Hourly data were only used for the validation step . TROPOMI - based NO2 and SO2 data were compared with TAQMS ground - measurements for three test sites ( urban , urban - traffic and suburban ) in the city of Istanbul . To evaluate MODIS AOD 470 nm data , we applied AERONET data ( AOD440 and AOD500 ) with a time tolerance of ±0.5 h. Since AERONET does not provide AOD 470 nm , the mean value of AERONET AOD 440 nm and 500 nm values were calculated for AOD 470 nm retrieval relevant to MODIS AOD 470 nm ( Equation ( 1 ) ) . . 3 . The monthly images of tropospheric NO2 column density over Turkey .