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

New technological advances permit lunar-irradiance to be measured with classical photometers. Lunar photometry has appeared as a suitable technique to retrieve aerosol information in nocturnal period, increasing the potential to derive AOD at night-time. However, the fast change in illumination throughout the Moon’s cycle implies that an accurate and time-dependent knowledge of the extraterrestrial lunar irradiance is mandatory in lunar photometry.

The RIMO model (an implementation of the ROLO – RObotic Lunar Observatory – model) has been considered a useful tool to estimate the AOD at night-time (Barreto et al., 2019).

A new empirical correction factor to the RIMO model (RCF or RIMO Correction Factor) has been implemented in Roman et al. (2020) by means of Cimel CE318-T Sun–sky–Moon photometer for 98 pristine nights with low and stable AOD conditions at the Izaña Observatory. The expected day-to-night coherence allowed the authors to estimate the inaccuracies inherent to the RIMO model and provides a RIMO corrected lunar irradiance model. The RCF correction applied to the RIMO model varies both with the Moon Phase Angle (MPA) and with wavelength, ranging from 1.01 to 1.14.

Figure 1.-  AOD values from the Sun, Moon, and star photometer at Granada (Spain) from the first to third Moon quarters in July 2016. Panel (f) shows the Ångström Exponent calculated with the wavelengths of 440, 500, 675 and 870 nm (436, 500, 670, and 880 nm for the star photometer). Moon phase angle is represented with a black line in each panel. Reprinted from Román et al. (2020).

These results revealed an expected overall underestimation of the RIMO model. The AOD derived by this corrected method is compared with the independent and co-located two-year AOD dataset made by a star photometer at Granada (Spain) (Figure 1). Average differences between −0.015 and −0.005 were found, and standard deviations are between 0.03 and 0.04 for 440, 500, 675, and 870 nm. Differences were higher for the 380 nm spectral band. No significant dependences on the Moon–star AOD differences as a function of MPA were found (Roman et al., 2020).