One of the major hypotheses that has recently been proposed for the cause of the observed effect of particulate matter on human health is that high numbers of ultrafine particles (for example, particles, diameters less than 0.1 μm) are more problematic than the particle mass that is now the basis of the National Ambient Air Quality Standards. For example, significant associations of elevated cardiovascular and respiratory disease mortality with various fine (and ultrafine) particle indices have been found in one study based in Erfurt, Germany. Specifically, significant associations were found between mortality and ultrafine particle number (NC), ultrafine particle mass (MC), fine particle mass or SO2 concentrations. The correlation between MC 0.01-2.5 and NC 0.01-0.1 is only moderate, suggesting that it may be possible to partially separate effects of ultrafine and fine particles. Thus, measurements of the ultrafine particle concentrations as well as particle mass are needed to help provide more data to examine these relationships.
In addition, there are a variety of particle counting needs in industrial settings. With the increased emphasis on nanometer sized particles for the production of nanostructured materials, particle counters can be important in process control. There are currently only a limited number of instruments available to make such measurements.
One method or device that has shown to be an effective means of detecting such fine particles is the use of heterogeneous nucleation with a turbulent mixing condensation nuclei counter (that is, a “TMCNC”). In a TMCNC system, a gas (for example, air) containing the particles to be measured is turbulently mixed with a stream of air saturated with a condensable vapor so that the vapor cools and condensates, that is nucleates, onto the particles. The resulting droplets or nuclei then grow to a size whereby they can be effectively detected by, for example, light scattering. Although the concept of a TMCNC has been available for over 15 years (see for example, McMurry, 2000), the TMCNC has not been developed into a viable commercial instrument capable of particle detection down to 2 nanometers.
The Condensation Nuclei Counter (CNC), which grows primary particles (nuclei) up to a more easily detectable size, is one of the most widely used devices for studying particles below 0.1 micrometer (μm). A general description of CNC's is given in many books and reviews (for example, see Willeke and Baron, 1993 and McMurry, 2000). Several types of CNCs are used in researching aerosols, that is, suspensions of fine solid or liquid particles in a gas, typically, air. The main difference among these CNC designs is the way the devices produce supersaturation that leads to particle growth up to a predetermined size for subsequent detection. In an expansion-type CNC, supersaturation is generated by adiabatic cooling during pressure reduction. An expansion-type CNC is typically a batch instrument. Expansion-type CNCs have been used in atmospheric aerosol research for many years. A continuous CNC (for example, as disclosed by Agarwal and Sem, 1980) is widely used. In the continuous CNC, supersaturation is formed by cooling a laminar aerosol flow that is saturated with working fluid vapor. In the conductive cooling type continuous CNC the aerosol-containing sample is typically saturated with working fluid and then cooled whereby the working fluid condenses on the aerosol particles. However, one disadvantage of the conductive cooling CNC is its sensitivity to moisture in the sample gas. Moisture in the sample gas may also condense on the aerosol particles or otherwise interfere with the condensation of the working fluid and affect the measured particle count. Attempts to remove moisture from the sample gas typically can also remove aerosol particles, which hampers the accuracy of the particle measurements. Aspects of the present invention overcome this deficiency of the prior art.
The third type of CNC, known as a turbulent mixing CNC (or TMCNC), is based on turbulent mixing of a gas flow with particles with working fluid vapor. The TMCNC instrument has not yet been commercialized. One prior art TMCNC is described by Kogan and Burnashova (1960) and was further developed in different versions by Okuyama, et al. (1984); Ankilov, et al. (1991); Kousaka (1993); and Mavliev and Wang (1999). The major advantage of the TMCNC is the flexibility of generating supersaturation by simply mixing the aerosol flow and a separate gas flow saturated with the working fluid vapor.
The CNC is one of the most sensitive devices for detecting nanometer-size particles, for example, some CNC studies have reached a detection limit of 2-3 nanometers (nm) (Stolzenburg and McMurry, 1991; McDermont et al., 1991; Okuyama et al., 1984; Mavliev and Wang, 1999). In one commercially available CNC system that can detect 3 nm particles, the aerosol stream is directed through a capillary tube in order to align the particle stream within a small central zone of uniform saturation conditions. However, the capillary in this system is prone to problematic clogging when used to directly measure ambient aerosols and cleaning the capillary tube can be difficult.
The minimum detection efficiency of CNCs is very sensitive to the size of particles being detected (Makela, et al., 1996). The detection efficiency may also be sensitive to the composition of the particle (Mavliev et al., 2001) and to the nature of the working fluid (Lee et al., 2003; Mavliev et al., 2004).
Although the CNC is primarily devoted to measuring the number concentration of particles, some prior art CNCs have been shown to be able to measure size distribution of particles, for example, of nanometer-sized particles. In one prior art system, the size distribution of nuclei can be measured by means of changing the CNC's sensitivity (McDermont et al. 1991) and by means of measuring the size of grown particles (Ahn and Liu, 1990; Rebours, et al., 1996; Saros, et al., 1996). One prior art method is based on the fact that the growth of smaller particles is delayed because of the Kelvin effect that results in the final size of particles being dependent on initial nucleus size. However, these prior art devices provide unsatisfactory size resolution and their operational range is limited to a range of 3-10 nm. In addition, in some of these prior art systems, the growth time for particles of the same size depends strongly on spatial uniformity of supersaturation. For most continuous flow CNCs, the spatial distribution of supersaturation is not uniform because of the use of diffusive cooling in the laminar flow (Stolzenburg, 1988).
Moreover, some commercially available prior art CNCs were developed as laboratory research tools or to monitor clean rooms, and, as far as the present inventors know, no effort has yet been made to optimize their performance for ambient aerosol monitoring. For example, typically, the aerosol-laden air streams of such research CNCs are not conditioned in any way, nor is there any control or limitation of the introduction of large particles to these CNCs. In prior art ultrafine CNCs, large particles can clog the device, for example, the capillaries in such devices. In addition, in prior art systems there is typically no control of temperature and humidity in the inlet stream and both temperature and humidity can produce some variability in the instrumental response. These deficiencies in the prior art devices cause problems when using a CNC for ambient particle monitoring.
In addition, prior art CNCs typically use n-butanol as the working fluid. However, butanol is toxic, flammable, has a noxious odor, and is thus undesirable in a commercially-available device that may readably experience human contact, operation, and servicing.
Thus, in view of the deficiencies and disadvantages of prior art CNCs, there is a need for an improved stand-alone TMCNC aerosol particle counter that will provide improved sensitivity, for example, for particle sizes down to the order of about 2 nm, well-defined inlet characteristics, robust design, consistent and reproducible performance, and employ a working fluid that is less offensive and less dangerous to humans. Aspects of the present invention address these and other deficiencies of the prior art while providing improved detection efficiency, greater ease of use, and adaptability for use in ambient gas particulate measurements.