- Email: [email protected]
air mixtures
Journal of Aerosol Science 60 (2013) 67–72
Contents lists available at SciVerse ScienceDirect
Journal of Aerosol Science journal homepage: www.elsevier.com/locate/jaerosci
Technical Note
Performance of water-based CPC 3788 for particles from a propane-flame soot-generator operated with rich fuel/air mixtures Alejandro Keller a,n, Torsten Tritscher b, Heinz Burtscher a a
Institute for Aerosol and Sensor Technology, University of Applied Sciences Northwestern Switzerland, Klosterzelgstrasse 2, 5210 Windisch, Switzerland TSI GmbH, Particle Instruments, Neuk¨ ollner Str. 4, 52068 Aachen, Germany
b
a r t i c l e i n f o
abstract
Article history: Received 9 November 2012 Received in revised form 14 February 2013 Accepted 15 February 2013 Available online 4 March 2013
The new TSI Inc. Nano Water-based Condensation Particle Counter 3788 (N-WCPC Model 3788) is designed for counting particles down to a mobility diameter of dp ¼ 2:5 nm. Published studies have shown that the use of water as a working fluid does not have a significant effect on the instrument performance. With the exception of a few specific substances, like emery oil or dioctyl sebacate which present a somewhat larger cut-point diameter, data in peer-reviewed literature suggests that the composition of the particles is not an issue as long as the particle diameter falls within the working range of the device. However, we have observed a very reduced counting efficiency from this water condensation particle counter for particles from a propane-rich fuel mixture (i.e. high carbon-to-oxygen ratio) generated by means of a Combustion Aerosol STandard burner (CAST). The percentage of detected particles decreases for an increasingly richer fuel mixture. The reduced efficiency is still present for particles well above the cut-point of the instrument. The 3788 N-WCPC counted only half as many dp ¼ 70 nm particles when compared to two butanol based Condensation Particle Counters (CPC, models 3025A and 3022A, TSI Inc.), and the efficiency was even lower for smaller particles: only 0:14 7 0:001 and 0:002 7 0:0001 for monodisperse particles with a mobility diameter of dp ¼ 30 nm and dp ¼ 15 nm, respectively. Preconditioning the polydisperse sample using a thermodenuder at 100 or even 250 1C slightly increased the counting efficiency of the N-WCPC, but this still remained below the counts of the butanol CPC. No reduction in counting efficiency was detected for CAST particles from propane-lean fuel-mixtures (otherwise known as black soot) or for organic particles produced by the photo-oxidation of a-pinene. For this reason, we believe that the effect is caused by the combination of the hydrocarbon rich composition of the particles and their low oxidation state. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Water condensation particle counter Counting efficiency Propane flame soot
1. Introduction Condensation particle counters (CPCs) have been commercially available since the 1970s as a way of detecting single particles with diameters that can extend below dp ¼ 100 nm. These instruments saturate an aerosol sample with a vapor, typically butanol, and then cool it down in a condenser tube where the particles are activated and grow to sizes over 1 mm.
n
Corresponding author. Tel.: þ41 564624575; fax: þ 41 564624245. E-mail addresses: [email protected], [email protected] (A. Keller).
0021-8502/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jaerosci.2013.02.005
68
A. Keller et al. / Journal of Aerosol Science 60 (2013) 67–72
The large resulting particles can then easily be counted using optical methods. A review of this technique can be found under Cheng (2011, Chapter 17). Modern CPCs are able to detect particles down to a few nanometers. The smallest-particle-size detection limit is defined as the point of 50% counting efficiency otherwise known as the cut-point diameter, D50. Close to that limit in the single nanometer range not all particles will be detected. Water based CPCs have been introduced recently (Hering & Stolzenburg, 2005; Hering et al., 2005). These instruments can use water as a working fluid thanks to their novel condensation chamber design. When first introduced, water based CPCs could reach a cut-point diameter of D50 ¼ 4:3 nm. Nowadays, a new commercial water CPC (3788 N-WCPC, TSI Inc.) can measure particles down to D50 ¼ 2:5 nm. A comparison between an older water based CPC and a butanol based CPC by Biswas et al. (2005) reveals that the two different technologies agree within the nominal 10% uncertainty of these instruments for single particle counting, and within 20% for high concentrations that require the use of the photometric mode. The smallest-particle-size detection limit of a CPC for organic substances generated in the laboratory might be somewhat higher than the value specified for the calibration material used by the manufacturer. Hering & Stolzenburg (2005) found the largest cut-point diameter (D50 ¼ 30 nm) for the older TSI-3785 water CPC when measuring dioctyl sebacate particles. The study was, however, not intended to be an exhaustive analysis for all possible materials. Up to our knowledge, clean emery oil (EO, polyalphaolefin, PAO 4 cSt) is the only substance that has been reported to have a significantly larger cut-point diameter (D50 ¼ 17:2 nm) for the 3788 N-WCPC (Kupc et al., 2013). EO is a highly branched isoparaffinic polyalphaolefin, has a low affinity for water, is non-soluble, and consists of 82–85% C30H60 and 13–16% C40H80 polyalphaolefins by volume (Giechaskiel et al., 2009). The same study by Kupc et al. (2013) found only a slight increase of the cut-point diameter (D50 ¼ 3:79 nm) when measuring aerosol from the steady combustion of a tea candle. We report on a large disagreement between a N-WCPC and commonly used butanol CPCs for a specific test aerosol. During an instrument comparison we discovered that the 3788 N-WCPC was counting only a fraction of the particles originating from a propane flame aerosol generator using a rich fuel/air mixture. Further measurements with a comprehensive setup showed that this was caused by an unexpected large cut-point diameter. The counting efficiency is already 50% for particles as large as dp ¼ 75 nm and even less for smaller diameters. Our results question the use of water based CPCs as an universal tool for combustion generated nanoparticles and show that butanol based CPCs are better suitable for CAST soot operated with rich fuel/air mixture.
2. Methods 2.1. Aerosol generator The flame generated soot was produced in a diffusion flame by means of a Combustion Aerosol STandard burner (CAST; model CAST-00-4, Jing Ltd.) using propane as a fuel. This aerosol generator has the flexibility of allowing an independent adjustment of all the gas flows of the system by means of mass flow controllers. By doing so, the CAST can produce particles of different mean diameters as well as different chemical compositions. A lean fuel mixture produces particles consisting off almost exclusively elemental carbon (EC), called ‘‘black’’ soot within this paper, whereas a rich mixture produces particles with high organic carbon (OC) content, identified here as ‘‘brown’’ soot. The different set points used throughout our experiments are listed in Table 1. All the black soot samples have the same carbon-to-oxygen ratio, C/O¼0.26, which represents an equivalence ratio of f ¼ 0:88. They differentiate themselves due to the premixed N2 in the fuel mixture, which affects the residence time in the flame and thus results in different mean diameters. Brown soot particles, on the other hand, differentiate themselves in size as well as in their C/O mixtures, which was always above stoichiometry. Schnaiter et al. (2006) found the organic-carbon-to-total-carbon ratio (OC/TC) of the resulting aerosol for similar CAST set points to be between OC/TC¼0.085 (C/O ¼0.29) and OC/TC ¼0.49 (C/O¼ 0.50). Secondary organic aerosol (SOA) particles were generated by evaporating a-pinene under a flow of zero-air and subsequently exposing this precursor in a continuous-flow reactor to the UV radiation from a 172 nm Xe2 excimer lamp. Table 1 CAST gas-flow set-points in lpm, equivalence ratio (f), carbon-to-oxygen ratio (C/O) of the fuel/air mixture, and geometrical mean diameter (GMD, in nm) and geometrical standard deviation (GSD) of the resulting particles size distribution for the propane flame experiments, arranged from the richest to the leanest flame. The heading also indicates if the gas is added prior to the combustion, premixed with the fuel, or used for quenching. The GMD of the samples was measured during the experiments described within this work using a NanoScan SMPS except for the points marked y, which were measured with the SMPS system described in the text. Set point
Air combustion
Propane
N2 premixed
N2 quenching
f
C/O
GMD
GSD
brown1 brown2 brown3 brown4 black1 black2
0.8 0.685 0.78 1. 1.534 1.534
0.0567 0.047 0.047 0.0567 0.0567 0.0567
0 0 0 0 0.26 0.3
7.622 6.47 6.47 7.622 7.622 7.622
1.69 1.63 1.43 1.35 0.88 0.88
0.51 0.49 0.43 0.41 0.26 0.26
44 37 46y 65 57y 52
1.6 1.6 1.9 1.7 1.8 1.6
A. Keller et al. / Journal of Aerosol Science 60 (2013) 67–72
69
Test Aerosol Rotating Disc Diluter
CAST Burner
Instruments
Bypass
2x N-WCPC, 3x Butanol CPC, 1x SMPS / NanoScan SMPS
Low-Flow Thermodenuder SOA Generator
Nano DMA
Open End, Filtered Air MFC
Excess-Air (undiluted)
Filtered Dilution-Air
Fig. 1. Schematic representation of the experimental setup. The box marked as MFC represents the mass flow controller that sets the clean/diluted-flow of the rotating disc diluter. The open end provides filtered air to compensate for the difference between the total flow of the instruments and the sample flow.
Table 2 List of the CPCs and N-WCPCs, and their operating conditions. The column labeled D50 represents the nominal cut-point of the instrument (i.e. the particle diameter at which only 50% of the aerosol particles are detected). The N-WCPC3788a was calibrated by the manufacturer prior to the measurements reported here. The last row shows the details for the NanoScan SMPS used to determine the size distribution of the samples. Identifier
Working fluid
D50 (nm)
Model
Manufacturer
Flow (lpm)
CPC3022 CPC3025 CPC3010 N-WCPC3788a N-WCPC3788b NanoScan SMPS
Butanol Butanol Butanol Water Water Isopropanol
7.0 3.0 10.0 2.5 2.5 10.0
CPC 3022A UCPC 3025A CPC 3010 N-WCPC 3788 N-WCPC 3788 NanoScan SMPS 3910
TSI TSI TSI TSI TSI TSI
1.5 1.5 1.0 1.5 1.5 0.75
Inc. Inc. Inc. Inc. Inc. Inc.
The 172 nm Xe2 lamp produces reactive species like O3 that oxidize the precursor molecule. The resulting molecules nucleate and condensate forming a highly oxidized organic aerosol. This process is similar to the formation of SOA in the atmosphere but at a much faster rate (seconds instead of hours). A description and details of the use of a similar reactor can be found in Keller & Burtscher (2012). 2.2. Experimental setup The experimental setup used in this study is shown in Fig. 1. First, a defined aerosol is produced by applying the methods described above. The test aerosol is then diluted by means of a rotating disc diluter (Matter Aerosol, model MD19). Depending on the type of experiment, the sample consists of (1) a polydisperse aerosol, when using the bypass line, (2) polydisperse but preconditioned in a low-flow thermodenuder (Fierz et al., 2012), or (3) monodisperse particles selected by means of Nano DMA (TSI Inc., model 3085). The dilution was chosen for a final concentration between 104 and 105 particles/cc for the polydisperse experiments and above 102 particles/cc for the monodisperse case. The mass flow controller regulating the flow of clean air into the diluter was set to 2 lpm when using the Nano DMA (sheath air 20 lpm), to 1 lpm for the thermodenuder and to 4.5 lpm for the bypass experiments. When the sample arrives to the instruments section, the particles are counted simultaneously by up to four different CPCs with a time resolution of 1 s. At the same time, a size distribution spectrum is registered every one or two minute by means of a NanoScan SMPS (TSI Inc., model 3910) or a SMPS system using a long DMA (TSI Inc., model 3081), respectively. The operating conditions and types of the CPCs are given in Table 2. The length of the tube connecting the different CPCs was selected to have the same residence time between the flow splitter and the classifiers. An open connection before the flow splitter provides filtered air to compensate for the difference between the total flow of the instruments and the sample flow. 3. Results and discussion Fig. 2 shows a comparison of the counting efficiency of the different CPCs and N-WCPCs. The data is normalized by the counts reported by the CPC3022. On the first part of the figure, it can be seen that the counts from the different butanol CPCs are very similar. The number concentration from the CPC3010 was between 80% and 100% of what was reported by the CPC3022, and the CPC3025 was within 99% and 127% of the CPC3022. The large width of the latter interval is caused by a single data point with a large uncertainty, corresponding to the black1 sample. Without this sample, the number concentration measured by the CPC3025 was within 107–121% of the values from the CPC3022.
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A. Keller et al. / Journal of Aerosol Science 60 (2013) 67–72
Fig. 2. Counting efficiency of two N-WCPC3788 and 2 butanol-based CPCs, when compared against the counts from a CPC3022 for different polydisperse soot-samples from a CAST burner. (a) shows experiments with the raw samples and (b) and (c) show experiments with samples preconditioned in a lowflow thermodenuder. The error bars represent the standard error of the measurement. The N-WCPC3788a was not available for the two samples marked with n (i.e. black1n and brown3n). (a) Without thermodenuder, (b) thermodenuer at 250 1C and (c) thermodenuer at 100 1C
It can be said that the agreement between the three butanol CPCs is very good. The nominal accuracy of the instruments is 10% and the trend in absolute counts can be explained by the different cut-point diameters of the three models: The CPC3010 has the largest cut-point diameter and presents the lowest counts, whereas the CPC3025 has the lowest cut-point diameter and presents the highest counts. Following the same argument for the cut-point diameter, we would expect to have a similar or higher particle count for the two N-WCPCs. However, the N-WCPCs reported always lower counts for the brown soot samples when compared to the butanol based CPCs. The best agreement was for the brown4 sample, where the N-WCPCs detected only 20% of the particles counted by the CPC3022. The counts were the lowest, less than 5%, for the brown2 sample. The counting efficiency for the polydisperse brown soot was generally lower for increasingly higher equivalence ratio, f. The only exception, the brown1 sample, showed counts that were comparatively higher than for brown2. As we will see below, this difference can be explained by the slightly larger average diameter of the brown1 sample (see Table 1). The low counting efficiency was not observed for any of the black soot samples. The setup with the thermodenuder was intended for finding out if the difference in temperature of the saturator chamber from the N-WCPC is responsible for the lower counting efficiency. The saturator of the N-WCPC operates at a higher temperature than the butanol CPCs (701 instead of 35 1C). The higher temperature may change the partition of the volatile substances through desorption of material from the aerosol phase. If this was the case, the difference in counting efficiency should disappear when preconditioning the sample in a thermodenuder at a temperature above the working temperature of the N-WCPC. This is, however, not the case as can be seen in Fig. 2b and c. After preconditioning the sample, the counts of the N-WCPC and the butanol CPC were closer. However, the counting efficiency of the N-WCPC still remained well below 50% for the brown2 sample even when using the thermodenuder at 250 1C. The average size of the aerosol, as measured by the NanoScan SMPS, was slightly reduced by preconditioning for the brown2 sample (from the original GMD¼37 down to GMD ¼32 nm at 250 1C). The aerosol size remained the same for the brown4 sample, which has a lower OC fraction. The reduced counts can hardly be subscribed to an instrument malfunction. The two instruments agreed well with each other, the difference between them was in general not statistically significant, and one of them had been recently calibrated by the manufacturer. Further insight can be obtained when measuring size selected, quasi-monodisperse particles. Fig. 3a shows an experiment with brown2 soot. This sample was selected because it presented the lowest counting efficiency during the polydisperse experiment. The counting efficiency of the N-WCPCs was already below 50% for the largest measured size (i.e. dp ¼ 75 nm) and even less for smaller diameters. The counts from the N-WCPCs were only about 0.14 70.001 and 0.00270.0001 of the counts reported by the CPC3022 for particles with dp ¼ 30 and dp ¼ 15 nm, respectively. The counts of the other butanol CPC (i.e. the CPC3025) were always in excellent agreement with the CPC3022.
A. Keller et al. / Journal of Aerosol Science 60 (2013) 67–72
71
Fig. 3. Counting efficiency of two N-WCPC3788 and the CPC3025, when compared against the counts from a CPC3022 for size selected particles. (a) shows the experiments with brown2 soot from the CAST burner, whereas (b) shows particles from the photo-oxidation of a-pinene. Note the difference in scale, which is logarithmic on (a) and linear on (b). The error bars represent the standard error of the measurement. (a) brown2 and (b) SOA from a–pinene.
As mentioned above, this steep drop in counting efficiency at large sizes explains why the polydisperse sample with the largest f, i.e. brown1, is counted more efficiently than brown2: Samples with higher f have lower counting efficiencies but, as can be seen in Table 1, brown2 particles have a size distribution with smaller GMD and are thus less likely to be detected by the N-WCPC. Finally we show in Fig. 3b an example of particles consisting of pure OC that are well detected by the N-WCPC. We can see that the SOA aerosol is counted equally by the butanol and the water CPCs and that the counts are well within the 10% accuracy of the devices. Oxidized hydrocarbons are hygroscopic, and a-pinene SOA is produced by the oxidation of the precursor molecule whereas brown soot, produced by means of a propane-rich fuel-mixture, is probably not oxidized at all and therefore likely to be rather hydrophobic. 4. Conclusions We observed a very reduced counting efficiency for a N-WCPC 3788 when measuring soot particles from a propane diffusion-flame using a rich fuel/air mixture produced by means of a CAST burner. The counting efficiency decreased for an increasingly richer propane-mixture, which is known to produce particles with a higher organic carbon content (i.e. ‘‘brown’’ soot). On the other hand, black-soot particles created by a lean mixture, and thus consisting mainly of elemental carbon, and SOA particles produced by the photo-oxidation of a-pinene were detected with the same efficiency than with the butanol-based CPCs. Preconditioning the polydisperse brown soot samples by means of a thermodenuder using temperatures up to 250 1C increased the counting efficiency, but this still remained well below the counts of the butanol based CPCs. The effect is caused by an extremely high cut-point diameter. For the monodisperse brown2 sample, the N-WCPC counted only half of the dp ¼ 75 nm particles detected by the butanol CPC, and only 0.14 70.001 and 0.0027 0.0001 for monodisperse particles with dp ¼ 30 nm and dp ¼ 15 nm, respectively. As a comparison, the nominal cut-point diameter of the N-WCPC 3788 is D50 ¼ 2:5 nm. We believe that the low counting efficiency is a consequence of the hydrocarbon rich composition of brown soot together with its very low oxidation state. Our results question the use of water based CPCs as an universal tool for combustion generated nanoparticles and show that butanol based CPCs are better suitable for CAST soot operated with rich fuel/air mixture.
Acknowledgments The authors would like to thank Hans Christen from DELTATECH AG for his support and for providing a N-WCPC 3788 for this measurements. References Biswas, S., Fine, P.M., Geller, M.D., Hering, S.V., & Sioutas, C. (2005). Performance evaluation of a recently developed water-based condensation particle counter. Aerosol Science and Technology, 39(5), 419–427. URL /http://www.tandfonline.com/doi/abs/10.1080/027868290953173S.
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Cheng, Y.-S. (2011). Condensation particle counters. In: Aerosol measurement: Principles, techniques, and applications (3rd ed.). John Wiley & Sons: Hoboken, New Jersey, pp. 381–392. Fierz, M., Vernooij, M.G., & Burtscher, H. (2007). An improved low-flow thermodenuder. Journal of Aerosol Science, 38(11), 1163–1168. URL /http://www. sciencedirect.com/science/article/pii/S0021850207001383S. Giechaskiel, B., Wang, X., Horn, H.-G., Spielvogel, J., Gerhart, C., Southgate, J., Jing, L., Kasper, M., Drossinos, Y., & Krasenbrink, A. (2009). Calibration of condensation particle counters for legislated vehicle number emission measurements. Aerosol Science and Technology, 43(12), 1164–1173. URL /http://www.tandfonline.com/doi/abs/10.1080/02786820903242029S. Hering, S.V., & Stolzenburg, M.R. (2005). A method for particle size amplification by water condensation in a laminar, thermally diffusive flow. Aerosol Science and Technology, 39(5), 428–436. URL /http://www.tandfonline.com/doi/abs/10.1080/027868290953416S. Hering, S.V., Stolzenburg, M.R., Quant, F.R., Oberreit, D.R., & Keady, P.B. (2005). A laminar-flow, water-based condensation particle counter (WCPC). Aerosol Science and Technology, 39(7), 659–672. URL /http://www.tandfonline.com/doi/abs/10.1080/02786820500182123S. Keller, A., & Burtscher, H. (2012). A continuous photo-oxidation flow reactor for a defined measurement of the SOA formation potential of wood burning emissions. Journal of Aerosol Science, 49(0), 9–20. URL /http://www.sciencedirect.com/science/article/pii/S0021850212000468S. Kupc, A., Bischof, O., Tritscher, T., Beeston, M., Krinke, T., & Wagner, P.E. (2013). Laboratory characterization of a new nano-water-based CPC 3788 and performance comparison to an ultrafine butanol-based CPC 3776. Aerosol Science and Technology, 47(2), 183–191. URL /http://www.tandfonline.com/ doi/abs/10.1080/02786826.2012.738317S. ¨ Schnaiter, M., Gimmler, M., Llamas, I., Linke, C., Jager, C., & Mutschke, H. (2006). Strong spectral dependence of light absorption by organic carbon particles formed by propane combustion. Atmospheric Chemistry and Physics, 6(10), 2981–2990. URL /http://www.atmos-chem-phys.net/6/2981/2006/S.
Contents lists available at SciVerse ScienceDirect
Journal of Aerosol Science journal homepage: www.elsevier.com/locate/jaerosci
Technical Note
Performance of water-based CPC 3788 for particles from a propane-flame soot-generator operated with rich fuel/air mixtures Alejandro Keller a,n, Torsten Tritscher b, Heinz Burtscher a a
Institute for Aerosol and Sensor Technology, University of Applied Sciences Northwestern Switzerland, Klosterzelgstrasse 2, 5210 Windisch, Switzerland TSI GmbH, Particle Instruments, Neuk¨ ollner Str. 4, 52068 Aachen, Germany
b
a r t i c l e i n f o
abstract
Article history: Received 9 November 2012 Received in revised form 14 February 2013 Accepted 15 February 2013 Available online 4 March 2013
The new TSI Inc. Nano Water-based Condensation Particle Counter 3788 (N-WCPC Model 3788) is designed for counting particles down to a mobility diameter of dp ¼ 2:5 nm. Published studies have shown that the use of water as a working fluid does not have a significant effect on the instrument performance. With the exception of a few specific substances, like emery oil or dioctyl sebacate which present a somewhat larger cut-point diameter, data in peer-reviewed literature suggests that the composition of the particles is not an issue as long as the particle diameter falls within the working range of the device. However, we have observed a very reduced counting efficiency from this water condensation particle counter for particles from a propane-rich fuel mixture (i.e. high carbon-to-oxygen ratio) generated by means of a Combustion Aerosol STandard burner (CAST). The percentage of detected particles decreases for an increasingly richer fuel mixture. The reduced efficiency is still present for particles well above the cut-point of the instrument. The 3788 N-WCPC counted only half as many dp ¼ 70 nm particles when compared to two butanol based Condensation Particle Counters (CPC, models 3025A and 3022A, TSI Inc.), and the efficiency was even lower for smaller particles: only 0:14 7 0:001 and 0:002 7 0:0001 for monodisperse particles with a mobility diameter of dp ¼ 30 nm and dp ¼ 15 nm, respectively. Preconditioning the polydisperse sample using a thermodenuder at 100 or even 250 1C slightly increased the counting efficiency of the N-WCPC, but this still remained below the counts of the butanol CPC. No reduction in counting efficiency was detected for CAST particles from propane-lean fuel-mixtures (otherwise known as black soot) or for organic particles produced by the photo-oxidation of a-pinene. For this reason, we believe that the effect is caused by the combination of the hydrocarbon rich composition of the particles and their low oxidation state. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Water condensation particle counter Counting efficiency Propane flame soot
1. Introduction Condensation particle counters (CPCs) have been commercially available since the 1970s as a way of detecting single particles with diameters that can extend below dp ¼ 100 nm. These instruments saturate an aerosol sample with a vapor, typically butanol, and then cool it down in a condenser tube where the particles are activated and grow to sizes over 1 mm.
n
Corresponding author. Tel.: þ41 564624575; fax: þ 41 564624245. E-mail addresses: [email protected], [email protected] (A. Keller).
0021-8502/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jaerosci.2013.02.005
68
A. Keller et al. / Journal of Aerosol Science 60 (2013) 67–72
The large resulting particles can then easily be counted using optical methods. A review of this technique can be found under Cheng (2011, Chapter 17). Modern CPCs are able to detect particles down to a few nanometers. The smallest-particle-size detection limit is defined as the point of 50% counting efficiency otherwise known as the cut-point diameter, D50. Close to that limit in the single nanometer range not all particles will be detected. Water based CPCs have been introduced recently (Hering & Stolzenburg, 2005; Hering et al., 2005). These instruments can use water as a working fluid thanks to their novel condensation chamber design. When first introduced, water based CPCs could reach a cut-point diameter of D50 ¼ 4:3 nm. Nowadays, a new commercial water CPC (3788 N-WCPC, TSI Inc.) can measure particles down to D50 ¼ 2:5 nm. A comparison between an older water based CPC and a butanol based CPC by Biswas et al. (2005) reveals that the two different technologies agree within the nominal 10% uncertainty of these instruments for single particle counting, and within 20% for high concentrations that require the use of the photometric mode. The smallest-particle-size detection limit of a CPC for organic substances generated in the laboratory might be somewhat higher than the value specified for the calibration material used by the manufacturer. Hering & Stolzenburg (2005) found the largest cut-point diameter (D50 ¼ 30 nm) for the older TSI-3785 water CPC when measuring dioctyl sebacate particles. The study was, however, not intended to be an exhaustive analysis for all possible materials. Up to our knowledge, clean emery oil (EO, polyalphaolefin, PAO 4 cSt) is the only substance that has been reported to have a significantly larger cut-point diameter (D50 ¼ 17:2 nm) for the 3788 N-WCPC (Kupc et al., 2013). EO is a highly branched isoparaffinic polyalphaolefin, has a low affinity for water, is non-soluble, and consists of 82–85% C30H60 and 13–16% C40H80 polyalphaolefins by volume (Giechaskiel et al., 2009). The same study by Kupc et al. (2013) found only a slight increase of the cut-point diameter (D50 ¼ 3:79 nm) when measuring aerosol from the steady combustion of a tea candle. We report on a large disagreement between a N-WCPC and commonly used butanol CPCs for a specific test aerosol. During an instrument comparison we discovered that the 3788 N-WCPC was counting only a fraction of the particles originating from a propane flame aerosol generator using a rich fuel/air mixture. Further measurements with a comprehensive setup showed that this was caused by an unexpected large cut-point diameter. The counting efficiency is already 50% for particles as large as dp ¼ 75 nm and even less for smaller diameters. Our results question the use of water based CPCs as an universal tool for combustion generated nanoparticles and show that butanol based CPCs are better suitable for CAST soot operated with rich fuel/air mixture.
2. Methods 2.1. Aerosol generator The flame generated soot was produced in a diffusion flame by means of a Combustion Aerosol STandard burner (CAST; model CAST-00-4, Jing Ltd.) using propane as a fuel. This aerosol generator has the flexibility of allowing an independent adjustment of all the gas flows of the system by means of mass flow controllers. By doing so, the CAST can produce particles of different mean diameters as well as different chemical compositions. A lean fuel mixture produces particles consisting off almost exclusively elemental carbon (EC), called ‘‘black’’ soot within this paper, whereas a rich mixture produces particles with high organic carbon (OC) content, identified here as ‘‘brown’’ soot. The different set points used throughout our experiments are listed in Table 1. All the black soot samples have the same carbon-to-oxygen ratio, C/O¼0.26, which represents an equivalence ratio of f ¼ 0:88. They differentiate themselves due to the premixed N2 in the fuel mixture, which affects the residence time in the flame and thus results in different mean diameters. Brown soot particles, on the other hand, differentiate themselves in size as well as in their C/O mixtures, which was always above stoichiometry. Schnaiter et al. (2006) found the organic-carbon-to-total-carbon ratio (OC/TC) of the resulting aerosol for similar CAST set points to be between OC/TC¼0.085 (C/O ¼0.29) and OC/TC ¼0.49 (C/O¼ 0.50). Secondary organic aerosol (SOA) particles were generated by evaporating a-pinene under a flow of zero-air and subsequently exposing this precursor in a continuous-flow reactor to the UV radiation from a 172 nm Xe2 excimer lamp. Table 1 CAST gas-flow set-points in lpm, equivalence ratio (f), carbon-to-oxygen ratio (C/O) of the fuel/air mixture, and geometrical mean diameter (GMD, in nm) and geometrical standard deviation (GSD) of the resulting particles size distribution for the propane flame experiments, arranged from the richest to the leanest flame. The heading also indicates if the gas is added prior to the combustion, premixed with the fuel, or used for quenching. The GMD of the samples was measured during the experiments described within this work using a NanoScan SMPS except for the points marked y, which were measured with the SMPS system described in the text. Set point
Air combustion
Propane
N2 premixed
N2 quenching
f
C/O
GMD
GSD
brown1 brown2 brown3 brown4 black1 black2
0.8 0.685 0.78 1. 1.534 1.534
0.0567 0.047 0.047 0.0567 0.0567 0.0567
0 0 0 0 0.26 0.3
7.622 6.47 6.47 7.622 7.622 7.622
1.69 1.63 1.43 1.35 0.88 0.88
0.51 0.49 0.43 0.41 0.26 0.26
44 37 46y 65 57y 52
1.6 1.6 1.9 1.7 1.8 1.6
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Test Aerosol Rotating Disc Diluter
CAST Burner
Instruments
Bypass
2x N-WCPC, 3x Butanol CPC, 1x SMPS / NanoScan SMPS
Low-Flow Thermodenuder SOA Generator
Nano DMA
Open End, Filtered Air MFC
Excess-Air (undiluted)
Filtered Dilution-Air
Fig. 1. Schematic representation of the experimental setup. The box marked as MFC represents the mass flow controller that sets the clean/diluted-flow of the rotating disc diluter. The open end provides filtered air to compensate for the difference between the total flow of the instruments and the sample flow.
Table 2 List of the CPCs and N-WCPCs, and their operating conditions. The column labeled D50 represents the nominal cut-point of the instrument (i.e. the particle diameter at which only 50% of the aerosol particles are detected). The N-WCPC3788a was calibrated by the manufacturer prior to the measurements reported here. The last row shows the details for the NanoScan SMPS used to determine the size distribution of the samples. Identifier
Working fluid
D50 (nm)
Model
Manufacturer
Flow (lpm)
CPC3022 CPC3025 CPC3010 N-WCPC3788a N-WCPC3788b NanoScan SMPS
Butanol Butanol Butanol Water Water Isopropanol
7.0 3.0 10.0 2.5 2.5 10.0
CPC 3022A UCPC 3025A CPC 3010 N-WCPC 3788 N-WCPC 3788 NanoScan SMPS 3910
TSI TSI TSI TSI TSI TSI
1.5 1.5 1.0 1.5 1.5 0.75
Inc. Inc. Inc. Inc. Inc. Inc.
The 172 nm Xe2 lamp produces reactive species like O3 that oxidize the precursor molecule. The resulting molecules nucleate and condensate forming a highly oxidized organic aerosol. This process is similar to the formation of SOA in the atmosphere but at a much faster rate (seconds instead of hours). A description and details of the use of a similar reactor can be found in Keller & Burtscher (2012). 2.2. Experimental setup The experimental setup used in this study is shown in Fig. 1. First, a defined aerosol is produced by applying the methods described above. The test aerosol is then diluted by means of a rotating disc diluter (Matter Aerosol, model MD19). Depending on the type of experiment, the sample consists of (1) a polydisperse aerosol, when using the bypass line, (2) polydisperse but preconditioned in a low-flow thermodenuder (Fierz et al., 2012), or (3) monodisperse particles selected by means of Nano DMA (TSI Inc., model 3085). The dilution was chosen for a final concentration between 104 and 105 particles/cc for the polydisperse experiments and above 102 particles/cc for the monodisperse case. The mass flow controller regulating the flow of clean air into the diluter was set to 2 lpm when using the Nano DMA (sheath air 20 lpm), to 1 lpm for the thermodenuder and to 4.5 lpm for the bypass experiments. When the sample arrives to the instruments section, the particles are counted simultaneously by up to four different CPCs with a time resolution of 1 s. At the same time, a size distribution spectrum is registered every one or two minute by means of a NanoScan SMPS (TSI Inc., model 3910) or a SMPS system using a long DMA (TSI Inc., model 3081), respectively. The operating conditions and types of the CPCs are given in Table 2. The length of the tube connecting the different CPCs was selected to have the same residence time between the flow splitter and the classifiers. An open connection before the flow splitter provides filtered air to compensate for the difference between the total flow of the instruments and the sample flow. 3. Results and discussion Fig. 2 shows a comparison of the counting efficiency of the different CPCs and N-WCPCs. The data is normalized by the counts reported by the CPC3022. On the first part of the figure, it can be seen that the counts from the different butanol CPCs are very similar. The number concentration from the CPC3010 was between 80% and 100% of what was reported by the CPC3022, and the CPC3025 was within 99% and 127% of the CPC3022. The large width of the latter interval is caused by a single data point with a large uncertainty, corresponding to the black1 sample. Without this sample, the number concentration measured by the CPC3025 was within 107–121% of the values from the CPC3022.
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Fig. 2. Counting efficiency of two N-WCPC3788 and 2 butanol-based CPCs, when compared against the counts from a CPC3022 for different polydisperse soot-samples from a CAST burner. (a) shows experiments with the raw samples and (b) and (c) show experiments with samples preconditioned in a lowflow thermodenuder. The error bars represent the standard error of the measurement. The N-WCPC3788a was not available for the two samples marked with n (i.e. black1n and brown3n). (a) Without thermodenuder, (b) thermodenuer at 250 1C and (c) thermodenuer at 100 1C
It can be said that the agreement between the three butanol CPCs is very good. The nominal accuracy of the instruments is 10% and the trend in absolute counts can be explained by the different cut-point diameters of the three models: The CPC3010 has the largest cut-point diameter and presents the lowest counts, whereas the CPC3025 has the lowest cut-point diameter and presents the highest counts. Following the same argument for the cut-point diameter, we would expect to have a similar or higher particle count for the two N-WCPCs. However, the N-WCPCs reported always lower counts for the brown soot samples when compared to the butanol based CPCs. The best agreement was for the brown4 sample, where the N-WCPCs detected only 20% of the particles counted by the CPC3022. The counts were the lowest, less than 5%, for the brown2 sample. The counting efficiency for the polydisperse brown soot was generally lower for increasingly higher equivalence ratio, f. The only exception, the brown1 sample, showed counts that were comparatively higher than for brown2. As we will see below, this difference can be explained by the slightly larger average diameter of the brown1 sample (see Table 1). The low counting efficiency was not observed for any of the black soot samples. The setup with the thermodenuder was intended for finding out if the difference in temperature of the saturator chamber from the N-WCPC is responsible for the lower counting efficiency. The saturator of the N-WCPC operates at a higher temperature than the butanol CPCs (701 instead of 35 1C). The higher temperature may change the partition of the volatile substances through desorption of material from the aerosol phase. If this was the case, the difference in counting efficiency should disappear when preconditioning the sample in a thermodenuder at a temperature above the working temperature of the N-WCPC. This is, however, not the case as can be seen in Fig. 2b and c. After preconditioning the sample, the counts of the N-WCPC and the butanol CPC were closer. However, the counting efficiency of the N-WCPC still remained well below 50% for the brown2 sample even when using the thermodenuder at 250 1C. The average size of the aerosol, as measured by the NanoScan SMPS, was slightly reduced by preconditioning for the brown2 sample (from the original GMD¼37 down to GMD ¼32 nm at 250 1C). The aerosol size remained the same for the brown4 sample, which has a lower OC fraction. The reduced counts can hardly be subscribed to an instrument malfunction. The two instruments agreed well with each other, the difference between them was in general not statistically significant, and one of them had been recently calibrated by the manufacturer. Further insight can be obtained when measuring size selected, quasi-monodisperse particles. Fig. 3a shows an experiment with brown2 soot. This sample was selected because it presented the lowest counting efficiency during the polydisperse experiment. The counting efficiency of the N-WCPCs was already below 50% for the largest measured size (i.e. dp ¼ 75 nm) and even less for smaller diameters. The counts from the N-WCPCs were only about 0.14 70.001 and 0.00270.0001 of the counts reported by the CPC3022 for particles with dp ¼ 30 and dp ¼ 15 nm, respectively. The counts of the other butanol CPC (i.e. the CPC3025) were always in excellent agreement with the CPC3022.
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Fig. 3. Counting efficiency of two N-WCPC3788 and the CPC3025, when compared against the counts from a CPC3022 for size selected particles. (a) shows the experiments with brown2 soot from the CAST burner, whereas (b) shows particles from the photo-oxidation of a-pinene. Note the difference in scale, which is logarithmic on (a) and linear on (b). The error bars represent the standard error of the measurement. (a) brown2 and (b) SOA from a–pinene.
As mentioned above, this steep drop in counting efficiency at large sizes explains why the polydisperse sample with the largest f, i.e. brown1, is counted more efficiently than brown2: Samples with higher f have lower counting efficiencies but, as can be seen in Table 1, brown2 particles have a size distribution with smaller GMD and are thus less likely to be detected by the N-WCPC. Finally we show in Fig. 3b an example of particles consisting of pure OC that are well detected by the N-WCPC. We can see that the SOA aerosol is counted equally by the butanol and the water CPCs and that the counts are well within the 10% accuracy of the devices. Oxidized hydrocarbons are hygroscopic, and a-pinene SOA is produced by the oxidation of the precursor molecule whereas brown soot, produced by means of a propane-rich fuel-mixture, is probably not oxidized at all and therefore likely to be rather hydrophobic. 4. Conclusions We observed a very reduced counting efficiency for a N-WCPC 3788 when measuring soot particles from a propane diffusion-flame using a rich fuel/air mixture produced by means of a CAST burner. The counting efficiency decreased for an increasingly richer propane-mixture, which is known to produce particles with a higher organic carbon content (i.e. ‘‘brown’’ soot). On the other hand, black-soot particles created by a lean mixture, and thus consisting mainly of elemental carbon, and SOA particles produced by the photo-oxidation of a-pinene were detected with the same efficiency than with the butanol-based CPCs. Preconditioning the polydisperse brown soot samples by means of a thermodenuder using temperatures up to 250 1C increased the counting efficiency, but this still remained well below the counts of the butanol based CPCs. The effect is caused by an extremely high cut-point diameter. For the monodisperse brown2 sample, the N-WCPC counted only half of the dp ¼ 75 nm particles detected by the butanol CPC, and only 0.14 70.001 and 0.0027 0.0001 for monodisperse particles with dp ¼ 30 nm and dp ¼ 15 nm, respectively. As a comparison, the nominal cut-point diameter of the N-WCPC 3788 is D50 ¼ 2:5 nm. We believe that the low counting efficiency is a consequence of the hydrocarbon rich composition of brown soot together with its very low oxidation state. Our results question the use of water based CPCs as an universal tool for combustion generated nanoparticles and show that butanol based CPCs are better suitable for CAST soot operated with rich fuel/air mixture.
Acknowledgments The authors would like to thank Hans Christen from DELTATECH AG for his support and for providing a N-WCPC 3788 for this measurements. References Biswas, S., Fine, P.M., Geller, M.D., Hering, S.V., & Sioutas, C. (2005). Performance evaluation of a recently developed water-based condensation particle counter. Aerosol Science and Technology, 39(5), 419–427. URL /http://www.tandfonline.com/doi/abs/10.1080/027868290953173S.
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