A comparison of lognormal and gamma size distributions for characterizing the stratospheric aerosol phase function from optical particle counter measurements

Nyaku, Ernest; Loughman, Robert; Bhartia, Pawan K.; Deshler, Terry; Chen, Zhong; Colarco, Peter R.

doi:https://doi.org/10.5194/amt-13-1071-2020

Articles | Volume 13, issue 3

https://doi.org/10.5194/amt-13-1071-2020

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

https://doi.org/10.5194/amt-13-1071-2020

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

Articles | Volume 13, issue 3

Research article

|

06 Mar 2020

Research article |

| 06 Mar 2020

A comparison of lognormal and gamma size distributions for characterizing the stratospheric aerosol phase function from optical particle counter measurements

Ernest Nyaku, Robert Loughman, Pawan K. Bhartia, Terry Deshler, Zhong Chen, and Peter R. Colarco

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Final revised paper (published on 06 Mar 2020)
Preprint (discussion started on 12 Apr 2019)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review of amt-2019-119', Anonymous Referee #1, 19 Apr 2019
- AC1: 'Reactions to Specific Comments from Anonymous Referee #1:', Ernest Nyaku, 26 Sep 2019
RC2: 'Review on Nyaku et al. (2019)', Anonymous Referee #2, 29 May 2019
- AC2: 'Reactions to Specific Comments from Anonymous Referee #2', Ernest Nyaku, 26 Sep 2019
RC3: 'Review', Anonymous Referee #3, 04 Jun 2019
- AC3: 'Reactions to Specific Comments from Anonymous Referee #3', Ernest Nyaku, 26 Sep 2019

Peer-review completion

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

AR by Ernest Nyaku on behalf of the Authors (27 Sep 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Oct 2019) by Troy Thornberry

RR by Anonymous Referee #1 (25 Oct 2019)

Suggestions for revision or reasons for rejection

General comments

In my opinion the authors did a good job responding to my comments on the previous version. I have some additional (mainly minor) comments and ask the authors to consider them. The paper is very well written and should be accepted after a minor revision.

Specific comments

Page 1, line 10: “.. the gamma distribution also fits the CARMA ..”

Why “also”? I think “also” doesn’t fit here.

Page 2, line 3: “Antartic” -> “Antarctic”

Page 2, line 9: “years has” -> “years have”

Page 2, line 14: “pyrocumulonumbus“ -> “pyrocumulonimbus“ or just “pyrocumulus“

Page 3, line 34: “AE values greater than 2 are indicative of small particles“

Without mentioning specific sizes this statement does not convey much information. What is „small“? Please be more specific or remove the statement. Or perhaps you just want to say that the AE depends on particle size and that AE decreases with increasing particle size (at least in a limited size range)?

Page 4, line 13: “when the exact AE used for the radiance simulations is applied in the extinction retrievals“

I read this sentence several times, but I’m not sure I understand what it means. Can you clarify?
How is the AE used?

Page 6, equation (2): The solid dots in the denominators are unusual? This should be $\times$, right?

Page 6, line 10: “the number of usable measurements decreases“

Not really clear, what this refers to? A decrease of the number of usable measurements over time, with increasing channel etc.? Please specify.

Page 7, line 15: „radii size“ sound somewhat tautological.

Page 8, line 21: “number size distribution“ is not a fully correct description and may be misleading.

Page 9, line 1: “aerosols loads“ -> “aerosol loads“ ?

Page 10, line 14: “The shape of the phase functions has been observed to depend on magnitude of the median radius (rm) in the case of the UMLN distribution“

The shape of the phase function certainly also depends on the distribution width, right?

Page 12, caption Fig. 3: “SCIAMACY“ -> “SCIAMACHY“

Page 13, line 6: Please remove parentheses after “is nearly constant for scattering angles OF“

Page 13, line 7: “ Errors in estimating the number of particles within this range by the function used for fitting the data will lead to uncertainties in the phase function as shown by the X = 1 plot in Figure 5.“

It's not fully correct to state or imply that the X=1 line represents the phase function error if the small particles are neglected. The contribution of these smaller particles to the overall scattering is small because of the strong size dependence of the scattering cross section.

Page 14, Figure 5: Something is wrong here. The case with X=10 should have the most pronounced forward scattering peak, i.e. a larger value of the phase function at a scattering angle of 0 deg compared to the cases. Please check.

Page 15, line 4: “compered“ -> “compared“

Page 16, caption Fig. 6: “SCIAMACY“ -> “SCIAMACHY“

Page 17, line 14: “and degassing volcanoes that are not explosive in nature“

What about the eruptions of Kasatochi, Sarychev Peak, Nabro, Calbuco etc. that occurred during this period of time? They were not particularly strong, but they were explosive, injected sulfur compounds into the stratosphere and affected the aerosol size distribution. If this eruptions were not considered, how can the size distribution be modelled accurately?

Page 17, line 18: “distributions fits“ -> “distribution fits“

Page 17, line 22: “are computed“ -> “is computed“

Same sentence: Have you tested how the chi^2 results change if only the bins are considered which are used for the fitting process? The results are probably different, but would be more consistent with the results obtained when fitting the OPC measurements. I suggest checking.

Page 18, line 17: “The good comparison .. provides evidence for the agreement“ sounds tautological.

Page 18, line 23: “Space Station )“ -> “Space Station)“

Page 21, line 6: “the scattered radiance arises from the aerosol phase function“

This is not well phrased, I think. The radiance arises (in part) from scattering by aerosols - whose angular distribution is described by the phase function …
But the scattered radiance does not arise from the phase function.

Page 21, line 22: “some what“ -> “somewhat“

Same line: “because of its insensitivity to particles between“

Is this really the case? What kind of “insensitivity“ do you mean? I don’t really understand this statement.

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RR by Anonymous Referee #3 (01 Nov 2019)

RR by Anonymous Referee #2 (04 Nov 2019)

ED: Publish subject to minor revisions (review by editor) (15 Nov 2019) by Troy Thornberry

AR by Ernest Nyaku on behalf of the Authors (06 Dec 2019) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (20 Jan 2020) by Troy Thornberry

I think overall the manuscript is in good shape for publication, although the conclusion/summary section perhaps needs some attention as was noted by a reviewer last fall.

The conclusions/summary should include more quantitative results, at least to the level of the abstract (if not more). The discussion of the fit comparison in paragraph 2 (x^2 details) is wordy and repetitive. Worth noting again that the particle size measurements are expressed as radii. What would be good to have (as noted by a reviewer previously) would be some estimate of the importance (significance) of the Pa(theta) measurement/model difference (Fig 10) on LS retrievals—doesn’t need to be extensive, but it would seem to be what is needed to assess the overall impact of the work.

A few specific comments to consider, beyond what will likely be addressed in copyediting:

P2: SO2 doesn’t need to be italicized.
P2,L7: “SO2 is also injected”—the “also” doesn’t have a referent since you have not mentioned other SO2 pathways to the stratosphere
P2,L7-13: the volcanic role in stratospheric aerosol is missing nuance—blanket statements about injection height and duration of impact are simplistic. Should at least express as ranges: “effects can last up to several years”, “injections can reach”, might mention impact dependence on latitude
Good additional references for the role of moderate volcanic activity in the stratospheric aerosol layer would be:
R. R. Neely III, O. B. Toon, S. Solomon, J. P. Vernier, C. Alvarez, J. M. English, K. H. Rosenlof, M. J. Mills, C. G. Bardeen, J. S. Daniel, J. P. Thayer, Recent anthropogenic increases in SO2 from Asia have minimal impact on stratospheric aerosol Geophysical Research Letters, doi:10.1002/grl.50263, 2013.
Mills, M. J., et al. (2016), Global volcanic aerosol properties derived from emissions, 1990–2014, using CESM1(WACCM), J. Geophys. Res. Atmos.,121, doi:10.1002/2015JD024290.
P2:L14: “pyrocumulus clouds from large wildfires that can inject”
P2:L15: see Neeley et al ref on ASM SO2 source. There are recent Pengfei Yu et al and Felix Ploeger et al papers that would be useful to reference for the ASM statement.
P2:L22: RI of 1.45+0i is assumed for all wavelengths?
P3,L34: “retrieval” should be “retrievals”
P5,L24: “aerosol concentration measurements”
P5,L27: “sulfuric acid vapor”?
P6,L4: calibration is used in addition to Mie theory to convert signal to particle size, correct?
P21,L7: suggest “molecular scattering and trace species absorption”

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AR by Ernest Nyaku on behalf of the Authors (29 Jan 2020) Author's response Manuscript

ED: Publish as is (03 Feb 2020) by Troy Thornberry

AR by Ernest Nyaku on behalf of the Authors (03 Feb 2020) Manuscript

Short summary

This paper shows the importance of the nature of the aerosol phase function used in the retrieval of the stratospheric aerosol extinction from limb scattering measurements. The aerosol phase function is derived from the parameters using either a unimodal lognormal or gamma aerosol size distribution. These two distributions were fitted to the same aerosol concentration measurements at two altitudes, and depending on the nature of the measurements, each distribution shows its strengths.