Atmospheric Measurement Techniques Discussions www.atmos-meas-tech-discuss.net 1867-8610 3 4 2010 10.5194/amtd-3-3425-2010 http://www.atmos-meas-tech-discuss.net/3/3425/2010/ http://www.atmos-meas-tech-discuss.net/3/3425/2010/amtd-3-3425-2010.html http://www.atmos-meas-tech-discuss.net/3/3425/2010/amtd-3-3425-2010.pdf 3425 3453 2010-08-17 Evaluating the assumptions of surface reflectance and aerosol type selection within the MODIS aerosol retrieval over land: the problem of dust type selection T. Mielonen tero.mielonen@fmi.fi R. C. Levy V. Aaltonen M. Komppula G. de Leeuw J. Huttunen H. Lihavainen P. Kolmonen K. E. J. Lehtinen A. Arola Finnish Meteorological Institute, Kuopio Unit, Kuopio, Finland Science Systems and Applications Inc., Lanham, MD, USA NASA/Goddard Space Flight Center, Greenbelt, MD, USA Finnish Meteorological Institute, Climate Change, Helsinki, Finland Department of Physics, University of Helsinki, Helsinki, Finland Department of Physics and Mathematics, University of Eastern Finland, Kuopio, Finland Aerosol Optical Depth (AOD) and Ångström exponent (AE) values derived with the MODIS retrieval algorithm over land (Collection 5) were compared with ground based sun photometer measurements in Europe, Asia, Africa, North America and South America. In Finland (Jokioinen and Sodankylä) measurements were done with Precision Filter Radiometer (PFR), while in Estonia (Toravere), Italy (Ispra, Rome Tor Vergata), India (Kanpur), China (Xianghe), GSFC (USA), Mexico (Mexico City), Zambia (Mongu) and Brazil (Alta Floresta) Cimel (AERONET, level 2) measurements were used. Comparison results for AOD were generally good, although there seems to be room for improvement in the MODIS aerosol model selection, particularly how dust is taken into account. At all studied sites, the MODIS algorithm often selects the dust aerosol model even when dust does not seem to be present and the air masses are not coming from arid regions. This happens especially when AOD values are relatively small (<0.3). The selection of the dust model reduces the correlation between ground based and MODIS AOD measurements in dust-free situations. Moreover, the current aerosol model selection scheme produces unphysical AE values. Our study suggests that the aerosol model combining is sensitive to the ratio of 660 nm and 2130 nm surface reflectances (slope(660/2130)). Furthermore, the value of the slope in the algorithm is mainly dependent on the Normalized Difference Vegetation Index (NDVI). 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