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Title:
PULSED CONDENSATION PARTICLE COUNTER
Document Type and Number:
WIPO Patent Application WO/2021/002908
Kind Code:
A2
Abstract:
A method and apparatus to create water vapor supersaturation and particulate counts from an air sample. The method and apparatus include introducing an air sample into a chamber by passing a flow into the chamber through the inlet by pumping at the outlet. The method further includes closing the inlet while continuing the pumping to exhaust the air sample from the chamber through the outlet. The pumping is performed at a rate operable to reduce pressure inside the chamber such that the air sample in the central portion of the chamber cools, and water vapor from walls of the chamber has time to diffuse into the air sample in the chamber from the walls. The cycles are repeated by continuously repeating the introducing and closing. The walls of the chamber may be wetted or dry.

Inventors:
HERING SUSANNE VERA (US)
LEWIS GREGORY STEPHEN (US)
SPIELMAN STEVEN RUSSEL (US)
Application Number:
PCT/US2020/026641
Publication Date:
January 07, 2021
Filing Date:
April 03, 2020
Export Citation:
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Assignee:
AEROSOL DYNAMICS INC (US)
Attorney, Agent or Firm:
VIERRA, Larry E. (US)
Download PDF:
Claims:
CLAIMS

We claim:

1. A method for measuring a concentration of particles in air, comprising

introducing an air sample into a particle chamber through an inlet;

closing the inlet to isolate the particle chamber;

exhausting the air sample in the isolated particle chamber through a nozzle connected to an optical detector, said exhausting of the flow performed at a flow rate chosen to cause an expansion of any of the air sample inside the particle chamber, reduce a temperature of the any of the air sample inside the particle chamber, increase a relative humidity of any of the air sample inside the particle chamber, and cause fluid to condense on particles suspended in the any of the air sample inside the particle chamber, thereby forming droplets;

counting the droplets in the air sample as it is exhausted through the optical detector;

measuring a pressure from which an amount of air exiting the chamber can be assessed; and

determining a particle concentration as a ratio of a number of droplets detected to the amount of air that has exited the chamber based on the counting and the measuring.

2 The method of claim 1 wherein walls of the particle chamber are wetted with water.

3. The method of claim 1 wherein further comprising configuring the optical detector is to count particles larger than about 400 nm during the introducing of the air sample.

4. The method of claim 1 wherein the counting and measuring steps are performed simultaneously. 5. The method of claim 1 wherein walls of the particle chamber are dry.

6. The method of claim 1 further including humidifying the air sample prior to introducing the air sample into the particle chamber.

7. The method of claim 1 in which walls of the particle chamber are formed from a membrane comprising sulfonated tetrafluoroethylene based fluoropolymer- copolymer on a first side, and having a second side in contact with water or high humidity air, and wherein the method further includes pausing between isolating of the particle chamber and the exhausting of the flow, such that the air within the particle chamber becomes humidified prior to expansion.

8. The method of claim 1 wherein the counting, measuring, and determining are performed during the exhausting.

9. The method of claim 8 comprising continuously repeating the introducing and exhausting.

10. A method to create water vapor supersaturation within a wet-walled chamber having an inlet and an outlet; comprising:

introducing an air sample into the chamber by passing a flow into the wet walled chamber through the inlet by pumping at the outlet;

closing the inlet while continuing the pumping to exhaust the air sample from the chamber through the outlet, the pumping performed at a rate operable to reduce pressure inside the chamber such that the air sample in a central portion of the chamber cools, and water vapor from walls of the chamber has time to diffuse into the air sample in the chamber from the walls.

11. The method of claim 10 comprising continuously repeating the introducing and closing.

12. The method of claim 11 further including pausing between introducing and the closing, such that the air sample within the wet-walled chamber becomes humidified prior to expansion.

13. The method of claim 12 further including:

counting droplets in the air sample as it is exhausted through an optical detector;

measuring a pressure from which an amount of air exiting the chamber can be assessed; and

determining a particle concentration as a ratio of a number of droplets detected to the amount of air that has exited the chamber based on the counting and the measuring.

14. A particle counting apparatus, comprising:

an inlet;

a first valve coupled to the inlet;

a particle chamber coupled to the first valve and having an output;

an optical detector at the output of the particle chamber and having a detector outlet;

a pump coupled the detector output and having a pump outlet;

a second valve coupled between the detector output and the pump; and a controller executing code instructing the controller to:

open the first and second valves, and cause the pump to introduce air into the particle chamber and pump a flow of air through the chamber from the inlet, through to the detector outlet; and

close the first valve and cause the pump at the outlet to pull air out of the chamber thereby reducing pressure inside the chamber at a flow rate selected to cause the air in a central portion of the chamber to cool and allow water vapor from walls of the chamber to diffuses into the air in the chamber from wet walls.

15. The apparatus of claim 14 wherein the controller executes code instructing the controller to close the second valve for less than two seconds while the first valve is closed and prior to causing the pump to reduce pressure inside the chamber.

16. The apparatus of claim 14 wherein the pump is coupled to the detector output by two flow paths between the detector output, one flow path including the second valve and another flow path including a third valve, such that a flow rate at which air is introduced into the particle chamber, and a rate at which air is exhausted from the chamber once the first valve is closed, may be independently controlled.

17. The apparatus of claim 14 wherein the optical detector is configured to count particles larger than about 400 nm when the first and second valves are open, and the flow is drawn through the particle chamber.

18. The apparatus of claim 14 wherein the controller executes code instructing the controller to:

count droplets in the air sample as it is exhausted through a optical detector; measure a pressure from which an amount of air exiting the chamber can be assessed; and

determine a particle concentration as a ratio of a number of droplets detected to the amount of air that has exited the chamber based on the counting and the measuring.

19. The apparatus of claim 14 wherein the controller executes code instructing the controller to repeatedly: open the first and second valves, and cause the pump to introduce air into the particle chamber; and close the first valve and cause the pump at the outlet to pull air out of the chamber.

Description:
PULSED CONDENSATION PARTICLE COUNTER

Inventors

Susanne Vera Hering

Gregory Stephen Lewis

Steven Russel Spielman

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Patent Application Number 62/830,346 filed on April 5, 2019.

GOVERNMENT RIGHTS

[0002] This technology was made with support from the US Department of Energy, under STTR grant number DE-SC0020496 and the National Institutes of Health grant OH10515. The government has certain rights in this technology.

FIELD

[0003] The technology pertains to the measurement of particles suspended in air or other gas.

BACKGROUND

[0004] Airborne particles in the ultrafine size range, loosely defined as those with diameters from 5 nm to 100 nm, are ubiquitous in urban air. Inhalation of these particles is considered a health risk and is implicated in shortening human lifetimes. Additionally, ultrafine particles are the dominant contributor to the total number of particles found in the atmosphere. Some ultrafine particles grow very rapidly, and can act as seeds for cloud formation, and thus play a role in global climate.

[0005] Typically, the number concentration of airborne particle number concentration measurements that encompass the ultrafine size range are done by condensation particle counting, wherein individual particles are enlarged to optically detectable sizes through vapor condensation. This approach enables the detection of particles as small as a few nanometers in diameter, which otherwise are too small to be seen by optical scattering.

[0006] Over the decades many types of condensation particle counters have been developed. Some detect particles as small 2 to 3 nm. Others are compact, but have a limited period of operation, 4-8 hours, as a result of the depletion of the condensing fluid. None are both portable and capable of extended operation.

SUMMARY

[0007] Technology for aerosol analysis is provided. One general aspect includes a method for measuring a concentration of particles in air. The method includes introducing an air sample into a particle chamber through an inlet and closing the inlet to isolate the particle chamber. The method also includes exhausting the air sample in the isolated particle chamber through a nozzle connected to an optical detector, said exhausting of the flow performed at a flow rate chosen to cause an expansion of any of the air sample inside the particle chamber, reduce a temperature of the any of the air sample inside the particle chamber, increase a relative humidity of any of the air sample inside the particle chamber, and cause fluid to condense on particles suspended in the any of the air sample inside the particle chamber, thereby forming droplets. The method also includes counting the individual droplets in the air sample as it is exhausted through an optical detector and measuring a pressure from which an amount of air exiting the chamber can be assessed. The method also includes determining a particle concentration as a ratio of a number of droplets detected to the amount of air that has exited the chamber based on the counting and the measuring. The method also includes the method where walls of the particle chamber are wet. Other embodiments of this aspect include corresponding computer systems, apparatus, and computer programs recorded on one or more computer storage devices, each configured to perform the actions of the method.

[0008] In a further aspect, the method may include performing the counting and measuring steps simultaneously. The method may further include performing the method where walls of the particle chamber are dry. The method may further include humidifying the air sample prior to introducing the air sample into the particle chamber. The method may further include a method in which walls of the particle chamber are formed from a membrane including sulfonated tetrafluoroethylene based fluoropolymer-copolymer on a first side, and having a second side in contact with water or high humidity air, and where the method further includes pausing between isolating of the particle chamber and the exhausting of the flow, such that the air within the particle chamber becomes humidified prior to expansion. The method may further include the counting, measuring, and determining performed during the exhausting. The method may further include continuously repeating the introducing and exhausting.

[0009] Another general aspect includes a method to create water vapor supersaturation within a wet-walled chamber having an inlet and an outlet; including: introducing an air sample into the chamber by passing a flow into the wet walled chamber through the inlet by pumping at the outlet; and closing the inlet while continuing the pumping to exhaust the air sample from the chamber through the outlet, the pumping performed at a rate operable to reduce pressure inside the chamber such that the air sample in a central portion of the chamber cools, and water vapor from walls of the chamber has time to diffuse into the air sample in the chamber from the walls. Other embodiments of this aspect include corresponding computer systems, apparatus, and computer programs recorded on one or more computer storage devices, each configured to perform the actions of the methods.

[0010] Implementations of this aspect may include one or more of: the method including continuously repeating the introducing and closing; and the method further including pausing between introducing and the closing, such that the air sample within the wet-walled chamber becomes humidified prior to expansion. The method may further include: counting droplets in the air sample as it is exhausted through an optical detector, measuring a pressure from which an amount of air exiting the chamber can be assessed, and determining a particle concentration as a ratio of a number of droplets detected to the amount of air that has exited the chamber based on the counting and the measuring.

[0011] Another general aspect includes a particle counting apparatus, including: an inlet; a first valve coupled to the inlet; a particle chamber coupled to the first valve and having an output; an optical detector at the output of the particle chamber and having a detector outlet; a pump coupled the detector output and having a pump outlet; a second valve coupled between the detector output and the pump; and a controller executing code instructing the controller to: open the first and second valves, and cause the pump to introduce air into the particle chamber and pump a flow of air through the chamber from the inlet, through to the detector outlet; and close the first valve and cause the pump at the outlet to pull air out of the chamber thereby reducing pressure inside the chamber at a flow rate selected to cause the air in a central portion of the chamber to cool and allow water vapor from walls of the chamber to diffuses into the air in the chamber from wet walls..

[0012] Implementations may include the apparatus where the controller executes code instructing the controller to close the second valve for less than two seconds while the first valve is closed and prior to causing the pump to reduce pressure inside the chamber. Implementations may include the apparatus where the pump is coupled to the detector output by two flow paths between the detector output, one flow path including the second valve and another flow path including a third valve, such that a flow rate at which air is introduced into the particle chamber, and a rate at which air is exhausted from the chamber once the first valve is closed, may be independently controlled. Implementations may include the apparatus where the optical detector is configured to count particles larger than about 400 nm when the first and second valves are open, and the flow is drawn through the particle chamber. Implementations may include the apparatus where the controller executes code instructing the controller to: count droplets in the air sample as it is exhausted through an optical detector, measure a pressure from which an amount of air exiting the chamber can be assessed, and determine a particle concentration as a ratio of a number of droplets detected to the amount of air that has exited the chamber based on the counting and the measuring. Implementations may include the apparatus where the controller executes code instructing the controller to repeatedly: open the first and second valves, and cause the pump to introduce air into the particle chamber; and close the first valve and cause the pump at the outlet to pull air out of the chamber.

[0013] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 A is a first embodiment of a Pulsed Condensation Particle Counter (Pulsed CPC).

[0015] Figure 1 B shows a second implementation of the Pulsed CPC with a separate expansion transport path.

[0016] Figure 2 illustrates the operational states of the Pulsed CPC.

[0017] Figure 3 is a display of the modeled saturation ratio within a particle of a Pulsed CPC chamber and its downstream focusing nozzle during the expansion from 1000 to 750mbar over 0.5 s.

[0018] Figure 4 is a graph of pressure, temperature and saturation ratio, vs. time and shows results of model calculations of temperatures and saturation ratio at the centerline of a nozzle entrance for varying particle chamber diameters for a fixed expansion rate (mean of ~500mbar/s) when the walls of the particle cell are wet, and the relative humidity input into the cell is 50%.

[0019] Figure 5 is a graph of pressure, temperature and saturation ratio, vs. time and shows results of model calculations of temperatures and saturation ratio at the centerline of a nozzle entrance at varying expansion rates for fixed particle chamber geometry of an 8.5 mm diameter cylinder with wet walls.

[0020] Figure 6 is a graph of pressure, temperature and saturation ratio, vs. time and shows results of model calculations of temperatures and saturation ratio at the centerline of the nozzle entrance for varying particle chamber geometries for a fixed expansion rate (mean of ~500mbar/s) when the walls of the particle cell are dry, and input air is at 90%RFI before the expansion. [0021] Figure 7 is a graph showing raw particle count and pressure data from a Pulsed CPC, where the overlay indicates the alternating purge and expansion steps.

[0022] Figure 8 is a graph of pressure, temperature and saturation ratio, vs. time and shows the reduction of the data from the first of the pulses in Figure 7, wherein the volume sampled is calculated from the measured pressure, and the particle number concentration at each that point in the expansion state is derived from the total number of droplets detected , divided by the volume sampled.

[0023] Figure 9 is a graph of the sampled volume, sum of counts and particle concentration vs. time and shows the pressure in the particle chamber and detected particle concentration during a single expansion of a wet-walled, 6mm diameter particle chamber.

[0024] Figure 10 is a graph of pressure, and particle concentration vs. time and shows the pressure in the particle chamber and detected particle concentration during a single expansion of a dry-walled, 9mm diameter particle chamber.

[0025] Figure 1 1 is a graph of particle concentration vs. time and shows particle concentration as measured by the Pulsed CPC as compared to a benchtop, commercial condensation particle counter, for sampling ambient air.

[0026] Figure 12 is a graph showing measured detection efficiency of a Pulsed CPC as a function of particle size at two different expansion rates.

[0027] Figure 13 is a graph comparing the response of a Pulsed CPC to a benchtop condensation particle counter for 20 nm sulfate aerosols over a range of particle concentrations.

[0028] Figure 14 is a graph comparing the response of the Pulsed CPC during the purge state to a commercially available benchtop optical particle spectrometer, which has a threshold at particle diameters of 300nm and 400 nm. DETAILED DESCRIPTION

[0030] The technology pertains to the measurement of particles suspended in air or other gas. More specifically, the technology pertains to devices and methods in which the size of particles is enlarged through condensation of water vapor onto the particle. These particle condensation techniques are most commonly applied to the detection, collection or inertial manipulation of airborne particles that are smaller than a few micrometers, or a few hundred nanometers in diameter. The Pulsed Condensation Particle Counter (Pulsed CPC) of this technology is a new approach for enlarging ultrafine particles through water vapor condensation. The Pulsed CPC utilizes the cooling created by adiabatic expansion, optionally in combination with the diffusive transport of water vapor from wet walls, to create the supersaturation for particle growth. The resulting condensationally enlarged particles are individually counted as the flow exits through an optical chamber. The single particle counting of this technology removes uncertainty associated with the ensemble scattering approach of prior adiabatic expansion instruments.

[0031] Generally, condensation particle counters work by condensing a material, such as water or alcohol, onto particles that are suspended in the air, and subsequently detecting the droplets formed by optical means. This technology uses this same basic principle of condensational enlargement and optical detection.

[0032] For small particles, the underlying physics of condensational growth requires supersaturation, defined as a region in which the vapor pressure of the condensing vapor is higher than its saturation value. Simple saturation is not sufficient to initiate condensational growth because the equilibrium vapor pressure above the curved surface is higher than over a flat surface of the same chemical composition. This is due to the surface energy, a phenomenon described by the Kelvin relation. The level of required supersaturation increases as the inverse of the particle diameter, so that higher supersaturation values are required for smaller particles. The relative humidity needed to activate the condensational growth of a 6 nm particle is in the range of 140%, depending on particle chemical composition. [0033] Super-saturated conditions are inherently a non-equilibrium state. The flow at walls of the bounding container cannot be supersaturated as any excess water vapor will simply deposit. However, it is possible to create supersaturated conditions within the core of the flow, or in the core of a confined volume. Methods of achieving this include: (1 ) the rapid (generally turbulent) mixing of saturated flows of differing temperatures; (2) laminar-flow diffusion; and (3) adiabatic expansion of a nearly saturated flow.

[0034] Most condensation particle counters in use today employ either the first two of these methods, namely turbulent mixing or laminar flow diffusion. These are continuous flow devices and offer the advantage of‘single particle counting’, wherein the droplets formed from condensational growth are passed through a light source and detected and counted individually. These approaches offer high sensitivity and precision. Some devices are capable of detection of particles as small as 2nm to 3nm. Yet they also require maintaining regions of the system at either high, or low temperatures, which consumes energy. Generally, they also require liquid reservoirs, which makes them intolerant to motion and tipping.

[0035] Adiabatic expansion, the third of these methods, is the oldest, dating from the nineteenth-century None of the previous or existing adiabatic expansion instruments are continuous flow devices, and none are capable of single particle counting; that is, none directly detect and enumerate the individual droplets formed. Indeed, they are inherently non-steady flow instruments, requiring introduction of a sample into a volume that is subsequently sealed and expanded. The measured quantity is the light scattered from the ensemble, or cloud, of droplets formed inside the expansion chamber, from which particle number concentration is inferred. Such devices do not count individual droplets passing through a light beam, as in the continuous flow instruments described above. As such, they are sensitive to the extent of droplet growth, as well as the particle number concentration and therefore lack the precision of the single-counting instruments.

[0036] The present technology comprises a pulsed condensation particle counter 100 (Pulsed CPC) which uses adiabatic expansion to create cooling, and yet is configured to individually count the condensationally enlarged particles. It can be operated either with dry walls, or with wet walls. The latter enhances the super- saturation through the combined effect of cooling from adiabatic expansion and diffusive transport of water vapor from wetted walls. In either configuration the Pulsed CPC approach provides the single particle counting advantages of the continuous flow instruments, while maintaining the lower energy requirements of the adiabatic approach.

[0037] A first embodiment of a Pulsed CPC is illustrated in Figure 1A. Air to be sampled passes through an inlet 10 and valve 1 1 into the particle chamber 12, and exits through an optical detector assembly 20, exit tube 18, a flow-limiting device 16 and pump 17. Flow enters the optical detector 20 through a focusing nozzle 13, wherein the flow is directed across a focused light beam 22, configured such that much of the scattered light is directed onto a photodetector 23. A pressure sensor 15 monitors the pressure upstream of a flow-limiting device 16. A controller 50 may be coupled to the respective valve 1 1 , optical detector assembly 20, flow limiting device 16, pressure sensor 15 and pump 17 to enable the respective states of operation by operating these respective elements.

[0038] The Pulsed CPC 100 has two states of operation -- purge and expansion -- and continuously cycles between these two states. During the purge state, the inlet valve 1 1 is open, and the sample (air) flows through the particle chamber 12 and optical detector 20. The sample flow is exhausted through the pump 17. Pump 17 operates to pull the sample from the inlet 10 through device 100. In one embodiment, the inlet valve 1 1 controls access to the particle chamber through the inlet pathway for the sample air flow. Valve 1 1 is shown as a rotating valve in Figure 1 and is shown open to allow passage of the sample through the device 100. As the valve rotates, it closes to prevent sample flow from inlet 10 into chamber 12. While shown as a rotating valve, valve 1 1 may also be a simple on/off solenoid valve, or an automated ball valve, or any other type of valve allowing or inhibiting flow through device 100. The flow-limiting device 16 controls the flow rate of the air sample in the device, and may be a simple orifice, or an orifice in combination with a vacuum regulator, or an active flow controller. In the purge state, the particle chamber 12 is filled with ambient air. Simultaneously, the device 100 measures direct light scattering from individual particles that have not been condensationally enlarged. Some will be sufficiently large to scatter a detectable amount of light and be counted. Depending on the optical configuration, the lower limit of detected particle size is around 0.3-0.5 pm.

[0039] The expansion state immediately follows the purge state. During the expansion state the inlet valve 1 1 is closed and the particle chamber 12 is partially evacuated through the optical detector 20 and exit tube 18 by pump 17. As air is removed from the particle chamber and optical detector, the pressure drops. The partial evacuation causes the air remaining within the chamber to expand. If the sampled air has been humidified, or if the walls of the particle chamber are wet, and if the expansion rate is appropriate, air within the particle chamber cools, initiating the formation of droplets around individual particles suspended in the air. During the expansion process, the droplets that have formed are carried through the nozzle 13, and through a focused light beam 22, and the individual droplets are detected and counted by means of the scattered light captured by a photo sensor 23. During the evacuation process, the pressure in chamber 12 and detector 20 is monitored by the pressure sensor 15, which is generally placed in exit tube 18 between the optical detector 20 and the flow-limiting device 16. Because the upstream pressure drop is small, the pressure sensor 15 reading is close to that of the particle chamber 12. The volume of air passing through the optical detector 20 at any moment is calculated from the pressure reading and the volume of the particle chamber 12. Typically, the expansion process occurs over a period of a few seconds, or less, and data are acquired at a rate of 16-64Hz.

[0040] Figure 1 B shows a second implementation of the Pulsed CPC with a separate expansion flow transport path 33 and purge flow pathway 34. The expansion rate can be controlled independently of the flow during the purge state by means of a secondary expansion pathway 33, which includes an expansion switching valve 31 and expansion flow limiting device 32. The flow-limiting device 32 controls the evacuation rate. This may be a simple orifice, or an orifice in combination with a vacuum regulator, or an active flow controller. Alternatively, a vacuum regulator may be placed immediately upstream of the pump 17, on exit tube 36, with appropriately sized orifices serving as the flow-limiting devices 16 and 32. The expansion pathway 33 may also include a ballast volume to aid in setting the desired evacuation rate. The purge flow pathway 34 may contain an independent shutoff valve 35. Generally, the necessary expansion flow is higher than the purge flow, and it is not necessary to close the purge flow pathway 34, and valve 35 may be eliminated. Yet when operating with a wet-walled particle chamber, it may be desirable to allow the air within the chamber to humidify prior to expansion. This can be accomplished by isolating the chamber by closing the purge valve 35 prior to opening the transport valve 31 , such that there is no flow through the chamber prior to the expansion. Alternatively, this may be accomplished by placing valve 35 on the exit tube 36. A controller 55 may be coupled to the respective valves 1 1 , 31 and 35, optical detector 20, flow limiting devices 16 and 32, pressure sensor 15 and pump 17 to enable the respective states of operation by operating these respective elements.

[0041] Each controller 50/55 may comprise a general-purpose processor, special purpose processor or programmable circuit executing code adapted to cause the controller to perform the methods herein by controlling the various elements of each Pulsed CPC described herein. In one embodiment, the controller is a microprocessor with a custom firmware program. Data output from the Pulsed CPC may, or may not, be stored internally to the controller. The controller may output a data output stream for transmitting data to external storage for later analysis. In another embodiment, the controller may comprise an electrical circuit or circuitry coupled to the various elements of the Pulsed CPC which is configured to operate the Pulsed CPC to perform any of the methods described herein. It should be understood that elements of the Pulsed CPC may include additional circuitry such as circuitry to control the a focused light beam 22 (which may comprise a laser), circuitry to operate the photodetector 23 and process its output prior to input to the controller.

[0042] Figure 2 illustrates the operational states of the Pulsed CPC. Initially, in the Pulsed CPC is configured in the purge state, with the inlet valve 1 1 open, and the expansion valve 31 , if present, is closed. Air flows through the particle chamber and exits through the optical detector. For configuration 100, the flow then exits through exit Iine18. For configuration 1 10, which has two downstream flow paths, the flow exits through purge pathway 34 and exit line 36. There is no expansion, and no condensational growth during the purge state. Instead, airborne particles are carried by the flow, through the optical detector 20. Each particle will scatter light as it passes through the optical detector, but only some will be of sufficient diameter to scatter enough light to be detected by the optical sensor. Generally, the threshold for detection is larger than 300 nm, and can be as high as 2000 nm. Typically, most particles suspended in the ambient air are smaller than this threshold and are too small to be detected. In normal operation the duration of the purge state is less than 60 seconds.

[0043] Subsequently, the Pulsed CPC transitions to the expansion state. This transition may be direct or may include an isolation state for a time of less than 2 seconds duration. In the isolation state, the valves are closed to stop flow through the particle chamber. In configuration 1 10 the valves are configured to close flow through the purge line 33 and flow through expansion line 34, such as by closing valves 31 and 35. In configuration 100 there is no provision for an isolation state, but this could be accommodated by adding a valve on exit tube 18. The inlet valve 1 1 may be either open or closed. There is no flow through the particle chamber 12, and there is no evacuation of the particle chamber 12. If the walls of the particle chamber 12 are wet or formed from a material such as sulfonated tetrafluoroethylene-based fluoropolymer- copoiymer, this isolation state allows the air within the particle chamber to be humidified. Alternatively, the isolation state may be bypassed, and the Pulsed CPC transitions directly to the expansion state.

[0044] In the expansion state the inlet valve 1 1 is closed, and flow through chamber 12, detector 20 and other elements to the pump 17 is open such that the air within the chamber 12 is partially evacuated through the optical detector 20. In configuration 100, this is accomplished by closing the inlet valve 1 1 , such that the flow through exit port 18 becomes the expansion flow path. In configuration 1 10, the inlet valve 1 1 is closed and the expansion valve 31 is open; valve 35 may be either open or closed. For either configuration 100 or 1 10, in the expansion state air in the particle chamber is exhausted through the optical detector 20, the pressure within the particle chamber 12 decreases, and correspondingly the remaining air in the chamber expands. With the appropriate selection of the evacuation rate, this expansion approximates an adiabatic process, and the temperature of much of the air within the chamber decreases. The net effect of the expansion is to create super-saturated conditions within the particle chamber, such that particles present grow by water condensation, forming droplets. Typically, particles as small as 5-10 nm in diameter will be activated to grow, and the droplets formed are generally more than one micrometer in diameter. As the air from the chamber continues to discharge through the optical detector 20, each droplet scatters light, producing a pulse of light that is detected and counted. During the expansion state the pressure inside the particle chamber 12 drops from an initial value close to the ambient air pressure, typically 1 atmosphere, to a final pressure in the range of 0.2 to 0.8 atmospheres. Once reaching the final pressure the expansion valve 35, if present, is closed, and the inlet valve 1 1 is opened, returning the instrument to the purge state. The adiabatic expansion of a volume of air that is nearly saturated (>90%RH), leads to super-saturated conditions. This is due to the decrease in temperature inside the volume as the flow expands, combined with the non-linear character of the saturation vapor pressure. During the adiabatic expansion both the temperature and the vapor pressure drop. The saturation vapor pressure, which is only a function of the temperature, also drops, and because the saturation vapor curve is non-linear, the saturation vapor pressure drops more quickly than does the vapor pressure, resulting in supersaturated conditions. This supersaturation persist until the flow warms due to non-adiabatic effects, i.e. heat transfer from the walls of the container. Very high supersaturations may be achieved with this approach, making the activation of growth of particles as small as a few nanometers feasible.

[0045] The Pulsed CPC of this technology can be operated with either dry, or wet walls. For operation with dry walls, higher supersaturations are achieved when humidifying the air prior to expansion. This is achieved by humidifying the air stream as it is introduced. Alternatively, it is achieved by forming the walls of the particle chamber 12 with a suifonated tetrafluoroethylene based fluoropolymer- copoiymer membrane, the opposite side of which is in contact with water or high humidity air, and waiting a brief moment between closing the inlet valve and opening the expansion valve, such that the flow becomes humidified prior to expansion. When operating with wet walls, as may be obtained by lining the walls with a wetted wick, the condensational growth is enhanced. With the wet walls, both heat and water vapor diffuse into the flow when the flow is expanded. The water vapor diffusion is driven by the rapid drop in pressure while the thermal diffusion, which is slower, is driven by the adiabatic cooling. Under this circumstance, the supersaturation is created through a combination of the cooling from adiabatic expansion and the transport of water vapor from the walls. [0046] The evolution of the saturation profiles within the particle chamber 12 during the expansion process, as calculated using Comsol Multiphysics®, a commercial finite element heat and mass transfer model available from Comsol, Inc. Using this tool, one solves the time-dependent differential equations for water vapor concentration c and temperature T

[0049] where D the mass diffusivity of water vapor, u is the flow velocity obtained from solving the Navier Stokes equations k, p, C P and a P are the thermal conductivity, density, constant pressure heat capacity and compressibility of air.

[0050] This model can be applied to several geometries for the particle chamber, and particle chambers of varying dimensions, with wet walls, and with dry walls. Figure 3 shows the results for a 12-mm diameter wet-walled particle chamber of Figure 1 B. The sampled air is at 50% RFI and 25°C, and the nozzle 13 has dry walls. The inlet valve is at the bottom (not shown), and during expansion the flow is upward. These 7 images show the calculated saturation ratio as it evolves during the expansion. Each image is a result is for one-half of the tube, with the centerline on the left edge of the image (because the system is cylindrically symmetric, only one half is modeled). Grayscale indicates the saturation ratio, with dark for S=0.5, and white for S>1 .8. At time t=0s the inlet valve 1 1 and transport flow valve 31 and purge valve 35 are shut. The first image 302 shows starting conditions of 50% relative humidity throughout, while the second image 304 shows the increase to near 100% as water vapor diffuses into the cell from the wet walls. At t=0.5s, the transport flow valve 31 is opened to evacuate the particle chamber. The pressure in the particle chamber decreases exponentially with time, as would happen for constant volumetric flow, with a pressure drop of 250 mbar within the cell during the subsequent 0.5s (0.5s< t < 1 s). During the expansion, the temperature within the core of the cell drops due to mechanical work, creating supersaturated conditions throughout the length of the tube. The increase in the saturation ratio throughout the expansion is due both to the drop in temperature and water vapor transport from the walls. The simultaneous heat and water vapor which are transported from the walls enhances the supersaturation because the mass diffusivity of water is higher than the thermal diffusivity of air. In this example, the saturation ratio reaches 1.95 in the core of the flow (T=10.6°C), which corresponds to a Kelvin diameter of 3.4 nm.

[0051] To quickly map the dependence on geometry and expansion rate, one may extract from the two-dimensional model the temperature and saturation ratio at one point, at the centerline of the nozzle entrance. The time evolution of the temperature and saturation at this one point allows comparison of different scenarios, with differing cell geometries, wall conditions and expansion rates. Figure 4 shows this comparison for wet-walled particle chambers of varying geometric proportion, operated at equal expansion rates. Shown are results for cylindrically shaped particle chambers of diameters of 6.0mm, 8.5mm and 12mm. The input air sample is initially at 50% RH. At time t=0s the flow is stopped by closing the flow to the pump 17 by closing valves 31 and 35 and closing the inlet valve 1 1 , and the humidity of the air within the chamber increases due to diffusion from the wet walls. At time t=0.5s, the inlet valve 1 1 is closed, and the pressure and temperature inside the particle chamber 12 drops. The modeling shows that lower temperatures, and higher peak supersaturations, are achieved with wider diameter tubes, as the heat transfer from the walls becomes less important. Figure 5 examines the effect of the expansion rates for a wet-walled, 8.5mm diameter particle chamber. In this embodiment, lower temperatures, and hence higher supersaturation are achieved at the higher expansion rates. The expansion rate is indicated by the initial rate of pressure decrease, in mbar/s.

[0052] Figure 6 shows the calculated super-saturation achieved through expansion when the particle chamber walls are dry. Again, modeling is for a cylindrically-shaped particle chamber with diameters of 6.0mm, 8.5mm and 12mm. The expansion starts at time t=0.5s, and the input air sample is at 90%RFI prior to the expansion. The temperature profile is essentially the same as obtained at the same expansion rate (500mbar/s), but the supersaturation achieved is lower. This indicates an enhancement in the saturation achieved by wetting the walls of the particle chamber. The enhancement over that achieved from expansion alone is due to the water transport from the walls.

[0053] The operational principal of the Pulsed CPC has been verified experimentally. Data were obtained with a particle chamber coupled to a commercial optical detector, and with solenoid valves on the inlet and on the expansion pathway. Figure 7 shows raw data obtained with a cylindrically shaped, 6-mm diameter particle chamber with a volume of 3cm 3 and wet walls. This figure illustrates the cycling between purge and expansion modes. Displayed are measurements of the particle chamber pressure and the number of droplets detected in each measurement interval. Data were recorded at 64Flz, i.e. each measurement interval is 1/64 s. The sample was drawn from the ambient air in the room.

[0054] In the purge mode, the inlet valve 1 1 is open, and the expansion pathway valve 31 is closed. Valve 35 was not present. The pressure in the particle chamber 12 is constant, and essentially no particles are detected. This is because there are too few particles of sufficient size to be seen by the optical system employed. In the expansion mode the inlet valve 1 1 is closed, and the expansion pathway valve 31 is opened, and the pressure in the particle chamber 12 drops. With each expansion state cycle, a burst of particles, or droplets, are seen by the optical detector. As the flow expands, and cools, the droplets passing through the nozzle 13 into the optical detector 20 are counted. To verify that this burst of particles was not generated from the closure of the inlet valve, the measurement was repeated with filtered air, and no particles are detected.

[0055] Figure 8 illustrates how these data are processed to retrieve the ambient particle number concentration. Data are taken from the first expansion cycle of Figure 7. The sampled volume-- the volume of air that has passed through the focusing nozzle 13 into the optical detector 20 since the beginning of the expansion--is calculated from the pressure reading and the volume of the particle chamber 12. Specifically, the sampled air volume is estimated as (1 -P/Po) * V 0 , where V 0 is the particle chamber volume, P is the pressure measured at the moment of the reading, Po the pressure before the expansion, and V 0 is calculated from the mechanical drawings of the system. This calculation ignores the temperature change (which is less than ~3% based on modeling). Then, the total number of particles detected during each expansion state is summed. The ratio of the number of detected particles to the sampled air volume is the indicated particle number concentration. During the expansion state the indicated particle concentration rises, and then plateaus. The plateau value is taken to be the measured particle concentration.

[0056] Particle detection is possible with either wet-walled or dry walled particle chambers. When using a wet-walled chamber, the humidification prior to expansion is provided by the walls themselves. When using a dry-walled chamber, humidification of the air sample prior to expansion is needed. Figure 9 shows the indicated particle concentration from a single pulse obtained with a narrow, cylindrically shaped, wet- walled particle chamber with a diameter of 6-mm, and a volume of 3cm 3 . Figure 10 shows analogous data obtained with a dry-walled, 9-mm diameter particle chamber. Both wet-walled and dry-walled approaches produce particle counts, although the dry- walled configuration required a wider tube, and a larger expansion rate.

[0057] Comparisons to a benchtop condensation particle counter are shown in Figure 1 1 for sampling ambient air. Data are from the 6-mm diameter, wet-walled system. The Pulsed CPC approach yields a particle number concentration within 10% of a benchtop condensation particle counter that has a nominal threshold of 5 nm.

[0058] Figures 12 presents calibration for a Pulsed CPC system showing particle- size dependent detection efficiency. Data were obtained using monodisperse ammonium sulfate aerosol generated by atomization, dried, charge equilibrated, and size classified using a differential mobility analyzer (TSI). The reference is a TSI Model 3789 versatile water condensation particle counter, configured for 2 nm particle detection. For our Pulsed CPC approach, concentrations are calculated from the maximum reading from each pulse, as described above. Detection efficiency as a function of particle size are shown for two different expansion rates, with the higher evacuation rate exhibiting a 50% detection at 10 nm in particle size. Figure 13 shows the response for varying concentrations of 20nm test particles, showing agreement within 15% of the reference instrument over a wide range in particle concentrations.

[0059] Figure 14 shows data from the Pulsed CPC during the purge cycle, with comparison to a commercial optical particle spectrometer (such as one manufactured by Climet Instruments, Redlands CA). During the purge cycle there is no condensational growth, and ambient particles pass through the optical detector without condensational enlargement. Those particles larger than about 300-400 nm scatter sufficient light to be counted without condensational enlargement, and these are counted directly. In this mode, the Pulsed CPC acts simply as many optical particle spectrometers, counting and sizing particles based on the directly scattered light. Typically, these counters detect only those particles larger than 300-400 nm. In the purge mode the Pulsed CPC provides data similar to other commercial optical particle spectrometers. This added measurement enables a dual-use instrument which can act as both a condensation particle counter for total particle number, and an optical sensor for counting and sizing ambient particles above a 300-400 nm threshold This latter measurement is what is used by current low-cost sensors to track PM2.5 mass. This offers the advantage of two types of measurements in one instrument.

[0060] Advantages of the Pulsed CPC over existing methods are portability, low power requirement and inherently compact size. The extent of expansion required is low - an adiabatic expansion of 10% of saturated air will activate the condensational growth of particles as small as 6 nm. At an initial humidity of 80%, a range that is easily achievable with Nafion-based humidifiers, a 15% expansion is sufficient for 6-nm particle activation. These values are readily obtained with a small diaphragm pump, such as a Sensidyne model 3A pump. In such pumps, the power requirement is small. For example, the Sensidyne model 3A pump, requires only 240mW to pull 0.8atm at a flow of 0.2L/min. This is an order of magnitude reduction on the 2.2 W power required for an energy-optimized, miniature version of a laminar-flow water-based CPC, where three Peltier heat pumps were used to maintain the required temperature differences along the growth tube. The housing of the Pulsed CPC is isothermal, and all at ambient temperature, eliminating the need for heaters, coolers and fans. The upstream valve 1 1 need not be air-tight but must block the flow sufficiently to allow the expansion, in other words the leak rate across the valve must be small compared to the evacuation rate. The Pulsed CPC is also insensitive to flow rate, as the measured air volume is derived from a pressure measurement and the particle chamber geometry.

[0061] The Pulsed CPC approach is technically feasible because of the relatively fast rate of droplet growth and as compared to evaporation. Once initiated, particle condensational growth is rapid, but droplet evaporation relatively slow. Activation of condensational growth requires supersaturated conditions, yet once growth is initiated the driving force is large. Growth to a few micrometers occurs within 20-100ms. Once the droplet reaches micrometer diameters, evaporation is relatively slow even as the saturation ratio drops. At 100% RH the lifetime of a 1 pm droplet is of the order of 1 second. This gives ample time to count the droplets once formed.

[0062] All prior adiabatic expansion instruments determine particle number concentrations from the scattering from the ensemble of droplets formed in the particle chamber. Instead, the Pulsed CPC counts individual droplets during the expansion process. As such, it is not subject to the uncertainties of ensemble counting, where the signal depends on the size of the droplets within the chamber as well as on the total number concentration. Another unique aspect is that when operated with wet walls in the particle chamber, the supersaturation obtained during the expansion is enhanced by the transport of water vapor from the walls during the expansion process itself. In this mode of operation, the Pulsed CPC does not rely strictly on adiabatic expansion but is enhanced by diffusion of water vapor from the walls. A third feature is its dual capability to measure larger (>300nm) airborne particles during the purge state, and then measure all particles, as small as 10nm in the expansion state. This dual capability provides an optical estimate of the PM2.5 parameter of interest due to government regulation, and in the same instrument, measure also the ultrafine particle concentration, of interest to public health.

[0063] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.