Trends in particle-phase liquid water during the Southern Oxidant and Aerosol Study

Nguyen, T. K. V.; Petters, M. D.; Suda, S. R.; Guo, H.; Weber, R. J.; Carlton, A. G.

doi:https://doi.org/10.5194/acp-14-10911-2014

Articles | Volume 14, issue 20

https://doi.org/10.5194/acp-14-10911-2014

© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-14-10911-2014

© Author(s) 2014. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 14, issue 20

Research article

|

16 Oct 2014

Research article |

| 16 Oct 2014

Trends in particle-phase liquid water during the Southern Oxidant and Aerosol Study

T. K. V. Nguyen, M. D. Petters, S. R. Suda, H. Guo, R. J. Weber, and A. G. Carlton

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Final revised paper (published on 16 Oct 2014)
Supplement to the final revised paper
Preprint (discussion started on 18 Mar 2014)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC C2417: 'Review of Nguyen et al', Anonymous Referee #1, 16 May 2014
RC C2455: 'a review of Nguyen et al.', Anonymous Referee #2, 17 May 2014
AC C4826: 'Response to reviews', Markus Petters, 14 Jul 2014

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Markus Petters on behalf of the Authors (14 Jul 2014) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Jul 2014) by Nicole Riemer

RR by Anonymous Referee #1 (28 Jul 2014)

RR by Anonymous Referee #2 (25 Aug 2014)

Suggestions for revision or reasons for rejection

The authors have addressed main points of the previous reviews. Addition of chemical composition data is certainly a significant improvement relative to the previous version. There are a few relatively minor points remaining, which are listed below with a reference to the author numbering of their replies.

R1 Issue #2 “(2) Revised paragraph”: I doubt that an example of glyoxal is a good one in this case. It is water-soluble; a large fraction of it could be lost from the gas to the wet walls in the cooling section. This is a minor question, since particle re-equilibration of semi-volatile material is hardly possible anyway (see below). The purpose of the example of high vapor pressure and low saturation ratio compounds is also not clear. Most of such compounds are in the gas phase anyway and probably will remain there even upon cooling.

R1 Issue #6: a 15% relative deviation from ambient RH is very far from being “slight”. At 50% RH it translates to a 7.5% RH deviation. For hygroscopic growth, this is a very significant difference and, actually, could translate to “huge” underestimation of water content, especially at RH > 80%. The authors should state upfront that their measurements represent a low estimate of the actual ambient water content.

R1 Issue #7, “(1) Revised discussion”: The text starting with “The absence of this effect needs to (be) interpreted…” and any discussion related to semi-volatiles in other parts of the text should be thoroughly revised. As I have mentioned in my original review, it is not feasible to achieve gas/particle equilibrium at time scales of the measurements (a few minutes). The characteristic time, i.e., e-folding time, to achieve equilibrium at ambient concentrations (~1000 1/cm3 during SOAS) and the mean particle size of 100 nm is about 30 min (for example, see eq. 12.133 in Seinfeld and Pandis, 2nd edition). If the aerosol is solid or if the accommodation coefficient is less than 1, this time will be even longer. It is a well-established fact that it is impossible to reach equilibrium in thermodenuders (residence times up to 1 min) at ambient conditions (see, for example, Cappa and Jimenez, ACP 2010, 5409-5424). Any discussion about the effect of compound activity, physical state, C*, calibration with different model compounds, etc., is absolutely irrelevant, because it is hardly possible, even when working with an ideal liquid aerosol, to “extend the residence time between the cold trap and the SMPS” to at least one hour (remember, 30 min is the e-folding time, still quite far from equilibrium). I recommend the authors remove any discussion that suggests that something could be done to quantify semi-volatiles using this method or that the observed results could be due to one or the other factor that affects aerosol volatility. The relatively long equilibration time is the main reason why no noticeable evaporation of semi-volatile material was observed. Actually, this is a good thing for the method because the interference of semi-volatile material in water determination is fundamentally limited.

As a side note, a question about the effect of removing most particle mass when selecting particles of a single size is quite misinformed too. As long as the gas chemical composition remains the same, removal or addition of particles of the same composition does not affect equilibrium. This follows from C_{g,i} = Csat,i Ca,i/Ca,tot (the basic equation of absorptive partitioning for an ideal solution, uniform molar mass and other usual assumptions), in which Cg,i, Ca,i and Csat,i are gas, particle and saturation concentrations of compound i, respectively, and Ca,tot is the total particle concentration. If Ca,i/Ca,tot remains constant, as is the case when one removes some particles from the aerosol, Cg,i will remain constant too. In other words, the aerosol remains in equilibrium. The misunderstanding of Reviewer 1, which the authors have apparently followed, comes from confusing the effect of pre-existing particle mass with the (non)effect of removing or adding particles of the same composition.

R1 Issue #8 and #9. As was discussed above, all of this discussion is irrelevant. Characteristic equilibration times for ambient aerosols, including those observed at SOAS, are of the order of one hour or longer, even if one assumes no kinetic limitations. I would also respectfully advise the authors to avoid citing the infamous Vaden et al., 2011 paper. There is much better evidence for existence of solid aerosols. This particular paper, however, is full of errors and misinterpretation (a discussion of its numerous problems can be found in the interactive discussion of a rejected ACPD paper by that group http://www.atmos-chem-phys-discuss.net/11/20107/2011/acpd-11-20107-2011-discussion.html).

R 2 Issue #3: Please confirm that the losses discussed here are not limited to the DMA, but to the complete system, including its inlet.

R2 Issue #5: The effect of residual water should be mentioned before the relevant equations. The equation for relative error in k should use a different symbol (epsilon_k or something like that), otherwise the symbol overlaps with k definition itself. It would be also useful to provide a more detailed explanation of how the values were obtained.

R2 Issue #6 (1) and (3): I am not sure why the authors mention elemental carbon in this context. It is hydrophobic and does not affect water volume. Crustal material also has only a limited effect on water absorption.

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ED: Reconsider after minor revisions (Editor review) (25 Aug 2014) by Nicole Riemer

AR by Markus Petters on behalf of the Authors (04 Sep 2014) Author's response Manuscript

ED: Publish as is (08 Sep 2014) by Nicole Riemer

AR by Markus Petters on behalf of the Authors (08 Sep 2014)

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