Abstract: Water Vapour total column (H2O):

The Global Ozone Monitoring Experiment (GOME) was launched on April 1995 on board ESA's second European Remote Sensing Satellite (ERS-2).
This instrument can measure a range of atmospheric trace constituents, with the emphasis on global ozone distributions. Furthermore cloud properties and intensities of ... ultraviolet radiation are retrieved. These data are crucial for monitoring the atmospheric composition and the detection of pollutants.

DLR generates the operational GOME/ERS-2 level 1 and level 2 products on behalf ESA. GOME Near Real Time (NRT) products are available three hours after sensing. The NRT service was started in 1997 by setting up the GOME Data Processor at Kiruna station. Nowadays the GOME raw data is transmitted form all ERS-2 receiving stations, including the DLR Antarctic station at O'Higgins, to the DLR for further processing.

Algorithm:
The operational H2O total column products are generated using the algorithm GDP (GOME Data Processor) version 4.x integrated into the UPAS (Universal Processor for UV/VIS Atmospheric Spectrometers) processor for generating level 2 trace gas and cloud products.
The total H2O column is retrieved from GOME solar backscattered measurements in the red wavelength region (614-683.2 nm), using the Differential Optical Absorption Spectroscopy (DOAS) method. A description of the algorithm and validation is given in the reference documents listed in the GOME-2 documentation pages.

DOAS slant column fitting:
The fitting window 614-683.2 nm was selected for GOME-2 [Wagner et al. (2003)]. An effective H2O cross-section for a single temperature of 290 K is included in the fit, as well as the cross-sections of O2 and of the interfering trace gas O2-O2. A Ring reference spectrum and 3 vegetation spectra are included as an additive fitting parameter. The detailed DOAS settings used for GOME-2 H2O slant columns retrieval are given in the [DLR/GOME-2/ATBD].

AMF and VCD determination:
The AMF of H2O is set equal to the AMF of the O2-C band times a correction factor [Wagner et al. (2006)]. The AMF for O2 is calculated from the known O2 VCD and the fitted O2 slant column. The fitted H2O and O2 slant columns are corrected for saturation effects. The correction factor is calculated for a single H2O profile, as function of viewing geometry and surface albedo. The surface albedo in the retrieval is taken from the Koelemeijer database, except over ocean where it is set to 0.02.

Acknowledgments:
GDP 4.x for total H2O retrieval is being developed by scientists from MPI-Chemie in Mainz and DLR
• MPI: Thomas Wagner, Steffen Beirle, Kornelia Mies
• DLR: Sander Slijkhuis, Bernd Aberle, Sunil Emmadi, Diego Loyola
and is funded by EUMETSAT O3M-SAF and national contributions.