GDP 4.0 transfer to SGP 3.0 for SCIAMACHY ozone column processing: Verification with SDOAS / GDOAS prototype algorithms and delta-validation with NDACC and WOUDC network data

Until mid 2006, SCIAMACHY data processors for the operational retrieval of ozone (O3) vertical column data were based on the historical version 2 of the GOME Data Processor (GDP). On top of known problems inherent to GDP 2, ground-based validations of SCIAMACHY 03 data also revealed issues specifie to SCIAMACHY, like a large cloud-dependent bias occurring in Northern fall. In 2006, the GDOAS prototype algorithm of the improved GDP version 4 was transferred to the off-line SCIAMACHY Ground Processor (SGP) version 3.0. In parallel, the calibration of SCIAMACHY radiometric data was upgraded. Before operational switch-on of SGP 3.0 and public release of upgraded SCIAMACHY O 3 columns, we investigated the accuracy of the algorithm transfer: (a) by checking the consistency of SGP 3.0 with prototype algorithms and with GDP 4.0; and (b) by comparing SGP 3.0 O3 column data with ground-based observations performed by the Brewer, Dobson and UVVIS networks. This delta-validation study showed that the Northern fall bias has disappeared and that the GDP 2 inherited solar zenith angle and latitude dependences have decreased significantly, concluding that SGP 3.0 is a significant improvement with respect to the previous processor IPF 5.04.