Dust specific extinction cross-sections over the Eastern Mediterranean using the BSC-DREAM model and sun photometer data: the case of urban environments

In this study, aerosol optical depth (AOD) measurements, from a MFR sun photometer operating in Athens, were compared with columnar dust loading estimations, from the BSC-DREAM model, during identified dust events, in order to extract the typical specific extinction crosssection for dust over the area. The selected urban environment of Athens provided us with the opportunity to investigate the mixing of dust and urban pollution and to estimate the contribution of the latter. The specific extinction cross-section for dust at 500 nm was found to be equal to σ ∗ 500=0.64±0.04 m 2 g, typical for medium to large distances from dust sources, with weak wavelength dependence in the visible and near infrared band (0.4–0.9μm). The model showed a tendency to underpredict AOD levels for increasing values of the Ångström exponent, indicative of fine particles of anthropogenic origin inside the boundary layer. On average we found an AOD under-prediction of 10–15% for Ångström exponents in the range of 0 to 1 and 30–40% in the range of 1 to 2. Additionally, modelled surface concentrations were evaluated against surface PM10 measurements. Model values were lower than measured surface concentrations by 30% which, in conjunction with large scatter, inCorrespondence to: E. Gerasopoulos ([email protected]) dicated that the effect of the boundary layer anthropogenic contribution to columnar dust loadings is amplified near the ground.