Retrieval of the optical properties of tropospheric aerosols over Athens, Greece combining a 6-wavelength Raman-lidar and the CALIPSO VIS-NIR lidar system: Case-study analysis of a Saharan dust intrusion over the Eastern Mediterranean

The National Technical University of Athens (NTUA) 6-wavelength (355, 387, 407, 532, 607 and 1064 nm) Raman lidar system has been used to derive the aerosol optical properties (e.g. the lidar ratio, the aerosol backscatter and extinction profile) and the water vapour mixing ratios over Athens, Greece during the CALIPSO space lidar overpasses over our area at 355, 532 and 1064 nm. These data have been analyzed for the case of a Saharan dust intrusion over the Eastern Mediterranean, occurred on January 26, 2007, using concurrent aerosol optical depth (AOD) data at 550 nm from the Moderate Resolution Imaging Spectroradiometer (MODIS) and forecasted data from the Dust Regional Atmospheric Modeling (DREAM) model

Optical-microphysical Properties of Saharan Dust Aerosols and Composition Relationship Using a Multi-wavelength Raman Lidar, in Situ Sensors and Modelling: a Case Study Analysis

A strong Saharan dust event that occurred over the city of Athens, Greece (37.9° N, 23.6° E) between 27 March and 3 April 2009 was followed by a synergy of three instruments: a 6-wavelength Raman lidar, a CIMEL sun-sky radiometer and the MODIS sensor. The BSC-DREAM model was used to forecast the dust event and to simulate the vertical profiles of the aerosol concentration. Due to mixture of dust particles with low clouds during most of the reported period, the dust event could be followed by the lidar only during the cloud-free day of 2 April 2009. The lidar data obtained were used to retrieve the vertical profile of the optical (extinction and backscatter coefficients) properties of aerosols in the troposphere. The aerosol optical depth (AOD) values derived from the CIMEL ranged from 0.33-0.91 (355 nm) to 0.18-0.60 (532 nm), while the lidar ratio (LR) values retrieved from the Raman lidar ranged within 75-100 sr (355 nm) and 45-75 sr (532 nm). Inside a selected dust layer region, between 1.8 and 3.5 km height, mean LR values were 83 ± 7 and 54 ± 7 sr, at 355 and 532 nm, respectively, while the Ångström-backscatter-related (ABR 355/532) and Ångström-extinction-related (AER 355/532) were found larger than 1 (1.17 ± 0.08 and 1.11 ± 0.02, respectively), indicating mixing of dust with other particles. Additionally, a retrieval technique representing dust as a mixture of spheres and spheroids was used to derive the mean aerosol microphysical properties (mean and effective radius, number, surface and volume density, and mean refractive index) inside the selected atmospheric layers. Thus, the mean value of the retrieved refractive index was found to be 1.49( ± 0.10) + 0.007( ± 0.007)i, and that of the effective radiuses was 0.30 ± 0.18 μm. The final data set of the aerosol optical and microphysical properties along with the water vapor profiles obtained by Raman lidar were incorporated into the ISORROPIA II model to provide a possible aerosol composition consistent with the retrieved refractive index values. Thus, the inferred chemical properties showed 12-40% of dust content, sulfate composition of 16-60%, and organic carbon content of 15-64%, indicating a possible mixing of dust with haze and smoke. PM10 concentrations levels, PM10 composition results and SEM-EDX (Scanning Electron Microscope-Energy Dispersive X-ray) analysis results on sizes and mineralogy of particles from samples during the Saharan dust transport event were used to evaluate the retrieval

Six-month ground-based water vapour raman lidar measurements over Athens, Greece and system validation

Water vapour is one of the most important greenhouse gases, since it causes about two third of the natural greenhouse effect of the Earth's atmosphere. To improve the understanding of the role of the water vapour in the atmosphere, extensive water vapour profiles with high spatio-temporal resolution are therefore necessary. A ground-based Raman lidar system is used to perform water vapour measurements in Athens, Greece (37.9°N, 23.6°E, 200 m asi.). Water vapour mixing ratio measurements are retrieved from simultaneous inelastic H2O and N2 Raman backscatter lidar signals at 387 nm (from atmospheric N2) and 407 nm (from H2O). Systematic measurements are performed since September 2006. A new algorithm is used to retrieve water vapour vertical profiles in the lower troposphere (0.5-5 km range height asl.). The lidar observations are complemented with radiosonde measurements. Radiosonde data are obtained daily (at 00:00 UTC and 12:00 UTC) from the Hellenic Meteorological Service (HMS) of Greece which operates a meteorological station at the "Hellinikon" airport (37. 54° N, 23.44° E, 15m asl) in Athens, Greece. First results of the systematic intercomparison between water vapour profiles derived simultaneously by the Raman lidar and by radiosondes are presented and discussed

Aerosol absorption profiling from the synergy of lidar and sun-photometry : The ACTRIS-2 campaigns in Germany, Greece and Cyprus

© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).Aerosol absorption profiling is crucial for radiative transfer calculations and climate modelling. Here, we utilize the synergy of lidar with sun-photometer measurements to derive the absorption coefficient and single scattering albedo profiles during the ACTRIS-2 campaigns held in Germany, Greece and Cyprus. The remote sensing techniques are compared with in situ measurements in order to harmonize and validate the different methodologies and reduce the absorption profiling uncertainties.Peer reviewe

An adaptive base point algorithm for the retrieval of aerosol microphysical properties

We propose a new iterative algorithm for the retrieval of the microphysical properties of stratospheric and tropospheric aerosols from multiwavelength lidar data. We consider the basic equation as an ill-posed problem and solve the system derived from spline collocation via a Padé iteration. The algorithm takes special care of the fact that the reconstruction of the distribution via spline collocation is very sensitive to the choice of base points of the chosen spline basis. The algorithm makes use of this fact by changing the base points used for the spline collocation at certain iteration steps. In addition, the effects of projection to ensure a nonnegative solution are examined. We tested how well this and other algorithms are suitable for retrieving the complex refractive index of the particles as well. We also examine whether the algorithm is capable of distinguishing between different, very small imaginary parts of the refractive index, which is often a main problem in practice. Finally, the algorithm is applied to real multiwavelength Raman lidar data and our results are partially validated by the thermodynamic chemical model Isorropia II

Inter-comparison of lidar and ceilometer retrievals for aerosol and planetary boundary layer profiling over Athens, Greece

This study presents an inter-comparison of two active remote sensors (lidar and ceilometer) to determine the mixing layer height and structure of the Planetary Boundary Layer (PBL) and to retrieve tropospheric aerosol vertical profiles over Athens, Greece. This inter-comparison was performed under various strongly different aerosol loads/types (urban air pollution, biomass burning and Saharan dust event), implementing two different lidar systems (one portable Raymetrics S.A. lidar system running at 355 nm and one multi-wavelength Raman lidar system running at 355 nm, 532 nm and 1064 nm) and one CL31 Vaisala S.A. ceilometer (running at 910 nm). Spectral conversions of the ceilometer's data were performed using the Ångström exponent estimated by ultraviolet multi-filter radiometer (UV-MFR) measurements. The inter-comparison was based on two parameters: the mixing layer height determined by the presence of the suspended aerosols and the attenuated backscatter coefficient. Additionally, radiosonde data were used to derive the PBL height. In general, a good agreement was found between the ceilometer and the lidar techniques in both inter-compared parameters in the height range from 500 m to 5000 m, while the limitations of each instrument are also examined

Three+two Raman lidar system configuration for space-borne active remote sensing system validation over Athens, Greece, in the frame of the EARLINET-ASOS and ESA-CALIPSO projects

Routine lidar measurements of the aerosol vertical distribution have been performed over Athens, Greece using a multiwavelength (355-387-407-532-607-1064 nm) Raman lidar system, since 2006 in the frame of the EARLINET-ASOS (2006-2011) project. Additionally, since June 2006, correlative measurements for CALIOP space-borne lidar are performed. The aim of these measurements is to provide validation profiles for the CALIOP instrument in the lower and middle troposphere (0.5-8 km) in terms of the aerosol backscatter coefficients at 532 nm and 1064 nm, the color ratio (532/1064 nm) and the depolarization ratio at 532 nm, but mainly to provide information about the aerosol extinction profiles and the corresponding lidar ratios at 532 nm. From the available correlative CALIOP level-2 and multiwavelength Raman lidar aerosol data over Athens, we selected to present cases that have been identified as Saharan dust outbreaks and large biomass burning events, using air mass backward trajectories in order to characterize the source of the aerosols. We found that the vertical profiles of the aerosol optical properties between CALIOP and NTUA lidars were not always in a good agreement during the exact time of the satellite overpass, especially for daytime measurements, when the distance between the two instruments was greater than 40 km. An improvement was noticed when ground-based lidar measurements where performed previously or later than the CALIPSO overpass time. For the nighttime intercomparison the agreement between the two instruments was better during the CALIPSO overpass time. This was attributed mainly to the closer nighttime satellite track over the Athens lidar station

First water vapor measurement over Athens, Greece, obtained by a combined Raman-elastic backscatter lidar system

Water vapor is one of the most important greenhouse gases, since it causes about two third of the natural greenhouse effect of the Earth's atmosphere. To improve the understanding of the role of the water vapor in the atmosphere, extensive water vapor profiles with high spatio-temporal resolution are, therefore, necessary. A recently install ground-based Raman lidar system is used to perform systematic water vapor measurements in the lower troposphere (500-5000 m asl.) over Athens, Greece (37.9°N, 23.6°E, 200 m asl.) since September 2006. Water vapor mixing ratio measurements are retrieved from simultaneous inelastic H2O and N2 Raman backscatter lidar signals at 387 nm (from atmospheric N2) and 407 nm (from H2O). The lidar observations are intercompared with radiosonde data obtained at the "Hellinikon" airport (37. 54° N, 23.44° E, 15m asl.) by the Hellenic Meteorological Service (HMS). First preliminary results of the systematic intercomparison between water vapor profiles derived, simultaneously, by our Raman lidar and by the HMS radiosondes are presented and show that the absolute differences generally remain less than 10% up to 5000 m height. Selected cases of water vapor vertical distributions in the troposphere are presented extensively and discussed, in conjunction with water vapor data obtained by the AIRS spaceborn sensors

The relationship between aerosol backscatter coefficient and atmospheric relative humidity in an urban area over Athens, Greece, using raman lidar and radiosonde data

In this article a statistical assessment concerning the relationship between the aerosol backscatter coefficient (β aer) and the relative humidity (RH) in the lower andmiddle troposphere, over Athens (Greece), is presented. For the purpose of this study, correlative radiosonde and aerosol backscatter lidar data were analysed for a period of 4 years (January 2003-December 2006), as obtained in the framework of the European Aerosol Lidar Network (EARLINET) project. The vertical profiles of the aerosol backscatter coefficients were measured by a combined Raman/elastic lidar system at ultraviolet (355 nm) and visible (532 nm) wavelengths. The correlation coefficient (R) of the vertical profiles of the RH against the backscatter coefficient of aerosols was investigated in altitudes within the free troposphere (0-6000 m). The altitude range was divided into three areas: 0 m up to the top of the Planetary Boundary Layer (PBL); PBL up to PBL + 2000 m; and PBL + 2000 m up to 6000 m. The properties and seasonal variations of the height of the PBL were also studied. The annual mean PBL height over Athens was found to be (1320 ± 480) m, while during the warm period of the year (spring-summer) the PBL was higher than during the cold period (autumn-winter). Regarding the correlation coefficient (R), low (0-0.5) and medium (0.5-0.8) R values were mostly observed during the warmmonths of the year. For the aerosols originating from the Balkan area the highest correlation was observed at both wavelengths (R = 0.71 at 355 nm and R = 0.41 at 532 nm), especially during the years 2003 and 2005 (R = 0.61 at 355 nm and R = 0.93 at 532 nm). The almost linear correlation of this type of aerosols can be attributed to the fact that these remained for a longer time in a coherently alternating atmosphere, therefore having the tendency to become homogenized