Retrieval of AOD Products from MAX-DOAS Measurements: Improvements and Sinergy with CIMEL

The institute for the atmospheric and climate science of the Italian national research council (CNR-ISAC) operates a Multi AXis – Differential Optical Absorption Spectroscopy (MAX-DOAS) instrument, capable to measure diffuse solar spectra in the Visible (VIS) and Ultra-Violet (UV) spectral ranges. This instrument, a SkySpec-2D system (https://airyx.de/item/skyspec/), has been installed at the Giorgio Fea observatory in San Pietro Capofiume (SPC), in the middle of the Po Valley, since 1st October 2021. It represents the first Italian MAX-DOAS instrument compliant with the Fiducial Reference Measurements for ground-based DOAS (FRM4DOAS) requirements.Its spectra are routinely provided to the FRM4DOAS central facility, where are processed using the reference retrieval codes Mexican MAX-DOAS Fit (MMF) and the MAinz Profile Algorithm (MAPA) to derive aerosol extinction and NO2 vertical profiles. In the last couple of years, in the frame of a European Space Agency (ESA) funded project, we developed an independent retrieval code, the DOAS optimal Estimation Atmospheric Profile (DEAP), for the retrieval of aerosol extinction and trace gases profiles from MAX-DOAS spectra. The DEAP performances have been assessed through the comparisons with the MMF and MAPA reference codes. The MAX-DOAS VIS spectra acquired at SPC are routinely processed by the DEAP code to retrieve aerosol extinction at 477 nm and NO2tropospheric profiles. Both the profiles can be vertically integrated to obtain the tropospheric Aerosol Optical Depth (AOD) and NO2 Vertical Column Density (VCD). Since 2023, the Giorgio Fea observatory has also been equipped with a Cimel CE-318-T Sun-Sky Lunar Multispectral photometer. Being part of the AErosol RObotic NETwork (AERONET), its measurements are centrally processed to derive aerosol information including AOD, with a higher accuracy compared to the MAX-DOAS ones. A comparison between the AERONET (Cimel) and DEAP (SkySpec-2D) AOD revealed a systematic underestimation of MAX-DOAS AOD, especially during the days with a high aerosol load. In this work, we present updates to the DEAP code aimed at improving the AOD products from MAX-DOAS spectra, investigating the sources of the observed discrepancy with respect to the AERONET products.