The paper presents a newly designed sampling device for atmospheric ice nucleating particles (INPs). It is designed for use in research aircraft. The design is described in detail and test results are presented to show the sampling efficiency and reliability of the device. The significance of the results is the prospect that it will aid in obtaining research results on atmospheric INPs at higher spatial and temporal resolution than has been possible with other systems. This can be specially useful when coupled with cloud and precipitation studies. The paper addresses the relevant questions thoroughly, although with some limitations and with some superfluous material. The topic of the paper fits well within the scope of AMT.

The manuscript presents an evaluation of the HERA sampling unit specifically for INPs. To some extent this is justified as the stated goal for HERA is to collect samples for INP studies. On the other hand, it makes the paper somewhat convoluted and focus is lost on the fact that the novelty is the HERA sampling, not the filter processing. The latter issue has been extensively treated in previous publications. Some new data are included here on pore-size dependence but really that should be a topic examined on its own.

Except for rather special situations, INPs can be expected to be dynamically just like other aerosol particles. Thus, transmission and collection efficiencies for INPs of given sizes, densities and shapes can be expected to be the same as for similar particles of other kinds. In view of that, it would be better if the paper separated the HERA sampling efficiency from INP processing issues. In fact, it is curious why the authors focus only on INPs and do not leave open the use of HERA for aerosol sampling for other types of aerosol analyses.

The paper also gets complicated by not clearly defining that installation-dependent other factors, specially inlet characteristics, are not subjects of this paper. Only in the discussion of the flight result (Section 4) do these issues have a place, as a specific instance of information needed to interpret the results.

The HERA unit is aimed to facilitate the collection of filter samples for INP analyses by eliminating in-flight handling of the filters. This is a worthwhile goal, although controls used in many previous cases showed that manual exchanges of filters didn't produce noticeable contamination. The potential of HERA will be more fully evident once the operation is fully automated and the need for a technician attending to the collector is eliminated. Then, the remaining limitation will be the number of filters per flight to six. Selection of six truly meaningful sampling intervals will require difficult decisions by the flight directors. The paper could be more realistic about these issues. Also, it should be clarified if filter holders could be exchanged during flights or not. Perhaps this is already in the paper and was missed in reading it.

The design of HERA is described in great detail. Yet, one misses information about overall size, weight and power requirements. Those are important factors for aircraft deployment.

Sampling efficiency is presented in terms of theoretical calculations. It would have been good to have support for the computed efficiencies from actual tests. The tests with Snowmax are in that direction but for smaller sizes than the calculations (Fig. 2).

A design criteria of 0.5 μm lower limit for particle sizes is given and transmission efficiencies are calculated for size above that limit. Small particles are less subject to losses. This is fine, but the justification given deserves some comments. Two references are cited in support of the decision for INPs. Both references show correlations of INP concentrations with particle sizes >0.5 μm but also show temperature dependence and variations with ambient conditions. None of the correlations are strong at higher nucleation temperatures and, in any case, correlations can arise from source or transport similarities without the INPs actually being >0.5 μm. Both studies were made with samples at ground level and there are other limitation as well in the two references. It is out of place for this manuscript to review the literature about INP sizes. It should however not give the appearance of a firm justification for the choice. Results obtained with the use of the HERA will have to be evaluated with the size question in mind, specially since the impact of INP sizes is also an issue for INP extraction and detection with the filter method.

With the foregoing in mind, it is recommended that the Introduction focus on the sampling issues from aircraft, that Section 3.1 be re-written to focus on showing the lack of sensitivity to flow-rate for small particles (as expected) for one filter setup. Sections 3.2, 3.3 and 4 are the main results to present. The use of 'deposition efficiency' should be avoided and it is easily confused with deposition nucleation tests. The discussion about cell fragments versus protein aggregates isn't effective because of the differences in physical dimensions; the sources of differences are more likely to be a question of sample aging and other treatment differences. The observed drop-off in detection at the higher temperatures is concerning but not a point to be treated in this paper.

In Section 3.3. the experimental setup would be of interest and the reason for the wind-speed dependence more understandable. This is more to the central point of the paper (sampling efficiency).

In summary, this is paper about an important new instrument and its characterization for INP sampling. Much good material is presented. Improvements can be gained by simplification of the paper.

The manuscript describes an automated 6-filter sampling system (called HERA) for collecting atmospheric ice nucleating particles (INPs) for offline immersion freezing analysis. Results are presented for HERA at relatively modest flow rates (~40 L/min) for laboratory INPs (Arizona Test Dust, SNOMAX), ambient ground sampling, and airborne measurements on the HALO. Samples were collected alongside the HALFBAC single-filter holder, which was designed for balloon-borne observations. The experimental design for validating aspects of the HERA system in some cases does not seem particularly well posed. For example, the effort to characterize the collection efficiency across varying filter pore sizes focuses on offline filter extraction and immersion freezing INP measurements, which have their own sources of uncertainty. Quantifying size-dependent filtration efficiency of large particles could've been more straightforwardly accomplished by using a size classifier and particle counters. Similarly, the theoretical calculations of particle transmission efficiency focus on the relatively short transport tube lengths within the sampler, but neglect the important aircraft inlet and long tube lengths that are going to be the limiting factors. Overall, the manuscript is well written albeit quite long. The topic is relevant for Atmospheric Measurement Techniques. While I do see some value in having an instrument paper to describe a specific airborne instrument, I'm struggling to identify what is novel in this work or, at least, of utility to the community with regard to airborne filter sampling. Consequently, I can't recommend the paper for publication, unless substantial efforts are made to deepen the level of characterization and analysis with regard to the HERA system itself.

Specific Comments:

What aspects of the HERA make it "next generation"? Frankly, it seems like a pretty straightfoward, moderate-flowrate (40 L/min), filter sampler, albeit with multiple cartidges instead of one.

Pg. 5, Line 155: "adapt the sample flow velocity to the wind speed". This doesn't read quite right. Suggest instead something like "match the inlet face velocity to the velocity of the surrounding air (often represented as the aircraft true air speed or for stationary sampling, wind speed)"

Figure 1 is very busy and shows a lot of unrelated information. Suggest combining the HALO aircraft diagram (panel a) with Figure 2 as it adds little value about the actual HERA instrument itself and splitting panels b and c into separate figures that are large enough to be legible. Additional labels on the filter holder cross-section, would be valuable -- what is the orange shaded region?

Following up on the previous comment, I would think that the most important sampling transmission concerns would be from the inlet(s) and long lengths of transport tubing, which Figure 2 and related discussion do not account for. This is a flaw, and Figure 2 is currently somewhat misleading in only accounting for the very short tube lengths within the instrument itself. Please include the inlet aspiration efficiency for the isokinetic and CVI inlets in the calculations as well as a rough transport tubing length. I suspect that the inflection point in Figure 2 will be shifted dramatically toward smaller sizes than what is presented now.

Pg. 7, Line 181: How long does it take to remotely switch between the filters? Closer to a second is fast, but closer to a minute is not particularly fast for aircraft sampling.

Section 2.3: Can filters be swapped out in-flight in order to sample more than 6 filters?

Pg. 7, Line 189: The paper seems to shift at this point away from the HERA operation to discussion of offline analyses of the filter extracts. This should be a new section.

Pg. 8, Lines 207-215: Do these reported 1-minute and 10-minute sampling time recommendations correspond to a filter flow rate of 40 L/min. Could a higher flow rate be used?

Pg. 10, Line 250: Now the flow rate is 15 L/min. Does this change the filter collection efficiency?

Pg. 10, Line 255: Why 15 minutes? This strikes me as quite a long time for each filter.

Pg. 10, Line 268: Why would there be differences in aspiration efficiency? Where the inlets and flow rates meaningfully different?

Pg. 14, Lines 341-342: Strike the sentence "In conclusion..." as I don't think a statement with this level of strength or clarity is appropriate to summarize what are rather messy experimental data.

Pg. 15, Lines 368-369: Strike the sentence, "From this...". The attribution of observational differences solely to differences in aspiration efficiency is speculative. 

Section 3: The Particle Loss Calculator can be used to compute inlet aspiration losses. Given the reported (relatively low wind speeds), what is a reasonable discrepency associated with differences in the inlet aspiration? This could be used to uncertainties to the data in Figure 4 and Figure 7.

Page 16, Line 381: What was the inlet tip diameter? What was a typical total air flow rate?

Page 16, Lines 383-384: If the CVI flow was only 5 L/min and HERA was pulling 40 L/min, where did the makeup air flow come from?

Page 18, Lines 416-418: Were such high flow rates investigated during this work? I would think that that would be very useful! What is the maximum flow rate for the 800-micron filters?

Page 18, Line 419: I am skeptical that a 7 micron particle can be efficiently sampled through an aircraft inlet and actually make it to the filter to be collected. Please provide information about particle collection efficiency that would be expected for the deployment of HERA on the HALO for CIRRUS-HL.

Page 19, Line 427-428: First, how do these findings suggest efficient sampling of INPs? Second, what might cause alteration of the particles' immersion freezing properties? What has been ruled out?

Section 3.3, Pg. 19, Line 432-433: Since the confounding results are due to a flaw in the experimental design, could the experiment be redone so that both instruments are sampling from a common inlet?

Appendix A: I don't understand the need for this section and suggest that it be removed. Much of the discussion seems to be focused on some experiments with SNOMAX to assess the contribution of any leaks when the system is operating at the same pressure as its surroundings, and it is found that the system was not leaking. For the aircraft campaign, where the differential pressure between the cabin and the system can be significant, the manuscript merely notes that leaks are avoided by leak testing the system at low vacuum and that any transient leaks would be identified in flight.