Aerosol Charging with a Piezoelectric Plasma Generator

Round 1

Reviewer 1 Report

This manuscript discusses a Piezoelectric Plasma Generator used to generate gaseous ions of both polarities (positive and negative), which can subsequently be used for bipolar diffusion charging of aerosols. For aerosol technologies such common (and ISO standard) aerosol measurement methods, charging is highly important. The application of a new type of charger for aerosols is always interesting, particularly since there are cost, size and regulatory advantages of this method. Overall, the paper is concise and well-structured. The charger is described with sufficient detail. The experimental set-up is more than suitable for a preliminary evaluation of the technology. Results are encouraging. The discussion of limitations is appropriate and appreciated. The quality of the language throughout is fine, except for early in the introduction (see below). The authors could go a little further to describe the experimental findings as discussed below. I highly recommend publication of this manuscript in Plasma nearly in its present form and suggest the following revisions.

Comments:

Unfortunately, while I appreciate the author’s intent, the first part of the introduction contains so many language mistakes it distracts from the quality of the work. I suggest removing P1L14-P1L21 or at least re-word it significantly more carefully since it is at the beginning of the manuscript.

It is a missed opportunity not to mention cost as a motivator in the introduction. Typical radioactive chargers cost thousands per charger and require paperwork. Even still, realistically, TSI will most likely not move to a charger like this for the SMPS. However, in theory this charger could be used in low-cost, high-volume applications (possibly cite ref https://pubs.acs.org/doi/10.1021/acssensors.9b02143) where reliable charging is highly desired.

P6L225 If desired, it would be straightforward to apply Gunn’s equation as in Appendix A2 (https://doi.org/10.1016/0095-8522(56)90050-2 should be cited) at a different ratio of ion mobilities (though only valid >50 nm, to predict charge fractions for +/-1, 2 charges, plot it on Figure 5 and see if the ratio of ion mobilities can adequately explain the differences between the CeraPlas and Kr-85. However, Figure 5b looks a little out of the ordinary that +2 charged fraction would be higher for smaller particles with the CeraPlas. If so, comparison to the above theory might help to narrow discussion as to possible reasons why that result was found.

P6L232 These ion concentrations (positives being higher concentrations than negatives) are not surprising and this is found in Kr-85 chargers so this paper could be cited as support for those findings and show similarity to a Kr-85 charger which is desired by the community (see Sec 4.4 of https://doi.org/10.1016/j.jaerosci.2020.105526). P7L236-239 Also, the calculation of Nt very much depends on flow rates so it is unsurprising the ionometer did not exact quantification of Nt product if the gas flow rate was not defined well. Discussion on P7L248, the authors can be more favourable. Reaching equilibrium only depends on quantities of particles relative to ions so, 10^6 may be enough, though I expect it is the ionometer and calculation that is providing a value that is too low for Nt product. The real Nt product is probably much higher close to the charger.

Typographical

The very first sentence of the introduction does not make sense. Also should be ‘than’ not ‘then’.

P1L8 Change to “m^-3 s” from “s m^-3” for Nt product in line with literature (Biskos, Schmidt-Ott, etc.)

P1L32 Change from “such safety issues” to “such as safety considerations”

P2L78 pre-exsiting

P6L225 Change “positive ones” to “positive ions”

P7L241-243 Throughout the manuscript, X-ray is mentioned quite often, but a Kr-85 is used as well. Please check if X-ray and Kr-85 are being mentioned in the correct spots.

P7L248 and throughout Ni t should be N_i t

Author Response

Dear Reviewer,

thank you very much for your valuable comments and suggestions, we appreciate them very much and addressed them in the following way:

• We revised the first part of the introduction
• We mentioned the cost aspect and the possible low-cost, high-volume application in the second paragraph of the introduction
• We did not apply the Gunn equation as suggested, since the ratio of the ion mobilities for the CeraPlas is not known and it was not possible for us to determine it within the extended revision period of this manuscript. We added a passage to the second paragraph of section 3.2 where we discuss the applicability of the plotted theoretical curve and the possible difference of the produced ions. Thank you very much for this suggestion.
• Thank you very much, we included the mentioned aspects into the second paragraph of chapter 4.

Reviewer 2 Report

The grammatical mistakes make it hard to read. You can use Grammarly to improve many of the issues in the introduction and throughout the test.  Also, verify the consistency of notation "Nit" is used inconsistently throughout the text.

Author Response

Dear Reviewer,

thank you very much for your valuable comment, we appreciate it very much. We improved the language of the manuscript, improved the introduction and reviewed the consistency of the notation.

Reviewer 3 Report

line 62 - "... for DMAs neutral aerosols are needed ..." - this is inaccurate. DMAs do require charging of particles in order to be deflected by the electric field and collected by the detector. In fact, while is may be common that aerosols are largely neutral at sub-100 nm particles, a (hypothetical) aerosol with 100% charge particles will only increase the output concentration of a DMA. 

line 65 - Fuchs' limiting sphere charging model has been challenged & more accurate formulations are available; please see these papers for many of the shortcomings of the limiting sphere model 

R. Gopalakrishnan and C. J. Hogan, Physical Review E 85, 026410 (2012).

R. Gopalakrishnan, T. Thajudeen, H. Ouyang, and C. J. Hogan, Journal of Aerosol Science 64, 60 (2013).

H. S. Chahl and R. Gopalakrishnan, Aerosol Science and Technology 53, 933 (2019).

Also, these papers present a complete experimentally validated model to describe the diffusion charging of spherical and non-spherical particles

L. Li, H. S. Chahl, and R. Gopalakrishnan, Journal of Aerosol Science 140, 105481 (2020).

L. Li and R. Gopalakrishnan, Journal of Aerosol Science 151, 105678 (2021).

• Figure 5 is promising. It establishes that CeraPlas(R) is capable of being used for bipolar charging. However, nothing is said about repeatability and reproducibility of the data. Can the authors provide data that can convince that the charger produces the same charge fraction when operated on different days?
• An associated concern is the composition of the carrier gas - ambient air vs compressed air? dry air vs a fixed RH air? inert gas vs air? Though it may be argued that this is a feasibility study, it will be worthwhile for the authors to recognize these issues and mention/address them in the paper.
• line 233, 234 - the authors use a recombination coefficient of 1.6x10¹² m³s^-1 is unfounded - they should state how they got this value (cite/provide details of the estimate). It seems to be very important in the estimation of the N_it product.
• line 248, 252 -  N_it is mistyped.
• line 248 - 252 - the Discussion presented here has no basis because it is not clear how the N_it product estimate is justified as the authors themselves note in lines 265 - 267
• line 273 - it is not clear how much adding nitrogen helped in suppressing ozone generation. No data is presented to substantiate this claim.
• line 285, 286 - the authors claim that there is no particle contamination and ozone generation. However, no data or quantitative proof is given as to how these possibilities were ruled out.

Overall, I feel that except the use of a commercial device CerPlas(R) that standardizes the non-thermal plasma generation, the presented work is not very different from characterizing a home made corona discharge source that when combined with a housing and an electrode, can be turned into a unipolar or bipolar ion source. 

Unless, the repeatability, ion composition, ion number concentration or N_it product of the charge housing are quantified, this study does not demonstrate a new concept for a charging device that is not already known. 

Author Response

Dear Reviewer,

thank you very much for giving us the opportunity to improve our manuscript with your valuable comments and suggestions. We appreciate your feedback and addressed it in the following way:

• Thank you very much for raising this point, we agree with your position. We removed this passage from the manuscript, since a detailed description of DMAs does not add value to it.
• We mention the advanced models in the second paragraph of section 2.1 and cited two of the listed works. Thank you very much for calling our attention of the work of Dr. Gopalakrishnan’s group.
• We conducted measurements over several days to clearly show the reproducibility of the produced charge fraction by measuring particle size distributions of particles which have been charged by the CeraPlas®. Further we also added date regarding the repeteability of subsequent measurement. We included the description to the last paragraph of section 2.3, added a paragraph into section 3.1 and added statements to the discussion and the conclusion.
• Thank you very much for the remark on the gas composition, we are aware of the very important influence of the carrier gas composition. A comprehensive investigation in this regard is beyond the scope of this feasibility study. We added a comment to the manuscript to mention possible influences.
• We added the source for the recombination coefficient we use.
• We included into the discussion of manuscript an explicit statement, that this work is a feasibility study and the determined value for the N_it-product Is an estimate, which we can give from the available knowledge. Further work is necessary to be able to give a comprehensive characterization of the CeraPlas®.
• We included a statement into the manuscript, that ozone (O₃) is not produced by the absence of oxygen. Operating the CeraPlas® under N₂ atmosphere thus suppresses O₃ generation
• We included an elaborate statement regarding the prove for no particle contamination by the CeraPlas® and added a statement regarding the suppression O₃ generation in the last paragraph of section 2.3.
• We highly appreciate your feedback and extended the revision period twice to conduct further measurements for the reproducibility and repeatability of the behavior of the CeraPlas®. Determination of the ion composition, investigations of the ion number concentration under different conditions and reliable determination of the N_it-product we be done in future work.

Round 2

Reviewer 3 Report

Thank you for considering my comments. I find the manuscript acceptable in its current form.