Analysis of the change in mineral dust optical properties over the Eastern Mediterranean with source location using SEAWIFS imagery Tulay Cokacar 1 , Cyril Moulin 2 , Nilgun Kubilay 1 and Temel Oguz 1 (1)Middle East Technical university, Institute of Marine Sciences, Erdemli Mersin TURKEY (2)Laboratoire des Sciences du Climat et de l'Environnement, CEA-CNRS, Gif sur Yvette, France Abstract Mineral dust from surrounding arid regions is the major aerosol in the Mediterranean atmosphere during spring and summer. The Eastern Mediterranean is particularly interesting because mineral dust may come from three different sources, originate from the central Sahara in spring, the eastern Sahara in summer and the Middle East/Arabian peninsula in autumn. We obtain the best agreement between SeaWiFS and Sun-Photometer derived aerosol optical thickness by considering the opt ical properties dust transported from different sources. Dust optical properties over the Eastern Mediterranean can be separated in two main categories with different absorption efficiencies. By using the TOMS absorbing aerosol product to locate the dust sources, we found that mineral dust that comes from Middle East Arabian Peninsula has lower absorption in blue and green parts of the solar spectrum than dust coming from Sahara. In addition, we found that the dust coming from Sahara over the Eastern Mediterranean has optical properties similar to that transported over the Atlantic. This new set of mineral dust optical properties will be used to improve ocean color estimates in the Eastern Mediterranean Sea. Keywords-aerosol;mineral dust;refractive index I. INTRODUCTION Saharan dust plumes are the major source of terrigeneous material in the atmosphere of the Mediterranean Sea and North Atlantic Ocean, and they also influence the chemistry and biology of the water column. The optical properties of these mineral particles still remain poorly known. The most striking and unknown characteristic of mineral dust in the solar spectrum is its ability to absorb in the violet, blue, and green portions of the spectrum (400-500 nm). Moreover, this absorption, characterizedby the imaginary part of the refractive index (the absorption index) of dust particles, is potentially highly variable since it depends on the amount of iron oxides (mainly hematite) Sokolik et al., 1993; Claquin et al., 1999] in the originating soil. Mineral dust constitutes an extreme case in atmospheric correction of ocean color because it often combines very high optical depths with a strong absorption capacity in the blue [Patterson, 1981]. The present study uses continuous surface-based sun photometer ground-based aerosol optical measurements measurements at the IMS-METU site at Erdemli (36° 33' N, 34°15' E) along the Turkish coast of the northeastern Mediterranean from January 2000 to June 2001 The data are conducted within the framework of the Aerosol Robotic Network (AERONET) program. The measurements are used to identify major characteristic features of mineral dust optical properties. All these aerosol optical data from the region, when pooled together, ultimately serve a basis for improving regional dust correction algorithm [Moulin et al., 2001a,b]. II RESULTS DATA SELECTION AND PROCESSING In this work, we used the method of Moulin et al. [2001] to estimate dust optical properties (including absorption) over the Eastern Mediterranean from SeaWiFS spectral measurements.Briefly, a set of N candidate aerosol models is used along with a model of water-leaving radiance. In this context, an aerosol model is comprised of a particle size distribution and index of refraction, with the radiative properties computed using mie theory and a vertical distribution of aerosol We applied the SMA (Gordon et al. 1997) to SeaWiFs on a pixel-by-pixel basis. We selected the most intense springtime and summertime dust events that occured in 2000. For pixels with high optical thickness (> 1), we assumed that the marine contribution to the SeaWiFS measurement is negligible in the visible, so that the measured reflectance spectrum is solely due to aerosol and molecular scattering. After molecular scattering removal, we computed aerosol reflectances for various dust absorbing efficiencies until these modeled reflectances match the measured ones. Comparisons between in_situ AOD showed that the Atlantic 0-7803-7929-2/03/$17.00 (C) 2003 IEEE 1285