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:

1. 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.
2. 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).

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?