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
atmosphere.
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
over Greece.
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.
Lidar techniques represent the most suitable tool to obtain information on the aerosol vertical distribution and therefore
to close this kind of observational gap. Lidar networks are fundamental to study aerosol on large spatial scale and to
investigate transport and modification phenomena. These are the motivations why EARLINET, the European Aerosol
Research Lidar Network, was established in 2000. At present, EARLINET consists of 25 lidar stations: 7 single
backscatter lidar stations, 9 Raman lidar stations with the UV Raman channel for independent measurements of aerosol
extinction and backscatter, and 9 multiwavelength Raman lidar stations (elastic channel at 1064 nm, 532 nm, 355 nm,
Raman channels at 532 nm and 355 nm, plus depolarization channel at 532 nm) for the retrieval of aerosol microphysical
properties.
EARLINET data can significantly contribute to the quantification of aerosol concentrations, radiative properties, long-range
transport and budget, and prediction of future trends on European and global scale. It can also contribute to
improve model treatment on a wide range of scales and to a better exploitation of present and future satellite data.
EARLINET is playing an important role in the validation and in the full exploitation of the CALIPSO mission.
EARLINET started correlative measurements for CALIPSO since June 2006. A strategy for correlative measurements
has been defined on the base of the analysis of the high resolution ground track data provided by NASA. Results in terms
of comparisons between EARLINET and available CALIPSO products, both level 1 and level 2 data, are presented.
Coordinated lidar observations of Saharan dust over Europe are performed in the frame of the EARLINET-ASOS
(2006-2011) project, which comprises 25 stations: 16 Raman lidar stations, including 8 multi-wavelength
(3+2 station) Raman lidar stations, are used to retrieve the aerosol microphysical properties. Since
the launch of CALIOP, the two-wavelength lidar on board the CALIPSO satellite (June 2006) our lidar
network has been performing correlative aerosol measurements during CALIPSO overpasses over the
individual stations. In our presentation, we report on the correlative measurements obtained during Saharan
dust intrusions in the period from June 2006 to June 2008. We found that the number of dust events is
generally greatest in late spring, summer and early autumn periods, mainly in southern and south-eastern
Europe. A measurement example is presented that was analyzed to show the potential of a ground based lidar
network to follow a dust event over a specific study area, in correlation with the CALIOP measurements. The
dust transport over the studied area was simulated by the DREAM forecast model. Cross-section analyses of
CALIOP over the study area were used to assess the model performance for describing and forecasting the
vertical and horizontal distribution of the dust field over the Mediterranean. Our preliminary results can be
used to reveal the importance of the synergy between the CALIOP measurement and the dust model, assisted
by ground-based lidars, for clarifying the overall transport of dust over the European continent.
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 present knowledge of the aerosol distribution is not sufficient to estimate the aerosol influence on global and
regional environmental conditions and climate. This observational gap can be closed by using advanced laser remote
sensing. EARLINET (European Aerosol Research Lidar Network) is the first aerosol lidar network, established in 2000,
with the main goal to provide a comprehensive, quantitative, and statistically significant database for the aerosol
distribution on a continental scale. EARLINET is a coordinated network of European stations (25 at present) using advanced lidar methods for the vertical profiling of aerosols. The network activity is based on simultaneous scheduled
measurements, a rigorous quality assurance program addressing both instruments and evaluation algorithms, and a
standardised data exchange format. Further observations are performed to monitor special events.
EARLINET-ASOS (Advanced Sustainable Observation System) is a five year EC Project started in 2006, based on the
EARLINET infrastructure. The main objectives are: to make EARLINET a world-leading instrument for the observation
of the 4-D aerosol distribution on continental scale; to foster aerosol-related process studies, validation of satellite
sensors, model development and validation, assimilation of aerosol data into operational models; and to build a
comprehensive climatology of the aerosol distribution.
This study describes the atmospheric aerosol load encountered over a number of sites from the Southern Balkan region
with a relatively well-known air quality factor. Using the aerosol optical depth AOD, retrieved from the two Moderate
Resolution Imaging Spectroradiometers, MODIS, on board the Terra and Aqua NASA satellites, the aerosol content of
numerous sites is investigated under the scope of local pollution sources, inter-regional transport and large scale dust
and/or biomass burning events. The wide time range of seven full years of MODIS/Terra measurements permits the
discussion of possible climatological aspects as well. The MODIS AOD is further validated using ground-based Brewer
spectrophotometer measurements over a metropolis of Northern Greece, Thessaloniki. Thessaloniki is situated in a
unique sea-side location which inflicts it with high humidity and sea-salt particles, and is furthermore frequently affected
by biomass burning and desert dust aerosols arriving from surrounding sources. Local and regional pollution further
influences the quality of the local air and the observed tropospheric optical depth. The air masses responsible for either
transporting polluted air into the free troposphere or circulating boundary layer aerosol load around the region of Norther
Greece have been identified and discussed in detail.
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.
EARLINET-ASOS (European Aerosol Research Lidar Network - Advanced Sustainable Observation System) is a 5-year EC Project started in 2006. Based on the EARLINET infrastructure, it will provide appropriate tools to improve the quality and availability of the continuous observations. The EARLINET multi-year continental scale data set is an excellent instrument to assess the impact of aerosols on the European and global environment and to support future satellite missions. The project is addressed in optimizing instruments and algorithms existing within EARLINET-ASOS, exchanging expertise, with the main goal to build a database with high quality aerosol data. In particular, the optimization of the algorithms for the retrieval of the aerosol optical and microphysical properties is a crucial activity. The main objective is to provide all partners with the possibility to use a common processing chain for the evaluation of their data, from raw signals to final products. Raw signals may come from different types of systems, and final products are profiles of optical properties, like backscatter and extinction, and, if the instrument properties permit, of microphysical properties. This will have a strong impact on the scientific community because data with homogeneous well characterized quality will be made available in nearly real time.
EARLINET, the European Aerosol Research Lidar Network, is the first aerosol lidar network, established in 2000, with the main goal to provide a comprehensive, quantitative, and statistically significant data base for the aerosol distribution on a continental scale. At present, 23 stations distributed over Europe are part of the network. The EARLINET-ASOS (Advanced Sustainable Observation System) EC Project, starting on the EARLINET infrastructure, will contribute to the improvement of continuing observations and methodological developments that are urgently needed to provide the multi-year continental scale data set necessary to assess the impact of aerosols on the European and global environment and to support future satellite missions. The main objective of EARLINET-ASOS 5-year project, started on 1 March 2006, is to improve the EARLINET infrastructure resulting in a better spatial and temporal coverage of the observations, continuous quality control for the complete observation system, and fast availability of standardized data products. This will be reached by defining and using common standards for instruments, operation procedures, observation schemes, data processing including advanced retrieval algorithms, and dissemination of data. The expected outcome is the most comprehensive data source for the 4-D spatio-temporal distribution of aerosols on a continental scale.
Spectral measurements of solar global and direct UV irradiance are routinely conducted at Thessaloniki, Greece with a double monochromator Brewer spectroradiometer. From absolutely calibrated direct and global irradiance spectra, aerosol optical depth and diffuse irradiance spectra between 300 nm and 365 nm are derived. The modification of the global and diffuse irradiance and of the relationship between direct and diffuse irradiance by the aerosol single scattering albedo (SSA) is investigated with respect to aerosol optical depth and solar zenith angle using radiative transfer model calculations. Model calculations were used also to investigate the effect of SSA on the radiance distribution in the ultraviolet. The model-derived relations are compared with measurements, with the aim to establish an indirect method of deriving an effective single scattering albedo from spectral measurements of the direct and global irradiance. The uncertainties introduced by different sources into the SSA estimates are discussed. Finally the effective SSA is determined for two days with different aerosol amounts and composition. The overall accuracy in determining indirectly the SSA depends strongly on the amount of aerosols varying between 0.1 and 0.2 units of SSA.
A sensitivity analysis is performed for the expected changes to the photodissociation rates of various key species that are relative to tropospheric chemistry, due to changes in the radiative transfer, caused by contrails and clouds generated from the use of hydrogen as fuel. The results are compared to those which correspond to contrails from conventional aircraft. For the determination of cirrus clouds as a model input the cirrus clouds climatology developed in the frame of the ISCPP has been used. The optical parameters of the contrails generated by conventional fuel and hydrogen fuel are adopted from the recent literature. The mean total ozone field, which corresponds to present status and has been used as an input is based on both TOMS and ground-based data, while for the future projections the IPCC 1999 scenario has been used. The Tropospheric Ultraviolet and Visible (TUV) radiative transfer model has been used, to calculate the actinic flux in the UV and visible part of the spectrum, at 3-km height steps in the troposphere. For this purpose, the model has been initially tested against spectral measurements, and an agreement of +/- 10% has been achieved when the appropriate input parameters are well documented. The different optical properties of the aerosols and clouds have been considered in the model calculations. For the calculation of the photodissociation rates the latest available chemicokinetical data have been incorporated into the model. It was found that the additional perturbations induced from contrails are larger for the kerosene contrails compared to the ones induced by LH2 contrails, since they are expected to have smaller optical depths.
The diurnal variation of the Planetary Boundary Layer (PBL) structure and of the vertical profiles of the suspended particlates attenuation coefficient, are presented for the case of a semi-rural area near the north-eastern exit of the Athenian basin. These data were obtained using a ground-based LIDAR system operating at 355 nm and 532 nm, during the MEDCAPHOT-TRACE Campaign in the summer of 1994. The LIDAR system was pointing vertically and was operated during various air pollution (low up to severe photochemical episodes) and meteorological conditions (synoptic flow circulation, land-, and sea-breeze circulation). The LIDAR data are intercompared with meteorological and photochemical pollution data obtained by an instrumented aircraft overflying the Athens basin.
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