African desert dust eventes impact over southeastern Spain (2005-2010): aerosol radiative properties and forcing

The aim of this PhD thesis is to analyze the radiative properties of the atmospheric aerosol during the African dust events registered at Granada from 2005 to 2010, as well as their influence on radiative forcing both at the surface and at the top of the atmosphere (TOA). To accomplish this goal we have used mainly the measurements made with a sun-photometer. In contrast to measurements performed by in-situ or chemical analysis, the advantage of this remote sensing technique is the no modification of the aerosol properties during the measurement process. The sun-photometer used is included in the AERONET network (AErosol RObotic NETwork), sponsored by NASA, and operated continuously in radiometric station located at CEAMA (Andalusian Center of Environment) as part of the instrumentation used by the Atmospheric Physic Group of the University of Granada (GFAT). Major advances presented in this work are related to the implementation and application of a new methodology for deriving optical and microphysical properties of the aerosol in the atmospheric column using sky radiance in the principal plane configuration, and considering, from a methodological point of view, that the particles are not spherical. The following paragraphs summarize briefly the main specific objectives discussed in this work. After the introduction, where we show the relevance of the PhD thesis; in next chapters, the radiometric magnitudes and the procedure for deriving the Radiative Transfer Equation (ETR) used in the Earth's atmosphere studies are defined. In addition, the atmospheric aerosol and its possible classification by sources, the chemical composition and size distribution are described in detail. From instrumental point of view, the sun-photometer CIMEL, the methodology for measurements and its calibration processes are also detailed. Finally, other instruments used in this PhD thesis are also briefly described. In the Methodology Section, a detailed description of the procedure to derive the aerosol optical depth is included, besides the algorithm used for ETR inversion using the sky radiance measurements in principal plane configuration and taking into account the non sphericity of particles. Also, we analyzed the sensitivity of the inversion algorithm and the atmospheric aerosol properties obtained with those provided by the AERONET network. Additionally, the quality control procedure of the experimental data, the criteria used for the selection of days with African dust events affecting our radiometric station, and the methodology used to derive the radiative forcing at surface and TOA are also described in detail. The analysis of the optical and microphysical aerosol properties during African dust events is carried out far away from the source regions. In this sense, it may produce efficient mixing of mineral particles with other particles from sources in the air masses path. Also, the aerosol properties may change depending on the African dust source regions. To account for these effects, we classified the optical and microphysical properties according to potential origin sources and also applying a cluster analysis. Moreover, it is interesting to analyze the atmospheric aerosol properties at some distance of its source regions before mixing with particles originated in other regions. In this sense, the Alboran Island, located in between the North African coast and the Southern Iberian Peninsula, offers an appropriate place to perform such studies. The GFAT have installed in spring-summer-autumn a sun-photometer instrument at the remote place of the Alboran Island. Therefore, taking into account these sun-photometer measurements, we have analyzed the properties of atmospheric aerosol transported from North Africa before contacting air masses from other regions. The atmospheric aerosol radiative forcing computed at surface and TOA, at daily and seasonal temporal scales, is estimated using aerosol properties according to the classification of the origin sources. To carry out this objective, we have used the radiative transfer model SBDART in spectral range 310-2800 nm, using as input parameters the aerosol optical depth and the inversion parameters: single scattering albedo and asymmetry parameter. In addition, the aerosol radiative forcing efficiency (aerosol radiative forcing per unit of aerosol optical depth) has been computed and analyzed. Taking into account that our atmospheric station includes in-situ instrumentation for estimating atmospheric aerosol properties on the surface, we have compared the aerosol properties derived in the atmospheric column and at surface for African dust events and dust free days. This is performed using measurements made with a PSAP (Particle Soot Absorption Photometer) and an integrating nephelometer. These measurements enable to evaluate if the aerosol properties retrieval in the atmospheric column are reproduced at surface. Furthermore, in the literature, there are few studies that analyze the effects of atmospheric aerosol on UV spectral range during African dust events. In this sense, the results shown UVER transmittance (erythemal) as well as the results of applying an aerosol absorption post-correction method to the UVER algorithm employed in the OMI (Ozone Monitoring Instrument) sensor.