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List of Abstracts 195aerosols and clouds, and their influence on climate. Satellite measurements usually present very largeuncertainties when compared to in situ measurements. We have analyzed data from 2000 to 2007 acquiredby the radiometers CERES and MODIS onboard the Terra satellite, which together provide shortwave andlongwave radiative fluxes on the top of the atmosphere, aerosol optical depth and surface reflectance amongother critical data. For the year 2000, a combined analysis of CERES and MODIS provides a value of -14 +or - 5 W/m2 for the shortwave direct radiative forcing of the whole Amazon region during the dry season.For the in situ validation, we have used the AERONET sun photometer network data, which provides verydetailed information on aerosol distribution and optical properties. We have used local solar fluxmeasurements from pyranometers from the SolRad-Net network, which are collocated to each sunphotometer.In an attempt to reduce the uncertainty of the calculated radiative forcing we have used theradiative transfer code SBDART and the above mentioned in situ measurements. We have also comparedCALIOP and AERONET AOD retrievals for four Amazon sites within 50km and 100km radius for threeyears, and have found that CALIOP consistently underestimates the AOD. The observed bias is a high -47%.This bias seems to have occurred due to the proximity of clouds near the area where measurements weretaken and also due to the misidentification of biomass burning aerosols with clouds.P-Observations 2.59 ID:4562 10:30The Organic Fraction of Aerosols in the CaribbeanFlavia Morales-García 1 , Anne Kasper-Giebl 2 , Hans Puxbaum 2 , Swen Metzger 3 , Stefano Decesari 4 , OlgaL. Mayol-Bracero 11 Institute for Tropical Ecosystem Studies, University of Puerto Rico, San Juan, Puerto Rico2 Institute for Chemical Technologies and Analytics, Vienna University of Technology, Vienna, Austria3 Department of Atmospheric Chemistry, Max Planck Institute for Chemistry, Mainz, Germany4 Institute of Atmospheric Sciences and Climate, CNR, Bologna, ItalyContact: f.portalatin@gmail.comAs part of the Rain in Cumulus over the Ocean Experiment, we investigated the organic fraction of aerosolparticles collected simultaneously at two tropical marine sites in the Caribbean region: Dian Point, Antiguaand Cape San Juan, Puerto Rico. Two size-resolved low-pressure impactors (Dp from ~0.054 to ~18 µm),stacked-filter units (Dp < 1.7 µm) and a high-volume sampler were used to collect the samples. Thermalopticalanalysis was used to determine the concentrations of aerosol total carbon, organic carbon, andelemental carbon. Neutral compounds, mono-, di-, and poly-acids were analyzed using an HPLC-TOC andacetate, formate, malonate, and oxalate using ion chromatography. The water-soluble organic carbon(WSOC) was determined with a total organic carbon (TOC) analyzer. The EQSAM3 (EQuilibriumSimplified Aerosol Model) was used to determine the water uptake of the collected aerosol samples. Airmasses influenced by marine (M) aerosols, African dust (AD), and anthropogenic pollution (AP) from NorthAmerica were studied. Particulate organic matter (POM) was up to 12% of the total aerosol mass and up to27% of the non-sea salt aerosol mass, and its concentrations were ~80 ng/m^3 on average during the M andAP periods. WSOC as well as oxalate tend to be higher for the fine fraction particularly during the presenceof AD. POM previous results generated also in Puerto Rico showed up to 4 times higher concentrations ofPOM. These discrepancies might be due to seasonal variations or a long trend. Results showed that thechemical composition of the organic fraction of aerosols, its contribution to the total aerosol mass, and theeffect they have on cloud properties are still not well understood. Answer to questions such as the organicaerosols’ temporal/spatial variation, dependence on the seasonal biological activity, mixing state, andsources are needed to fully understand their role in the Caribbean climate and their true extent and origin.iCACGP-IGAC 2010 14 July, 2010