[1] One of the difficulties arising when intercomparing independent measurements of atmospheric constituent profiles consists in homogenizing their respective profile coordinates in order to allow quantitative comparison results. Special care should be devoted in particular to the homogenization of remote sensor measurements, those being indeed intricately bound to their respective numerical grids through discretization rules implied for the evaluation of the retrieval algorithms. Recently, a method of intercomparing remote sounders while allowing for different observational characteristics was proposed by Rodgers and Connor (2003). However, at the time of publication, application of this method was restricted to comparisons of identical state vectors. We propose to relax this condition by the use of linear transformation functions to homogenize the products of independent retrievals. We combine this technique with the Rodgers and Connor procedure to compare independent ozone profile measurements by the Global Ozone Monitoring Experiment (GOME) and a ground-based microwave radiometer (MW). Verification of the achieved results is obtained by considering a second series of MW retrievals, evaluated directly on the GOME numerical grid.

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TROPOMI (Tropospheric Ozone-Monitoring Instrument) is a five-channel UV-VIS-NIR-SWIR non-scanning nadir viewing imaging spectrometer that combines a wide swath (114°) with high spatial resolution (10 × 10 km 2 ) . The instrument heritage consists of GOME on ERS-2, SCIAMACHY on Envisat and, especially, OMI on EOS-Aura. TROPOMI has even smaller ground pixels than OMI-Aura but still exceeds OMI's signal-to-noise performance. These improvements optimize the possibility to retrieve tropospheric trace gases. In addition, the SWIR capabilities of TROPOMI are far better than SCIAMACHY's both in terms of spatial resolution and signal to noise performance.TROPOMI is part of the TRAQ payload, a mission proposed in response to ESA's EOEP call. The TRAQ mission will fly in a non-sun synchronous drifting orbit at about 720 km altitude providing nearly global coverage. TROPOMI measures in the UV-visible wavelength region (270-490 nm), in a near-infrared channel (NIR) in the 710-775 nm range and has a shortwave infrared channel (SWIR) near 2.3 µm. The wide swath angle, in combination with the drifting orbit, allows measuring a location up to 5 times a day at 1.5-hour intervals. The spectral resolution is about 0.45 nm for UV-VIS-NIR and 0.25 nm for SWIR. Radiometric calibration will be maintained via solar irradiance measurements using various diffusers. The instrument will carry on-board calibration sources like LEDs and a white light source. Innovative aspects include the use of improved detectors in order to improve the radiation hardness and the spatial sampling capabilities. Column densities of trace gases (NO 2 , O 3 , SO 2 and HCHO) will be derived using primarily the Differential Optical Absorption Spectroscopy (DOAS) method. The NIR channel serves to obtain information on clouds and the aerosol height distribution that is needed for tropospheric retrievals. A trade-off study will be conducted whether the SWIR channel, included to determine column densities of CO and CH 4 , will be incorporated in TROPOMI or in the Fourier Transform Spectrometer SIFTI on TRAQ.The TROPI instrument is similar to the complete TROPOMI instrument (UV-VIS-NIR-SWIR) and is proposed for the CAMEO initiative, as described for the U.S. NRC Decadal Study on Earth Science and Applications from Space. CAMEO also uses a non-synchronous drifting orbit, but at a higher altitude (around 1500 km). The TROPI instrument design is a modification of the TROPOMI design to achieve identical coverage and ground pixel sizes from a higher altitude. In this paper capabilities of TROPOMI and TROPI are discussed with emphasis on the UV-VIS-NIR channels as the TROPOMI SWIR channel is described in a separate contribution [5].

In the ultraviolet and visible part of the spectrum, measurements of space‐borne grating spectrometers are in general sensitive to the state of polarization of the observed light. The correction for this polarization sensitivity is based on broadband polarization measurements. In parts of the spectrum where the state of polarization is varying rapidly with wavelength this correction is not sufficient and severely limits the accuracy of the atmospheric parameters retrieved from the polarization corrected measurements. In this paper we demonstrate that the problems due to instrument polarization sensitivity can be solved in a natural way by the use of polarization modeling. For the forward model of a retrieval algorithm we propose the combination of a vector radiative transfer model to simulate the transport of radiation in the probed atmosphere and a straightforward simulation of the instrument polarization sensitivity by use of the Mueller matrix formalism. The use of a vector radiative transfer model also overcomes another common bias in retrieval algorithms, caused by the widely used scalar approximation of atmospheric radiative transfer. The capability and need of the proposed approach are demonstrated for ozone profile retrieval from measurements of the Global Ozone Monitoring Experiment (GOME). A comparison of retrieved profiles with 123 ozonesonde profiles shows that the use of a polarization forward model yields a significant improvement in root‐mean square difference of about a factor 1.5 in the stratosphere as well as in the troposphere. Also, a solar zenith angle dependence in the differences is reduced significantly.