Articles | Volume 10, issue 8
Atmos. Meas. Tech., 10, 3007–3019, 2017
Atmos. Meas. Tech., 10, 3007–3019, 2017

Research article 21 Aug 2017

Research article | 21 Aug 2017

Assessment of nocturnal aerosol optical depth from lunar photometry at the Izaña high mountain observatory

África Barreto1,2,3, Roberto Román4,5, Emilio Cuevas3, Alberto J. Berjón2, A. Fernando Almansa1,2,3, Carlos Toledano2, Ramiro González2, Yballa Hernández3, Luc Blarel6, Philippe Goloub6, Carmen Guirado2,3, and Margarita Yela7 África Barreto et al.
  • 1Cimel Electronique, Paris, France
  • 2Grupo de Óptica Atmosférica, Universidad de Valladolid, Valladolid, Spain
  • 3Izaña Atmospheric Research Center, State Meteorological Agency of Spain (AEMET), Santa Cruz de Tenerife, Spain
  • 4Department of Applied Physics, University of Granada, Granada, Spain
  • 5Andalusian Institute for Earth System Research, IISTA–CEAMA, University of Granada, Junta de Andalucía, Granada, Spain
  • 6Laboratoire d'Optique Atmosphérique (LOA), Université de Lille, Villeneuve d'Ascq, France
  • 7Instrumentation and Atmospheric Research Department, National Institute for Aerospace Technology (INTA), Madrid, Spain

Abstract. This work is a first approach to correct the systematic errors observed in the aerosol optical depth (AOD) retrieved at nighttime 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 the 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 Robotic Lunar Observatory (ROLO) model could be deduced from the comparison in this 59-night period of the CE318-T calibration performed by means of the lunar Langley calibration 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 AErosol RObotic NETwork (AERONET) 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 the 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.

Short summary
This work involves a first analysis of the systematic errors observed in the AOD retrieved at nighttime using the Sun–sky–lunar CE318-T photometer. In this respect, this paper is a first attempt to correct the AOD uncertainties that currently affect the lunar photometry by means of an empirical regression model. We have detected and corrected an important bias correlated to the Moon's phase and zenith angles, especially at longer wavelength channels.