Three-year ground based measurements of aerosol optical depth over the Eastern Mediterranean: the urban environment of Athens

E. Gerasopoulos,P. Kokkalis,C. S. Zerefos,H. El-Askary,V. Amiridis,K. Eleftheratos,T. W. Andreae,M. O. Andreae,S. Kazadzis

doi:10.5194/acp-11-2145-2011

Abstract

Abstract. Three years (2006–2008) of ground-based observations of the Aerosol Optical Depth (AOD) in the urban environment of Athens, in the Eastern Mediterranean, are analysed in this work. Measurements were acquired with a Multi-Filter Rotating Shadowband Radiometer at five wavelengths. The daily average AOD at 500 nm is 0.23, and the mean Ångström coefficient calculated between 415 and 867 nm is 1.41. The annual variability of AOD has a spring maximum dominated by coarse dust particles from the Sahara (AOD 0.34–0.42), while the diurnal pattern is typical for urban sites, with AOD steadily increasing throughout the day. The greatest contribution to the annually averaged AOD, accounting for almost 40%, comes from regional and local sources (namely the Istanbul metropolitan area, the extended areas of biomass burning around the north coast of the Black Sea, power plants spread throughout the Balkans and the industrial area in the Po valley, with average daily AOD in the range of 0.25–0.35). An additional important contribution (23%) is dust from Africa, whereas the rest of Europe contributes another 22%. The geographical distribution of the above sources in conjunction with the prevailing synoptic situation and contribution of local sources, lead to mixed types of aerosols over Athens, with highly variable contribution of fine and coarse particles to AOD in the range 10%–90%. This is the first long-term, ground based data set available for Athens, and it has also been used for the validation of satellite derived AOD by MODIS, showing good agreement on an annual basis, but with an overestimation of satellite AODs in the warm period.

Highlights

Aerosols have long been identified as and included among the major controllers of the Earth’s climate (e.g. IPCC, 2001)
The methodology followed for the extraction of the Aerosol Optical Depth (AOD) values from direct solar irradiance is thoroughly described in Gerasopoulos et al (2003)
The daily average AOD at 500 nm for the whole period is 0.23 ± 0.17, with 50% of the values lying in the range 0.12–0.29 and maximum frequency class 0.1–0.15

Summary

Introduction

Aerosols have long been identified as and included among the major controllers of the Earth’s climate (e.g. IPCC, 2001). Despite progress achieved during the last decades in understanding the effects of aerosols on climate, their large spatial-temporal variability and heterogeneity still causes significant uncertainties at global scales (IPCC, 2007). Aerosols can affect the climate via radiation forcing and interaction with cloud and degrade air quality, visibility and public health. The high content of atmospheric aerosols in the area, on the order of 2 to 10 times higher than over the least polluted environments at northern latitudes (Lelieveld et al, 2002), has been demonstrated by a number of ground based studies E. Gerasopoulos et al.: Three-year ground based measurements of aerosol optical depth

Methods

Results

Conclusion