Fast mixing condensation nucleus counter

Details Fast mixing condensation nucleus counter Fast mixing condensation nucleus counter

2000-10-12

2003-05-20

Lee, John R. (Department: 2881)

Optics: measuring and testing

With sample preparation

Condensation nuclei detector

C356S335000, C356S440000, C073S865500, C073S028040, C073S031020, C073S863210

Reexamination Certificate

active

06567157

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to a fast mixing condensation nucleus counter for use in determining the physical characteristics of aerosol particles.
BACKGROUND OF THE INVENTION
Atmospheric particles influence climate change, radiative transfer, visibility, and air quality. Atmospheric aerosols include particles that are emitted directly to the atmosphere and those that are formed in the atmosphere by the reactions of gaseous pollutants and certain natural compounds. At high concentrations, they become the haze that reduces visibility a becomes a health hazard. Aerosols also play an important role in the global atmosphere. They scatter sunlight back to space, producing a cooling effect that partially offsets the warming induced by greenhouse gases such as CO
2
.
Aerosol measurements characterize the size, concentration and composition of particles suspended in the atmosphere. The ability to measure the concentration and size distribution of fine particles is essential to the understanding of the dynamics of aerosols in the atmosphere, in combustion systems, or in technological applications. The importance of characterizing fast transient aerosols has increased in recent years. For example, rapid transients in aerosol systems can arise due to dynamic response, such as in diesel engine particle emissions, or as a result of high speed traversing through different air masses, commonly a problem in airborne measurements. A continuing focus of aerosol research, then, is the development of measurement methods that have the time and size resolution necessary to resolve rapid aerosol dynamics in the atmosphere and in technological systems.
Detection and analysis of aerosols using a condensation nucleus counter (CNC) is well known. The CNC is also used as the primary detector for obtaining particle size distributions, for example in scanning electrical mobility spectrometers (SEMS), also known as scanning mobility particle sizers (SPMS). However, traditional CNC designs have slow detector response times, limiting the speed at which particle size distributions can be obtained, and thus rendering them impractical for obtaining time sensitive particle size distributions.
The condensation nucleus counter detects particles by condensing a vapor on the particles to grow them to large enough size that they can be counted optically. This measurement involves four steps: i) the production of sufficient quantities of vapor; ii) creation of the supersaturation necessary to activate the particles; iii) maintenance of the particles in the supersaturated state long enough to grow to detectable size; and iv) detection of the grown particles. The time required for a CNC to respond to changes in the aerosol concentration is constrained by the sum of the relevant times.
Another problem with these traditional CNCs is that stable flow recirculations are created in these systems. Stable flow recirculations operate to randomly trap some of the sample particles within the CNC. Thus, while some particles immediately exit the mixing region and enter the detector, other particles continue to recirculate inside the CNC and randomly exit at some later time, introducing an exponentially decaying distribution of delays between the time a particle enters the CNC and when it is detected. This was not a problem for early uses of CNCs, but has important consequences when such detectors are used for time sensitive measurements. In particular, the distribution of delay times smears scanning DMA size distribution measurements so the full potential of SEMS systems has not yet been realized. These stable flow recirculations create mixing and detection delays of up to 1 s, making scans shorter than 3 s impractical in these CNC systems.
In these traditional CNC designs, the aerosol sample is first passed through a saturation chamber wherein a sufficient quantity of vapor-laden gas is produced, and then to a condensation chamber for supersaturation and growth. In later designs, the sample aerosol bypasses the saturation chamber and is fed directly into the condensor where it mixes, under laminar flow conditions, with a pre-saturated flow of gas from the saturation chamber. This simple plumbing change eliminates the time delay associated with vapor production step above, and increases the detection speed of the CNC dramatically. For example, in a CNC using the original design, such as the TSI Model 3010, a typical particle size distribution scan (with data inversion to correct for smearing of the data) can be taken in 30 to 45 s. Meanwhile, scans up to 10 times faster can be obtained with ultrafine CNC (UCNC) devices, such as the TSI Model 3025, utilizing the saturation chamber bypass design.
While scanning times are faster in these UCNC systems, such UCNC devices generally have a very small aerosol flow rates, up to 33 times smaller than the standard CNCs, reducing the count rate obtainable with these detectors and making such devices practical only for aerosols with extremely high number concentrations or long sample times. This is particularly true at the low end of the particle size distribution where the charging efficiency of the spectrometer is low. As a result of the low signal strength of such devices, particles in a single mobility channel must be scanned for a longer time, either by reducing the scan rate, or by summing the counts acquired during a number of scans. While either of these solutions will increase count rates, both of these solutions also increase the length of time needed to obtain a scan, rendering the device less than ideal for obtaining particle size distributions where small fast transients are involved.
An alternative design for continuous-flow CNCs is the mixing CNC (MCNC). In this instrument a cold aerosol flow is mixed with a comparable flow of hot, vapor-laden gas. The mixed gas then passes from the mixing chamber into a chamber that provides sufficient residence time for the supersaturated particles to grow to optically detectable sizes. In these MCNC systems, rapid, nearly adiabatic mixing is facilitated by making the mixing region turbulent. Turbulent mixing can achieve compositional homogeneity quickly and without the use of a cooler. However, until now, large mixing chamber volumes have been employed to prevent thermophoretic deposition of the aerosol particles in the mixing chamber. The large mixing chamber volumes employed in these MCNC systems also create stable recirculation zones within the mixing chamber, resulting in long residence times for the aerosol in the mixing chamber rendering these MCNCs too slow for use as a DMA detector.
Accordingly, a need exists for a system that provides a fast response CNC which would allow accelerated SEMS measurements by reducing the residence time of aerosol particles in the system while maintaining high sample flow rates that enable high count rates by the detector.
SUMMARY OF THE INVENTION
The present invention is directed to a fast mixing condensation nucleus counter (FMCNC), for use in obtaining particle size distributions of fast transient aerosol systems over a wide range of particle sizes. This invention utilizes the turbulent mixing technology of the MCNC systems to provide fast particle growth and high sample flow rate and signal strength, but restricts the size of the mixing chamber to minimize the detector delay associated with traditional MCNC systems. This invention is also directed to novel methods for obtaining particle size distributions of fast transient aerosol systems over a wide range of particle sizes using the FMCNC of the invention.
In one embodiment, the invention is directed to a fast mixing condensation nucleus counter comprising a detector and a mixing condensation device having a mixing chamber adapted to allow gas to flow within it along a preselected path to an outlet, wherein the outlet directs the gas flow to the detector. The mixing chamber has an inlet for introducing vapor-laden gas into the chamber and at least one nozzle for introducing a sample gas having particles entrained therein i

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