Assessment of nocturnal Aerosol Optical Depth from lunar photometry at Izaña high mountain Observatory.

This work is a first approach to correct the systematic errors observed in the aerosol optical depth (AOD) retrieved at night-time using lunar photometry and calibration techniques dependent on the lunar irradiance model. To this end, nocturnal AOD measurements were performed in 2014 using the CE318-T master Sun-sky-lunar photometer (Lunar-Langley calibrated) at Izaña high mountain Observatory. This information has been restricted to 59 nights characterized as clean and stable according to lidar vertical profiles. A phase angle dependence as well as an asymmetry within the Moon's cycle of the ROLO model could be deduced from the comparison in this 59-nights period of the CE318-T calibration performed by means of the Lunar-Langley and the calibration performed every single night by means of the common Langley technique. Nocturnal AOD has also been compared in the same period with a reference AOD based on daylight AOD extracted from the AERONET network at the same station. Considering stable conditions, the difference ΔAODfit, between AOD from lunar observations and the linearly interpolated AOD (the reference) from daylight data, has been calculated. The results show that ΔAODfit values are strongly affected by Moon phase and zenith angles. This dependency has been parameterized using an empirical model with two independent variables (Moon phase and zenith angles) in order to correct the AOD for these residual dependencies. The correction of this parameterized dependency has been checked at four stations with quite different environmental conditions (Izaña, Lille, Carpentras and Dakar) showing a significant reduction of the AOD dependence on phase and zenith angles, and an improved agreement with daylight reference data. After the correction, absolute AOD differences for day-night-day clean and stable transitions remain below 0.01 for all wavelengths. [ABSTRACT FROM AUTHOR]