Patent Description:
The inventive subject matter disclosed herein relates to particulate matter sensors, and, more specifically, to optical particle spectrometers calibrated in substantially real time by mass concentration sensors.

Airborne particulate matter (PM) pollutants are small solid particles or liquid droplets suspended in the atmosphere. The particles or droplets may include, for example, diesel exhaust, tobacco smoke, volcanic ash, bacteria, mold spores, and pollen. PM pollutants have diameters ranging from many tens of micrometers (µm) down to a few nanometers. PM pollutants measuring, for example, <NUM> in diameter or less (PM<NUM>), are particularly harmful to humans as they can penetrate deep into human respiratory systems, and may even get into the bloodstream. A determination of particulate matter relates the mass of particles per unit volume, indicated as a mass concentration value.

Therefore, the mass concentration provides an indication of the actual mass of particulate matter per unit volume in a given environment (e.g., within a tunnel on an interstate highway system or other transportation routes with heavy traffic (e.g., automobiles, diesel-powered trains, bus routes, etc.), the interior of an automobile or bus, the interior of a factory floor, or a number of other environments). Mass concentration values are typically reported in units of micrograms per cubic meter (µg/m<NUM>). For example, a mass concentration of particulate matter in a large, congested or polluted city can be approximately <NUM>µg/m<NUM> or higher. Mass concentration values may also be related to a given particle diameter such as PM<NUM> (<NUM> and smaller), PM<NUM> (<NUM> and smaller), or PM<NUM> (<NUM> and smaller). Public health agencies typically report mass concentration statistics with a <NUM>% accuracy, or better.

Mass concentration values can be contrasted with a particle count (e.g., as reported by an optical particle counter (OPC) or optical particle spectrometer (OPS)) as the OPC may simply provide a total number of particles, or the total number of particles sorted by particle size ranges (as with an OPS). Consequently, an OPC or an OPS does not measure true mass, does not account for the density of particles measured, generally does not account for the reflectivity of the particles, and so on. However, often these devices are used to provide an estimate of true mass by making assumptions about the particle density and reflectivity, but the accuracy of such an estimate can be off by a factor of two or more. Nonetheless, OPC and OPS devices are generally more compact, less expensive, and easier to operate and maintain than many true mass concentration measurement devices. Moreover, recently developed miniature devices that measure mass concentration based on a resonant frequency change as particles are deposited and suffer from the fact that the miniature devices load with particles over time, eventually changing their response characteristics, and are thus not useful for continuous use over long periods of time. In contrast, OPS and OPC devices measure particles that pass through them, so they do not load with particles. Therefore, what is needed is a way to accurately and precisely correlate the total number concentration of particles reported by an OPC or OPS with true mass concentration values.

Prior art is found in <CIT> which discloses a particulate mass monitor which includes two mass sensors, such as an optical sensor (e.g., a light scattering photometer or nephelometer) and a beta radiation attenuation sensor for substantially continuous monitoring of ambient particulate matter. During operation, the first mass sensor references the time-averaged measurement of the second mass sensor such that the second mass sensor calibrates the response of the first mass sensor. If the first sensor is an optical sensor, as it detects the presence of particulate matter within a fluid, the mass concentration measurement (e.g., signal output) provided by the optical sensor is altered using a ratio of concentration measurements of the second mass sensor and the optical sensor. The combined use of the two mass sensors provides accurate mass measurements of ambient particulate matter with a relatively high time resolution.

<CIT> describes a system and a method of measuring a particle's size in a select aerosol using the optical diameter of the particle to perform a mobility and/or aerodynamic diameter conversion without any knowledge about the particle's shape and its optical properties in the aerosol being characterized. In one example embodiment of the invention, the method includes generating a set of calibration data and finding the optimal refractive index and shape that best fits the calibration data. In addition, the method includes creating a new calibration curve that provides a mobility-equivalent or aerodynamic-equivalent diameter.

<CIT> describes an apparatus and method for estimating size segregated aerosol mass concentration in real time and using a single detector. A beam of electromagnetic radiation is passed through a particle stream made of a test or field aerosol. The single detector outputs an electrical signal proportional to the electromagnetic radiation scattered thereupon. The electrical signal may be conditioned to produce an integrated signal for measuring radiation scattered from all the particles in the interrogation volume, a pulse height from an individual particle within the volume, and/or a time-of-flight measurement from the individual particle. The integrated signal can be correlated to particle mass concentration. The pulse height signal and the time-of-flight signal may be converted to infer the size of the individual particle. Attendant size distributions for the particle sizes may also be obtained. Using known or assumed particle properties, the mass concentration may be estimated from the size distribution.

The invention is expressly defined in the claims of this patent.

As noted above, optical particle spectrometer (OPS) devices are frequently used to determine an approximation of the mass concentration of particulate matter (PM) in a given environment. The disclosed subject matter combines an OPS with a particle mass concentration measurement device, such as a film bulk acoustic resonator (FBAR) or a quartz crystal monitor (QCM, also known as a quartz crystal microbalance), in order to provide correction factors to the "equivalent-mass" concentration as determined by the OPS, thus calibrating the OPS for an effective mass concentration measurement of one or more of the reported particle size ranges. One characteristic of other devices, such as FBAR and QCM devices, is that they can be made very small and at low cost for use in applications where miniaturization and high-volume production are desired. However, unlike OPS devices, FBAR and QCM devices tend to load with particles over time, thereby resulting in inaccurate reporting values.

In embodiments, the calibration of the OPS (for one or more of the reported particle size ranges by the OPS) is performed with a single correction factor (e.g., a single point for a given PM value, such as PM<NUM>). Typically, the correction factor is off by a constant multiple or factor, so that difference can be determined using the single correction factor. However, in other embodiments described herein, the calibration can be performed with multiple correction factors (e.g., at PM values of PM<NUM> and PM<NUM>). Additional PM values (e.g., PM<NUM>) may also be used to provide one or more correction factors. As is known in the art, the PM value is not a bin or size range of particles. Instead, the PM value relates to the entire mass of particles below a certain size (e.g., PM<NUM> relates to the entire mass of particles below <NUM>). Consequently, a more accurate and precise measurement of PM mass concentrations by the OPS is possible using the systems and methods provided herein.

Various types of OPS devices are available and usable with the various embodiments described herein. For example, a TSI Model <NUM> Optical Particle Sizer or a TSI Model <NUM> DUSTTRAK™ Aerosol Monitor (both available from TSI Incorporated, Shoreview, Minnesota, USA) uses light scattering technology to determine mass concentration in real-time. An aerosol sample is drawn into the sensing chamber in a continuous stream. One section of the aerosol stream is illuminated with a small beam of laser light. Particles in the aerosol stream scatter light in all directions. In some cases, a lens at, for example, about <NUM>° to both the aerosol stream and laser beam, collects some of the scattered light and focuses it onto a photodetector. The detection circuitry converts the light into a voltage that is proportional to the amount of light scattered which is, in-turn, proportional to the mass concentration of the aerosol. The voltage is read by a processor and multiplied by an internal calibration constant to yield mass concentration. The internal calibration constant is determined from the ratio of the voltage response of the <NUM> OPS or the DUSTTRAK™ monitor to the known mass concentration of a test aerosol (the monitor may be calibrated against a gravimetric reference using the respirable fraction of ISO Standard <NUM>-<NUM>, A1 test dust (e.g., such as "Arizona Road Dust")). If highly-accurate mass concentration readings are required, the <NUM> OPS or the DUSTTRAK™ monitors can be recalibrated for an environment where a specific aerosol-type predominates. Similar types of calibration may also be accomplished with OPS-type devices available from other manufacturers.

Other types of particle measurement sensors are also available. For example, PM<NUM> particle sensors, such as the PMS <NUM>, are available from Plantower (Houshayu, Shunyi District, Beijing, China) and a number of other manufacturers.

Mass measuring devices and mass concentration measurement devices are known in, for example, the atmospheric aerosol sciences. Mass concentration devices can include, for example, the true-mass, filter-based TEOM® microbalance (available from Thermo Fisher Scientific; Franklin, Massachusetts, USA). Other types of mass measurement concentration devices include, for example, a QCM or FBAR device, as noted above.

The QCM is used for micro weighing and consists of a quartz plate having a mechanical resonance frequency that is inversely proportional to the thickness of the plate. Because of the very high Q value (low internal friction) of quartz, the resonant frequency may be measured electrically through a piezoelectric effect. If a mass to be measured is applied to the resonator, e.g. in the form of PM, it will have an effect on frequency very nearly that of an increase in an equivalent mass of quartz. The added mass may be determined by translating frequency changes into an equivalent thickness of quartz and then into mass by means of the known density of quartz.

In one form, an FBAR device may be fabricated by, for example, sputter deposition of a piezoelectric material, such as zinc oxide (ZnO) or aluminum nitride (AlN), on to a thin membrane formed on a semiconductor substrate. The combination of the piezoelectric layer and thin membrane forms an acoustic structure that is resonant at a specific frequency. A ZnO film having a thickness of a few microns yields a resonator with a fundamental frequency of around <NUM>. As particles adhere to a mass-sensitive element of the FBAR device, the fundamental frequency of the element decreases in proportion to the mass of particles that reach and adhere to the element. As particles are deposited onto the mass-sensitive element, the frequency at which the device oscillates decreases proportionally and an added amount of mass due the particles is calculated from the reduction in frequency.

As described in more detail below, the OPS is used as the continuous particle "mass" measuring component of the inventive subject matter, and the mass concentration measurement device (mass sensor) can be used either intermittently or continuously to measure true mass in the same environment, allowing the determination of a correction factor, Cf to correct the OPS to a true mass measurement value. The combination of these two device types provides the benefits of, for example, (<NUM>) the robustness of the OPS measurement; with (<NUM>) the improved accuracy and precision enabled by the true mass measurement device, while concurrently overcoming shortcomings of the OPS such as, for example, (<NUM>) the potentially less accurate OPS measurement if the true mass is not known; and (<NUM>) the fact that true mass measuring components may not be suitable for continuous undiluted measurements because they load with particles over time, potentially losing sensitivity.

Therefore, the disclosed subject matter provides an intermittent or continuous, substantially real-time, in situ, true mass measurement to enable determination of an accurate correction factor for equivalent mass concentrations reported by an OPS. The inventive subject matter described herein will find use in many applications where increased mass concentration measurement accuracy is desired, and also where miniaturization and low cost of the instrumentation are required, such as automobile cabin air-quality sensing. It is well-known that the particle mass concentration calculated using an OPS can be in error by <NUM>% or more, depending on differences between the material being sampled and that used to calibrate the OPS. This error is usually compensated for using a correction factor that is a multiplier constant or function that brings the OPS output more in line with the actual mass being measured. The actual mass is generally determined using a bulky and expensive reference device such as a tapered element oscillating microbalance (TEOM) noted above, which measures true particle mass concentration or a beta attenuation monitor (BAM), which is a US-based Federal equivalent mass concentration measuring method that determines mass based on the absorption of beta radiation by solid particles extracted from flowing air.

With reference now to <FIG>, a diagram of a particulate matter sensor calibration system <NUM>, is shown in accordance with an embodiment of the disclosed subject matter. The particulate matter sensor calibration system <NUM> is shown to include an OPS <NUM>, a particle diverter mechanism <NUM>, an optional mass filter <NUM> which is non-optionally included in the claimed invention, and a mass sensor <NUM>. A particle-laden airstream <NUM> is drawn into an inlet of the OPS <NUM> to begin the particle measurement and calibration process. Each of the OPS <NUM>, mass filter <NUM>, and mass sensor <NUM> devices are also shown to include respective outlet ports <NUM>, <NUM>, <NUM>. Although not shown, each of the respective outlet ports <NUM>, <NUM>, <NUM> may be coupled to an absolute filter (not shown) on a downstream side of the respective ports.

A calibration communication loop <NUM> is coupled from the mass sensor <NUM> to the OPS <NUM> (e.g., through an electrical connection or a wireless connection) to provide actual mass concentration readings from the mass sensor <NUM> to a processor (not shown) within the OPS <NUM>. In this embodiment, the processor with the OPS <NUM> calculates a calibration factor, Cf. In other embodiments, the OPS <NUM> provides the measured mass concentration value to the mass sensor <NUM>. In this embodiment, the calibration factor, Cf, is calculated (e.g., by a processor within the mass sensor <NUM>) at the mass sensor <NUM> and forwarded to the OPS <NUM>. In either embodiment, the calibration communication loop <NUM> provides calibration data. Examples of using measurements from the mass sensor <NUM> to calibrate the OPS <NUM> are disclosed in more detail below.

The particle diverter mechanism <NUM> may be any type of fluidic switching-device to divert intermittently at least a portion (e.g., a controlled and predetermined fraction) of the particle-laden airstream <NUM> directly to the mass filter <NUM> or from the OPS <NUM> to the mass filter <NUM>. In other embodiments, the particle-laden airstream may be coupled from the OPS <NUM> to the mass sensor <NUM> without using the intervening and optional mass filter <NUM>. The particle diverter mechanism <NUM> can therefore comprise, for example, an electrically-activated solenoid valve or other types of fluidic valve that can be programmed or otherwise controlled to divert the airstream at predetermined time intervals. For example, the particle diverter mechanism <NUM> may divert the particle-laden airstream <NUM> to the mass filter <NUM> for one second each minute, one minute in each <NUM>-minute monitoring interval, or other fraction of a predetermined monitoring time interval. In other embodiments, the particle diverter mechanism <NUM> comprises one or more pumps switched on and off at appropriate intervals. In still other embodiments, the particle diverter mechanism <NUM> comprises a valve coupled to a downstream pump to stop airflow to the mass filter <NUM> or mass sensor <NUM> combination at appropriate intervals. Overall, a determination as to how frequently to intermittently divert the airstream to the mass filter <NUM> can be dependent on factors such as, for example, a flow rate of the particle-laden airstream (or diverted fraction thereof), an aerosol concentration of the ambient environment being measured, a particle size-distribution, and a required accuracy of calibration of the OPS <NUM>.

The mass filter <NUM> is intended to create size fractions of particles within the airstream. In various embodiments, the mass filter <NUM> may comprise a cascade impactor. In a cascade impactor, several particle collection impaction devices are placed serially in fluid communication with each other. The cascade impactor is based on accelerating a particle-laden airstream, at a known volumetric flowrate, through a series of increasingly-smaller nozzles, each nozzle being directed at an impaction plate. As the nozzle sizes decrease, the velocity of the particle-laden airstream increases, thereby increasing the inertia of particles. Particles with a given inertia can no longer follow the streamlines to successive stages and are impacted onto one of the series of impaction plates. For example, the particle-laden stream is directed into an inlet of the cascade impactor. Particles larger than approximately <NUM> micrometers (Dp > <NUM>) are impacted onto an impaction plate in the first stage. Particles smaller than approximately <NUM> continue on to stage two of the impactor. Stage two can be designed such that particles less than <NUM> but greater than approximately <NUM> (<NUM> < Dp < <NUM>) are impacted from the particle stream. Subsequently, a third stage can be designed such that particles less than <NUM> but greater than approximately <NUM> (<NUM> < Dp < <NUM>) are impacted from the particle stream. In the example three stage impactor, particles less than approximately <NUM> exit the mass filter <NUM> through the outlet port <NUM> and, in some embodiments, into an absolute filter (not shown but known in the art). In other embodiments, additional stages can be added to the cascade impactor.

In this embodiment utilizing a cascade impactor, each of the impaction plates can be one of the various mass sensing devices as noted above (e.g., each plate comprises a separate FBAR or QCM device). The governing equations for an inertial impactor, known in the art, are used to calculate a given particle cut-size for each of the impaction plates (e.g., a measured particle mass concentration output after Stage I relates to a PM<NUM> size, a measured particle mass concentration output after Stage II relates to a PM<NUM> size, etc.). The actual mass concentration of particles, derived from the inertial impactor governing equations, can then be fed back to the OPS <NUM> and used to correlate the measured particle count to the mass concentrations, using a correction factor, Cf, for one or more particle size ranges (e.g., PM<NUM>) correlated to the mass of all particles below a certain particle size. The correction factor applied to the OPS-measured particle concentration can be, for example, a ratio of particle concentration measured by the mass sensor <NUM> divided by a particle concentration measured by the OPS <NUM>.

In other embodiments, the mass filter <NUM> can also be a virtual impactor or cyclonic-type separator. A virtual impactor is closely related to the inertial impactor, discussed above. However, a virtual impactor separates out particles that would be collected by an impaction plate (a small fraction of the total inlet flow, the minor flow), but that are instead simply redirected. Again, smaller particles more readily follow the streamlines of the particle-laden airstream as the particles pass out the sides (a larger fraction of the total inlet flow <NUM>, the major flow) of a virtual impactor. Hence, like the inertial impactor, particle size ranges within both the major flow and the minor flow can be tailored to a given size range by tailoring geometrical and fluid flow parameters of the virtual impactor. Particle mass concentrations within either or both of the minor flow and the major flow can then be measured by a mass sensor <NUM>. The actual measured mass may then be fed back through the communication loop <NUM> to the OPS <NUM>. Other types of mass filters are described below.

In other embodiments, the mass filter <NUM> is a physically-rotating filter (e.g., a cylindrical filter), referred to as a centrifugal filter, where the rotating filter rotates along the axis parallel to the airstream. The rotational speed determines the particle cutoff size. Additionally, the collection efficiency of the centrifugal filter is adjustable by changing the rotational speed without changing the pressure drop across the rotating filter.

With reference now to <FIG>, a diagram of a particulate matter sensor calibration system <NUM> is shown in accordance with an embodiment of the disclosed subject matter. The particulate matter sensor calibration system <NUM> is shown to include an OPS <NUM>, a particle diverter mechanism <NUM>, an optional mass filter <NUM> which is non-optionally included in the claimed invention, and a mass sensor <NUM>. A particle-laden airstream <NUM> is drawn into an inlet of the optional mass filter <NUM> to begin the particle measurement and calibration process. Each of the OPS <NUM>, the mass filter <NUM>, and the mass sensor <NUM> devices is also shown to include respective outlet ports <NUM>, <NUM>, <NUM>. Although not shown, each of the respective outlet ports <NUM>, <NUM>, <NUM> may be coupled to an absolute filter (not shown) on a downstream side of the respective ports.

As compared with the particulate matter sensor calibration system <NUM> of <FIG>, the mass filter <NUM> of the particulate matter sensor calibration system <NUM> is configured so that the mass filter <NUM> is upstream of remaining components of the system <NUM>. In the particulate matter sensor calibration system <NUM> of <FIG>, the OPS <NUM> was upstream of the mass sensor <NUM>. In <FIG>, the mass filter <NUM> is shown to be upstream of the OPS <NUM>. In various embodiments, the mass filter <NUM> may be the same as or similar to the mass filter <NUM>.

Upon reading and understanding the disclosure provided herein, the person of ordinary skill in the art will understand when to use the optional mass filter <NUM>, if at all. If used, the skilled artisan will further recognize which configuration (e.g., the mass filter <NUM> is upstream or downstream of the OPS <NUM>) will provide a more particularly relevant sampling scheme for a given environment. For example, if the particulate matter sensor calibration system <NUM> or <NUM> will be used where both the OPS <NUM> and the mass sensor <NUM> will be used to perform, for example, a PM<NUM> measurement, configuring the mass filter <NUM> to be upstream of the OPS <NUM> may be preferable.

However, there are other situations in which it may be desirable for the OPS <NUM> to measure the entirety of the particle number concentration and the mass sensor <NUM> to measure only particle mass concentration below a selected particle size cutoff value. In still other situations, it may be desirable for both the OPS <NUM> and the mass sensor <NUM> to measure all particle sizes in a particular environment. In still other situations in which a natural size distribution of particles (e.g., particles already binned or sized (e.g., by a size-selective inlet), or a monodisperse or limited particle size range) may be present so the optional mass filter <NUM> may not be needed. At least each of these situations, as well as other situations, is contemplated by various embodiments presented herein.

<FIG> shows an embodiment of an example calibration method <NUM> usable with the systems of <FIG>. At operation <NUM>, a mass concentration from the particle-laden airstream <NUM> is measured and reported (e.g., displayed or logged) by the OPS. If a calibration factor, Cf, is specified initially for the OPS, or if it has already been calculated from an actual received value of mass concentration, transmitted over the calibration communication loop, the reported mass concentration value already includes the calibration factor.

With concurrent reference to <FIG>, operation <NUM> is an optional step usable with the particulate matter sensor calibration system <NUM>. At operation <NUM>, a controlled fraction of the particle-laden airstream <NUM> is intermittently diverted to the optional mass filter <NUM> through the particle diverter mechanism <NUM>. The particle-laden airstream <NUM> is diverted at predetermined time intervals. For example, the particle diverter mechanism <NUM> may divert the particle-laden airstream <NUM> to the mass filter <NUM> for one minute in each <NUM>-minute monitoring interval. A ratio of total flow to diverted flow is used in a determination of the calibration factor.

At operation <NUM>, at least one mass concentration number is measured at one or more cut-off values. For example, mass concentration number may be measured at a PM<NUM> cut-off value, a PM<NUM> cut-off value, and a PM<NUM> cut-off value. In other embodiments, a single mass concentration number is measured at a single cut-off value such as, for example, a PM<NUM> cut-off value.

At operation <NUM>, each of the one or more measured mass concentration numbers is then sent to a processor in the OPS to compare the OPS-measured mass concentration number with the true mass concentration number as determined by the mass sensor. In other embodiments, the OPS sends a measured mass concentration number to the mass sensor and the mass sensor sends the calibration factor, Cf, to the OPS via the calibration communication loop.

Claim 1:
A system (<NUM>) to measure a sampled particle-laden airstream (<NUM>), the system comprising:
an optical particle spectrometer (<NUM>) configured to measure a concentration of particulate matter in the sampled particle-laden airstream;
a particle diverter (<NUM>) in fluid communication with the optical particle spectrometer, the particle diverter configured to divert at least a portion of the particle-laden airstream at predetermined intervals;
a mass sensor (<NUM>) configured to measure a mass of a fraction of particles within the diverted particle-laden airstream received from the particle diverter;
a calibration communication loop (<NUM>) configured to provide the measured mass of the particles to the optical particle spectrometer; and
a processor configured to calculate a calibration factor, Cf, from mass concentration readings provided from the mass sensor via the calibration communication loop or mass concentration values provided from the optical particle spectrometer via the calibration communication loop, wherein the system further comprises either:
a) a mass filter (<NUM>) coupled upstream of the mass sensor so as to receive the portion of the particle-laden airstream and being configured to filter a fraction of the particles within the particle-laden airstream that are above a predetermined particle size, or
b) a mass filter coupled upstream of the optical particle spectrometer so as to receive the sampled particle-laden airstream and being configured to filter a fraction of the particles within the particle-laden airstream that are above a predetermined particle size.