Counting Efficiency of the TSI Model 3020 Condensation Nucleus Counter

The counting efficiency of the TSI model 3020 condensation nucleus counter (CNC) was determined as a function of aerosol flow rate and trigger level using aerosols of known size and an aerosol electrometer. When the aerosol flow rate dropped from 300 to 200 mL/min, counting efficiencies increased significantly in the single-particle counting mode for particles with diameter < 20 nm while those for larger particles remained constant. However, the photometric mode counting efficiency for particles with diameter > 20 nm increased and exceeded unity. When the aerosol flow rate was reduced to 100 mL/min, the counting efficiencies for both counting modes decreased regardless of particle size. Varying the trigger level of the CNC did not influence the photometric mode counting efficiency. However, the counting efficiency of the single-particle counting mode increased with decreasing trigger level, especially for particles < 20 nm in diameter. Characteristics for individual instruments need to be measured because counting efficiencies of two CNCs with the same trigger level and flow rate were not identical.     

A Laminar-Flow,Water-Based Condensation Particle Counter (WCPC)

A new water-based condensation particle counter (WCPC) is presented. The WCPC is a thermally diffusive, laminar flow instrument. Condensational enlargement is achieved through the introduction of a saturated airflow into a “growth tube” with wetted walls held at a temperature higher than that of the entering flow. An unsheathed, 1 L/min instrument utilizing this principle has been evaluated with various aerosols. The particle size detected with an efficiency of 50% is at or below 4.8 nm for particles sampled from vehicular emissions or ambient air, and for various laboratory-generated inorganic salts. The cut point is higher for the organic materials tested, ranging from 8 nm to 30 nm depending on the compound and purity level. An empirically determined dead-time correction factor is applied to the coincidence correction, which allows extension of the single-count mode to higher concentrations. The counting efficiencies for 80 nm oil and salt aerosols are equal, and above 97% for concentrations approaching 10 ⁵ cm ^?3 . When subject to a step-fucntion change in input concentration the time required to attain 90% of the final value, including a 0.5 s lag, is 1.3 s. The corresponding exponential time constant is 0.35 s. The WCPC evaluated here is marketed as the TSI Model 3785.     

Inter-Comparison of a Fast Mobility Particle Sizer and a Scanning Mobility Particle Sizer Incorporating an Ultrafine Water-Based Condensation Particle Counter

An Ultrafine Water-based Condensation Particle Counter (UWCPC), a Scanning Mobility Particle Sizer (SMPS) incorporating an UWCPC, and a Fast Mobility Particle Sizer (FMPS) were deployed to determine the number and size distribution of ultrafine particles. Comparisons of particle number concentrations measured by the UWCPC, SMPS, and FMPS were conducted to evaluate the performance of the two particle sizers using ambient particles as well as lab generated artificial particles. The SMPS number concentration was substantially lower than the FMPS (FMPS/SMPS = 1.56) measurements mainly due to the diffusion losses of particles in the SMPS. The diffusion loss corrected SMPS (C-SMPS) number concentration was on average ～ 15% higher than the FMPS data (FMPS/C-SMPS = 0.87). Good correlation between the C-SMPS and FMPS was also observed for the total particle number concentrations in the size range 6 nm to 100 nm measured at a road-side urban site (r² = 0.91). However, the particle size distribution measured by the C-SMPS was quite different from the size distribution measured by the FMPS. An empirical correction factor for each size bin was obtained by comparing the FMPS data to size-segregated UWCPC number concentrations for atmospheric particles. The application of the correction factor to the FMPS data (C-FMPS) greatly improved the agreement of the C-SMPS and C-FMPS size distributions. The agreement of the total particle concentrations also improved to well within 10% (C-FMPS/C-SMPS = 0.95).     

An Ultrafine,Water-Based Condensation Particle Counter and its Evaluation under Field Conditions

An ultrafine, water-based condensation particle counter (U-WCPC, TSI Model 3786) has been compared to a butanol-based ultrafine counter (U-BCPC, TSI Model 3025) for measurement of atmospheric and freeway-tunnel aerosols. The U-WCPC utilizes a warm, wet-walled growth tube to activate and grow particles through water condensation in a laminar-flow. It has an aerosol sampling rate of 0.3 L/min, and a nominal detection limit near 3 nm. Several field comparisons were made to the butanol-based instrument with the same nominal detection limit. For measurements of size-selected aerosols with diameters of 5 nm and larger the two instruments generally agreed, with a mean response within 5%. At 3 nm particle size differences were observed, and these differences varied with the data set. Measurements of ambient aerosol in Boulder, Colorado showed higher counting efficiency at 3 nm with the U-BCPC, while in a California freeway tunnel the opposite trend was observed, with higher counting efficiencies at 3 nm observed by the U-WCPC. For direct measurement of atmospheric aerosols, the two types of instruments yielded equivalent concentrations, independent of particle number concentration.     

Laboratory Evaluation of a TSI Condensation Particle Counter (Model 3771) Under Airborne Measurement Conditions

The performance of a condensation particle counter (CPC, Model 3771, TSI Inc.), which has a nominal minimum detectable particle size (d ₅₀) of 10 nm, has been tested in the laboratory for the purpose of airborne measurements. First, the effects of particle coincidence at concentrations above the upper limit specified by the manufacturer (>10⁴ cm^-3 were evaluated. By applying a correction factor derived from experimental results, the CPC can quantify particle concentrations of as high as 5 × 10⁴ cm– 3. Second, the effects of inlet pressure (p) on the size dependence of the detection efficiency were investigated (particle diameter (d)= 8–100 nm, p= 1010–300 hPa). The asymptotic detection efficiency and d ₅₀ showed decreasing and increasing trends with decreasing pressure, respectively, especially at p < 600 hPa. It is likely reduction of the 1-butanol saturation ratio in the condenser at decreased pressures can explain the observed pressure dependence. Finally, the temporal variation of the detection efficiency during continuous operation of the CPC without the supply of 1-butanol was investigated (d= 10 and 100 nm, p = 1010, and 600 hPa). The detection efficiencies did not show significant change, at least over 6 h, without the supply of 1-butanol, which ensures stable performance of the CPC for flight durations of 4–5 h. Based on our laboratory evaluations, possible errors in airborne measurements were estimated assuming typical particle number size distributions of ambient aerosols.     

Characterization of a Modified Expansion Condensation Particle Counter for Detection of Nanometer-Sized Particles

A newly developed condensation particle counter provides measurements of aerosol particle number densities for size diameters as low as 3 nm. This Expansion Condensation Particle Counter (ECPC) operates based on fast adiabatic expansion with specialized detection and evaluation of the temporal development of light scattered by the ensemble of growing droplets. In its new configuration the ECPC has been modified such that a previously needed calibration factor became obsolete. In this article the new design is described which now includes a fast pressure sensor for monitoring the pressure drop inside the measurement chamber. Extensive laboratory experiments for characterizing the ECPC are described where sulfuric acid droplets with diameters between ～2.5 nm and 23 nm have been utilized. Water as well as butanol are demonstrated to be suitable working fluids. One experiment using tungsten oxide (WOx) particles shows that a 50% cut-off size diameter as low as 2.5 nm can be reached for this ECPC with a detection efficiency of several percent for particles as small as 1.4 nm. High and low supersaturations are experimentally examined and the corresponding different cut-off sizes are obtained. Measurements of ambient urban air in Mainz (Germany) obtained by this ECPC are juxtaposed to those from a TSI UCPC 3025A with satisfactory agreement. Similarly, in-situ data recorded with two ECPC units in the city of Isfahan (Iran) are shown to demonstrate the suitability of the technique for traffic related pollution measurements. Also, in future applications coarse information on the chemical nature of nucleated particles can be obtained by simultaneously using various condensing liquids in different channels of the ECPC setup.     

Calibration of the TSI 3760 Condensation Nucleus Counter for Nonstandard Operating Conditions

The counting efficiency of the TSI 3760 condensation nucleus counter was tested for operation at (a) reduced flow rates and (b)reduced pressures. Circumstances often dictate that these conditions are encountered in sampling atmospheric aerosols. Results indicate that the counting efficiency of the instrument for particles in the range of 0.02–0.1 μm in diameter is not attenuated in operation at flow rates between 0.2 and 1.4 L/min. Furthermore, there does not appear to be any attenuation in the instrument's counting efficiency when operated at pressures between roughly 250 mb and atmospheric pressure.     

Performance of TSI 3760 Condensation Nuclei Counter at Reduced Pressures and Flow Rates

This article describes an experimental study of the performance of the TSI model 3760 clean room condensation nuclei counter (CNC) at various pressures and flow rates. Studies were made to determine the counting efficiency of the instrument in the pressure range of 0.1–1 atm and flow rate range of 0.15–1.4 L/min. The counting efficiency curves were found to be shifted to larger particle sizes as the pressure or flow rate was reduced. The low pressure and low flow rate limits of the instrument were also determined.

The numerical model developed in our previous study (Zhang and Liu, 1990) was used to predict the performance of the CNC. The numerical results were compared with the experimental data and found to agree well in the pressure range of 0.2–1.0 atm and flow rate range of 0.3–1.4 L/min. Discrepancies were found to be more significant at the lower pressures and flow rates.     

Minimizing Concentration Effects in Water-Based,Laminar-Flow Condensation Particle Counters

Concentration effects in water condensation systems, such as used in the water-based condensation particle counter, are explored through numeric modeling and direct measurements. Modeling shows that the condensation heat release and vapor depletion associated with particle activation and growth lowers the peak supersaturation. At higher number concentrations, the diameter of the droplets formed is smaller, and the threshold particle size for activation is higher. This occurs in both cylindrical and parallel plate geometries. For water-based systems, we find that condensational heat release is more important than vapor depletion. We also find that concentration effects can be minimized through use of smaller tube diameters, or more closely spaced parallel plates. Experimental measurements of droplet diameter confirm modeling results.

© 2013 American Association for Aerosol Research     

Characterization of an Automated,Water-Based Expansion Condensation Nucleus Counter for Ultrafine Particles

The design and experimental characterization of a condensation nucleus counter (CNC) is presented. The counter produces supersaturation by means of fast volume-controlled adiabatic expansion. The aerosol number concentration is derived from observing scattered laser light in the forward direction under a solid angle between 1.1° and 4.4° over the full annular sector. The number concentration is derived by application of Mie theory from the characteristic pattern in the temporal evolution of the detected signal during the droplet growth process. The equation for calculation of the aerosol number density by this method is presented. Theoretical considerations for the smallest aerosol particles that can be activated indicate a lower size cut-off between 2.5 and 3.0 nm. Model calculations of the expected Mie scatter signal during expansion agree very well with the experimental observations. The Expansion-CNC can be operated fully automated under computer (PC) control in 10-second sample cycles. For characterization it is compared with a TSI 3025A Ultrafine-CPC (TSI UCPC) for measurements of monodisperse sodium chloride and sulfuric acid aerosol particles, indicating good agreement between the two counters down to particle sizes as low as 3.5 nm under laboratory conditions. In addition, ambient aerosol measurements in urban air show excellent agreement with simultaneous TSI UCPC measurements for particle number concentrations ranging from roughly 50 cm^{? 3} to 130000 cm^{? 3}.     

Causes of Concentration Differences Between a Scanning Mobility Particle Sizer and a Condensation Particle Counter

Accurate aerosol concentration measurement is important in many applications of aerosol science. Here we compare aerosol concentration measurements of classified NaCl aerosol in the size range of 20 to 80 nm (diameter) between a scanning mobility particle sizer (SMPS) and a condensation particle counter (CPC). The SMPS systematically measured higher concentrations than the CPC, with the difference increasing with decreasing particle size. Experiments suggest several causes for the discrepancy. First, the factory calibration of the SMPS impactor flow was incorrect for the study site at 780 mbar. Second, the neutralizer used in the SMPS was inefficient in bringing the classified aerosol to charge equilibrium, and third, there were significant losses of charged aerosol within the CPC. The comparisons were improved with proper impactor flow calibration and proper charge neutralization of the classified aerosol before measurement by the SMPS and CPC. The results of this study point to the importance of proper conditioning of aerosol below about 100 nm for measurement with the SMPS and condensation-based particle counters.     

Performance of a TSI Aerodynamic Particle Sizer

Calibration curves of the aerodynamic particle sizer (APS) under different sets of operating conditions (i.e., pressure drop across the nozzle, flow rate, and ambient pressure) were obtained. Materials used included oleic acid (OA), dioctyl phthalate (DOP), polystyrene latex (PSL), and fused aluminosilicate particles (FAP). The effect of particle density on the calibration was not found to be significant among test aerosols (in the density range from 0.89 to 2.3 g/cm³). Calibration curves obtained at reduced ambient pressure were different from the manufacturer's curve, indicating that recalibration of the APS is required if other than standard operating conditions are used. However, all the curves can be consolidated into a unique curve that relates the Stokes number at the nozzle exit to the normalized particle velocity (particle velocity divided by gas velocity). Methods for calculating gas velocity, particle velocity, and other pertinent parameters for the APS were developed and the results are presented. Consequently, these parameters together with the unique curve can be used to generate calibration curves for any set of operating conditions without performing the experimental calibration in the laboratory. The geometric standard deviations of monodisperse aerosols measured by the APS are generally in good agreement (< 2%) with those determined by other methods, thus demonstrating the good resolution of the instrument.     

Dependence of the Performance of TSI 3020 Condensation Nucleus Counter on Pressure,Flow Rate,and Temperature

A theoretical study has been carried out to investigate the performance of the TSI 3020 condensation nucleus counter (CNC) at various pressures and flow rates by assuming a parabolic velocity profile in the condenser tube and solving the heat and mass transfer equations using the finite difference method. Calculations have been performed for pressures ranging from 0.03 to 10 atm and sampling flow rates from 0.5 to 50 mL/s. The results indicate that the counting efficiency of the CNC is a function of pressure and flow rate due to changes in heat and mass transfer rates. The counting efficiency can be correlated with a single parameter, ζ, which combines the effects due to pressure, sampling flow rate, and the length and diameter of the condenser tube. The cut size of the instrument, Dp₅₀, defined as the particle size at which the counting efficiency is 50%, has been found to vary with pressures, reaching a minimum at a pressure of approximately 1 atm. The cut size of the CNC has been found to be most sensitive to the temperature difference between the saturator and condenser but relatively insensitive to the flow rate and the saturator temperature.     

Sizing Small Sulfuric Acid Particles with an Ultrafine Particle Condensation Nucleus Counter

The sizing capability of an ultrafine particle condensation nucleus counter (which uses butanol as the condensing fluid) equipped with pulse height analysis was evaluated in terms of particle composition for sulfuric acid aerosol and sulfuric acid aerosol to which gas-phase ammonia had been added. The response of the counter depended on composition for a range of particle sizes when the water partial pressure was low. For water partial pressures < 5 Torr and for particles > 4 nm in diameter, the response (pulse heights) of the instrument to particles of a given size was substantially different for sulfuric acid particles and those that were neutralized with ammonia. For water partial pressures > 5 Torr, however, neutralizing the particles with ammonia had little effect on pulse height distributions. For particles smaller than 4 nm diameter the pulse heights were insensitive to exposure to ammonia.     

Modification of Laminar Flow Ultrafine Condensation Particle Counters for the Enhanced Detection of 1 nm Condensation Nuclei

This paper describes simple modifications to thermally diffusive laminar flow ultrafine condensation particle counters (UCPCs) that allow detection of ～1 nm condensation nuclei with much higher efficiencies than have been previously reported. These non-destructive modifications were applied to a commercial butanol-based UCPC (TSI 3025A) and to a diethylene glycol-based UCPC (UMN DEG-UCPC). Size and charge dependent detection efficiencies using the modified UCPCs (BNL 3025A and BNL DEG-UCPC) were measured with high resolution mobility classified aerosols composed of NaCl, W, molecular ion standards of tetra-alkyl ammonium bromide, and neutralizer-generated ions. With negatively charged NaCl aerosol, the BNL 3025A and BNL DEG-UCPC achieved detection efficiencies of 37% (90× increase over TSI 3025A) at 1.68 nm mobility diameter (1.39 nm geometric diameter) and 23% (8× increase over UMN DEG-UCPC) at 1.19 nm mobility diameter (0.89 nm geometric diameter), respectively. Operating conditions for both UCPCs were identified that allowed negatively charged NaCl and W particles, but not negative ions of exactly the same mobility size, to be efficiently detected. This serendipitous material dependence, which is not fundamentally understood, suggests that vapor condensation might sometimes allow for the discrimination between air “ions” and charged “particles.” As a detector in a scanning mobility particle spectrometer (SMPS), a UCPC with this strong material dependence would allow for more accurate measurements of sub-2 nm aerosol size distributions due to the reduced interference from neutralizer-generated ions and atmospheric ions, and provide increased sensitivity for the determination of nucleation rates and initial particle growth rates.

Copyright 2012 American Association for Aerosol Research     

Particle counting efficiencies of new TSI condensation particle counters

In this study calibration measurements of new TSI condensation particle counter models, using either butanol or water as working fluid, are described. Experiments were carried our at ambient, laboratory conditions for two particle materials, silver and sodium chloride. The obtained counting efficiency curves are presented.     

The Detection Efficiency of the Single Particle Soot Photometer

A single particle soot photometer (SP2) uses an intense laser to heat individual aerosol particles of refractory black carbon (rBC) to vaporization, causing them to emit detectable amounts of thermal radiation that are used to quantify rBC mass. This approach is well suited for the detection of the majority of rBC mass loading in the ambient atmosphere, which occurs primarily in the accumulation mode (～ 1–300 fg-rBC/particle). In addition to operator choices about instrument parameters, SP2 detection of rBC number and/or mass can be limited by the physical process inherent in the SP2 detection technique — namely at small rBC mass or low laser intensities, particles fail to heat to vaporization, a requirement for proper detection. In this study, the SP2's ability to correctly detect and count individual flame-generated soot particles was measured at different laser intensities for different rBC particle masses. The flame-generated soot aerosol used for testing was optionally prepared with coatings of organic and non-organic material and/or thermally denuded. These data are used to identify a minimum laser intensity for accurate detection at sea level of total rBC mass in the accumulation mode (300 nW/(220-nm PSL)), a minimum rBC mass (～ 0.7-fg rBC-mass corresponding to 90 nm volume-equivalent diameter) for near-unity number detection efficiency with a typical operating laser intensity (450 nW/(220-nm PSL)), and a methodology using observed color temperature to recognize laser intensity insufficient for accurate rBC mass detection. Additionally, methods for measurement of laser intensity using either laboratory or ambient aerosol are presented.     

Moderated,Water-Based,Condensational Particle Growth in a Laminar Flow

Presented is a new approach for laminar-flow water condensation that produces saturations above 1.5 while maintaining temperatures of less than 30°C in the majority of the flow and providing an exiting dew point below 15°C. With the original laminar flow water condensation method, the particle activation and growth occurs in a region with warm, wetted walls throughout, which has the side-effect of heating the flow. The “moderated” approach presented here replaces this warm region with two sections—a short, warm, wet-walled “initiator,” followed by a cool-walled “moderator.” The initiator provides the water vapor that creates the supersaturation, while the moderator provides the time for particle growth. The combined length of the initiator and moderator sections is the same as that of the original, warm-walled growth section. Model results show that this new approach reduces the added heat and water vapor while achieving the same peak supersaturation and similar droplet growth. Experimental measurements confirm the trends predicted by the modeling.

Copyright 2014 American Association for Aerosol Research     

A Variable Supersaturation Condensation Particle Sizer

A prototype variable supersaturation condensation particle sizer (VSCoPS) capable of measuring particle size distributions from 5 to 30 nm has been developed. This system design is adapted from existing condensation particle counter (CPC) technology with three significant differences: (1) the working fluid is a perfluorinated organic compound that is nonreactive toward, and not an effective solvent for, most laboratory or ambient particle compositions; (2) the vapor pressure of the working fluid is controlled by dilution of saturated air with vapor-free air at the same temperature; and (3) the optical block and condenser are located below the saturator, so that fluid condensed on the condenser walls does not flow back toward the saturator. By using fast-response flow controllers to vary the ratio of saturator and dilution air while keeping total flows and temperatures constant, the vapor saturation ratio in the condenser can be controlled with time constants of ～ 1 s. The nucleation threshold diameter is changed by stepping through small increments in saturation ratio. The particle size distribution can be recovered by inverting the measured concentration using the known instrument response for each saturation ratio. Further development of the system may allow size distribution measurements to smaller particle diameters and scan times of < 30 s at total particle concentrations as low as ～ 100 cm^{? 3} .     

Ash Vaporization and Condensation During Combustion of a Suspended Coal Particle

The results of a theoretical study of the formation and growth of the submicron flyash aerosol around a single burning coal particle are presented. The vaporization of ash and subsequent aerosol formation near the coal particle are studied because the local combustion environment influences these processes strongly. A mathematical model is developed that describes the transport of ash vapor and and the growth of the aerosol. The ash aerosol calculation is superimposed on an existing solution to the combustion problem. Included in the model are the effects of convective transport and of both homogeneous and heterogeneous condensation of the ash vapor. The results of the calculations show that refractory compounds with low surface tension, like silica, nucleate very near the coal particle's surface and produce a substantial mass loading of aerosol. The presence of the aerosol does not greatly affect the ash vaporization rate, which is primarily a function of combustion conditions. The size and amount of the submicron ash aerosol are determined by both the local combustion conditions and the ash's physical properties.