Abstract:
An absolute reference aerosol sampler system and method based on isokinetic and isoaxial sampling principles wherein the position of a collection inlet for an aerosol sampling head is adjusted relative to the actual wind direction in real time. In addition, flow rate of the air at a collection inlet within the absolute-reference sampler is matched to the real time actual wind speed. These adjustments and controls enable the absolute reference sampler system to obtain particulate samples from the immediate environment without biasing the size distribution in any manner, resulting in a fully unbiased, representative sample.

Description:
GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used and/or licensed by or for the government of the United States of America. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an aerosol sampler system. More particularly, the aerosol sampler system of the present invention provides an aerosol sampling that does not disturb the environment upstream of the collected sample. Most particularly, the aerosol sampler system collects aerosol samples in an unbiased manner with respect to aerosol particle size. 
     2. Brief Description of the Related Art 
     Sample size bias occurs within most commercially available sample collection systems. This is most commonly a result of the competition between air drag and inertial effects on the aerosol particle due to non-uniformities in the airflow at the entrance of the sample collection system. This problem is addressed by assigning a parameter known as aerodynamic equivalent diameter (AED) which equates a given particle to that of a theoretical spherical particle with a specific standard density, i.e., the theoretical spherical particle displays an experimental aerodynamic behavior that is the same as the real given particle. As such, the particle size is equated with the “aerodynamic equivalent diameter,” i.e., the size of a unit density sphere having the same settling velocity as the particle in question, of whatever size, shape or density. Simply viewed, as the flow into a collection entrance becomes increasingly restricted, less massive particles are proportionately forced around the entrance, while the particles having a greater mass are more disproportionately forced into the entrance, resulting in sampling errors. 
     The currently available sampling devices introduce biasing factors into the collected sample, due to various imperfections and limitations in their design. These samplers generally fall into two categories, omni-directional samplers and fixed direction samplers. Omni-directional samplers generally under-sample larger particle sizes (&gt;10 microns), as the larger particles tend to deviate from the air stream direction due to inertial effects when entering the inlet, resulting in impaction on the inner walls of the inlet. Further complicating the collection process is the fact that the magnitude of the loss of large particles is a function of the ambient wind speed. Similarly, fixed directional samplers generally under-sample larger particles when not aligned with the ambient wind direction, or when sampling at an inlet velocity greater than that of the ambient wind speed. The magnitude of these losses is also a function of wind speed. As a result, even when these sampling flaws are apparent for a given experiment, the affect of these flaws on the collected sample remains unknown. 
     Accordingly, there is a need to provide an improved aerosol sampling system that collects aerosol samples in an unbiased manner with respect to aerosol particle size. The present invention addresses this and other needs. 
     SUMMARY OF THE INVENTION 
     The present invention includes an absolute reference sampler system comprising (A) a positioning component comprising a sampling head having a collection inlet, means for determining wind direction wherein the wind direction is determined in real time, means for comparing the position of the collection inlet and the determined wind direction wherein a differential value is created and means for positioning the sampling head reactive to the differential value and operably connected to the sampling head wherein the means for positioning the sampling head rotates the sampling head for isoaxial alignment with the determined wind direction, and (B) an intake component comprising a mass flow controller, means for determining wind speed wherein the wind speed is determined in real time and means for drawing an effective amount of ambient air into the collection inlet for mass flow controller operation wherein the mass flow controller receives an input for the determined wind speed causing the mass flow controller to regulate ambient air flow into the collection inlet to an air speed substantially equivalent to the determined wind speed. 
     The present invention also includes a method for ambient aerosol sampling comprising the steps of providing an absolute reference sampler system comprising (A) a positioning component comprising a sampling head having a collection inlet, means for determining wind direction wherein the wind direction is determined in real time, means for comparing the position of the collection inlet and the determined wind direction wherein a differential value is created and means for positioning the sampling head reactive to the differential value and operably connected to the sampling head wherein the means for positioning the sampling head rotates the sampling head for isoaxial alignment with the determined wind direction, and (B) an intake component comprising a mass flow controller, means for determining wind speed wherein the wind speed is determined in real time and means for drawing an effective amount of ambient air into the collection inlet for mass flow controller operation wherein the mass flow controller receives an input for the determined wind speed causing the mass flow controller to regulate ambient air flow into the collection inlet to a speed substantially equivalent to the determined wind speed, and drawing an effective amount of ambient air into the collection inlet wherein the differential value relative to the wind direction and sampling head position is calculated causing the means for. positioning the sampling head to rotate the sampling head proportionally to the differential value to a calculated position and wherein the mass flow controller regulates the ambient air flow into the collection inlet to a speed substantially equivalent to the determined wind speed. 
     The present invention further includes an ambient aerosol sampling product generated by the process comprising the steps of positioning a collection inlet into isoaxial alignment with a determined wind direction in real time and regulating ambient air flow into the collection inlet to a speed substantially equivalent to a determined wind speed in real time wherein an amount of collected aerosol comprises an unbiased environmental sample. 
     The present invention isoaxially aligns the collection inlet of the aerosol sampling system with the wind direction and draws an isokinetic sample at an inlet speed equivalent to the wind speed, allowing the collection of an unbiased aerosol sample with respect to aerosol particle size. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the absolute reference sample system of the present invention having a positioning component and intake component, with FIG. 1A illustrating in detail the shroud shown in FIG. 1, FIG. 1B illustrating in detail the airtight sealed bearing system of the absolute reference sample system shown in FIG. 1, and FIG. 1C providing an exploded view of said sealed bearing system; 
     FIG. 2 is a schematic representation of the comparator circuit of the present invention for controlling the positioning component; 
     FIG. 3 is an operational schematic representation of the absolute reference sampler of the present invention; 
     FIG. 4 is graphical representation of the wind direction compared to the collection inlet position of the present invention over time; and, 
     FIG. 5 is graphical representation of the wind speed compared to the inlet speed of the present invention over time. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a system and method to collect a representative sample of ambient aerosols from an open environment. The sampler of the present invention is a device used in the field of aerosol science to collect aerosol particles from the surrounding environment. Once the sampler collects a particle sample, the collected particle sample may then be tested to yield qualitative or quantitative information about the environment from which the sample was collected. The sampler does not disturb the environment upstream of the collected sample, points into the wind, i.e., aligns isoaxially with the wind direction, and ingests samples with an inlet speed equivalent, generally equal, to the ambient wind speed, i.e., achieves an isokinetic state with the wind speed. As the sampler attains this criterion, the collected aerosol sample becomes unbiased for differing aerosol particle sizes within the ambient air. The sampler provides a representative sample of the immediate environment by collecting a sample of the ambient aerosols in the surrounding environment without biasing the sample size distribution, i.e., the collected sample is representative of the surrounding environment. Collection of unbiased samples permits an accurate determination of aerosol concentrations, such as aerosol contaminants within the ambient air. 
     As seen in FIG. 1, an absolute reference sampler system  10  comprises a positioning component  12  and an intake component  14 . The positioning component  12  of the absolute reference sampler system  10  functions to continuously align a sampling head  22  having a collection inlet  24  in substantially isoaxial alignment with a given wind direction  110  as measured within the immediate environment of the absolute reference sampler system  10 . The intake component  14  draws ambient air  100  into the collection inlet  24  at a substantially isokinetic rate to a given wind speed  120 . Although the position of the sampling head  22  is changed, the sampling head  22  maintains a sealed flow between the collection inlet  24  and a means for drawing  48  an effective amount of ambient air  100  through the use of a sealed bearing system  60 . 
     Referring to FIG. 1A, beyond the leading edge or tip of the collection inlet  24 , a shroud  26  is preferably used to facilitate the ingress of the air sample into the inlet probe  24 A of the collection inlet  24 . The shroud  26  includes support fins  26 B supporting the outer edges of the shroud  26 . Shroud  26  acts to decelerate the incoming ambient air  100 . Preferably, the shroud  26  decelerates the ambient air  100  by a factor of from about 2 to about 5, more preferably a factor of from about 3.5 to about 4.5. The proper size and performance characteristics for the shroud  26  for a given absolute reference sampler system  10  are determinable by those skilled in the art, particularly in light of the disclosure of shrouds  26  in the articles “A Predictive Model for Aerosol Transmission through a Shrouded Probe” by Gong et al., Environmental Science &amp; Technology, Vo. 30, No. 11, Pgs. 3192-3198, and “Shrouded Probe Performance: Variable Flow Operation and Effect of Free Stream Turbulence” by Chandra et al., Aerosol Science and Technology, 26:2 February 1997, pgs. 111-126, with the disclosures of these two articles incorporated herein by reference in their entirety. As discussed within these articles, when the shroud  26  is added over the tip of the collection inlet  24 , the air entering the collection inlet  24  becomes decelerated by an appropriate factor, as previously described, depending on the geometry of the collection inlet  24  and shroud  26 . This deceleration allows the absolute reference sampler system  10  to either utilize less capable mechanisms for drawing air into the collection inlet  24  for the absolute reference sampler system  10  to function in the same range of wind speeds, or to expand the range of wind speeds covered by the same mechanisms for drawing air into the absolute reference sampler system  10 , depending on the numerical factor of deceleration provided by the shroud  26 . For example, if the shroud  26  decelerates the inlet air velocity by a factor of four, the range of wind speeds measurable by the absolute reference sampler system  10  without the shroud  26  having a range of from 0 to 10 mph advances to isokinetic wind speeds sampling of 0 to 40 mph (4×10) for the absolute reference sampler system  10  with the addition of the shroud  26 . 
     As further seen in FIGS. 1 and 1A, within the sampling head  22 , an inspection component  28  is placed along conduit  74  for the collection inlet  24  to collect (retain) or analyze the aerosol particles that entered the collection inlet  24 . The inspection component  28  may comprise a collection mechanism, such as a fixed filter holder  30  containing a standard fiber or membrane type filter  32  for collection of the aerosol sample. Once the trial is concluded, the filter  32  is removed from the filter holder  30  and analyzed. Preferably, however, the inspection component  28  comprises a mechanism for immediately analyzing the aerosol sample using an analyzing component  34 , such as an optical particle counter/sizer. Exemplary devices include those commercially available as the MET ONE from Pacific Scientific of Grants Pass, Oregon or the TSI Aerodynamic Particle Sizer, with proper mounting and configuration into the sampling head  22  of the absolute reference sampler system  10  determinable by those skilled in the art, such as removing the optical particle detector mechanism to mount it directly to the sampling head  22  while remaining attached electronically through a cable or other means such as a wireless transmitter to the rest of the commercial system, which performs particle counting/sizing functions in a normal manner allowing the user to monitor the data through a dedicated computer  50 , shown in FIG.  3 . The MET ONE particle counting optics detect the aerosol sample as it passes through a MET ONE optical scanner  36  which is connected to the MET ONE data processing component  38  through a data cable or wireless modem, which then produces size and quantity data on the detected particles. 
     Preferably, the sampling head  22  comprises a swiveling sampling head  22  that rotates into two directions within a plane above and parallel to the ground, i.e., along the horizontal plane. Within the swiveling sampling head  22 , the inspection component  28  is appropriately placed to collect aerosol samples from the drawn ambient air  100 . Proper placement of the inspection component  28  is determinable by those skilled in the art in light of the disclosure herein, the main criteria being to avoid turbulence within the collection inlet  24  while creating enough distance from the blunt face of the inspection component  28  to minimize any effect on the particle flow at the inlet tip. Generally, inspection component  28  placement will most appropriately collect or analyze a representative, i.e., unbiased, aerosol sample, such as perpendicular placement directly in the path of the drawn ambient air  100  with the inspection component  28  holding the filter  32  or analyzing component  34  from about 5 inches to about 15 inches from the tip of the collection inlet  24  on the swiveling sampling head  22 . 
     The positioning component  12  moves through a bearing system  60  (detailed in FIG. 1B) located below the positioning component  12 . The bearing system  60  of the absolute reference sampler system  10  contains the ambient air  100  flow within the sealed conduit  74  to maintain vacuum for pulling the ambient air  100  through the collection inlet  24 . As such, the bearing system  60  seals the conduit  74  between and connecting a means for drawing  48  ambient air  100  and the collection inlet  24 . This permits the bearing system  60  to allow efficient rotation of the sampling head  24  while ensuring that only sampled ambient air  100  gets into the intake component  14 . The bearing system  60  may include any appropriate mechanism sufficient to accomplish proper functioning of the present invention in light of the teaching herein, with a preferred bearing system  60  comprising a Browning Model FBE920 Flange mounted ball bearing  72  having a one inch (1 in.) diameter shaft  62  press fitted into the bearing hole  70 . The shaft  62  protrudes approximately two inches (2 in.) out of the base  66  of the bearing, and mates with a second shaft  64 . The interconnection between the bottom of shaft  62  and the top of second shaft  64  is sufficiently aligned and lubricated to form an airtight seal  68  within the tee-shaped collar  65  sitting on support plate  67 , while allowing the bearing mounted shaft  62  to swivel inside collar  65 , with the second shaft  64  fixed in place within the same collar  65 . Both shaft  62  and second shaft  64  have a three-quarters inch (¾ in.) inner diameter through which the ambient air  100  passes as the ambient air  100  is pulled through the conduit  74  by a pump  48 . An O-ring seal  63  at the top of shaft  62 , underneath the flange mounted ball-bearing  72  and along the outer wall of shaft  64 , provides a leak proof seal at these points. The configuration of shaft  62  and second shaft  64  allows the sampling head  22  to rotate while maintaining the conduit  74  sealed for the ambient air  100  to pass, with variations of this design determinable by those skilled in the art in light of the disclosure herein. 
     Wind direction  110  and wind speed  120  are determined by on-site measurement. Preferably, the absolute reference sampler system  10  includes a portable meteorological station  16  that includes a means for determining wind direction  52  and a means for determining wind speed  54 , as well as other devices for measuring and recording environmental conditions such as temperature, true barometric pressure, and relative humidity in the immediate area of the absolute reference sampler system  10 . As part of the positioning component  12  of the absolute reference sampler system  10 , the means for determining wind direction  52  determines the ambient air  100  wind direction  110  in real time. This provides a reference measurement to maneuver the sampling head  22  in a particular direction to maintain the collection inlet  24  in isoaxial alignment with wind direction  110 , which is subject to rapid and repeated changes. Additionally, the means for determining wind speed  54  determines the ambient air  100  wind speed  120  in real time for an additional reference for calibrating a correct ambient air  100  draw into the collection inlet  24 . Preferably, both the means for determining wind direction  52  and means for determining wind speed  54  are housed within the meteorological station  16 , which may be located in a suitable place for actual wind condition experienced by the absolute reference sampler system  10 , with the proper location determinable by those skilled in the art. Preferably, the meteorological station  16  includes several environmental measuring devices for extensive analysis of the collected aerosol sample. While the meteorological station  16  provides real time wind speed  120  and wind direction  110  information for the absolute reference sampler  10  to properly adjust, additional environmental conditions are monitored and/or recorded with these environmental measuring devices in the portable meteorological station  16  such as, without limitation, temperature, barometric pressure and relative humidity. Although preferably portable, the meteorological station  16  may be permanently fixed within a location, as desired, preferably remaining within a three-foot radius of the absolute reference sampler system  10 . Portability provides easy relocation of the absolute reference sampler system  10 , for either use as a single device in each of several areas and/or use in remote areas. Portable infers a compactness and weight that readily permits transport to and set-up of the absolute reference sampler system  10  in a desired location, such as by transport vehicle or carried by individuals. Preferably, this includes weights of 60 pounds or less, preferably from 30 pounds or less, and sizes not exceeding from about 4 meters 2 , preferably from about 2 meters 2 . Real time wind speed  120  data, wind direction  110  and other relevant environment conditions are monitored and recorded using such devices as the ASCII Data Logger  20  and the portable computer  50 , which preferably utilize a Metrabyte Data Acquisition or similar card and compatible software such as LabView or LabTech Notebook software for viewing and logging of real time data. Real time determination of the wind direction  110  and wind speed  120  occurs with any appropriate commercially available meteorological system, such as a meteorological sensing device sold under the tradename Anemometer and Meteorological Sensor, Model 500CM-6 manufactured by Cossonay Meteorology Systems of Reading, Pa. for measuring wind speed and direction, temperature, pressure, and relative humidity. 
     Measurement of real time wind speed  120  permits proper regulation of the ambient air  100  by the intake component  14  into the collection inlet  24 . The intake component  14  of the absolute reference sampler system  10  includes a mass flow controller  42  for controlling the intake component  14 , i.e., the draw of the ambient air  100  into the absolute reference sampler system  10  and regulating the flow of ambient air  100  into the collection inlet  24 . As the means for drawing  48  ambient air  100 , such as a pump, draws air into the absolute reference sampler system  10 , the mass flow controller  42  ensures ambient air  100  draw remains referenced to the actual wind speed  120  of the ambient air  100  outside of the absolute reference sampler system  10 . The mass flow controller  42  preferably comprises a flow meter  42 A and a valve  42 B for referencing the ambient air  100  draw into the absolute reference sampler system  10 . The flow meter  42 A component of the mass flow controller  42  measures the amount of ambient air  100  passing through the mass flow controller  42 . This measurement of ambient air  100  by the flow meter  42 A is used, in real time, to control the valve  42 B. The valve  42 B of the mass flow controller  42  allows the mass flow controller  42  to vary this amount of ambient air  100  passing through the mass flow controller  42 . In combination, the flow meter  42 A and valve  42 B function to equate the draw of ambient air  100  into the absolute reference sampler system  10  with the external environmental wind speed  120  conditions. Preferably, the two functional parts of the mass flow controller  42 , i.e., the flow meter  42 A and the flowmeter valve  42 B, are integrated into a single unit. The valve  42 B preferably comprises a proportioning valve  42 B. Selection of a suitable mass flow controller  42  is determinable by those skilled in the art in light of the disclosure herein, for example, a mass flow controller  42  being commercially available under the tradename of Matheson, manufactured by Matheson Gas Products of Montgomeryville, Pa. 
     In one preferred embodiment, the mass flow controller  42  responds to a command voltage input (determined by the wind speed) ranging from 0 to 5 volts dc. At an input of 0 volts dc, the valve is fully closed, while for an input of 5 volts dc the valve is fully open. All voltages in between 0 and 5 respond in a linear manner, i.e., for an input of 2.5 volts the valve is half opened. For a vacuum pump  48  and mass flow controller  42  with a maximum flow capacity of 50 standard liters per minute (slpm) throughput, an input of 2.5 volts dc would yield a throughput of 25 slpm, an input of 1.25 volts dc yields 12.5 slpm, and other voltages being likewise linearly proportional to the throughput of the valve  42 B. 
     If the collection inlet  24  does not include the shroud  26 , the velocity of the air entering the collection inlet is set equal to the ambient wind speed which corresponds to a flow meter  42 A reading by the equation I below, where flowrate and wind speed are referenced to standard temperature at 20° C. and standard pressure at 1 atmosphere: 
     
       
         Flowrate=(π/4)(Diameter Inlet ) 2 ×Wind Speed  (I) 
       
     
     This encompasses the condition of isokinetic inlet sampling in which the wind velocity at the collection inlet  24  equals the ambient air speed  120 . When the shroud  26  is placed over the collection inlet  24 , with the inlet probe diameter 30% of the shroud diameter, the required system flowrate is reduced by a factor of 4 relative to the unshrouded inlet, and is described by equation II below:                    Flowrate   =       (     π   /   4     )            (     Diameter   Inlet     )     2     ×       (     Wind                 Speed     )     /   4                   =       (     π   /   4     )            (     0.3        Diameter   Shroud       )     2     ×       (     Wind                 Speed     )     /   4                   =     0.0225          π        (     Diameter   Shroud     )       2     ×       (     Wind                 Speed     )     /   4                     (   II   )                                
     Accordingly, in the case of a shrouded probe, the inner probe  24 A has a flowrate only 0.0225 times the ambient flow  100  entering the shroud  26 . This provides a substantially equivalent wind speed at the shrouded probe compared to the actual wind speed conditions to compensate for an isokinetic condition. As such the flowrate of the shrouded probe is shown in equation III below: 
     
       
         Flowrate=xπ(Diameter Shroud ) 2 ×(Wind Speed)/4  (III) 
       
     
     where x is determinable by those skilled in the art for a given shroud configuration and size. 
     The intake component  14  further includes the means for drawing  48  an effective amount of ambient air  100  into the collection inlet  24 . An effective amount includes sufficient ambient air  100  flow for mass flow controller  42  operation to match the speed of the ambient air  100  flow into the collection inlet  24  with the real time determined wind speed  120 . The means for drawing  48  includes any suitable vacuum device that functionally draws enough ambient air  100  into the collection inlet  24 . Preferably, the means for drawing  48  ambient air  100  into the collection inlet comprises a pump  48 . Such devices include, without limitation, mechanical, electrical or pneumatic pumps, with the proper selection of pump or other suitable drawing device determinable by those skilled in the art in light of the disclosure herein. For proper continuous functioning, the selected pump  48  should be capable of pulling against a complete vacuum, i.e., against a closed system, without sustaining damage. 
     FIG. 2 shows a schematic representation of a comparator circuit  46  used by the present invention for controlling the positioning component  12 . As seen in FIGS. 1 and 2, the means for comparing the position of the collection inlet  24  and the determined wind direction  110  preferably comprises the comparator circuit  46  for controlling the sampling head  22  position. Wind direction  110  data is used to control a stepper motor feedback system  58 A to swivel the sampling head  22  which is achieved through the comparator circuit  46  which compares the output of a position sensor, attached to the shaft of the sampling head  22 , with the wind direction  110  data, instructing a stepper motor  58  to turn either clockwise or counter clockwise until the desired position is reached, at which time the stepper motor  58  is disabled until the wind direction  110  changes. 
     As seen in FIG. 2, the absolute reference sampler system  10  comparator circuit  46  acts as a means for comparing  56  the position of the collection inlet  24  and the determined wind direction  110 , to ensure isoaxial alignment of the collection inlet  24  with the actual wind direction  110 . Position comparison between the position of the collection inlet  24  and the wind direction  110  yields a differential value that is created to represent any variance between the position of the collection inlet  24  and the determined wind direction  110 , i.e., the degree of non-isoaxial alignment. With the determination of the differential value, the absolute reference sampler system  10  employs a stepper motor  58  for positioning the sampling head  22 , and concurrently the collection inlet  24 , that uses, or reacts, to the differential value to adjust the sampling head  22 . The stepper motor  58  for positioning is operably connected to the sampling head  22 , allowing the stepper motor  58  to turn, swivel or otherwise rotate the sampling head  22  into the wind direction  110  and fixing the collection inlet  24  in isoaxial alignment with the determined wind direction  110 , i.e., the wind direction  110  data is used to control a stepper motor feedback system  58 A (shown in FIG. 1) which swivels the sampling head  22 . 
     Rotation of the sampling head  22  is achieved using a means for positioning  58  the sampling head  22 , such as the stepper motor  58  in combination with the stepper motor feedback system  58 A. Any suitable motor  58  may be used for moving the sampling head  22 , with proper selection of the motor  58  being determinable by those skilled in the art in light of the disclosure herein. An example of such motor  58  includes a Hurst Model SAS4004-014 stepper motor controlled by a Hurst Model 220006 Stepper Motor Controller, manufactured by Hurst Manufacturing Division of Princeton, Ind., which is controlled by standard CMOS 5 volt logic level inputs. The stepper motor feedback system  58 A preferably comprises a US Digital Corporation A2 absolute encoder attached to the shaft of the sampling head  22 . The A2 encoder produces an analog voltage of 0 to 4.096 volts dc. These values represent the position or bearing of the collection inlet  24 , with 0 Vdc representing 0 degrees, and 4.096 Vdc representing 359 degrees. All values between 0 Vdc and 4.096 Vdc are linearly related to the position of the collection inlet  24 , i.e., 90 degrees is represented by 1.024 Vdc, 180 degrees is represented by 2.048 Vdc, 270 degrees is represented by 3.072 Vdc, etc. As further seen in FIG. 2 in the first comparison step of the comparator circuit  46 , the representative voltage of the collection inlet  24  (shaft encoder) is compared electronically with the voltage from the wind direction sensor (wind direction), such as a Fidelity Model 500 wind direction sensor, manufactured by Cossonay Meteorology Systems of Reading, Pa. The voltage for the wind direction sensor has the same characteristics as that of the shaft encoder, with 0 Vdc representing 0 degrees, 1.024 Vdc representing 90 degrees, 2.048 Vdc representing 180 degrees, 3.072 Vdc representing 270 degrees, 4.096 Vdc representing 359 degrees, and all interposed values corresponding linearly. When the wind direction voltage is greater than that of the shaft encoder voltage, the stepper motor  58  (shown in FIG. 1) is directed to turn clockwise until the difference between the two voltages is minimal, at which time the stepper motor  58  is disabled until the wind direction changes again. When the wind direction voltage is smaller than that of the shaft encoder voltage, the stepper motor  58  is directed to turn counter-clockwise until the difference between the two voltage values becomes minimal, at which time the stepper motor  58  is disabled until the wind direction changes again. 
     Problematic with the comparator circuit  46 , large excursions of the sampler head  22  may occur when attempting to cross from 359 degrees to 1 degree, or any time it is required to cross 0 degree. This problem is alleviated by computing the difference voltage between the wind direction sensor and the shaft encoder sensor. For occasions when the shaft position crosses 0 degrees, which would result in large 300+ degree excursions of the sampling head  22 , an absolute magnitude of the difference between the wind direction voltage and shaft encoder voltage is computed. As such, when the difference is smaller than 2.048 Vdc (180 degrees), the stepper motor  58  reacts in the direction as previously described. An absolute magnitude of the voltage difference that exceeds half of the full scale voltage (2.048 Vdc) indicates that the sampling head  22  and the wind direction  110  are more than 180 apart, and that it would be shorter for the sampling head  22  to turn in the opposite direction. Accordingly, when the voltage difference is greater than 2.048 Vdc, the stepper motor  58  is instructed to turn in the opposite direction of the initial comparison, until the difference between the two voltages is minimal, at which time the stepper motor  58  is disabled. For example, when the wind direction  110  is at 10 degrees and the sampler head  22  is at 350 degrees (a difference of 340 degrees, and being greater than 180 degrees ), the sampler head  22  moves 20 degrees clockwise instead of 340 degrees counter-clockwise to realign to collection inlet  24  in isoaxial alignment with the wind direction  110 . 
     Referring further to FIG. 2, the proper direction of turn for the sampling head  22  is accomplished at the second comparator, whose output is Exclusive-Ored (X-OR) with the output of the first comparator. If the output of the second comparator is logic level  1 , the output of the X-OR gate will be the inverse of the input from the first comparator, therefore instructing the stepper motor  58  to turn in the opposite direction. The stepper motor  58  continues turning until the voltage difference falls below a threshold voltage set at the third comparator. When this occurs, the output of the third comparator goes to a high logic level and disables the stepper motor  58 , as the sample head  22  becomes essentially in alignment with the wind direction  110 . Once disabled, the stepper motor  58  remains disabled until the difference voltage exceeds the threshold voltage due to a change in wind direction  110 . 
     FIG. 3 is an operational schematic representation of the absolute reference sampler system  10 . In a preferred embodiment as seen in FIG. 3, meteorological data is accumulated in real time by the Fidelity MET Sensor  16 A and Fidelity Relative Humidity Sensor  16 B. The information collected includes wind speed  120 , wind direction  110 , relative humidity, barometric pressure, and temperature. This information passes to the Fidelity Junction Box  18  through electrical cabling in the form of binary data. The data is processed in the Junction Box  18 , where corrections are made to the wind speed data based on the current temperature, relative humidity, and barometric pressure. Four output signals originate from the Junction Box  18 . One includes an ASCII formatted serial data stream  18 A sent to an ASCII data logger  20  manufactured by Acumen Instruments of Ames, Iowa, which stores the data relating to wind speed  120 , wind direction  110 , relative humidity, and barometric pressure and temperature. ASCII comprises a standard format used for transmitting binary data between digital systems, preferably through RS-232 or RS-422 links. The data logger  20  stores the ASCII data for the length of the test, and can be analyzed later with a personal computer to determine whether the absolute reference sampler system  10  is properly functioning. The other three signals  18 B from the junction box  18  pass to an electronic control box  44  through electrical cabling, and include three control signals: a serial data signal, data clock signal, and data select signal. These signals  18 B are processed by the control box  44  to control the position of the sampling head  22 . The feedback circuit, i.e., comparator circuit  46  (shown in FIG. 2) compares the wind direction  110  of the ambient air  100  information with the sampler head  22  position, and generates the appropriate control signals to the stepper motor  58  which turns the collection inlet  24  to the correct isoaxial position with the wind direction  110 . These signals also contain the wind speed  120  information used to determine the proper pump  48  rate. The pump  48  pulls at a constant rate of revolutions per minute (rpm), yielding a free air capacity of greater than 100 liters per minute (lpm), using a mass flow controller  42  internal to the control box  44  restricts the amount of air passing through the collection inlet  24  in proportion to the wind speed  120 . A set of analog voltage outputs is sent from the control box  44  to an external computer  50  containing a data acquisition card. These outputs are dc voltages ranging from 0 to +5 Volts dc which vary according to the sensor input, and represent wind speed  120 , wind direction  110 , sampling head  22  position, and inlet flow rate. The computer  50  converts these signals to a real time graphical display, allowing the operator to monitor the absolute reference sampler system  10  performance in real time, with the computer  50  additionally logging the collected data for later analysis. 
     In operation, the absolute reference sampler system  10  samples ambient air  100  by drawing an effective amount of ambient air  100  into the collection inlet  24 . This effective amount of ambient air  100  occurs with the differential value relative to the wind direction  110  and sampling head  22  position is calculated causing the means for positioning  58  the sampling head  22  to rotate the sampling head  22  proportionally to the differential value to a calculated sampling head  22  position, with the mass flow controller  42  regulating the ambient air  100  flow into the collection inlet  24  to a speed substantially equivalent to the determined wind speed  120 , e.g., a fixed rate pump pulls continuously at a rate of greater than 100 liters per minute free air capacity, while a 50 standard liters per minute (standard liters per minute) mass flow controller  42  varies or limits the flow rate at the tip of the collection inlet  24  in accordance with the wind speed  120  data determined by the meteorological station  16 . Once the aerosol sample is collected in the filter  32 , the filter  32  is removed from the swiveling sampling head  22  and the aerosol sample therefrom is analyzed. 
     While modulation of ambient air  100  intake occurs, the sampling head  22  is rotated for isoaxial alignment of the collection inlet  24  with the determined wind direction  110 . Isoaxial alignment includes those angles that minimize or eliminate bias of a collected sample, as determinable by those skilled in the art in light of the disclosure here. Preferably isoaxial alignment ranges from equal to or less than about −3° to about +3°. Most preferably, the isoaxial alignment of the collection inlet  24  to the determined wind direction  110  is approximately 0°, with an unavoidable but minimal real time lag for positioning. 
     The absolute reference sampler  10  achieves proper directional alignment in real time through the use of the swiveling sampling head  22 , and proper real time system flow rate through the use of a mass flow controller  42 . 
     EXAMPLE 1 
     An absolute reference sampler prototype was built and tested. The absolute reference sampler was constructed using a mass flow controller transducer manufactured by Matheson Gas Products of Montgomeryville, Pa., an A2 Absolute Encoder with a shaft angle within a 360 degree range manufactured by US Digital Corporation of Vancouver, Wash., a Hurst Stepping Motor Controller, Model 220006, manufactured by Hurst of Princeton, Ind., and a meteorological sensing device of an Anemometer and Meteorological Sensor, Model 500CM-6 manufactured by Cossonay Meteorology Systems of Reading, Pa. for measuring wind speed and direction, temperature, pressure, and relative humidity. Critical parameters, such as wind speed, wind direction, flow rate and sampling head position were monitored in real time through use of a portable computer and data acquisition card. These parameters were logged to the computer&#39;s hard drive for analysis and archival purposes using Microsoft Excel. The prototype was portable, measuring 4.5 feet in height, 18 inches in depth and 12 inches in width, with a weight of 25 pounds. 
     Results of the tests are shown in FIGS. 4 and 5. As seen in the graph depicted in FIG. 4, the actual recorded wind direction is compared to the collection inlet position of the system of the present invention over a time period of 2.5 minutes. The alignment between the actual wind direction and sampler head has a maximum deviation of less than 100 degrees for less than 2 seconds (occurring at time about 109 seconds) with an overall mean deviation of approximately 5.9 degrees. For the hundreds of corrections evidenced within the time period of 2.5 minutes, the maximum deviation occurred when the present invention corrected in the opposite direction of the wind direction change, followed by an immediate recovery to the correct alignment. FIG. 5 is a graphic representation of the wind speed compared to the inlet speed of the present invention over time. The plot of FIG. 5 shows the system flow rate of the present invention matches the actual wind speed in real time over the test period of 2.5 minutes with close alignment. The correlation between the actual wind speed and the inlet air velocity has a maximum deviation of less than three standard liters per minute (slpm) (occurring at time about 1.5 minutes) with an overall mean deviation of approximately 0.23 slpm. With these preliminary data points of the performance of the actual sampling accuracy of the absolute reference sampler, the close correlation of sampler head position and system flow rate with the ambient wind conditions permit the absolute reference sampler to accurately collect ambient aerosol samples from a given environment in an unbiased manner. 
     EXAMPLE 2 
     (Flow Rate) 
     Prophetic 
     An absolute reference sampler of the present invention is constructed with fixed rate pump having a continuous pull rate of greater than 100 liters per minute free air capacity. Within the sampler, a 50 standard liters per minute (standard liters per minute) mass flow controller varies the flow rate at the sampler tip to match the wind speed data, in real time, collected from an attached meteorological station. The mass flow controller proportions the valve to proportion air  100  flow through it in response to an electrical input of a 0-5 volt command voltage. An operational amplifier circuit computes the 0 to 5 Vdc command voltage based on the wind speed data. A 0 volt input produces a flow of 0 liters per minute and a 5 volt input produces a flow of 50 liters per minute, with input values between 0 and 5 corresponding in a linear manner (giving a proportion of the control voltage to the desired flow rate into the absolute reference sampler as a ratio of approximately 1:10). For example, a wind speed of 10 mph provides a flow rate of 34 (10×3.4) liters per minute from a Matheson command voltage of 3.4 (34/10) volts. The required flow rate for the mass flow controller is determined in the unshrouded probe configuration by the formula Flow rate=(π/4)(Diameter inlet ) 2 ×Wind Speed, where flowrate and wind speed are referenced to standard temperature (20° C.) and standard pressure (1 atmosphere). With an absolute reference sampler having an inlet with a diameter of 0.50 inches, the mass flow controller restricts the flow rate as seen below in Table 1: 
     
       
         
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 wind 
                 0 mph 
                 5 mph 
                 7.35 mph 
                 10 mph 
                 14.7 mph 
                 22.05 
                 29.4 
               
               
                 speed 
                   
                   
                   
                   
                   
                 mph 
                 mph 
               
               
                 flow 
                 0 lpm 
                 17 lpm 
                 25 lpm 
                 34 lpm 
                 50 lpm 
                 75 
                 100 
               
               
                 rate 
                   
                   
                   
                   
                   
                 lpm 
                 lpm 
               
               
                   
               
             
          
         
       
     
     As seen in Table 1, a wind speed of 7.35 miles per hour (mph) requires a flow rate of 25 liters per minute (lpm), a wind speed of 14.7 mph requires a flow rate of 50 lpm, a wind speed of 22.05 mph requires a flow rate of 75 lpm, and a wind speed of 29.4 mph requires a flow rate of 100 lpm. In the configuration within this Example 2, the maximum wind speed that can be accurately sampled without a shroud is 14.7 mph, as the mass flow controller can only handle a maximum of 50 liters per minute flow rate. A flow controller with a higher upper limit allows for accurate sampling at higher wind speeds. 
     The process of the present invention collects an unbiased aerosol sample from a given environment. As such, the present invention minimizes, or eliminates, the size distribution biases commonly introduced by currently available samplers. 
     The foregoing summary, description, examples and drawings of the invention are not intended to be limiting, but are only exemplary of the inventive features which are defined in the claims.