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International audience; The authors regret that there is an error in equation 12. The right equation for κλ, as is defined in Barreto et al. (The new Sun-sky-lunar Cimel CE318-T multiband photometer - a comprehensive performance evaluation, Atmos. Meas. Tech., 9, 631-654, 10.5194/amt-9-631-2016, 2016), is the reversed form of equation 12. That is: κλ = V0,λ*G/E0,λ.

The first multi-instrument nocturnal aerosol optical depth (AOD) intercomparison campaign was held at the high-mountain Izaña Observatory (Tenerife, Spain) in June 2017, involving 2-min synchronous measurements from two different types of lunar photometers (Cimel CE318-T and Moon Precision Filter Radiometer, LunarPFR) and one stellar photometer. The Robotic Lunar Observatory (ROLO) model developed by the U.S. Geological Survey (USGS) was compared with the open-access ROLO Implementation for Moon photometry Observation (RIMO) model. Results showed rather small differences at Izaña over a 2-month time period covering June and July, 2017 (±0.01 in terms of AOD calculated by means of a day/night/day coherence test analysis and ± 2% in terms of lunar irradiance). The RIMO model has been used in this field campaign to retrieve AOD from lunar photometric measurements. No evidence of significant differences with the Moon's phase angle was found when comparing raw signals of the six Cimel photometers involved in this field campaign. The raw signal comparison of the participating lunar photometers (Cimel and LunarPFR) performed at coincident wavelengths showed consistent measurements and AOD differences within their combined uncertainties at 870 nm and 675 nm. Slightly larger AOD deviations were observed at 500 nm, pointing to some unexpected instrumental variations during the measurement period. Lunar irradiances retrieved using RIMO for phase angles varying between 0° and 75° (full Moon to near quarter Moon) were compared to the irradiance variations retrieved by Cimel and LunarPFR photometers. Our results showed a relative agreement within ± 3.5% between the RIMO model and the photometer-based lunar irradiances. The AOD retrieved by performing a Langley-plot calibration each night showed a remarkable agreement (better than 0.01) between the lunar photometers. However, when applying the Lunar-Langley calibration using RIMO, AOD differences of up to 0.015 (0.040 for 500 nm) were found, with differences increasing with the Moon's phase angle. These differences are thought to be partly due to the uncertainties in the irradiance models, as well as instrumental deficiencies yet to be fully understood. High AOD variability in stellar measurements was detected during the campaign. Nevertheless, the observed AOD differences in the Cimel/stellar comparison were within the expected combined uncertainties of these two photometric techniques. Our results indicate that lunar photometry is a more reliable technique, especially for low aerosol loading conditions. The uncertainty analysis performed in this paper shows that the combined standard AOD uncertainty in lunar photometry is dependent on the calibration technique (up to 0.014 for Langley-plot with illumination-based correction, 0.012–0.022 for Lunar-Langley calibration, and up to 0.1 for the Sun-Moon Gain Factor method). This analysis also corroborates that the uncertainty of the lunar irradiance model used for AOD calculation is within the 5–10% expected range. This campaign has allowed us to quantify the important technical difficulties that still exist when routinely monitoring aerosol optical properties at night-time. The small AOD differences observed between the three types of photometers involved in the campaign are only detectable under pristine sky conditions such as those found in this field campaign. Longer campaigns are necessary to understand the observed discrepancies between instruments as well as to provide more conclusive results about the uncertainty involved in the lunar irradiance models. This work has been developed within the framework of the activities of the World Meteorological Organization (WMO) Commission for Instruments and Methods of Observations (CIMO) Izaña Testbed for Aerosols and Water Vapour Remote Sensing Instruments. AERONET sun photometers at Izaña have been calibrated within the AERONET Europe TNA, supported by the European Union’s Horizon 2020 research and innovation program under grant agreement no. 654109 (ACTRIS‒2). CE318-T linearity check has been performed as part of the ESA-funded project “Lunar spectral irradiance measurement and modelling for absolute calibration of EO optical sensors” under ESA contract number: 4000121576/17/NL/AF/hh. LunarPFR has been performing measurements since 2014 in Norway thanks to Svalbard Science Forum funded project, 2014–2016. The authors would like to thank AERONET team for their support and also to NASA’s Navigation and Ancillary Information Facility (NAIF) at the Jet Propulsion Laboratory to help the implementation of the “SPICE” ancillary information system used in this study. We also thank Izaña's ITs for their work to implement the RIMO model in the free-access server. Special thanks should be given to Tom Stone, who has kindly provided us with the USGS/ROLO irradiance values used in the model comparison analysis. This work has also received funding from the European Union’s Horizon 2020 research and innovation programme and from Marie Skłodowska-Curie Individual Fellowships (IF) ACE-GFAT (grant agreement no. 659398). The authors are grateful to Spanish MINECO (CTM2015-66742-R) and Junta de Castilla y León (VA100P17).

A comprehensive comparison of more than 70000 synchronous 1-minute aerosol optical depth (AOD) data from three Global Atmosphere Watch-Precision Filter Radiometer (GAW-PFR) and 15 Aerosol Robotic Network-Cimel (AERONET-Cimel) radiometers was performed for the four nearby wavelengths (380, 440, 500 and 870 nm) in the period 2005–2015. The goal of this study is to assess whether, despite the marked differences between both networks and the number of instruments used, their long term AOD data are comparable and consistent. AOD traceability established by the World Meteorological Organization (WMO) consists in determining the percentage of synchronous data within specific limits. If, at least, 95 % of the AOD differences of an instrument compared to the WMO standards lie within these limits, both data populations are considered equivalent. The percentage of traceable data is 92.7 % (380 nm), 95.7 % (440 nm), 95.8 % (500 nm) and 98.0 % (870 nm). When small misalignments in GAW-PFR sun-pointing were fixed (period 2010–2015), the percentage of traceable data increased. The contribution of calibration related aspects to comparison outside the 95 % traceability limits is insignificant in all channels, except in 380 nm. The simultaneous failure of both cloud screening algorithms might occur only under the presence of cirrus, or altostratus clouds on the top of a dust-laden Saharan air layer. Differences in the calculation of the optical depth contribution due to Rayleigh scattering, and O3 and NO2 absorption have a negligible impact. For AOD > 0.1, a non-negligible percentage (≈ 1.9 %) of the AOD data outside the 95 % traceability limits at 380 nm can be partly assigned to the different field of view of the instruments. The comparison of the Angström exponent (AE) shows that under non-pristine conditions (AOD > 0.03 and AE