An overview of the ACE-2 clear sky column closure experiment (CLEARCOLUMN)

As 1 of 6 focused ACE-2 activities, a clear sky column closure experiment ( CLEARCOLUMN) took place in June/July 1997 at the southwest corner of Portug al, in the Canary Islands, and over the eastern Atlantic Ocean surrounding and linking those sites. Overdetermined sets of volumetric, vertical profil e and columnar aerosol data were taken From the sea surface to similar to 5 km asl by samplers and sensors at land sites (20-3570 m asl), on a ship, a nd on 4 aircraft. In addition, 5 satellites measured upwelling radiances us ed to derive properties of the aerosol column. Measurements were made in a wide range of conditions and locations (e.g., the marine boundary layer wit h and without continental pollution, the free troposphere with and without African dust). Numerous tests of local and column closure, using unidiscipl inary and multidisciplinary approaches, were conducted. This paper summariz es the methodological approach, the experiment sites and platforms, the typ es of measurements made on each, the types of analyses conducted, and selec ted key results, as a guide to the more complete results presented in other papers in this special issue and elsewhere. Example results include determ inations of aerosol single scattering albedo by several techniques, measure ments of hygroscopic effects on particle light scattering and size, and a w ide range in the degree of agreement found in closure tests. In general, th e smallest discrepancies were found in comparisons among (1) different tech niques to measure an optical property of the ambient, unperturbed aerosol ( e.g., optical depth, extinction, or backscatter by sunphotometer, lidar, an d/or satellite) or (2) different techniques to measure an aerosol that had passed through a common sampling process (e.g., nephelometer and size spect rometer measurements with the same or similar inlets, humidities and temper atures). Typically, larger discrepancies were Found between techniques that measure the ambient, unperturbed aerosol and those that must reconstruct t he ambient aerosol by accounting for (a) processes that occur during sampli ng (e.g., aerodynamic selection, evaporation of water and other volatile ma terial) or (b) calibrations that depend on aerosol characteristics (e.g., s ize-dependent density or refractive index). A primary reason for the discre pancies in such cases is the lack of validated hygroscopic growth models co vering the necessary range of particle sizes and compositions. Other common reasons include (1) using analysis or retrieval techniques that assume aer osol properties (e.g., density, single scattering albedo, shape) that do no t apply in all cases and (2) using surface measurements to estimate column properties. Taken together, the ACE-2 CLEARCOLUMN data set provides a large collection of new information on the properties of the aerosol over the no rtheast Atlantic Ocean. CLEARCOLUMN studies have also pointed to improved t echniques For analyzing current and future data sets (including satellite d ata sets) which will provide a more accurate and comprehensive description of the Atlantic-European-African aerosol. Thus they set the stage for an im proved regional quantification of radiative forcing by anthropogenic aeroso ls.