A substantial improvement of a commercial particle sizing instrument in the nm range - the A10 PSM - is described which consists in the extension of the size range to 1-12 nm and the increase in detection efficiency. The PSM 2.0  provides important overlap to other sizing instruments. The optimization of the flows in the instrument prevents shortcomings of the precursor instrument such as instabilities due to variability of the saturator flow. The mixing of sample flow and saturated flow was optimized by keeping the total flow to the mixing point constant but with different ratios of saturated and dry air. Finally, a size calibration and a field comparison with other instruments, among them mobility based instruments, is presented and the effect of charging in the bespoke size range is discussed.

1. Does the paper address relevant scientific questions within the scope of AR? YES
2. Does the paper present novel concepts, ideas, tools, or data? YES
3. Are substantial conclusions reached? YES
4. Are the scientific methods and assumptions valid and clearly outlined? YES, see comments.
5. Are the results sufficient to support the interpretations and conclusions? PARTIALLY, see comments.
6. Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)? PARTIALLY, see comments.
7. Do the authors give proper credit to related work and clearly indicate their own new/original contribution? YES
8. Does the title clearly reflect the contents of the paper? PARTIALLY, see comments.
9. Does the abstract provide a concise and complete summary? YES
10. Is the overall presentation well structured and clear? YES
11. Is the language fluent and precise? YES
12. Are mathematical formulae, symbols, abbreviations, and units correctly defined and used? PARTIALLY, see comments.
13. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated? YES, see comments.
14. Are the number and quality of references appropriate? YES
15. Is the amount and quality of supplementary material appropriate? No supplementary material was provided.

detailed comments:

• Title: For a technical note paper speaking of ”a new decade” is a bit overdone and early in my point of view. The below discussion shows that some aspects such as time resolution are not yet properly characterized. Matter of taste of course.
• From line 84 on and also in Fig. 1 the PSM 2.0 design and work principle is outlined. I miss several details here which should make it easier to comprehend the pros of the new design, moreover, the nomenclature is not consistent. Is the “mixing section” in line 86 the same as the “T-block” in Fig. 1? There is some inconsistency in the description of the flows in the instrument: Lines 86-87: “…two mass flow controllers are operated in tandem to produce a constant 2 lpm flow rate…”. I understand this in the way that 2 lpm are constantly fed into the T-block. Lines 90-91: “In the prototype PSM 2.0, the total saturator flow rate has been increased to 2 lpm, of which at most 1.9 lpm can be wet saturator flow.” So, what are actually the saturator flow, dry flow, and outlet flow values?  These should be described. What is meant by “wet saturator flow”? Is it the flow downstream of the saturator? Having mentioned that, the following statement in Line 90f is not easy to understand: “This means that the ratio of saturator flow to total flow in the instrument has increased almost 60 % compared to the A10….”. The term “Total flow” is not defined. If it is saturator flow plus inlet flow, in my understanding for the A10 this is a varying quantity while in PMS 2.0 it is described as constant. The authors mentioned possible temperature instabilities in the A10. However, the temperature control in the PSM 2.0 is not sufficiently described. The temperatures or temperature ranges of inlet, flow heater and condenser should be described. Is there a need or an option to cool the inlet flow under hot ambient conditions?
• I would like to ask for comments about possible effects of sample heating at the inlet of the PSM 2.0.
• There is another inconsistency in terminology: Line 112 uses the term “PSM growth tube”, this is obviously the “Condenser” in Fig. 1. I recommend to thoroughly check the manuscript for consistent language and terminology.
• Line 85 states that in the PSM 2.0 “the only parameter which is varied is the ratio of vapor amount arriving to the mixing section from the saturator”. In Fig. 2a and 2b, A10 and PSM 2.0 are compared, and the abscissa of the diagrams is QReaders may question why the diagrams are not plotted as functions of the ratio. Q_saturator is probably defined differently for the PSM 2.0 and the A10 and the reader cannot comprehend how the “ratio of vapor” depends on Q_saturator.
• It is probably beyond the scope of this paper, but I miss some information on the time resolution of the new PSM 2.0. Is there a gain in time resolution of the PSM 2.0 by faster reaching stability after changing saturator and dry flows?
• As to field measurements and Figs. 3, 4 and 5, statements on concentration uncertainties: “The concentration uncertainties were calculated in the same way as in Kangasluoma et al., (2020).” This puzzled me a bit because Kangasluoma (2020) very comprehensively discussed factors of uncertainty but did not present any calculations; instead, a Monte Carlo simulation of uncertainties was presented. It should be explained in more detail if and how this simulation was used for the uncertainty calculations of the data presented. I refrain from further commenting on the field measurements because of lack of expertise in this field of atmospheric research.

The particle size magnifier (PSM) was a major advance for aerosol formation studies, as it unveiled the sub-3 nm size range at an unprecedented level (enabling neutral cluster detection at high sensitivities). However, the PSM as presented back in 2011 had major shortcomings: 1) Its operation was from time to time cumbersome and required very experienced researchers for best data output, 2) its counting efficiency was highly seed-material dependent introducing major uncertainties and 3) its size range was limited preventing efficient comparison with other instruments and 4) its time resolution was rather low. Sulo et al. present the PSM 2.0, which promises to solve 1) and 3). The PSM 2.0 is certainly an important update and has the potential to become an essential instrument in sub-10 nm particle studies. The paper is well-written, concise, and certainly presents novel insights suitable for publication in AR. However, in this technical note I am missing a discussion of 2) and 4). Moreover, I think that the improvements on 1) are also insufficiently discussed. Therefore, I can only recommend publication of the manuscript after the following minor points have been revised.

• 1 misses some important details. It would be helpful if the flow arrows show the typical flow rate ranges (or fixed values) as already written in the text. Also, all heated/cooled segments should be labelled with the default temperature settings. For the condenser I would illustrate the separation between sample and excess flow via core-sampling somehow also in the Figure. The DEG-trap is not mentioned in the text (what is it, why is it there and how does this improve the PSM2.0?) and while the reader familiar with the PSM might not wonder, I think it is necessary to mention that a CPC is used for subsequent particle detection, and it should be mentioned what type was used in this study.
• As the authors claim “Additionally, PSM2.0 is significantly more stable than PSM A10 due to (…)” in their conclusion I ask them to provide additional information on how they proof this claim. This could be e.g., long-term time-series of both instruments, stability of background during several scans, overall drift of regular background measurements (as performed with an Airmodus Diluter for example, see Lampimäki et al., 2023, J. Aerosol Sci.) or variability with external conditions (such as humidity). I don’t request the authors to perform more experiments, but just to support the claim of improved stability to some extent, maybe with the data they already have at hand. Otherwise, the statements on improved stability need to be removed.
• In the instrument description I am missing information on scan-time and procedure and the inversion procedure applied. How many size-bins can be obtained from the PSM 2.0 within the 1-12 nm range, or what resolution is expected (The steepness of Fig. 2b lets me think that it might be quite tricky to resolve above 5 nm into too many bins)?
• Can the authors already comment on the see-material dependence of the PSM2.0 at larger sizes? If not, they should at least mention that this is an important subject for future studies. They should also shortly explain why the plateau of the detection efficiency now reaches 1 (and never reached this value in the A10). This also greatly reduces the total concentration uncertainties obtained from PSM2.0.
• I think Fig. 4 and Fig.5 with their more than four orders of magnitudes in concentration on the y-axis do not fully enable a judgement on the concentration agreement. Can you add a plot which shows the ratio between the instruments over time in the SI? Moreover, it should be mentioned that Fig.4a most likely shows such a good agreement between NAIS and PSM2.0 for the wrong reasons. NAIS underestimates below 3 nm but overestimates above 4 nm, which altogether might be the reason for the good agreement. In fact, the comparison shown in Fig. 5 is very close to the results form Kangasluoma et al. 2020 who used more sophisticated sub-10 nm DMPS systems. In that sense, the PSM2.0 seems to possibly provide pretty good agreement with such sub-10 nm DMPS systems, and I would love to see a comparison to one of those instruments in the future. With respect to that it could also be mentioned that the reduced agreement with the “standard”-DMPS below 5 nm is most likely due to reduced counting statistics as recently shown by Stolzenburg et al., 2023, Atmos. Meas. Tech.
• Last, I think the title of the manuscript might be a bit too provocative. If it really pushed aerosol measurements to a new decade needs to be decided in a decade.

Technical:

• Please label x-axis in Fig.3 and 4 with “Time”.
• 2: Please explain D₅₀ in the caption.
• Check that you name condenser and growth tube consistently.
• In line 70 it is already stated that the activation occurs in the condenser and only later this statement (line 99) is supported by a reference.
• Line 127: Delete “of” between “setup” and “used”