Abstract:
A wide range particle counter has sections which separately detect coarse and fine particles in an aerosol. The coarse particles are counted and sized in an optical particle counter. The fine particles are classified with a differential mobility analyzer to determine size and then passed through a vaporizer and condenser and also counted. Using the different sections permits counting and measuring a wide range of particle size in a single instrument with high accuracy and reliability.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and apparatus for measuring the size distribution of aerosols over a wide particle size range. Specifically, the invention relates to the measurement of particles suspended in a gas, which is referred to as an aerosol. The most common carrier gas is air, but other gases, such as nitrogen, helium, argon, CO 2 , and other gases, may also be the media for particle suspension. The particles can be solid, liquid, or a mixture of both. 
     In the ambient atmosphere, particles may exist over a size range from about 2 nanometers (nm) to over 50,000 nm in diameter, with particles in the 10 nm to 10,000 nm range being the most important from a health and safety standpoint. No single device currently exists that can measure particles over this range. The wide-range particle counter (WPC) described herein makes this possible. 
     Particle counters now available have a limited operable range of sizes, and several different particle counters are needed to properly analyze aerosols. 
     Aerosols occur both in nature and in the human environment. They are important in scientific research and in technical applications. Aerosol particles in the atmosphere can scatter light and affect atmospheric visibility. When inhaled, the suspended particles can deposit in the lungs to cause potential health effects in humans. Aerosol particles often need to be measured so the sources of the particles can be controlled or precautions taken if the sources cannot be controlled. 
     Aerosols are also generated on purpose for scientific and technical applications. In laboratory studies, for instance, aerosols with controlled size distribution are needed to test filters and other particle collectors to determine their efficiency. In medical applications, drug compounds are frequently generated in aerosol form for delivery to the lungs for disease treatment. The particle size distribution is important because particle size determines the specific regions of the lungs where the inhaled particles will deposit, hence the effectiveness and efficacy of the inhaled drugs. In all cases in this specification, a gas containing suspended particles shall be referred to as an aerosol, with no limitation being made as to the chemical nature of the particles and that of the gas, and their respective physical states. 
     One of the most widely used aerosol-measuring instruments presently is the optical particle counter (OPC) first described in U.S. Pat. No. 2,732,753 (O&#39;Konski). In an OPC, the aerosol is passed through a beam of light to cause optical scattering. The scattered light signal from each particle is then detected and related to particle size. The OPC is capable of detecting particles to a lower size limit of about 100 nm in diameter, with some special OPCs having been designed to detect particles as small as 60 nm in diameter or a characteristic dimension. 
     Another particle-measuring instrument is the condensation nucleus counter (CNC), also referred to as a condensation particle counter. The most widely used CNC is that based on U.S. Pat. No. 4,790,650 (Keady). In this CNC, the aerosol is first saturated with the vapor of a working fluid at an elevated temperature. A typical working fluid is butyl alcohol, and a typical saturator temperature is 35° C. The vapor-laden aerosol then passes through a condenser, typically kept at 5° C. to cool the gas and cause the vapor to condense on particles to form droplets. The droplets are then counted by optical scattering, as in a conventional OPC. The CNC is capable of detecting particles below the lower size limit of the OPC, since droplets formed by vapor condensation are considerably larger than the particles themselves, thus making them easier to detector by light scattering. 
     Since a CNC is only capable of counting particles, but not measuring the particle size, a CNC must be combined with a size-analyzing device, such as a mobility analyzer, in order to both determine the size and the particles count. A differential mobility analyzer (DMA) is usually used for size determination. The DMA method of size classification is based on the electrical mobility of singly charged particles, i.e. particles carrying a single electron unit of charge. Liu and Pui (1974) and Knutson and Whitby (1975) were the developers of the DMA for this application. The publications explaining this DMA method are: “A Submicron Aerosol Standard and the Primary, Absolute Calibration of the Condensation Nuclei Counter,” Benjamin Y. H. Liu, David Y. H. Pui,  Journal of Colloid and Interface Science , vol. 47, No. 1, Apr. 1974; and “Aerosol Classification by Electric Mobility: Apparatus, Theory, and Applications,”  Journal of Aerosol Science , 1975 pp. 443-451, W. O. Knutson and K. T. Whitby. 
     Recent improvements in the DMA are described in the article “Design and Testing of an Aerosol/Sheath Inlet for High Resolution Measurements with a DMA,” Da-Ren Chen, David Y. H. Pui, George W. Mulholland, and Marco Fernandez,  Journal of Aerosol Science , Vol. 30, No. 8, pp. 983-999, 1999 by Chen et al (1995). The development of the nano-DMA for particle measurement below 50 nm in particle diameter is disclosed by Pui et al in U.S. Pat. No. 6,230,572 B1. These recent developments further improved the accuracy and range of the DMA devices. 
     The DMA method of size classification relies on the fact that the electrical mobility of a singly charged particle is inversely related to particle size. A polydisburse aerosol containing singly charged particles over a range of sizes can be classified according to size in an electric field and produce a nearly monodisburse aerosol within a narrow range of electrical mobilities and thus the produced aerosol contains particles of substantially the same size. The classified aerosol can then be counted by a CNC. The DMA is generally limited to particles smaller than about 500 nm in diameter. 
     All aerosol-measuring instruments have certain inherent size limits. In the case of the DMA, the limit is due to the low electrical mobility of large particles. As the particle size increases, the electrical voltage needed to classify the particle by electrical mobility also increases. At the usual flow rate used in differential mobility analysis, voltages as high as 10,000 Volts may be needed to classify particles at a diameter of 500 nm. For this reason, mobility analysis is seldom used beyond an upper size limit of about 500 nm. 
     On the other hand, the OPC is limited in the particle size it can satisfactorily detect due to the scattered light signal from a particle generally decreasing with decreasing particle size. Below about 100 nm, the scattered light signal begins to enter the so-called Rayleigh scattering regime, where the signal varies approximately as the sixth power of particle size. A factor-of-two reduction in particle size would thus lead to approximately a 64-fold reduction in the scattered light signal. Detecting small particles below 100 nm becomes increasingly more difficult, even when using a high-powered lasers as light sources, collecting optics with a high numerical aperture, and sensitive photo-detectors. Although optical particle counters have been designed to detect particles as small as 60 nm in diameter, the equipment needed is generally large and expensive. For this reason, high sensitivity optical particle counters are not widely used. 
     In principle, optical particle counters can be further improved to detect particles smaller than 60 nm. With further advance, even smaller particles may be detectable. However, advances in optical particle counting technology have not made the technology more useful for aerosol measurement over a wide size range. Designers of optical particle counters have not recognized the issues related to wide range particle counting and the special requirements that must be met in order to measure particles over a wide size range. A requirement that is illustrated with the following example. 
     In the ambient atmosphere, the aerosol size distribution generally follows Junge&#39;s law, which states that the concentration of aerosol particles larger than a certain size is inversely proportional to the 3 rd  power of particle size. If the atmospheric particle concentration larger than 50 nm is, say 30,000 particles per cc, then the concentration of particles larger than 500 nm would be a factor of 1,000 lower, or on the order of 30 particles per cc. For particles larger than 5,000 nm, the concentration would be a million times lower, or on the order of 0.03 particles per cc. 
     The sharply declining concentration of large particles in the atmosphere indicates that even if a single detector is developed that can detect particles over a wide size range, say, from 50 nm to 10,000 nm in diameter, the detector, when operated at a specific sampling flow rate, would result in very high particle count rates in the small particle range, and a very low count rate in the large particle range. 
     For instance, at a sampling flow rate of 1 liter per minute (1 pm), i.e. 1,000 cc per minute, each minute would give rise to 30,000,000 particles in the 50 nm to 500 nm diameter range that need to be counted. Such a count rate is generally too high and would exceed the count rate limitation of the current optical counter technology. On the other hand, each minute of sampling by the detector would only yield 30 counts for particles in the greater than 5,000 nm range. Such a particle count is usually too low for statistically accurate purposes. 
     In order to count atmospheric fine particles in the 50 nm to 500 nm range at a more reasonable rate, the flow rate of the detector may be reduced to, say, 0.1 lpm so that only 3,000,000 particles need to be counted each minute. At such a sampling flow rate, the detector would yield only 3 particle counts in the greater than 5,000 nm range each minute, thus worsening the statistical accuracy of the large particle count. On the other hand, if the sampling flow rate is increased to, say, 10 pm so that 300 particles in the greater than 5,000 nm range can be counted each minute to improve the statistical counting accuracy for large particles, 300,000,000 particles would need to be counted in the 50 nm to 500 nm range, thus worsening the count-rate requirement of the counter for fine particles. 
     This example illustrates why the OPC is unable to measure aerosols over a wide size range, and why the conventional OPC by itself is inherently incapable of making such measurements with accuracy over the entire particle size range of interest in aerosols. 
     SUMMARY OF THE INVENTION 
     The present invention is a single measuring instrument built on a common chasis or a single platform, using multiple sensors provided with appropriate flow rates to detect and measure aerosol particles over a wide size range, typically from 10 nm to 10,000 nm in diameter, and greater for example, from 2 nm to 50,000 nm. Instrument sections measuring the 10 nm to 10,000 nm particle size are termed wide-range particle counters (WPC) and the 2 nm to 50,000 nm range instrument sections are termed ultra-wide range, particle counters (UWPC). These instruments make it possible to carry out measurements that are not possible with currently available instrumentation. 
     The WPC described in this specification is based on the novel combination of multiple sensors or detectors that combine optical detection with electrical mobility analysis to form a single device covering a wide particle size range. Each sensor is optimized in terms of particle measurement range, aerosol flow rate, reduced particle loss in sampling lines, and optical and electrical designs. 
     The measuring instrument of the present invention is simple in design and yet capable of performing the measurement automatically over a wide size range. 
     The lower limit is preferably 2 nm to 20 nm, and the upper limit can be anywhere between 5,000 nm and 50,000 nm. 
     The instrument includes controls for controlling operating parameters in order to insure reliable instrument operation, accuracy of measurement, and ease of use. 
     The number of sensors, flowmeters, pumps, and other components are minimized so that a rather complicated instrument like the WPC can be simplified and manufactured at a reasonable cost. 
     The resulting instrument described herein is estimated to weigh less than 35 lbs., making the device quite portable and convenient to use. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a wide range particle counter instrument showing a chassis or housing; 
     FIG. 1A is a schematic diagram of one embodiment of the present invention; 
     FIG. 2 is a schematic diagram of a second embodiment of the invention using a different sensor arrangement; 
     FIG. 3 is a cross-sectional view of one form of a light scattering droplet counter used with the particle counter of the present invention; 
     FIG. 4 is a cross-sectional view of an optical particle sensor used with the present invention using collection adapter at 90° to a laser beam; 
     FIG. 5 is a top sectional view of the optical particle counter of FIG. 4; taken on line  5 — 5  in FIG. 4; and 
     FIG. 6 is a cross-sectional view of a differential mobility analyzer used in the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, a typical wide particle counter  10  is illustrated. The components shown in FIGS. 1 and 2 are on a chassis or platform  11  that houses the components. The chassis platform provide a support structure for the components. The controls that are described later can receive operator inputs from control or function buttons  11 A. Display screen  11 B is provided to display messages or readouts from an internal processor or computer forming the controls. 
     An inlet nozzle or tube  11 C is also shown and leads to the internal lines and counters. 
     FIG. 1A is a schematic diagram of the wide range particle counter  10  in a first preferred embodiment. The particle counter  10  is on the chassis or housing  11  shown in FIG.  1 . This includes two light scattering particle sensors  12  and  14 , a differential mobility analyzer  16 , a saturator  18 , a condenser  20 , an ionizer  22 , and associated pumps  24 ,  26 ,  28 , flow meters  30 ,  32  and  33 , and a particle filter  34 . The saturator  18  condenser  20  and light scattering particle counter  14  can be made as a sub assembly, as indicated by the dotted lines. 
     The first particle sensor  12  is a light-scattering particle counter (LPC) to detect coarse particles larger than a certain size, typically 300 nm in diameter. Aerosol is drawn through a line  38  from inlet tube  11 C and through the LPC  12  at a flow rate of Q 1  liters per minute (1 pm), by the pump or flow generator  24 . This aerosol airflow also passes through the flowmeter  30  to measure the flowrate of the aerosol. The output signal from flow meter  30 , indicating flow rate is used in conjunction with a controller  40  to vary the speed of pump  24  to maintain a constant flow, Q 1 , through the light scattering particle counter  12 . 
     At the same time a second airflow, Q 2  is provided in a line  42  by tapping into line  38  so the same aerosol source is provided to both branches of flow. The flow in line  42  carries particles for detection below a certain limiting size, typically 300 nm in diameter, by the method of the present invention. This flow is established by pump  26 . The flow Q 2  flows through ionizer  22 , and line  42  connects to differential mobility analyzer (DMA)  16 . An output line from the DMA  16  connect to a saturator  18 , a condenser  20 , and then to the light-scattering particle sensor  14  used as a light-scattering droplet counter (LDC). The flowmeter  32  is connected to the output or exit line  31  of the light-scattering particle sensor  14  and to pump  26 . The output, of flow meter  32  is used in conjunction with an electronic controller  40  to vary the speed of the pump  26  to maintain Q 2  at the desired value. The electronic controller  40  can be part of an overall system  39  having a power supply, signal and data processing capabilities and control electronics. The system  39  is mounted on the chassis. 
     The third pump  28  connected to the sheath flow outlet of DMA  16 , maintains a steady airflow, Q 3  in a line  50 , which passes into the DMA to provide the clean sheath gas flow needed for the DMA. The flow sensor and controller needed to maintain this flow at a constant value are not shown for simplicity and clarity. The sheath flow is drawn from the annular space  49  surrounding the high voltage electrode  53 . A high efficiency particle filter,  34  is used in line  50  to remove unwanted particulate contaminants in the flow Q 3  before it is introduced back into the DMA  16  in the sheath flow inlet chamber  51 . After passing through the DMA, the sheath flow, Q 3 , passes through a flowmeter  33 , before the flow goes to the inlet of pump  28  to complete the flow loop. The output flow rate signal from flowmeter  33  is used in conjunction with electronic controller  40  to vary the speed of pump  28  to maintain the constant sheath flow, Q 3 . 
     The flow rate Q 1  in line  38  for the coarse particle detector or counter  12  generally must be higher than the flow rate Q 2 , for the fine particle detector or DMA  16 . For atmospheric measurements, a flow ratio of Q 1 /Q 2  of 10 to 1 is both reasonable and quite achievable. For other applications, flow ratios of 2 to 1 or even 1 to 1 may suffice. 
     Due to Junge&#39;s law of atmospheric size distribution, flow ratios below 1 to 1, i.e. with Q 1 &lt;Q 2 , would not be very useful for aerosol size distribution analysis using the multiple sensor approach described herein. In the preferred embodiment shown in FIG. 1, typical values for the flow rates are Q 1 =3 lpm, and Q 2 =0.3 lpm. The typical clean sheath flow rate for the DMA  16  is typically and preferably Q 3 =3 lpm. 
     There is an intermediate range of particle sizes that are accurately measured by either the fine particle detector or the coarse particle detector. This intermediate size range can be from 90 nm to 600 nm. So, for a coarse particle detector that has a nominal lower limit of 200 nm the coarse detector operates into this intermediate range. The fine particle detector having an upper limit of 300 nm also operates in the intermediate range. 
     In addition to the embodiment of FIG. 1A, other embodiments also are usable. FIG. 2 shows a three-sensor system  59  that is mounted on the chassis  11  as shown in FIG. 1 for aerosol measurement over a 10 nm to 10,000 nm particle diameter range. In this embodiment, two light-scattering particle counters  60  and  62  are used to cover the 1,000 nm to 10,000 nm diameter range, and the 100 nm to 1,000 nm diameter range, respectively. A fine particle counter consisting of a CNC, which is shown as a light scattering droplet counter  64  receives flow from a DMA  66  through line  78 , which flow passes through a saturator  68  and condenser  70 , connected in a series in line  72 . The fine particle counter  64  is used to cover the 10 nm to 100 nm diameter range. Again, both the fine particle counter and the coarse particle counter assembly of the two counters overlap the intermediate size range. 
     The input flow to the system  59  is through a line  76 , carrying flow Q 1  from a source  77  to the input of the light scattering particle counter  60 . The line  76  can be coupled to inlet tube  11 C of FIG.  1 . The output is connected with a line  78  through a flow meter  80 , and then to an inlet side of a pump  82 . The flow rate signal from the flow meter  80  is provided to a controller  84  and this signal is used to control the pump  82  so that the appropriate flow rate Q 1  is established in line  76 . The pump  82  discharges the flow back to the atmosphere remote from the inlet of line  76 . The controller  84  can be part of an overall control system  85  based on a computer for controlling the needed functions. 
     A line  86  is connected to line  76 , and carries a particle carrying flow Q 2 , which is lower than flow Q 1 , provided to the counter  60 . Flow Q 2  is provided to a second light scattering particle counter  62 . The output flow from light scattering counter  62  is connected through a flow meter  88  to an inlet of a pump  90 . Flow meter  88  also is connected to provide a flow signal to the controller  84  and the controller will adjust the pump  90  to establish the appropriate flow Q 2  in the line  86 . The output of pump  90  is discharged to atmosphere, again remote from the inlet line  76 . 
     A line  94  is connected to line  86  on an input side of the light scattering particle counter  62  and carries a flow Q 3  through an ionizer  96  to a differential mobility analyzer  66 . The flow Q 3  is established by a pump  98  that is on an output side of the counter  64  and carries a flow Q 3  through the line  72  and an output line  100  from counter  64 , through a flow meter  102  to the pump  98 . The condenser  70  causes vapor generated by saturator  68 A to condense on particle nuclei to form droplets that are counted by the light scattering counter  64 . 
     The flow meter  102  also provides a signal to the controller  84  for controlling the pump  98  to provide the desired level of flow through the lines  72  and  94 . The flow from pump  98  is discharged into the atmosphere. 
     In FIG. 2, three separate pumps,  82 ,  90  and  98 , are shown being used with individual flow sensors and controllers to maintain the gas flow Q 1  Q 2 , and Q 3  at their respective constant values. Another way of maintaining constant gas flows is to use critical orifices connected to a common vacuum source maintained by a single vacuum pump. When the vacuum is higher than about ½ of an atmosphere when sampling gas from atmospheric pressure, the flow becomes choked and reaches a constant value. By choosing proper orifice sizes the flows, Q 1  Q 2 , and Q 3  can be maintained constant without separate sensors and variable speed pumps. 
     The sheath flow that is needed for the differential mobility analyzer is provided along a line  104 , and is represented at Q 4 . The sheath flow enters a chamber  103  in the DMA and is directed through an annular sheath flow passageway  107  and flows down around the center high voltage electrode  105 . 
     Flow line  104  comes from an output side of a pump  106  that has an input line  108  leading from the sheath flow annular passageway  107  of the differential mobility analyzer, through a flow meter  110 . A high efficiency filter  112  is in line  104  on the output side of the pump  106 , so that the flow Q 4  is maintained very clean. The flow meter  110  also provides a signal to controller  84 , for controlling the pump  106  to establish the appropriate flow rate. 
     The respective particle carrying flow rates for the three-sensor combination  59  are represented at Q 1 , Q 2  and Q 3 , and the flows have the relationship, Q 1 &gt;Q 2 &gt;Q 3 . This particular embodiment of the invention has the advantage of further improving the statistical counting accuracy over the entire size range. At the same time the arrangement of the embodiment shown in FIG. 2 reduces the size range of particles that must be classified by the DMA  66 , leading to further reduction in the high voltage required for the center electrode  105  of the DMA as will be explained, as well as the physical dimensions and weight of the DMA  66 . 
     In yet another embodiment of particle counters assembled in the same schematic diagram as FIG. 2, the two light scattering particle counters  60  and  62  may be used to measure particles in the 5,000 nm to 50,000 nm, and 500 to 5,000 nm diameter ranges, respectively, and the DMA-CNC combination for the fine particle counter is used to measure particles in the 10 nm to 500 nm diameter range. The size range of the modified ultra-wide range particle counter (UWPC) is 10 nm to 50,000 nm, and thus even wider than that of the wide particle counter shown in the specific example of FIG.  2 . The possible flow rates for the alternate form of the particle counters of FIG. 2 to form an ultra wide particle counter are: Q 1 =30 liters per minute (lpm), Q 2 =3 lpm, and Q 3 =0.3 lpm. Q 4 , the sheath flow, will remain essentially the same as in the specific form of FIG.  2 . 
     In addition to the above, other coarse and fine particle sensors may be used in combination to overcome the fundamental limitations of the individual sensors when applied to aerosol measurement that in some cases may span nearly five decades-i.e. 2 nm to 100 nm diameter in particle size, and more than ten decades in concentration i.e. from less than 0.001 particle per cc to over 10 7  particles per cc. A single measuring assembly having particle counting sensors as described herein makes such measurement possible. 
     It should be clear to those with ordinary skill in the art of particle counting that other sensor combinations, including the type and number of sensors used in the combination, may be varied to accomplish the objective of wide-range particle counting for different purposes and/or different applications without substantially deviating from the basic principle and approach of this invention. 
     With respect to the specific optical particle counters that can be used in the wide range particle counter, FIG.  3  and FIGS. 4 and 5 show two possible designs. In FIG. 3, the optical sensor is used as a Light-Scattering Droplet Counter (LDC) for the CNC such as that shown at  14  in FIG. 1A and 64 in FIG.  2 . The LDC makes use of forward-scattering optics and a solid-state diode laser  120  as the light source. The laser  120  has a suitable projection lens (not shown), to project a nearly parallel beam of collimated light through a condensing lens  122  mounted in a wall of a housing  124 . Lens  122  is a cylindrical lens that brings the laser beam represented at  126  to a focus at the axis  125  of an aerosol inlet nozzle  128 , and an aerosol outlet tube  130 . The beam  126  widens after passing its focal point at axis  125 , and projects to a lens  132  that has an opaque, light absorbing surface portion  134  that absorbs the laser beam, and thus serves as a beam stop in the center of the lens. 
     The aerosol is passed into the LDC housing  124  through the nozzle  128 . The nozzle tapers to be smaller toward its tip and when the aerosol reaches the nozzle tip, the cross-sectional flow area is greatly reduced over the main portion of the line so the aerosol is accelerated to a high velocity. This high velocity aerosol, which is a gas containing particles to be detected, then passes through across the focused laser beam  126  and flows out of the light-scattering particle counter housing  124  through the outlet tube,  130 . As each particle passes through the focused laser beam in region  127 , the particle scatters light in all directions. The collecting lens  132  then collects the scattered light, within the angular range of the scattered light subtended by the lens  132  onto photodiode detector  136 . A signal from photodiode  136  proportional to the light received is then processed electronically by a suitable pulse height analysis circuitry  138 . Although a single lens  132  is shown as the collecting lens for the scattered-light, it is to be understood that more than one lens, or a multi-element lens, can be used as the collector to improve the performance. 
     FIGS. 4 and 5, an optical particle sensor  150 , using 90° scattering optics is shown. The optical particle sensor  150  is used as a Light-Scattering Particle Counter such as that shown at  12 ,  60  and  62  in FIGS. 1A and 2 to measure the sizes of particles by sensing the scattered light signal. In sensor  150 , a housing  152  mounts a cylindrical lens  154  that focuses a laser beam  156  in the housing along the axis of the beam produced by a diode laser  158 . A collecting lens,  160  is mounted in a collar or tube on a sidewall of the housing  152  and will collect light scattered from particles passing across the laser beam focal point  164 . The aerosol is carried into the housing  152  through an inlet nozzle  162  that narrows the aerosol to a narrow stream as it passes across the focal region or point  164  of the laser beam  156 . The gas stream exits through a tube  166 . The common axis of the inlet nozzle  162  and tube  166  is at  90  to the axis of collecting long  160 . 
     The scattered-light from the particles provides light signals in the generally 90° direction from the laser beam as the particles pass through the focused laser beam. These scattered light signals are collected by the collecting lens  160 , and detected by a photodiode detector  168 . A conical cavity  169  in the far end wall of the housing open to receive the laser light beam  156 , serves as a light trap to absorb the laser light. As each particle crosses the focus region  164  of laser beam  156 , the scattered light signal from the particle is detected by the photo-diode detector,  168 . It is to be noted that lens  160  is focused such that light scattering from particles passing through the focal point of the laser beam is directed to fall on the sensing surface in the photodiode detector  168 . 
     Both the optical particle sensors with the forward scattering optics shown in FIG.  3  and the 90° scattering optics sensor of FIG. 4 and 5 can be used to detect the scattered light from particles. However, for particle sizing, it is generally preferred to use light scattering optics that excludes scattered light within a certain narrow angle in the forward direction from the optical axis of the laser beam, or in other words, scattered light that travels in the same general direction of the laser beam, or deviates only by a small angle from the direction of travel of the laser beam. 
     Light-scattering particle sensors with forward scattering optics that maximize the collection of scattered light signal in the forward direction are more sensitive, but give rise to a scattering signal that is not a monotonic function of particle size. The phenomenon, known as Mie resonance, can cause ambiguity in the measured particle size. For this reason, light-scattering particle sensors with 90° scattering optics (FIGS.  4  and  5 ), or with the optical axis of the collecting lens placed at some finite angle, such as 30°, 45°, 60°, or the like are preferred. Light-scattering particle sensors that make use of mirrors instead of lenses can also be used provided the collecting optics is designed to exclude the scattered light signal in the near forward direction of the light beam. 
     The airflow, Q 2 , established by pump  26  in FIG. 1A passes through the ionizer  22 , the DMA  16 , the saturator  18 , the condenser  20 , and then the light-scattering droplet counter  14 . The ionizer  22  usually contains a small, low-level radioactive source, such as radioactive Krypton 85, or polonium 210. The alpha, beta or gamma radiation emanating from the ionizing source causes air (gas) molecules to be ionized. The ionized gas molecules then collide with the aerosol particles to cause a low-level electrical charge to appear on the particles. 
     When the charged particles reach a state of charge equilibrium with the ions, referred to as Boltzmann equilibrium, the charged particles will bear a certain relationship to the total particles (charged and uncharged) in the gas. At Boltzmann equilibrium, particles of a specific size and carrying, say, a single electronic charge will be in a fixed ratio to the total number of particles of that size in the gas. Since this ratio is known from theory, by measuring the singly charged particles of that size, the total number of particles of that size in the gas can be determined. 
     There are various designs of differential mobility analyzers for aerosol classification by electrical mobility. The basic principals of operation of DMA&#39;s are well known. A schematic diagram of a preferred design for a DMA is shown in FIG. 6 at  180 . It is to be understood that the DMA shown in FIG. 6 is the preferred form of the DMA&#39;s  16  and  66  in FIGS. 1A and 2. In this design a central metal cylinder  182  forming an electrode is concentratic with an outer tubular primary cylinder  184  and a shorter nested outer cylinder  186 . The inner and outer cylinders  182  and  184  and  186  are at different electrical potentials chosen to establish a radial electrical field in the annular space  200  between the inner cylinder  182  and the outer cylinders  184  and  186 . The inner metal electrode cylinder  182  is held at a high voltage V 1 , while the nested outer cylinders  184  and  186  are grounded. Outer cylinders  184  and  186  have the same internal diameters so their internal surfaces form a single cylindrical surface of uniform internal diameter. The inner cylinder  182  is supported on an upper insulator support  181  and on a lower insulator support  183 . Thus, the central high voltage electrode is insulated from the outer metal cylinders. 
     A polydisperse aerosol source  188 , carrying particles in Boltzmann charge equilibrium, is introduced into the aerosol inlet  190  at the top of the DMA  180 . The aerosol inlet is separate from the sheath flow inlet. This polydisburse aerosol flows radially outward in the horizontal gap space  192  forming a passage between the end walls of outer cylinders  184  and  186 . The aerosol then flows through the short upper annular space  194  between short outer cylinder  186  and the upper part of primary outer cylinder  184  and emerges through the gap or space  196  between the lower end of cylinder  186  and a shoulder  198  formed on the interior of primary outer cylinder  184 . The aerosol flows into an annular space  200  between the interior surface  202  of cylinder  184  and the outer surface  204  of the inner high voltage electrode cylinder  182 . A radial electric field is established between surfaces  202  and  204  and is used for mobility classification. 
     The clean sheath gas flow needed for mobility classification, as mentioned previously, is introduced into the DMA from a source  206  through the clean sheath flow inlet tube  208  in the top wall  210  of primary outer cylinder  184 . The tube  208  carries the sheath gas flow across chamber  192  and into a chamber  211  that opens to annular space  200 . The sheath gas flow passes through a fine mesh screen  212  that distributes the flow evenly over the cross sectional area of the annular space  200  and establishes a laminar flow below the screen  212 . As this laminar sheath gas flow merges with the laminar polydisbursed aerosol flow emerging through the slit  196 , the flows combine to form a single laminar flow stream that flows down the annular space  200  between the high voltage central electrode cylinder  182  and the grounded cylinders  184  and  186 . 
     Part of the flow in the annular space  200  can exit through a slit or a passageway  220  in the central electrode  182 , that connects to an outlet bore  222  in the central electrode that opens through the insulator support  182  and leads to a particle counter. Additionally, there are a number of spaced exit holes  224  in a flange  226  of support  183 . The flange  226  serves to block the annular passage  200 , except for holes  224 . The flow then exits out an opening  228  in an end wall  230  formed by the insulator support  183 . 
     Particles from the polydisperse source  188  with a charge having an electrical polarity that is opposite to the polarity of the high voltage of the inner cylinder  182  are attracted to the cylinder. If the central electrode is provided with a positive polarity, the charged particles attracted to the outer surface  204  of the inner high voltage electrode cylinder  182  would be negatively charged. As the charged particles from source  188  move across and through the laminar flow in the space  200  between the cylindrical surfaces  202  and  204 , the particles are classified, i.e., separated, according to electrical mobility. Small particles with high electric mobility move through the laminar sheath flow more quickly than the larger particles and the small particles are deposited on the outer surface  204  cylinder  182  above the exit slit  220 . Particles that are larger than a selected size with lower mobility and moving at a lower speed, do not reach the outer surface  204  of cylinder  198 . These larger particles (above the design cut off size) are carried with excess flow through the flow distributing holes  224  in the flange  226  of lower insulator support  183  into a flow plenum  227  and then exhausted through opening  228 . 
     There is a small aerosol gas flow drawn through the exit slit  220 . This flow is generated by a flow generator, as shown in FIGS. 1A and 2, pumps  26  and  98 , respectively. Particles within a narrow range of electrical mobility and thus within a narrow range of size, are deflected to the vicinity of slit  220  and are carried into the outlet passage  222  as a monodisburse aerosol by the small airflow generated. 
     For size distribution analysis of an aerosol, the high voltage on the inner electrode  182  is adjusted through a sequence of voltage values with a voltage controller  234 . At each high voltage setting, the monodisperse particles at the exit are counted by a CNC, which comprises a saturator, a condenser and a light scattering droplet counter as shown in FIGS. 1A and 2. The result can then be analyzed to give the size distribution of the aerosol. 
     The saturator, such as saturator  18  and  68 , in a CNC usually is made of a porous material saturated with a working fluid, usually butyl alcohol in liquid form, and is shown as taken from a source  240  (in both FIGS.  1 A and  2 ). The porous material is kept at a suitably high temperature, typically 35° C. with a heater  242 . A passageway in the porous material, such as passageways  18 A and  68 A in FIGS. 1 and 2, allows the aerosol passing through the passageway to be heated and saturated with the vapor of the working fluid as it flows through. 
     The condensers  20  and  70  comprise of one or more flow passageways in a solid metal block kept at a low temperature, typically 5° C. with a cooler  244 . As the heated and vapor-laden aerosol flows through the condenser  20  or  70 , the gas cools, causing the vapor or gas to become supersaturated. The supersaturated vapor then condenses on the particles in the aerosol to form droplets, which are then detected by the respective light-scattering droplet counter  14  and  64 . The droplets formed are larger than the particles and more easily counted. 
     Since the purpose of wide-range particle analysis using the WPC is to characterize an aerosol over nearly its entire particle size range, it is important that the sample drawn in by the WPC for size distribution analysis is a representative sample of the atmosphere to be analyzed. For atmospheric aerosol measurement, it is usually preferred to draw the total flow of the aerosol sample through the same common inlet, and divide this total flow into two sub-fractions, Q 1 , and Q 2 , for size distribution analysis by the respective coarse-particle and fine-particle detectors shown in FIG.  1 A. If the atmosphere to be analyzed is the interior space of a room with uniformly distributed airborne particles, it may be permissible to draw the sample flows, Q 1  and Q 2 , through separate sampling inlets for analysis by the two separate particle counters, since the atmosphere is generally uniform throughout the room. 
     Since the sample flow, Q 1  in FIG. 1A for coarse particle detection carries coarse particles up to 10,000 nm in diameter for analysis, it is preferable to make the flow passageway between the sampling inlet and particle detection zone in the coarse particle counter relatively short. In addition, it is important to design the flow passageway such as line  38  to be relatively straight, so that the sample flow undergoes little change in flow direction to avoid particle deposition in the sampling line due to particle inertia which can occur when the flow direction changes. 
     On the other hand, for the airflow Q 2 , carrying particles for detection by the fine particle detector, with a typical upper particle size limit of 200 nm, there is less concern for particle loss during sampling and transport. The sampling line  42  for Q 2 , can be relatively longer and can include one, two, or more 90-degree turns. The schematic flow diagram of FIG. 1 showing the preferred embodiment indicates how this can be accomplished. The coarse particle flow, Q 1 , is shown to enter the coarse particle detector directly along line  38  with no substantial change in flow direction, while the fine-particle flow, Q 2 , is sampled from the inlet with line  42  undergoes two 90 degree turns before entering the inlet of the fine particle detector, including the DMA  16 . 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.