Fractionation of Aerosols by Particle Size and Material Composition Using a Classifying Aerodynamic Lens

Response to Reviewer 1 Comments

1. Summary

We would like to extend our sincere gratitude for your thorough and constructive feedback on our manuscript. We appreciate the time and effort you have invested in reviewing our work and providing valuable suggestions. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Can be improved

We added a few clarifying sentences to the introduction.

Are all the cited references relevant to the research?

Can be improved

We would happily include any specific or general reference, if the reviewer would provide detail about what is missing in their opinion.

Is the research design appropriate?

Can be improved

The clarifications made to the aim of the research and the answers provided in the Point-by-Point response should clarify eventual questions on the design of the research.

Are the methods adequately described?

Can be improved

We added more details to the methodology section, regarding the setup and the overall design of the separation device. This includes an additional Figure and Table added to the appendix.

Are the results clearly presented?

Can be improved

We made edits to a confusing figure.

Are the conclusions supported by the results?

Can be improved

We added a reference to the presented results in to support the conclusions.

3. Point-by-point response to Comments and Suggestions for Authors

Comment 1: For the separation of copper and silicon particles, this method can inherently have limitations from oxidation of copper and silicon particles. The density difference between copper and silicon would be decreased.

Response 1: It is a valid concern, that oxidization would alter the density of the particles and thus alter their expected behavior in the CAL. It is, however, not necessarily clear, that the separation efficacy will be hindered by this. Dependent on the particle size, the difference in aerodynamic diameter can either increase or decrease with a change of particle densities. 

Moreover, the EDX measurements did not yield significant amounts of oxygen. Typically, the so-called native oxide layer is several nanometers thin which is a small volume in comparison to the nonoxidized core of particles of the sizes used in this research – for nanoparticles oxidation would be a valid concern. Only a high-temperature treatment in oxygen would lead to the higher concentrations, but we are not aware of such a treatment. However, we added a figure to the appendix, showing the expected product mass ratios when the densities changed to CuO (6.315 g/cm3) and SiO2 (2.196 g/cm3), as well as a table including the EDX measurements of the feed materials. Next to copper and silicon, aluminum and carbon are found on the samples, oxygen is only present in miniscule amounts. We think that this indicates that the particles are not oxidized. Both added materials are referred to in the main text with a short added paragraph added to Section 5.1 :  “An additional concern could be the purity of the particles itself. Cu and Si are prone to surface oxidization. However, only trace amounts of oxygen were observed in the EDX. Figure A3, in the appendix, illustrates how the model prediction changes, when the particle densities are altered to reflect the materials (ρ^CuO = 6.315g/cm³ and ρ^SiO2 = 2.196g/cm³). The prediction based on fully oxidized particles seems to agree better with the EDX data, especially for the samples taken at 400 Pa. However, this would contradict the EDX measurements of the pure materials, given in Table A5. Severe oxidization during the aerosolization process also seems unlikely, due to the room temperature and nitrogen being used as carrier gas.”  

Comment 2: Regarding Figure 7, it is not easy to find difference between the result of feed stream and that of product stream. Do you have any specific reason to use the result of 400 Pa? Maybe the result of pressure less than 300 Pa be more distinguishable. And extreme size particles would be eliminated by loss especially for larger particles not by aerodynamic lens.

Response 2:  Although we agree with the reviewer on the difficulty to differentiate the Feed and Product stream histograms provided in Figure 7, we like to give our reasoning to use these here: The most likely cause, for the similarities between the feed and product stream representations is that they depict the normalized number fraction. It is easier to see the difference between the results when the counts are only normalized by the bin width and not also by the total count of the respective species. The problem with this alternative re-presentation is that it gives a misrepresentation of the actual distribution in the mixture, because of the limited available particle counts. The number of counted particles for this sample was over all the highest of the samples taken after the CAL, and we had chosen to use the CFD simulation of 400 Pa for the representation in the manuscript. However, we could add additional (not normalized) histograms of corresponding to each CAL sampling pressure to the appendix, if reviewers and editor deem this necessary.  

Comment 3: The difference between those values needs to be explained in the Figure 8. The word ‘product’ may hider reader’s understanding. How about using ‘prediction’?

Response 3: We agree with the Reviewer on this point and followed this advice, as it helps to reduce potential confusion. The caption of figure 8 has been altered to help with this as well. It now reads: "The measured product mass fractions are compared to the prediction by the model. Measurements are given as points, whereas the model predictions are given as dashed lines, for copper in red and for silicon in blue."

Regarding the differences between "those values", we are not quite sure which values those are. We relate the difference between model prediction and measurements to low particle counts and difficult to access shapes, which complicate the conversion to stokes diameter, on which the model is based.

Comment 4: Regarding Table 4, standard deviations of copper and silicon particles are as same as each other. It that right?

Response 4: Yes, this is correct. It is a result of restricting the EDX analysis to just copper and silicon, so that their sum is always 100 %. The one sigma standard deviations describes the deviation over 3 samples which were measured at 5 points each, with again 5 subsamples takes, as described in the method section. 

The values of copper and silicon content are measured simultaneously, but will, through the restriction, always be mirroring each other. This means their standard deviation will also be equal. If the restriction is removed, the sum of the Cu and Si percentages could deviate from 100 %, and we likely would also see a variation in the standard deviation. In an early version of the manuscript, we had included an additional error estimation provided by the EDX instrument itself, which is given for each measurement, so this resulted in a standard deviation of an error estimation. We found this to be more confusing than helpful, but would also include them in the appendix, if needed.

As editing tables (especially in LaTeX) can often introduce errors, we thank the reviewer again, for having an eye on these.

4. Response to Comments on the Quality of English Language

Point 1:

Response 1:  Reviewer 1 made no comments on the Quality of English Language.

5. Additional clarifications

Thank you again for your valuable contribution to this manuscript. Please feel free to contact us if there is any need for further clarifications.

Response to Reviewer 2 Comments

1. Summary

We would like to extend our sincere gratitude for your thorough and constructive feedback on our manuscript. We appreciate the time and effort you have invested in reviewing our work and providing valuable suggestions. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

yes

We still added a few clarifying sentences to the introduction. Which should address concerns the reviewer raised. More on this is found in the point-by-point answers.

Are all the cited references relevant to the research?

yes

Is the research design appropriate?

Can be improved

The clarifications made to the aim of the research and the answers provided in the Point-by-Point response should relief eventual questions on the design of the research.

Are the methods adequately described?

yes

We still added more details to the methodology section, regarding the setup and the overall design of the separation device.

Are the results clearly presented?

Can be improved

We made edits to a confusing figure.

Are the conclusions supported by the results?

Can be improved

We added a reference to the presented results to support the conclusions.

3. Point-by-point response to Comments and Suggestions for Authors

Comment 1: It is confused whether the study has accomplished its objective, to investigate aerodynamic fractionation process taking into account their multiple properties. In the conclusion, it is mentioned that “Particularly, at lower operating pressures, around 200 Pa, this study efficiently separated silicon particles from copper ones” but it is not clear which result supports this statement. It might be referring to Figure 6 and 7 but it is not same as Figure 2, mixed particles are in Figure 6b and 7b after separation

Response 1: The reason that we claim the separation is more efficient at lower pressures, is found in Figure 8, which shows the model predictions compared to the actual results.  The CFD simulations were mostly used as input to the model by providing a basis for the theoretical transfer function. We added a clarification to that statement, in the conclusion, which refers to Figure 8. It reads “Particularly, at lower operating pressures, below 300 Pa, the CAL efficiently separated silicon from copper particles, as indicated by the results presented in Figure 8.”

The question regarding the objective was raised again later in Comment 5; it will be answered to there.

Comment 2: Line 228-229 it may be not a necessary sentence.

Response 2:  We agree. Therefore, we eliminated the sentence and restructured the sections and subsection in the method part so that a section heading is not immediately followed by a subsection heading. (That had been the reason for this sentence in the first place). Consequently, all subsequent headings in this section were moved up one level. We marked all changed subheadings in the manuscript.

Comment 3: Line 301-310 the reference samples were collected from main aerosol stream line in a different direction and would it be isokinetic sampling? Can it be possible to over or under estimation of the airborne particles?

Response 3: We agree with the reviewer, that here might be a possible over or under estimation the number of airborne particles, but this should not really matter for the results and the prediction, as long as the mass fractions can be discerned. Moreover, the flowrates into CAL and reference are similar, as is the pipe diameter up to the respective entrances; and the filter housings are identical.

However, the question might show that there is need for an additional clarification in the manuscript, which we added to Section 5.1, by referring to an additional table provided in the appendix: “The aerosol production unit leads the aerosols to a distribution chamber, from which the CAL sampling line and the reference samples are fed. The dimensions of this chamber are given in Table~\ref{tab: Distributordimensions} found in the appendix.” The Reynolds number of the flow into this distribution chamber > 1300, so sampling for both CAL and reference is essentially carried out from a turbulent mixing chamber.

Comment 4: Line 330-352 why not utilizing direct reading instruments for monitoring between product line and excess line? Also, why not testing the CAL with two different sizes of sphere particles with simulated particle sizes, 100, 400, and 800 nm to see clear separation?

Response 4: As the reviewer points out, there are some improvements to this study, which could be added in the future. We would have very much liked to be able to utilize direct reading (or online) measurement instruments, but as the exit of the CAL is in a lower pressure level this was not possible for us at the time. Differentiating between the different species, while measuring the particle count and mass load simultaneously, would have been ideal. We are open for suggestions for future measurements.

Regarding the tests of simulated particle sizes: it would probably be a good idea to do experiments with spherical model particles of known sizes. However, finding an adequate test material is somewhat more complex, as we would require two materials with a greater difference in density. Spherical particles are often produced from latex, alternatively there are oil-droplets, and sintered metal particles. That could be an option for future experiments as well. We also simulated particle trajectories for other stokes diameters, but we chose to exclude those from the figure, because it became unreadable.

Comment 5:  Figure 6 and 7 Would it be better to present distributions of the product line and excess line, which is main objective of the study?

Response 5: We thank the reviewer for raising the question regarding the main objective of the study, as we agree that is was not stated clearly enough. Therefore, we edited the following paragraphs in the introduction: "The objective of this study is to investigate an aerodynamic fractionation process taking into account multiple properties of airborne particles." and "CALs have been used to fractionate nanoparticles by size before \cite{REF-DAPS-Babick}; this work investigates the use of a CAL as tool for material and size fractionation of microparticles."

Regarding the questions concerning Figures 6 and 7, we want to provide some additional information. The excess or waste outlet line is also located at the end of the CAL - in a low-pressure environment. In theory, particles from the excess line can also be collected, in practice however, the collection is somewhat hindered by the shape of the exit. Whereas the sampling outlet is continuing the aerosol stream from the CALs entrance in one straight line on the filter, the closest point for collection after the excess outlet is after an almost 90° bent. This leads to more losses. We attached a render annotated image of the CAL in the appendix of the manuscript).    

4. Response to Comments on the Quality of English Language

Point 1: Minor editing of English language required

Response 1:  Reviewer 2 asked for minor editing regarding the quality of the English Language in the manuscript. As the reviewer did not point out any specific or general points to where and how the quality of English language could be improved, we hope that the edits and additions we made in response to the points raised by the reviewer are sufficient.