Evaluation and advancement of the SKYNET Improved Langley Plot method in the Mediterranean area

Accurate calibration of the Sun-sky radiometers is essential for the reliable retrieval of the aerosol optical properties. The standard Langley plot (Shaw, 1983) method is widely used to calibrate Sun-sky radiometers. Although it provides us with the most reliable calibration, the instrument has to be transported to a pristine location at high altitude to perform this procedure. An improved Langley method (ILP) was proposed by Nakajima et al. (1996) and Campanelli et al. (2004) to address this issue. Currently, ILP is widely used for PREDE POM radiometers in the SKYNET. In ILP, the Sun-sky radiometer can be calibrated on site without requiring transport to high altitude. Radiance data in the almucantar plane is used to obtain the AOD using Skyrad 4.2 (Nakajima et al., 1996). Later, this AOD is used to calculate the calibration value. Because the AOD is allowed to vary, unlike the Standard Langley plot method, which assumes AOD is constant, the calibration can be obtained even at non-ideal sites.  

In this study, we upgrade the Improved Langley plot (method) by replacing the old Skyrad 4.2 inversion algorithm with the latest Skyrad MRI v2 (Kudo et al., 2021). Skyrad MRI v2 employs an optimisation of the inversion technique similar to the one used by the AERONET network. It also employs a dynamic cost function rather than the static cost function used in Skyrad 4.2. Moreover, it can handle non-spherical particles. Overall, Skyrad MRI v2 helps ILP obtain a more accurate AOD, enabling a better estimate of the calibration constant. We also propose a modification to the ILP method that uses an iterative method to calculate the calibration value. We used the data from the QUAlity and TRaciability of Atmospheric aerosol Measurements (QUATRAM, www.euroskyrad.net/quatram) campaigns and from the Skynet sites of Burjassot and Valencia, and showed that the upgraded ILP method using the iterative method improved the calibration values. The comparison of the direct AOD computed using the new calibration with co-located AERONET and PFR measurements showed an increase in the number of points in agreement within the WMO limits imposed for AOD. The mean difference in direct AOD is also reduced to within ±0.01, indicating improved consistency. The improvement was significant for wavelengths below 500 nm, whereas it was minor for wavelengths at or above 500 nm. This indicates the robustness of the ILP at longer wavelengths. At the same time, this highlights the need for a more robust approach at shorter wavelengths, which is addressed by the proposed methodology.

Acknowledgement

The current analysis has been done in the frame of the COST Action CA21119 HARMONIA, supported by COST (European Cooperation in Science and Technology). The Spanish Ministry of Economy and Competitiveness also funded the research through project PID2022-138730OB-I00. The participation of G. Kumar has been supported by the Santiago Grisolia program fellowship GRI-SOLIAP/2021/048. We thank AERONET, PHOTONS and SKYNET for their scientific and technical support