Seasonal variability of optical properties of aerosols in the Eastern Mediterranean

Abstract

The aerosol optical properties (scattering and absorption coefficients) were investigated at two remote locations in the Eastern Mediterranean in conjunction with aerosol ion composition measurements: Finokalia in the Crete Island in Greece (March 2001-June 2002) and Erdemli in Turkey (July 1999-June 2000). Ambient light-scattering coefficient (σsp-532 nm) at Finokalia had a mean value of 50±23 Mm-1 while at Erdemli this value was 90±160 Mm-1, due to a severe dust event that occurred from 17 to 19 April 2000. Scattering coefficients up to 5000 Mm -1 were encountered during the transition periods (spring and autumn) and were associated with dust storm events. During these events significant correlations were observed between dust and σsp and mass scattering efficiencies of 0.21 and 0.96 m2g-1 were calculated for dust for Finokalia and Erdemli, respectively. Significant correlations were also observed at both locations between non-sea-salt sulphate (nss-SO42-); σsp and mass scattering efficiencies of 5.9±1.8 and 5.7±1.4 m2g-1 were calculated for the nss-SO42- at Finokalia and Erdemli, respectively. At Finokalia absorption measurements were also performed at the same time and the mean absorption coefficient (σ ap-565 nm) was found to be 5.6±3.6 Mm-1. Maxima of absorption coefficient were associated with two distinct meteorological situations indicative of pollution transported from northern Europe and Saharan dust events. Saharan dust can therefore significantly contribute to both scattering and absorption of solar radiation, the latter due to its hematite content. Based on scattering and absorption measurements, an annual mean single-scattering albedo (ω) adjusted at 550 nm of 0.89±0.04 was calculated for Finokalia. Finally, radiative forcing efficiency (RFE) over the sea at 550 nm induced by aerosols has been calculated for Finokalia. RFE follows a clear seasonal variation, with the lowest mean values during summer (-73W m-2) and the highest during winter (-30W m2). Using aerosol optical thickness measurements in the area, we obtain radiative forcing estimates at the top of the atmosphere (TOA) ranging from -12.6 to -2.3 W m 2 for summer and winter, respectively. These values are up to five times higher than that induced by the greenhouse gases (2.4 Wm-2) but opposite in sign. © 2005 Elsevier Ltd. All rights reserved.