E. Giannakaki

EARLINET correlative measurements for CALIPSO: First intercomparison results

A strategy for European Aerosol Research Lidar Network (EARLINET) correlative measurements for Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) has been developed. These EARLINET correlative measurements started in June 2006 and are still in progress. Up to now, more than 4500 correlative files are available in the EARLINET database. Independent extinction and backscatter measurements carried out at high-performance EARLINET stations have been used for a quantitative comparison with CALIPSO level 1 data. Results demonstrate the good performance of CALIPSO and the absence of evident biases in the CALIPSO raw signals. The agreement is also good for the distribution of the differences for the attenuated backscatter at 532 nm ((CALIPSO-EARLINET)/EARLINET (%)), calculated in the 1–10 km altitude range, with a mean relative difference of 4.6%, a standard deviation of 50%, and a median value of 0.6%. A major Saharan dust outbreak lasting from 26 to 31 May 2008 has been used as a case study for showing first results in terms of comparison with CALIPSO level 2 data. A statistical analysis of dust properties, in terms of intensive optical properties (lidar ratios, Angstrom exponents, and color ratios), has been performed for this observational period. We obtained typical lidar ratios of the dust event of 49 ± 10 sr and 56 ± 7 sr at 355 and 532 nm, respectively. The extinction-related and backscatter-related Angstrom exponents were on the order of 0.15–0.17, which corresponds to respective color ratios of 0.91–0.95. This dust event has been used to show the methodology used for the investigation of spatial and temporal representativeness of measurements with polar-orbiting satellites.

EARLINET observations of the 14-22-May long-range dust transport event during SAMUM 2006: validation of results from dust transport modelling

We observed a long-range transport event of mineral dust from North Africa to South Europe during the Saharan Mineral Dust Experiment (SAMUM) 2006. Geometrical and optical properties of that dust plume were determined with Sun photometer of the Aerosol Robotic Network (AERONET) and Raman lidar near the North African source region, and with Sun photometers of AERONET and lidars of the European Aerosol Research Lidar Network (EARLINET) in the far field in Europe. Extinction-to-backscatter ratios of the dust plume over Morocco and Southern Europe do not differ. Ångström exponents increase with distance from Morocco. We simulated the transport, and geometrical and optical properties of the dust plume with a dust transport model. The model results and the experimental data show similar times regarding the appearance of the dust plume over each EARLINET site. Dust optical depth from the model agrees in most cases to particle optical depth measured with the Sun photometers. The vertical distribution of the mineral dust could be satisfactorily reproduced, if we use as benchmark the extinction profiles measured with lidar. In some cases we find differences. We assume that insufficient vertical resolution of the dust plume in the model calculations is one reason for these deviations.

EARLINET correlative measurements for CALIPSO

The European Aerosol Research Lidar Network (EARLINET) was established in 2000 to derive a comprehensive, quantitative, and statistically significant data base for the aerosol distribution on the European scale. At present, EARLINET consists of 25 stations: 16 Raman lidar stations, including 8 multi-wavelength Raman lidar stations which are used to retrieve aerosol microphysical properties. EARLINET performs a rigorous quality assurance program for instruments and evaluation algorithms. All stations measure simultaneously on a predefined schedule at three dates per week to obtain unbiased data for climatological studies. Since June 2006 the first backscatter lidar is operational aboard the CALIPSO satellite. EARLINET represents an excellent tool to validate CALIPSO lidar data on a continental scale. Aerosol extinction and lidar ratio measurements provided by the network will be particularly important for that validation. The measurement strategy of EARLINET is as follows: Measurements are performed at all stations within 80 km from the overpasses and additionally at the lidar station which is closest to the actually overpassed site. If a multi-wavelength Raman lidar station is overpassed then also the next closest 3+2 station performs a measurement. Altogether we performed more than 1000 correlative observations for CALIPSO between June 2006 and June 2007. Direct intercomparisons between CALIPSO profiles and attenuated backscatter profiles obtained by EARLINET lidars look very promising. Two measurement examples are used to discuss the potential of multi-wavelength Raman lidar observations for the validation and optimization of the CALIOP Scene Classification Algorithm. Correlative observations with multi-wavelength Raman lidars provide also the data base for a harmonization of the CALIPSO aerosol data and the data collected in future ESA lidar-in-space missions.

Estimation of the microphysical aerosol properties over Thessaloniki, Greece, during the SCOUT‐O3 campaign with the synergy of Raman lidar and Sun photometer data

[1] An experimental campaign was held at Thessaloniki, Greece (40.6°N, 22.9°E), in July 2006, in the framework of the integrated project Stratosphere‐Climate Links with Emphasis on the Upper Troposphere and Lower Stratosphere (SCOUT‐O3). One of the main objectives of the campaign was to determine the local aerosol properties and their impact on the UV irradiance at the Earth’s surface. In this article, we present vertically resolved microphysical aerosol properties retrieved from the inversion of optical data that were obtained from a combined one‐wavelength Raman/two‐wavelength backscatter lidar system and a CIMEL Sun photometer. A number of assumptions were undertaken to overcome the limitations of the existing optical input data needed for the retrieval of microphysical properties. We found acceptable agreement with Aerosol Robotic Network retrievals for the fine‐mode particle effective radius, which ranged between 0.11 and 0.19 for the campaign period. It is shown that under complex layering of the aerosols, general assumptions may result in unrealistic retrievals, especially in the presence of aged smoke aerosols. Furthermore, with this instrument setup, the inversion algorithm can also be applied successfully for the complex refractive index in cases of vertically homogeneous layers of continental polluted aerosols. For these inversion cases, the vertically resolved retrievals for the single‐scattering albedo resulted in values around 0.9 at 532 nm, which were in very good agreement with estimates from airborne in situ observations obtained in the vicinity of the lidar site.

EARLINET observations of the Eyjafjallajökull ash plume over Europe

EARLINET, the European Aerosol Research Lidar NETwork, established in 2000, is the first coordinated lidar network for tropospheric aerosol study on the continental scale. The network activity is based on scheduled measurements, a rigorous quality assurance program addressing both instruments and evaluation algorithms, and a standardised data exchange format. At present, the network includes 27 lidar stations distributed over Europe. EARLINET performed almost continuous measurements since 15 April 2010 in order to follow the evolution of the volcanic plume generated from the eruption of the Eyjafjallajökull volcano, providing the 4-dimensional distribution of the volcanic ash plume over Europe. During the 15-30 April period, volcanic particles were detected over Central Europe over a wide range of altitudes, from 10 km down to the local planetary boundary layer (PBL). Until 19 April, the volcanic plume transport toward South Europe was nearly completely blocked by the Alps. After 19 April volcanic particles were transported to the south and the southeast of Europe. Descending aerosol layers were typically observed all over Europe and intrusion of particles into the PBL was observed at almost each lidar site that was affected by the volcanic plume. A second event was observed over Portugal and Spain (6 May) and then over Italy on 9 May 2010. The volcanic plume was then observed again over Southern Germany on 11 May 2010.

Evaluation of CALIPSO’s Aerosol Classification Scheme During the ACEMED Experimental Campaign Over Greece: The Case Study of 9th of September 2011

In order to assess the validity of CALIPSO’s aerosol classification scheme, an experimental campaign called ACEMED (Evaluation of CALIPSO’s aerosol classification scheme over Eastern Mediterranean) has been organized over Greece on September 2011. In this study, we concentrate on the characterization of the aerosol load over Greece on 9th of September, using advanced in-situ aircraft instrumentation (onboard the FAAM-Bae146 aircraft of the UK Met Office). The analytical evaluation of CALIPSO’s aerosol-type classification scheme that is performed using synchronous/collocated satellite/airborne measurements, show a qualitatively reasonable performance of the CALIPSO’s aerosol classification scheme in the complex aerosol environment of the case under study, where smoke, continental, urban and dust aerosol components are present.

Vertical Separation of Aerosol Types Using of CALIPSO Level-2 Products

A first attempt is made to vertically separate profiles of optical parameters due to different aerosol types over Africa. The method applied makes use of particle backscatter profiles at 532 nm and vertically resolved linear particle depolarization ratio measurements at the same wavelength measured by space-borne CALIPSO lidar. Values of particle depolarization ratio of ‘pure’ aerosol types were taken from literature. A case of CALIPSO space-borne lidar system was selected on the basis of different mixing state of the atmosphere. To identify the origin of air-masses 4-day air mass back trajectories were computed using HYbrid Single-Particle Langrangian Integrated Trajectory (HYSPLIT) model, for different arrival heights, for the location and time under study.

Forest Fire Aerosols: Vertically Resolved Optical and Microphysical Properties and Mass Concentration from Lidar Observations

The influence of smoke on the aerosol loading in the free troposphere from EARLINET observations are examined in this paper. Several cases during 2001–2011 were identified over Thessaloniki and Athens, Greece, when very high aerosol optical depth values in the free troposphere were observed with a UV-Raman lidar. Particle dispersion modeling (FLEXPART) and satellite hot spot fire detection (ATSR) showed that these high free tropospheric aerosol optical depths are mainly attributed to the advection of smoke plumes from biomass burning regions. The biomass burning regions were found to extend across Russia in the latitudinal belt between 45°N and 55°N, as well as in Eastern Europe. The highest frequency of agricultural fires occurred during the summer season (mainly in August). Emphasis is also given on the 2007 wild fires surrounding Athens and earlier studies performed in the frame of EARLINET. The data collected allowed the optical and microphysical characterization of the smoke aerosols that arrived over Greece, where limited information has so far been available and in synergy with AERΟNET and CALIPSO observation a first attempt is made for the vertically resolved mass concentration of the smoke plumes.

Validation of CALIPSO level-2 products using a ground based lidar in Thessaloniki, Greece

We present initial aerosol validation results of the space-borne lidar CALIOP -onboard the CALIPSO satellite - Level 2 extinction coefficient profiles, using coincident observations performed with a ground-based lidar in Thessaloniki, Greece (40.5° N, 22.9° E, 50m above sea level). A ground-based backscatter/Raman lidar system is operating since 2000 at the Laboratory of Atmospheric Physics (LAP) in the framework of the European Aerosol Research LIdar NETwork (EARLINET), the first lidar network for tropospheric aerosol studies on a continental scale. Since July 2006, a total of 150 coincidental aerosol ground-based lidar measurements were performed over Thessaloniki during CALIPSO overpasses. The ground-based measurements were performed each time CALIPSO overpasses the station location within a maximum distance of 100 km. The duration of the ground-based lidar measurements was approximately two hours, centred on the satellite overpass time. The analysis was performed for 4 different horizontal resolutions of 5, 25, 45 and 105 km. For our analysis we have used Atmospheric Volume Description (AVD) array to screen out everything that is not an aerosol. Also, the cloud-aerosol discrimination (CAD) score, which provides a numerical confidence level for the classification of layers by the CALIOP cloud-aerosol discrimination algorithm was set between -80 and -100. CALIPSO extinction QC flags, which summarize the final state of the extinction retrieval, was also used. In our analysis we have used those measurements where the lidar ratio is unchanged (extinction QC = 0) during the extinction retrieval or it the retrieval is constrained (extinction QC = 1). The comparison was performed both for extinction and backscater coefficient profiles. For clear sky conditions, the comparison shows good performances of the CALIPSO on-board lidar.

Aerosol single scattering albedo retrieval with various techniques in the UV and visible wavelength range

The most important aerosol properties for determining aerosol effect in the solar radiation reaching the earths surface are the aerosol extinction optical depth and the single scattering albedo (SSA). Most of the latest studies, dealing with aerosol direct or indirect effects, are based on the analysis of aerosol optical depth in a regional or global scale, while SSA is typically assumed based on theoretical assumptions and not direct measurements. Especially for the retrieval of SSA in the UV wavelengths only limited work has been available in the literature. In the frame of SCOUT-O3 project, the variability of the aerosol SSA in the UV and visible range was investigated during an experimental campaign. The campaign took place in July 2006 at Thessaloniki, Greece, an urban environment with high temporal aerosol variability. SSA values were calculated using measured aerosol optical depth, direct and diffuse irradiance as input to radiative transfer models. The measurements were performed by co-located UV-MFRSR and AERONET CIMEL filter radiometers, as well as by two spectroradiometers. In addition, vertical aerosol profile measurements with LIDAR and in-situ information about the aerosol optical properties at ground level with a nephelometer and an aethalometer were available. The ground-based measurements revealed a strong diurnal cycle in the SSA measured in-situ at ground level (from 0.75 to 0.87 at 450nm), which could be related to the variability of the wind speed, the boundary layer height and the local aerosol emissions. The reasons for SSA differences obtained by different techniques are analyzed for the first time to provide recommendations for more accurate column SSA measurements.