Optimization of the SKYNET calibration with an iterative Improved Langley method and its validation in the Mediterranean area

Abstract. The Improved Langley plot (ILP) method was developed for on-site calibration of PREDE-POM radiometers within SKYNET. It eliminated the need to ship instruments to remote high-altitude sites as required by the Standard Langley plot (SLP) method, reducing data gaps, transport-related risks, and operational costs while maintaining field instrument uncertainty below 2.4 %. The study aims to update the ILP method by adopting the latest Skyrad MRI v2 and introducing an iterative procedure. Testing with the data from the QUATRAM (QUAlity and TRaceabiliy of Atmospheric aerosol Measurements) campaigns and the Burjassot site in Spain showed an overall improvement of the ILP method. The median differences in direct aerosol optical depth (AOD) relative to GAW-PFR (Precision Filter Radiometers) decreased to approximately −0.01 at the 500 and 870 nm channels. AOD differences within the World Meteorological Organization (WMO) limits increased by 15-fold and 4-fold at 500 and 870 nm channels, respectively. The PREDE-POM and AERONET Level 2 Version 3 direct AODs at the Burjassot site showed good agreement, with median AOD differences within ±0.01 across all wavelengths, except at 340 nm. However, the fraction of points within WMO limits increases by over 10 % at wavelengths below 500 nm, while remaining nearly unchanged (variations below 6 %) at longer wavelengths. These results confirm the robustness of the ILP currently used in SKYNET and highlight the possibility of an improvement at shorter wavelengths. This limitation is addressed in the present study through the proposed iterative ILP method based on Skyrad MRI v2.