PASSIVE REMOTE-SENSING OF TROPOSPHERIC AEROSOL AND ATMOSPHERIC CORRECTION FOR THE AEROSOL EFFECT

The launch of ADEOS in August 1996 with POLDER, TOMS, and OCTS instrum ents on board and the future launch of EOS-AM 1 in mid-1998 with MODIS and MISR instruments on board start a new era in remote sensing of ae rosol as part of a new remote sensing of the whole Earth system (see a list of the acronyms in the Notation section of the paper). These pla tforms will be followed by other international platforms with unique a erosol sensing capability, some still in this century (e.g., ENVISAT i n 1999). These international spaceborne multispectral, multiangular, a nd polarization measurements, combined for the first time with interna tional automatic, routine monitoring of aerosol from the ground, are e xpected to form a quantum leap in our ability to observe the highly va riable global aerosol. This new capability is contrasted with present single-channel techniques for AVHRR, Meteosat, and GOES that although poorly calibrated and poorly characterized already generated important aerosol global maps and regional transport assessments. The new data will improve significantly atmospheric corrections for the aerosol eff ect on remote sensing of the oceans and be used to generate first real -time atmospheric corrections over the land. This special issue summar izes the science behind this change in remote sensing, and the sensiti vity studies and applications of the new algorithms to data from prese nt satellite and aircraft instruments. Background information and a su mmary of a critical discussion that took place in a workshop devoted t o this topic is given in this introductory paper. In the discussion it was concluded that the anticipated remote sensing of aerosol simultan eously from several space platforms with different observation strateg ies, together with continuous validations around the world, is expecte d to be of significant importance to test remote sensing approaches to characterize the complex and highly variable aerosol field. So far, w e have only partial understanding of the information content and accur acy of the radiative transfer inversion of aerosol information from th e satellite data, due to lack of sufficient theoretical analysis and a pplications to proper field data. This limitation will make the antici pated new data even more interesting and challenging. A main concern i s the present inadequate ability to sense aerosol absorption, from spa ce or from the ground. Absorption is a critical parameter for climate studies and atmospheric corrections. Over oceans, main concerns are th e effects of white caps and dust on the correction scheme. Future impr ovement in aerosol retrieval and atmospheric corrections will require better climatology of the aerosol properties and understanding of the effects of mixed composition and shape of the particles. The main ingr edient missing in the planned remote sensing of aerosol are spaceborne and ground-based lidar observations of the aerosol profiles.