Abstract:
An impactor inlet for a system for monitoring an aerosol including a plurality of particles is provided. Each of the particles has a size. The inlet includes a housing defining a chamber to receive and direct a flow of the aerosol. A high flow rate nozzle receives the aerosol at a flow rate of greater than 20 liters per minute (lpm) and accelerates and directs the aerosol flow toward a impaction plate. The aerosol flow separates into a minor flow including particles having a size greater than about 10 um, and a major flow including the remaining particles. The minor flow impacts on the impaction plate, and the major flow is emitted from an outlet.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application incorporates by reference in its entirety U.S. non-provisional application Ser. No. 09/992,554 filed Nov. 13, 2001. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to particulate matter monitors, and more particularly to inlets for monitoring of particulate matter.  
         BACKGROUND  
         [0003]    Since the advent of the particulate matter (PM) standards by the United States Environmental Protection Agency (US EPA), particle sampling has become a primary goal of both scientists and lawmakers. The addition of the PM2.5 (fine particle) and the soon to be developed PM10-PM2.5 (coarse particle) standards to the PM10 standard has created a need for reliable continuous coarse and fine PM measurement devices.  
           [0004]    An essential component of a modern PM monitoring device is a size pre-selective inlet. This is even more important when the size range to be removed prior to sampling consists of large particles. If the inlet allows even a small fraction of the undesirable PM into the measuring device, the error could be large. The reason for this is that large particles have large masses, which may heavily influence the measurement of a mass-based monitor.  
           [0005]    Because of their large mass, hence inertia, coarse particles are difficult to sample and collect. When these heavy particles are accelerated in an impactor jet, their substantial inertia causes them to hit the impactor collection plate. Many times the particle will bounce off this plate and become re-entrained in the air stream. This may cause overestimation of the mass downstream of the inlet. Re-entrainment of particles may also be related to the flow rate of the aerosol flowing into the inlet. As the flow rate increases, re-entrainment increases.  
           [0006]    Another problem that occurs because of the inertia of these particles is the underestimation of particle mass that results from anisokinetic sampling. Anisokinetic sampling is a condition in which the mean velocity of the flowing air differs from the mean velocity of the air entering the inlet of the sampling probe. In addition, unlike the PM in smaller size ranges, coarse PM is not as uniformly dispersed in the atmosphere. It settles and becomes resuspended due to localized events (i.e. high wind episodes).  
         SUMMARY  
         [0007]    In one aspect, an impactor inlet for a system for monitoring an aerosol including a plurality of particles is provided. Each of the particles has a size. The inlet includes a housing defining a chamber to receive and direct a flow of the aerosol. A high flow rate nozzle receives the aerosol at a flow rate of greater than 20 liters per minute (lpm) and accelerates and directs the aerosol flow toward a impaction plate. The aerosol flow separates into a minor flow including particles having a size greater than about 10 um, and a major flow including the remaining particles. The minor flow impacts on the impaction plate, and the major flow is emitted from an outlet.  
           [0008]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0009]    [0009]FIG. 1 is a two-dimensional perspective view of an aspect of an impactor inlet.  
         [0010]    [0010]FIG. 2 is a two-dimensional perspective view of an aspect of an impactor inlet nozzle.  
         [0011]    [0011]FIG. 3 is a graph showing penetration distance versus particle diameter.  
         [0012]    [0012]FIG. 4 is a block diagram of an embodiment of a system for monitoring an aerosol.  
         [0013]    [0013]FIG. 5 is a two-dimensional diagram of a system for monitoring an aerosol.  
         [0014]    [0014]FIG. 6 is a block diagram of an embodiment of a system for monitoring an aerosol.  
         [0015]    [0015]FIG. 7 is a flow diagram of a process for monitoring an aerosol. 
     
    
       [0016]    Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0017]    [0017]FIG. 1 shows an aspect of a high flow rate inlet  100  for receiving an aerosol containing particulate matter. The high flow rate inlet  100  is configured to receive the aerosol at a flow rate of about 50 lpm and to have a high efficiency cutpoint. The high flow rate inlet  100  may separate the particulate matter as a function of size. For example, particulate matter having an aerodynamic diameter that is greater than or equal to 10 um may be separated from particulate matter having an aerodynamic diameter that is less than 10 um. The smaller diameter particulate matter may then be passed through the impactor inlet  100  to a monitoring device.  
         [0018]    The high flow rate inlet  100  includes a housing  112  to provide an enclosure and to direct the flow of the aerosol. An air intake  114  in the housing  112  receives the aerosol into the high flow rate inlet  100  and is configured to reduce anisokinetic sampling errors. The air intake  114  may comprise an opening extending about the circumference of the housing so that air may flow into the housing from any direction. The opening may be discontinuous so long as air from substantially all directions may be received into the housing  112 . One such air intake  114  may be located along a vertical surface of the housing beneath an upper surface  117 . An insect screen  116  may optionally be provided at the air inlet  114  to prevent insects from contaminating the aerosol.  
         [0019]    A deflection cone  118  may redirect the aerosol towards a nozzle  120 . The deflection cone  118  provides one alternative for receiving an aerosol from a sidewardly facing air intake while directing the aerosol flow in a downward direction.  
         [0020]    A nozzle  120  receives the flow of aerosol and concentrates and directs that flow towards an impaction plate  122 . FIG. 2 shows an expanded view of the nozzle  120  in relation to the impaction plate  122 . The nozzle  120  preferably has a cutpoint of about 10 um at a flow rate of about 50 lpm. However, the nozzle may include other cutpoints ranging from 0.25 um. Conventional nozzles that have a cutpoint of 10 um are generally limited to a flow rate of 16.7 lpm. The invention recognizes that to attain a higher flow rate while maintaining a sharp cutpoint at 10 um, the nozzle diameter, W,  124  may be widened, the nozzle length, L,  126  may be shortened, and the distance,  128  from the jet-to-impaction plate may be increased. For example, in one embodiment, the nozzle  120  may have a diameter  124  of about 1.7 cm, a length  126  of about 4.7 cm, and a distance  128  to the impaction plate of about 1.1 cm.  
         [0021]    To determine a figure of merit for particle capture, the Stokes number, St, of a particle having a 50% probability of impacting may be computed. The Stokes number is defined as follows:  
         S                 t     =         τ                 U     W     =         ρ   p          C   c          d   p   2        U       9                 μ                 W                               
 
         [0022]    where U is the jet velocity, t is the relaxation time, W is the nozzle diameter, p p  is the particle density, u is the dynamic viscosity of the air, d p  is the particle diameter in um, and C c  is the Cunningham slip correction factor.  
         C   c     =     I   +       2     P                   d   p              [     6.32   +     2.01                   exp     (                          -   0.1095                   P            d   p       )           ]                               
 
         [0023]    where P is the absolute pressure in the impaction region (in cm Hg) and the corresponding jet velocity for a flow rate of 50 lpm is 367 cm/s. The computed Stokes number corresponding to a cut-point of 10 um and the exemplary nozzle is about 0.16.  
         [0024]    [0024]FIG. 3 shows a graph of the efficiency of one aspect of a high flow rate inlet in accordance with the principles described herein. A waveform  140  of the penetration efficiency versus aerodynamic diameter shows that particle penetration may be 90% or higher for particles in the range of 2.5 to 8 um. Penetration decreases sharply to about 50% at 9.5-9.7 um and further to less than 10% for particles larger than 11 um in aerodynamic diameter. A geometric standard deviation (σg) may be determined, to use as an estimate of the sharpness of the particle penetration. This may be used to indicate very sharp aerodynamic particle separation characteristics (σ g =1.1) for the high flow rate inlet.  
         [0025]    [0025]FIG. 4 shows an aspect of a particulate matter monitor  10  that includes an aspect of the above described high flow rate inlet. The monitor  10  is particularly suitable for continuous monitoring of coarse particulate matter having an aerodynamic diameter of 2.5 um to 10 um. However, the monitor  10  may be used to monitor aerosols that include particulate matter having other size ranges including aerodynamic diameters between 0.25 um to 100 um. The monitor system  10  includes an impactor assembly  12  coupled to a first sensor  18 . The impactor assembly  12  removes particles having an aerodynamic diameter outside of a predetermined band such as the band between 2.5 um to 10 um. The impactor assembly  12  includes a first impactor  14  and a second impactor  16  connected in series. At least one of the impactors  14  and  16  includes an high flow rate inlet for receiving the aerosol at a flow rate of about 50 lpm. Each of the impactors  14  and  16  removes a band of particles based upon the size of the particles. For example, the first impactor  14  may remove particles having an aerodynamic diameter greater than 10 um, and the second impactor  16  may remove particles having an aerodynamic diameter less than a minimum particle size. The remaining portion of the particles in the aerosol have an aerodynamic diameter predominantly in the range between the minimum and maximum particle size, although there may be a minimal quantity of particles within the remaining portion that may have an aerodynamic diameter outside the range of 2.5 um to 10 um.  
         [0026]    The impactors  14  and  16  may also concentrate the quantity of particles having a particle size between the minimum and maximum particle size so that an enriched aerosol is supplied to the first sensor  18 . The particles may be concentrated by receiving an aerosol into the impactor assembly  12  at an inlet flow rate, and supplying the enriched aerosol to the first sensor  18  at an outlet flow rate that is less than the inlet flow rate. The level of concentration is a function of the ratio of the inlet flow rate to the outlet flow rate. For example, at an inlet flow rate of 50 lpm and an outlet flow rate of 2 lpm, a remaining portion of particles in a band between the minimum and maximum particle size may be concentrated by a factor of about 25 while maintaining the quantity of particles that are less than the minimum particle size at about ambient concentrations.  
         [0027]    The first sensor  18  receives the enriched aerosol from the impactor assembly  12  and continuously measures a predetermined characteristic of the particles in the aerosol. The first sensor  18  may measure particle characteristics including mass and chemical composition. Devices that may be used for measuring mass include tapered element oscillating microbalances (TEOMs), light scattering photometers, beta attenuation monitors, and optical counters. For measuring chemical composition, devices include ion chromatographs for sulfate, nitrate, sodium, and ammonium; inductively-coupled plasma mass spectrometers and graphite furnaces for trace elements and metals; thermal desporption units for organic concentrations; and mass spectrometers for detection of biologically active compounds in airborne coarse particles.  
         [0028]    A third impactor  20  may be coupled to the impactor assembly  12  to extract another band of particle sizes to be measured by another sensor  22 . The third impactor  20  may be arranged to receive a portion of the aerosol that includes particles having an aerodynamic diameter that is either less than the minimum particle size or greater than the maximum particle size. The third impactor  20  removes another portion of the particles based upon particle size so that the remaining portion includes particles having sizes within a another band of particle sizes such as between the minimum particle size and a smaller particle size that is less than the minimum particle size. For example, the impactor assembly  12  may remove particles having sizes outside the range of 2.5 um to 10 um. The third impactor  20  may then receive the exhaust aerosol from the impactor assembly  12  including particles having sizes that are less than 2.5 um. The third impactor  20  may remove particles from the exhaust aerosol having particle sizes that are less than 1.5 um. A remaining portion of the aerosol having particles with sizes in the range of 1.5 um to 2.5 um is sent to the second sensor  22  to be measured. In addition, the third impactor  20  may concentrate the particles having the selected range of sizes by emitting the aerosol at an outlet flow rate that is less than the flow rate into the third impactor  20 . Further fractionate portions of the particles may be obtained by coupling further impactors and sensors to the third impactor  20  or to the impactor assembly  12 .  
         [0029]    [0029]FIG. 5 shows a two-dimensional view of an embodiment of a particulate matter (PM) monitor  30  for measuring a characteristic of PM. The PM is suspended in an aerosol that is drawn into a high flow rate inlet  32  at about 50 lpm. The high flow rate inlet  32  removes matter that has an aerodynamic diameter that is greater than 10 um. A first virtual impactor  34  is coupled to the high flow rate inlet  32  to remove particles that have an aerodynamic diameter that is less than 2.5 um. The first virtual impactor  34  has a major flow  36  with a flow rate of 48 lpm, and a minor flow  38  with a flow rate of 2 lpm. The major flow  36  includes the particles that have a size that is less than 2.5 um. The minor flow  38  includes an enriched aerosol of the particles ranging in size between 2.5 um and 10 um. The concentration of the selected particles is about 25 times the ambient level. The enriched aerosol additionally includes an ambient level of particles having a size that is less than 2.5 um. The minor flow  38  is coupled to a TEOM  40  that measures the mass of the PM.  
         [0030]    The major flow  36  is coupled to a pump  50  that draws the aerosol through the PM inlet  32  and the virtual impactor  34 . Alternatively, the major flow  36  may be coupled to a second virtual impactor (not shown) to extract a fractionate portion of particles from the first virtual impactor major flow  36 . The pump  50  would then be coupled to the second virtual impactor.  
         [0031]    [0031]FIG. 6 shows another embodiment of a PM monitor  60  for measuring a characteristic of PM. The configuration of the PM monitor  60  is similar to PM monitor  30  except that the aerosol is drawn into a virtual impactor  62  before flowing through a high flow rate inlet impactor  64  and into a sensor  66 . The aerosol is drawn into the virtual impactor  62  at an inlet flow rate. A major flow  68  of the virtual impactor  62  exhausts particles that are smaller than a minimum particle size. A minor flow  70  of the virtual impactor  62  outputs an enriched aerosol at an outlet flow rate that is less than the inlet flow rate. The enriched aerosol includes particles that have an aerodynamic diameter that is greater than the minimum particle size. The particles are concentrated at a level above the ambient level by a factor approximately equal to the ratio of the inlet flow rate to the outlet flow rate of the virtual impactor  62 . The inlet impactor  64  removes particles from the enriched aerosol that have a particle size that is greater than the maximum particle size. The remaining portion of the aerosol includes an enriched portion containing particles ranging in size between the minimum particle size and the maximum particle size, as well as an ambient level of particles that are less than the minimum particle size.  
         [0032]    [0032]FIG. 7 shows a flow chart for a PM measurement process. Starting at block  70 , an aerosol including PM is drawn into a high flow rate inlet at an inlet flow rate equal to or greater than 20 lpm. Continuing at block  72 , a first portion of particles having particle sizes in a first range is removed from the aerosol. At block  74 , another portion of particles having particle sizes in a second range is also removed from the aerosol so that the remaining portion has particles that have particle sizes between a minimum particle size and a maximum particle size. Continuing to block  76 , the remaining portion of the aerosol is emitted at an outlet flow rate that is less than the inlet flow rate so that the particles between the minimum particle size and the maximum particle size are concentrated in the remaining aerosol. Finishing at block  78 , measuring a characteristic of the remaining aerosol.  
         [0033]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.