A new aerosol retrieval method called Synergetic Aerosol Retrieval (SYNAER), using simultaneous measurements of the radiometer Along Track Scanning Radiometer (ATSR-2) and the spectrometer Global Ozone Monitoring Experiment (GOME) in the visible and near-infrared spectra, was developed. Both instruments are flown onboard the European Remote Sensing (ERS-2) satellite. SYNAER delivers boundary layer aerosol optical thickness (BLAOT) and aerosol type both over land and over ocean, the latter as BLAOT percentage of six representative components from the Optical Parameters of Aerosols and Clouds (OPAC) data set. The high spatial resolution of ATSR-2 permits accurate cloud detection. It allows BLAOT calculation over automatically selected dark pixels and surface albedo correction for a set of boundary layer aerosol mixtures. After spatial integration and colocation to GOME pixels, these parameters are used to simulate GOME spectra for the same set of aerosol mixtures. A least squares fit of these spectra to the measured and cloud-corrected GOME spectrum chooses the aerosol mixture. First validation studies are presented in part 2 of this paper <Holzer-Popp et al., 2002>. The method will be used for the future sensor pairs Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY)/Advanced ATSR (AATSR) on Envisat and GOME-2/Advanced Very High Resolution Radiometer (AVHRR) on METOP. Thus, SYNAER holds the potential to extract a long-term climatological data set.