Backscatter lidar observations such as those provided by the CALIPSO mission are expected to give complementary information to long-used radiometric observations for aerosol properties characterization important to climate and environment issues. However, retrieving aerosol optical depth (AOD) and profiling the aerosol extinction cannot be done accurately applying a standard inversion procedure to the backscatter lidar measurements, without a precise knowledge of aerosol properties on the vertical. The objective of this first study is to propose a new approach to quantify the AOD over the ocean combining the surface return signals from the lidar and radar onboard the CALIPSO and CloudSat platforms, respectively. Taking advantage of the satellite formation within the AQUA-train, first comparisons of AODs retrieved with our method and MODIS ones at tropical latitudes show an overall bias smaller than 1%, and a standard deviation of about 0.07. These first results are presented and error sources are discussed.