Retrieval of single scattering albedo using spectral solar measurements from a precision spectroradiometer (PSR)
Authors: Karanikolas Angelos , Raptis Panagiotis Ioannis , Kouremeti Natalia , Gröbner Julian , Doppler Lionel , Fountoulakis Ilias , Kouklaki Dimitra , Papachristopoulou Kyriaki , Kazadzis Stelios 
Published: July 2024

One of the key parameters affecting the levels of solar radiation reaching the ground is the single scattering albedo (SSA). The SSA shows how strongly aerosols absorb radiation and its value between 0 (no absorption) and 1 (absorption of the total incident radiation). The spectral SSA corresponding to the aerosol scattering of the total direct solar irradiance extinction through the total atmospheric aerosol column can be retrieved through sun-photometric measurements. Currently, the main columnar SSA product worldwide is the Aerosol Robotic Network (AERONET) product retrieved from a combination of spectral AOD and sky radiance measurements using an inversion modelling approach. Outputs are provided at 4 selected wavelengths between 440&#8211;1020 nm. The quality assured dataset (L2.0 AERONET product) is restricted to observations under Aerosol Optical Depth (AOD) above 0.4 at 440 nm, at solar zenith angles of 54 - 75&#176; so they are limited to a small number of retrievals per day. This criterion is estimated to exclude around 95% of the annual global AOD values at 440 nm and 89% over land [Andrews et al., 2017].In this study, we retrieve the SSA by the inversion of the direct to global spectral solar irradiance ratio [Kazadzis et al., 2016]. The solar irradiance measurements are provided by a precision spectroradiometer (PSR) deployed in 2 locations: Thiseio, Athens, Greece during the ASPIRE (Atmospheric parameters affecting SPectral solar IRradiance and solar Energy) campaign and Meteorologisches Observatorium Lindenberg (MOL-RAO), Lindenberg, Germany. The goal is to investigate whether the PSR measurements can be used to increase the data availability, accuracy, spectral range and spectral resolution of SSA retrievals. To retrieve the SSA, we compare the ratio between the direct and global spectra of the PSR with the respective values derived by radiative transfer model simulations for different AOD conditions. The PSR is able to measure with high frequency direct and global solar irradiance (approximately every 5 minutes) within the spectral range of 300 nm-1020 nm, at 1024 wavelengths. Its calibration is traceable to SI and its direct normal spectral irradiance has an expanded uncertainty of 1.7-2% [Gr&#246;bner &amp; Kouremeti, 2019].ReferencesAndrews, E., Ogren, J.A, Kinne, S. and Samset, B., Comparison of AOD, AAOD and column single scattering albedo from AERONET retrievals and in situ profiling measurements, Atmospheric Chemistry and Physics, 17(9), 6041-6072, 2017.Gr&#246;bner, J., and Kouremeti, N., The Precision Solar Spectroradiometer (PSR) for direct solar irradiance measurements. Solar Energy, 185, 199-210, 2019.Kazadzis, S., Raptis, P., Kouremeti, N., Amiridis, V., Arola, A., Gerasopoulos, E., and Schuster, G. L.: Aerosol absorption retrieval at ultraviolet wavelengths in a complex environment, Atmos. Meas. Tech., 9, 5997&#8211;6011, https://doi.org/10.5194/amt-9-5997-2016, 2016.