Atmospheric Chemistry and Physics Interpretation of FRESCO cloud retrievals in case of absorbing

Abstract. Cloud and aerosol information is needed in trace gas retrievals from satellite measurements. The Fast REtrieval Scheme for Clouds from the Oxygen A band (FRESCO) cloud algorithm employs reflectance spectra of the O2 A band around 760 nm to derive cloud pressure and effective cloud fraction. In general, clouds contribute more to the O2 A band reflectance than aerosols. Therefore, the FRESCO algorithm does not correct for aerosol effects in the retrievals and attributes the retrieved cloud information entirely to the presence of clouds, and not to aerosols. For events with high aerosol loading, aerosols may have a domi-nant effect, especially for almost cloud free scenes. We have analysed FRESCO cloud data and Absorbing Aerosol In-dex (AAI) data from the Global Ozone Monitoring Exper-iment (GOME-2) instrument on the Metop-A satellite for events with typical absorbing aerosol types, such as volcanic ash, desert dust and smoke. We find that the FRESCO ef-fective cloud fractions are correlated with the AAI data for these absorbing aerosol events and that the FRESCO cloud pressure contains information on aerosol layer pressure. For cloud free scenes, the derived FRESCO cloud pressure is close to the aerosol layer pressure, especially for optically thick aerosol layers. For cloudy scenes, if the strongly ab-sorbing aerosols are located above the clouds, then the re-trieved FRESCO cloud pressure may represent the height of the aerosol layer rather than the height of the clouds. Com-bining FRESCO and AAI data, an estimate for the aerosol layer pressure can be given.