Optimization of the SKYNET calibration with an iterative Improved Langley method and its validation in the Mediterranean area

Kumar, Gaurav; Momoi, Masahiro; Campanelli, Monica; Garcia-Suñer, Meritxell; Kouremeti, Natalia; Kudo, Rei; Pedrós, Roberto; Estellés, Víctor

doi:10.5194/egusphere-2026-4267

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https://doi.org/10.5194/egusphere-2026-4267

Preprints

23 Jul 2026

| 23 Jul 2026

Status: this preprint is open for discussion and under review for Atmospheric Measurement Techniques (AMT).

Optimization of the SKYNET calibration with an iterative Improved Langley method and its validation in the Mediterranean area

Gaurav Kumar, Masahiro Momoi, Monica Campanelli, Meritxell Garcia-Suñer, Natalia Kouremeti, Rei Kudo, Roberto Pedrós, and Víctor Estellés

Abstract. The Improved Langley plot (ILP) method was developed for on-site calibration of PREDE-POM radiometers within SKYNET. It eliminated the need to ship instruments to remote high-altitude sites as required by the Standard Langley plot (SLP) method, reducing data gaps, transport-related risks, and operational costs while maintaining field instrument uncertainty below 2.4 %. The study aims to update the ILP method by adopting the latest Skyrad MRI v2 and introducing an iterative procedure. Testing with the data from the QUATRAM (QUAlity and TRaceabiliy of Atmospheric aerosol Measurements) campaigns and the Burjassot site in Spain showed an overall improvement of the ILP method. The median differences in direct aerosol optical depth (AOD) relative to GAW-PFR (Precision Filter Radiometers) decreased to approximately −0.01 at the 500 and 870 nm channels. AOD differences within the World Meteorological Organization (WMO) limits increased by 15-fold and 4-fold at 500 and 870 nm channels, respectively. The PREDE-POM and AERONET Level 2 Version 3 direct AODs at the Burjassot site showed good agreement, with median AOD differences within ±0.01 across all wavelengths, except at 340 nm. However, the fraction of points within WMO limits increases by over 10 % at wavelengths below 500 nm, while remaining nearly unchanged (variations below 6 %) at longer wavelengths. These results confirm the robustness of the ILP currently used in SKYNET and highlight the possibility of an improvement at shorter wavelengths. This limitation is addressed in the present study through the proposed iterative ILP method based on Skyrad MRI v2.

Received: 16 Jul 2026 – Discussion started: 23 Jul 2026

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Gaurav Kumar, Masahiro Momoi, Monica Campanelli, Meritxell Garcia-Suñer, Natalia Kouremeti, Rei Kudo, Roberto Pedrós, and Víctor Estellés

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RC1:
'Comment on egusphere-2026-4267', Anonymous Referee #1, 14 Aug 2026 reply
GENERAL COMMENTS
The manuscript by Kumar et al. describes an update to the Skynet method (ILPU) for the in-situ calibration of POM sun/sky radiometers. In this update, a more recent model (Skyrad MRI v2) is employed, together with an interactive technique. The method is tested using data from the QUATRAM campaigns and a longer dataset from Burjassot. The results show that, in most cases, the updated method performs better than the previous version, while in general it does not lead to any deterioration in the results. As the authors honestly acknowledge, the update to the calibration method does not resolve all discrepancies between the POM measurements and those from the other instruments used as references. Nevertheless, it represents a step towards further progress and, for this reason, I believe that the manuscript is worthy of publication. The manuscript is generally well written in English. I would recommend publication after the following minor issues have been addressed.
SPECIFIC COMMENTS
It is not entirely clear to me how the algorithm converges. After the first iteration (n = 0), the retrieved F0 may still be affected by errors, meaning that the retrieved AOD may also contain errors, which will depend on the airmass. At n = 1, which is the final iteration in the scheme proposed by the authors, AOD is used in addition to normalised radiance as input. This means that the inversion at n = 1, and hence the new estimate of F0, will also be affected by the error in the AOD from the initial iteration. Why, then, should this new estimate be more accurate than the previous one? In other words, what is the mechanism that leads to convergence? This point may also be somewhat confusing for a reader who is not familiar with the details of the algorithm.

So far, the algorithm accuracy appears to have been assessed mainly through comparisons with other reference instruments and with full retrievals. A "synthetic" study could be very useful for investigating the sensitivity of the algorithm and gaining a better understanding of its potential and limitations. For example, the authors could use the forward model to simulate almucantar measurements for known aerosol properties and then apply the ILPU to check if they obtain similar values. By varying the input aerosol properties and the initial conditions assumed by the algorithm, they could gain further insight into its behaviour. I am not suggesting that such a test should be added to the manuscript. However, if the authors already have such material available, it could be included for completeness; otherwise, the test could be mentioned as a possible direction for future work.

Related to the previous point, I find that a sensitivity analysis with respect to both the initial conditions within the iterative scheme and the actual aerosol properties in the atmosphere is somewhat missing from the manuscript. Perhaps a short dedicated paragraph or section, even only based on the relevant literature, could be included in the discussion. Alternatively, if the authors already have suitable data available, the discussion could be supported by those results.

How is the "switch" between the inversion based only on normalised radiance data and the inversion that also uses transmittance data implemented technically?

Regarding the statement, "Moreover, diffuse sky radiance measurements with a scattering angle ranging from 4.5 degrees to 35 degrees is used in this study", I could not find an explanation for this choice. How does it compare with the scattering-angle range usually employed in the ILP?

Are the authors confident that the aerosol (and atmospheric) conditions encountered during the QUATRAM campaigns and at Burjassot are representative of other sites and periods in which the ILPU method might be applied? For example, how does the method perform under conditions of substantially lower or higher AOD?

To ensure that the results are reproducible, could the authors also describe which values of the atmospheric gases were included in the model and how these values were obtained (e.g. from measurements, reanalyses, climatological values, etc.)? How sensitive are the results to this choice?

Why is the sensitivity test performed only by increasing the baseline calculation by 3%, rather than also considering a 3% decrease? Does the algorithm also converge in this case?

Regarding the article structure, the fundamental improvements introduced in Skyrad MRI v2 are described rather late in the manuscript, at line 239: "the iterative approach works better thanks to the algorithm update [...] The retrieval of the aerosol optical parameters is optimised to the transmittance data rather than to the input direct extinction AOD. This allows the retrieved extinction and scattering AOD to adjust freely during the iteration process". Without this explanation, the transition from Skyrad v4.2 to Skyrad MRI v2 appears to be motivated mainly by the fact that the latter is a "newer" version, rather than by specific improvements that make it better suited to the purpose of this study. I would therefore suggest introducing this explanation earlier in the manuscript.

TECHNICAL REMARKS
Line 16: Please also mention the possibility of transferring calibration information from reference instruments. More generally, I would revise the description of the benefits of the Improved Langley Plot (ILP). In my view, the main advantage is not primarily that it eliminates the need to ship the instrument (this latter could be easily replaced by another instrument), but rather that it enables continuous monitoring of the instrument's sensitivity and performance, rather than providing such an assessment only once a year or every two years.

Line 19: Please include information on the locations of the QUATRAM campaigns. Also, specify that Skyrad MRI v2 is a model, as this is not yet clear to the reader at this point in the manuscript.

Lines 26–27: Please clarify whether the reported improvement at wavelengths mainly below 500 nm is primarily due to the higher AOD at shorter wavelengths and to the way the WMO limits are defined (i.e. independently of AOD and wavelength).

Line 32: Please note that the terminology used to describe aerosol effects has evolved since IPCC AR5, with the terms "aerosol–radiation interactions" (ARI) and "aerosol–cloud interactions" (ACI) now being commonly used.

Line 40: "Aerosol measurements" should be qualified here to make clear that it refers to measurements of the entire atmospheric vertical column. I would also replace "measure direct Sun aerosol optical depth (AOD)" with "derive aerosol optical depth (AOD) from measurements of direct-Sun irradiance".

Line 42: Please note that SSA is a derived product, as opposed to the previous ones.

Lines 57–58: Please note that international networks can provide replacement instruments during the calibration period, thereby largely overcoming this limitation.

Line 71: Could you clarify what is meant by the "annual trend" here?

Line 72: What factor changes from 0.5% to 2.4%?

Line 74 (and other occurrences): Please use \citet rather than \citep, i.e. remove "()" around citations.

Lines 76–87: Please clarify whether these studies applied the ILP method to determine F0 using only the period of the campaign or over a longer period (e.g. including measurements before and/or after the campaign).

Line 89: Skyrad is introduced here for the first time; however, it is not yet clear how the models are used to determine the calibration factor. Please provide some clarification at this point.

Line 114: Why is the 940 nm wavelength listed for POM-01 but not for POM-02? Moreover, since the authors have introduced the different wavelength ranges as the main difference between the POM models, do they think that this difference could also have an impact on the ILPU calibration?

Line 118: Are the instruments thermostated or simply heated? How might the temperature dependence affect the results presented in this paper?

Lines 119–121: Please remove one of the two similar sentences (likely a typo).

Sections 2.2 and 2.3: Please clearly state that these instruments are described here because they are used as reference instruments. Also, since the study concerns calibration, please describe how these reference instruments are calibrated. Note also that there are erroneously two sections numbered "2.3".

Section 2.4: Please clearly state the period used for the calibration. Does it coincide with the campaign periods?

Line 145: "Two PREDE-POM": were these POM-01 or POM-02 instruments, based on the description given earlier?

Lines 163–165: This seems to be a fundamental point and, in my view, should be introduced much earlier in the manuscript.

Line 168: "he" → "the".

Section 3.1: Interestingly, the paper does not mention the cross-IL (XIL) method described by Nakajima et al. (2020), which was developed to correct an underestimation by the IL method. Does this mean that the XIL method is not effective for the application considered here?

Line 172: What do you mean by "formalised"?

Line 174: "Similar to...": I guess this is "similar" because the observed signal, as described by the Bouguer–Lambert–Beer equation, does not depend on the calibration method.

Lines 176–177: Strictly speaking, different atmospheric constituents have different airmasses (m). This is particularly relevant at large airmasses.

Line 179: The expression "m = 1/cos(θ)" applies only under the plane-parallel approximation.

Lines 176–181: It appears that τ_g has not been defined or described.

Equation 4: The sign appears to be incorrect: there should be a "+" rather than a "−".

Line 190: Is the ILP, and therefore the ILPU, based on the assumption that SSA remains constant throughout the Langley event (day or half-day)? If so, how might this assumption affect the results?

Equations 6–7: Please define and explain all terms in these equations.

Line 197: Is the "atmospheric" phase function the sum of the aerosol and Rayleigh phase functions?

Line 204: Should this be "the" radiative transfer model or "a" radiative transfer model?

Lines 207–208: Please explain why this is important for the analysis.

Line 218: Why "daily"? Are the morning and afternoon Langley "events" (i.e. two per day) combined?

Figure 1: It is somewhat confusing to see arrows connecting both methods. As currently presented, the figure could suggest that one method depends on the other, rather than that the second method represents an evolution of the first. Also, the text is too small. Please use "(a)" and "(b)" to label the two panels if they are referred to separately in the text (e.g. line 331).

Line 229: Should this be "from" or "with respect to"? Also, what exactly are the "limitations" referred to here?

Line 234: Can the authors explain why including τ_ext^dir improves the retrieval? Is the information contained in the radiance measurements insufficient to estimate F0 accurately?

Lines 256–263: One of the first results that the reader is likely to be interested in is the difference between the F0 values obtained with ILP and ILPU. This is effectively presented later in Table 2, but it could also be mentioned here.

Line 269: The perturbation is described as being chosen "arbitrarily". Is 3% representative of any specific uncertainty or expected variation?

Lines 275–277: "Dashed", not "dotted".

Figure 2: Please use letters to label the different subfigures. Do the authors have an explanation for the slower convergence at shorter wavelengths? The wavelength dependence could point towards an effect related to molecular scattering or to the higher AODs at shorter wavelengths.

Table 2 and Equation 8: Usually, relative differences are calculated as (tested quantity − reference quantity) / reference quantity. Why are the differences here calculated relative to the tested quantity instead?

Lines 318–320: Consider moving these lines earlier, as they explain what is meant by the "full" inversion mentioned above.

Figures 3–4: Is there a more informative quantity than "Measurement index" that could be used on the x-axis? Why not use the date? Also, since the WMO criterion is mentioned, could a plot as a function of airmass be included somewhere? Finally, do percentage differences in AOD have a clear physical meaning, or would absolute differences be more appropriate here, given that AOD is derived from a logarithmic relationship?

Line 350: What are the two "AOD types"?

Tables 4–5: Please also include "%" in the row headers.

Line 386: Is this τ^dir or τ^dif?

Line 399: Please avoid the wording "direct AOD" in the section title.

Lines 402–403: Please revise this sentence; it appears that a verb is missing.

Line 406: Please refer to the SUNRAD "code".

Figures 5–6: Please use colours consistently across the different figures (e.g. red versus green), and explain the meaning of the colours in the captions.

Figure 5, caption: How can the WMO acceptance criteria be represented in this plot, given that the criterion depends on the airmass?

Line 425: Should this be "most" or "all"? Also, please repeat the airmass range during the campaigns in this paragraph, as the results may depend on the range considered.

Table 7: These results are somewhat disappointing compared with the other results presented in the manuscript. Do the authors have an explanation for this, or any ideas on how this aspect could be investigated further?

Figure 7: Please explicitly state which quantity is considered for the bias and RMSD (AOD?). When referring to the bias, clearly specify the order in which the difference is calculated, as the result is not commutative. Also, it appears that the last lines of the caption belong to Figure 8 instead.

Line 470: The order appears to be incorrect: MBD is shown in Figure 7, whereas RMSD is shown in Figure 8.

Lines 470–476: The observed wavelength dependence is also likely related to the fact that AOD is larger at shorter wavelengths. This could be mentioned explicitly.

Section 5: The "Summary and conclusions" section is too long. Please try to summarise the most important findings without repeating all the results presented in the previous sections.

Line 486: Please rewrite this sentence, as it is not clear which algorithm "adopted" refers to.

Lines 487–489: Please explain more explicitly what changes between the model versions, beyond simply changing the model name.

Lines 512–515: Please specify the dates and sites of the campaigns. Also, the results should be critically assessed in this section. For example, is a 1.3% difference considered satisfactory, and on what basis?

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Citation: https://doi.org/10.5194/egusphere-2026-4267-RC1
RC2: 'Comment on egusphere-2026-4267', Anonymous Referee #2, 25 Aug 2026 reply

General assessment
This manuscript presents a useful update to the SKYNET Improved Langley calibration method. The approach is well motivated, the datasets are appropriate, the topic is relevant to AMT, however, several issues must be resolved before the claimed improvement is demonstrated convincingly. The recommended revision is mostly minor except possibly for the uncertainty assessment (main comment 1).
Main comments
1. Provide an uncertainty assessment. Calibration changes are around 0.9-1.9 % between ILP and ILPU (Table 2, row 3) and may be comparable to calibration-transfer and measurement uncertainties; the ILP literature values quoted in the manuscript are themselves 1.0-2.5 % (Campanelli et al., 2004) and 0.6 - 2.5 % RMSE (Uchiyama et al., 2018), and the abstract states that ILP maintains "field instrument uncertainty below 2.4 %". No corresponding figure is given for ILPU. For a calibration paper in AMT this might be the single number a reader is most likely to look for and is a critical number to assess the improvement of ILPU over ILP.
2. Clarify the number of iterations. The manuscript states that two or three iterations are required below 500 nm, but uses only one iteration throughout. This is particularly important because the largest claimed benefits occur at short wavelengths, precisely those that are being under-iterated. Maybe give a numerical convergence criterion, rather than a qualitative judgement. The statement at l. 278-279 that the calibration values "ultimately converge to the same point or within a small range" is overstated for 340 nm, where a 0.62 % discrepancy remains after six iterations and the two rounds are still approaching each other monotonically in Fig. 2. Please soften this and state explicitly that 340 nm has not converged within six iterations. If feasible for a single month, extending the 340 nm channel alone to n = 10 would indicate where the two rounds converge and would close this question.
Also refine the quantification of the computation cost, which is the sole justification offered for Nmax = 1. "Several hours of computing time on a standard desktop computer" (l. 290-293) is not sufficient: per month, per instrument, per iteration? What is “standard” nowadays? Scalability might be highly relevant, and in general I would like an explicit operational recommendation, whether SKYNET should adopt ILPU, at which wavelengths and sites, and with how many iterations.
3. Correct the interpretation of the WMO analysis. The WMO criterion is defined as ±(0.005 + 0.01/m) and is therefore airmass-dependent, but the shaded band in Figs. 5 and 6 is fixed. The text at l. 422-423 describes that band as "the median difference at ±0.01", whereas the caption of Fig. 6 describes the same band as the WMO acceptance criteria. Since the principal result of Sects. 4.5 and 4.6 is the fraction of points falling inside this band, please state unambiguously which criterion was applied in the statistics and what is drawn in the figures, and confirm that the two are the same. Please also confirm the intended form of the criterion (the usual expression is 0.005 + 0.010/m) and give the reference. Absolute percentage-point changes are more informative than "fold" increases when the initial fraction is near zero; the phrasing at l. 24 of the abstract ("AOD differences within the WMO limits increased by 15-fold") also inverts the meaning, since it is the fraction of points falling within the limits that increased.
4. Discuss results that do not improve. At Burjassot, Table 8 shows decreases from 88 % to 82 % at 675 nm, 91 % to 89 % at 870 nm, and 90 % to 89 % at 1020 nm. The conclusion that ILPU improves all wavelengths is therefore not supported. A six-percentage-point loss at 675 nm on a sample of 788 points may not be negligible, and is opposite in sign to the paper's central claim. The text (l. 457-458) reports only that variations are "below 6 %", and the conclusions mention only the gains at 340 and 380 nm. Please report the decreases explicitly, state whether they lie within the sampling noise of the comparison and, if so, give the confidence interval that justifies that statement (see comment 1).
5. Improve reproducibility and operational guidance. Please provide the retrieval settings, filtering and cloud-screening details, calibration sample sizes, and equations and sign conventions for MBD and RMSD. Is the code in a public repository with a DOI?
Specific comments
Abstract. Please rewrite so that the existing ILP method, the proposed ILPU method, and their respective results are clearly distinguished. As written, the sequence of results in l. 19-28 reads as though it describes the performance of the existing method, and the closing sentence ("This limitation is addressed in the present study through the proposed iterative ILP method based on Skyrad MRI v2") then implies that the new method's results have not yet been given. The rewrite should (i) state in one sentence what ILPU is and how it differs from ILP, (ii) report the quantitative outcomes with unambiguous attribution to ILPU, and (iii) close with the practical recommendation to the network.
Eqs. (1) and (2). Eq. (1) includes a gas optical depth term τ_g which is absent from Eq. (2) without explanation. Please reconcile.
L. 206 vs. l. 213 and l. 319-320. The text states that the volume size distribution is retrieved from radiances at scattering angles "up to 30 degrees", but elsewhere the range used in this study is given as 4.5 to 35 degrees. Please clarify the choice of this range and the operational ILP configuration, since the difference between this restricted range and the full 2.5-180 degrees range is later invoked (l. 319-321) to explain the ILPU/full-inversion discrepancy.
L. 308-310. "more precise" should probably be "more accurate", since the claim concerns bias rather than scatter.
L. 342-346. "the differences are more pronounced with the CNR instrument compared to the UV, but this could be due to instrument-specific effects" is vague. If no explanation can be offered, please say so plainly; if a specific cause is suspected (filter degradation, temperature …), name it.
L. 389-390. Please give the provenance of the nominal combined uncertainty of 0.02 and state how the combination was performed.
L. 393 and l. 525-526. A coefficient of determination of 0.98-0.99 is said to indicate excellent agreement. R² measures correlation, not agreement, the MBD and RMSD values are what support the agreement claim. Please rephrase.
L. 523-524. The MBD values quoted do not match Table 6: the ILPU rows appear to have been exchanged between the two instruments (the ILP rows are correct as given). Table 6 gives, for ILPU, -0.0002 to 0.0003 for POM-CNR and -0.0007 to -0.0003 for POM-UV, whereas the text reports -0.0007 to 0.0003 for POM-CNR and -0.0003 to 0.0 for POM-UV. Please check and correct.
L. 451-452. The one-year Burjassot comparison retains only 788 collocated points, which follows from the 10 s coincidence window used here against the 10 min window used in Sect. 4.4. Please comment on whether so strict a criterion is necessary for direct sun data, and whether relaxing it to, say, one minute changes the statistics in Table 8.
L. 477-481. The seasonal pattern is attributed to long-range dust transport. Since AERONET Ångström exponent and, presumably, coarse-mode information are available at Burjassot, a brief quantitative check (e.g. stratifying the comparison by AE) would turn this from a plausible association into a supported statement. This is optional but would add value.
L. 489. Seven wavelengths are listed as "six".
L. 132 and l. 140. Two consecutive subsections are both numbered 2.3. Renumber.
Table 1. The rows of round B are labelled F0A⁽¹⁾ to F0A⁽⁶⁾; these should presumably be F0B.
Table 2. The "POM-CNR" label sits in a merged cell at the right-hand edge and its scope is unclear. If the whole table refers to POM-CNR only, please say so in the caption; if the corresponding POM-UV values exist, please add them, since the two instruments behave differently elsewhere in the paper.
Table 6. The caption reads "POV-UV".
Figs. 7 and 8. The text near l. 470 refers to "the monthly RMSD (Fig. 7) and MBD (Fig. 8)", but Fig. 7 is captioned as MBD and Fig. 8 as RMSD. Please correct the cross-references. The Fig. 7 caption likewise states that the y-axis indicates the monthly RMSD, and describes "the black line" as showing two different things (zero bias and the reported uncertainty).
Fig. 2. The y-axis is labelled only "F0" with no units; please add units and clarify the per-panel ×10⁻ⁿ multipliers, which are easy to misread.
L. 419-429. The description of Fig. 5 largely re-reads the figure and could be shortened (optional).
References. Several entries are corrupted, notably Holben et al. (1998) ("Tan&, : D" … ). The Nakajima et al. (2020) entry appears to lack journal name, volume and year. Please regenerate the bibliography.
Technical corrections
L. 119-121: the sentence giving the InGaAs operating range (0 to −25 °C) is duplicated.
L. 253: "sufficient for and accurate calibration" should read "sufficient for an …".
L. 267: "Round A" and "round A", use consistent capitalisation throughout (also round B).
L. 291: "so only one iteration will be used for the rest of the study" , tense inconsistency with the surrounding past-tense narration.
L. 349-352: "the differences between the two AOD types are strikingly similar for both instruments" is confusing; "AOD types" presumably means the ILPU and full-inversion AODs. Please clarify the terminology, which is used inconsistently with Sect. 4.2.
L. 352: sentence ends without a full stop.

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Citation: https://doi.org/10.5194/egusphere-2026-4267-RC2
RC3: 'Comment on egusphere-2026-4267', Anonymous Referee #3, 01 Sep 2026 reply

General assessment
The manuscript offers a significant improvement over the current SKYNET ILP calibration method and fits therefore well to the scope of the AMT. The basic principles of the method have previously been presented, and with the improvement of computer efficiency, it has now been possible to test the effectiveness of the method in practice. The authors demonstrate improvements in the method compared to the previous version in a variety of ways. However, there are some issues which may need improvement.

Main comments
1. The wavelengths shorter than 500 nm may need more than 2-3 iterations (according to figure 2). Does it take several hours for one wavelength, one iteration, one month? Should the number of iterations be left to individual users to decide, or should SKYNET provide a recommendation for the (minimum) number of iterations?
2. In the section 4.5 (row 412), the authors give the WMO acceptance criteria, ±0.005 + 0.01/m, which means that that the airmass is varying (you may include a reference here). In figure 5, the pink colored area requires a constant value of m=2, which is somewhat strange. What are the airmasses of the data used in the boxplot?

Specific comments
1. Equation (7) - especially the beta - is not introduced in the text. If it is not necessary and does not add value, please consider leaving it out from the text.
2. Improved Langley Plot (ILP) method is presented in Section 3.1, where it is said (row 210)that "[T]his study used the latest Skyrad MRI v2 for the inversion procedure". However, according to the Figure 1, Skyrad 4.2 is still used in ILP. The reader who is not completely familiar with the subject is left to wonder whether an error has occurred here, and the use of the correct version should be clearly and uniformly indicated both in the text and in the diagram.
3. Table 7 shows that points within WMO limit during QUATRAM II (ILP) is 0 % at 500 nm, while the text (row 436) says "from nearly zero". Please provide some decimals for the percentage, unless it is exactly zero. Same applies to row 533, with "nearly 0 %".
4. Eq. 8 would perhaps look better if mathematical symbols were used instead of text.
5. X axis in Figure 3, measurement index, maybe unclear to someone, please clarify in row 343 what does it mean.
6. Row 406, SUNRAD is mentioned first time here. Therefore, please introduce here what it is.
7. Row 451, 10 seconds is quite a short time. Maybe you could extend the allowed time window to get more common data points. On the other hand, row 388 says "within 10-minute time interval". Why you use a short interval for one measurement and a longer one for another?
8. You mentioned the CIMEL instrument number first time here. Maybe better to leave it out unless you prefer to mention it in the very beginning of the paper, together with instrument number of the PREDE and PFR.

Technical corrections:
Please use term "sun photometer" in a unified manner throughout the text. Now the term are written in different ways: rows 49, 52, 379 sun-photometers; whereas rows 122, 123 sun photometer; rows 163, 165 Sun photometer
row 33: Solar, whereas row 46 solar => please use a consistent way of writing the term everywhere
rows 132 and 140, same section numbering in both sections
row 168: he should probably be "the"
row 254 "for and accurate calibration" => for an accurate calibration?
row 403, word "that" may probably be unnecessary here?
row 495, "above 400 nm. the discrepancy..." should perhaps be written as "above 400 nm. The discrepancy..."
row 565, "NK-" should be "NK:"
row 592, Ångström is written with capital letters.

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Citation: https://doi.org/10.5194/egusphere-2026-4267-RC3

Gaurav Kumar, Masahiro Momoi, Monica Campanelli, Meritxell Garcia-Suñer, Natalia Kouremeti, Rei Kudo, Roberto Pedrós, and Víctor Estellés

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Short summary

Aerosols are tiny particles suspended in the atmosphere, affecting the air quality and climate. Accurate aerosol measurements are thus important for understanding their effects. Sky radiometers are widely used instruments for the aerosol measurements. We developed a method to improve the calibration coefficients of sky radiometers, using the latest data-processing algorithm and an iterative procedure. The proposed method produced more accurate results, improving long-term aerosol monitoring.


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