Vapor wall deposition in Teflon chambers

Zhang, X.; Schwantes, R. H.; McVay, R. C.; Lignell, H.; Coggon, M. M.; Flagan, R. C.; Seinfeld, J. H.

doi:https://doi.org/10.5194/acp-15-4197-2015

Articles | Volume 15, issue 8

https://doi.org/10.5194/acp-15-4197-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/acp-15-4197-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 15, issue 8

Research article

|

23 Apr 2015

Research article |

| 23 Apr 2015

Vapor wall deposition in Teflon chambers

X. Zhang, R. H. Schwantes, R. C. McVay, H. Lignell, M. M. Coggon, R. C. Flagan, and J. H. Seinfeld

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Final revised paper (published on 23 Apr 2015)
Supplement to the final revised paper
Preprint (discussion started on 24 Oct 2014)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC C10426: 'Reviewer comment to Zhang et al.', Anonymous Referee #1, 23 Dec 2014
- AC C12000: 'Response to Reviewer #1', xuan zhang, 07 Feb 2015
SC C10518: 'Organic Compound Losses in teflon Chambers', Paul Ziemann, 26 Dec 2014
- AC C12002: 'Response to Prof. Ziemann', xuan zhang, 07 Feb 2015
RC C10977: 'Review of Zhang et al.', Anonymous Referee #2, 08 Jan 2015
- AC C12001: 'Response to Reviewer #2', xuan zhang, 07 Feb 2015

Peer-review completion

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

AR by Xuan Zhang on behalf of the Authors (07 Feb 2015) Author's response

ED: Referee Nomination & Report Request started (17 Feb 2015) by V. Faye McNeill

RR by Anonymous Referee #2 (12 Mar 2015)

Suggestions for revision or reasons for rejection

The authors have done a very thorough job of responding to the reviewer comments, as well as the comments by Paul Ziemann, including performing a new experiment. This paper should be published once the authors address the minor issues below.

The addition of the CIMS measurements prior to when the lights were turned on in Fig. 3 is very useful. However, it seems as if Fig. 3 is showing the non-background subtracted signals. The authors should note this in the caption.

Regarding the comparison between the current study and the work from Ziemann and co-workers, there remains an unfortunate discrepancy. It is very difficult to reconcile the results from the two studies, and there is overlap in the vapor pressure ranges considered, even if the functional group types here are more diverse. Certainly, this issue is not going to be worked out from this single study. Nonetheless, in the authors response they note that they tentatively attribute the discrepancy to differences in mixing between the two chambers. I think that there are two main arguments against this being the predominant aspect.

1. The theoretical loss rate of gases, absent accommodation limitations, is governed by mixing and diffusion. This has not been measured here for the gases, but this group has previously reported wall loss rates for particles as a function of size. These particle wall loss rates provide constraints on the potential for differences in mixing to be responsible for the differences. Although the Caltech chamber may operate in a static mode (no active mixing), the loss rates of particles are nonetheless substantial, even for the smallest particles for which sedimentation is not an issue. I believe the consideration of these particle wall loss rates places bounds, at least approximate ones, on mixing effects. The particle loss rates reported in Loza et al. (2012) for the smallest particles sampled (~10 nm) were on the order of 10 4 s 1. This is much larger than many of the values observed in the current study, suggesting that diffusion (i.e. mixing) is not a limiting factor.
2. Although the authors rightly note that Ziemann and co-workers use “active” mixing, the action is very limited in scope. Specifically, it appears that after injection into the Ziemann chamber, a small fan is turned on for a few 10’s of seconds before it is turned off for the remainder of the experiment. Thus, the “active” mixing is only at the very, very beginning of the experiment. In the current study, the authors fill the bag and allow the system to sit for ~1hr prior to turning on the lights. Mixing, i.e. homogenization, will occur during this period thus that at the point when the lights are turned on the system may, in fact, be reasonably well mixed. As such, the differences in the experiments may not be as large as one might think.

My point is not that one of the studies is right or wrong, but simply that I don’t think that differences in mixing will ultimately prove to be the final answer to the disparity between these studies. The authors have obviously done their due diligence in that they performed an additional experiment as recommended by Ziemann and this certainly provides further confidence in the results from the current study. In any case, I would suggest that the authors update their “tentative attribution” statement. Hopefully this discrepancy will be fully resolved through future studies, as this is an important topic.

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RR by Anonymous Referee #3 (25 Mar 2015)

Suggestions for revision or reasons for rejection

The manuscript addresses an important topic of vapor wall losses and presents a protocol for constraining vapor wall interactions in Teflon chamber. In general the manuscript is well written, presents quality research and the topic suits well for ACP. Therefore publication in ACP can be recommended. Several parts of the manuscript had been improved in the revised version making the manuscript more clear. However, I find that couple of issues in the modelling framework still require clarification before publication of the manuscript. Below are my specific comments.

P6 eq. 6: I agree with reviewer #1 in that the it is not clear how Eq. 6 is obtained and this was not clarified by the reply of the authors’ or the revised manuscript. The first part of Eq. 6 is based on diffusion and concentration gradient and describes the net molecular flux towards wall (difference between those coming from gas phase and those evaporating from the wall). The right hand side of the Eq. 6 is the kinetic flux of molecules to the wall with non-perfect accommodation accounted for. However, as it is written in Eq. 6 that these two expressions are equal, it seems that evaporation from the wall was included in the alpha_w. This means that alpha_w is not the actual accommodation coefficient defined in the similar way as for particles (=fraction of molecules that accommodate on the surface from those colliding it) but an “effective” accommodation coefficient including the evaporation. In order for alpha_w in Eq. 6 to represent the actual accommodation coefficient the right hand side of the equation should include evaporation term as noted already by reviewer #1. If alpha_w includes evaporation then it should indeed depend on the concentration of i in the wall and the volatility of the compounds. In that case it is not clear if the derivation of Eq. 12 for the evaporation flux of i is consistent as Eq. 10b assumes that k_w,depo,i does not include evaporation (which it seems to include in alpha_w). Therefore, I am wondering if the Eq. 6, 8 and 12 are inconsistent or if there was some additional assumptions incorporated in the deriving of them which are not stated in the text. Could the authors clarify their definition of alpha_w in the text?

P7 L265-283: It is not clear from the text which values of the gas phase concentration were used from the measurements in solving Eq. 16 and 17. Was the concentration @298K taken just before the temperature was increased and concentration @318K taken at the end of the experiment? The Eq. 16 and 17 are valid only for equilibrium situation. In Supplementary Material it is stated based on model results that gas-wall equilibrium is reach in 20-25 hours. However, based on Fig. 3 it seems like gas-wall equilibrium was not reached during the experiment as the concentrations do not reach constant value for most compounds. In some cases equilibrium was almost reached at 298K but none of the cases show clear equilibration at 318K. Could this cause error or uncertainty in the C_w and alpha_w values and could the effect be dependent on the volatility of the compound?

Minor comments:
Table 2: The equation numbers in the footnotes seem to refer to wrong equations.
P35, Second last line of figure caption of Fig. 3: It says that red circles represent ‘re-evaporation’. Maybe the ‘re-evaporation’ is a typo as the concentrations are going down.
Figure 1: The figure shows different expression for J_w,i compared to the text (Eq. 12). It seems that the equation given in the Fig. 1 was not used in the calculations and therefore I suggest removing that expression from the Fig. 1.

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ED: Publish subject to technical corrections (25 Mar 2015) by V. Faye McNeill

AR by Xuan Zhang on behalf of the Authors (09 Apr 2015) Author's response Manuscript

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Short summary

We present an experimental protocol to constrain the nature of organic vapor--wall deposition in Teflon chambers and develop an empirical model to predict the wall-induced deposition rate of intermediate/semi/non-volatility organic vapors in chambers.