity control equipment (such as that taught in copending, commonly assigned U.S. Ser. No. 09/014,252, now U.S. Pat. No. 4,151,953 incorporated, in full, herein by reference), and other gas stream and/or particulate matter conditioning apparatus may also be used with the differential particulate mass monitor. The readings provided by the two mass detectors may comprise frequency, mass, mass rate, mass concentration or other parameter(s). These and other variations which fall within the scope of the appended claims are considered to be part of the present invention.

## Claims

1. Apparatus for measuring the mass of particulate matter in a particle laden gas stream, comprising: substantially identical to said particle laden gas stream; particle free gas stream to alternately engage said first mass detector and said second mass detector respectively during an alternating sequence of successive measurement time periods; and by said first mass detector and a second reading provided by said second mass detector for each of said successive measurement time periods, which difference intrinsically corrects for volatilization losses occurring during said measurement time periods, and for determining from said difference a measure of the mass or concentration of particulate matter in the particle laden gas stream.

2. The apparatus of claim 1 wherein said first means comprises particle removal means for removing substantially all particulate matter from said particle laden gas stream to provide said particle free gas stream.

3. The apparatus of claim 2 wherein said particle removal means removes said particulate matter from said particle laden gas stream while maintaining gas stream temperature, pressure and flow rate substantially the same.

4. The apparatus of claim 3 wherein said particle removal means comprises an electrostatic precipitator.

5. The apparatus of claim 3 wherein said particle removal means comprises a pair of electrostatic precipitators with one of each precipitator placed upstream of each mass detector, and said switching means comprises means for controllably activating only an alternate one of said pair of electrostatic precipitators during each of said successive measurement time periods.

6. The apparatus of claim 5 wherein said first mass detector and said second mass detector each comprises an oscillating element microbalance.

7. The apparatus of claim 6 wherein said first mass detector and said second mass detector each comprises an element oscillating in a clamped-free mode.

8. The apparatus of claim 7 wherein the oscillating element of each detector is hollow and supports a filter at a free end of the element, the filter serving to collect particulate matter from the particle laden gas stream when the particle laden gas stream engages the detector.

9. The apparatus of claim 8 further comprising flow control means for maintaining a substantially identical gas stream flow at the filter of each detector during a measurement period.

10. The apparatus of claim 6 wherein readings provided by said first and second mass detectors are based upon detected change of frequency of oscillation of an oscillating element of the microbalance with respect to time.

11. The apparatus of claim 1, wherein said alternating sequence comprises an even numbered ones set and an odd numbered ones set of measurement time periods, and wherein said switching means causes: said particle free gas stream to simultaneously engage said second mass detector during each of even numbered ones in the sequence of said successive measurement time periods, and said particle free gas stream to simultaneously engage said first mass detector during each of odd numbered ones in the sequence of said successive measurement time periods; represents mass lost during the even numbered ones of said successive measurement time periods, and said first reading represents mass lost and said second reading represents mass gained during said odd numbered ones of said successive measurement time periods; and for each successive measurement time period to determine the measure of the mass or concentration of particulate matter in the particle laden gas stream.

12. The apparatus of claim 11 wherein the mass lost represents mass of volatile component of said particulate matter.

13. The apparatus of claim 12 wherein each of said successive measurement time periods lasts for no more than about fifteen minutes.

14. The apparatus of claim 13 wherein each of said successive measurement time periods lasts for no more than about a minute.

15. The apparatus of claim 1 wherein the first reading provided by the first mass detector and the second reading provided by the second mass detector each comprises a mass rate reading dm/dt.

16. The apparatus of claim 15 wherein said alternating sequence comprises an even numbered ones set and an odd numbered ones set of measurement time periods, and wherein said measure of the mass of particulate matter in the particle laden gas stream comprises a corrected mass concentration determined in accordance with the following relationships: /.DELTA.t).sub.n =(.DELTA.m/.DELTA.t).sub.n and where: stream engages the first mass detector, and stream engages the second mass detector, detector, detector, period n,

17. The apparatus of claim 4 wherein said electrostatic precipitator operates in a low current, positive corona discharge mode.

18. The apparatus of claim 1 further comprising a dryer for reducing water vapor in the particle laden gas stream and in the particle free gas stream.

19. In a differential particle mass measurement system in which a particle laden gas stream engages a first mass detector and a particle free gas stream engages a second mass detector, and the second mass detector is used as a reference to cancel out detector instrument effects from a reading provided by said first mass detector, the improvement comprising: particle free gas stream to alternately engage said first mass detector and said second mass detector respectively during an alternating sequence of successive measurement time periods, whereby correction is intrinsically provided for volatilization losses occurring during said successive measurement time periods.

20. Apparatus for measuring the mass of particulate matter, including volatile components thereof, in a particle laden gas stream, comprising: second stream; detector and for directing said second stream to continually engage said second mass detector; for removing substantially all particulate matter from said first stream when said first stream particle removal means is activated; for removing substantially all particulate matter from said second stream when said second stream particle removal means is activated; particle removal means and said second stream particle removal means over an alternating sequence of successive measurement time periods; and first mass detector and a second reading provided by said second mass detector for each of said successive measurement time periods, which difference intrinsically corrects for volatilization losses occurring during said measurement time periods, and for determining from said difference a measure of the mass or concentration of particulate matter in the particle laden gas stream.

21. The apparatus of claim 20 wherein at least one of said first stream particle removal means and said second stream particle removal means comprises an electrostatic precipitator.

22. In a differential particle mass measurement method in which a particle laden gas stream engages a first mass detector, the first mass detector collects a current particle sample from said stream during a current measurement time period and measures mass gain due thereto, and a second mass detector engaged by a particle free gas stream is used as a reference to cancel out detector instrument effects, the improvement comprising: volatilization losses occurring during said current measurement time period.

23. The method of claim 22 wherein said change in mass comprises a loss of mass due to volatilization losses of collected particles.

24. The method of claim 23 wherein mass lost due to volatilization losses as measured by said second mass detector is added to the mass gain measured by said first mass detector to yield a corrected particle mass measurement for said current measurement time period.

25. The method of claim 24 wherein said measured loss of mass occurs during said current measurement time period in an earlier collected particle sample, said earlier collected particle sample having been collected by said second mass detector in a preceding measurement time period.

26. The method of claim 25 wherein said current measurement time period and said preceding measurement time period are of such short duration as to ensure substantially identical volatilization losses during said current measurement time period of said earlier collected sample and said current particle sample.

27. Apparatus for measuring the mass of particulate matter in a particle laden gas stream, comprising: said particle laden gas stream; free gas stream to alternately engage said first mass detector and said second mass detector respectively during an alternating sequence of successive measurement time periods; and by said first mass detector and a second reading provided by said second mass detector for said successive measurement time periods, which difference intrinsically corrects for volatilization losses occurring during said measurement time periods, and determining from said difference a measure of the mass or concentration of particulate matter in the particle laden gas stream.

28. Apparatus for measuring the mass of particulate matter in a particle laden gas stream, comprising: substantially identical to said particle laden gas stream; particle free gas stream to alternately engage said mass detector during an alternating sequence of successive measurement time periods; and provided by the mass detector for a current measurement time period and a second reading, which difference intrinsically corrects for volatilization losses occurring during the current measurement time period, and for determining from said difference a corrected measure of the mass or concentration of particulate matter in the particle laden gas stream.

29. The apparatus of claim 28 wherein said first means comprises particle removal means for removing substantially all particulate matter from said particle laden gas stream to provide said particle free gas stream.

30. The apparatus of claim 29 wherein said particle removal means removes said particulate matter from said particle laden gas stream while maintaining gas stream temperature, pressure and flow rate substantially the same.

31. Apparatus for measuring the mass of particulate matter in a particle laden gas stream, comprising: otherwise substantially identical to said particle laden gas stream; and particle free gas stream to alternately engage said mass detector during an alternating sequence of successive measurement time periods.

32. The apparatus of claim 31 wherein said electrostatic precipitator operates in a low current, positive corona discharge mode.

33. The apparatus of claim 28 further comprising a dryer for reducing water vapor in the particle laden gas stream and in the particle free gas stream.

34. The apparatus of claim 28 wherein a reading provided by the mass detector comprises a mass rate reading.

35. Apparatus for measuring the mass of particulate matter, including volatile components thereof, in a particle laden gas stream, comprising: substantially all particulate matter from said stream when said particle removal means is activated while maintaining said gas stream otherwise substantially the same; and successive time periods.

36. Apparatus for measuring the mass of particulate matter, including volatile components thereof, in a particle laden gas stream, comprising: substantially all particulate matter from said stream when said particle removal means is activated; and successive measurement time periods.

37. In a particle mass measurement method in which a particle laden gas stream engages a mass detector, the mass detector collects a current particle sample from said stream during a current measurement time period and measures mass gain due thereto, the improvement comprising: particles for said current measurement time period; and measurement time period by adding said measured loss of mass to said measured mass gain.

38. The improved method of claim 37, wherein said loss of mass is measured by a second mass detector during the current measurement time period.