GB2371362A - Monitoring apparatus and method for detecting particles in a gas stream using ionisation means and a SAW detector - Google Patents

Abstract

The apparatus comprises a surface acoustic wave generator 10 for generating surface acoustic waves along a propagation path that includes a detector surface area 17, a gas flow generator 31 for forming a stream of gas to be sampled over the detector surface area; and precipitation means 34 for electrically charging particles in the gas stream prior to incidence of the particles onto the detector surface area. By monitoring changes in the resonant frequency of the surface acoustic wave generator, continuous measurement of the level of gas-borne particles is achieved.

Description

METHOD AND APPARATUS FOR MONITORING ATMOSPHERIC PARTICULATE MATTER The present invention relates to methods and apparatus for the detection and monitoring of atmospheric, or airborne particulate matter, and in particular for the quantitative measurement and analysis of such particulate matter.

I Atmospheric particulates play an important role in atmospheric chemistry. They can directly affect the amount of solar radiation reaching the Earth's surface or indirectly affect the amount as a consequence of their effects on clouds by acting as cloud condensation nuclei. Particles can also affect the gas phase chemistry of the atmosphere by acting as sinks or sources of many reactive species. Particulates less than 10 pm in size can be deposited in the pulmonary and tracheobronchial regions of lungs and have a significant impact on human health.

To assess the importance of atmospheric particulates, high quality atmospheric measurements of the nature and size of atmospheric particulates are required. A variety of prior art techniques are known for characterisation of particulate matter in terms of total mass per unit volume of air, size

distribution and chemical composition (see, eg. W. C. Hinds, 1998, Aerosol Technology : Properties, behaviour, and measurement of airborne particles, J. Wiley and Sons).

Real time measurements with high spatial resolution are required to elucidate the sources and chemistry of atmospheric particulates and the processes involved in their formation and fate. Techniques such as laser ionisation (see, eg. E. Gard et al, 1997, Real time analysis of individual

atmospheric aerosol particles : Design and performance of a portable ATOFMS, Anal. Chem 69, 4083-4091) and aerosol lens-time-of-flight mass

spectrometry (see, eg. J. T. Jayne et al, 2000, Development of an aerosol mass spectrometer for size and composition analysis of submicron particles, Aerosol Sci. Technol. 33, 49-70) allow the real time measurement of particulate size, number density and composition. However, tools to provide real time total mass measurements remain a challenge. Current measurements of total mass of particulate matter, such as gravimetric methods, require particle-loading times of the order of hours to days. In addition, TOF-mass spectrometry remains an expensive and cumbersome technique, not suited to low cost, frequent and widespread use in environmental monitoring.

The majority of available data on atmospheric particulate matter has been obtained using technology based on filter, impactor or electrostatic collection of particles. Whilst such techniques are sufficiently inexpensive to allow a wide spatial coverage, they do not allow real time analysis.

The present invention is directed towards the provision of a low cost, low power, analytical tool for enabling the real time measurement of atmospheric or airborne particles.

Throughout the present specification, the conventional expression"airborne particles" is intended to include particles or particulate matter generally found in a gaseous medium, such as air, although not exclusively limited to air as the gaseous medium. The principles of the invention are therefore applicable not simply to the monitoring of atmospheric particulate matter, but also generally to the monitoring of particles in any gas stream.

According to one aspect, the present invention combines an electrostatic precipitator with a surface acoustic wave detector to provide an apparatus for the monitoring of airborne particles. Surface acoustic wave (SAW) devices

are highly surface mass sensitive, inexpensive and can provide information in-situ and rapidly.

According to another aspect, the present invention provides apparatus for monitoring gas-borne particles comprising: a surface acoustic wave generator for generating surface acoustic waves along a propagation path that includes a detector surface area; a gas flow generator for forming a stream of gas to be sampled over the detector surface area; and precipitation means for electrically charging particles in the gas stream prior to incidence of the particles onto the detector surface area.

According to another aspect, the present invention provides a method of monitoring gas-borne particles comprising the steps of : generating a surface acoustic wave along a propagation path that includes a detector surface area; forming a stream of gas to be sampled over the detector surface area; electrically charging particles in the gas stream prior to incidence of the particles onto the detector surface area; and determining a magnitude of change in a physical property of said acoustic wave to establish a mass loading of charged particles being collected on the detector surface area Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a surface acoustic wave (SAW) device suitable for use in a particle monitoring apparatus according to the present invention; Figure 2 is an exemplary frequency response curve of the SAW device of figure 1;

Figure 3 is a schematic diagram of a particle monitoring apparatus according to a preferred embodiment of the present invention ; Figure 4 is an exemplary graph showing particle size distribution and cumulative distribution as monitored by the apparatus of figure 4; and Figure 5 is a schematic diagram of a multiple SAW element particle monitoring apparatus for determining airborne particle size distribution.

The present invention combines an electrostatic precipitator (EP) device with a SAW detector for the real-time monitoring and sizing of atmospheric particulate matter. The low cost of SAW devices is exploited to provide simultaneous mass determination and particulate sizing by operating a set of EP-SAW devices simultaneously. The relatively small size of SAW devices enables the EP-SAW devices to be conveniently used in a portable field instrument.

With reference to figure 1, a surface acoustic wave device 10 provides a combined mechanical and electromagnetic field disturbance 11 that is localised to around one wavelength of the surface of a solid substrate 1 along a propagation path 12. The field disturbance is hereinafter referred to as a surface acoustic wave.

On a piezoelectric substrate 1 the wave can be generated by a set of metal interdigitated transducer elements 13,14 (IDT's) fabricated on the surface of the substrate, having an RF voltage applied across them. The spacing of the fingers of the IDT 13 or 14 determines the wavelength À and this together with the substrate orientation-which defines the propagation speed vdetermines the frequency of operation.

For a strong piezoelectric substrate, such as y-z lithium niobate, the speed is around 3500 mus-'ad a frequency of 100 MHz can be achieved using a

wavelength of 35 um. SAW devices are widely used as the frequency control element in communications devices (mobile phones, etc.) and in such mass manufactured applications costs have been reduced substantially.

The SAW device 10 as shown in figure 1 is in a delay line configuration and provides a free surface area between pairs ofIDT's 13,14 and 15,16 that can be used as a sensor or detector surface area 17. As the surface acoustic wave 11 is surface localised, any rigid mass deposited on the propagation path 12 changes the speed of propagation and hence the resonant frequency of the device. Thus, at least a portion of the propagation path 12 is used as a particle detector surface area 17.

An exemplary response of the SAW device 10 is shown in figure 2, where a resonant frequency response 20, at frequency of the SAW device having a "clean"detector surface area is contrasted with the resonant frequency response 21, at frequency-g of the device when the detector surface area is"loaded"with particulate matter. The magnitude of the frequency shift, Of provides a measure of the quantity of particulate matter that has deposited on the detector surface area 17.

The downward shift in the frequency response of the device is preferably made entirely dependent on surface mass loading by metallising the propagation path with a metallisation layer to form the detector surface area, and holding this area at a constant DC bias, thus shorting the electric field boundary conditions.

For a mass response, the shift in frequency of the SAW device is proportional to the square of the frequency, so that fabricating IDT's with smaller wavelengths provides enhanced sensitivity. The effectiveness of SAW devices for field based atmospheric applications has been

demonstrated by V. P. Ostanin et al, 2000,"A surface acoustic wave frost point hygrometer for measurements of stratospheric water vapour", who have used the mass sensing principle to determine the dew point variation with altitude. In this work, the substrate temperature of a SAW device is varied until a large change in resonance is detected due to condensation of water vapour on the substrate.

In order to be able to function as a monitor for atmospheric particles, the SAW device of the present invention must be provided with an efficient means for effecting a predictable loading of the particulate material onto the detector surface area 17, in real time, during use of the detector.

The present invention provides such a deposition mechanism using electrostatic precipitation. Atmospheric particles can generally be charged by introducing them into a chamber in which a corona discharge has been created or alternatively using a radioactive source to produce positive ions that collide with, and ionise, the atmospheric particles. A voltage applied to a collector plate causes the charged particles to deposit on the collector plate.

Electrostatic precipitators have high collection efficiencies for particulate matter (99-100%) over a wide range of sizes (0. 05-5 pom).

With reference to figure 3, a combined SAW device and electrostatic precipitator according to a preferred configuration is shown and now described.

The apparatus of figure 3 generally provides a gas-borne particle monitoring apparatus 30, comprising a gas flow inlet 31 incorporating a particle filter 32 and an ionization gas inlet 33 incorporating an ionisation source 34. A detection chamber 35 is connected to both the gas flow inlet 31 and the ionisation gas inlet 33 and houses a SAW device 10 as described in

connection with figure 1. The SAW device 10 is mounted on a heater 36 and is coupled to a frequency monitor 37. The detection chamber 35 is also coupled to a gas outlet 38 coupled to a pump, not shown in the drawing.

The basic principle of operation of the monitoring apparatus of figure 3 is to use electrostatic precipitator embodied in the ionisation source 34 to adhere particulate matter to the metallised detector surface area 17 of the surface acoustic wave device 10 propagation path 12. This metallised detector surface area 17 is the mass-sensing element that determines the frequency shift of the SAW device 10 which itself determines the oscillation frequency of a resonator. To ensure that electrostatically charged particles adhere to the detector surface area 17, the metallised surface is held at a DC potential and operated at a slightly elevated temperature, provided by heater 36, to ensure that only the charged particles of interest adhere to the surface.

Air or other gaseous medium to be monitored for particulates is drawn into the detection chamber 35 through inlet 31 by a pump downstream of the outlet 38. The filter 32 ensures that only particles below a desired detection threshold size (or"pass size") are detected, the passage of larger particles into the detection chamber 35 being prevented. A suitable clean, inert gas such as nitrogen N2 is also drawn into the detection chamber 35 from ionisation gas inlet 33. Due to the action of radioactive bombardment from a Po alpha emitter in the ionisation source 34, the nitrogen carrier gas is ionised to produce N2 molecules. The collision of N2+ molecules with atmospheric particles produces positively charged particles that are collected on the detector surface area 17 owing to the applied negative voltage on the SAW.

Once charged and collected on the metallised detector surface area 17 in the propagation path 12 of the SAW device surface, the particles are

immediately detected by observing the shift Of in the resonant frequency of the SAW device as detected by the frequency monitor 37 that is coupled to the interdigitated transducers 13-16. The metallised detector surface area is preferably held at a constant DC bias. The mass of particulate matter collected is evaluated from the magnitude of the induced frequency shift of the SAW resonator.

To inhibit SAW device sensitivity to mass loading by gaseous material, the device is preferably operated at the elevated temperature provided by heater 36 so that only charged particulates will remain bound to the device surface, uncharged gas molecules being thermally driven off the detector surface area 17.

In the preferred embodiment, described in connection with figure 1, the SAW device comprises a dual IDT delay line resonator fabricated on a lithium niobate substrate designed to operate at an initial resonant frequency of 100MHz. The overall surface area of the SAW device is preferably of the order of 1 cm x 0.4 cm.

Working at higher frequencies is possible and increases the mass sensitivity of the devices, but a frequency of 100 MHz is chosen in a preferred embodiment to allow simpler RF circuit design and greater stability when using standard, commercially available RF amplifiers and power splitters. A range of SAW device designs is possible, and together with variations in the operating temperature, DC bias voltage and operating frequency, can be used to generally optimise the performance, sensitivity and cost of the particle detector in any given application. In addition, varying the relative location and efficiency of the ionisation source 34 and the detection chamber 35 configuration may influence the performance of the particle detector.

Real time particle monitoring is obtained with the apparatus described since, as particulate matter is adhered to the detector surface area, there will be a continuing shift in the resonant frequency. In practice, considerable thicknesses of particulate matter may be built up over time on the detector surface area 17, continuously varying the resonant frequency without significant loss in sensitivity.

It will be understood from the above description that the particle detector 30 is suitable for monitoring, in real time, the arrival in the detection chamber 35 of a single class of particulates having a size range dmax to dmin where dmax is equal to the filter 32 pass size, and dmin is at the detection threshold or sensitivity of the SAW device. The magnitude of the resonant frequency shift, Of, detected by the frequency monitor, will rise with time as a function of the number of particulates present in the gas stream.

To obtain particle size selectivity (ie. to break down the range d to d, into multiple"bins", multiple particle detectors 30 can be used in parallel.

Figure 5 shows one suitable arrangement of apparatus for monitoring multiple particle sizes.

The particle-size-selective monitoring apparatus 50 comprises a gas flow inlet manifold 51 feeding a plurality of separate gas flow inlets 31a... 31e of a corresponding plurality of parallel particle detectors 30a... 30e. Although five such detectors are illustrated, it will be understood that any convenient number may be used.

Each gas flow inlet 31a... 31e supplies its respective particle detector 30a... 30e via a filter 32a... 32e, each filter having a different particle size

threshold. For example, filter 32a may have a particle size threshold of dl, 32b a threshold of d2, and so on down to filter 32e having a particle size

threshold of d5. Of course, any suitable thresholds may be chosen, and these need not be at regular intervals.

The gas outlet 38a... 38e of each detector 30a... 30e may be connected to a common gas outlet manifold 58 leading to a pump (not shown). Preferably, each detector 30a... 30e includes its own respective electrostatic precipitator in the form of an ionisation source 34, although these separate ionisation sources may be connected to a common nitrogen supply 33.

The output (resonant frequency shift, or alternatively, particle count) of each detector 30a... 30e is fed to a central processor 52, which stores the cumulating particle counts into corresponding memory bins 53-1, 53-2,...

53-5 of a memory 53.

The difference in the count between each detector 30a... 30e will thus provide real time measurement of the total mass of atmospheric particulate matter over defined size ranges.

The set of filters 32a... 32e act as size exclusion inlets, so that detector 30a will receive all particle sizes from dmin up to du, while detector 30b will receive all particle sizes up to d2, etc, and detector 30e will only receive particle sizes up to d5, where d5 < d4 < < d2 < . The principle of the multibin system of figure 5 is described with reference to figure 4.

For a given particulate size distribution 41, the size range between, for example, A and B can be determined by measuring the total number of particles in the size ranges up to A and up to B. The difference in the cumulative distribution 42 up to B and up to A then gives the particles in the range between A and B. In the example, the number of counts in the bin d2 to d3 is determined by the difference in counts between detectors 30b and

30c. The counts are, of course, determined by the magnitude of the shift in resonant frequency for each detector 30a... 30e.

It will be understood that the quantitative output of the particle monitoring apparatus may be provided as a total or cumulative mass loading in each particle size range. Alternatively, the quantitative output may be provided as a"number of particles"in each size range, by pre-determining an average mass for a given range of particle sizes for a given expected particulate material being collected and computing a number of particles therefrom.

Coarse particle sizing and fine particle sizing is possible by designing the monitoring apparatus 50 to have an interchangeable filter set. Thus an initial determination of size distribution can be performed with a coarse filter set, and then an optimised filter set can be deployed.

Further advantages of the monitoring apparatus design can be realised.

Because of the low cost of the SAW devices 10, these can be provided as plug-in modules to enable easy analysis of the chemical nature of the particulates gathered at a later date. The chemical nature can be analysed after collection by washing the particles off the SAW device surface and using suitable analytical techniques such as ion chromatography.

The detector response as a function of mass and size of particulate matter can be calibrated using tracer particles and with aerosols generated in the laboratory using commercially available atomizers, such as described in O.

Lindquist et al, 1982,'Low temperature thermal oxidation of nitric oxide in

polluted air", Atmos. Environ. 16, pp. 1957-1972, or by comparison with existing aerosol lens-TOF mass spectrometers.

Preferred embodiments of the invention have been described above. However, it will be apparent that various modifications can be made to these embodiments without departing from the scope of the invention as defined with reference to the appended claims.

For example, although the preferred embodiments use an ionisation source as the means for charging particles in the gas stream, an alternative would be to use a corona discharge. However, this may be disadvantageous in some aspects as it could present an electric shock hazard and a possible source of particle loss as a consequence of electric sparking.

Although the preferred embodiment of the present invention determines a shift in resonant frequency of the propagation path of the SAW device in order to determine the mass loading of particles on the detector surface area, it is possible to use other techniques, in the time or frequency domain, to establish a quantitative measure of such mass loading, as will be understood by those skilled in the art.

For example, generally monitoring changes in measurements of the speed of propagation of the surface acoustic wave along the propagation path will provide a quantitative measure of changes in mass loading on the detector surface area.

In another embodiment, monitoring a phase and/or amplitude shift in the surface acoustic wave received at the end of the propagation path may be used to provide a quantitative measure of changes in mass loading on the detector surface area. More generally, any physical property of the surface acoustic waves travelling along the propagation path that varies as a function of the mass loading of particles onto the detector surface area may be monitored.

In another embodiment, the propagation path may include two traversals of the surface acoustic wave across the detector surface area by providing a SAW transducer on a first side of the detector surface area and reflective structure on the opposite side. Waves transmitted to the reflective structure are returned to the transducer in order that propagation delay, amplitude or phase shift, or other parameter may be measured for the reflected wave.

The presently preferred embodiment uses a lithium niobate substrate because this has a very high coupling coefficient, providing a high efficiency for converting voltage changes at the IDT into an acoustic signal. However, other materials, particularly quartz, could also be chosen also with good results.

Claims (24)

1. Apparatus for monitoring gas-borne particles comprising : a surface acoustic wave generator for generating surface acoustic waves along a propagation path that includes a detector surface area; a gas flow generator for forming a stream of gas to be sampled over the detector surface area; and precipitation means for electrically charging particles in the gas stream prior to incidence of the particles onto the detector surface area.

2. Apparatus according to claim 1 in which the surface acoustic wave generator includes means for determining changes in the resonant frequency of the propagation path.

3. Apparatus according to claim 2 in which the means for determining changes in resonant frequency of the propagation path includes means for determining magnitude of the resonant frequency shift as a function of time.

4. Apparatus according to claim 1 in which the SAW generator includes interdigitated, electrically conductive transducers situated on either side of the detector surface area, having the surface acoustic wave propagation path extending therebetween.

5. Apparatus according to claim 1 in which the detector surface area includes an electrically conductive surface layer.

6. Apparatus according to claim 5 in which the electrically conductive surface layer comprises a metallised layer.

7. Apparatus according to claim 5 or claim 6 further including a bias generator for electrically biasing the electrically conductive surface layer of the detector surface.

8. Apparatus according to claim 7 in which the bias generator is adapted to maintain a negative voltage on the detector surface area.

9. Apparatus according to claim 1 further including a heater for heating the detector surface area.

10. Apparatus according to claim 1 in which the surface acoustic wave generator comprises a pair of interdigitated transducer delay line resonators formed on a lithium niobate substrate.

11. Apparatus according to claim 1 in which the precipitation means comprises an electrostatic precipitator.

12. Apparatus according to claim 11 in which the electrostatic precipitator comprises an ionisation source for generating positive ions for collision with particles in the gas stream.

13. Apparatus according to claim 12 in which the ionisation source comprises an alpha particle emitting source.

14. Apparatus according to claim 11 in which the electrostatic precipitator comprises a corona discharge generator.

15. Apparatus according to claim 1 in which the gas flow generator further includes a particle filter for preventing particles larger than a predetermined size entering the gas stream.

16. Apparatus according to claim 1 further including a plurality of said surface acoustic wave generators each arranged in one of a plurality of gas stream chambers.

17. Apparatus according to claim 16 in which each one of the plurality of gas stream chambers is restricted allow passage only of particle sizes below a predetermined size by a separate filter.

18. Apparatus according to claim 17 further including processing means for receiving an output from each of said plurality of surface acoustic wave generators, the processing means adapted to compute an amount of particulates matter corresponding to each one of a plurality of discrete particle size ranges.

19. Apparatus according to claim 1 in which the detector is removable from the apparatus to allow compositional analysis to be performed on the particles adhered to the detector surface area.

20. Apparatus according to claim 1 in which the surface acoustic wave generator further includes means for determining changes in velocity of propagation of the surface acoustic waves along the propagation path.

21. Apparatus according to claim 1 in which the surface acoustic wave generator further includes means for measuring a phase and/or amplitude shift in the surface acoustic waves transmitted along said propagation path.

22. A method of monitoring gas-home particles comprising the steps of : generating a surface acoustic wave along a propagation path that includes a detector surface area;

forming a stream of gas to be sampled over the detector surface area ; electrically charging particles in the gas stream prior to incidence of the particles onto the detector surface area; and determining a magnitude of change in a physical property of said acoustic wave to establish a mass loading of charged particles being collected on the detector surface area.

23. Apparatus substantially as described herein with reference to the accompanying drawings.

24. A method of monitoring gas-home particles substantially as described herein with reference to the accompanying drawings.