Ground-based MFRSR UV-Vis spectral retrievals of Saharan dust absorption at Izaña Observatory

Abstract. This paper presents a multi-instrument synergistic technique to retrieve atmospheric dust aerosol columnar effective imaginary refractive index (k), single scattering albedo (SSA), and absorption aerosol optical depth (AAOD). The technique combines: (a) aerosol information derived from the narrow field-of-view measurements by filter sun-moon-sky radiometer within the Aerosol Robotic Network (AERONET): spectral aerosol optical depth (AOD) and inversion properties; (b) the total, direct, and diffuse sky irradiance measurements from UV- and Vis-Multifilter Rotating Shadowband Radiometers (MFRSR); (c) trace gas columns from satellite measurements (OMI and OMPS). The approach is demonstrated on the data collected at the Izaña Atmospheric Observatory (IZO), located at an altitude of 2.4 km on Tenerife Island, a unique site for Saharan dust column optical properties retrievals due to very clean background condition for calibrating the instrument. This multi-instrument synergy enables consistent column absorption retrievals from ultraviolet (UV) to visible (VIS) wavelengths, while effectively accounting separately for aerosol and gaseous (Ozone-O₃, and Nitrogen Dioxide-NO₂) absorption. The MFRSR calibration procedure relies on observations acquired on cleaner days (AOD<0.1 at 440 nm) to eliminate the observed dependency of the calibration constant on increasing dust aerosol loading leading to an inefficient correction for the forward scattering (aureole effect). The retrieval algorithm 1) integrates the temporally collocated AERONET-retrieved particle size distribution and the real part of the refractive index into the radiative transfer simulations, while accounting for the pre-defined spheroidal shape distribution of the dust aerosols, and 2) fits the measured ratio of diffuse to direct-normal irradiance for discrete wavelengths (325 nm to 440 nm) to the pre-calculated, on-the-fly look-up table to retrieve the spectral imaginary part of the refractive index. The retrieved SSA at 440 nm shows good agreement with AERONET inversions, mostly within ±0.03 for AOD>0.2, and ±0.02 at higher AOD (>0.4). This close correspondence confirms the consistency between the two fundamentally distinct inversion techniques and enhances confidence in the concurrent MFRSR UV wavelength inversions. We present a multi-year (2019–2023) MFRSR aerosol absorption record revealing enhanced dust absorption at UV wavelengths with noticeable intraseasonal and interannual variabilities, which are indicative of a varying composition of minerals (iron oxides) in the dust. The spectral aerosol absorption effects reduce the amount of surface-reaching UV radiation and slow down tropospheric photochemistry, which can have implications for air quality, human health, and ecosystem dynamics. The ongoing AERONET and MFRSR measurements currently made at the Santa Cruz ground-level site on Tenerife Island will continue to produce a unique, long-term ground-based UV spectral Saharan dust absorption dataset, providing a valuable reference for evaluating space-based UV aerosol absorption retrievals from instruments such as DSCOVR-EPIC, S5P-TROPOMI, and the most recently launched PACE-OCI. In addition to deriving spectral absorption properties, the enhanced sensitivity of UV measurements to the dust spectral absorption, demonstrated with the MFRSR inversion in this work, can be exploited for inferring the mineralogical composition of the dust aerosols, which is critical to improving the dust representation in Earth System Models.