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Picture of the CE318-TP Cimel #1235 installed on the roof of the Observatory

Cimel photometer

The CE 318 is a multi-wavelength automatic sun-sky-moon photometer developed by Cimel Electronique, measuring direct solar irradiance and sky radiance at nine bands centered at 340 nm, 380 nm, 440 nm, 500 nm, 675 nm, 870 nm, 937 nm, 1020 nm, and 1640 nm (10 nm FWHM), with a field of view of 1.2°. Lunar direct irradiance measurements are performed from the first to the last quarter at the bands centered at 440 nm, 500 nm, 675 nm, 870 nm, 937 nm, 1020 nm, and 1640 nm.
 
The photometer is programmed to perform either direct sun or sky radiance measurements several times during the day, for airmass values below 7.
 
The direct sun measurements are made at all nine wavelength bands. The aerosol optical depth is derived at 340 nm, 380 nm, 440 nm, 500 nm, 675 nm, 870 nm, 1020 nm, and 1640 nm using the Beer-Bouguer law, after removing the extinction due to Rayleigh scattering, ozone and other gas absorption. The 937 nm channel is used to estimate the column water vapor.
 
Additionally, the instrument measures sky radiance in four bands (440, 675, 870, and 1020 nm) along the solar principle plane (at fixed azimuth angle equal to the sun azimuth and changing the scattering angle) and along the solar almucantar (at fixed elevation angle and changing the azimuth angle). Using the inversion of Dubovik and King [A flexible inversion algorithm for the retrieval of aerosol optical properties from sun and sky radiance measurements, JGR, 105, 20673-20696, 2000], the aerosol size distribution in the size range 0.1-15 µm, the phase function and refractive indices are derived.
 
Different photometers have been operated at the observatory, all part of the Aerosol Robotic Network (AERONET, https://aeronet.gsfc.nasa.gov/).
 
The first instrument, model CE318N, identified with the #172, was first operated since 2010 jointly by ENEA and the University of Modena and Reggio Emilia. Since 2018 two CE318-TP models (#1212 and #1235), with polarization in three directions, alternatively collect measurements to ensure continuity in the dataset when one of the two instrument is under calibration.
 
The annual calibration of the Cimel photometers is carried out at the Grupo de Óptica Atmosférica (GOA) of the University of Valladolid (Spain) installation of the AERONET-EUROPE Calibration Service Centre in the context of ACTRIS Transnational Access (supported by the European Commission under the Horizon 2020 - Research and Innovation Framework Programme, H2020-INFRADEV-2019-2, Grant Agreement number: 871115, ACTRIS-IMP).