Evaluation and Intercalibration of Various Low-Cost Sun-Photometers During the RACE-EarthCARE Validation Experimental Field Campaign in Thessaloniki, Greece

Accurate and harmonized aerosol observations are essential for climate studies, validation of satellite products, and assessment of aerosol impacts on surface solar radiation. Low-cost hand-held sun-photometer sensors facilitate a high monitoring density, providing near-real time and spatially distributed information. Their compact design and portability make them ideal for mobile monitoring observations. They can complement global monitoring networks by enabling aerosol observations in data-sparse regions, where conventional sun-photometric observations are limited. These capabilities are particularly valuable for target field campaigns, as they enable spatially and temporally collocated aerosol observations that support the validation of satellite-derived aerosol products.

This study presents results from the RACE-ECV (Radiation closure experiments for EarthCARE Validation) field campaign conducted in Thessaloniki, Greece between 24 April and 21 May 2025. The campaign included coordinated calibration and validation activities focusing on low-cost, hand-held sun-sky photometers and the evaluation of Aerosol Optical Thickness (AOT) products from the EarthCARE mission, with particular emphasis on the EarthCARE Multi-Spectral Imager (MSI) and Atmospheric Lidar (ATLID) AOT satellite products.

Six hand-held sun-photometers, three Microtops II and three Calitoos, participated in the campaign. Measurements were performed at three locations: a central urban site of the city at the Laboratory of Atmospheric Physics (LAP) of the Aristotle University of Thessaloniki (AUTH) (40.38° N, 22.57° E, 60 m a.s.l.), an urban-suburban location at Center for Interdisciplinary Research and Innovation (KEDEK) (40.33° N, 22.59° E, 63 m a.s.l.) of AUTH and a rural location in Epanomi (40.20° N, 22.58° E, 17 m a.s.l.).

Cloud-free measurements were conducted on 17–18 May 2025 at the Laboratory of Atmospheric Physics (LAP) station of (AUTH), Greece, using these handheld sun photometers. These observations were part of a coordinated calibration exercise, where the hand-held instruments were calibrated against a reference Cimel sun–sky photometer which is regularly maintained and calibrated according to AERONET standards. New calibration factors for the instruments were also derived from AOD measurements obtained with a collocated Global Atmosphere Watch precision-filter radiometer (GAW-PFR) at the KEDEK station in multiple wavelengths.

Finally, for the validation of the EarthCARE Aerosol Optical Thickness (AOT) from MSI and ATLID, mobile observations were conducted at six locations along the EarthCARE overpass tracks on 25 April and 20 May 2025 in Thessaloniki district with Microtops II and Calitoo instruments. These measurements complemented the fixed-station observations, enhancing the spatial coverage of the campaign. Results regarding the calibration of the hand-held instruments and validation of EarthCARE AOT products will be presented.

Acknowledgements: The authors acknowledge the project RACE-ECV, (SBFI-633.4-2021-2024/PMOD - EarthCARE 202/2) supported by SBFI the CERTAINTY project funded from Horizon Europe programme under Grant Agreement No 101137680. The study is supported by the ATARRI project funded by the European Union’s Horizon Europe Twinning Call (HORIZON-WIDERA-2023-ACCESS-02) under the grant agreement No 101160258 and the ‘EXCELSIOR’: ERATOSTHENES: H2020 Widespread Teaming project. This work supported by HARMONIA COST Action CA21119 - International network for harmonization of atmospheric aerosol retrievals from ground based photometers, supported by COST (European Cooperation in Science and Technology).