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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2020-293
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-293
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  10 Aug 2020

10 Aug 2020

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This preprint is currently under review for the journal AMT.

Correction of a lunar irradiance model for aerosol optical depth retrieval and comparison with star photometer

Roberto Román1, Ramiro González1, Carlos Toledano1, África Barreto2,3,1, Daniel Pérez-Ramírez4,5, Jose A. Benavent-Oltra4,5, Francisco J. Olmo4,5, Victoria E. Cachorro1, Lucas Alados-Arboledas4,5, and Ángel M. de Frutos1 Roberto Román et al.
  • 1Group of Atmospheric Optics (GOA-UVa), Universidad de Valladolid, 47011, Valladolid, Spain
  • 2Izaña Atmospheric Research Center, Meteorological State Agency of Spain (AEMET), Izaña, Spain
  • 3Cimel Electronique, Paris, France
  • 4Department of Applied Physics, Universidad de Granada, 18071, Granada, Spain
  • 5Andalusian Institute for Earth System Research, IISTA-CEAMA, Granada, Spain

Abstract. The emergence of Moon photometers is allowing measurements of lunar irradiance over the world and increasing the potential to derive aerosol optical depth (AOD) at night-time, that is very relevant in polar areas. Actually, new photometers implement the latest technological advances that permit lunar irradiance measurements together with classical Sun photometry measurements. However, a proper use of these instruments for AOD retrieval requires accurate time-dependent knowledge of the extraterrestrial lunar irradiance over time, due to its fast change throughout the Moon's cycle. This paper uses the RIMO model (an implementation of the ROLO model) to estimate the AOD at night-time assuming that the calibration of the solar channels can be transferred to the Moon by a vicarious method. However, the obtained AOD values using a Cimel CE318-T Sun/sky/Moon photometer for 98 pristine nights with low and stable AOD at the Izaña Observatory (Tenerife, Spain) are not in agreement with the expected (low and stable) AOD values, estimated by linear interpolations from daytime values obtained during the previous evening and the following morning. Actually, AOD calculated using RIMO shows negative values and with a marked cycle dependent on the optical airmass. The differences between the AOD obtained using RIMO and the expected values are assumed to be associated with inaccuracies in the RIMO model, and these differences are used to calculate the RIMO correction factor (RCF). The RCF is a proposed correction factor that, multiplied by RIMO value, gives an effective extraterrestrial lunar irradiance that provides the expected AOD values. The RCF varies with the Moon phase angle (MPA) and with wavelength, ranging from 1.01 to 1.14, which reveals an overall underestimation of RIMO to the lunar irradiance. These obtained RCF values are modeled for each photometer wavelength to a second order polynomial as function of MPA. The AOD derived by this proposed method is compared with the independent AOD measurements obtained by a star photometer at Granada (Spain) for two years. The mean of the Moon-star AOD differences are between −0.015 and −0.005 and the standard deviation between 0.03 and 0.04 (which is reduced to about 0.01 if one month of data affected by instrumental issues is not included in the analysis), for 440, 500, 675 ad 870 nm; however, for 380 nm, the mean and standard deviation of these differences are higher. The Moon-star AOD differences are also analyzed as a function of MPA, showing no significant dependence.

Roberto Román et al.

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Short summary
Atmospheric aerosol and gaseous properties can be derived at night-time if the lunar irradiance at ground is measured. To this end, the knowledge of the lunar irradiance at the top of the atmosphere is neccesary. This extraterrestrial lunar irradiance is usually calculated by models since it varies with several geometric factors mainly depending on time and location. This paper proposes a correction on the most used lunar irradiance model to be applied for atmospheric aerosol characterization.
Atmospheric aerosol and gaseous properties can be derived at night-time if the lunar irradiance...
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