Diurnal variation of the aerosol optical depth for two distinct cases in the Athens area, Greece G.M. Giavis a,b , H.D. Kambezidis b, * , N. Sifakis c , Z. Toth d , A.D. Adamopoulos e , D. Zevgolis f a Department of Physics, University of Patras, GR-26500 Patras, Greece b Atmospheric Research Team, Institute for Environmental Research and Sustainable Development, National Observatory of Athens, PO Box 20048, GR-11810 Athens, Greece c Institute for Space Studies and Remote Sensing, National Observatory of Athens, I. Metaxa and V. Pavlou, GR-15236 P. Penteli, Greece d Hungarian Meteorological Service, 1525 Budapest, Hungary e Directorate for Air Pollution and Noise Control, Division of Air Quality, Ministry of Environment, Physical Planning and Public Works, Patission 147, GR-11251 Athens, Greece f Hellenic Open University, Tsamadou 29 and R. Fereou, GR-26222 Patras, Greece Received 10 September 2004; accepted 22 March 2005 Abstract This work uses the A ˚ ngstro ¨ m turbidity coefficient to investigate the variation of the aerosol optical depth (AOD) in the Athens area during baerosol-clearQ (low particulate-matter concentration) and baerosol-burdenQ (high particulate-matter concentration) atmospheric conditions. The AOD is estimated at wavelengths in the band 310–1100 nm from direct-beam spectral irradiance measurements using ground-based instrumentation during an experimental campaign performed in the period 20 September to 1 October 2002. All data collected were recorded under clear-sky conditions near the city center of Athens; the AODs were estimated with calculated optical air masses in the range 1.3–2.0. The study shows that the AOD is influenced by meteorological parameters such as humidity and wind speed and direction. Also, traffic seems to play a significant role on the concentration of the aerosols in the atmosphere of urban environments. Therefore, the AODs are higher with low relative humidity, low south wind, high traffic and human activity. On the contrary, the AODs are found to be low with high relative humidity, higher south winds and low 0169-8095/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2005.03.003 * Corresponding author. Tel.: +30 210 3490119; fax: +30 210 3490113. E-mail address: harry@env.meteo.noa.gr (H.D. Kambezidis). Atmospheric Research 78 (2005) 79 – 92 www.elsevier.com/locate/atmos