Reply on Owen Coopers comment

Abstract

We present a new tropospheric ozone data set based on TROMOMI/Sentinel-5 Precursor (S5P) total ozone mea- surements combined stratospheric ozone data from the Belgian Assimilation System for Chemical ObsErvations (BASCOE). BASCOE is constrained by assimilating ozone observations from the microwave limb sounder (MLS). The tropospheric ozone algorithm is similar to the well established OMI-MLS or OMPS-MERRA-2 retrieval. Compared to this we gain spatial resolu- tion when applying the algorithm to TROPOMI data (5.5 × 3.5 km2) and extend these data record into the future. Compared to the OMPS-MERRA-2 data a mean positive bias of ≈ 3 DU is found. A small negative bias of about 2 DU is observed in the tropics relative to the operational TROPOMI tropospheric (S5P_O3_TCL) data based on the CCD algorithm. The new tropospheric ozone data (S5P-BASCOE) is compared to a set of globally distributed ozone sondes data integrated up to the tropopause level. For the comparison both the mean of the satellite observations around the sounding station and the closest column data are used. Depending on the latitude the S5P-BASCOE deviate from the sondes and between 0 and 5 DU, indicating a good agreement. However, some exceptional larger positive deviation up to ±10 DU are found. The monthly mean tropospheric column as well as time series for selected places showed the expected spatial and temporal pattern.