Influence of the dry aerosol particle size distribution and morphology on the cloud condensation nuclei activation. An experimental and theoretical investigation

Wu, Junteng; Faccinetto, Alessandro; Grimonprez, Symphorien; Batut, Sébastien; Yon, Jérôme; Desgroux, Pascale; Petitprez, Denis

doi:https://doi.org/10.5194/acp-20-4209-2020

Articles | Volume 20, issue 7

Atmos. Chem. Phys., 20, 4209–4225, 2020

https://doi.org/10.5194/acp-20-4209-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Atmos. Chem. Phys., 20, 4209–4225, 2020

https://doi.org/10.5194/acp-20-4209-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 20, issue 7

Research article 08 Apr 2020

Research article | 08 Apr 2020

Influence of the dry aerosol particle size distribution and morphology on the cloud condensation nuclei activation. An experimental and theoretical investigation

Junteng Wu et al.

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Final revised paper (published on 08 Apr 2020)
Preprint (discussion started on 22 Feb 2019)

Interactive discussion

Status: closed

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

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RC1: 'Review', Anonymous Referee #1, 06 May 2019
RC2: 'Review', Anonymous Referee #2, 02 Jul 2019
AC1: 'Answer to the Reviewers', Alessandro Faccinetto, 13 Aug 2019

Peer-review completion

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

AR by Alessandro Faccinetto on behalf of the Authors (19 Aug 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (20 Aug 2019) by Ari Laaksonen

RR by Anonymous Referee #1 (03 Sep 2019)

Suggestions for revision or reasons for rejection

The authors refocused the work and added citations about previous work to the manuscript given by both referees. However, the authors failed to properly implement those previous works in their analysis. Important experimental details are not given. As a result, the current way the results are analyzed remains insufficient to warrant publication. Specifically, the authors fail to properly account for DMA transfer effects. For example, Figure 4 shows a kappa frequency distribution for ammonium sulfate. If the effects of non-monodisperse size (not even accounting for multiple charges) had been taken into account properly, the kappa distribution should approach a Dirac delta function. It does not, and thus viewed from a high-level, the result is simply incorrect. The conclusion that “The main advantage of using a morphology-corrected is that the effects of the particle size and morphology on activation are decoupled from the particle chemistry, and therefore is preserved as a “chemistry-only” indicator.” is thus not true.

The manuscript is lacking proper treatment of the experimental details. For example, it is unclear where neutralizers were used and where not. Multiple charges are fitted through Eq. (12) but DMA theory relating to charging efficiencies and sizes is not included. The attribution to +1 and +2 contributions shown in Figure 3c are incorrect.

I continue to believe that kappa distributions (or shape factor distributions) from these data could be obtained. Closure of DMA/SMPS/CCN derived shape distributions with TEM would be valuable contribution to the literature. However this requires precise treatment of the tandem DMA and CCN measurements that removes the ambiguity of size, which requires appropriate non-trivial inversion of the data. Successful inversion should remove the effect of the DMA transfer function and retrieve the true hygroscopicity distribution, which in case of the soot, might be all attributed to shape or to oxidation state. Details about neutralizer placement and DMA transfer models should be discussed. The quality of the inversion should be assessed perhaps for a case with synthetic data. Methodology should all be discussed prior to results.

Other Comments

“To avoid this problem, kappa has been obtained from the fitting of the activation curve with generic sigmoid functions that do not take into account the particle size distribution or a distribution of values of kappa. More specifically, kappa has been calculated from the critical supersaturation (F = 0.5) by using an analytical approximation of Eq. (2) only valid for k > 0.2, which is not always the case for soot particles.”

Who did this for what type of aerosol? Please provide citations to studies that wrongly calculated kappa for soot particles. Even if this were true the above seems to not fully capture what has been done in the literature.

ammonium sulfate nanoparticles dispersed in nitrogen → did you account for viscosity differences of using pure N2 and air?

What is the justification for Eq. (4)? Why would kappa take on this distribution? Should an inversion not retrieve the distribution without prior specification?

Figure 1 shows a DMA transfer function, but the specifics have not been discussed.The manuscript should be organized in a more sequential manner.

Figure 1 x-axis should be Dve?

Section 3.1: what is the purity of the water?

pg. 8: extremely low kappa → what does that mean? Provide a number and reference?

so called → please avoid this phrasing

pg.9 please provide sheath to sample flow ratios for all DMAs. Also, please provide the aerosol neutralization method and activity of sources.

SMSP at regular time intervals → SMPS?

Section 3.4 Diagnostics: “The DMA can be used independently to select aerosol particles of the desired mobility.” → was the SMPS DMA ever used this way? If not, please omit as it is confusing.

pg. 10, Eq. (12), and Figures 3 + 4:

Eq. (12) appears to be an empirical two mode fit that is applied to the observed mobility distribution. There are numerous questions that need to be addressed.

(1) Was a neutralizer used in the SMPS? Or were the particles sampled without reneutralization from the chamber?

(2) Was the size distribution shown in Figure 2 inverted? If so, what is the justification for the two charge fit? If not, what is the justification to work with raw spectra?

(3) The number concentration and size of the +2 charged particles should not be allowed to be “floating”. The number concentration of +2 charged particles is determined strictly by the charging efficiency and that is known a priori.

(4) The kappa distribution shown in Figure 4 cannot possibly be correct. All ammonium sulfate particles have the same kappa. This means that the shown kappa distribution does not remove the effect of the DMA transfer function. What I shown here appears to be the kappa one would retrieve not properly accounting for the distribution of sizes produced by the DMA. Such a kappa distribution is not particularly meaningful as it folds in the effect of particle size.

The authors should discuss observed shape effects for AS in the literature (various sources provide a dynamic shape factor for AS particles).

While kappa has it’s application, perhaps even for soot, the authors should at least discuss the various adsorption theories that have been proposed (e.g. Henson, 2007 JGR, Sorjamaa and Laaksonen, 2007, ACP, Kumar et al., 2009, ACP), and how those approach differ from the ones used here.

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RR by Anonymous Referee #2 (23 Jan 2020)

ED: Reconsider after major revisions (23 Jan 2020) by Ari Laaksonen

AR by Alessandro Faccinetto on behalf of the Authors (05 Mar 2020) Author's response Manuscript

ED: Publish subject to technical corrections (10 Mar 2020) by Ari Laaksonen

AR by Alessandro Faccinetto on behalf of the Authors (12 Mar 2020) Author's response Manuscript

Post-review adjustments

AA: Author's adjustment | EA: Editor approval

AA by Alessandro Faccinetto on behalf of the Authors (07 Apr 2020) Author's adjustment Manuscript

EA: Adjustments approved (07 Apr 2020) by Ari Laaksonen

Short summary

Soot particles released during anthropogenic activities may lead to positive direct or negative indirect climate forcing depending on their aging in the atmosphere. The latter occurs whenever soot particles act as cloud condensation nuclei (CCN) and trigger the formation of persistent clouds. Herein, we investigate the impact of the size distribution and morphology of freshly emitted soot particles on their aging process and propose a model to quantitatively predict their efficiency as CCN.