DOI: 10.23646/384592a6-1d67-46f6-995d-8202bf51f190
title: ABC-Rad (AERONET-BSRN-CAMS-Radiation)

description: Several datasets have been compiled from different sources comprising measurements of the downwelling surface solar irradiance, modelled values of the clear-sky downwelling surface solar irradiance, modelled values of the circumsolar irradiance, reference ground-based aerosol optical properties, and modelled aerosol optical depth. The data are all quality-controlled and cloud-screened. This dataset is of value to scientists or practitioners studying the effects of aerosols on the downwelling surface solar irradiance. It may also be used for benchmarking exercises of solar or aerosol data. The inclusion of modelled circumsolar data is also of use to scientists or practitioners working with such variable. The datasets herein are saved for each station independently, and the files follow the naming convention: ABC-Rad_xxx_yyy.nc. ABC-Rad stands for AERONET, BSRN, CAMS, and Radiation; xxx represents the BSRN station acronym, and yyy indicates the AERONET product, either DSA (Direct Sun Algorithm) or INV (Inversions). The selection of stations was based on availability of reference data of aerosol optical properties from AERONET (https://doi.org/10.1016/S0034-4257(98)00031-5) and co-located ground measurement of the downwelling surface solar irradiance from the BSRN network (https://doi.org/10.1175/1520-0477(1998)079&lt;2115:BSRNBW&gt;2.0.CO;2). To match in time with the clear-sky downwelling surface solar irradiance data of the McClear model (https://doi.org/10.5194/amt-6-2403-2013) of the CAMS Radiation Service (https://doi.org/10.24381/5cab0912), only stations with overlapping measurements after 2004 were retained. This resulted in 24 stations. 

AERONET DATA — The reference aerosol optical properties available in this dataset were retrieved from the AERONET data (https://aeronet.gsfc.nasa.gov/). They were downloaded from the Version 3 Direct Sun Algorithm and Inversion Algorithm products, all in Level 2 to ensure quality-assured and cloud-screened data. Additionally, the aerosol optical depth (AOD) at the exact wavelength of 550 nm is not provided from AERONET but was computed for the Direct Sun Algorithm data by utilizing a second-order polynomial fit (https://doi.org/10.1175/1520-0469(1976)033&lt;2242:AMFITO&gt;2.0.CO;2).

 BSRN DATA — The ground-measured downwelling surface solar irradiance data are available as one-minute averages at all 24 stations. Measurements of the BSRN network are collected using Class A thermopiles, and the global horizontal irradiance (GHI), beam (or direct) normal irradiance (BNI) and diffuse horizontal irradiance (DHI) are measured using separate radiometers. The pyrheliometers used for BNI measurements have aperture half-angles of either 2.5° or 2.9°. The quality-control tests presented in Forstinger et al. (https://doi.org/10.18086/swc.2021.38.02) were applied to filter any suspicious reference data from the analysis. The applied tests were namely the K-tests (https://doi.org/10.1016/j.solener.2017.05.004 and https://doi.org/10.1016/j.egypro.2015.03.205), along with the closure, extremely rare limits, physically possible limits and tracker off tests (https://doi.org/10.2174/1874282300802010023 and https://bsrn.awi.de/fileadmin/user_upload/bsrn.awi.de/Publications/BSRN_recommended_QC_tests_V2.pdf).

CAMS RADIATION DATA — The clear-sky McClear global horizontal irradiance (GHI), beam (or direct) normal irradiance (BNI), beam horizontal irradiance (BHI) and diffuse horizontal irradiance (DHI) are freely available through the Atmosphere Data Store of CAMS (https://doi.org/10.24381/5cab0912). The data was downloaded at a time step of 1 min from the CAMS solar radiation service dataset, where the sky type was selected as cloud-free, which corresponds to McClear values. The data was downloaded in “expert mode”, where the following outputs are also provided: solar zenith angle, total column content in water vapor and ozone, partial optical depths at 550 nm for black carbon, dust, sea salt, organic matter, sulphate, nitrate, ammonium and secondary organics, the parameters fiso, fvol and fgeo, ground albedo. 

MATCHING THE DATA — The datasets herein are only concerned with clear-skies. The matching with AERONET data already takes care of having no clouds in the direction of the Sun. To further account for clouds across the sky vault, a restrictive cloud-screening algorithm for BSRN radiation observations was applied. The applied clear-sky detection algorithm is based on the correlation between measured GHI and a modelled clear-sky irradiance model (modified Lambert Beer) fitted to measurement over a moving time window. Following the selection of clear-sky instances of BSRN data, the quality-assured AERONET data were matched in time to their corresponding BSRN stations, such that at each timestamp the maximum absolute time difference between them was no longer than one minute. The corresponding McClear data was then matched to them at its native one-minute time steps. 

CIRCUMSOLAR IRRADIANCE DATA — Strictly using the McClear estimates, the parametric model of Eissa et al. (https://doi.org/10.1016/j.solener.2018.02.015) was applied to determine circumsolar irradiance (at normal and horizontal incidence) for an opening half-angle of either 2.5° or 2.9° (depending on the reference BSRN BNI measurement). This is done to allow for correcting the McClear beam and diffuse irradiance estimates (which do not account for the circumsolar irradiance). The Eissa et al. model was applied using the version where the coefficients were established using data from two sites mixed. To apply the circumsolar correction, the CSNI is added to the McClear BNI value, and the CSHI is subtracted from the McClear DHI value. This implies an increase in the modelled BNI and a decrease in the modelled DHI. 

ACKNOLEDGMENT - The research leading to these results has received funding from SciDoSol, a sponsorship chair at Mines Paris – PSL funded by TotalEnergies, RTE, Somfy, TSE and CalibSun, and from the Copernicus Atmosphere Monitoring Service, a program being operated by the European Centre for Medium-Range Weather Forecasts (ECMWF) on behalf of the European Union. It also took place within the Task 16 “Solar Resource for High Penetration and Large Scale Applications” of the Photovoltaic Power Systems Programme (PVPS) of the International Energy Agency (IEA). The authors also thank the PIs of AERONET and BSRN stations, along with all the personnel who installed and maintained the instruments.