the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
13 Jan 2021
13 Jan 2021
Consistency of total column ozone measurements between the Brewer and Dobson spectroradiometers of the LKO Arosa and PMOD/WRC Davos
- 1Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Davos Dorf, Switzerland
- 2Meteoswiss, Payerne, Switzerland
- 1Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Davos Dorf, Switzerland
- 2Meteoswiss, Payerne, Switzerland
Abstract. Total column ozone measured by Brewer and Dobson spectroradiometers at Arosa and Davos, Switzerland, have systematic seasonal variations of around 1.5 % using the standard operational data processing. Most of this variability can be attributed to the temperature sensitivity of approx. +0.1 %/K of the ozone absorption coefficient of the Dobson spectroradiometer (in this study D101). While the currently used Bass&Paur ozone absorption cross-sections produce inconsistent results for Dobson and Brewer, the use of the ozone absorption cross-sections from Serdyuchenko et al. (2013) in conjunction with an effective ozone temperature dataset produces excellent agreement between the investigated four Brewers (of which two double Brewers), and Dobson D101. Even though other ozone absorption cross-sections available in the literature are able to reduce the seasonal variability, all of those investigated produce systematic biases in total column ozone between Brewer and Dobson of 1.1 % to 3.1 %. The highest consistency of total column ozone from Brewers and Dobson D101 at Arosa/Davos of 0.1 % is obtained by applying the Rayleigh scattering cross-sections from Bodhaine et al. (1999), the ozone absorption cross-sections from Serdyuchenko et al. (2013), the effective ozone temperature from either ozonesondes or ECMWF, and the measured line-spread functions of Brewer and Dobson. The variability between Brewer and Dobson for single measurements of 0.9 % can be reduced to less than 0.5 % for monthly means and 0.3 % on yearly means. As show here, the proposed methodology produces consistent total column ozone datasets between Brewer and Dobson spectroradiometers of better than 1 %. For colocated Brewer and Dobson spectroradiometers, as is the case for the Arosa/Davos total column ozone times series, this allows the merging of these two distinct datasets to produce a homogeneous time series of total column ozone measurements. Furthermore, it guarantees the long-term future of this longest total column ozone time-series, by proposing a methodology how to eventually replace the ageing Dobson spectroradiometer with the state-of-the art Brewer spectroradiometer.
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Julian Gröbner et al.
Status: closed
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CC1: 'Comment on amt-2020-497', James Kerr, 28 Jan 2021
I found this paper by Gröebner et al, 2021 (G2021) interesting and informative. However, I would like to draw to the attention of the authors the research that was carried out many years ago investigating the effects of ozone temperature on ozone absorption and the resulting impact on Brewer/Dobson measurements.
At least two papers that are entirely relevant to the subject matter of G2021 should be appropriately included:
1) Kerr et al., 1988: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JD093iD09p11129
This is the first reported long term Brewer/Dobson colocated comparison of ozone measurements showing the systematic annual difference between Dobsons and Brewers. It is also the first to suggest that the seasonal difference in measured values could be due to effects of ozone temperature on ozone absorption coefficients. However, this early paper is not included in G2021, whereas, several other following references are cited on lines 35-40.
2) Kerr, 2002 (K2002): https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2001JD001227 This paper found that the convoluted absorption coefficients of Bass and Paur, 1985 required modification in order to “best fit” observed data at MLO and Toronto. It also found that the temperature dependence of the Brewer algorithm using the modified Bass/Paur coefficients is essentially zero. Table 2 of K2002 reports the calculated (unrevised) Bass/Paur temperature dependence (based on quadratic fits) as 0.094%/C in good agreement with that of the IGQ given in Table 2 of G2021 (0.104%/K). K2002 also reports the revised temperature dependence as -0.005%/C in good agreement with the IUP and IUP_A in Table 2 of G2021 (0.010%/K and 0.001%/K).
I should also point out that findings of K2002 are based on measurements made of absorption by atmospheric ozone using a Brewer instrument and the sun as a light source. The conclusions made by R2014 and G2021 are based on high quality (IUP) measurements made of absorption by ozone in the laboratory using a high resolution spectrometer and a lamp as a light source. The laboratory measurements are then convoluted with the Brewer slit functions to determine the differential absorption coefficients. I find it quite remarkable that both the laboratory based measurements and the field based measurements have arrived to the same conclusion: i.e. the Brewer operational algorithm for measuring total ozone has little dependence on effective ozone temperature.-
AC1: 'Reply on CC1', Julian Gröbner, 13 Mar 2021
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2020-497/amt-2020-497-AC1-supplement.pdf
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AC1: 'Reply on CC1', Julian Gröbner, 13 Mar 2021
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RC1: 'Comment on amt-2020-497', Anonymous Referee #1, 07 Feb 2021
The paper presents a thorough comparison of total ozone measurements based on Dobson and Brewer spectrophotometers operating at the combined Arosa/Davos station. The paper shows how different ozone absorption cross sections can affect the data retrieved for each instrument, taking also into account the temperature dependence of the cross sections, the recalculated instrument-specific Rayleigh scattering coefficients, and the effective ozone height. This study contains very useful information, essential for assessing the accuracy of a combined total ozone time series for this station. As such I think it is appropriate for publication in AMT.
The only weaknesses of this study are related to the presentation quality. The language used in the paper is sometimes oversimplified introducing difficulties in understanding by non-expert readers. Some examples are given in the comments below (listed by line number) which aim mainly at improving the clarity of the paper.
Specific Comments:
21: Rephrase because as it written now it implies that all instruments (including Brewers) were installed in 1926!
24: In addition to horizontal, state the vertical displacement of the two stations.
27: In the abstract it is stated that there is a seasonal variability of 1.5%, while here that there a consistency of within 1%. Which of the two is more accurate? Furthermore, in line 87 this number is further reduced to 0.5%.
64: Aerosols and NO2 also absorb in this range. Although for Davos and Arosa their effect should be negligible, these species should be mentioned for completeness.
68: Actually, α(λ) is the absorption coefficient and not the cross section
109: This sentence is unclear for non-experts, please rephrase: “…does not coincide with the emission lines of the spectral lamps, the line…”
212: The procedure for determining the error in the total ozone due to the use of different Rayleigh cross sections could be slightly expanded so that inexperienced readers can follow it better. Alternatively, a reference could be provided to improve understanding.
238: However, if new Rayleigh cross sections are used, then the calibration of the instrument would change so this offset of about 2.4 DU would be finally compensated.
248: The term “ozone airmass” cannot be understood by non-expert readers. Line 69 defines it as “effective airmass for ozone absorption” so the term could also be used here.
253: Please explain what is meant by 95% variability. Does it refer to the 95% of the data?
280: In addition to noise in the cross sections another reason could be the different wavelengths used in Brewers compared to Dobsons in conjunctions with the spectral variability of the cross sections.
307: The average deviation for IUP_ATMOS is the same (though of opposite sign) with the Operational settings. However, the spread becomes smaller amplitude 0.09 vs 0.75 and this should be mentioned.
315: ECMWF does not provide the effective temperature but the temperature profiles from which the effective temperature can be calculated. Moreover the ozone profiles that are needed for the calculation of Teff are available from other sources which should be mentioned.
332: Since a supplement already exists, I suggest to include this figure in the supplement, to demonstrate the difference of the stray-light effect of the single Brewers.
335: The conclusions section starts very abruptly. Please start by at least identifying the instrument’s location.
343-345: I think this sentence is somewhat misleading. It is not clear what is meant by “precludes their use as common ozone absorption cross-sections”. If I understand correctly, the same cross-section can be used in both types of instruments as long as their temperature sensitivity is taken into account for each instrument.
Finally, Please make sure that the captions of all Figures are placed below the figure.
Technical comments:
61: Replace slant pass with slant path (also in the caption of Figure 7 and at lines 325, 327, 331)
72: Please add after “…attenuate” “for each pair”
90: Replace “consist of” with “are”
99: Replace Bewer by Brewer
113: Please make clear that the tuneable portable source is the TuPS device already mentioned above.
114: Please state the name of the project.
119: Replace cross-sections with coefficients
266: “the variability of the effective ozone temperature”
325: Caption of Figure 7, third line: Replace “airmasses” with “ozone slant path”
337: Replace “effective ozone coefficient” with “ozone absorption coefficient”
342: Add also the original IUP cross section dataset
360: Replace “a series of” with “three”
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AC2: 'Reply on RC1', Julian Gröbner, 13 Mar 2021
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2020-497/amt-2020-497-AC2-supplement.pdf
-
AC2: 'Reply on RC1', Julian Gröbner, 13 Mar 2021
-
RC2: 'Comment on amt-2020-497', Alberto Redondas, 11 Feb 2021
This is an important work confirming the compatibility between Brewer and Dobson observations when new ozone cross-section, line spread function, and temperature correction are used. In general, it is a very good manuscript that will be making an impact on future works. An open question is how the trend calculations of the total ozone long term series from Arosa-Davos will be affected when the ozone layer temperature correction is applied to the observations and how the possible trend in effective ozone temperatures.
I suggest accepting it for publication after minor revisions:
Most of the datasets used are not available and will difficult the reproducibility of the results, this include, ozone cross-section of the EMPR-ATMOZ, line Spread functions, and the Brewer /Dobson Ozone datasets.
Straylight correction is applied to the single brewer, this should be detailed in the methodology section. Also, the straylight on Dobson can be explained Can be estimated by TUPS measurements?
Specific comments and technical corrections
Section 2.1 Total ozone measurements: Aerosol term and its cancelation is missing from the discussion.
line 100: Calibration reports can be cited by his DOI in particular 2017 https://dx.doi.org/10.31978/666-20-019-9 and 2018 https://dx.doi.org/10.31978/666-20-018-3 calibrations,
line 130: Please explain the normalization of the ozone sounding.
line 138: please correct the link ( the correct one ends in .php)
line 150: Brewer and Dobson use different ozone effective heights on the operational procedure for the air mass calculation the effect of the ozone height is different, even if the effect is reduced due to the horizon minor please clarify.
line 230: Units missing for Rayleigh coefficients.
line 275: Explanation for the large difference on the offset of ACS dataset.
Figure 5: BOp and Bop are confusing terms of the first panel, please change.
line 325: To explicit the straylight, could usefully use a common calibration for both instruments, Brewer is calibrated against the Dobson or vice versa, using low OSC conditions and then see the comparison at high OSC conditions. We have to take into account that the Dobson has a considerably bigger FOV (Dobson nominal from FOV 7º to 8 º whereas the Brewer is around 2º-3º) and is more affected by atmospheric straylight.
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AC3: 'Reply on RC2', Julian Gröbner, 13 Mar 2021
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2020-497/amt-2020-497-AC3-supplement.pdf
-
AC3: 'Reply on RC2', Julian Gröbner, 13 Mar 2021
Status: closed
-
CC1: 'Comment on amt-2020-497', James Kerr, 28 Jan 2021
I found this paper by Gröebner et al, 2021 (G2021) interesting and informative. However, I would like to draw to the attention of the authors the research that was carried out many years ago investigating the effects of ozone temperature on ozone absorption and the resulting impact on Brewer/Dobson measurements.
At least two papers that are entirely relevant to the subject matter of G2021 should be appropriately included:
1) Kerr et al., 1988: https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JD093iD09p11129
This is the first reported long term Brewer/Dobson colocated comparison of ozone measurements showing the systematic annual difference between Dobsons and Brewers. It is also the first to suggest that the seasonal difference in measured values could be due to effects of ozone temperature on ozone absorption coefficients. However, this early paper is not included in G2021, whereas, several other following references are cited on lines 35-40.
2) Kerr, 2002 (K2002): https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2001JD001227 This paper found that the convoluted absorption coefficients of Bass and Paur, 1985 required modification in order to “best fit” observed data at MLO and Toronto. It also found that the temperature dependence of the Brewer algorithm using the modified Bass/Paur coefficients is essentially zero. Table 2 of K2002 reports the calculated (unrevised) Bass/Paur temperature dependence (based on quadratic fits) as 0.094%/C in good agreement with that of the IGQ given in Table 2 of G2021 (0.104%/K). K2002 also reports the revised temperature dependence as -0.005%/C in good agreement with the IUP and IUP_A in Table 2 of G2021 (0.010%/K and 0.001%/K).
I should also point out that findings of K2002 are based on measurements made of absorption by atmospheric ozone using a Brewer instrument and the sun as a light source. The conclusions made by R2014 and G2021 are based on high quality (IUP) measurements made of absorption by ozone in the laboratory using a high resolution spectrometer and a lamp as a light source. The laboratory measurements are then convoluted with the Brewer slit functions to determine the differential absorption coefficients. I find it quite remarkable that both the laboratory based measurements and the field based measurements have arrived to the same conclusion: i.e. the Brewer operational algorithm for measuring total ozone has little dependence on effective ozone temperature.-
AC1: 'Reply on CC1', Julian Gröbner, 13 Mar 2021
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2020-497/amt-2020-497-AC1-supplement.pdf
-
AC1: 'Reply on CC1', Julian Gröbner, 13 Mar 2021
-
RC1: 'Comment on amt-2020-497', Anonymous Referee #1, 07 Feb 2021
The paper presents a thorough comparison of total ozone measurements based on Dobson and Brewer spectrophotometers operating at the combined Arosa/Davos station. The paper shows how different ozone absorption cross sections can affect the data retrieved for each instrument, taking also into account the temperature dependence of the cross sections, the recalculated instrument-specific Rayleigh scattering coefficients, and the effective ozone height. This study contains very useful information, essential for assessing the accuracy of a combined total ozone time series for this station. As such I think it is appropriate for publication in AMT.
The only weaknesses of this study are related to the presentation quality. The language used in the paper is sometimes oversimplified introducing difficulties in understanding by non-expert readers. Some examples are given in the comments below (listed by line number) which aim mainly at improving the clarity of the paper.
Specific Comments:
21: Rephrase because as it written now it implies that all instruments (including Brewers) were installed in 1926!
24: In addition to horizontal, state the vertical displacement of the two stations.
27: In the abstract it is stated that there is a seasonal variability of 1.5%, while here that there a consistency of within 1%. Which of the two is more accurate? Furthermore, in line 87 this number is further reduced to 0.5%.
64: Aerosols and NO2 also absorb in this range. Although for Davos and Arosa their effect should be negligible, these species should be mentioned for completeness.
68: Actually, α(λ) is the absorption coefficient and not the cross section
109: This sentence is unclear for non-experts, please rephrase: “…does not coincide with the emission lines of the spectral lamps, the line…”
212: The procedure for determining the error in the total ozone due to the use of different Rayleigh cross sections could be slightly expanded so that inexperienced readers can follow it better. Alternatively, a reference could be provided to improve understanding.
238: However, if new Rayleigh cross sections are used, then the calibration of the instrument would change so this offset of about 2.4 DU would be finally compensated.
248: The term “ozone airmass” cannot be understood by non-expert readers. Line 69 defines it as “effective airmass for ozone absorption” so the term could also be used here.
253: Please explain what is meant by 95% variability. Does it refer to the 95% of the data?
280: In addition to noise in the cross sections another reason could be the different wavelengths used in Brewers compared to Dobsons in conjunctions with the spectral variability of the cross sections.
307: The average deviation for IUP_ATMOS is the same (though of opposite sign) with the Operational settings. However, the spread becomes smaller amplitude 0.09 vs 0.75 and this should be mentioned.
315: ECMWF does not provide the effective temperature but the temperature profiles from which the effective temperature can be calculated. Moreover the ozone profiles that are needed for the calculation of Teff are available from other sources which should be mentioned.
332: Since a supplement already exists, I suggest to include this figure in the supplement, to demonstrate the difference of the stray-light effect of the single Brewers.
335: The conclusions section starts very abruptly. Please start by at least identifying the instrument’s location.
343-345: I think this sentence is somewhat misleading. It is not clear what is meant by “precludes their use as common ozone absorption cross-sections”. If I understand correctly, the same cross-section can be used in both types of instruments as long as their temperature sensitivity is taken into account for each instrument.
Finally, Please make sure that the captions of all Figures are placed below the figure.
Technical comments:
61: Replace slant pass with slant path (also in the caption of Figure 7 and at lines 325, 327, 331)
72: Please add after “…attenuate” “for each pair”
90: Replace “consist of” with “are”
99: Replace Bewer by Brewer
113: Please make clear that the tuneable portable source is the TuPS device already mentioned above.
114: Please state the name of the project.
119: Replace cross-sections with coefficients
266: “the variability of the effective ozone temperature”
325: Caption of Figure 7, third line: Replace “airmasses” with “ozone slant path”
337: Replace “effective ozone coefficient” with “ozone absorption coefficient”
342: Add also the original IUP cross section dataset
360: Replace “a series of” with “three”
-
AC2: 'Reply on RC1', Julian Gröbner, 13 Mar 2021
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2020-497/amt-2020-497-AC2-supplement.pdf
-
AC2: 'Reply on RC1', Julian Gröbner, 13 Mar 2021
-
RC2: 'Comment on amt-2020-497', Alberto Redondas, 11 Feb 2021
This is an important work confirming the compatibility between Brewer and Dobson observations when new ozone cross-section, line spread function, and temperature correction are used. In general, it is a very good manuscript that will be making an impact on future works. An open question is how the trend calculations of the total ozone long term series from Arosa-Davos will be affected when the ozone layer temperature correction is applied to the observations and how the possible trend in effective ozone temperatures.
I suggest accepting it for publication after minor revisions:
Most of the datasets used are not available and will difficult the reproducibility of the results, this include, ozone cross-section of the EMPR-ATMOZ, line Spread functions, and the Brewer /Dobson Ozone datasets.
Straylight correction is applied to the single brewer, this should be detailed in the methodology section. Also, the straylight on Dobson can be explained Can be estimated by TUPS measurements?
Specific comments and technical corrections
Section 2.1 Total ozone measurements: Aerosol term and its cancelation is missing from the discussion.
line 100: Calibration reports can be cited by his DOI in particular 2017 https://dx.doi.org/10.31978/666-20-019-9 and 2018 https://dx.doi.org/10.31978/666-20-018-3 calibrations,
line 130: Please explain the normalization of the ozone sounding.
line 138: please correct the link ( the correct one ends in .php)
line 150: Brewer and Dobson use different ozone effective heights on the operational procedure for the air mass calculation the effect of the ozone height is different, even if the effect is reduced due to the horizon minor please clarify.
line 230: Units missing for Rayleigh coefficients.
line 275: Explanation for the large difference on the offset of ACS dataset.
Figure 5: BOp and Bop are confusing terms of the first panel, please change.
line 325: To explicit the straylight, could usefully use a common calibration for both instruments, Brewer is calibrated against the Dobson or vice versa, using low OSC conditions and then see the comparison at high OSC conditions. We have to take into account that the Dobson has a considerably bigger FOV (Dobson nominal from FOV 7º to 8 º whereas the Brewer is around 2º-3º) and is more affected by atmospheric straylight.
-
AC3: 'Reply on RC2', Julian Gröbner, 13 Mar 2021
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2020-497/amt-2020-497-AC3-supplement.pdf
-
AC3: 'Reply on RC2', Julian Gröbner, 13 Mar 2021
Julian Gröbner et al.
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