[1] The temporal variability of aerosol optical properties is investigated over the broader Mediterranean basin, with emphasis on aerosol optical depth (AOD) that is an effective measure of aerosol load. The study is performed using Collection 005 Level-3 mean daily spectral aerosol data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on board the Terra and Aqua satellites, which cover the 6-year period from 2000 to 2006. The results of our analysis reveal a significant interannual variability of AOD in the study region. Specifically, the regional mean visible AOD over land and ocean has decreased over the period 2000-2006 by 20% in relative percentage terms (or by 0.04 in absolute terms). This tendency is statistically significant according to the Man-Kendall test. However, the decreasing tendency of AOD is not uniform over the whole basin. It appears mainly in the western parts of Iberian, Italian, and Balkan peninsulas (and coastal areas), as well as in the southern Anatolian peninsula. The analysis for summer (June to September) and winter (November to March) seasons revealed different tendencies in both AOD and precipitation. The summer-period AOD has decreased by 0.04 (or by 14%) probably due to decreased emission rates of anthropogenic pollution. In contrast, the winter AOD has increased by 0.03 (or 19%) mainly related to decreased precipitation (associated with an increasing tendency in the NAO index). The decreasing tendency in MODIS AOD is in good agreement with corresponding AOD tendencies based on data from Aerobot Robotic Network (AERONET) stations in the study region and ground based PM10 measurements at selected stations.

The global marine organic aerosol budget is investigated by a 3-dimensional chemistry-transport model considering recently proposed parameterisations of the primary marine organic aerosol (POA) and secondary organic aerosol (SOA) formation from the oxidation of marine volatile organic compounds. MODIS and SeaWiFS satellite data of Chlorophyll-a and ECMWF solar incoming radiation, wind speed, and temperature are driving the oceanic emissions in the model. Based on the adopted parameterisations, the SOA and the submicron POA marine sources are evaluated at about 5 Tgyr−1(∼1.5 Tg Cyr−1) and 7 to 8 Tgyr−1(∼4 Tg Cyr−1), respectively. The computed marine SOA originates from the dimethylsulfide oxidation (∼78%), the potentially formed dialkyl amine salts (∼21%), and marine hydrocarbon oxidation (∼0.1%). Comparison of calculations with observations indicates an additional marine source of soluble organic carbon that could be partially encountered by marine POA chemical ageing.

For the first time, the direct radiative effect (DRE) of aerosols on solar radiation is computed over the entire Mediterranean basin, one of the most climatically sensitive world regions, using a deterministic spectral radiation transfer model (RTM). The DRE effects on the outgoing shortwave radiation at the top of atmosphere (TOA), DRE_TOA, on the absorption of solar radiation in the atmospheric column, DRE_atm, and on the downward and absorbed surface solar radiation (SSR), DRE_surf and DRE_netsurf, respectively, are computed separately. The model uses input data for the period 2000–2007 for various surface and atmospheric parameters, taken from satellite (International Satellite Cloud Climatology Project, ISCCP-D2), Global Reanalysis projects (National Centers for Environmental Prediction – National Center for Atmospheric Research, NCEP/NCAR), and other global databases. The spectral aerosol optical properties (aerosol optical depth, AOD, asymmetry parameter, <i>g</i>_aer and single scattering albedo, &omega;_aer), are taken from the MODerate resolution Imaging Spectroradiometer (MODIS) of NASA (National Aeronautics and Space Administration) and they are supplemented by the Global Aerosol Data Set (GADS). The model SSR fluxes have been successfully validated against measurements from 80 surface stations of the Global Energy Balance Archive (GEBA) covering the period 2000–2007. <br><br> A planetary cooling is found above the Mediterranean on an annual basis (regional mean DRE_TOA = &minus;2.4 W m⁻²). Although a planetary cooling is found over most of the region, of up to −7 W m⁻², large positive DRE_TOA values (up to +25 W m⁻²) are found over North Africa, indicating a strong planetary warming, and a weaker warming over the Alps (+0.5 W m⁻²). Aerosols are found to increase the absorption of solar radiation in the atmospheric column over the region (DRE_atm = +11.1 W m⁻²) and to decrease SSR (DRE_surf = &minus;16.5 W m⁻² and DRE_netsurf&minus;13.5 W m⁻²) inducing thus significant atmospheric warming and surface radiative cooling. The calculated seasonal and monthly DREs are even larger, reaching −25.4 W m⁻² (for DRE_surf). Within the range of observed natural or anthropogenic variability of aerosol optical properties, AOD seems to be the main responsible parameter for modifications of regional aerosol radiative effects, which are found to be quasi-linearly dependent on AOD, &omega;_aer and <i>g</i>_aer

Recent variations in surface solar radiation (SSR) at the beginning of the 21st century (2000-2007) were determined at scales ranging from local/regional to hemispherical/global, on the basis of radiative transfer computations and information from satellites, reanalyses and surface measurements. Under all-sky conditions, in the Northern Hemisphere (NH) there is no clear dimming/brightening signal after 2000, whereas in the SH there is a more clear dimming arising from both increasing clouds and aerosols. Dimming is observed over land and ocean in the Southern Hemisphere (SH), and over oceans in the NH, whereas a slight brightening occurred over NH land. However, opposite tendencies are found even within the same continent, indicating the need to assess SSR changes at regional/local scales apart from hemispherical/global ones.