Characterization and longitudinal study of aerosol particles using a synergy of lidar techniques and passive remote sensing methods in the Mediterranean basin

Abstract

The aim of this Doctoral Thesis is the study of the tropospheric aerosols by means ground based remote sensing and space-borne techniques, through statistical analysis of the retrieved aerosol optical properties. At the same time, advanced mathematical algorithms are applied to retrieve the aerosol microphysical properties while predictive models are used to obtain additional information, such as the source regions of the transported aerosols and their radiative forcing effect. In the opening chapter, Chapter 1, a theoretical background on atmospheric aerosols, their role in atmospheric physics and the different types of aerosols are presented, giving an overall view of the studied field. There is a variety of aerosol sources and particle sizes and therefore, aerosols can be found at different heights in the atmosphere. All atmospheric aerosols scatter incoming solar radiation, while a few ones can also absorb it. In the atmosphere, there is a mixture of both scattering and absorbing aerosols, and their net effect on Earth's energy budget depends on surface and cloud characteristics. All these aspects are shortly presented in this Chapter. In Chapter 2, except the vertical atmospheric structure, the fundamental aspects of atmospheric physic and optics are mentioned, focusing on the mechanisms of the atmospheric substances (aerosols and molecules) and their interactions with light. Absorption, transition, scattering, extinction, depolarization and fluorescence of light are the basic phenomena discussed here as a brief outline of the fundamental laws governing the transmission of light in the atmosphere centred around the Beer-Lambert law. The aerosol remote sensing techniques are included in Chapter 3, along with the available instrumentation used to obtain our results. Firstly, the lidar technique is schematically analysed and a full description of the lidar equation is presented. The relevant detection modes of the lidar signals and the different types of lidar instruments are presented as well as the instrumentation that is available at the Laser remote sensing unit of NTUA. Additionally, the lidar pre-processing methods along with aerosol data products are mentioned. Finally, a brief description of the Aerosol Robotic Network (AERONET) of sun photometer measurements is presented. Tools and modelling that exploit lidar satellite measurements which were used to enhance our findings are introduced and shortly presented in Chapter 4. The EARLINET Single Calculus Chain (SCC) is the tool used for retrieving the aerosol optical properties and the Spheroidal Inversion eXperiments (SphInX) software tool, developed at the University of Potsdam, provided the microphysical retrievals from lidar data inputs. Useful information about the HYSPLIT model which simulates the backward trajectory analysis, the Dust Regional Atmospheric Model (BSC-DREAM8b v2.0), the Library for radiative transfer (Libradtran) tool and satellite data is also provided. Our results and a comprehensive analysis are presented in Chapter 5. Firstly, we present a comprehensive analysis of the seasonal variability of the vertical profiles of the optical and geometrical properties of Saharan dust aerosols, observed in the height region between 1000 and 6000 m, over Athens, Greece, from February 2000 to September 2017. These nighttime observations were performed by the EOLE Raman lidar system under cloud-free conditions. Moreover, 4 years of lidar measurements of Saharan dust intrusions over the Mediterranean basin (2014-2017), obtained from 4 selected EARLINET stations (Granada, Potenza, Athens, Limassol) are studied in terms of aerosol optical, geometrical, mixing state and microphysical properties. Specific case studies are further analysed. Finally, simulations of the regional radiative forcing of dust events over Mediterranean are presented. The concluding remarks are given in Chapter 6, which is the final chapter of this Thesis.