A New MODIS C6.1 and MERRA-2 Merged Aerosol Products: Validation over The Eastern Mediterranean Region

In this study, we used long-term (2003–2018) aerosol datasets acquired from Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2),  along with two products of MODerate resolution Imaging Spectroradiometer (MODIS)/Terra and Aqua, Collection 6.1(C6.1) and Level 2 (L2), to conduct a comprehensive validation of Aerosol Optical Depth (AOD) using 9 AERONET sites with continuous observations for at least 1 year over the Eastern Mediterranean (EM) region. In order to reduce the number of missing cells from the satellite instruments and produce a near-daily dataset, the “Dark _Target _Deep_Blue_AOD _550_Combined” (DTDB) for both Tera and Aqua at 550 nm were merged, and named as MODIS AOD. The MODIS and MERRA-2 AODs at high spatial resolution (10 km × 10 km and 50 km× 62.5 km, respectively) were resampled to 100 km spatial grid, using the nearest neighbor interpolation technique to overlap the pixels of each other, and to match the full field view of AERONET.Using the 9 collected AERONET observations, the overall performance of the daily MODIS and MERRA-2 AODs over the EM region was validated first. The results showed a significant spatial agreement between these AODs and ground-based AOD, with an acceptable bias (For MODIS: R=0.71, RMSE=0.12, MAE= 0.09, MFE= 40.3% and IOA= 0.81), (For MERRA-2: R=0.75, RMSE=0.10, MAE= 0.06, MFE= 32.1% and IOA=0.83) Moreover, the monthly mean of these AODs were also validated using the monthly mean of the AERONET observations.  The results also showed significant spatial agreement, with an acceptable bias (For MODIS: R=0.78, RMSE=0.06, MAE= 0.05, MFE= 23.7% and IOA =0.79) and (For MERRA-2: R=0.83, RMSE=0.04, MAE= 0.03, MFE= 19.5% and IOA=0.86).The obvious over estimation of daily and monthly mean in the performance of MODIS AOD [i.e., (for daily; RMB=1.2, FGE=24.4%) and (for monthly; RMB=1.21, FGE=20.1%)], and the under estimation of MERRA-2 AOD [ i.e., (for daily; RMB=0.86, FGE=-9.34% ) and (for monthly RMB=0.87, FGE=-13.2%)], should not be overlooked. Due to these systematic under-over estimation error, a new merged AOD product of MERRA-2 and MODIS named as MERRA-2 MODIS Merged AOD (MMM) was generated, on daily time series data during the years 2003-2018 using the combination method. The MMM performance results not only indicated an agreement between the MMM AOD and AERNONT AOD higher than MODIS and MERRA-2 [ i.e., (for daily; R=0.77, RMSE=0.09, MAE= 0.05, MFE= 31.3% and IOA=0.86) and (for monthly; ( R=0.86, RMSE=0.04, MAE= 0.02, MFE= 14.03% and IOA=0.92)] , but also it showed an efficient estimation closer to line1:1 [ i.e., (for daily; RMB=0.99 and FGE=6.6%) and (for daily; RMB=1.03 and FGE=5.5%)].