Dynamic consideration of smog chamber experiments

Chuang, Wayne K.; Donahue, Neil M.

doi:https://doi.org/10.5194/acp-17-10019-2017

Articles | Volume 17, issue 16

https://doi.org/10.5194/acp-17-10019-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

The CERN CLOUD experiment (ACP/AMT inter-journal SI)

https://doi.org/10.5194/acp-17-10019-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 17, issue 16

Research article

|

28 Aug 2017

Research article |

| 28 Aug 2017

Dynamic consideration of smog chamber experiments

Wayne K. Chuang and Neil M. Donahue

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Final revised paper (published on 28 Aug 2017)
Supplement to the final revised paper
Preprint (discussion started on 09 Dec 2016)

Interactive discussion

Status: closed

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

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RC1: 'Referee Comment', Anonymous Referee #1, 19 Dec 2016
RC2: 'Review of "Dynamic Consideration of Smog Chamber Experiments" by Wayne K. Chuang and Neil M. Donahue', Anonymous Referee #2, 05 Jan 2017
AC1: 'Authors' Response', Wayne Chuang, 26 Apr 2017

Peer-review completion

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

AR by Wayne Chuang on behalf of the Authors (12 May 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (16 May 2017) by Kari Lehtinen

RR by Anonymous Referee #2 (29 May 2017)

Suggestions for revision or reasons for rejection

Comments on the revised manuscript:

The authors' replies and modifications made to the manuscript have clarified and improved the manuscript. I'd still ask the authors to address the following points in the final version, as I found the replies somewhat incomplete:

1) Nanoparticle coagulation: It is stated that coagulation of small particles cannot be important for the CLOUD growth rate observations for organics, as opposed to the results of Lehtipalo et al. for acid-base mixtures. The reason for bringing up the Lehtipalo et al. work was that the study discusses the significance of self-coagulation for growth also with respect to the saturation vapor pressure of the clustering vapor. Lehtipalo et al. conclude that the observed growth rates can be explained by assuming Psat = 0, at which coagulation significantly contributes to the growth. Test model runs, shown in Fig. S6 in that paper, show however that essentially the same growth rates are reached already at Psat = 5x10^-9 Pa as self-coagulation processes become important when cluster evaporation is very low.
The lowest saturation vapor pressures assumed in the current manuscript are of the order of around 10^-14 (ELVOCs)...10^-6 (LVOCs) Pa, but in their replies the authors state that that cluster self-coagulation is negligible. If self-coagulation is important at Psat = 5x10^-9 Pa, how can it not be important below it? While it may be that coagulation is not significant for organics in the actual CLOUD experiment, it must be significant in particle modeling with these model parameters - Psat values used for the ELVOCs lead to elevated concentrations of stable small particles.
(A small sidenote about the model assumptions: The statement about critical sizes being smaller than the smallest modeled size, which is the size of one molecule, is hardly justifiable - or if they are approximately the size of a monomer unit, clustering should be in practice kinetically controlled which brings us back to the coagulation question.)

2) Effects of ions and recombination: The observed growth rates are indeed mostly of the same order of magnitude for runs with and without ions, but it is not direct evidence about the particle formation mechanism being the same. The main point is that if there are elevated concentrations of small ion particles as the data show, recombination processes are likely to lead to faster formation of somewhat larger, neutral particles. Recombination rate constants are expected to be 3-4 orders of magnitude higher than vapor condensation rate constants, which makes the collision frequencies of small charged clusters of opposite polarities comparable to those of vapor molecules.

To summarize, it is clear that the condensation modeling is a valid approach for larger sizes which are the main topic of the study. It may, however, be useful to acknowledge that dynamic processes other than simple vapor condensation may contribute to the observed high growth rates of especially the smallest particles. In this case, the least volatile species don't need to be as low-volatile as assumed, and/or they don't need to exist at as high concentrations as assumed. This remark is not meant as criticism at the present modeling study, but as an attempt to suggest additional reasons to explain the rapid appearance of subsequent particle size classes in the CLOUD chamber.

Minor comment:
Constant HOM and increasing HOM: The explanation about the HOM reaching steady state in 10 minutes is not evident in the CLOUD data - it seems to take much longer according to the figures in Kirkby et al. and Tröstl et al., and this is the reason for asking the original question. The time scale can of course be assessed from the loss rate assuming a constant source -although the HOM source isn't exactly constant as the alpha-pinene concentration increases- and negligible losses on particles: reaching e.g. 90% of the steady-state value should take about a half an hour (during which particles should already be appearing). But the "constant HOM" case in Fig. 3 in Tröstl et al. does not show any increase during the first 10 minutes, or at any times, and the only experiment shown where [HOM] does not increase is the ion run (Fig. S8 in Tröstl et al.)
I understand that this point is more relevant to the Tröstl et al. work, but as I could not find in that paper any explanation for the "constant" and "increasing" HOM cases, it is appropriate to clarify them in the current manuscript since these cases are simulated and discussed here.

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ED: Reconsider after minor revisions (Editor review) (07 Jun 2017) by Kari Lehtinen

AR by Wayne Chuang on behalf of the Authors (17 Jun 2017) Author's response Manuscript

ED: Publish as is (27 Jun 2017) by Kari Lehtinen

AR by Wayne Chuang on behalf of the Authors (05 Jul 2017)

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Short summary

Experiments on organic aerosol formation are important for our understanding of climate. Recent experiments on the reaction of ozone with alpha-pinene showed high production of extremely low-volatility organics. This appeared to contradict prior volatility distributions derived from equilibrium partitioning. We examined this using a dynamic volatility basis set model and found that the delay between the production and condensation of organics is integral to reconciling this difference.