An improved algorithm for the retrieval of total and tropospheric nitrogen dioxide ( NOsub2/sub ) columns from the Global Ozone Monitoring Experiment-2 (GOME-2) is presented. The refined retrieval will be implemented in a future version of the GOME Data Processor (GDP) as used by the EUMETSAT Satellite Application Facility on Atmospheric Composition and UV Radiation (AC-SAF). The first main improvement is the application of an extended 425–497?nm wavelength fitting window in the differential optical absorption spectroscopy (DOAS) retrieval of the NOsub2/sub slant column density, based on which initial total NOsub2/sub columns are computed using stratospheric air mass factors (AMFs). Updated absorption cross sections and a linear offset correction are used for the large fitting window. An improved slit function treatment is applied to compensate for both long-term and in-orbit drift of the GOME-2 slit function. Compared to the current operational (GDP?4.8) dataset, the use of these new features increases the NOsub2/sub columns by ～1 – 3×10sup14/sup molec?cm sup2/sup and reduces the slant column error by ～24 %. In addition, the bias between GOME-2A and GOME-2B measurements is largely reduced by adopting a new level 1b data version in the DOAS retrieval. The retrieved NOsub2/sub slant columns show good consistency with the Quality Assurance for Essential Climate Variables (QA4ECV) retrieval with a good overall quality. Second, the STRatospheric Estimation Algorithm from Mainz (STREAM), which was originally developed for the TROPOspheric Monitoring Instrument (TROPOMI) instrument, was optimised for GOME-2 measurements to determine the stratospheric NOsub2/sub column density. Applied to synthetic GOME-2 data, the estimated stratospheric NOsub2/sub columns from STREAM shows good agreement with the a priori truth. An improved latitudinal correction is introduced in STREAM to reduce the biases over the subtropics. Applied to GOME-2 measurements, STREAM largely reduces the overestimation of stratospheric NOsub2/sub columns over polluted regions in the GDP?4.8 dataset. Third, the calculation of AMF applies an updated box-air-mass factor (box-AMF) look-up table (LUT) calculated using the latest version 2.7 of the Vector-LInearized Discrete Ordinate Radiative Transfer (VLIDORT) model with an increased number of reference points and vertical layers, a new GOME-2 surface albedo climatology, and improved a priori NOsub2/sub profiles obtained from the TM5-MP chemistry transport model. A large effect (mainly enhancement in summer and reduction in winter) on the retrieved tropospheric NOsub2/sub columns by more than 10?% is found over polluted regions. To evaluate the GOME-2 tropospheric NOsub2/sub columns, an end-to-end validation is performed using ground-based multiple-axis DOAS (MAXDOAS) measurements. The validation is illustrated for six stations covering urban, suburban, and background situations. Compared to the GDP?4.8 product, the new dataset presents improved agreement with the MAXDOAS measurements for all the stations.
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