AERONET observations of direct and indirect aerosol effects over a South European conurbation

Sun photometer measurements over the Thessaloniki AErosol RObotic NETwork (AERONET) site were conducted during 2005-2007 and provide a method for determining the temporal variability of aerosol optical properties. Monthly mean aerosol optical thickness (AOT) showed higher values during the warmer months and lower values during the colder months. The single scattering albedo (SSA) and also the imaginary part of the refractive index (k) indicate more absorbing particles during autumn/winter than summer/spring. Aerosol direct radiative forcing measurements are also presented. The monthly change in aerosol radiative forcing was examined at the top of the atmosphere (TOA; from -8.0 to -28.0 W m-2), at the bottom of the atmosphere (BOA; from -19.0 to -55.0 W m-2) and within the atmosphere (ATM; from 11.0 to 35.0 W m-2); the ATM is affected by both the SSA and k. We also investigated the interaction between aerosol optical properties over Thessaloniki and cloud parameters over a region of the Northern Aegean. For the autumn and spring mean monthly values, high correlation coefficients were found between AOT and cloud fraction, the SSA and cloud fraction, AOT and cloud condensation nuclei per square centimetre (CCN/cm2), and cloud optical depth and AOT, and finally a negative correlation was found between AOT and cloud effective radius.