the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 Nov 2023
Influence of soot aerosol properties on the counting efficiency of PN-PTI instruments
Abstract. In this work, we investigated the influence of different types of soot aerosol on the counting efficiency (CE) of instruments employed for the periodic technical inspection (PTI) of diesel vehicles. Such instruments report particle number (PN) concentration. Combustion aerosols were generated by a prototype bigCAST, a miniCAST 5201 BC, a miniCAST 6204 C and a miniature inverted soot generator (MISG). For comparison purposes, diesel soot was generated by a Euro 5b diesel test vehicle with by-passed diesel particulate filter (DPF). The size-dependent counting efficiency profile of six PN-PTI instruments was determined with each one of the aforementioned test aerosols. The results showed that the type of soot aerosol affected the response of the PN-PTI sensors in an individualised manner. Consequently, it was difficult to identify trends and draw conclusive results about which laboratory-generated soot is the best proxy for diesel soot. Deviations in the counting efficiency remained typically within 0.25 units when using laboratory-generated soot compared to Euro 5b diesel soot of similar mobility diameter (~50–60 nm). Soot with a mobility diameter of ~100 nm generated by the MISG, the lowest size we could achieve, resulted in similar counting efficiencies as that generated by the different CAST generators for most of the PN-PTI instruments, showing that MISG may be a satisfactory – and affordable- option for PN-PTI verification.
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CC1: 'Comment on ar-2023-16', O. F. Bischof, 16 Nov 2023
This manuscript is topical as several countries of Europe have started to implement new periodic technical inspections (PTI) in their vehicle emission legislations. It describes an important topic related to the continued adoption and prospective harmonization of the particle number or PN-PTI test of vehicles. It is a comprehensive study and a very well written and insightful paper.
Main comments:
- The authors state that “In this study, we challenged six different DC-based PN-PTI instruments…”. Please explain why you chose to only investigate DC-based instruments and not complement their performance with CPC-based PN-PTI models such as the MAHLE PMU 400 (respectively the Brainbee PMU-400) or the BEA 090 made by Robert Bosch.
- It was surprising to see that in case of the MISG, only particles with a GMD of 100 nm were generated, so only one particle size distribution. Have you tried to operate it with a different air/fuel ratio (AFR) or with a fuel other than the mixture of dimethyl ether and propane? It seems odd to see only one data point for the MISG, e.g. in Fig. 3. Note that Bischof et al. (2019) have used the MISG as a calibration aerosol source to determine the counting efficiency of CPCs with DMA-selected particles down to the small nanoparticle size range.
- You state that mobility particle size distributions were measured simultaneously by SMPS, but you do not show any. A particle size distribution of the test aerosol would be interesting to see, e.g. to show the full size range, its concentrations as well as its mode. As is, we can only assume the aerosol was mono-modal and did not have a nucleation mode (which PN-PTI instruments might experience in real life tests).
- The CAP 3070 instrument is based on the so-called escaping current principle, so it detects the current leaving its sensor on charged particles rather than measuring the total net charge carried by particles after collection on a diffusion screen as most DC-sensors do; see Lehtimäki (1983). Could this difference in measurement principle also be a reason for the larger difference in the counting efficiency observed in chapter 3.2, in addition to “an overestimated internal correction factor”?
References
Bischof, O., Weber, P., et al. (2019). Characterization of the Miniaturized Inverted Flame Burner as a Combustion Source to Generate a Nanoparticle Calibration Aerosol, Emiss. Control Sci. Technol., 6, 37–46.
Lehtimäki, M. (1983). Modified Electrical Aerosol Detector. In: Aerosols in the Mining and Industrial Work Environments, Vol. 3, Ann Arbor Science Publishers, 1135–1143.
Citation: https://doi.org/10.5194/ar-2023-16-CC1 - AC1: 'Reply on CC1', Konstantina Vasilatou, 20 Nov 2023
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CC2: 'Comment on ar-2023-16', Martin Fierz, 26 Dec 2023
Disclosure: I am co-founder of naneos, we build the HEPaC, one of the six devices mentioned in this paper. We also supplied the a similar sensor for PN-PEMS applications a few years before PN-PTI.
This paper nicely shows the effect of using different soot generators on the measured particle counting efficiency for six different instruments based on diffusion charging, something that we know well from the development of such sensors. In my opinion, some more background would be helpful for a better understanding of the observations in the paper, in particular:
1) The influence of particle morphology on diffusion charging (unipolar and bipolar) is well known, so it is obvious that instruments based on diffusion charging will - at least potentially - be sensitive to particle morphology (you should add one or more references to this). To my knowledge, the fractal dimension of the particles is usually quoted when discussing effects on diffusion charging (rather than the density - because the density depends on the particle diameter). It would therefore be better to give an estimate of the fractal dimension of the particles rather than the density, also for comparison with previous studies on particle charging. Also, it is known from the literature that the differences regarding charging due to morphology increase with increasing particle diameter. Since the regulations include counting efficiency limits at 200nm, where the issue of particle morphology is probably even more critical than at 100nm, it would have been nice to extend the measurements up to 200nm. Did you just decide not to measure larger particles, or were there issues of generating a sufficent number of such large particles with your soot generators (just like it is difficult to generate enough smaller particles with the MISG)?
2) Pure diffusion charging is unsuitable for particle counting, as larger particles acquire more charge than smaller ones. There are multiple different ways to achieve a more uniform counting efficiency with a diffusion charger so that it can fulfil the PN-PTI specifications. The method chosen to achieve this is crucial for understanding the behavior of the instrument in experiments like this - e.g. how the device will react to precharged particles of either polarity, and also how it will react to the higher/lower charge that particles with higher or lower fractal dimensions acquire. Therefore, you should briefly explain the operation principle of the different devices. For example, the HEPaC and the DiTEST device use the same principle of operation, so it can be expected that they react in a similar way, whereas at least one of the other 4 devices uses a very different principle of operation and can be expected to react in a different way. Grouping the instruments by principle of operation might help explain the different types of instrument responses you observed.
Citation: https://doi.org/10.5194/ar-2023-16-CC2 -
AC2: 'Reply on CC2', Konstantina Vasilatou, 13 Feb 2024
The comment was uploaded in the form of a supplement: https://ar.copernicus.org/preprints/ar-2023-16/ar-2023-16-AC2-supplement.pdf
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AC2: 'Reply on CC2', Konstantina Vasilatou, 13 Feb 2024
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RC1: 'Comment on ar-2023-16', Anonymous Referee #1, 14 Jan 2024
Comments on “Influence of soot aerosol properties on the counting efficiency of PN-PTI instruments”
This study examines the counting efficiency of several diffusion charging-based instruments for PTI testing using various soot aerosols. The test aerosols were characterized in terms of size distribution, effective density, EC/TC ratio, and morphology (e.g., TEM images and derived primary particle size). This topic is timely and of interest to the aerosol and emissions measurement community. However, in my opinion, this paper suffers from two major caveats in its current form:
- While the paper presents the counting efficiency results (in different ways), it falls short of discussing in enough depth the “effect” of soot aerosols on the counting efficiency, as the title suggests, and “why” those effects are observed.
- It is not clear why the studied particle properties (e.g., EC/TC ratio or primary particle size) would affect the instrument counting efficiency. I understand that diffusion charging depends on the size of the particle and its morphology (if it is not spherical), but particle effective density (at one size), primary particle diameter, and EC/TC ratio do not provide meaningful insight into particle morphology.
Other specific comments:
- Why did the authors choose TSI NPET 3795 as the reference particle counter? The authors state that NPET was calibrated according to ISO 27891, so why didn’t they use the same reference instrument used to calibrate NPET as the reference particle counter? According to the specifications of NPET, it has a counting efficiency of < 50% at 23 nm and > 50% at 41 nm. The relatively low detection efficiency of NPET at 23 nm and even at 41 nm can potentially lead to unknown counting efficiency of PTI instruments, as some of the (smaller) soot aerosols may not be measured by NPET. Since this study is done by METAS, I suggest a reference CPC (with a lower d50 size) and a diluter with a known dilution factor be used as the reference particle counter.
- The authors have shown TEM images of different soot aerosols, which provide qualitative insight about particle morphology. For example, it is clear from these images that soot from MISG has a very compact structure, while soot from other sources are fractal aggregates. However, it is important to quantify the morphology of particles too to allow studying its effect on counting efficiency. This is typically done by determining the fractal dimension (e.g., through image analysis) or mass-mobility exponent (e.g., measuring particle mass or effective density over a range of particle sizes). The use of effective density at one particle size (100 nm) or primary particle diameter cannot give meaningful information about particle morphology.
- The recommendations given in Section 4 do not seem to be directly drawn based on the results of this study. Rather, some of the recommendations are generic and seem to be based on the results of other or previous studies.
- Figures S6 – S9 are not referenced in the main text of the paper.
- Table S1: It seems that EC/TC ratio is typed mistakenly as “EC/OC ratio”. In any case, EC/TC ratio is also given in Table 1, so I suggest providing this information in one place only (either in Table 1 or S1).
Citation: https://doi.org/10.5194/ar-2023-16-RC1 -
AC3: 'Reply on RC1', Konstantina Vasilatou, 13 Feb 2024
The comment was uploaded in the form of a supplement: https://ar.copernicus.org/preprints/ar-2023-16/ar-2023-16-AC3-supplement.pdf
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RC2: 'Comment on ar-2023-16', Anonymous Referee #2, 19 Jan 2024
The aim of this paper is to characterise the response of different diffusion-charging particle number sensors used for the periodic technical inspection of diesel vehicles based on diffusion charging to different soot generators and diesel exhaust. The response of the different instruments is characterised by comparing the PN counts at different particle sizes relative to a CPC-based PN counter calibrated traceable to ISO 27891.
The subject is of interest to the particle emissions community. However, the following aspects need to be addressed:
The paper examines the properties of soot that influence the counting efficiency of instruments for periodic technical inspection. The instruments used in the study are all based on diffusion charging. Since diffusion charging by itself is particle size dependent, different operating strategies are used to limit the influence of particle size. Therefore, the operating principles of the instruments differ significantly. A comprehensive description of the measurement principles used and explanations for the different responses to soot based on the measurement principle would be a very valuable contribution.
As the size dependence is crucial, it would have been very interesting to see larger sizes (e.g. up to 200 nm) as some legislation requires CEs up to 200 nm.
It is well known that (diffusion) charge-based sensor principles are sensitive to particle morphology. A discussion of this influence as well as a morphological characterisation (e.g. fractal dimension) of the soot produced by the different generators is missing.
The rationale for the selection of particle properties (EC/TC mass fraction, effective density, primary particle size) to assess the counting characteristics of the PN-PTI instruments is unclear and should be explained.
Chapter 4 makes recommendations based on the results of the study. What specific results led to the recommendations? Why is soot the best calibration aerosol? It seems that BigCAST Aerosol gives different CE results (e.g. 1.4 in Fig. 5a and 1.0 in Fig. 5b for CAP3070).
Minor: Why is a rather complicated aerosol aftertreatment (CS, dehumidifier, 1:10 diluter, blower, dilution bridge, …) after the soot generators used? Why is it different for the EU5b exhaust?
Citation: https://doi.org/10.5194/ar-2023-16-RC2 -
AC4: 'Reply on RC2', Konstantina Vasilatou, 13 Feb 2024
The comment was uploaded in the form of a supplement: https://ar.copernicus.org/preprints/ar-2023-16/ar-2023-16-AC4-supplement.pdf
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AC4: 'Reply on RC2', Konstantina Vasilatou, 13 Feb 2024
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