Aerosol Optical Depth Levels Research Articles

Inventory of African desert dust events over the southwestern Iberian Peninsula in 2000–2005 with an AERONET Cimel Sun photometer

Routine aerosol measurements are carried out at El Arenosillo station (37.1°N, 6.7°W), in Huelva (Spain), since February 2000 with a Cimel Sun photometer included in the AERONET network. This station, located in the coast of the southwestern Iberian Peninsula, is an appropriate site for the detection and characterization of desert dust aerosols arriving from North Africa. The aerosol database allows us to make an inventory of desert dust events over 6 years corresponding to the period 2000–2005. The inventory is based on threshold values for the aerosol optical depth (AOD) and Ångström exponent, the analysis of the size distributions, a manual inspection, and the use of air mass back trajectories as complementary information. This inventory shows the presence of desert aerosols 65 d yr−1 (18% of days), although there are significant differences among years. Desert dust events have a mean duration of 4 days, with mean AOD (440 nm) of 0.31 and mean Ångström exponent of 0.66. The desert dust episodes are more frequent in February, March, and the summer months. During autumn, winter, and April the episodes are less frequent. This seasonal pattern of the desert events influences the AOD at El Arenosillo, which presents an analogous seasonal pattern. Because of the desert events, AOD levels increase 28% at 440 nm and 48% at 870 nm. Desert dust intrusions have been also catalogued according to four basic meteorological scenarios. The analysis of the desert inventory has made it possible to establish an automatic method for the detection of the desert dust intrusions over the Iberian Peninsula with Sun photometer data. Finally, a classification of air masses (based on back trajectories) has been compared with the occurrence of desert dust intrusions.

Diurnal variation of the aerosol optical depth for two distinct cases in the Athens area, Greece

This work uses the Ångström turbidity coefficient to investigate the variation of the aerosol optical depth (AOD) in the Athens area during “aerosol-clear” (low particulate-matter concentration) and “aerosol-burden” (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 human activity and traffic. Also, the AOD levels show a gradual decrease with wavelength and time of the day.