Atmospheric aerosols play very important roles in climate change and air particulate pollution. Lidars based on elastic scattering have been widely used to measure aerosol spatial distribution and to retrieve the profiles of aerosol optical properties by an assumption of the aerosol extinction-to-backscatter ratio. High Spectral Resolution Lidar (HSRL) is one of methods that can be used to measure aerosol optical properties without a-priori hypotheses. Compared to Raman lidar, HSRL has the advantage of day and night measurements and can be adapted to many kinds of carrying platforms.
Unlike ordinary elastic backscatter lidar, HSRL needs to separate the Mie signal scattered by atmospheric aerosol and the Rayleigh signal scattered by atmospheric molecules. Due to small spectral difference between Mie and Rayleigh signals, there are three difficulties: firstly, the laser source must have a narrow bandwidth, high energy and stable center wavelength; secondly, the receiver should have a very narrow spectral filter to separate aerosol scattering and molecular scattering; thirdly, the center wavelength of the receiver must be real-time locked to laser source.
In order to study the influence of system parameters on the measurement accuracy of a high spectral resolution lidar and to optimize their values, a simulation and analysis has been done and will be presented in this paper. In this paper, the system parameters including the linewidth of emission laser, the bandwidth of the Fabry–Pérot interferometric filter in the receiver and the spectral tracking accuracy between the receiver and laser are mainly analyzed. At the same time, several environmental factors have been considered, including atmospheric temperature and wind, pointing accuracy of platform, aerosol concentration range etc. A typical vertical distribution of atmospheric aerosol optical properties is considered and the received signals of high spectral channels are simulated. From the simulated signals, the aerosol optical properties are retrieved and the deviation relative to the input values is obtained. Under the conditions of given environmental factors, the relationship between system parameters of high spectral resolution lidar and relative error of retrieved aerosol optical properties is carried out.
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.
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 optical properties and the spatial distribution of the tropospheric aerosols over Naples under Saharan dust outbreaks
conditions have been studied by means of lidar measurements performed between May 2000 and August 2003 in the
frame of the EARLINET project. Climatological analysis of sand plume has been done by comparing normal and dust
affected conditions. Results in terms of backscattering and extinction coefficient as well as their integrated quantities
show that the aerosol load from the ground level up to 2 Km during Saharan dust transport events is almost the same of
normal conditions. This is probably due to the relevant widespread of local aerosol sources, such as vehicular traffic,
industrial activities, etc. Nevertheless, when sand outbreaks occur, the extinction to backscattering ratio, i.e. the lidar
ratio, clearly shows that the aerosol type in the lowest atmospheric layer changes. Moreover, Saharan dust transport
events strong increase both integrated backscatter and optical dept above 2 km.
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.
The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite had been launched in April 2006. Its main goal is to probe the vertical structure and to measure the properties of thin clouds and aerosols plume of the Earth's atmosphere. In order to validate the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) products, correlative measurements between CALIPSO and EARLINET stations have been planned in the framework of the validation campaign. At Napoli EARLINET station correlative measurement have been performed starting from the beginning of June 2006. Results obtained both during daytime with elastic lidar and nighttime with Raman lidar operating at two wavelengths (355nm and 532nm) are shown. The results of the application of a new algorithm to retrieve aerosol basckscattering and extinction coefficient backscattering from space and ground based elastic lidar signals are presented. Case study of Saharan dust outbreaks and cirrus clouds captured during correlative measurement runs are discussed.
During August-October 2005, several lidar measurements have been performed at many Italian lidar stations during the
Launch (International Lindenberg campaign for Assessment of hUmidity aNd Cloud profiling systems and its impact on
High-resolution modelling) campaign according with the IOP (Intensive Operational Period) schedule.
IOP 4 episode (1-3 October 2005) has been selected and investigated with a special attention to the variability of water
vapour mixing ratio vertical distribution to better understand the associated circulation in the coastal waters of the Gulf
of Naples (Tyrrhenian Sea).
A simulation is performed using the mesoscale model MM5 (version 3) model from PSU/NCAR1,2 for this study. Water
vapour mixing ratio observations have been detected in the Naples site (40°50'N, 14°11'E, 118 m asl) by a water vapour
Raman lidar which is also able to measure the backscatter profile at 355 and 532 nm and the extinction profile at 355
nm.
Water vapour mixing ratio vertical profiles retrieved by lidar have been compared with the MM5 water vapour mixing
ratio vertical profiles. The agreement between the model and the lidar data is good; all the modelled and the lidar profiles
are able to capture the cold air intrusion, that is the sharp decrease of the water vapour mixing ratio at about 2000 m.
Also the water-layer between 3000 and 8000 m is well captured. Moreover, both the model and the observations are able
to capture the land/sea breeze phenomena.
The characterization of a plasma plume is a key issue in laser ablation and deposition studies. Combined diagnostic
measurements by Optical Emission Spectroscopy, Fast Imaging have been used to study the dynamics and composition
of laser ablation plume produced during ultrashort laser irradiation of metals, in vacuum. Our results show that, in the
laser fluence range of 0.1-1.0 J/cm2, the process of matter removal results in a plasma plume which is mainly composed
of two different populations: atoms and nanoparticles. The nanoparticles dynamics during expansion has been analyzed
through their structureless continuum optical emission, while atomic species have been identified by their characteristic
emission lines. The presence of a fast atomic component emitted from the sample surface as a result of the supercritical
state induced by the intense ultrashort laser pulse irradiation has been also observed both by optical emission
spectroscopy and fast imaging techniques. Finally, atomic force microscopy analysis of the material deposited at room
temperature has allowed the characterization of the nanoparticles size distribution.
The international experiment EAQUATE (European AQUA Thermodynamic Experiment) was held in September 2004 in Italy and in the United Kingdom. The Italian phase, performed in the period 6-10 September 2004, was mainly devoted to assessment and validation of performances of new IR hyperspectral sensors and benefits from data and results of measurements of AQUA and in particular of AIRS. It is also connected with the preparatory actions of MetOp mission with particular attention to calibration and validation of IASI products (as water vapour and temperature profiles), characterization of semitransparent clouds and study of radiative balance, demonstrating the role of ground-based and airborne systems in validation operations.
The Italian phase of the campaign was carried out within a cooperation between NASA Langley Research Center, University of Wisconsin, the Istituto di Metodologie per l'Analisi Ambientale (CNR-IMAA), the Mediterranean Agency for Remote Sensing (MARS) and the Universities of Basilicata, Bologna and Napoli. It involved the participation of the Scaled Composites Proteus aircraft (with NAST thermal infrared interferometer and microwave radiometer, the Scanning HIS infrared interferometer, the FIRSC far-IR interferometer), an Earth Observing System-Direct Readout Station and several ground based instruments: four lidar systems, a microwave radiometer, two infrared spectrometers, and a ceilometer. Radiosonde launches for measurements of PTU and wind velocity and direction were also performed as ancillary observations. Four flights were successfully completed with two different AQUA overpasses. The aircraft flew over the Napoli, Potenza and Tito Scalo ground stations several times allowing the collection of coincident aircraft and in- situ observations.
Systematic Lidar measurements of tropospheric aerosols optical properties have been carried out in the urban area of Napoli (Southern Italy, 40°50’18”N, 14°10’59”E, 118 m above sea level), in the framework of EARLINET project. Lidar systematic measurements of aerosol backscattering and extinction profiles have been performed at laser wavelength of 351nm and were obtained from simultaneously measurements of elastic and N2 Raman signals. Following the EARLINET protocol, regular measurements have been performed three times a week in two days and they provide information on aerosol optical properties, with a final spatial resolution of 60m and a temporal resolution between 1 and 30 min. A statistical analysis in terms of integrated backscattering (BI), optical depth (OD), extinction to backscattering ratio (LR), and Dust Layer height (DL), obtained from measurements carried out in clear sky conditions over 30 months, has been realized. Further measurements have been performed during Saharan Dust transport events and some detailed observation of complete diurnal cycle has been carried out, in order to know the dynamic and the evolution of the Planetary Boundary Layer
Multiple-scattering effects sometime bias the ground-based lidar measurements, in particular for density aerosol and cirrus cloud. Both analytical and Montecarlo methods are very useful tools to study this influence. However, for analytical solution, it needs to make some hypotheses and the Montecarlo simulation is only a forward method. In this paper, an itinerative method is introduced based on Montecarlo simulation. Both extinction and backscattering coefficients, obtained by Raman lidar, are corrected for the multiple-scattering influence. For the typical cirrus cloud, the error of the multiple-scattering influence on extinction can be as large as 100%. However, it is negligible of the influence on backscattering coefficient. Therefore, the lidar ratio is also sensitive to the multiple-scattering effect.
An accurate transmitter-receiver alignment of a lidar system is essential to obtain correct results. At same time, the effective telescope area can influence the lidar returning signal, especially at the low range. In order to solve this problem, a fast and precise automatic alignment and data correction lidar system is presented, based on an emission- detection overlap model and ray tracing method. Using it we can get an aligned and whole range corrected lidar signal. A centering precision of few mrad has been obtained through a dedicated software automatic controlling a gimbals-mounted mirror. In particular the correctness of the center and of its error has been determined. While through the effective telescope area correction, got the atmosphere parameter as low as 60 m from the telescope. This system has been used in single wavelength direct detection mode and in tunable two- wavelength DIAL measurement. They were respectively applied for continuous monitoring of tropospheric aerosols and continuous measurement of the NO2.
A lidar system with the characteristic of simultaneously monitoring several atmospheric parameters such as atmospheric aerosols load, water vapor content, temperature and density, atmospheric transmissivity, and atmospheric pollutants concentration, has been realized. This system, based on two OPA lasers pumped by a Nd:YAG laser operating at maximum repetition rate of 100 Hz, is primary dedicated to tropospheric measurements. The system is continuously tunable over the spectral range 410 nm - 2500 nm, allowing DIAL measurements in a region where most of atmospheric pollutants display absorption lines. Preliminary measurements have been carried out. We report the simultaneous temporal evolution of the aerosol backscattering coefficient profiles at 355 nm and 829 nm in a rural area. A method to select optimal couples of wavelength for DIAL measurements in the near/mid IR spectral region has been developed. A sensitivity analysis has been carried out in order to find the minimum detectable concentration, that has been demonstrated of the order of few tenths up to few part per million in volume.
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