Aerosol climatology and discrimination of different types over Athens, Greece, based on MODIS data

A long-term (2000-2005) monitoring of aerosol data from the moderate resolution imaging spectroradiometer (MODIS) is analyzed focusing on the Greater Athens Area (GAA) in the Eastern Mediterranean region. The MODIS aerosol optical depth standard product (AOD at 550 nm) and its respective ratio attributed to fine-mode (FM) particles are employed to evaluate the seasonal variability of the aerosol properties over Athens. The climatological trend of both parameters in the period 2000-2005 is nearly absent, while remarkable year-to-year variability can be observed. The seasonal analysis reveals a significant AOD variability over Athens, with minimum values in winter (AOD(550) similar to 0.2), and maximum in summer (AOD(550)similar to 0.45). Regarding the FM fraction, maximum values are present in spring and minimum in summer, thus revealing the dominance of FM and coarse-mode particles, respectively. For the whole data set, a method is implemented to distinguish the main aerosol types (urban/industrial (hereafter UI), clean maritime (hereafter CM type) and desert dust (hereafter DD) over Athens, based on both AOD and FM values. Because of the mixing processes in the atmosphere the majority of the cases (46.6%) belong to a mixed (hereafter MT) aerosol type. The U aerosols are more frequent in spring (41.2%) and less in winter (9.1 %), while the coarse particles, probably DD, more frequent in summer (35.8%) and less in winter (3.5%). In contrast, the clean atmospheric conditions are more frequent in winter (23.9%), when the mixing processes are also well established (63.5%). For each aerosol type, the mean AOD(550) and FM values are also computed. Their seasonal variability exhibits a clear summer maximum for UI, CM and MT aerosols, while the DD exhibits maximum in spring. As regards the FM values of the different aerosol types they exhibit a rather constant variation with small fluctuations from season to season. (c) 2007 Elsevier Ltd. All rights reserved.