Learnings of measurement campaigns in photometry instrumentation and inversion methods, a COST/HARMONIA study
Authors: Doppler Lionel , Kazadzis Stelios , Kouremeti Natalia , Masoom Akriti , Barreto Africa , Cuevas Emilio , Toledano Carlos , Roman Roberto , Campanelli Monica , Ritter Christoph 
Published: July 2023

Since four decades the Aerosol Optical Depth (AOD) is retrieved operationally using the photometry technique. Photometers are operated during day pointing the sun (solar photometers) or during night pointing the moon (lunar photometers) or the stars (stellar photometers). There are different networks of photometers, using different kind of instruments and methods of inversion. The three main networks are AERONET, SkyNet and GAW/PFR. HARMONIA (International network for harmonization of atmospheric aerosol retrievals from ground-based photometers) is a COST action that gathers a large panel of actors/stakeholders of photometry and has the ambition to describe the state of the art of the techniques, measurements and inversion methods, pointing out their diversity and suggesting some harmonized standard procedures of measurements and inversion techniques.
One milestone of COST/HARMONIA presented here is to report on recent (2017 &#8211; 2022) campaigns using different kind of photometers and methods of inversion: 1) The Nocturnal AOD Intercomparison of Iza&#241;a 2007. Two types of lunar photometers and one stellar photometer have been involved. The quality of lunar measurements to the AOD stellar measurements and the lunar exo-atmospheric irradiance model have been evaluated. 2) QUATRAM (QUAlity and TRaceabiliy of Atmospheric aerosol Measurements) were five campaigns (2017-2021) involving three different type of sun photometers (Cimel CE318, Prede POM and PMOD PFR). 3) ANACC (Arctic Night Aerosol Characterization Campaign) was a campaign during the polar night (February 2020) in Ny &#197;lesund, involving two kinds of lunar photometers, a stellar photometer and a Raman-Lidar. In addition to the instrument intercomparison, this campaign could focus on Arctic Haze and Polar Stratospheric Clouds, whose optical properties could be investigated. 4) SCILLA (Summer Campaign for Intercomparison of Lunar measurements of Lindenberg&#8217;s Aerosol) was a nocturnal AOD campaign in Summer 2020, involving lunar photometers of all three types (Cimel, PFR, Prede), two stellar photometers, a Raman lidar, and some COBALD balloon-carried AOD radiosondes. The aim was to estimate the differences of AOD obtained with lunar photometers of the same type and compare them to the differences of AOD obtained from instrument of other types and the stellar photometers. Also, a focus was set on the synergy total column measurements (AOD from photometers) with profiling measurements (LIDAR, COBALD). 5) FRC-V (Fifth Filter Radiometer Comparison) was a WMO solar photometer campaign in Davos. Thirty-two filter radiometers and spectroradiometers from 12 countries participated. 6) The MAPP campaign in September 2017 in Iza&#241;a (MAPP: Metrology for aerosol optical properties; a project part of the EURAMET EMPIR program). One aim was to obtain extra-terrestrial solar and lunar spectral irradiance traceable to the International System. The campaign involved the most sophisticated instrumentation for measuring solar and lunar irradiance, including QASUME, the reference spectral radiometer of PMOD-WRC, Fourier transform Spectrometers and more than 30 instruments.