The new sun-sky-lunar Cimel CE318-T multiband photometer – a comprehensive performance evaluation
Authors: Barreto A. , Cuevas E. , Granados-Muñoz M. J. , Alados-Arboledas L. , Romero P. M. , Gröbner J. , Kouremeti N. , Almansa A. F. , Stone T. , Sorokin M. , Holben B. , Canini M. , Yela M. 
Published: October 2015

Abstract. This paper presents the new photometer CE318-T, able to perform daytime and nighttime photometric measurements using the sun and the moon as light source. Therefore, this new device permits to extract a complete cycle of diurnal aerosol and water vapor measurements valuable to enhance atmospheric monitoring. In this study we have found significantly higher triplets precision when comparing the CE318-T master and the Cimel AErosol RObotic NETwork (AERONET) master (CE318-AERONET) triplets as a result of the new CE318-T tracking system. Regarding the instrument calibration, a new methodology to transfer the calibration from a master (Sun Ratio technique) is presented and discussed. It allows us to reduce the previous complexities inherent to nocturnal calibration. A quantitative estimation of CE318-T AOD uncertainty by means of error propagation theory during daytime revealed AOD uncertainties (uDAOD) for Langley-calibrated instruments similar to the expected values for other reference instruments (0.002–0.009). We have also found uDAOD values similar to the values reported in sun photometry for field instruments (~ 0.015). In the case of nighttime period, the CE318-T estimated uncertainty (uNAOD) is dependent not only on the calibration technique but also on illumination conditions and the instrumental noise. These values range from 0.011–0.019 for Lunar Langley calibrated instruments to 0.012–0.021 for instruments calibrated using the Sun Ratio technique. A subsequent performance evaluation including CE318-T and collocated measurements from independent reference instruments has served to assess the CE318-T performance as well as to confirm its estimated uncertainty. Daytime AOD evaluation performed at Izaña station from March to June 2014, encompassed measurements from a reference CE318-T, a CE318-AERONET master, a Precision Filter Radiometer (PFR) and a Precision SpectroRadiometer (PSR) prototype, reporting low AOD discrepancies between the four instruments (up to 0.006). The nocturnal AOD evaluation was performed using CE318-T and star photometer collocated measurements and also by means of a day/night coherence transition test using the master CE318-T and the CE318 daytime data from the CE318-AERONET master. Results showed low discrepancies with star photometer at 870 and 500 nm channels (≤ 0.013) and differences with AERONET daytime data (1 h after and before sunset and sunrise) in agreement with the estimated uNAOD values at all illumination conditions in case of channels within the visible spectral range, and only for high moon's illumination conditions in case of near infrared channels. Precipitable water vapor (PWV) validation showed a good agreement between CE318-T and Global Navigation Satellite System (GNSS) PWV values for all illumination conditions, within the expected precision for sun photometry. Finally, two case studies have been included to highlight the ability of the new CE318-T to capture the diurnal cycle of aerosols and water vapor as well as short-term atmospheric variations, critical for climate studies.