## 2 Materials and Methods
### MERRA-2 Data  
The FORTH RTM input dust optical depth, single scattering albedo, and asymmetry parameter, as well as other surface and atmospheric properties, were taken from the MERRA-2 (Modern-Era Retrospective analysis for Research and Applications, Version 2), which is the latest version of global atmospheric reanalysis for the satellite era produced by NASA's Global Modeling and Assimilation Office (GMAO) [62]. It uses both ground-based and satellite aerosol observations, namely: (i) ocean-only reflectances from the Advanced Very High-Resolution Radiometer (AVHRR) instruments, (ii) reflectances from the Moderate Resolution Imaging Spectroradiometer (MODIS), (iii) AOD from the Multi-angle Imaging SpectroRadiometer (MISR) over the bright desert regions, and (iv) AOD observations from the AErosol RObotic NETwork (AERONET) surface station network, and assimilates them using the Goddard Earth Observing System (EOS) Model, Version 5 (GEOS-5) with its Atmospheric Data Assimilation System (ADAS), version 5.12.4. Data are produced on a 0.500\({}^{\circ}\)\(\times\) 0.625\({}^{\circ}\) latitude-longitude grid resolution at 72 atmospheric layers between the surface and 0.01 hPa, for the climatological period from 1980 up to the present, and they are available through the NASA's Goddard Earth Sciences Data and Information Services Center (GES DISC) website ([[https://disc.gsfc.nasa.gov/](https://disc.gsfc.nasa.gov/)]([https://disc.gsfc.nasa.gov/](https://disc.gsfc.nasa.gov/)), accessed on 1 June 2023).  
It should be noted that the vertically and spectrally (at 25 wavelengths between 0.25 and 8.5 \(\upmu\)m) resolved optical properties, which are required for the RTM calculations, are not directly available from MERRA-2 database, but they are calculated in this study using vertically resolved (in 72 layers), 3-hourly instantaneous aerosol mixing ratios and spatiotemporally collocated relative humidity data, both taken from the \"M2 ISNVAER\" product, along with look-up-tables providing the extinction and efficiency per aerosol type, size bin, and relative humidity [5; 63].  
MERRA-2 AOD data have been evaluated against corresponding satellite and ground-based data and have been found to operate well against both of them [64]. However, since AOD is a key determining parameter for aerosol DREs [51; 55], an evaluation of AOD data was performed for the first two days of the DAEC (16 and 17 June 2016) over 31 AERONET stations (Figure S1) and the results are shown in Figure S3. In this evaluation, the third day (18 June 2016) is excluded since data were not available. MERRA-2 AOD data correlate well with AERONET retrievals (correlation coefficient R equal to 0.85, Figure S3a), while MERRA-2 slightly (by \(-\)0.01 or 6.4%) underestimates AOD compared to AERONET. Moreover, as shown in Figure S3b, MERRA-2 closely follows the daily course of AOD, in terms of averages over the 31 AERONET stations and the corresponding MERRA-2 pixels, with relative percent differences mostly within \(\pm\)20%. The scatter plot comparison between the SSA of MERRA-2 and AERONET over 8 stations in the MB during the study episode (16-18 June 2016), based on a low number of compared data pairs (N = 33), reveals an underestimation of MERRA-2 SSA (by \(-\)0.01 or \(-\)1.2%) against AERONET measurements (Figure S4).