There are a number of cross-cutting issues relevant for all ESA atmospheric missions which can only be addressed through suitable airborne measurements. The missions include ADM/Aeolus, EarthCARE and Sentinel-3/-4/-5/-5p which deliver different aerosol related products requiring both detailed research in the development of the retrieval methods and the independent validation/evaluation of products following launch. Aerosol characterisation is also needed for non-atmospheric missions, e.g. for atmospheric correction purposes.  

There is an objective need to develop a suitable airborne lidar instrument to perform profile measurements of aerosol extinction, backscatter and depolarization at different wavelengths, capable of providing extensive properties (e.g. aerosol optical depth) but also intensive aerosol properties (e.g. particle depolarization, Angström exponent, lidar ratio, colour ratio). Multi-wavelength measurements are required also for aerosol type characterization and aerosol-cloud discrimination. These properties cannot be derived by conventional backscatter lidars, due to limitations regarding the lidar ratio assumption and lidar equation instabilities. High Spectral Resolution Lidar (HSRL) capabilities are required to discriminate aerosol and molecular signals and thereby to measure aerosol extinction and backscatter independently. Moreover, nearly-coincident airborne in-situ and lidar measurements are required to support the study of the relationship between the in-situ measured optical/physical aerosol properties and lidar-derived optical properties. A possible solution employing a horizontally-pointing airborne lidar configuration, where the aerosol is first detected via lidar and then measured quasi-simultaneously by the in-situ instrumentation should be investigated.