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- ItemAn overview of the ACE-2 clear sky column closure experiment (CLEARCOLUMN)(Milton Park : Taylor & Francis, 2016) Russell, Philip B.; Heintzenberg, JostAs 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 Portugal, in the Canary Islands, and over the eastern Atlantic Ocean surrounding and linking those sites. Overdetermined sets of volumetric, vertical profile and columnar aerosol data were taken from the sea surface to~5 km asl by samplers and sensors at land sites (20–3570 m asl), on a ship, and on 4 aircraft. In addition, 5 satellites measured upwelling radiances used to derive properties of the aerosol column. Measurements were made in a wide range of conditions and locations (e.g., the marine boundary layer with and without continental pollution, the free troposphere with and without African dust). Numerous tests of local and column closure, using unidisciplinary and multidisciplinary approaches, were conducted. This paper summarizes the methodological approach, the experiment sites and platforms, the types of measurements made on each, the types of analyses conducted, and selected key results, as a guide to the more complete results presented in other papers in this special issue and elsewhere. Example results include determinations of aerosol single scattering albedo by several techniques, measurements of hygroscopic effects on particle light scattering and size, and a wide range in the degree of agreement found in closure tests. In general, the smallest discrepancies were found in comparisons among (1) different techniques to measure an optical property of the ambient, unperturbed aerosol (e.g., optical depth, extinction, or backscatter by sunphotometer, lidar, and/or satellite) or (2) different techniques to measure an aerosol that had passed through a common sampling process (e.g., nephelometer and size spectrometer measurements with the same or similar inlets, humidities and temperatures). Typically, larger discrepancies were found between techniques that measure the ambient, unperturbed aerosol and those that must reconstruct the ambient aerosol by accounting for (a) processes that occur during sampling (e.g., aerodynamic selection, evaporation of water and other volatile material) or ( b) calibrations that depend on aerosol characteristics (e.g., sizedependent density or refractive index). A primary reason for the discrepancies in such cases is the lack of validated hygroscopic growth models covering the necessary range of particle sizes and compositions. Other common reasons include (1) using analysis or retrieval techniques that assume aerosol properties (e.g., density, single scattering albedo, shape) that do not 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 northeast Atlantic Ocean. CLEARCOLUMN studies have also pointed to improved techniques for analyzing current and future data sets (including satellite data sets) which will provide a more accurate and comprehensive description of the Atlantic–European–African aerosol. Thus they set the stage for an improved regional quantification of radiative forcing by anthropogenic aerosols.
- ItemSize-segregated chemical, gravimetric and number distribution-derived mass closure of the aerosol in Sagres, Portugal during ACE-2(Milton Park : Taylor & Francis, 2016) Neusüß, C.; Weise, D.; Birmili, W.; Wex, H.; Wiedensohler, A.; Covert, D.S.During the ACE-2 field campaign in the summer of 1997 an intensive, ground-based physical and chemical characterisation of the clean marine and continentally polluted aerosol was performed at Sagres, Portugal. Number size distributions of the dry aerosol in the size range 3–10 000 nm were continuously measured using DMPS and APS systems. Impactor samples were regularly taken at 60% relative humidity (RH) to obtain mass size distributions by weighing the impactor foils, and to derive a chemical mass balance by ion and carbon analysis. Hygroscopic growth factors of the metastable aerosol at 60% RH were determined to estimate the number size distribution at a relative humidity of 60%. A size segregated 3-way mass closure study was performed in this investigation for the first time. Mass size distributions at 60% RH derived from number size distribution measurements and impactors samples (weighing and chemical analysis) are compared. A good agreement was found for the comparison of total gravimetrically-determined mass with both number distribution-derived (slope=1.23/1.09; R2>0.97; depending on the parameters humidity growth and density) and chemical mass concentration (slope=1.02; R2=0.79) for particles smaller than 3 mm in diameter. Except for the smallest impactor size range relatively good correlations (slope=0.86–1.42) with small deviations (R2=0.76–0.98) for the different size fractions were found. Since uncertainties in each of the 3 methods are about 20% the observed differences in the size-segregated mass fractions can be explained by the measurement uncertainties. However, the number distributionderived mass is mostly higher than the chemically and gravimetrically determined mass, which can be explained by sampling losses of the impactor, but as well with measurement uncertainties as, e.g., the sizing of the DMPS/APS.