Patent Publication Number: US-8966958-B2

Title: Particle classifier

Description:
This application is a 371 National Entry Application of PCT/CA2010/000995, which has an international filing date of Jul. 2, 2010 and is entitled “Particle Classifier”, and which claims benefit of U.S. Provisional Application Ser. No. 61/222,890, filed Jul. 2, 2009 and entitled “Aerodynamic Particle Classifier”. 
    
    
     TECHNICAL FIELD 
     Aerosol particle classifiers. 
     BACKGROUND 
     Aerosol classifiers are used to produce a monodisperse aerosol, that is, they select a narrow range of particles from a larger distribution of particles. This method is used for many applications including; nano-particle generation, measuring distributions of particles in air, measuring the deposition of particles in filters and other devices, sampling ambient aerosols, and many others. These measurements are often done in research areas as diverse as: nano-technology, pharmaceutical research, health-effects studies, inhalation toxicology, bio-aerosol detection, filter testing, indoor-air quality studies, industrial hygiene, energy and combustion research, automotive emissions measurements, and atmospheric and climate-change research. 
     Currently, the most commonly used classifier is called the Differential Mobility Analyzer (DMA, Knutson and Whitby 1975). The DMA classifies particles based on their electrical mobility, that is, the motion of a charged particle in an electrostatic field. By controlling the electrostatic field and the flow between two cylinders the particles are classified by their electrical mobility, which is related to the number of electric charges on the particle and the drag experienced by the particle, which is a function of the particle&#39;s size and shape. For non-spherical particles an equivalent diameter, called the electrical mobility equivalent diameter is defined for these particles, which have the same electrical mobility of a spherical particle of the same size. To classify particles with this instrument an electric charge must be placed on these particles using charging methods such radioactive-source charge neutralizers or corona discharge. However, with all charging methods not a single charge is placed on each particle but rather a distribution of charges are placed on the population of particles. For example, particles may obtain one, two, three, or more positive charges; one, two, three, or more negative charges or no charge at all. The electrical mobility of the particles is a function of the number of charges on the particle and its drag. Therefore, a smaller particle with one charge will have the same electrical mobility as a larger particle with two charges. Thus, the aerosol sample that is classified by the DMA will not be truly monodisperse in terms of particle size, but rather it will have a mix of sizes corresponding to an integer number of charged particles. Techniques are used to minimize the number of charge states but the DMA can never produce a truly monodisperse aerosol. For some applications (like measuring size distributions) the error introduced by the charge distribution can be corrected using inversion techniques, but it can never be fully eliminated. In other applications and experiments, these extra particle sizes can degrade performance or skew results. 
     Another technique has been used to classify particles by their mass-to-charge ratio is an instrument called the Aerosol Particle Mass analyzer (APM; Ehara et al. 1996; Ehara 1995) or the Couette Centrifugal Particle Mass Analyzer (Couette CPMA; Rushton and Reavell 2004; Olfert and Collings 2005). With these instruments charged particles are classified between two rotating cylinders with electrostatic and centrifugal forces. A similar charging mechanism is applied to charge the particles. Therefore, particles of the same mass-to-charge ratio will be classified. For example, a particle with one charge will be classified at the same time as a particle with twice the mass and twice the number of charges. Therefore, the APM or Couette CPMA do not produce a truly monodisperse aerosol. 
     Other aerosol and particle instruments are based on measuring what is called the ‘aerodynamic’ diameter of the particle. The aerodynamic equivalent diameter is defined as the diameter of a spherical particle with a density of water that has the same terminal velocity as the actual particle. Instruments that measure the aerodynamic size of particles include various kinds of impactors (Marple et al., 1991; Keskinen et al., 1992), virtual impactors (Conner, 1966), and aerodynamic lenses (Liu et al., 1995a, 1995b). However, these methods only provide a means of dividing the aerosol sample in half, where particles larger than the cut-off point are classified in one direction (i.e., impacted onto the impaction plate) and particles smaller than the cut-off point continue with the flow. Often, several of these stages are stacked together to provide classification into several large bins. There is currently no instrument that classifies particles by their aerodynamic diameter and produces a monodisperse aerosol. 
     SUMMARY 
     The applicant has devised a new instrument, called the Aerodynamic Particle Classifier (APC) that provides classification of particles. In an embodiment, a method of classification of particles suspended in a fluid is provided comprising the steps of providing a carrier flow of a fluid, supplying particles into suspension in the carrier flow, providing an acceleration to the flow at an angle to the velocity of the flow to cause the particles to follow trajectories determined by the acceleration and drag on the particles caused by the fluid, and classifying the particles according to the trajectories of the particles. The particles may be classified for example by splitting a flow containing the particles or by detecting impacts of the particles on boundaries of a flow channel containing the flow. 
     The following are features any or all of which may be provided in combination with the above method of classification of particles: the fluid may be a gas such as air; the carrier fluid may be caused to rotate around an axis by the rotation of one or more conveying flow channels; the acceleration may be centripetal acceleration; the step of supplying particles into suspension in the carrier flow may comprise merging a flow of a fluid containing suspended particles into the carrier flow; the step of classifying particles according to the trajectory of the particles may comprise splitting the carrier flow into two or more flows; and the step of classifying the particles according to the trajectory of the particles may comprise supplying a surface at which particles may impact depending on their trajectory. 
     Also provided is an apparatus for classifying particles suspended in a fluid, the apparatus comprising: elements defining one or more carrier flow channels, a source of a carrier fluid flow into the carrier flow channel, a source of particles connected to supply the particles into suspension in the carrier fluid in the carrier flow channel, a drive connected to operate on the elements defining the carrier flow channel to supply an acceleration to the elements defining the flow channel at an angle to the flow of fluid through the carrier flow channel, and a classification system for classifying the suspended particles according to their trajectories. 
     The following are features all or any of which may be provided in combination with the above apparatus for classifying particles: the carrier fluid may be a gas; the carrier flow may be caused to flow through one or more flow channels caused to rotate around an axis: the flow channels may be sectors or the whole of an annular space defined by inner and outer walls which are surfaces of revolution around an axis close to the axis of rotation; the surfaces of revolution may be substantially cylindrical in shape; the drive may comprise a motor connected to cause rotation of the elements defining the carrier flow channel; the source of particles connected to supply the particles into suspension in the carrier fluid in the carrier flow channel or channels may comprise elements defining a suspension flow channel or channels which intersect the carrier flow channels, the suspension flow channels being capable of directing a fluid containing suspended particles into the carrier flow channels; the classification system may comprise elements defining a split of each of the carrier flow channels into two or more channels; the classification system may comprise a surface in each carrier flow channel at which particles suspended in fluid in the carrier flow channel may impact depending on their trajectory; and the surface may be an element defining or partially defining the carrier flow channel. 
     These and other aspects of the device and method are set out in the claims, which are incorporated here by reference. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Embodiments will now be described with reference to the figures, in which like reference characters denote like elements, by way of example, and in which: 
         FIG. 1  is a schematic of an Aerosol Particle Classifier (APC) (not to scale) with a cylindrical flow path; 
         FIG. 2  is a diagram showing details of the particle trajectory and flows between the cylinders in the embodiment of  FIG. 1 ; 
         FIG. 3A  is a graph of the normalized transfer function of the APC of  FIG. 1 ; 
         FIG. 3B  is a graph of the transfer function of the APC of  FIG. 1  in terms of aerodynamic diameter for the operating conditions given in the description; 
         FIG. 4  is a schematic of an APC (not to scale) with a partial cylinder flow path; 
         FIG. 5  is a schematic of an APC (not to scale) with a curved flow path with boundaries shaped as surfaces of revolution; 
         FIG. 6  is a schematic of an APC (not to scale) with detectors on an outer cylinder defining the flow path; 
         FIG. 7A  is a schematic showing an aerodynamic classifier with a particle counter; and 
         FIG. 7B  is a schematic showing a particle charger with an aerodynamic classifier of the embodiment of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  show diagrams of an exemplary embodiment of the APC, generally denoted by  100 . The APC disclosed here comprises elements defining a carrier flow channel, here two concentric cylinders, an inner cylinder  102  and an outer cylinder  104  rotating in the same direction and at a similar rotational speed (normally the two cylinders would be rotating at the same rotational speed although different speeds can also be used, see below). Other surfaces of revolution (axially symmetric shapes, synonymously surfaces of rotation) than cylinders may also be used. In an embodiment a flow channel may be defined by partial cylinders, e.g. sectors of a cylinder that do not extend in a full circle around the central axis of the cylinders, or partial surfaces of revolution, and by substantially radial surfaces between the inner and outer surfaces. If a flow channel is defined by partial cylinders or partial surfaces of revolution then the surfaces could form a single element defining the flow channel. Referring to  FIG. 1 , in the embodiment shown the cylinders are attached to a rotating shaft  120  mounted on bearings  122  and rotated via pulley  124 . These elements act as a drive to operate on the elements defining a carrier flow channel (in this embodiment by rotating them) to supply an acceleration (here a centripetal acceleration) to the elements defining the flow channel at an angle to the flow of fluid through the carrier flow channel. Referring to  FIG. 2 , a source of particles is connected to supply particles into suspension in the carrier fluid, in this embodiment slit  118  acting as a source. The particles, carried along by the aerosol flow  106  with flow rate Q a , enter the gap between the two cylinders through slit  118  in the inner cylinder wall. A sheath flow  108  with flow rate Q sh  is also introduced from a source of carrier fluid flow into the carrier flow channel between the two cylinders. In this embodiment an initial flow channel  126  acts as a source of carrier fluid by introducing the sheath flow into the carrier flow channel. It is assumed that flow is laminar and incompressible, which is a reasonable assumption for the geometry, flow rates, and gas pressure used in normal operation. In this embodiment the flow is axial in the frame of reference of the rotating cylinders, and tangential to the cylinders or to an imaginary cylinder coaxial with the cylinders; in an embodiment with cylinders rotating at different speeds the flow may still be tangential. In the absence of any centrifugal force (due to centripetal acceleration of the fluid containing the particles) the particles would travel between the two cylinders between the inner cylinder wall and the aerosol streamline  110 . However, when the cylinders are rotated, the particles experience a centrifugal force in the direction of the outer cylinder and a drag force toward the centre of rotation. The centrifugal force both supplies the particles into the carrier flow and imparts a component of velocity across the carrier flow. Thus, in this example, the particles are not pre-mixed. The particles will also travel in the axial direction carried along by the aerosol flow and sheath flow. Therefore, the velocity of particles in the radial (v r ) and axial (v z ) direction will be: 
                       v   r     =         ⅆ   r       ⅆ   t       =           C   c       3   ⁢   πμ   ⁢           ⁢     d   p         ⁢   m   ⁢           ⁢     ω   2     ⁢   r     =       τω   2     ⁢   r           ⁢     
     ⁢   and   ⁢     
     ⁢         v   z     =         ⅆ   z       ⅆ   t       =     u   z         ,             (   1   )               
where r is the radial position of the particle, ω is the rotational speed of the cylinders, m is the mass of the particle, d p  is the diameter of the particle, μ is the viscosity of the carrier gas, C c  is the Cunningham slip correction factor for the particle, and u z  is the velocity of the carrier gas in the axial direction. It will be assumed that the velocity profile is uniform (i.e., u Z  is constant). The particle relaxation time, τ, is defined as,
 
                     τ   =           C   c       3   ⁢   π   ⁢           ⁢   μ   ⁢           ⁢     d   p         ⁢   m     =           C   c     ⁢     ρ   p     ⁢     d   p   2         18   ⁢   μ       =         C   c     ⁢     ρ   0     ⁢     d   ae   2         18   ⁢   μ             ,           (   2   )               
where ρ p  is the true particle density, ρ 0  is unit density (1000 kg/m 3 ), and d ae  is the so-called aerodynamic diameter of the particle.
 
     Using the chain rule and differentiating, the radial position of the particle can be found as a function of the axial position, 
                       r   ⁡     (   z   )       =       r   in     ⁢     exp   (         τω   2     ⁢   z       u   z       )         ,           (   3   )               
where r in  is the initial position of the particle when it enters the classifier.
 
     A classification system classifies the suspended particles according to their trajectories. In the embodiment shown particles are classified according to whether their trajectories bring them through sampling exit  114 . The transfer function of the instrument (the distribution of particles that leave the classifier at any given operating condition) can be found by determining the trajectory of the particles. A sample flow  112  with flow rate Q s  exits the classifier through sampling exit  114 . In the embodiment shown, the sample flow is part of the sheath flow. The remainder of the sheath flow and the aerosol flow exit the classifier as exhaust flow  116 . Defining r 1  as the outer radius of the inner cylinder, r 2  as the inner radius of the outer cylinder, r 3  as the outer radius of the aerosol flow, and r 4  as the inner radius of the sample flow, the largest particle (i.e., the largest τ) that will pass through the classifier, exiting the classifier in the sample flow  112 , will start at r in =r 1  and will reach r 2  at the end of the classifier (z=L). Therefore, 
     
       
         
           
             
               
                 
                   
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     The smallest particle that will be classified, τ min , will enter the classifier at r in =r 3  and will reach r 4  at the end of the classifier. The radii r 3  and r 4  can be related to the radii r 1  and r 2 , realizing that for uniform flow, 
                     u   z     =           Q   sh     +     Q   a         π   ⁡     (       r   2   2     -     r   1   2       )         =         Q   sh       π   ⁡     (       r   2   2     -     r   3   2       )         =         Q   s       π   ⁡     (       r   2   2     -     r   4   2       )         .                 (   5   )               
Therefore,
 
     
       
         
           
             
               
                 
                   
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     Particles with τ&gt;τ max  will intercept the outer cylinder wall before reaching the exit slit and will adhere to the cylinder surface, while particles with τ&lt;τ min  will flow past the exit slit and be carried out of the instrument with the exhaust flow. The particles adhere to the wall of the outer cylinder due to van der Waals forces (Friedlander, 2000) and will remain there until the cylinder is cleaned. (Like the DMA, under normal operating conditions and aerosol concentrations, the cylinder will only need to be cleaned once every few months.) Between the maximum and minimum relaxation times, only a fraction of the particles will be classified. A particle must migrate into the sample flow, defined by the sample streamline (r 4 ≦r&lt;r 2 ), by the time the particle has reached the end of the classifier. For particles with τ&gt;τ min , only particles with an initial radial position r c ≦r&lt;r 3  will be classified, where r c  is called the critical radius. The limiting trajectory for τ&gt;τ min  will be the particle that starts at r c  and reaches r 4 . Substituting this condition into Eq. 3 and solving for the aerosol fraction, f 1 , that is classified reveals, 
                     f   1     =                 Q   sh     +     Q   a     -     exp   (         -   2     ⁢     τω   2     ⁢   L   ⁢           ⁢     π   ⁡     (       r   2   2     -     r   1   2       )             Q   sh     +     Q   a         )                 (       Q   sh     +     Q   a     -       Q   s     ⁡     (     1   -       r   1   2     /     r   2   2         )         )               Q   a     ⁡     (     1   -       r   1   2     /     r   2   2         )         -         Q   sh       Q   a       .               (   7   )               
Likewise, for particles with τ&lt;τ max , the particles starting at the critical radius, r c , must reach r 2  by the end of the classifier. In this case the fraction of the aerosol, f 2 , that is classified is,
 
                     f   2     =           Q   sh     +     Q   a         Q   a       ⁢       (     1   -       1   -     exp   (         -   2     ⁢     τω   2     ⁢   L   ⁢           ⁢     π   ⁡     (       r   2   2     -     r   1   2       )             Q   sh     +     Q   a         )         1   -       r   1   2     /     r   2   2             )     .               (   8   )               
Furthermore, if the sample flow rate is smaller than the aerosol flow rate, then the transfer function cannot be larger than, f 3 =Q s /Q a .
 
     The transfer function, Ω, will be the minimum of these three fractions or one. Therefore, the transfer function can be expressed as, Ω=max[0,min(f 1 ,f 2 ,f 3 ,1)]. 
     The normalized transfer function is shown in  FIG. 3A , where the normalized particle relaxation time is defined as τ/τ*. The value τ* is the particle relaxation time at the centre of the transfer function and is defined as τ*=(τ max +τ min )/2. The half-width of the transfer function is defined as, Δτ=(τ max −τ min )/2. 
     It can be shown, that when the gap between the cylinders is small and Q a =Q s , then the relative width of the transfer function, Δτ/τ*, is Just the ratio of the aerosol to sheath flow rates, Δτ/τ*=Q a /Q sh . To produce a highly monodisperse aerosol this ratio should approximately be in the range of 0.05 to 0.1. 
     As an example some sample dimensions and operating conditions that are well suited for most applications are: r 2 =37 mm, r 1 =35 mm, L=200 mm, Q sh =3 L/min, and Q a =Q s =0.3 L/min. In general, it is beneficial to keep the gap between the cylinders relatively small (i.e. (r 2 −r 1 )&lt;&lt;r 1 ), as smaller gaps increase the height of the transfer function. During operation the flow rates maybe changed to vary the width of the transfer function as desired as long as the flow remains laminar. An example of a transfer function of the APC is shown in  FIG. 3B  using the given dimensions. In this example, when the rotational speed is 5650 rpm the centre of the non-diffusion transfer function will be at d ae *=100 nm, with the minimum and maximum sizes classified at 92 nm and 108 nm, respectively. A more complicated model was also developed which accounts for the effects due to particle diffusion within the classification region. The diffusion transfer function is also shown in  FIG. 3   b  and it shows that the transfer function becomes broader and shorter when diffusion is included. This affect will become more prevalent for smaller particle sizes. 
     Thus, this embodiment of the APC will produce a very narrow, or monodisperse, size distribution. Using these same dimensions, the APC would be able to classify particles over an extremely wide range for example 10 nm to 10 μm using rotational speeds ranging from 20,000 to 95 rpm and smaller particle sizes could be classified by using higher rotational speeds (by comparison the DMA is typically used over a range of approximately 2.5 to 1,000 nm). 
     It should be noted that the analysis used here is very similar to the proven theoretical analysis used in the DMA, with the exception that the APC has a centrifugal force instead of an electrostatic force to classify the particles. Therefore, we have a sound basis for predicting that this theoretical model of the APC will closely match experimental data once a prototype is developed. 
     The above description of the embodiment of the APC of  FIG. 1  is just one way the instrument could be designed or configured. Other alternative designs can be envisioned depending on the application or requirements. These include: 1) the use of rotating channels instead of a continuous cylindrical section, 2) two cylinders rotating a slightly different rotational speeds, and 3) ways to measure aerosol size distributions. 
     The embodiment of  FIG. 1  uses concentric cylinders to classify the particles. However, other rotating geometries may also be used. For example, partial cylindrical sections such as sectors of a cylinder may be used as elements defining one or more carrier flow channels or other long channels attached to a rotating shaft may be used as the carrier flow channels. However, the analysis of the transfer function will change with these different geometries, where the cylindrical geometry is the simplest to analyze. Referring to  FIG. 4 , an APC  200  is shown with a rotating flow channel (multiple flow channels may be included around the central axis, but only one is shown in the figure). The flow channel may be defined by an inner partial cylindrical section  202  and an outer partial cylindrical section  204  although other shapes than partial cylindrical sections are possible. The embodiment may operate in a similar way as the embodiment of  FIG. 1  except that the flow channel does not extend all the way around the central axis. In particular rotating shaft  220 , bearings  222 , pulley  224 , sheath flow  208 , aerosol flow  206 , initial flow channel  226 , sample flow  212  and exhaust flow  216  may be similar to their counterparts of  FIG. 1  and cooperate similarly, except that the portions of the sheath flow and sample flow within the rotating parts of the classifier may not extend all the way around the central axis. Referring to  FIG. 5 , an APC  300  is shown with a flow channel defined by an inner surface  302  and an outer surface  304  which are not cylinders. The surfaces may be surfaces of revolution. However, it would also be possible to use different shapes including partial surfaces of revolution that extend only part of the way around the central axis, as in  FIG. 4 . Further shapes other than partial surfaces of revolution may also be used. The embodiment shown in  FIG. 5  may operate in a similar way as the embodiment of  FIG. 1  except for the different shape of the flow channel. In particular, rotating shaft  320 , bearings  322 , pulley  324 , sheath flow  308 , aerosol flow  306 , initial flow channel  326 , sample flow  312  and exhaust flow  316  may be similar to their counterparts of  FIG. 1  and cooperate similarly, except where a different shape is appropriate to accommodate the shape of the flow channel. 
     In the analysis relating to  FIGS. 1 and 2  it was assumed that the cylinders were rotating at the same rotational speed. However, cylinders rotating at slightly different speeds can also be used. In this case it would be preferable to rotate the inner cylinder slightly faster than the outer cylinder. When this is done, and the speed difference is large enough, the centrifugal force will decrease as its radial position increases. This causes the particle trajectories in the classifier to slightly ‘converge’ near the end of the classifier, resulting in a higher transfer function. A similar method is used in the Couette CPMA to improve its transfer function and it is the key difference between it and the APM. However, as shown in the example above ( FIG. 3B ) the peak of the transfer function without the speed difference is already 0.95, therefore the added complexity of rotating the cylinders at slightly different speeds is mostly likely not worth the slight improvement in transfer function. If different rotational speeds are used it should be noted that the speed ratio must satisfy the Rayleigh criterion (i.e., it must not be the case that (r 1 /r 2 ) 2 &gt;ω 2 /ω 1 ) beyond which the flow becomes unstable, thereby disturbing the classification of the particles. 
     Thirdly, the above description describes how the APC can be used to produce a monodisperse aerosol based on the particle&#39;s aerodynamic diameter, much like how a DMA is used to produce a quasi-monodisperse aerosol based on the particle&#39;s electrical mobility diameter. DMA&#39;s are often combined with a condensation particle counter (CPC) to measure the number concentration of the quasi-monodisperse aerosol. Typically, the voltage controlling the electrostatic field in the DMA is ‘scanned’ over the range of the instrument, and by completing a data inversion of the CPC data, the size distribution of the aerosol can be determined. This combination of DMA and CPC is normally called, a Scanning Mobility Particle Sizer (SMPS). The same method can be employed by combining the APC with a CPC, or any other particle counting device, and by continuously ‘scanning’ the rotational speed or intermittently stepping the rotational speed. That is, a particle counter could be placed in or connected to the particle classification system or the outlet of the particle classification system in order to detect the concentration of particles of an aerodynamic diameter or other derived metric allowing them to reach the particle counter; the acceleration provided to the flow could be varied continuously or in steps, the aerodynamic diameter (or other metric related to the aerodynamic diameter) required to reach the particle counter changing with the acceleration provided to the flow, so that the particle counter measures a spectrum of aerodynamic diameter (or other metric related to the aerodynamic diameter) versus concentration as the acceleration provided to the flow varies. Referring to  FIG. 7A , an aerodynamic classifier  500 , which may be of any of the embodiments described above, is shown with a particle counter  540 . An initial aerosol flow  536  enters the classifier which acts on the aerosol flow to produce a classified flow  538  containing a selected portion of the aerosol particles present in the initial aerosol flow. The classified flow may be for example a sampling flow. The particle counter may be for example a condensation particle counter. The embodiment shown in  FIG. 7A  may include embodiments used to measure a spectrum of aerodynamic diameter versus concentration or other embodiments. 
     Another way the APC can be used to measure aerosol size distributions is by eliminating the aerosol exit slit and placing in the flow channel one or more detectors at which the particles may impact depending on their trajectory and measuring the number of particles impacting the one or more detectors. In an embodiment a detector may comprise a conductor connected to an electrometer circuit, for example in an annular embodiment, the detector may comprise a conducting ring connected to an electrometer. The ring may be electrically isolated from both the remainder of the surface defining the flow channel and any other detection rings which may be present. The detectors may be situated at different axial locations along the outermost surface of the flow channel. Referring to  FIG. 6 , an APC  400  is shown having detectors  428  located in the flow channel at which particles may impact depending on their trajectory. In the embodiment shown the flow channel is defined by cylinders as in  FIG. 1 , although other shapes would also work. In this embodiment the detectors may be rings, electrically connected to electrometer circuits, extending around the inside of the outer cylinder. No sampling exit is necessary to classify particles when detectors are used to detect impacting particles, although the detectors could also be used in embodiments with a sampling exit. The embodiment shown in  FIG. 6  may otherwise operate similarly to the embodiment of  FIG. 1 , in particular, the rotating shaft  420 , bearings  422 , pulley  424 , sheath flow  408 , aerosol flow  406 , initial flow channel  426 , and exhaust flow  416  may be similar to their counterparts of  FIG. 1  and cooperate similarly, except that the articles of the aerosol flow are charged. In this system, the particles to be measured would be charged (most likely with a corona discharge-type charger or any other particle charging method, see Hinds,  Aerosol Technology , Wiley, 1999). The charged particles would move down the classification section and impact the electrometer rings on the outer cylinder, thereby causing a measurable current in the electrometer ring, where the current is proportional to the number concentration of particles impacting the electrometer. Larger particles would impact the electrometer rings near the aerosol entrance and smaller particles would impact the rings near the aerosol exit. Referring to  FIG. 7B  an APC  400  with electrometers, such as for example the APC shown in  FIG. 6 , is shown with a particle charger  430 . An uncharged aerosol flow  432  enters the particle charger  430  to produce a charged aerosol flow  434  comprising charged aerosol particles. The APC  400  operates on the charged aerosol flow  434  to classify the charged aerosol particles. By using a data inversion routine, the aerosol size distribution can be determined. Similar techniques have been used in DMA-like instruments like the differential mobility spectrometer (Reavell et al., A fast response particulate spectrometer for combustion aerosols.  Society of Automotive Engineers,  2002) and the engine exhaust particle sizer (Johnson et al., An engine exhaust particle sizer spectrometer for transient emission particle measurements.  Society of Automotive Engineers,  2004); where electrometer rings have been placed inside a DMA-like classification column. 
     Thus the applicant has devised a new instrument, called the Aerodynamic Particle Classifier (APC). As indicated, a detailed theoretical model has been developed for the instrument. The model shows the instrument can have excellent classification properties (i.e. wide range, high resolution, and high penetration efficiency) without requiring particle charging. This results in an instrument that in an embodiment can produce a true monodisperse aerosol without classifying multiply-charged particles like the DMA, APM, or CPMA. An APC could be combined in series with a DMA or CPMA in order to measure other important particle properties including: mobility diameter, particle mass, effective density, fractal-like dimension, and dynamic shape factor. 
     Immaterial modifications may be made to the embodiments described here without departing from what is covered by the claims. 
     In the claims, the word “comprising” is used in its inclusive sense and does not exclude other elements being present. The indefinite article “a” before a claim feature does not exclude more than one of the feature being present. Each one of the individual features described here may be used in one or more embodiments and is not, by virtue only of being described here, to be construed as essential to all embodiments as defined by the claims.