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Direct aerosol radiative forcing ARF and cloud radiative forcing CRF over the Aegean Sea are investigated using 1 year of almost continuous observations of aerosol optical properties, cloud data and Clouds and the Earth's Radiant Energy System CERES satellite short-wave SW and long-wave LW fluxes during 2005–2006. Aerosol optical depth AOD values from moderate resolution imaging spectroradiometer MODIS over the Aegean Sea are found to be high during summer and spring months. This period of the year, the air masses come predominantly from the eastern and northeastern part of Europe. The monthly mean ARF at the top of the atmosphere TOA over the Aegean Sea resulted in net cooling and ranged from −6.0 to −15.8 W m−2, within the range of results from previous studies. The presence of fine particles greatly influenced the ARF. The CRF was also characterized by net cooling and ranged from −23 to −84 W m−2 and appeared to be driven primarily by changes in cloud fraction. An apparent aerosol weekly cycle with lower values during the weekend as well as a cloud fraction weekly cycle with lower values during the weekend is observed over the Aegean Sea, which, if confirmed, could result in net TOA forcing of around −20 W m−2.

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.

[1] An experimental campaign was held at Thessaloniki, Greece (40.6°N, 22.9°E), in July 2006, in the framework of the integrated project Stratosphere‐Climate Links with Emphasis on the Upper Troposphere and Lower Stratosphere (SCOUT‐O3). One of the main objectives of the campaign was to determine the local aerosol properties and their impact on the UV irradiance at the Earth’s surface. In this article, we present vertically resolved microphysical aerosol properties retrieved from the inversion of optical data that were obtained from a combined one‐wavelength Raman/two‐wavelength backscatter lidar system and a CIMEL Sun photometer. A number of assumptions were undertaken to overcome the limitations of the existing optical input data needed for the retrieval of microphysical properties. We found acceptable agreement with Aerosol Robotic Network retrievals for the fine‐mode particle effective radius, which ranged between 0.11 and 0.19 for the campaign period. It is shown that under complex layering of the aerosols, general assumptions may result in unrealistic retrievals, especially in the presence of aged smoke aerosols. Furthermore, with this instrument setup, the inversion algorithm can also be applied successfully for the complex refractive index in cases of vertically homogeneous layers of continental polluted aerosols. For these inversion cases, the vertically resolved retrievals for the single‐scattering albedo resulted in values around 0.9 at 532 nm, which were in very good agreement with estimates from airborne in situ observations obtained in the vicinity of the lidar site.

Spectral measurements of the aerosol optical depth (AOD) and the Ångström coefficient were conducted at Thessaloniki, Greece (40.5° N, 22.9° E) between January 1997 and December 2005 with a Brewer MKIII double-monochromator spectroradiometer. The dataset was compared with collocated measurements of a second spectroradiometer (Brewer MKII) and a CIMEL sun-photometer, showing correlations of 0.93 and 0.98, respectively. A seasonal variation of the AOD was observed at Thessaloniki, with AOD values at 340 nm of 0.52 and 0.28 for August and December respectively. Back trajectories of air masses for up to 4 days were used to assess the influence of long-range transport from various regions to the aerosol load over Thessaloniki. It is shown that part of the observed seasonality can be attributed to air masses with high AOD originating from North-Eastern and Eastern directions during summertime. The analysis of the long-term record (9 years) of AOD showed a downward tendency. A similar decreasing tendency was found in the record of the PM$_{10}$ aerosol measurements, which are conducted near the surface at 4 air-quality monitoring stations in the area of the city of Thessaloniki.