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.
A lidar-based method was used to separate profiles of optical parameters due to different aerosol types over different
European Aerosol Research LIdar NETwork (EARLINET) stations. The method makes uses of particle backscatter
profiles at 532 nm and vertically resolved linear particle depolarization ratio measurements at the same wavelength.
Values of particle depolarization ratio of 'pure' aerosol types (Saharan dust, biomass burning aerosols, anthropogenic
aerosols, Volcanic ash aerosols) were taken from literature. Cases of CALIPSO space-borne lidar system were selected
on the basis of different mixing state of the atmosphere over EARLINET stations. To identify the origin of air-masses
four-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 was used. Also, the Dust REgional
Atmospheric Modeling (DREAM) model was used to identify cases where dust from Saharan region was affecting the
place under study. For our analysis we have used Atmospheric Volume Description (AVD), Cloud-Aerosol
Discrimination (CAD) and extinction Quality Control (QC) flags to screen out CALIOP data. The method was applied
for different horizontal resolution of 5, 25, 45 and 105 km. The height-resolved lidar results were finally compared with
column-integrated products obtained with Aerosol Robotic Network Sun photometer (AERONET) in order to see to
what extent Sun photometer columnar data are representative when different aerosol layers are present in the
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.
The arrival of the volcanic ash plume of the Eyjafjallajökull eruption was observed over Greece almost one week after its
major eruption (on April 14, 2010) with two multi-wavelength Raman lidar systems, members of the EARLINET
network. Intensive lidar measurements were performed throughout the event over Thessaloniki and Athens to derive the
optical properties of the ash aerosols in the troposphere. During April 21, 2010 two layers of volcanic ash were present
over Thessaloniki, one around 2.5 and one around 5 km height after circulating over central Europe. The first layer was
persistent but with variable thickness, while the thin layer observed at 5 km height disappeared after some hours. Later
on and at higher altitudes thin layers of ash were observed between 5 and 8 km, directly associated with the volcanic
eruption. The observed layer at around and 3 km was persistently observed till April 28. The volcanic ash was observed
over Athens, after passing over Southern Italy, during April and May 2010, in two height regions: between 6-10 km
height and between 4 km and the ground level. We found that this was directly linked to the maximum height of the
emitted volcanic ash. The most intensive period for ash presence over Athens was between April 21 and 23. In most
cases, ash layers were very well stratified in the form of filaments starting around 3-4 km down to 1.5 km height. Mixing
of ash with locally produced aerosols was frequently observed during the measuring period resulting to enhanced PM10
concentrations at ground level. Volcanic ash was also observed during May 10-11 and 17-19, 2010, after being
transported over Spain and Northern Italy. Both over Athens and Thessaloniki Saharan dust particles were mixed with
volcanic ones on certain days of May 2010, which resulted to more complicated structures of the aerosol layers observed
The most important aerosol properties for determining aerosol effect in the solar radiation reaching the earth's 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.
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.
Measurements performed with a backscatter and Raman lidar at Thessaloniki, Greece were used to characterize cirrus
clouds and aerosol layers by determining their optical properties. This is achieved through the application of different
post-processing algorithms. We retrieved the cirrus cloud's optical properties by using three independent mathematical
methods. In the first method, an iterative procedure was used based on the criterion that forward and backward
integration coincide to the desired degree of accuracy. In the second method, the optical depth of the cirrus cloud can be
determined by comparing the backscattering signals just bellow and above the cloud if the lidar signals are correctly
represent the scattering medium. The third method, the well known Raman method, is applicable to night time
measurements and is capable for determining the vertical profile of lidar ratio. The results are considerably influenced by
multiple scattering effects, that not taken into account and this leads to a significant underestimation of calculated cirrus
optical depths and lidar ratios. To estimate and correct this effect we have applied a radiative transfer model that
calculates the multiple scattering contributions for each cirrus case analyzed. We have compared the resulting optical
depths and lidar ratios and found a good agreement between these methods. The comparison has been performed to the
effective values of optical depth and lidar ratio.