Title:                 Reduced seed-material dependence of a condensation particle counter (Wlasits et al., AR-2024-2)

This well written manuscript describes the characterization of a newly-developed CPC model (A30, Airmodus Oy, Helsinki, Finland) for its operation with two different working fluids. The authors tested this CPC for its performance when operated with butanol and propylene glycol. The study was conducted for four different particle materials (sodium chloride, silver, ammonium sulfate and caryophyllene, which is a plant volatile found in essential oils). The authors found an influence of the particle materials on the counting efficiency and lower cut-off size of the CPC operated with butanol, while they did not observe any similar dependence of the CPC operated with propylene glycol. They then came to the somewhat hasty conclusion that the observed effect is only due to the working fluid, while they do not seem to consider that e.g. the very different operational temperatures used inside the CPC will also have played an important role. This is especially surprising as they later on decrease the saturator temperature of the propylene glycol-based CPC, but did not try to increase it in the butanol-based A30. The latter could have revealed if this new A30 CPC is possibly operated below its optimum supersaturation when using butanol as its working fluid. That said, the final conclusion is very sound and useful: “The existence of a working fluid-specific supersaturation profile associated with no significant composition dependence of the counting efficiency should be validated by inclusion of other working fluids”.

Main comments:

• General comment: I am personally struggling with the term “seed material”, which seems to say that seeds are to be measured by a CPC and possibly even imply that they are all of one and the same material. Instead, I would suggest to consider something along the lines of the “material dependence of particles acting as nuclei in a condensation particle counter”.
• Line 9: The authors state that “the composition dependence of particle activation is a source of profound uncertainty for the accurate assessment of particle number concentrations”, which is too general a statement the way it is phrased. Apart from the measurement of nucleation events in ambient studies, the number of particles near a CPC’s cut-off that either get or do not get activated and grow to be detected is typically a relatively small fraction of the total number concentration of particles that is measured over the complete size range of a CPC.
• Line 15: If “innovative choice of the working fluid” refers to propylene glycol then it should be made clear that propylene glycol or IUPAC has already been used in CPCs well over three decades ago (Sem, 2002). It was actually also used in a commercial CPC many years ago, e.g. the Model 3851 manufactured by KANOMAX that was introduced in 1986 (see McMurry, 2000).
• Line 115: “These conspicuous discrepancies between the two identically constructed CPCs must be attributed to changes related to the working fluids in use”. I am not convinced this is the (only) explanation. While the two A30 CPCs are “identically constructed”, the authors previously stated that “propylene glycol-based CPCs (are) operated at a higher saturator temperature and a decreased condenser temperature compared to the butanol-based models”. Tab. 2 shows a delta T of 36C for the IUPAC-A30, while that’s only 24C for the butanol-A30. I could well imagine that the newly developed A30 CPC simply does not operate with optimized supersaturation conditions for butanol (and would also benefit from e.g. a higher saturator temperature). In particular as the data from the well-characterized butanol-based CPC 3776 in Fig. S5 (in the supplement) show no visible difference for the exact same particle materials.
• Line 121: “These calculations revealed that the measured counting efficiencies cannot solely be attributed to diffusional losses within the instrument”. This is somewhat poorly phrased and hard to understand. Why would the “measured counting efficiencies… be attributed to diffusional losses” in the first place? Is there something missing here?
• Line 174: “The figure suggests that there is a working fluid-specific saturation ratio at which the composition dependence of particle activation vanishes.” Yes, this is getting to the core of it!
• Line 178: “While further completion of the figure by adding water- and DEG-based CPCs and boosters exceeds the scope of the presented study, this matter shall be urgently addressed in subsequent research”. Why limit this interesting sentence explicitly to water- and DEG-based CPCs? There are a lot of isopropanol CPCs in use and new substances such as dimethyl sulfoxide (DMSO) are increasingly investigated (Weber et al., 2023). Incidentally, the latter paper here in the AR journal also looked at the saturation ratio necessary to activate a sodium chloride particle of a given initial size for three working fluids: water, butanol and the new working fluid DMSO.

Minor Comments

• Line 86: “The overall design of the Airmodus A30 is presented in Fig. 1”. This is the first time that instrument is mentioned, hence this is where the manufacturer’s information should be shown.
• Line 95: “butanol-based variant and a variant of the A30 using propylene glycol”. The use of the term variant is inconsistent with the later statement that the actual instruments are identical. I believe it is better to say something like “the butanol-based A30 and the A30 operated with propylene glycol”.
• Line 165: A part of this sentence that says “the set of butanol-based CPC manufactured by TSI Inc.” is a bit awkwardly phrased and the location details of the manufacturer are missing.

References

McMurry, P. H. The History of Condensation Nucleus Counters, Aerosol Sci. Technol., 33:297–322, 2000

Sem, G. J. Design and performance characteristics of three continuous-flow condensation particle counters: a summary, Atmos. Res., 62: 3-4, 267-294, 2002

Weber, P., et al. A New Working Fluid for Condensation Particle Counters for Use in Sensitive Working Environments. Aerosol Res. 1, 1-12, 2023

The authors present a study on a newly developed Airmodus A30 CPC, which is the latest model of the manufacturer to detect aerosol particle concentration even below the size range of 200 nm, where classical optical detection method of particles is not possible. The tested CPC’s used 2 different working fluids namely the n-butanol and non-hazardous propylene glycol. During the experiments 4 different seed materials were applied (sodium chloride, silver, ammonium sulfate and caryophyllene). They experienced that the butanol-based CPC operated with the default saturator and condenser temperature had a strong dependence on the seed material, since a profound difference was measured between the counting efficiencies of different seed particles. During the experiments with the propylene glycol-based instrument no seed material dependence was observed at the default saturator temperature. When the saturator temperature of the propylene glycol-based CPC was decreased, the counting efficiency was decreasing as well. I fully agree with the other reviewer that it could have been interesting to modify the saturator temperature also in case of the butanol-based instrument. The results of this work could be very useful even for the manufacturers, regarding how to improve the performance of the newly developed instruments.

Main comments:

• Line 13: “Our results conclusively demonstrate that composition-dependent particle detection can be reduced to the brink of disappearance by innovative choice of the working fluid.”. Line 189: “Comparison to previous results and decreasing the saturator temperatures of the propylene glycol-based model confirmed that a composition dependence of the described kind is mainly linked to the supersaturation profile seeds are exposed to. By lowering peak saturation ratios, we were able to re-induce a composition dependence of the counting efficiency in case of the propylene glycol-based CPC”. Please clear, which is the main outcome of the work, whether the innovative choice of the working fluid, or dependence on the supersaturation profile seeds are exposed or both.

Minor Comments

• Line 13: The first time Airmodus A30 mentioned. Please put the manufacturer information as in line 95 written.