the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Mar 2025
Spatiotemporal variations of stratospheric aerosol size between 2002 and 2005 from measurements with SAGE III/M3M
Felix Wrana
Terry Deshler
Christian Löns
Larry W. Thomason
Christian von Savigny
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- Final revised paper (published on 27 Mar 2025)
- Preprint (discussion started on 30 Sep 2024)
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2024-2942', Anonymous Referee #1, 24 Oct 2024
Review of Wrana et al. manuscript submitted for publication in ACP, "Variability of stratospheric aerosol
size distribution parameters between 2002 and 2005 from measurements with SAGE III/M3M"This manuscript presents an analysis of large-scale variations in particle size distribution (PSD) within
the stratospheric aerosol layer, as derived from satellite measurements of aerosol extinction across
multiple wavelengths.The stratospheric aerosol PSD variations, from 4 years of the SAGE-III/M3M record (2002-2005), span the mid-latitudes
and high-latitudes of both hemispheres, and with the satellite's orbit measuring at high-latitudes through
the polar winter season, represent a substantial and ground-breaking analysis.The analysis includes three modest-SO2 large-magnitude explosive tropical eruptions, finding these smaller eruptions
(small compared to Pinatubo and Hunga) cause an increase in particle number, but a reduction in particle size,
The high-latitude analysis also identifies a clear decrease in particle size during polar winter, likely associated with
either the influx of meteoric smoke particles (e.g. Curtius et al., 2005; Weigel et al., 2014) and/or new sulphate aerosol
particle formation in late-winter/early-spring (e.g. Campbell and Deshler, 2014).The manuscript is mostly very well-written, particularly the results section 3, with excellent discussion of the potential
drivers for the observed PSD variations, and the topic will be of substantial interest to the stratospheric aerosol community.There are a few paragraphs in earlier parts of the manuscript (particularly section 2.2) that require some minor changes,
and some terminology requires sharpening up slightly -- e.g. text referring to "the solar occultation data set" needs
to be re-worded to be specific to the aerosol extinction retrievals, and the text "aerosol size retrieval" is not really
appropriate, since the aerosol extinction is already a retrieval, and the wording needs to be clear the PSD paramaeters
are derived from the aerosol extintion retrieval -- i.e. "derived aerosol size product" or "derived PSD parameters".However, this really is an excellent analysis, and once the set of minor revisions below are attended to, this will
represent an important and valuable paper, very suitable for publication in Atmospheric Chemistry and Physics.Main Minor Revisions
--------------------M-MR-1) "The solar occultation data set" -- Abstract line 1, section 1, line 56, section 2.2, line 108 (and elsewhere)
As mentioned above, the text refers to the SAGE-III/M3M data analysed as "the solar occultation data set", but this is
not sufficiently identifying the sub-set of the SAGE-III data being analysed. Remember that SAGE-III measures not only
aerosol but also stratospheric ozone, NO2 and water vapour, and all of these SAGE-III data-products are measured applying
the solar occultation technique. Please refer to "the multi-wavelength aerosol extinction data set" or
"stratospheric aerosol data set" to be clear it's the aerosol extinction that's being analysed.For the Abstract line 1, suggest to change "derived from the solar occultation data of the" instead to "derived from
the multiple-wavelength aerosol extinction retrevals of the"For section 1, line 56, change "is investigated using the solar occultation data set of the" instead to
"is investigated using multiple-wavelength aerosol extinction retrevals of the"For section 2.2, line 108, change "The SAGE III/M3M solar occultation data set is used in this work to derive parameters..."
instead to "The SAGE III/M3M multiple-wavelength aerosol extinction data set is used in this work to derive parameters...",M-MR-2) "for the retrieval of all parameters" (Abstract line 3), "the aerosol size retrieval" -- section 2.2 lines 118 & 127,
and "the retrieval method" on section 2.2 line 113. This is the other "main minor-revision", to avoid the word "retrieval"
when referring to the particle size parameters.The individual-wavelength aerosol extinction is itself a retrieval, and although aerosol extinction at ~1 micron is quite
"clean" in relation to the majority of the extinction being aerosol, for the other wavelength aerosol extinctions analysed,
there is still some uncertainty even within the aerosol extinction (in relation to other species contributing to the measured
extinction at that wavelength, see e.g. McCormick (1987) Figure 4 and Chu and McCormick (1979) Figure 3).More generally, when the particle size is derived from multiple aerosol extinctions at different wavelengths, it is then
no longer appropriate to refer to the derived size products as a "retrieval". Please check through the manuscript, and
change "retrieval" instead to "derived product" or "derived PSD parameters" or similar where referring to the size.aFor Abstract line 3, change "allows for the retrieval of all parameters controlling the assumed monomodal lognormal..."
instead to "enables to derive a best estimate for the 3 parameters within an assumed monomodal lognormal..."For section 2.2 line 118, change "a common assumption in stratospheric aerosol size retrievals from satellite measurements"
to "a common assumption when deriving aerosol size distribution from satellite measurements".For section 2.2 line 121, change "there is not enough independent spectral information in satellite measurements to
facilitate a retrieval with a more complex model". That's roughly correct, but it's more to do with non-uniqueness
of the solution, i.e. the derivation to particle size is argued by some to not be sufficiently well-defined to warrant
a more complex particle size model. However, this is a much debated issue, and for example Thomason et al. (2008)
used a bimodal size distribution assumption within their derivation of Surface Area Density and PSD parameters.
In-situ optical particle measurements clearly show that after volcanic eruptions there is clearly a separate mode
with larger volcanic aerosol (see e.g. Deshler, 2008, Figure 4), and the text here needs to be changed
to reflect that there are differing opinions on this.This sentence (118 to 121) is already quite long , and then to clarify this, suggest to re-structure into 2 sentences.
Within lines 118-119, please delete "on the one hand", and put full-stop after "true conditions".
And then please add "However, in-situ optical particle measurements after the Pinatubo eruption clearly show the presence
of a second mode with much larger volcanic aerosol (see e.g. Deshler, 2008, Figure 4), with Thomason et al. (2008)
using a bimodal size distribution assumption within their method to derive Surface Area Density and PSD parameters."And then pls change ", but more importantly out of necessity, since usually there is not enough independent spectral
information in satellite measurements to facilitate a retrieval with a more complex model" to
"However methods remain largely mono-modal, and it remains unclear whether or not the derivation of particle size
from spectral aerosol extinction satellite measurements is precise enough to warrant applying a more complex
multi-modal particle size model".For section 2.2 line 139, change "The retrieval of median radius and mode width is independent of..."
instead to "The derived values of median radius and mode width are independent of..."For section 2.2 line 144, change "from the retrieved median radius..." to "from the derived median radius..."
For section 2.2 line 145, change "can be calculated from the retrieval results" to "can be calculated from the derived PSD parameters".
For section 2.2 line 155, change "noisy data in the retrieved quantities." to "noisy data in the derived PSD parameters."
M-MR-3) Improve the terms in the equations, and symbols used, within section 2.2 lines 145-154
Please use roman (not italics) for abbreviations "med" and "eff", and the natural logarithm & exponential functions (ln and exp),
For these equations 3 and 4, the abbreviations Rmed and Reff need to have the "med" and "eff" as roman text (not italics),
to be clear these are not additional variables or indices within a matrix etc. Please change the "med" and "eff" also to
roman style (rather than italics) in the references to Rmed and Reff in the text (e.g. line 148).
Please add "(Reff)" after "effective radius" on line 145, to introduce the abbreviation, at that first use.
Please also add "aerosol particle" before "effective radius" (line 145) to be clear this is not cloud effective radius.M-MR-4) Change text "the size of individual aerosol particles can be measured" to avoid potential confusion re: the OPC instrument.
The Wyoming OPC is described to a good level of detail in Deshler et al. (2003). It does not measure the size of
individual particles, but counts the number of particles crossing a beam of white light, via individual pulse-occurrences
of forward-scattered light at the 40 degree forward-scattering angle. The information on lines 179-192 suggest this is
understood, but the information on line 174 could lead some readers to misunderstand the operation of the instrument.Please change "Using an optical particle counter (OPC), the size of individual aerosol particles can be measured.
This is a major advantage over satellite measurements, whether the measured signal originates from many different aerosol sizes".Suggest to replace "Using an optical particle counter (OPC), the size of individual aerosol particles can be measured.
This is a major advantage over satellite measurements, whether the measured signal originates from many different aerosol sizes".
instead with "An optical particle counter (OPC) measures the number of aerosol particles at light-scattering sizes,
and such observations have provided the ground-truth for stratospheric aerosol satellite measurements since the advent
of the SAM-II and SAGE instruments (e.g. McCormick et al., 1977)."That 2nd sentence could also lead to misunderstanding from some readers, and in fact the OPC is measuring a range of particle
sizes, and whilst with this split-sentence, the statement there is not obviously incorrect, it would be misleading
as currently worded.Suggest to replace "This is a major advantage over satellite measurements, whether the measured signal originates from many
different aerosol sizes", instead to "The number of aerosol particles is a key microphysical property (e.g. for particle growth),
and although PSD parameters can be derived from satellite, comparisons to in-situ OPC measurements are required to validate
and refine aspects of the techniques (e.g. Oberbeck et al., 1989; Hervig and Deshler, 2002)."M-MR-5) Figure 7 --- please add pale-blue or very-pale-grey shading across the temperature range T = 195K and lower.
This then will have sharing between 21 and 25km in the December 4th 2002 Figures -- add this to all 4 rows of
that central column. That will then make this much clearer to the reader that this is a PSC being measured there.
(rather than an aerosol enhancement).Other Minor Revisions
----------------------O-MR-1) Title -- The word "Varibility" somehow to me suggests an analysis across a relatively short period.
Since this is a global-scale analysis of the particle size variations, suggest to replace "Variability of"
instead with "Large-scale variations in the", and delete "parameters" (don't need to specify that in the title).A change in title is obviously a decision for the authors, but I think "variation" is more scientific than simply
"variability", and think that "large-scale" or "hemisphere-scale" should be stated in the title
(to give an indication of the type of variation analysed), Could possibly add "spatio-temporal" also.O-MR-2) Abstract lines 1-2 -- change "is shown" and insert "analysed for" before "their evolution".
The Abstract needs to present the MS to be an analysis of the measurements, which it does, very well.O-MR-3) Abstract line 4 -- change "there were three smaller tropical eruptions" -- whilst it would be OK
to say "smaller eruptions" if caveated to "(than Pinatubo)", in order to reach the stratosphere,
the volcanic eruptions must already be "large-magnitude explosive", so re-word this here.
I think by "small" you mean the amount of SO2 emitted was low/modest, compared to Pinatubo.Suggest to re-word to "The 2002-2005 stratospheric aerosol layer was mostly at close to background
conditions, but included three moderate-magnitude tropical eruptions (Ruang, Reventador and Manam)".Then re-word the follow-on sentence instead to refer to the latitudes measured
"The SAGE III/M3M satellite measured only in mid- and high latitudes, but derived PSD parameters
indicate a reduction in particle size after all 3 eruptions (within an increased particle number)".
Or similar wording, to be consistent with the way you choose to summarise this initial finding here.O-MR-4) Abstract line 6 -- change "Apart from the likely effect of" to "In addition to this effect of..."
(The "Apart from" somehow seems negative, and "In addition" makes clear this is a separate effect.)O-MR-5) Introduction line 14 -- suggest to add cites to Solomon et al. (2011) and Kremser et al. (2016)
here re: the stratospheric aerosol layer (after "layer of aerosol particles").O-MR-6) Introduction line 15 -- change "It was first measured" to be clear you mean measured in-situ,
and also give the actual year the first stratospheric aerosol balloon measurements were made, 1957
(see Figure 14 of Junge et al., 1961). Also, it was the high-profile Science paper by Junge et al. (1961)
that brought the recognition of the stratospheric aerosol layer to the general science audience,
and then suggest to cite 1961a and 1961b, as listed in the References below.Suggest to replace "It was first measured and described by Junge et al. (1961) through balloon measurements"
with "The stratospheric aerosol layer was first measured from high-altitude balloon soundings in 1957
(Junge et al.,1961a, Junge et al., 1961b) ...O-MR-7) Section 2.1, line 79 -- There needs to be a cite to a paper for the SAGE III/M3M aerosol
measurements here, and suggest to cite Thomason and Taha (2003) here.O-MR-8) Section 2.1, line 80 -- Suggest to add mention to SAGE-II here, and then cite the 1987
McCormick paper (which you already cite) in the context of the range of different species measured.Specifically, suggest to add "essentially the same range of" before "different atmospheric" and
replace "like" with "as SAGE-II (see McCormick, 1987)"O-MR-9) Section 2.2, lines 109-110 -- Please cite also the original methods that were developed
by Yue et al. (1986) to derive the size/SADparameters from SAGE-II, and the 1996 paper that compared
the original method from Yue86 to the PCA method from Thomason and Poole (1993).Specifically, suggest to change "is in essence the same as the one used in" with "is similar to the
original methods for deriving size parameters from SAGE-II (see Yue et al., 1986, Yue et al., 1995),
and was used also in..."O-MR-10) Section 2.2, lines 113-114 -- This sentence is too colloquial (in some places), and suggest to
change "A number of assumptions underly the retrieval method and therefore also have to be kept in mind.."
with "There are a number of assumptions within the PSD parameters derived from SAGE-II, which must be
considered when interpreting the results.". (delete "later on")O-MR-11) Section 2.2, line 117 -- Please change "Here, the three parameters controlling the monomodal lognormal
size distribution are the median radius..."" with "In this form, there are 3 parameters that together describe
the aerosol particle size distribution: the median radius..."O-MR-12) Section 2.2, lines 151-152 -- Please change "It is much more useful than the mode width".
That's a rather subjective statement, and remember they're actually the same quantity, just that one is
an absolute measure of the size variation within the mode, and the other is a relative measure.I have seen that in previous papers the terminology is established to be "absolute mode width" when
referring to the (absolute) standard deviation. But the term sigma here would usually be expected
to denote that absolute quantity, the absolute standard deviation. To avoid confusion, within this paper,
I am requesting to add sub-script g in all instances of the sigma, which here is the geometric standard deviation.Please change all instances of sigma to be sigma-subscript-g, then being clear when you say "the mode width"
(without "absolute") you actually mean the geometric standard deviation (i.e. a mode width of 1.0 means there
is no variation in size within the lognormal, the mode is mono-disperse).O-MR-13) Section 2.2, lines 152-153 -- Further to the above, please change "It is more useful than...",
to "The absolute mode width provides the variation in nanometers, with variations then easier to interpret than..."O-MR-14) Section 2.2, line 155 -- add "for the derived PSD parameters" after "an accuracy parameter" and
change "to exclude noisy data" instead to "to be able to exlcude less reliable data".O-MR-15) Section 2.2, line 167 -- replace "in 0.5km steps" with "at 0.5km vertical resolution".
O-MR-16) Section 2.2, line 180 -- the reference to "12 size classes" here is mis-leading.
The 12 size channels are counting particles larger than 12 different minimum sizes
(see Deshler et al., 2019). So please change "in 12 size classes" to "larger than 12 size-cuts
(see Deshler et al., 2019)".
O-MR-17) Section 2.2, line 181 -- replace "For this, the scattering of white light by, ideally, single aerosol particles
is measured at an angle of 40o relative to the incident light of the incandescent lamp." with
"The instrument measured individual pulses of forward-scattered white light at an angle of 40 degrees,
a photomultiplier counting individual aerosol particles."O-MR-18) Section 2.2, lines 184-185 -- change "Aerosol size is retrieved from the measurements" -- it isn't.
There's a threshold size associated with each "size channel", but that's the lower-limit size for
particles being measured within the number concentration for that size-channel.I think it's best to delete the entire sentence there beginning "Aerosol size is retrieved..."
O-MR-19) Section 2.2, lines 186-187 -- the wording here re: the CN measurement needs improvement.
A suggested re-wording is to change "Also included are separate measurements of the total..."
with "All Kiruna W-OPC soundings analysed here also included a separate measurement of the total"
(I'm assuming that's the case, please check this, or clarify how many did included CNC as well as OPC).O-MR-20) Section 2.2, line 187-188 -- correct "glycole" to "glycol" and re-word "forcing the aerosol
particles to grow to a detectable size through the condensation of..." with "aerosol particles larger
than 10nm grown to light-scattering sizes via condensation of..."O-MR-21) Section 3.3, line 344 -- add "ice" before "frost point" and add also "(~3K below the NAT
frost point)". That then makes clear, that the STS formation temperature is interim between
TICE and TNAT.O-MR-22) Section 3.3, line 348 -- suggest to replace the Tritscher et al. (2021) review article cite
with the studies of Hoyle et al. (2013) and Engel et al. (2013).O-MR-23) Section 3.3, line 356 -- please change "plot" to "in Figure 6"
O-MR-24) Section 3.3, line 357 -- please change "of perturbed aerosol extinction"
instead to "of strongly elevated aerosol extinction".O-MR-25) Section 4, line 377 -- please insert "aerosol" before "extinction coefficient".
O-MR-26) Section 4, line 379 and Figure 7 -- please add labels "a)", "b)" etc. to the sub-panels
in Figure 7m and change "topmost" to refer to "Figure 7a" etc.O-MR-27) Section 4, line 385 -- change all instances of "collocations" to "co-locations"
References
----------Campbell and Deshler (2014)
Condensation nuclei measurements in the midlatitude (1982–2012) and Antarctic (1986–2010) stratosphere between 20 and 35km
J. Geophys. Res. Atmos, vol. 119, 137–152, https://doi.org/10.1002/2013JD019710.Chu and McCormick (1979)
Inversion of stratospheric aerosol and gaseous constituents from spacecraft solar extinction data in the 0.38-1.0 micron
wavelength region, Applied Optics, vol. 18, no. 9, 1404-1413, https://doi.org/10.1364/AO.18.001404Curtius et al. (2005)
Observations of meteoric material and implications for aerosol nucleation in the winter Arctic lower stratosphere derived
from in-situ particle measurements,
Atmos. Chem. Phys., 5, 3053–3069, https://doi.org/10.5194/acp-5-3053-2005Deshler (2008)
A review of global stratospheric aerosol: Measurements, importance, life cycle, and local stratospheric aerosol
Atmospheric Research, vol. 90, 223–232, https://doi.org/10.1016/j.atmosres.2008.03.016Deshler et al. (2019)
Retrieval of Aerosol Size Distributions From In Situ Particle Counter Measurements: Instrument
Counting Efficiency and Comparisons With Satellite Measurements
J. Geophys. Res.: Atmos, 124. https://doi.org/10.1029/2018JD029558Engel et al. (2013)
Heterogeneous formation of polar stratospheric clouds – Part 2: Nucleation of ice on synoptic scales
Atmos. Chem. Phys., 13, 10769–10785, https://doi.org/10.5194/acp-13-10769-2013Hervig and Deshler (2002)
Evaluation of aerosol measurements from SAGE II, HALOE, and balloon-borne optical particle counters
J. Geophys. Res., vol. 107, no. D3, 4031, https://doi.org/10.1029/2001JD000703Hoyle et al. (2013)
Heterogeneous formation of polar stratospheric clouds – Part 1: Nucleation of nitric acid trihydrate (NAT)
Atmos. Chem. Phys., 13, 9577–9595, https://doi.org/10.5194/acp-13-9577-2013Junge, C. E., Chagnon, C. W., and Manson, J. E. (1961a)
Stratospheric aerosols
J. Meteorol., vol. 18, 81-108, https://doi.org/10.1175/1520-0469(1961)018<0081:SA>2.0.CO;2Junge, C. E., Chagnon, C. W., and Manson, J. E. (1961b)
A worldwide stratospheric aerosol layer, Science, 133, 1478–1479, https://doi.org/10.1126/science.133.3463.1478-a.Kremser et al. (2016)
Stratospheric aerosol—Observations, processes, and impact on climate
Rev. Geophys., 54, https://doi.org/10.1002/2015RG000511McCormick et al. (1979)
Satellite studies of the stratospheric aerosol
Bulletin of the American Meteorological Society, vol. 60, no. 9, pp. 1038-1046
https://doi.org/10.1175/1520-0477(1979)060<1038:SSOTSA>2.0.CO;2McCormick (1987)
SAGE-II: An overview
Adv. Space Res., vol. 7, no. 3, 219-226, https://doi.org/10.1016/0273-1177(87)90151-7Oberbeck et al. (1989)
SAGE II Aerosol Validation: Selected Altitude Measurements, Including Particle Micromeasurements
J. Geophys. Res., vol. 94, no. D6, 8367-8380, https://doi.org/10.1029/JD094iD06p08367Solomon et al. (2011)
The Persistently Variable “Background” Stratospheric Aerosol Layer and Global Climate Change
Science, vol. 333, 866-870, https://doi.org/10.1126/science.1206027Thomason and Taha (2003)
SAGE-III aerosol extinction measurements: Initial results
Geophys. Res. Lett., vol. 30, no. 12, 1631, https://doi.org/10.1029/2003GL017317.Thomason and Poole (1993)
Use of Stratospheric Aerosol Properties as Diagnostics of Antarctic Vortex Processes
J. Geophys. Res,, vol. 98, no. D12, 23,003-23,012,
https://doi.org/10.1029/93JD02461Weigel et al. (2014)
Enhancements of the refractory submicron aerosol fraction in the Arctic polar vortex: feature or exception?
Atmos. Chem. Phys., 14, 12319–12342, https://doi.org/10.5194/acp-14-12319-2014Yue et al. (1986)
Retrieval of composition and size distribution of stratospheric aerosols
with the SAGE-II experiment,
Journal of Atmosphere and Ocean Technology, vol. 3, page 371-380
https://doi.org/10.1175/1520-0426(1986)003<0371:ROCASD>2.0.CO;2Yue et al. (1995)
Aerosol surface areas deduced from early 1993 SAGE II data
and comparisons with stratospheric photochemistry, aerosols,
and Dynamics Expedition measurements
Geophys. Res. Lett., vol. 22, no. 21, 2933-2936,
https://doi.org/10.1029/95GL02941Citation: https://doi.org/10.5194/egusphere-2024-2942-RC1 -
RC2: 'Reply on RC1', Anonymous Referee #1, 24 Oct 2024
I am here uploading PDF of my review, as although the page-formatting of my review
seemed fine when I clicked "Preview" before submitting this, the formatting here
has added double-spacing, and the text size has wrapped to multiple lines.
I've imported the .txt file into Microsoft Word, adjust the font-size, and saved to PDF. -
AC1: 'Reply on RC1', Felix Wrana, 17 Dec 2024
Thank you very much for your kind words as well as for your comments which helped to increase the precision of the manuscript's wording and to make it more understandable for the reader.
Here is the complete list of your comments with our answers:
Main Minor Revisions
M-MR-1) "The solar occultation data set" -- Abstract line 1, section 1, line 56, section 2.2, line 108 (and elsewhere)
Comment: As mentioned above, the text refers to the SAGE-III/M3M data analysed as "the solar occultation data set", but this is not sufficiently identifying the sub-set of the SAGE-III data being analysed. Remember that SAGE-III measures not only aerosol but also stratospheric ozone, NO2 and water vapour, and all of these SAGE-III data-products are measured applying the solar occultation technique. Please refer to "the multi-wavelength aerosol extinction data set" or "stratospheric aerosol data set" to be clear it's the aerosol extinction that's being analysed.
Answer: We changed it throughout the document according to your suggestion.
Comment: For the Abstract line 1, suggest to change "derived from the solar occultation data of the" instead to "derived from the multiple-wavelength aerosol extinction retrievals of the"
Answer: We changed it as suggested (subsequently we write only “Done”).
Comment: For section 1, line 56, change "is investigated using the solar occultation data set of the" instead to "is investigated using multiple-wavelength aerosol extinction retrievals of the"
Answer: Done.
Comment: For section 2.2, line 108, change "The SAGE III/M3M solar occultation data set is used in this work to derive parameters…" instead to "The SAGE III/M3M multiple-wavelength aerosol extinction data set is used in this work to derive parameters...",
Answer: Done.
M-MR-2)
Comment: "for the retrieval of all parameters" (Abstract line 3), "the aerosol size retrieval" -- section 2.2 lines 118 & 127, and "the retrieval method" on section 2.2 line 113. This is the other "main minor-revision", to avoid the word "retrieval" when referring to the particle size parameters.
Answer: Done.
Comment: The individual-wavelength aerosol extinction is itself a retrieval, and although aerosol extinction at ~1 micron is quite "clean" in relation to the majority of the extinction being aerosol, for the other wavelength aerosol extinctions analysed, there is still some uncertainty even within the aerosol extinction (in relation to other species contributing to the measured extinction at that wavelength, see e.g. McCormick (1987) Figure 4 and Chu and McCormick (1979) Figure 3).
More generally, when the particle size is derived from multiple aerosol extinctions at different wavelengths, it is then no longer appropriate to refer to the derived size products as a "retrieval". Please check through the manuscript, and change "retrieval" instead to "derived product" or "derived PSD parameters" or similar where referring to the size.
Answer: We do not necessarily agree that parameters (like the median radius) derived from previously retrieved quantities (like the aerosol extinction coefficients) cannot be called “retrieved” quantities themselves. We did call it that in previous papers as have other authors before. However there are also many papers which are treating the issue like you suggest, so we changed it throughout the text according to your wish.
Comment: For Abstract line 3, change "allows for the retrieval of all parameters controlling the assumed monomodal lognormal…" instead to "enables to derive a best estimate for the 3 parameters within an assumed monomodal lognormal...”
Answer: We changed the sentence to “The broad wavelength spectrum of the measurements enables to derive all 3 parameters within an assumed monomodal lognormal size distribution.”.
Comment: For section 2.2 line 118, change "a common assumption in stratospheric aerosol size retrievals from satellite measurements" to "a common assumption when deriving aerosol size distribution from satellite measurements".
Answer: Due to multiple of your comments, we rewrote this part of the section (see next comment).
Comment: For section 2.2 line 121, change "there is not enough independent spectral information in satellite measurements to facilitate a retrieval with a more complex model". That's roughly correct, but it's more to do with non-uniqueness of the solution, i.e. the derivation to particle size is argued by some to not be sufficiently well-defined to warrant a more complex particle size model. However, this is a much debated issue, and for example Thomason et al. (2008) used a bimodal size distribution assumption within their derivation of Surface Area Density and PSD parameters. In-situ optical particle measurements clearly show that after volcanic eruptions there is clearly a separate mode with larger volcanic aerosol (see e.g. Deshler, 2008, Figure 4), and the text here needs to be changed to reflect that there are differing opinions on this.
This sentence (118 to 121) is already quite long , and then to clarify this, suggest to re-structure into 2 sentences. Within lines 118-119, please delete "on the one hand", and put full-stop after "true conditions". And then please add "However, in-situ optical particle measurements after the Pinatubo eruption clearly show the presence of a second mode with much larger volcanic aerosol (see e.g. Deshler, 2008, Figure 4), with Thomason et al. (2008) using a bimodal size distribution assumption within their method to derive Surface Area Density and PSD parameters."
And then pls change ", but more importantly out of necessity, since usually there is not enough independent spectral information in satellite measurements to facilitate a retrieval with a more complex model" to "However methods remain largely mono-modal, and it remains unclear whether or not the derivation of particle size from spectral aerosol extinction satellite measurements is precise enough to warrant applying a more complex multi-modal particle size model".
Answer: Thomason et al. (2008) is not an example of a paper in which a bimodal lognormal size distribution was derived. They used one and two monodisperse modes to estimate a lowermost and an uppermost possible value for the Surface Area Density and when the second monodisperse mode was used, they additionally fixed the total number density. In other words they only derived 3 parameters (the radii of the two modes and the number density of the mode with the larger radius).
We argue that due to the limited amount of independent spectral information (limited in the amount of channels within a certain wavelength range but also in terms of wavelength range covered by the channels) only a limited amount of independent PSD parameters can be derived. As of now, we are not aware of a work which actually could derive the 6 parameters of a bimodal lognormal size distribution (median radius, mode width and number density for both modes) from a satellite aerosol data set. Instead when it is attempted, some parameters are usually fixed. Knepp et al. (2024) tried to retrieve a bimodal lognormal distribution from SAGE III/ISS data, but concluded this: “[…] while the OPC analysis looks promising, the theoretical evaluation in Table 7 demonstrates that there is not enough information within the SAGE extinction spectra to sufficiently constrain the solution space to accurately infer bimodal PSD parameters.”
We agree, that in-situ OPC data sets showed convincingly, that two lognormal modes frequently give a better fit to the in-situ measurements, but in this part of the text we only want to argue why a monomodal assumption still is often used when aerosol size is derived from satellite retrieval data. Still, we will add it in the text, since it makes sense to include the information here.
We also agree with you, that the reason why more complex retrievals such as a bimodal lognormal PSD is difficult ultimately due to the non-uniqueness of the solution, which we did not make clear enough before. Therefore, also keeping in mind your suggestion to shorten the sentence, we changed the section in the following way:
“ … At least after the strong Pinatubo eruption from 1991 a bimodal lognormal function yielded a better representation of the observed aerosol size distribution than a monomodal lognormal function in balloon-borne in situ measurements (Deshler, 2008). Despite this, monomodality is a common assumption when deriving stratospheric aerosol size distributions from satellite measurements, because it can often be a good model of the true conditions. Additionally, Knepp et al. (2024) showed that deriving bimodal lognormal PSD parameters from SAGE data is infeasible, stating that “[…] there is not enough information within the SAGE extinction spectra to sufficiently constrain the solution space to accurately infer bimodal PSD parameters”.
Comment: For section 2.2 line 139, change "The retrieval of median radius and mode width is independent of…" instead to "The derived values of median radius and mode width are independent of..."
Answer: The suggested change does not quite capture the essence of what we wanted to say. Keeping in mind your wish to not use the word “retrieval” in this context, we instead changed the sentence to:
“The median radius and mode width can be derived independent of the total Number density N0, because the latter cancels out when forming the extinction ratios.”
Comment: For section 2.2 line 144, change "from the retrieved median radius..." to "from the derived median radius..."
Answer: Done.
Comment: For section 2.2 line 145, change "can be calculated from the retrieval results" to "can be calculated from the derived PSD parameters".
Answer: Done.
Comment: For section 2.2 line 155, change "noisy data in the retrieved quantities." to "noisy data in the derived PSD parameters."
Answer: Done.
M-MR-3)
Comment: Improve the terms in the equations, and symbols used, within section 2.2 lines 145-154.
Please use roman (not italics) for abbreviations "med" and "eff", and the natural logarithm & exponential functions (ln and exp),
For these equations 3 and 4, the abbreviations Rmed and Reff need to have the "med" and "eff" as roman text (not italics), to be clear these are not additional variables or indices within a matrix etc. Please change the "med" and "eff" also to roman style (rather than italics) in the references to Rmed and Reff in the text (e.g. line 148). Please add "(Reff)" after "effective radius" on line 145, to introduce the abbreviation, at that first use. Please also add "aerosol particle" before "effective radius" (line 145) to be clear this is not cloud effective radius.
Answer: We changed it throughout the document.
M-MR-4)
Comment: Change text "the size of individual aerosol particles can be measured" to avoid potential confusion re: the OPC instrument.
The Wyoming OPC is described to a good level of detail in Deshler et al. (2003). It does not measure the size of individual particles, but counts the number of particles crossing a beam of white light, via individual pulse-occurrences of forward-scattered light at the 40 degree forward-scattering angle. The information on lines 179-192 suggest this is understood, but the information on line 174 could lead some readers to misunderstand the operation of the instrument.
Please change "Using an optical particle counter (OPC), the size of individual aerosol particles can be measured. This is a major advantage over satellite measurements, whether the measured signal originates from many different aerosol sizes".
Suggest to replace "Using an optical particle counter (OPC), the size of individual aerosol particles can be measured. This is a major advantage over satellite measurements, whether the measured signal originates from many different aerosol sizes". instead with "An optical particle counter (OPC) measures the number of aerosol particles at light-scattering sizes, and such observations have provided the ground-truth for stratospheric aerosol satellite measurements since the advent of the SAM-II and SAGE instruments (e.g. McCormick et al., 1977)."
That 2nd sentence could also lead to misunderstanding from some readers, and in fact the OPC is measuring a range of particle sizes, and whilst with this split-sentence, the statement there is not obviously incorrect, it would be misleading as currently worded.
Suggest to replace "This is a major advantage over satellite measurements, whether the measured signal originates from many different aerosol sizes", instead to "The number of aerosol particles is a key microphysical property (e.g. for particle growth), and although PSD parameters can be derived from satellite, comparisons to in-situ OPC measurements are required to validate and refine aspects of the techniques (e.g. Oberbeck et al., 1989; Hervig and Deshler, 2002)."
Answer: Thanks for the comments. You are right, our description of the way the OPC measured aerosol size was not quite precise enough. We rewrote much of Sect. 2.4 and included the citations you suggested and a more detailed description of the measurement process and how aerosol size above the 12 size thresholds is obtained.
M-MR-5)
Comment: Figure 7 --- please add pale-blue or very-pale-grey shading across the temperature range T = 195K and lower. This then will have sharing between 21 and 25km in the December 4th 2002 Figures -- add this to all 4 rows of that central column. That will then make this much clearer to the reader that this is a PSC being measured there. (rather than an aerosol enhancement).
Answer: Thanks for this suggestion. We implemented it.
Other Minor Revisions
Comment: O-MR-1) Title -- The word "Varibility" somehow to me suggests an analysis across a relatively short period. Since this is a global-scale analysis of the particle size variations, suggest to replace "Variability of" instead with "Large-scale variations in the", and delete "parameters" (don't need to specify that in the title).
A change in title is obviously a decision for the authors, but I think "variation" is more scientific than simply "variability", and think that "large-scale" or "hemisphere-scale" should be stated in the title (to give an indication of the type of variation analysed), Could possibly add "spatio-temporal" also.
Answer: Thank you for the suggestion. We decided to change the title of the paper to:
“Spatio-temporal variations of stratospheric aerosol size between 2002 and 2005 from measurements with SAGE III/M3M”
Comment: O-MR-2) Abstract lines 1-2 -- change "is shown" and insert "analysed for" before "their evolution". The Abstract needs to present the MS to be an analysis of the measurements, which it does, very well.
Answer: Done.
Comment: O-MR-3) Abstract line 4 -- change "there were three smaller tropical eruptions" -- whilst it would be OK to say "smaller eruptions" if caveated to "(than Pinatubo)", in order to reach the stratosphere, the volcanic eruptions must already be "large-magnitude explosive", so re-word this here. I think by "small" you mean the amount of SO2 emitted was low/modest, compared to Pinatubo.
Suggest to re-word to "The 2002-2005 stratospheric aerosol layer was mostly at close to background conditions, but included three moderate-magnitude tropical eruptions (Ruang, Reventador and Manam)".
Then re-word the follow-on sentence instead to refer to the latitudes measured "The SAGE III/M3M satellite measured only in mid- and high latitudes, but derived PSD parameters indicate a reduction in particle size after all 3 eruptions (within an increased particle number)". Or similar wording, to be consistent with the way you choose to summarise this initial finding here.
Answer: Mostly following your suggestion, we changed the two sentences to:
“The 2002-2005 stratospheric aerosol layer was mostly at close to background conditions, but included three moderate-magnitude tropical eruptions (Ruang, Reventador and Manam). The SAGE III/M3M instrument measured only in mid- and high latitudes, but derived particle size distribution (PSD) parameters indicate a reduction in particle size and an increase in particle number concentration after all 3 eruptions.”
To keep it consistent, we also changed the text throughout the document when referring to the “small eruptions”.
Comment: O-MR-4) Abstract line 6 -- change "Apart from the likely effect of" to "In addition to this effect of…" (The "Apart from" somehow seems negative, and "In addition" makes clear this is a separate effect.)
Answer: You are right. We changed it to: “In addition to this likely volcanic effect on the PSD, […] ”.
Comment: O-MR-5) Introduction line 14 -- suggest to add cites to Solomon et al. (2011) and Kremser et al. (2016) here re: the stratospheric aerosol layer (after "layer of aerosol particles").
Answer: Done.
Comment: O-MR-6) Introduction line 15 -- change "It was first measured" to be clear you mean measured in-situ, and also give the actual year the first stratospheric aerosol balloon measurements were made, 1957 (see Figure 14 of Junge et al., 1961). Also, it was the high-profile Science paper by Junge et al. (1961) that brought the recognition of the stratospheric aerosol layer to the general science audience, and then suggest to cite 1961a and 1961b, as listed in the References below.
Suggest to replace "It was first measured and described by Junge et al. (1961) through balloon measurements" with "The stratospheric aerosol layer was first measured from high-altitude balloon soundings in 1957 (Junge et al.,1961a, Junge et al., 1961b) ...
Answer: Done.
Comment: O-MR-7) Section 2.1, line 79 -- There needs to be a cite to a paper for the SAGE III/M3M aerosol measurements here, and suggest to cite Thomason and Taha (2003) here.
Answer: Done.
Comment: O-MR-8) Section 2.1, line 80 -- Suggest to add mention to SAGE-II here, and then cite the 1987 McCormick paper (which you already cite) in the context of the range of different species measured. Specifically, suggest to add "essentially the same range of" before "different atmospheric" and replace "like" with "as SAGE-II (see McCormick, 1987)"
Answer: Done.
Comment: O-MR-9) Section 2.2, lines 109-110 -- Please cite also the original methods that were developed by Yue et al. (1986) to derive the size/SADparameters from SAGE-II, and the 1996 paper that compared the original method from Yue86 to the PCA method from Thomason and Poole (1993).
Specifically, suggest to change "is in essence the same as the one used in" with "is similar to the
original methods for deriving size parameters from SAGE-II (see Yue et al., 1986, Yue et al., 1995),
and was used also in..."
Answer: We implemented your suggestion and added a second part. We changed it to:
“This aerosol size derivation method is similar to the original methods for deriving size parameters from SAGE-II (see Yue et al., 1986, Yue et al., 1995), adapted for the larger range of wavelengths covered by the SAGE III aerosol extinction channels. It was used also in […] ”
Comment: O-MR-10) Section 2.2, lines 113-114 -- This sentence is too colloquial (in some places), and suggest to change "A number of assumptions underly the retrieval method and therefore also have to be kept in mind.." with "There are a number of assumptions within the PSD parameters derived from SAGE-II, which must be considered when interpreting the results.". (delete "later on")
Answer: We changed the sentence to:
“There are a number of assumptions within the PSD parameters derived this way from SAGE III/M3M, which must be considered when interpreting the results.”
Comment: O-MR-11) Section 2.2, line 117 -- Please change "Here, the three parameters controlling the monomodal lognormal size distribution are the median radius..."" with "In this form, there are 3 parameters that together describe the aerosol particle size distribution: the median radius..."
Answer: Done.
Comment: O-MR-12) Section 2.2, lines 151-152 -- Please change "It is much more useful than the mode width". That's a rather subjective statement, and remember they're actually the same quantity, just that one is an absolute measure of the size variation within the mode, and the other is a relative measure.
I have seen that in previous papers the terminology is established to be "absolute mode width" when referring to the (absolute) standard deviation. But the term sigma here would usually be expected to denote that absolute quantity, the absolute standard deviation. To avoid confusion, within this paper, I am requesting to add sub-script g in all instances of the sigma, which here is the geometric standard deviation.
Please change all instances of sigma to be sigma-subscript-g, then being clear when you say "the mode width" (without "absolute") you actually mean the geometric standard deviation (i.e. a mode width of 1.0 means there is no variation in size within the lognormal, the mode is mono-disperse).
Answer: As requested, we changed every sigma to sigma-subscript-g and changed “mode width” to “geometric standard deviation” when talking about it in the text.
Comment: O-MR-13) Section 2.2, lines 152-153 -- Further to the above, please change "It is more useful than...", to "The absolute mode width provides the variation in nanometers, with variations then easier to interpret than..."
Answer: Done.
Comment: O-MR-14) Section 2.2, line 155 -- add "for the derived PSD parameters" after "an accuracy parameter" and change "to exclude noisy data" instead to "to be able to exclude less reliable data".
Answer: Done.
Comment: O-MR-15) Section 2.2, line 167 -- replace "in 0.5km steps" with "at 0.5km vertical resolution".
Answer: We do not agree with this change. There can be a difference between the actual vertical resolution of the measurements and the grid on which the retrieved parameters are provided. For SAGE II the vertical resolution is ~ 1 km (Damadeo et al., 2013), but the data is provided on a 0.5 km grid, which is what we wanted to say here in the text. Calling it a 0.5 km vertical resolution here can lead to misunderstandings in our view.
Comments: O-MR-16) Section 2.2, line 180 -- the reference to "12 size classes" here is mis-leading. The 12 size channels are counting particles larger than 12 different minimum sizes
(see Deshler et al., 2019). So please change "in 12 size classes" to "larger than 12 size-cuts
(see Deshler et al., 2019)".
O-MR-17) Section 2.2, line 181 -- replace "For this, the scattering of white light by, ideally, single aerosol particles is measured at an angle of 40° relative to the incident light of the incandescent lamp." with "The instrument measured individual pulses of forward-scattered white light at an angle of 40 degrees, a photomultiplier counting individual aerosol particles."
O-MR-18) Section 2.2, lines 184-185 -- change "Aerosol size is retrieved from the measurements" -- it isn't.
There's a threshold size associated with each "size channel", but that's the lower-limit size for
particles being measured within the number concentration for that size-channel.
I think it's best to delete the entire sentence there beginning "Aerosol size is retrieved…"
O-MR-19) Section 2.2, lines 186-187 -- the wording here re: the CN measurement needs improvement. A suggested re-wording is to change "Also included are separate measurements of the total…" with "All Kiruna W-OPC soundings analysed here also included a separate measurement of the total" (I'm assuming that's the case, please check this, or clarify how many did included CNC as well as OPC).
Answer: Based on your comments and discussions including Terry Deshler, we rewrote large parts of section 2.4 “Kiruna OPC measurements”, which you are referring to in your comments, including all this and adding some more information to make everything more clear and more correct.
Comment: O-MR-20) Section 2.2, line 187-188 -- correct "glycole" to "glycol" and re-word "forcing the aerosol particles to grow to a detectable size through the condensation of..." with "aerosol particles larger than 10nm grown to light-scattering sizes via condensation of..."
Answer: We corrected the typo from “glycole” to “glycol”. However, we do not agree with part of the suggested change in the second part of your comment. Namely, we disagree with the change from "detectable size" to "light-scattering sizes", since very small aerosol particles also do scatter light (as do the even smaller air molecules). The point we want to make here is that below a certain size the scattered light of single aerosol particles cannot be detected in such measurements. Therefore we changed the sentence to:
“This is achieved through the use of ethylene glycol vapor and a growth chamber, forcing aerosol particles larger than 10 nm to grow to optically detectable sizes via condensation of ethylene glycol.”
Comment: O-MR-21) Section 3.3, line 344 -- add "ice" before "frost point" and add also "(~3K below the NAT frost point)". That then makes clear, that the STS formation temperature is interim between TICE and TNAT.
Answer: Done.
Comment: O-MR-22) Section 3.3, line 348 -- suggest to replace the Tritscher et al. (2021) review article cite with the studies of Hoyle et al. (2013) and Engel et al. (2013).
Answer: Done.
Comment: O-MR-23) Section 3.3, line 356 -- please change "plot" to "in Figure 6"
Answer: Done.
Comment: O-MR-24) Section 3.3, line 357 -- please change "of perturbed aerosol extinction"
instead to "of strongly elevated aerosol extinction".
Answer: Done.
Comment: O-MR-25) Section 4, line 377 -- please insert "aerosol" before "extinction coefficient".
Answer: Done.
Comment: Section 4, line 379 and Figure 7 -- please add labels "a)", "b)" etc. to the sub-panels
in Figure 7m and change "topmost" to refer to "Figure 7a" etc.
Answer: Done.
Comment: O-MR-27) Section 4, line 385 -- change all instances of "collocations" to "co-locations"
Answer: We are unsure what is correct here and also consulted the Copernicus editorial team. The ACP guidelines direct to the dictionaries Cambridge, Merriam-Webster and Collins. In there, “co-location” does not appear, but as a derivative of “location” and the prefix “co-” it could be applicable here. The word “collocation” does appear in the dictionaries, but usually in a linguistic context. We therefore leave it as it is for now and let it be handled in the copy-editing step in case the manuscript is accepted for publication.
Miscellaneous: In addition to the changes in response to the reviewers we restructured and added a few sentences to one passage of Sect. 4 (“Comparison to OPC measurements”), which discussed whether or not the data compared for January 10th, 2004, can be classified as background conditions or not. Here, we changed the order of sentences to make the argument easier to follow and added some information on the possible influence of wildfires, latitudinal sampling and seasonal variation.
Sources mentioned:
Damadeo, R. P., Zawodny, J. M., Thomason, L. W. and Iyer, N.: SAGE version 7.0 algorithm: application to SAGE II. Atmos. Meas. Tech., 6, 3539–3561, doi:10.5194/amt-6-3539-2013, 2013.
Knepp, T. N., Kovilakam, M., Thomason, L., and Miller, S. J.: Characterization of stratospheric particle size distribution uncertainties using SAGE II and SAGE III/ISS extinction spectra. Atmos. Meas. Tech., 17, 2025-2054, https://doi.org/10.5194/amt-17-2025-2024, 2024.
Citation: https://doi.org/10.5194/egusphere-2024-2942-AC1
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RC2: 'Reply on RC1', Anonymous Referee #1, 24 Oct 2024
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RC3: 'Comment on egusphere-2024-2942', Filip Vanhellemont, 07 Nov 2024
Review of:
Variability of stratospheric aerosol size distribution parameters between 2002 and 2005 from measurements with SAGE III/M3M
Felix Wrana et al.
General Comments
This paper shows how the 3 parameters of the assumed log-normal size distribution of stratospheric aerosols at different altitudes are derived from the SAGEIII/M3M aerosol extinction data set at the 3 trusted wavelengths of 449, 755 and 1545 nm. The SAGEIII instrument and its orbit/coverage is adequately described, followed by a valid description of the theory and numerical procedure to derive the size distribution parameters. A very interesting data interpretation follows, with plausible identification of the aftermath of three tropical volcanic eruptions and winter condensation events, possibly associated with intrusion of meteoric smoke from the upper atmosphere. Sporadic elevated aerosol extinction in the northern hemisphere, coinciding with very cold temperatures, convincingly point in the direction of Polar Stratospheric Clouds. Comparisons with in situ optical particle counter measurements are for some cases surprisingly good, (considering the difficulties associated with these kinds of retrievals), lending credibility to the quality of the data. Finally, as a caveat, an additional section describes the fundamental limitation of the used method to derive PSD parameters when the assumed monomodal log-normal PSD does not correspond to the actual observed one. This last section handles an important topic of which researchers are often unaware.
This paper fits perfectly within the scope of ACP; it presents a new useful atmospheric data set and associated scientific interpretation, it’s not a purely technical paper on instrumentation and retrievals. The results are novel: a new data set is presented, and I was particularly intrigued by the potential signature of meteoric smoke entering the stratosphere. All methods are valid, clearly described (with one minor exception; see below) and are therefore reproducible by others. Adequate credit is given to previous work in the form of citations. Title, abstract, and the general sectioning of the paper is good. I would have preferred a more condensed language, but this is entirely subjective, and it furthermore shows that the authors have the wish to be precise. Mathematical notation (the font) needs to be adapted to become more readable (see below). As a last general comment, I very much appreciated how the authors make a clear distinction between unambiguous proof (volcanic/meteoric causes) and plausible explanation.
I therefore recommend publication with only minor revisions.
Specific comments
Line 89: ‘Since the scattering angle does not change between northern and southern hemisphere solar occultation measurements, the data quality of both is likely very similar.’ I don’t really understand. Solar occultation implies a scattering angle of zero degrees. Perhaps you are talking about straylight conditions. Can you clarify?
Line 96: Before ‘This setup results in … providing aerosol extinction …’ it would be good to mention that ozone, Rayleigh scattering, and other contributions have been removed, and that subsequently aerosol extinction is obtained as a rest product.
Line 117: There seems to be some ambiguity on the notation and the name of the parameter sigma. I would have preferred the notation ‘s’ and reserve ‘sigma’ for sigma = log(s). It’s not an important point; sigma is unambiguously used in the equation on line 116. Following this use, sigma is actually the geometric standard deviation. The name ‘mode width’ is found in other papers but I’m not certain what it means (while the meaning of e.g. ‘median radius’ is very clear from the name). The meaning of ‘geometric standard deviation’ is clear: 68% of the particles have sizes in the interval [rmed/sigma, rmed*sigma].
Line 142: I understand the method to get the number density, but could you specify at which wavelength you do it? There are three possibilities (449, 755 or 1545 nm).
Line 204: could you please specify the filtering method that was used? If it has been described in another paper, a citation is sufficient.
Line 247: It is clear from figures 2 and 3 that the eruption signatures are less visible in the Southern than Northern hemisphere, with the exception of Manam. Can it be explained by the fact that Manam is the only eruption that occurred in the Southern hemisphere (4°S), even though not far from the equator?
Section 4 (Comparison to OPC measurements): Comparisons such as these are intrinsically difficult since the particle size range where instruments are sensitive differs from instrument to instrument, e.g. UV instruments have more sensitivity to small particles than Visible instruments, an OPC has a different response to particle size than an occultation instrument etc. Ideally this should be reflected in comparisons by taking into account the error bars on effective radius, number density etc. but these are not presented in this study. As a minimum, please mention this problem in section 4 as a partial explanation of the discrepancies between the two data sets.
Section 5 (Discussion of monomodality). I fully agree with this analysis and the conclusions, but the problem is also strongly related to the wavelength range of the instrument and the associated noise on the aerosol extinction as function of wavelength. This has not been discussed. E.g. a larger extinction noise level at short wavelengths will also shift the derived monomodal distribution. Can you please mention this somewhere in the section? (PS: this is of course related to the previous comment)
Technical corrections
Throughout the paper:
In all mathematical notation, subscripts that are not scalar indices should be written in roman, not italics, e.g. for ‘r_med’, the ‘r’ is in italics, the ‘med’ is in roman. Also, mathematical functions are in roman, not italics (‘exp’, ‘ln’).
I’m still confused about when to use ‘aerosol’ or ‘aerosols’. Perhaps they are equally valid. Can you please check?
Everywhere in the paper: I believe it is written as ‘the Sun’, so capitalized.
Specific corrections/suggestions:
Line 43: … high measurement frequency and an almost global daily coverage. But they suffer from (1) the necessity to make assumptions about the stratospheric aerosol …. particle size, (2) the complicated light paths … and (3) resulting issues with, …
Line 49: … of the instrument. Typically, a few tens of profiles are measured per day.
Line 51: … stratospheric aerosol size: the high signal strength of the sun allows a small field of view, …
Line 55: … facilitates a straightforward way …
Line 58: … on the Russian satellite Meteor-3M (SAGE III/M3M).
Line 65: … restricted the possibility of unambiguous aerosol size retrievals, as discussed in Wrana et al. (2023).
Line 73: … be discussed in Sect. 3. Finally, after the …
Line 75: … as well as for aerosol size retrievals in general …
Line 78: The SAGE III/M3M solar occultation instrument was launched …
Line 82: Remove the entire sentence, it has been stated before.
Line 86: Sunrise and sunset is here …. Remove the sentence, the statement is obvious.
Line 94: Those consist of a CCD array covering the wavelengths from 280 nm to 1040 nm …
Line 101: … other satellite instruments as well as internal consistency.
Line 108: … parameters describing the size distribution of stratospheric …
Line 122: … inadequate in some cases, as will be illustrated in Sect. 5.
Line 125: … Li et al., 2023), the assumption is realistic.
Line 134: … provided. As already stated in the previous section, these SAGE III/M3M spectral channels …
Line 139: … total number density …
Line 146: … in the relevant size range the effective radius is more indicative of the … for most of the aerosol radiative effect.
Line 157: … values below 16 are excluded, as was done in …
Line 158: … of the retrievals overall, although much less in the maximum …
Line 162: … to August 2005, i.e. having considerable overlap …
Line 166: … Aerosol extinction coefficients are provided for the spectral channels …
Line 182: … The scattered light is focused onto a pair of …
Line 202: … The red line in each color plot …
Line 203: … The range of values for each parameter … : Remove the entire sentence, it is obvious.
Line 219: In the Northern hemisphere, …
Line 295: The extinction data were averaged …
Figure 4, right panel, title: ‘January 2003’
Line 297: The sampling of SAGE II also … Remove the entire sentence, and replace with: ‘Furthermore, the SAGE II sampling causes different latitude bins to represent different days within the considered month. ’
Line 322: … simply be a retrieval artifact rather than a real feature.
Line 328: … are indeed volcanic events; they may be comparable …
Line 340: Solid particles, such as nitric acid … are the most commonly occurring type of particle in a PSC.
Figure 6, caption: … aerosol extinction coefficient at 449 nm, temperature, …
Line 367: … PSC particles may differ strongly from the values …
Line 368: … in which case Mie theory would be inaccurate.
Line 373: … for validation of the retrieved data of this work.
Line 382: … measurements are sparse. That is why …
Line 425: SO2 not in italics, roman.
Line 441: … correspond to air volumes of hundreds of cubic kilometers …
Line 455: Using a Mie code ( …
Line 481: … an increase in the number of particles that is at least big enough ….
Line 483: … non-existent. Unfortunately it is at least highly difficult, if not impossible, to derive the size distribution …
Line 488: It is important to keep this possibly large source of errors …
Line 509: … when temperatures were particularly low.
Line 518: … (CN) layer, which forms seasonally …
Line 523: … happen at lower nonpolar latitudes.
Line 524: … Kiruna, Sweden, were compared to …
Line 530: Finally, the effects of the assumed size distribution …
Line 534: This stands in contrast with the high variability of real PSDs.
Line 546: Change ‘LW’ to LT or LWT
Citation: https://doi.org/10.5194/egusphere-2024-2942-RC3 -
AC2: 'Reply on RC3', Felix Wrana, 17 Dec 2024
We want to thank you very much for accepting to review this manuscript and for the time you took to thoroughly read through it. Thanks to your comments the manuscript has now improved in many places with regard to understandability, correctness and language. Thank you also for the kind words and the recommendation of the manuscript to be published with only minor revisions.
We implemented most of your suggested changes.
Below is the complete list of your comments with our answers to them.
Specific comments
Comment: Line 89: ‘Since the scattering angle does not change between northern and southern hemisphere solar occultation measurements, the data quality of both is likely very similar.’ I don’t really understand. Solar occultation implies a scattering angle of zero degrees. Perhaps you are talking about straylight conditions. Can you clarify?
Answer: Yes, our wording was unclear here. The thought originally was to express a contrast to limb scattering satellite measurements, where differing scattering angles in the northern and southern hemisphere can lead to large differences in data quality and stating that this does not affect solar occultation measurements. To reduce any possible confusion we just reduced the sentence to:
“The data quality of sunrise and sunset measurements is likely very similar.”
Comment: Line 96: Before ‘This setup results in … providing aerosol extinction …’ it would be good to mention that ozone, Rayleigh scattering, and other contributions have been removed, and that subsequently aerosol extinction is obtained as a rest product.
Answer: We added the following sentence:
“Aerosol extinction is retrieved from the calculated line-of-sight transmission profiles by removing the effects of Rayleigh scattering due to air molecules, ozone absorption and NO2 absorption (Thomason et al., 2010).”
Comment: Line 117: There seems to be some ambiguity on the notation and the name of the parameter sigma. I would have preferred the notation ‘s’ and reserve ‘sigma’ for sigma = log(s). It’s not an important point; sigma is unambiguously used in the equation on line 116. Following this use, sigma is actually the geometric standard deviation. The name ‘mode width’ is found in other papers but I’m not certain what it means (while the meaning of e.g. ‘median radius’ is very clear from the name). The meaning of ‘geometric standard deviation’ is clear: 68% of the particles have sizes in the interval [rmed/sigma, rmed*sigma].
Answer: We agree, that the term “geometric standard deviation” is unambiguous and therefore better here than “mode width”. Also in reaction to the first reviewers comments, we changed the symbol “sigma” to “sigma-subscript-g” and in the text we changed “mode width” to “geometric standard deviation”.
Comment: Line 142: I understand the method to get the number density, but could you specify at which wavelength you do it? There are three possibilities (449, 755 or 1545 nm).
Answer: We added the following sentence:
“The number density values shown in this work were calculated at λ = 755 nm, differences between this and results calculated at 449 or 1545 nm are negligible however.”
Comment: Line 204: could you please specify the filtering method that was used? If it has been described in another paper, a citation is sufficient.
Answer: No specific filtering method was applied here. The maximum and lower end of the depicted value range for each parameter was instead chosen in a way that the differences between large and small values became as visible as possible while losing as little visible data points as possible. For example, values that were so large that including them would darken the whole plot, although they themselves were not visible at all in the plot (e.g. because they were not existing in a patch but only as individual single values, not showing up in the contour plot) were excluded. Hopefully this is what you asked about.
Comment: Line 247: It is clear from figures 2 and 3 that the eruption signatures are less visible in the Southern than Northern hemisphere, with the exception of Manam. Can it be explained by the fact that Manam is the only eruption that occurred in the Southern hemisphere (4°S), even though not far from the equator?
Answer: Yes, we think that this may play a role. Thank you for the suggestion. We added the following sentences to the end of Sect. 3.1:
“It therefore appears that more of Manam's emissions were transported to the southern hemisphere than of Ruang's and Reventador's emissions before. Factors that may have contributed to this are the larger amount of SO2 injected into the stratosphere by Manam, its location further south and the timing of its main eruption at the start of the year rather than towards its end.”
Comment: Section 4 (Comparison to OPC measurements): Comparisons such as these are intrinsically difficult since the particle size range where instruments are sensitive differs from instrument to instrument, e.g. UV instruments have more sensitivity to small particles than Visible instruments, an OPC has a different response to particle size than an occultation instrument etc. Ideally this should be reflected in comparisons by taking into account the error bars on effective radius, number density etc. but these are not presented in this study. As a minimum, please mention this problem in section 4 as a partial explanation of the discrepancies between the two data sets.
Answer: We added the following sentence:
“Differences in the sensitivity to different sizes of aerosol particles will also play a role in the comparison made in this section. For example, the WOPC instrument measures scattered white light (see Sect. 2.4), while for the SAGE III/M3M PSD derivation method used in this work two distinct channels in the visible and one in the near-infrared are used, which will already result in differences in sensitivity.”
Comment: Section 5 (Discussion of monomodality). I fully agree with this analysis and the conclusions, but the problem is also strongly related to the wavelength range of the instrument and the associated noise on the aerosol extinction as function of wavelength. This has not been discussed. E.g. a larger extinction noise level at short wavelengths will also shift the derived monomodal distribution. Can you please mention this somewhere in the section? (PS: this is of course related to the previous comment)
Answer: We added the following part to the end of the section:
“All PSD parameters derived from remote sensing instruments will be affected by this issue, although different instruments will not be affected in exactly the same way since the use of different wavelengths and viewing geometries will affect the sensitivity to different parts of the PSD (i.e. smaller and larger particles). Measurement uncertainties at the individual wavelength channels will also impact this. A more specific analysis of an attempt to retrieve bimodal lognormal PSDs from the solar occultation SAGE III/ISS data set was done by Knepp et al. (2024).”
Technical corrections
Comment: Throughout the paper:
In all mathematical notation, subscripts that are not scalar indices should be written in roman, not italics, e.g. for ‘r_med’, the ‘r’ is in italics, the ‘med’ is in roman. Also, mathematical functions are in roman, not italics (‘exp’, ‘ln’).
Answer: We implemented your requested change (subsequently we write only “Done”).
Comment: I’m still confused about when to use ‘aerosol’ or ‘aerosols’. Perhaps they are equally valid. Can you please check?
Answer: Aerosol and aerosols is a tough distinction since aerosol is inherently plural. The convention often followed and which is followed here is that aerosol refers to a population of particles that all have roughly the same composition. Aerosols then refers to particle populations with differing compositions. So externally mixed particles. Thus it is fair to refer to stratospheric aerosol here in most places, since roughly it is all sulfuric acid and water. We only used “aerosols” in places where all types of aerosol, including ones with different compositions are meant.
Comment: Everywhere in the paper: I believe it is written as ‘the Sun’, so capitalized.
Answer: Done.
Specific corrections/suggestions:
Comments: Line 43: … high measurement frequency and an almost global daily coverage. But they suffer from (1) the necessity to make assumptions about the stratospheric aerosol …. particle size, (2) the complicated light paths … and (3) resulting issues with, …
Line 49: … of the instrument. Typically, a few tens of profiles are measured per day.
Line 51: … stratospheric aerosol size: the high signal strength of the sun allows a small field of view, …
Line 55: … facilitates a straightforward way …
Line 58: … on the Russian satellite Meteor-3M (SAGE III/M3M).
Answer: Done.
Comment: Line 65: … restricted the possibility of unambiguous aerosol size retrievals, as discussed in Wrana et al. (2023).
Answer: We implemented your suggestion, but used “derivation” instead of “retrievals”, due to the wish of the other reviewer to not use “retrieval” in this context.
Comment: Line 73: … be discussed in Sect. 3. Finally, after the …
Answer: Done.
Comment: Line 75: … as well as for aerosol size retrievals in general …
Answer: We chose to say “remote sensing aerosol size retrievals” on purpose, since in our view the issue discussed in Sect. 5 (assuming the wrong shape of the PSD) applies not or at least to a much lesser extent to in situ OPC measurements. With those, individual aerosol particles are counted into size bins, which means that an assumption on the shape of the PSD does not necessarily have to be made. Therefore, we would like to leave this part unchanged.
Comment: Line 78: The SAGE III/M3M solar occultation instrument was launched …
Answer: Following the intent of your comment, but adding “lunar occultation” in for completeness, we changed the sentence to:
“The solar (and lunar) occultation instrument SAGE III/M3M […] “
Comment: Line 82: Remove the entire sentence, it has been stated before.
Answer: Done.
Comment: Line 86: Sunrise and sunset is here …. Remove the sentence, the statement is obvious.
Answer: What we wanted to convey in this sentence is, that we are talking about sunrises and sunsets from the point of view of the satellite, not about whether or not it is a sunrise or sunset on Earth’s surface. It is not necessarily the same from both points of view. To make this point clearer we changed the sentence to:
“Sunrise and sunset here refer to orbital sunrise/sunset, i.e. as seen from the instrument, rather than local sunrise/sunset.”
Comment: Line 94: Those consist of a CCD array covering the wavelengths from 280 nm to 1040 nm …
Answer: Done.
Comment: Line 101: … other satellite instruments as well as internal consistency.
Line 108: … parameters describing the size distribution of stratospheric …
Answer: Done.
Comment: Line 122: … inadequate in some cases, as will be illustrated in Sect. 5.
Answer: We would like to not add this, since this addition would introduce a repetition, as the very next sentence states: “The effect this would have on the size retrieval of this work is described in Sect. 5.”
Comments: Line 125: … Li et al., 2023), the assumption is realistic.
Line 134: … provided. As already stated in the previous section, these SAGE III/M3M spectral channels …
Line 139: … total number density …
Line 146: … in the relevant size range the effective radius is more indicative of the … for most of the aerosol radiative effect.
Line 157: … values below 16 are excluded, as was done in …
Line 158: … of the retrievals overall, although much less in the maximum …
Line 162: … to August 2005, i.e. having considerable overlap …
Line 166: … Aerosol extinction coefficients are provided for the spectral channels …
Line 182: … The scattered light is focused onto a pair of …
Line 202: … The red line in each color plot …
Line 203: … The range of values for each parameter … : Remove the entire sentence, it is obvious.
Line 219: In the Northern hemisphere, …
Line 295: The extinction data were averaged …
Figure 4, right panel, title: ‘January 2003’
Line 297: The sampling of SAGE II also … Remove the entire sentence, and replace with: ‘Furthermore, the SAGE II sampling causes different latitude bins to represent different days within the considered month. ’
Line 322: … simply be a retrieval artifact rather than a real feature.
Line 328: … are indeed volcanic events; they may be comparable …
Answer: All done.
Comment: Line 340: Solid particles, such as nitric acid … are the most commonly occurring type of particle in a PSC.
Answer: If we implemented this suggestion, a verb would be missing in the sentence… We are not sure what exactly is intended by the reviewer here, so we changed the sentence to implement the suggested start of the sentence:
“Solid PSC particles, such as nitric acid trihydrate (NAT) (Voigt et al., 2000; Hanson and Mauersberger, 1988) and ice particles, as well as liquid supercooled ternary solution (STS) droplets exist (Carslaw et al., 1994; Tabazadeh et al., 1994;Deshler et al., 2003a), the latter of which are the most commonly occurring type of particle in a PSC.”
Comments: Figure 6, caption: … aerosol extinction coefficient at 449 nm, temperature, …
Line 367: … PSC particles may differ strongly from the values …
Line 368: … in which case Mie theory would be inaccurate.
Answer: Done.
Comment: Line 373: … for validation of the retrieved data of this work.
Answer: We changed it to “ […] for validation of the derived data of this work.” due to the wish of the other reviewer to not use “retrieve” in this context.
Comments: Line 382: … measurements are sparse. That is why …
Line 425: SO2 not in italics, roman.
Line 441: … correspond to air volumes of hundreds of cubic kilometers …
Line 455: Using a Mie code ( …
Answer: All done.
Comment: Line 481: … an increase in the number of particles that is at least big enough ….
Answer: The suggested change would change the intended meaning of the sentence. Instead we reworded this part and shortened the sentence to make it more understandable.
Comments: Line 483: … non-existent. Unfortunately it is at least highly difficult, if not impossible, to derive the size distribution …
Line 488: It is important to keep this possibly large source of errors …
Line 509: … when temperatures were particularly low.
Line 518: … (CN) layer, which forms seasonally …
Line 523: … happen at lower nonpolar latitudes.
Line 524: … Kiruna, Sweden, were compared to …
Line 530: Finally, the effects of the assumed size distribution …
Line 534: This stands in contrast with the high variability of real PSDs.
Line 546: Change ‘LW’ to LT or LWT
Answer: All done.
Citation: https://doi.org/10.5194/egusphere-2024-2942-AC2
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AC2: 'Reply on RC3', Felix Wrana, 17 Dec 2024
Peer review completion
