Comparison of the optical depth of total ozone and atmospheric aerosols in Poprad-Gánovce, Slovakia

Hrabčák, Peter

doi:https://doi.org/10.5194/acp-18-7739-2018

Articles | Volume 18, issue 10

https://doi.org/10.5194/acp-18-7739-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

Quadrennial Ozone Symposium 2016 – Status and trends...

https://doi.org/10.5194/acp-18-7739-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 18, issue 10

Research article

|

01 Jun 2018

Research article |

| 01 Jun 2018

Comparison of the optical depth of total ozone and atmospheric aerosols in Poprad-Gánovce, Slovakia

Peter Hrabčák

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Final revised paper (published on 01 Jun 2018)
Preprint (discussion started on 10 May 2017)

Interactive discussion

Status: closed

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

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RC1: 'Referee comment acp-2017-286', Anonymous Referee #2, 02 Jun 2017
- AC1: 'reply to referee comments', Peter Hrabčák, 04 Jul 2017
RC2: 'Referee comment', Anonymous Referee #1, 05 Jun 2017
- AC2: 'reply to referee comments', Peter Hrabčák, 04 Jul 2017

Peer-review completion

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

AR by Peter Hrabčák on behalf of the Authors (23 Nov 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (25 Nov 2017) by Alkiviadis Bais

RR by Anonymous Referee #2 (10 Dec 2017)

Suggestions for revision or reasons for rejection

With respect to the previous version, the paper has improved significantly. The author has followed the suggestions of the referees and provided reasonable answers to most of their questions. There are, however, points that still need further attention, as discussed below.

In my opinion, the main outstanding issues of the paper are:

1) On Sec. 3.1, the author presents a comparison between the AOD determined by three methods - his own, the one implemented in the standard Brewer operating software, and a Cimel photometer. This is a great addition which provides the necessary confirmation of the quality of the present data. I have, however, some questions and suggestions with regard to this section:

i) First, I suggest moving this section to just before Sect. 3.4. The current sections 3.2 and 3.3 provide information on the calibration and corrections used to determine the AOD, and section 3.4 provides data retrieved using them, so it should come after them, and just before the whole series is shown. This change would also make easier to understand Sect. 3.1, because the reader would have been introduced to the details of the ETCs determination which are currently provided in Sect. 3.2. Furthermore, I would also suggest changing the order of Sects. 3.2 and 3.3, because it seems that the determination of the ETCs in Sect. 3.2 requires the data corrections explained in Sect. 3.3.

In short, I suggest to reorder the sections in the following way: 3.3 (corrections), 3.2 (ETCs), 3.1 (validation), 3.4 (long AOD series)

ii) For the 2015-2016 period, the BSM-LPM fit on Fig. 2 doesn't seem to show any offset, the CSP-LMP fit on Fig. 3 shows a small one, and the CSP-BSM fit on Fig. 4 shows a large one. This seems strange at first sight. Can the author provide an explanation? What are the values of the intercepts (independent terms) of the fits?

2) The method used to calculate the stray light correction in pages 17-18 does not seem clear to me. In particular, on page 17 the author states that "A ratio of average count rates for four wavelengths in the region from 290 to 291.5 nm to the count rates for the monitored wavelength (one out of five) was determined for 3,386 spectral analyses in total, as well as for various zenith angles of the Sun within them". The author always refer to 306-302 nm as the Brewer operational range, so what are these four wavelengths from 290 to 291.5 nm? Why does the calculated ratio provide the stray light correction?

3) Figure 9 on page 18 provides a long TOC series from 1962 to 2016, made up of data from a Dobson spectrophotometer operating from 1964 to 1978 at a nearby site, satellite data from TOMS for the 1979-1993 period, and Brewer data for the years 1994 to 2016. Although I requested this figure in my previous review, now I have some concerns regarding it, in part because of the changes introduced in this revision of the manuscript:

i) If the period of operation of the Dobson is 1964-1978 as written on page 18, where does the data for the years 1962 and 1963 come from?

ii) Has the author checked that all the three datasets used in the figure are compatible? The Dobson and the Brewer instruments operated at different sites, and ground-based and satellite data can, in my experience, present substantial differences.

iii) Last but not least, what conclusions should the reader extract from Fig. 9? The focus of this new version of the paper seems to be the determination and analysis of a long Brewer AOD series at Poprad-Gánovce. This figure only seems to allow the author to write that the year 1994 is the minimum of the combined series, but this is doubtful because this year is the least reliable one of the Brewer AOD data, as discussed on page 19. I would thus now suggest removing this figure and related text to keep the focus on the main message of this revision of the paper - the Brewer AOD calibration and resulting data series.

Besides the main issues mentioned above, there are some smaller ones that should be also addressed:

4) The quality of the English has been greatly improved, but there are still some problems, like e.g. a missing "a" in front of "Cimel" on line 14 of the abstract, a missing "the" in front of "five" in the line 31 of page 5, or a missing "the" in front of "more" on line 7 of page 14, to mention a few. I recommend the author to check again the whole text.

5) In Eq. 2, two different symbols, "m" and the greek letter "\mu", are used to denote the airmasses of different contributions. Although the symbols are explained in the text, it would be easier for the reader to keep just one of them and use subindices to differentiate among the contributions.

6) The equation of point 8 at the top of page 9 should be explained in words. Also, why was the value 1.75 selected?

7) Eqs. 5-7 seem to correspond to the airmasses introduced in Eq. 2. Why "AMF" is used now instead of "\mu"? It would also be better to introduce these definitions just after Eq. 2.

8) On page 21, line 6, the author states that "the observed characteristics are typical for the central European location of the station". Why? Could the author explain this in more detail and provide some reference to back up this statement?

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RR by Anonymous Referee #1 (12 Dec 2017)

ED: Reconsider after major revisions (13 Dec 2017) by Alkiviadis Bais

AR by Peter Hrabčák on behalf of the Authors (20 Jan 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (23 Jan 2018) by Alkiviadis Bais

RR by Anonymous Referee #2 (05 Feb 2018)

Suggestions for revision or reasons for rejection

This new version of the paper shows many improvements in its organization and readability. I have some remaining questions and suggestions, but they are fairly minor. Once they have been taken into consideration by the author, I believe the paper will be suitable for publication.

1) General questions:
a) Could you insert in the manuscript a brief explanation for the approximation of the aerosol airmass presented on page 7, line 30 (that is, \mu_a \sim \mu_{H_2O})?
b) Could you provide some justification (a reference would suffice) for the sentence on page 8, line 2, stating that the SO_2 has a low impact on the AOD determination and its determination is not accurate?
c) Why do you call the algorithm presented in page 10 a “cloud screening”? It clearly removes a lot of bad measurements, but are these measurements only produced by clouds?
d) On page 16, line 14, it is stated that “it is not known if the BSM would take into account a change in the distance of Earth from the Sun”. Have you asked IOS?

2) Figures:
a) On Fig.4, the line used for the average ETC shows some sloped segments. However, the average ETC are constant values in every two-year period. The sloped segments should be removed, leaving a blank space if necessary. This would also help to identify the calibration periods.
b) Figs. 6, 7, and 8: I suggest adding the date range above on top of the figure for easier reference.
c) Figs. 7 and 8: the label on the vertical axes should be “AOD – CSP” and not “AOD – fotometer”

3) English: please check the following
a) Page 1, line 7: instead of “presented”, it should be “present”
b) P.1, l.10: remove the blank space between “MK” and “IV”, as in the rest of the paper
c) P.7, l.6: I think “mountainous” is more usual than “montane”
d) P.7, l.10: remove “a” from “is a part”
e) P.7, l.13: change “size” to “value”
f) P.7, l.14: change “reference instrument” to “reference Brewer instrument”
g) P.8, l.5: change “An NOAA” to “A NOAA”
h) P.8, l.23: change “Eq. (2) is adjusted” to “Eq. (2) is linearized”
i) P.8, l.31: change “The inclination of obtained line a” to “The slope a of the obtained line”
j) P.8, l.32: change “The natural logarithm of ETC ln(S0,\lambda)” to “The natural logarithm of the ETC, ln(S0,\lambda)”
k) P.11, l.19: change “was made pursuant to the recommendations” to “was made following the recommendations”
l) P.14, l.5: change “as well ETCs” to “as well as the ETCs”
m) P.18, l.15: change “by means of an average value of ETC” to “by means of the LPM method using an average ETC value for every 2-year period shown in Fig.4”
n) P.18, l.17: change “by analogy” to “in the same way but adding the value of the standard deviation”
o) P.20, l.5: change “In case of AOD” to “In the case of the AOD”
p) P.21, l.4: change “Figure 13 presents the same characteristics as in Fig. 12” to “Fig. 13 is the same as Fig. 12”
q) P.21, l.17: remove the commas enclosing “for which it was possible to determine the AOD”
r) P.22, l.21: change “Cloud screening employed” to “The cloud screening method employed”
s) P.23, l.5: remove “of the two compared methods”
t) P.23, l.7: change “longest of examined wavelengths” to “longest of the examined wavelengths”
u) P.23, l.9: change “application of correction” to “application of the correction”
v) P.23, l.14: change “The slightly” to “A slightly”

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RR by Anonymous Referee #1 (09 Feb 2018)

Suggestions for revision or reasons for rejection

MAIN ISSUES

The paper cannot be published as is, and major revisions are required. I do not mean that the main outcomes of the work (trends of ozone and aerosol optical depths) are wrong, but several issues should be settled, or at least discussed, before the paper can be accepted.

1/ The determination of the ETC appears as the main problem, along with the estimate of its uncertainty:

1a/ the uncertainty of the mean, better taking into account the number of ETC estimates, should be used instead of the standard deviation of the ETC estimates. Otherwise, as said in the paper, the uncertainty can be lower for a time interval with few ETC estimates than for one with several estimates;

1b/ in his reply to my review, the author states that "the values of the uncertainties of the annual averages do not enter the calculation of the uncertainty of the linear trend". Instead, I think that the uncertainties should enter the calculation of both the value and the uncertainty of the linear trend;

1c/ minimum data coverage thresholds (e.g., minimum measurement days per month, minimum measurement months per year) should be fixed before averaging the data (p. 11 l. 9-15), especially in case of large gaps (due to maintenance of the instrument or clouds) and presence of a seasonal cycle. Maybe this is not an issue, but it should be specified how long is the longest period without data, if any;

1d/ I could have missed this information, but I do not understand why "an advantage of the LPM is that it takes into account the change of ETCs during the intercalibration period" (p. 16 l. 19-20). From Fig. 4, I can understand that the ETCs estimated by the LPM are averaged and fixed for an intercomparison period, aren't they?

1e/ is the Langley plot performed by correcting S_lambda (Eq. 3) for O3 absorption or by including O3 in tau_lambda? In the first case, why the ozone cycle is mentioned as a source of error (p. 5)? In the second case, how is mu_w calculated (including ozone)?

1f/ since the BSM method is basically a calibration transfer from a reference instrument, the author should explain how Brewer #017 was calibrated;

2/ the AOD differences between the LPM and the BSM methods can be easily attributed to the ETC difference (AOD difference = 1/mu * log(ETC1/ETC2)). Therefore, the deviations between the two methods are mainly not "random" (contrary to what is written at p. 15 l. 14 and at p. 16 l. 12); the observed variability between summer and winter (p. 15 l. 21-23) is an obvious consequence of air mass variation during the year; the AOD differences, always positive in the first period and always negative in the second period, are dictated by the difference in ETC shown in Fig. 5. I would thus recommend to plot the AOD differences as a function of air mass to check that the ETC difference is the main cause of deviation;

3/ as I mentioned in my previous review, an additional source of errors can be the missing SL correction in the BSM, which is not mentioned among the sources listed at p. 16. However, according to the maintainer of the Brewer operating software: "Using the real-time AOD (or even ozone) data from the operating software for anything other than a rough idea about the AOD doesn't make any sense since no corrections are applied in real time for the SL ratio changes. It's likely worse than real time data for ozone due to potential changes in absolute sensitivity in Brewers (that do not affect ozone)" (V. Savastiouk, personal communication, 2017). This issue should be discussed and its potential impact estimated;

4/ even though it is probably a minor issue (especially at low air masses), the spectral stray light effect is inaccurately taken into account, in my opinion, for two reasons:

4a/ both the slits employed for UV scans and the grating positions are different from those of DS measurements, therefore the ratio between the irradiance at longer and shorter wavelengths in the "scanning mode" may not be representative of stray light in "slitmask" mode (DS measurements). The cited study of Arola & Koskela (2004), using this method, only aimed at a rough estimate of the effect;

4b/ the assumption of spectrally constant stray light is simplistic, as pointed out in Garane et al. (2006), and will lead to some uncertainty.

Therefore, if the author wants to keep the analysis of wavelengths shorter than 320 nm, they should point out that they could be affected by the stray light, rather than trying to correct them and jump to the conclusion that the Angstrom coefficient is negative;

5/ most of all, language needs major revisions, as well as the structure of the paper:

5a/ check the whole text for the correct use of articles "a" and "the";

5b/ remove all technical details that are not fundamental for the paper (e.g., discussion about wavelengths, p. 4) and cite some reference where the information can be found elsewhere (e.g., instead of the data reduction explained at p. 8);

5c/ split the theory ("Methods") from the results. E.g., p. 11 l. 19-26 are theory, as well as p. 12 l. 5-14;

5d/ use letters, e.g. (a), (b), etc., not "right/left", to identify the subfigures in the panels, according to the ACP guidelines (https://www.atmospheric-chemistry-and-physics.net/for_authors/manuscript_preparation.html).

TECHNICAL CORRECTIONS

- replace "Operational Brewer Program" with "Brewer Operating Software";

- better use "AERONET" instead of "Cimel", since the data are provided by AERONET;

- p. 1: use the long form "spectral stray light" instead of just "stray light" at the first occurrence, to differentiate it from other sources of stray light;

- p. 3: latitude °N and longitude °E (NW/EW?);

- p. 3: order the pollution sources by relevance;

- p. 4: unless very old instruments are considered, UV, SO2 and O3 estimates are performed by all instruments. NO2 instruments are performed only by MKIV Brewers, which are no longer manufactured;

- p. 5: it is said that LPM is not recommended for low latitude stations, however Poprad-Gánovce (706 m a.s.l.) cannot be considered a high altitude station. Please, clarify this point;

- p. 5: how were the air mass thresholds of 3 (line 12) and 4 (line 32) established?

- p. 5: explain better the "polarization effect" and use the form "internal polarization effect";

- p. 5: "corrections published in Cede at al. (2006)" -> which one of the methods described in the paper was used?

- p. 6: include the physical units for the Loschmidt constant;

- p. 7: remove repetitions in lines 1-3;

- p. 7: explain why the method works best at lower latitudes;

- p. 7: "implemented", not "developed" by Vladimir Savastiouk;

- p. 7: are the alternative Komhyr (1980) and Kasten and Young (1989) formulations used for O3 as well?

- p. 8: why was the stray light correction applied at that stage?

- p. 8: explain if ND filter spectral transmittance was taken into account for AOD calculations;

- p. 8: what are the "practical purposes"?

- p. 9: the conditions 1-3 are related to the single measurement, while conditions 4-9 concern the whole day. They should be splitted;

- p. 11: "more relevant" compared to what?

- p. 11: include a formula for the correction factor;

- p. 14: what criterium was followed to define "similar" the two ln(ETC)'s, sometimes deviating by 0.1 (l. 19)? Similarly, how can you quantitatively tell that the agreement is "excellent" (p. 15) and "high level"? Compare your results to the ones obtained in a recent paper about Brewer AOD in the UV range (of López-Solano et al., 2017, https://www.atmos-chem-phys-discuss.net/acp-2017-1003/);

- p. 16: "it is not known that the BSM would take into account a change in the distance of Earth from the Sun". Does it mean that the Brewer Operating Software does not take into account the Earth-Sun distance or that you do not know whether it takes this factor into account? In the latter case, examine the source code or ask to the maintainer of the Brewer operating software!

- p. 18: since only direct sun measurements are considered in the paper, only report the ozone trend for DS;

- p. 21: explain what agricultural activities can determine the increase of AOD (wildfires?).

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ED: Reconsider after major revisions (13 Feb 2018) by Alkiviadis Bais

AR by Peter Hrabčák on behalf of the Authors (13 Apr 2018) Manuscript

ED: Publish subject to technical corrections (30 Apr 2018) by Alkiviadis Bais

AR by Peter Hrabčák on behalf of the Authors (09 May 2018) Author's response Manuscript

Short summary

The paper presents results of a Brewer MKIV optical depth measurements from 1994 to 2016. The optical depth values were determined for the wavelengths of 306.3, 310, 313.5, 316.8 and 320 nm. The Langley plot method was applied to calculate the aerosols optical depth. A statistically significant decrease in the total optical depth of the atmosphere was observed with all examined wavelengths. Its root cause is the statistically significant decline in the optical depth of aerosols.