the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Mar 2026
An Assessment of Lunar Photometry in AERONET
Abstract. AERONET has been acquiring direct beam lunar observations at six wavelengths from 440 to 1640 nm from most newer model T CIMEL sun-sky radiometers in the network for many years and producing a night-time AOD data set, which currently includes observations at 492 sites dating back as far as 2014. The new dataset of lunar AOD now uses an updated empirical correction for the ROLO lunar irradiance model and has been extensively analyzed for all long-term lunar-capable sites by evaluating the continuity of AOD between solar and lunar measurements during limited temporal windows. Comparison of daytime and nighttime AOD measurements shows good agreement at all sites with more than a decade of data with mean differences typically within ±0.01 for AOD440 nm and similar or better agreement at longer wavelengths. Comparisons during 3-hour transition periods between day and night observations during conditions of low and stable AOD found statistically negligible differences of AOD at all common wavelengths as well as for Ångström Exponent, column water vapor and AODfine and AODcoarse parameters from SDA. The lunar AOD product demonstrates consistency with solar AOD across diverse aerosol conditions at AERONET sites globally, validating the empirical correction approach.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Measurement Techniques.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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CC1: 'Comment on egusphere-2026-1506', Matthijs Krijger, 26 Apr 2026
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AC1: 'Reply on CC1', Joel Schafer, 30 Apr 2026
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This is a valuable suggestion. A table of the derived ROLO correction coefficients for each CIMEL wavelength will be included as supplemental data. Thanks for the comment.
Citation: https://doi.org/10.5194/egusphere-2026-1506-AC1
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AC1: 'Reply on CC1', Joel Schafer, 30 Apr 2026
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CC2: 'Comment on egusphere-2026-1506', Hugh Kieffer, 04 May 2026
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Leading digits in comments are line number in the preprintThis is a significant paper and the large effort described will aid in approaching absolute lunar calibration.
------------------ Major -----------------
As you state, it is widely thought that ROLO model has biases. Better models have been available for several years.
You may find the following comprehensive review helpful.
"Activities to Promote the Moon as an Absolute Calibration Reference"
Z. Jing et al, Remote Sens. 2023, 15, 2431. https://doi.org/10.3390/rs1509243158: Use of the LIME model for this work would be problematic, as they are based upon the same thru-the-atmosphere methodology and share data observations. This work and LIME would best be derived in parallel! although there are no authors in common.
54+. There is a newer lunar spectral irradiance model that would be useful.
H. H. Kieffer, ''Multiple-instrument-based spectral irradiance of the Moon'',
J. Applied Remote Sensing, Vol. 16(3), 038502 (2022)
https://doi.org/10.1117/1.JRS.16.038502 (open access)
As listed in Table 7 of that SLIM (now SLIMM) paper, ROLO is 3 to 8% below SLIMM.Over 400 to 1040 nm, the SLIMM model is within 1% of the carefully calibrated air-LUSI results (discussed within GSICS, but not yet published)
Fig 1. These values look close to what the SLIMM model would forecast.
~75 The variation of lunar irradiance with sub-viewer lunar coordinates ("libration") is about 1% for modest phase and more at large phase. Libration should be considered in a correction LUT. 600 observations should be enough to assess the magnitude of libration from your data alone. Or you could use the model based on lunar maps (e.g., SLIMM Table 3). Hopefully, this will reduce the scatter in Figure 2.
Section 3.1 Good discussion of lunar irradiance properties.
Fig 5 and 6. Good figures!
Section 5: The near-in-time solar and lunar observations by the same instruments, especially at the two high-elevation sites, are key to this paper.
All things that might affect these need thorough and prominent quantitative discussion. E.g. ,instrument non-linearity, exposure timing resolution and accuracy, sensitivity to sky radiance level, ADC resolution at low light levels, ... . The sensitivity to AOD is already well discussed.Section 5.2 Overall bias message is clear, but the concept behind 440nm exclusion and Fig. 15 are hard to follow.
245. Say where is Palangkaraya and its elevation.
Fig 15a. Move both interior labels down so that 'Solar' does not look like a subtitle to the upper part
252:253 Meaning unclear. Is 'range ' a verb or noun here?Citation: https://doi.org/10.5194/egusphere-2026-1506-CC2 -
RC1: 'Comment on egusphere-2026-1506', Anonymous Referee #1, 13 May 2026
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This manuscript presents an extensive assessment of AERONET nighttime aerosol optical depth (AOD) retrievals based on lunar photometry and empirical correction of the ROLO lunar irradiance model. The work is timely and important because nighttime AOD observations are increasingly valuable for aerosol monitoring, air quality applications, wildfire smoke tracking, and studies of diurnal aerosol variability. The manuscript is generally well organized and contains a large amount of useful analysis based on long-term global AERONET observations. The effort to evaluate continuity between solar and lunar retrievals across hundreds of sites is particularly valuable.
Major comments:
The current validation strategy relies heavily on continuity between solar and lunar AOD during day–night transition periods under low and stable aerosol conditions (AOD440 ≤ 0.1 with low variability). While this is a reasonable approach for isolating calibration consistency and minimizing atmospheric variability, it may introduce a selection bias toward relatively homogeneous and low-loading atmospheric states. Consequently, the presented analysis primarily demonstrates agreement under favorable retrieval conditions rather than across the broader range of nighttime aerosol environments that are often of greatest scientific and operational interest.
Although the empirical correction is applied at the irradiance calibration level and is therefore not explicitly AOD-dependent, it is unclear whether retrieval performance remains equivalent under high-AOD conditions where signal levels are substantially reduced and atmospheric variability is often larger. Additional discussion of how calibration uncertainty, low-signal effects, and retrieval bias may vary with aerosol loading would strengthen the manuscript.
In addition, the manuscript later notes that high-AOD lunar observations are more likely to be excluded because of low-signal limitations, particularly at shorter wavelengths. This raises the possibility of an additional sampling bias in the validation dataset.
It would strengthen the manuscript if the authors discussed more explicitly how these filtering criteria may limit the representativeness of the validation framework, particularly for high-AOD, polluted, smoke-affected, or rapidly evolving nighttime conditions. Relatedly, would the reported agreement statistics change materially under elevated AOD conditions?
Minor:
Line 43: Vo should be V0
Figure 2: The legend titles should be made consistent (e.g., use either “channel” or “wavelength” for both figures). Additionally, why do the phase-angle dependence patterns differ between the two wavelengths?
Section 4: The manuscript would benefit from clearer specification of the data quality levels used throughout each analysis section. While Section 5 explicitly references Level 2 (L2) AOD data, it is less clear whether the transition analyses in Section 4 also rely exclusively on L2 observations. It would be helpful to clarify whether “L2” solar and lunar products should be interpreted as equivalent in quality assurance rigor and discuss any implications for the validation results.
Line 137: The statement that deviations “diminish with increasing wavelength” would be stronger if quantitative wavelength-dependent results or statistics were provided.
Line146: Figures 7 and 8 would benefit from additional discussion and interpretation. In particular, the spread of the day–night AOD differences appears to increase substantially at higher AOD values. This may reflect more rapidly evolving aerosol conditions during transition periods, but it could also indicate increasing retrieval uncertainty or residual bias under high-loading conditions where lunar signal limitations become more important. Please discuss the likely causes of the increased scatter and whether retrieval performance degrades systematically at elevated AOD.
Line 252: Please provide a quantitative description for “the great majority.” More generally, several statements throughout the manuscript rely on qualitative descriptors such as “good agreement,” “minor,” “very small,” “not notably dependent,” and “the great majority” without quantitative definition. I recommend providing explicit numerical thresholds, percentages, or statistical metrics wherever possible to improve interpretability and reproducibility.
Some figure labels and color-bar annotations are difficult to read because of small font sizes (e.g., Figures 7 and 8).
Citation: https://doi.org/10.5194/egusphere-2026-1506-RC1
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The authors provide a nice detailed description of all their calibration steps, including a novel LUT correction to the well-established ROLO model that seems to work well. In figure 1 the impact of the ROLO correction for 440, 500, 675, 870, 1020, 1640nm are shown, but in figure 2 the LUT corrections are only shown for 440nm and 675nm. Without knowledge of the correction at other wavelenghts it is impossible for other scientists to investigate whether or not the corrections are physical/reasonable and/or replicate the results.
As such please provide also the ROLO corrections for the other wavelenghts., either visually and/or preferable an open dataset for application by other users.