Synergetic use of active and passive remote sensing techniques to retrieve vertical profiles of atmospheric aerosol properties during day- and night-time

Resumen

This PhD dissertation focuses on the characterization of atmospheric aerosols properties during day- and night-time combining active and passive remote sensing measurements. The study provides both column-integrated and vertically resolved aerosol properties. The key for this thesis is the combination of advanced ground-based remote sensing instrumentation. By one hand, it is used a multiwavelength Raman lidar that which measures both elastic and Raman signals, therefore, independent backscatter and extinction profiles can be calculated. The system operates twice a week in the frame of ACTRISE - ARLINET (Aerosols, Clouds, and Trace gases Research InfraStructure Network - European Aerosol Research LIdar NETwork) and more intensively during special field campaigns. But due to the low signal-to-noise ratio Raman measurements are only available during night-time, being the daytime measurements based on the use of multiwavelength elastic measurements. Additionally, continuous measurements during the entire year have been registered with a ceilometer in the infrared range. Finally, sun/sky photometry was used to obtain direct estimation of aerosol optical properties during daytime and lunar photometry for direct estimation of spectral aerosol optical depths during night-time. Experimental measurements used were collected during ChArMEx/ADRIMED (Chemistry-Aerosol Mediterranean Experiment/Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region) field campaign in 2013, and during SLOPE I (Sierra Nevada Lidar AerOsol Profiling Experiment) field campaign in 2016. Both field campaigns were developed at the Andalusian Global ObseRvatory of the Atmosphere (AGORA) that deployed instrumentation at three different sites at different altitudes near the city of Granada (Southeastern Spain): the UGR station located at Andalusian Institute for Earth System Research (IISTA-CEAMA) in the city at 680 m a.s.l.; and two mountain stations located at short distance in Sierra Nevada mountains: Cerro Poyos (CP) station at 1820 m a.s.l. and Sierra Nevada Station (SNS) at 2500 m a.s.l.. The atmospheric aerosol properties analyzed in this thesis are mainly retrieved by Generalized Retrieval of Aerosol and Surface Properties algorithm (GRASP), which was developed in the Laboratoire d’Optique Atmosphérique, Lille (France). GRASP is a versatile and flexible algorithm for the retrieval of aerosol microphysical properties from their optical properties, and in this thesis, it uses the combination of sun/sky photometer with lidar measurements to retrieve enhanced columnar aerosol microphysical properties that allow the separation between fine and coarse mode properties, and also vertically-resolved microphysical properties. The retrievals from GRASP algorithm are evaluated versus in-situ airborne measurements during ChArMEx/ADRIMED field campaign for a desert dust episode. GRASP was run using as inputs lidar and sun/sky photometer measurements acquired either in UGR or CP stations to explore the effect of incomplete overlap associated with lidar measurements. The extinction profiles at 532 nm retrieved by GRASP present differences less than 20% with respect the in-situ measurements on board the ATR-42 aircraft by CAPS (Cavity Attenuated Phase Shift). In the case when the dust layer was coupled to the aerosol layer close to surface, the total volume concentration differences between airborne in situ data and GRASP retrievals are 15% or 36%, depending on the use of the sun/sky measurements at UGR or at CP for the retrievals. In contrast, in the case when dust layer was decoupled from the aerosol layer close to the surface the differences are around 17% for both retrievals. GRASP algorithm is also used for retrieving continuous day- and night-time aerosol properties during a dust event registered during the SLOPE I campaign. For extending the use of GRASP to night-time, three different schemes have been proposed in this thesis. The first one assumes that there is no change in the aerosol column-integrated properties along the night, in this way the retrieval is done combining the night time elastic lidar measurements with the closest sun/sky measurements registered the day before or the day after. The second approach considers that the aerosol load in the vertical column can be monitored by lunar photometry, but assumes that there are no changes in the aerosol column-integrated intensive properties, that is this approach considers that there are no changes in the aerosol type. In this way, the night-time elastic lidar signal is combined with the aerosol optical depth measured by lunar photometer, fixing the aerosol complex refractive index and spherical particle fraction to the values retrieved from the closest daytime retrieval. The last approach uses night-time elastic lidar signals and aerosol optical depth measurements retrieved from a lunar photometer in combination with relative sky radiances at the lunar aureole, retrieved from a Sky images. The different approaches are applied to the evolution of a Saharan dust outbreak registered during SLOPE I field campaign. The dust plume evolution has been monitored and the results discussed with the different approaches. Furthermore, considering the availability of independent measurements of some aerosol properties at SNS station, GRASP retrievals at 2500m a.s.l. are compared with in-situ measurements obtained at SNS station, assuming that the proximity of this station allow us to consider that it is in the same vertical column explored by the lidar system operated at UGR. GRASP retrievals show coherent values when compared with AERONET retrievals, being observed a smooth and coherent day-to-night evolution. GRASP and Raman retrieved values agrees quite well, with differences below 30%. Generally, both GRASP retrievals and in-situ measurements follow the same patterns and are sensitive to the arrival of Saharan dust particles. Finally, it is presented a novel approach for the estimation of vertically-resolved aerosol concentrations from GRASP retrievals using combined measurements of ceilometer and sun/sky photometer measurements. Sensitivity tests for this configuration are performed with synthetic data to show the performance of this new methodology, especially for coarse particles. This new configuration is applied to measurements at UGR station and the retrievals are compared with in-situ airborne measurements acquired during the ChArMEx/ADRIMED field campaign. This study shows that the retrieved aerosol volume concentration profiles agree well with in-situ airborne measurements, being the mean differences within the uncertainty of GRASP retrievals. An additional validation of the methodology is based on the use of in-situ aerosol volume concentration registered at SNS station during the SLOPE I field campaign station. In this sense, the aerosol volume concentration retrieved at 2500m a.s.l. from the combination of ceilometer and sun/sky photometer present a high correlation with some trends to overestimation of the in-situ measurements obtained at SNS station.