Pulse Condensation Particle Counter

Abstract

A method and apparatus for supersaturating an air sample with water vapor and counting particles. The method and apparatus include introducing an air sample into the chamber by pumping at the outlet to cause a flow of air to enter the chamber via the inlet. The method also includes closing the inlet while continuing to pump the air sample out of the chamber via the outlet. Pumping is performed at an operable rate to reduce the pressure in the chamber so that the air sample in the central portion of the chamber cools and water vapor from the chamber walls has time to diffuse from the walls into the air sample in the chamber. The cycle is repeated by continuously repeating the introduction and closing. The chamber walls may be wet or dry.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 62 / 830,346, filed on April 5, 2019. Technical Field

[0003] The technique involves the measurement of particles suspended in air or other gases. Background Art

[0004] Atmospheric particles in the ultrafine size range (roughly defined as particles with a diameter between 5nm and 100nm) are ubiquitous in urban air. Inhalation of these particles is considered a health risk and has been linked to a shortened human lifespan. In addition, ultrafine particles are a major contributor to the total number of particles found in the atmosphere. Some ultrafine particles grow very quickly and can act as seeds for cloud formation, thereby playing a role in global climate.

[0005] Typically, number concentration measurements of atmospheric particle number concentrations that encompass the ultrafine size range are accomplished by condensation particle counting, where individual particles are magnified to optically detectable sizes by vapor condensation. This method can detect particles as small as a few nanometers in diameter that would otherwise be too small to be seen by optical scattering.

[0006] Many types of condensation particle counters have been developed over the decades. Some can detect particles as small as 2 to 3 nm. Others are compact but have limited operating times of 4-8 hours due to depletion of the condensing fluid. None are both portable and capable of extended operation. Summary of the invention

[0007] Aerosol analysis techniques are provided. A general aspect includes a method for measuring the concentration of particles in air. The method includes introducing an air sample into a particle chamber via an inlet and closing the inlet to isolate the particle chamber. The method also includes discharging the air sample in the isolated particle chamber via a nozzle connected to an optical detector, the discharge of the air flow being performed at a selected flow rate to cause any air sample in the particle chamber to expand, reduce the temperature of any air sample in the particle chamber, increase the relative humidity of any air sample in the particle chamber, and condense the fluid on the particles suspended in any air sample in the particle chamber, thereby forming droplets. The method also includes counting individual droplets in the air sample when the air sample is discharged via the optical detector and measuring the pressure, from which the amount of air leaving the chamber can be evaluated. The method also includes determining the particle concentration as a ratio of the number of droplets detected to the amount of air that has left the chamber based on the counting and measurement. The method also includes a method in which the wall of the particle chamber is wet. Other embodiments of this aspect include corresponding computer systems, devices, and computer programs recorded on one or more computer storage devices, each configured to perform the actions of the method.

[0008] On the other hand, the method may include performing the counting and measuring steps simultaneously. The method may also include implementing a method in which the wall of the particle chamber is dry. The method may further include humidifying the air sample before introducing the air sample into the particle chamber. The method may also include a method in which the wall of the particle chamber is formed by a membrane, a first side of the membrane includes a fluorinated polymer-copolymer based on sulfonated tetrafluoroethylene, and the membrane has a second side in contact with water or high humidity air, and wherein the method also includes pausing between the isolation of the particle chamber and the discharge of the airflow so that the air in the particle chamber becomes humidified before expansion. The method may further include counting, measuring and determining performed during the discharge. The method may further include continuously repeating the introduction and discharge.

[0009] Another general aspect includes a method of producing water vapor supersaturation in a wet-wall chamber having an inlet and an outlet; comprising: introducing an air sample into the chamber by pumping at the outlet to cause a flow of air to enter the wet-wall chamber through the inlet; and closing the inlet while continuing to pump to exhaust the air sample from the chamber through the outlet, pumping at an operable rate to reduce the pressure in the chamber so that the air sample in the central portion of the chamber cools and water vapor from the chamber walls has time to diffuse from the walls into the air sample in the chamber. 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.

[0010] Implementations of this aspect may include one or more of the following: the method includes continuously repeating the introduction and closing; the method also includes pausing between the introduction and closing so that the air sample in the wet-wall chamber becomes humidified before expansion. The method may also include counting droplets in the air sample as it is exhausted via the optical detector, measuring the pressure from which the amount of air leaving the chamber can be estimated, and determining the particle concentration as a ratio of the number of droplets detected to the amount of air leaving the chamber based on the counting and the measurement.

[0011] Another general aspect includes a particle counting device comprising: an inlet; a first valve connected to the inlet; a particle chamber connected to the first valve and having an output end; an optical detector located at the output end of the particle chamber and having a detector outlet; a pump connected to the detector output end and having a pump outlet; a second valve connected between the detector output end and the pump; and a controller execution code that instructs the controller to: open the first valve and the second valve to cause the pump to introduce air into the particle chamber and pump the air flow from the inlet through the chamber to the detector outlet; close the first valve to cause the pump at the outlet to draw air out of the chamber, thereby reducing the pressure in the chamber at a selected flow rate to cool the air in the central portion of the chamber and allow water vapor from the chamber walls to diffuse from the wet walls into the air in the chamber.

[0012] Embodiments may include an apparatus wherein the controller executes code instructing the controller to close the second valve for less than two seconds when the first valve is closed and before causing the pump to reduce the pressure in the chamber. Embodiments may include an apparatus wherein the pump is coupled to the detector output via two flow paths between the detector output, one flow path including the second valve and the other flow path including the third valve, such that the rate at which air is introduced into the particle chamber and the rate at which air is exhausted from the chamber once the first valve is closed can be independently controlled. Embodiments may include an apparatus wherein the optical detector is configured to count particles greater than about 400 nm when the first valve and the second valve are open and airflow is drawn through the particle chamber. Embodiments may include an apparatus wherein the controller executes code instructing the controller to count droplets in the air sample as the air sample is exhausted via the optical detector, measure pressure from which the amount of air leaving the chamber can be estimated, and determine the particle concentration as a ratio of the number of droplets detected to the amount of air that has left the chamber based on the counting and the measurements. Implementations may include an apparatus wherein the controller executes code instructing the controller to repeatedly: open the first valve and the second valve to cause the pump to draw air into the particle chamber; and close the first valve to cause the pump at the outlet to draw air out of the chamber.

[0013] This summary is provided to introduce some concepts in a simplified form that will be further described in the detailed description below. 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] FIG. 1A is a first embodiment of a pulsed condensation particle counter (pulsed CPC).

[0015] FIG1B shows a second implementation of pulse CPC with independent extended transmission paths.

[0016] Figure 2 The operating states of the pulse CPC are illustrated.

[0017] Figure 3 Shown are the simulated saturation of particles within a pulsed CPC chamber and its downstream focusing nozzle during expansion from 1000 to 750 mbar in 0.5 s.

[0018] Figure 4 is a plot of the relationship between pressure, temperature and saturation and time, showing the model calculation results of the temperature and saturation at the centerline of the nozzle inlet for different particle chamber diameters at a fixed expansion rate (average of about 500 mbar / s) when the particle chamber wall is wet (relative humidity in the input chamber is 50%).

[0019] Figure 5 is a plot of pressure, temperature, and saturation versus time showing model calculation results for temperature and saturation at the nozzle inlet centerline for different expansion rates for a fixed particle chamber geometry of an 8.5 mm diameter cylinder with wetted walls.

[0020] Figure 6 is a graph of the relationship between pressure, temperature and saturation and time, showing the model calculation results of the temperature and saturation at the centerline of the nozzle inlet for different particle chamber geometries at a fixed expansion rate (average of about 500 mbar / s) when the particle chamber wall is dry and the relative humidity of the input air before expansion is 90%.

[0021] Figure 7 is a graph showing raw particle count and pressure data from pulsed CPC with an overlay showing alternating purge and expansion steps.

[0022] Figure 8 is a graph of pressure, temperature, and saturation versus time, and shows Figure 7 The data for the first pulse in the is reduced, where the sampled volume is calculated from the measured pressure and the particle number concentration at each point in the expanded state is given by the total number of droplets detected divided by the sampled volume.

[0023] Fig. 9 is a graph of sample volume, total counts, and particle concentration versus time, and shows the pressure in the particle chamber and the detected particle concentration during a single expansion of a wetted-wall, 6 mm diameter particle chamber.

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

[0025] Fig.11 is a graph of particle concentration versus time showing the particle concentration measured by Pulsed CPC compared to a benchtop commercial condensation particle counter used for ambient air sampling.

[0026] Fig.12 is a graph showing the detection efficiency of pulsed CPC measurements as a function of particle size at two different expansion rates.

[0027] Fig.13 is a graph comparing the response of a pulsed CPC to a benchtop condensation particle counter to 20 nm sulfate aerosol over a range of particle concentrations.

[0028] Fig.14 Figure 2 is a comparison of the response of pulsed CPC under purge conditions to a commercial benchtop optical particle spectrometer that has thresholds at particle sizes of 300 nm and 400 nm. DETAILED DESCRIPTION

[0029] The technology relates to the measurement of particles suspended in air or other gases. More specifically, the technology relates to devices and methods for increasing the size of particles by condensing water vapor onto the particles. These particle condensation techniques are most commonly used to detect, collect or inertially manipulate atmospheric particles with diameters less than a few microns or hundreds of nanometers. The technology's Pulsed Condensation Particle Counter (Pulsed CPC) is a new method for amplifying ultrafine particles by condensation of water vapor. Pulsed CPC uses cooling produced by adiabatic expansion, optionally combined with diffuse transport of water vapor through wet walls, to produce supersaturation for particle growth. As the gas stream exits through an optical chamber, the resulting condensation-enlarged particles are counted individually. The technology's single particle counting eliminates the uncertainties associated with the ensemble scattering method of previous adiabatic expansion instruments.

[0030] Typically, condensation particle counters work by condensing a substance such as water or alcohol onto particles suspended in the air and then detecting the resulting droplets optically. The technology uses the same basic principles of condensation growth and optical detection.

[0031] For small particles, the underlying physics of condensation growth requires supersaturation, which is defined as the region where the vapor pressure of the condensing vapor is above its saturation value. Simple saturation is not sufficient to initiate condensation growth, as the equilibrium vapor pressure above a curved surface is higher than a flat surface with the same chemical composition. This is due to surface energy, a phenomenon described by the Kelvin relation. The required supersaturation level increases with the inverse of particle size, so smaller particles require higher supersaturation values. The relative humidity required to activate condensation growth on 6nm particles is in the range of 140%, depending on particle chemistry.

[0032] Supersaturated conditions are inherently nonequilibrium. A flow at the walls of a bounding container cannot be supersaturated because any excess water vapor will simply settle out. However, it is possible to create supersaturated conditions within the core of a flow or within a finite volume. Methods for achieving this include: (1) rapid (usually turbulent) mixing of saturated flows of different temperatures; (2) laminar diffusion; or (3) adiabatic expansion of a nearly saturated flow.

[0033] Most condensation particle counters in use today employ the first two of these methods, either turbulent mixing or laminar diffusion. These are continuous flow devices that have the advantage of "single particle counting" where droplets formed by condensation growth pass through a light source and are individually detected and counted. These methods offer high sensitivity and precision. Some devices are able to detect particles as small as 2nm to 3nm. However, they also require areas of the system to be kept at high or low temperatures, which consumes energy. Generally, they also require liquid reservoirs, which makes them unable to withstand motion and dumping.

[0034] The third of these methods, adiabatic expansion, is the oldest, dating back to the nineteenth century. None of the previous or current adiabatic expansion instruments are continuous flow devices and are capable of single particle counting; that is, none directly detect and enumerate individual droplets as they form. In fact, they are essentially non-steady-state flow instruments that require the introduction of a sample into a volume, followed by sealing and expansion. The quantity measured is the light scattered from the cluster or cloud of droplets that forms within the expansion chamber, from which the particle number concentration can be inferred. Such devices do not count individual droplets that pass through the light beam, as in the continuous flow instruments described above. They are therefore sensitive to the extent of droplet growth, as well as particle number concentration, and therefore lack the precision of single counting instruments.

[0035] The present technology includes a pulsed condensation particle counter 100 (pulsed CPC) that uses adiabatic expansion to produce cooling and is also configured to count condensation-enlarged particles one by one. It can operate with either dry or wet walls. The latter enhances supersaturation through the combined effects of adiabatic expansion cooling and wet wall water vapor diffusion transport. In either configuration, the pulsed CPC method provides the single particle counting advantages of a continuous flow instrument while maintaining the lower energy requirements of an adiabatic method.

[0036] FIG1A shows a first embodiment of a pulse CPC. Air to be sampled enters a particle chamber 12 via an inlet 10 and a valve 11, and is discharged via an optical detector 20, an outlet pipe 18, a flow limiting device 16, and a pump 17. The flow enters the optical detector 20 through a focusing nozzle 13, where the flow is directed through a focusing beam 22, which is configured so that most of the scattered light is directed onto a photodetector 23. A pressure sensor 15 monitors the pressure upstream of the flow limiting device 16. A controller 50 can be coupled to corresponding valves 11, optical detectors 20, flow limiting devices 16, pressure sensors 15, and pumps 17 to achieve corresponding operating states by operating these corresponding elements.

[0037] The pulse CPC 100 has two operating states - purge and expansion - and cycles continuously between these two states. During the purge state, valve 11 is open and the sample (air) flows through the particle chamber 12 and the optical detector 20. The sample flow is discharged by pump 17. Pump 17 operates to extract the sample from the inlet 10 through the device 100. In one embodiment, valve 11 controls the sample air flow through the inlet channel into the particle chamber. Valve 11 is shown as a rotary valve in FIG. 1 and is shown open to allow the sample to pass through the device 100. When the valve rotates, it closes to prevent the sample from flowing from the inlet 10 into the particle chamber 12. Although shown as a rotary valve, valve 11 can also be a simple on / off solenoid valve, or an automatic ball valve, or any other type of valve that allows or prohibits flow through the device 100. The flow limiting device 16 controls the flow rate of the air sample in the device and can be a simple hole, a hole combined with a vacuum regulator, or an active flow controller. In the purge state, the particle chamber 12 is filled with ambient air. At the same time, the device 100 measures direct light scattering from uncondensed enlarged individual particles. Some are sufficient to scatter a detectable amount of light and be counted. Depending on the optical configuration, the lower limit of the detected particle size is about 0.3-0.5 μm.

[0038] The expansion state follows the purge state. During the expansion state, the valve 11 is closed and the particle chamber 12 is partially evacuated by the pump 17 via the optical detector 20 and the outlet pipe 18. As the air is exhausted from the particle chamber and the optical detector, the pressure drops. Partial evacuation causes the air left in the chamber to expand. If the sampled air has been humidified, or if the wall of the particle chamber is wet, and if the expansion rate is appropriate, the air in the particle chamber will cool and droplets will begin to form around the individual particles suspended in the air. During the expansion process, the formed droplets are carried through the nozzle 13 and pass through the focused light beam 22, and the scattered light captured by the photodetector 23 is detected and counted by the individual droplets. During the evacuation process, the pressure in the particle chamber 12 and the optical detector 20 is monitored by the pressure sensor 15, which is usually placed in the outlet pipe 18 between the optical detector 20 and the flow limiting device 16. Since the upstream pressure drop is small, the reading of the pressure sensor 15 is close to the reading of the particle chamber 12. The volume of air passing through the optical detector 20 at any time is calculated from the pressure reading and the volume of the particle chamber 12. Typically, the expansion process lasts a few seconds or less, and the data acquisition rate is 16-64Hz.

[0039] FIG. 1B shows a second embodiment of a pulsed CPC having a separate expansion channel 33 and a purge flow channel 34. During the purge state, the expansion rate can be controlled independently of the flow rate by the expansion channel 33, which includes a valve 31 and a flow limiting device 32. The flow limiting device 32 controls the evacuation rate. This can be a simple hole, or a hole in combination with a vacuum regulator or an active flow controller. Optionally, the vacuum regulator can be placed directly upstream of the pump 17, on the outlet pipe 36, with a hole of appropriate size as the flow limiting device 16 and 32. The expansion channel 33 may also include a ballast volume to help set the desired evacuation rate. The purge flow channel 34 may include an independent valve 35. Typically, the necessary expansion flow is higher than the purge flow, the purge flow channel 34 does not need to be closed, and the valve 35 can be eliminated. However, when operating with a wet wall particle chamber, it may be desirable to allow the air in the chamber to be humidified before expansion. This can be achieved by isolating the chamber by closing valve 35 before opening valve 31, so that no flow passes through the chamber before expansion. Alternatively, this can be achieved by placing valve 35 on outlet pipe 36. Controller 55 can be connected to corresponding valves 11, 31 and 35, optical detector 20, flow limiting devices 16 and 32, pressure sensor 15 and pump 17 to achieve corresponding operating states by operating these corresponding elements.

[0040] Each controller 50 / 55 may include a general purpose processor, a dedicated processor, or a programmable circuit that executes a code that is suitable for causing the controller to perform the methods herein by controlling the various elements of each pulse CPC described herein. In one embodiment, the controller is a microprocessor with a customized firmware program. The data output from the pulse CPC may or may not be stored inside the controller. The controller may output a data output stream for transferring the data to an external memory for later analysis. In another embodiment, the controller may include an electronic circuit or circuits connected to the various elements of the pulse CPC that are configured to operate the pulse CPC to perform any of the methods described herein. It should be understood that the elements of the pulse CPC may include additional circuits, such as circuits that control the focused beam 22 (which may include a laser), circuits that operate the photodetector 23 and process its output before inputting it into the controller.

[0041] Figure 2 The operating states of a pulsed CPC are illustrated. Initially, in the pulsed CPC configured for the purge state, valve 11 is open, then valve 31 (if present) is closed. Air flows through the particle chamber and exits via the optical detector. For configuration 100, the flow then exits via outlet pipe 18. For configuration 110 with two downstream flow channels, the flow exits via purge flow channel 34 and outlet pipe 36. There is no expansion and no condensation growth in the purge state. Instead, atmospheric particles are carried by the flow through the optical detector 20. Each particle scatters light as it passes through the optical detector, but only some particles are large enough in diameter to scatter enough light to be detected by the optical sensor. Generally, the detection threshold is greater than 300nm and can be as high as 2000nm. 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.

[0042] Subsequently, the pulse CPC switches to an expanded state. This transition can be direct or include an isolated state that lasts less than 2 seconds. In the isolated state, the valve is closed to stop the flow through the particle chamber. In configuration 110, the valve is configured to close the flow through the expansion channel 33 and the flow through the purge flow channel 34, for example by closing valves 31 and 35. In configuration 100, an isolated state is not provided, but this can be adapted by adding a valve on the outlet pipe 18. Valve 11 can be open or closed. There is no fluid flowing through the particle chamber 12, and there is no evacuation of the particle chamber 12. If the wall of the particle chamber 12 is wet or formed by a material such as a fluoropolymer-copolymer based on sulfonated tetrafluoroethylene, the isolated state allows the air in the particle chamber to be humidified. Alternatively, the isolated state can be bypassed and the pulse CPC directly switches to the expanded state.

[0043] In the expansion state, valve 11 is closed and the flow through the particle chamber 12, optical detector 20 and other elements to the pump 17 is open, so that the air in the particle chamber 12 is partially evacuated through the optical detector 20. In configuration 100, this is achieved by closing valve 11, so that the flow through the outlet pipe 18 becomes an expansion flow channel. In configuration 110, valve 11 is closed and valve 31 is open; valve 35 can be opened or closed. For configurations 100 or 110, in the expansion state, the air in the particle chamber is discharged through the optical detector 20, the pressure in the particle chamber 12 is reduced, and correspondingly, the remaining air in the chamber expands. By appropriately selecting the evacuation rate, this expansion approximates an adiabatic process and the temperature of most of the air in the chamber is reduced. The net effect of the expansion is to create supersaturated conditions in the particle chamber, so that the particles present grow by water condensation to form droplets. Typically, particles as small as 5-10nm in diameter are activated to grow, and the droplets formed are usually larger than 1 micron in diameter. As air from the chamber continues to be discharged 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 within the particle chamber 12 drops from an initial value close to ambient pressure, typically 1 atmosphere, to a final pressure in the range of 0.2 to 0.8 atmospheres. Once the final pressure is reached, valve 35 (if present) is closed and valve 11 is opened, returning the instrument to the purge state. Adiabatic expansion of a volume of air close to saturation (>90% relative humidity) results in a supersaturated condition. This is due to the decrease in temperature inside the volume as the flow expands, coupled with the non-linear characteristics of the saturation vapor pressure. During an adiabatic expansion, both the temperature and vapor pressure drop. The saturated vapor pressure, which is a function of temperature only, also drops, and because the saturation vapor curve is non-linear, the saturated vapor pressure drops faster than the vapor pressure, resulting in a supersaturated state. This supersaturation persists until the flow becomes hot due to non-adiabatic effects, i.e., heat transfer from the container walls. Very high supersaturations can be achieved using this method, which can activate particle growth as small as a few nanometers.

[0044] Pulsed CPC of this technology can operate with dry walls or wet walls. For dry wall operation, higher supersaturation can be achieved when the air is humidified before expansion. This is achieved by humidifying the air flow when it is introduced. Alternatively, it is achieved by forming the wall of the particle chamber 12 with a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer membrane, the other side of which is in contact with water or high humidity air, and waiting a moment between closing the inlet valve and opening the expansion valve to allow the air flow to become humidified before expansion. When operating with a wet wall, such as can be obtained by lining the wall with a wet core, condensation growth is enhanced. In the wet wall case, as the flow expands, both heat and water vapor diffuse into the flow. Water vapor diffusion is driven by a rapid drop in pressure, while the slower heat diffusion is driven by adiabatic cooling. In this case, supersaturation is produced by a combination of cooling from adiabatic expansion and transport of water vapor on the wall.

[0045] The evolution of the saturation distribution in the particle chamber 12 during the expansion process, such as using Comsol Calculated, Comsol is a finite element heat and mass transfer model available from Comsol. Using this tool, the time-dependent differential equations for water vapor concentration c and temperature T can be solved:

[0046]

[0047] where D is the mass diffusivity of water vapor and u is the flow rate obtained by solving the Navier-Stokes equations. k, ρ, Cp, and αp are the thermal conductivity, density, heat capacity at constant pressure, and compressibility of air.

[0048] The model can be applied to particle chambers of different geometries with wet and dry walls and of different sizes. Figure 3Shows the results of the 12 - millimeter - diameter wet - wall particle chamber of Figure 1B. The relative humidity of the sampled air is 50%, the temperature is 25 °C, and nozzle 13 has a dry wall. The inlet valve is at the bottom (not shown), and the flow is upward during expansion. These 7 images show the evolution of the calculated saturation during the expansion. Each image is the result of half of the tube, with the center - line at the left edge of the image (since the system is cylindrically symmetric, only half is modeled). The gray - scale represents the saturation ratio, black represents S = 0.5, and white represents S>1.8. At time t = 0 s, valves 11, 31, and 35 are closed. The first image 302 shows the starting condition of 50% relative humidity, while the second image 304 shows an increase to nearly 100% as water vapor diffuses from the wet wall into the chamber. At t = 0.5 s, valve 31 is opened to evacuate the particle chamber. The pressure in the particle chamber decreases exponentially with time, which is the same as what happens for a constant - volume flow. In the subsequent 0.5 s (0.5 s <t <1 s), the pressure drop in the chamber is 250 mbar. During the expansion, due to mechanical work, the temperature in the core of the chamber decreases, creating a supersaturated state along the entire tube length. The increase in saturation during the entire expansion is due to the temperature drop and the water - vapor transport from the wall. Since the mass diffusivity of water is higher than the thermal diffusivity of air, the simultaneous transport of heat and water vapor from the wall enhances the supersaturation. In this embodiment, the saturation in the core of the flow (T = 10.6 °C) reaches 1.95, which corresponds to a Kelvin diameter of 3.4 nm.

[0049] To quickly plot the dependence on geometry and expansion rate, the temperature and saturation ratio at a point on the nozzle - inlet center - line can be extracted from the two - dimensional model. The time evolution of temperature and saturation at this point allows for the comparison of different cases with different chamber geometries, wall conditions, and expansion rates. Figure 4 Shows a comparison of wet - wall particle chambers with different geometric ratios at the same expansion rate. The results of cylindrical particle chambers with diameters of 6.0 mm, 8.5 mm, and 12 mm are shown. The input air sample is initially at 50% relative humidity. At time t = 0 s, the flow to pump 17 is closed by closing valves 31 and 35 and closing valve 11, thus stopping the flow, and the humidity of the air in the chamber increases due to diffusion from the wet wall. At time t = 0.5 s, valve 11 is closed, and the pressure and temperature in particle chamber 12 decrease. The model shows that as heat transfer from the wall becomes less important, lower temperatures and higher peak supersaturations can be achieved using wider - diameter tubes. Figure 5 Examines the effect of the expansion rate on a wet - wall particle chamber with a diameter of 8.5 mm. In this embodiment, lower temperatures are achieved at higher expansion rates, and thus higher supersaturations are achieved. The expansion rate is represented by the initial pressure - drop rate in mbar / s.

[0050] Figure 6 The calculated supersaturation obtained by expansion when the particle chamber wall is dry is shown. Again, modeling was for cylindrical particle chambers with diameters of 6.0 mm, 8.5 mm, and 12 mm. Expansion began at time t=0.5 s, and the relative humidity of the input air sample before expansion was 90%. The temperature profile is essentially the same as that obtained under the same expansion rate (500 mbar / s), but the supersaturation achieved is lower. This shows that an increase in saturation is achieved by wetting the walls of the particle chamber. This is due to the transport of water from the wall compared to the enhancement obtained by expansion alone.

[0051] The operating principle of the pulsed CPC has been experimentally verified. Data were obtained using a particle chamber coupled to a commercially available optical detector and electromagnetic valves on the inlet and expansion channels. Figure 7 Shown using a cylindrical shape, 6mm diameter, and 3cm volume 3 Raw data obtained with a particle chamber with purge and wet walls. The figure illustrates the cycle between purge and expansion mode. Shown are the measured values ​​of the particle chamber pressure and the number of droplets detected in each measurement interval. The data were recorded at 64 Hz, i.e. each measurement interval was 1 / 64 second. The samples were taken from the ambient air in the chamber.

[0052] In purge mode, valve 11 is open and valve 31 is closed. Valve 35 is not present. The pressure in particle chamber 12 is constant and essentially no particles are detected. This is because there are too few particles large enough to be seen by the optical system employed. In expansion mode, valve 11 is closed, valve 31 is opened, and the pressure in particle chamber 12 drops. With each cycle of the expansion state, a burst of particles or droplets is seen by the optical detector. As the flow expands and cools, droplets are counted that pass through nozzle 13 and into optical detector 20. To verify that this burst of particles is not caused by the closure of the inlet valve, the measurement is repeated with filtered air and no particles are detected.

[0053] Figure 8 It is shown how to process these data to obtain ambient particle number concentrations. Data are taken from Figure 7 The sample volume is calculated from the pressure reading and the volume of the particle chamber 12 - the volume of air that has entered the optical detector 20 through the focusing nozzle 13 since the start of expansion. Specifically, the sampled air volume is estimated as (1-P / P o )*V o , where V o is the volume of the particle chamber, P is the pressure measured at the time of reading, P o is the pressure before expansion, V ois calculated based on the mechanical drawings of the system. This calculation ignores temperature changes (based on modeling, temperature changes are less than ~3%). The total number of particles detected during each expansion state is then summed. The ratio of the number of particles detected to the sampled air volume is the indicated particle number concentration. During the expansion state, the indicated particle concentration rises and then reaches a plateau. The plateau value is considered the measured particle concentration.

[0054] Particle detection can be performed with either wet-wall or dry-wall particle chambers. When using a wet-wall chamber, humidification prior to expansion is provided by the walls themselves. When using a dry-wall chamber, it is necessary to humidify the air sample prior to expansion. Fig. 9 Shows a 6mm diameter and 3cm volume 3 A single pulse obtained with a narrow cylindrical wet-wall particle chamber indicates the particle concentration. Fig.10 Similar data obtained with a 9mm diameter drywall particle chamber is shown. Both wet-wall and dry-wall methods produce particle counts, although the drywall configuration requires wider ducts and a greater expansion ratio.

[0055] Fig.11 A comparison with a benchtop condensation particle counter for sampling ambient air is shown. The data is from a 6mm diameter wetted wall system. The pulsed CPC method produces particle number concentrations within 10% of the benchtop condensation particle counter, which has a nominal threshold of 5nm.

[0056] Fig.12 The calibration of the pulsed CPC system is shown, showing the detection efficiency in dependence on particle size. The data were acquired using a monodisperse ammonium sulfate aerosol generated by nebulization, drying, charge balancing, and size classification using a differential mobility analyzer (TSI). The reference was a TSI Model 3789 Multifunction Water Condensation Particle Counter configured for 2nm particle detection. For our pulsed CPC method, the concentration was calculated from the maximum reading per pulse as described above. The detection efficiency is shown as a function of particle size for two different expansion rates, with the higher pumpdown rate showing 50% detection at a 10nm particle size. Fig.13 The responses for different concentrations of 20 nm test particles are shown, showing agreement with the reference instrument to within 15% over a wide range of particle concentrations.

[0057] Fig.14Data from the Pulsed CPC during a purge cycle is shown and compared to a commercially available optical particle spectrometer, such as the one manufactured by Climet Instruments of Redlands, California. During the purge cycle, there is no condensation growth and the ambient particles pass through the optical detector without condensation enlargement. Those particles larger than about 300-400nm scatter enough light to be counted without condensation enlargement, and these are counted directly. In this mode, the Pulsed CPC counts and measures particles based on directly scattered light, just like many optical particle spectrometers. Typically, these counters only detect those particles larger than 300-400nm. In purge mode, the Pulsed CPC provides data similar to other commercially available optical particle spectrometers. This added measurement makes it possible to use the instrument dual-purpose, either as a condensation particle counter for total particle counts or as an optical sensor for counting and measuring ambient particles above the 300-400nm threshold. The latter measurement is what low-cost sensors currently use to track PM2.5 mass. This provides the advantage of making both types of measurements in one instrument.

[0058] The advantages of the pulsed CPC over existing methods are portability, low power requirements, and inherent compact size. The required degree of expansion is low—adiabatic expansion of 10% saturated air will activate condensation growth of particles as small as 6nm. With an initial humidity of 80% (a range easily achieved by Nafion-based humidifiers), an expansion of 15% is sufficient to activate 6nm particles. These values ​​are easily achieved with small diaphragm pumps, such as the Sensidyne type pumps. In such pumps, the power requirements are small. For example, a Sensidyne type pump, at a flow rate of 0.2L / min, requires only 240mW to pull 0.8atm. This is an order of magnitude lower than the 2.2W power required for the energy-optimized micro-laminar water-based CPC, in which three Peltier heat pumps are used to maintain the required temperature difference along the detection tube. The housing of the pulsed CPC is isothermal, all at ambient temperature, and no heaters, coolers, or fans are required. The valve 11 does not need to be airtight, but must sufficiently block the flow to allow expansion, in other words, the leakage rate across the valve must be less than the evacuation rate. Pulsed CPC is also insensitive to flow rate because the measured air volume is derived from the pressure measurement and the particle chamber geometry.

[0059] The pulsed CPC method is technically feasible because droplet growth is relatively fast compared to evaporation. Once initiated, particle condensation grows rapidly, but droplet evaporation is relatively slow. Activation of condensation growth requires supersaturated conditions, however once growth begins, the driving force is large. Growth to several microns occurs within 20-100ms. Once a droplet reaches a micron diameter, evaporation is relatively slow even as saturation decreases. At 100% relative humidity, the lifetime of a 1μm droplet is about 1s. This gives ample time to count the droplets that form.

[0060] All existing adiabatic expansion instruments determine the particle number concentration based on scattering from a cluster of droplets formed in a particle chamber. In contrast, the Pulsed CPC counts individual droplets during the expansion process. It is therefore not affected by the uncertainty of cluster counting, where the signal depends on the size of the droplets in the chamber as well as on the total number concentration. Another unique aspect is that, when operating with wet walls in the particle chamber, the supersaturation obtained during the expansion process 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 strictly rely on adiabatic expansion, but is enhanced by the diffusion of water vapor on the walls. A third feature is its dual capability to measure larger (>300nm) atmospheric particles in the purge regime and then all particles, as small as 10nm in the expansion regime. This dual capability provides an optical assessment of PM2.5 parameters and, in the same instrument, also measures ultrafine particle concentrations of public health interest.

[0061] Although the subject matter has been described in language specific to structural features and / or methodological acts, it should 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.

Claims

1. A method for measuring the concentration of particles in air, the method comprising: introducing an air sample into the particle chamber via an inlet; Close the inlet to isolate the particle chamber; Expelling an air sample from the isolated particle chamber through a nozzle connected to an optical detector, the expelling of air flow at a selected flow rate to cause expansion of any air sample in the particle chamber, reduce the temperature of any air sample in the particle chamber, increase the relative humidity of any air sample in the particle chamber, and cause fluid to condense on particles suspended in any air sample in the particle chamber to form 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 leaving the chamber is estimated; as well as Based on the counting and measurement, the particle concentration is determined as the ratio of the number of droplets detected to the amount of air that has left the chamber.

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

3. The method of claim 1, further comprising configuring the optical detector to count particles larger than 400 nm during introduction of the air sample.

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

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

7. A method according to claim 1, wherein the wall of the particle chamber is formed by a membrane, the first side of the membrane comprising a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, and the membrane has a second side in contact with water or high humidity air, wherein the method also includes pausing between the isolation particle chamber and the exhaust air flow so that the air in the particle chamber becomes humidified before expansion. The method of claim 1 , wherein said counting, said measuring and said determining are performed during discharge.

9. The method according to claim 8, comprising continuously repeating the introducing and discharging.

10. A method of producing water vapor supersaturation in a wet wall chamber having an inlet and an outlet, comprising: introducing an air sample into the chamber by pumping at the outlet to cause air flow through the inlet into the wet-wall chamber; closing the inlet while continuing pumping to exhaust the air sample from the chamber through the outlet, the pumping being performed at an operable rate to reduce the pressure within the chamber so that the air sample in the central portion of the chamber cools and water vapor from the chamber walls has time to diffuse from the walls into the air sample in the chamber; Counting the droplets in the air sample as it is exhausted through the optical detector; measuring the pressure, from which the amount of air leaving the chamber is assessed; as well as Based on the counting and the measurements, the particle concentration is determined as the ratio of the number of droplets detected to the amount of air that has left the chamber.

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

12. The method of claim 11 further comprising pausing between introducing and closing to allow the air sample within the wet-wall chamber to become humidified prior to expansion.

13. A particle counting device comprising: Entrance; a first valve coupled to the inlet; a particle chamber coupled to the first valve and having an output end; an optical detector located at the output end of the particle chamber and having a detector outlet; a pump coupled to the output end of the detector and having a pump outlet; a second valve coupled between the detector output and the pump; and The controller executes code which instructs the controller to: opening the first valve and the second valve to allow the pump to introduce air into the particle chamber and pump the air flow from the inlet through the chamber to the detector outlet; as well as The first valve is closed and the pump at the outlet draws air from the chamber, thereby reducing the pressure in the chamber at a flow rate selected to cool the air in the central portion of the chamber and allow water vapor from the chamber walls to diffuse from the wet walls into the air in the chamber, wherein the optical detector is configured to count droplets formed around particles, thereby counting particles, and Wherein the controller executes code instructing the controller to determine the particle concentration as a ratio of the number of droplets counted to an amount of air that has left the chamber, the amount of air that has left the chamber being based on a pressure measurement of the air leaving the chamber.

14. The device according to claim 13, wherein: The controller executes code instructing the controller to close the second valve for less than two seconds when the first valve is closed and before causing the pump to reduce pressure in the chamber.

15. The apparatus according to claim 13, wherein: The pump is coupled to the detector output via two flow channels between the detector outputs, one flow channel comprising the second valve and the other flow channel comprising a third valve, such that the flow rate at which air is introduced into the particle chamber and the rate at which air is exhausted from the particle chamber once the first valve is closed are independently controlled.

16. The apparatus of claim 13, wherein the optical detector is configured to count particles larger than 400 nm when the first valve and the second valve are open and the gas flow is drawn through the particle chamber.

17. The apparatus of claim 13, wherein the controller executes code instructing the controller to repeatedly: open the first valve and the second valve and cause the pump to introduce air into the particle chamber; and close the first valve and cause the pump at the outlet to draw air out of the chamber.

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