The potential of spaceborne lidar to monitor aerosol layering and mixing with high vertical resolution is reviewed. An overview is presented on aerosol lidar techniques of past, present, and future NASA and ESA lidar missions. The potential of a standard backscatter lidar (LITE, 1994), a backscatter/polarization lidar (CALIPSO, since 2006), a polarization High Spectral Resolution Lidar (HSRL, ATLID, EARTHCARE), and of a multiwavelength polarization HSRL are discussed regarding their ability to derive height profiles of optical and microphysical properties of aerosols and to resolve aerosol types and mixtures as a function of height. The important role of ground-truth activities is emphasized. Measurement examples taken with ground-based lidars illustrate that these systems provide a detailed characterization of complex aerosol scenarios in contrast to the snapshot-like observations with spaceborne lidars.
Integrated approach has been adopted at the ADvanced Environmental Research Center (ADEMRC), Gwangju Institute
of Science and Technology (GIST), Korea for effective monitoring of atmospheric aerosol. Various active and passive
optical remote sensing techniques such as multi-wavelength (3β+2α+1δ) Raman LIDAR, sun-photometry, MAX-DOAS,
and satellite retrieval have been utilized. This integrated monitoring system approach combined with in-situ surface
measurement is to allow better characterization of physical and optical properties of atmospheric aerosol. Information on
the vertical distribution and microphysical properties of atmospheric aerosol is important for understanding its transport
characteristics as well as radiative effect. The GIST multi-wavelength (3β + 2α+1δ) Raman lidar system can measure
vertical profiles of optical properties of atmospheric aerosols such as extinction coefficients at 355 and 532nm, particle
backscatter coefficients at 355, 532 and 1064 nm, and depolarization ratio at 532nm. The incomplete overlap between
the telescope field-of-view and beam divergence of the transmitting laser significantly affects lidar measurement,
resulting in higher uncertainty near the surface where atmospheric aerosols of interest are concentrated. Differential
Optical Absorption Spectroscopy (DOAS) technique is applied as a complementary tool for the detection of atmospheric
aerosols near the surface. The passive Multi-Axis DOAS (MAX-DOAS) technique uses scattered sunlight as a light
source from several viewing directions. Recently developed aerosol retrieval algorithm based on O4 slant column
densities (SCDs) measured at UV and visible wavelengths has been utilized to derive aerosol information (e.g., aerosol
optical depth (AOD) and aerosol extinction coefficients (AECs)) in the lower troposphere. The aerosol extinction
coefficient at 356 nm was retrieved for the 0-1 and 1-2 km layers based on the MAX-DOAS measurements using the
retrieval algorithm. Ground-based measurements of tropospheric aerosol using multi-wavelength Raman lidar system
and a mobile MAX-DOAS system had been carried out at the Gwangju Institute of Science and Technology (GIST). To
evaluate the performance of the integrated measurement system (Lidar + MAX-DOAS), an aerosol retrieval method
called STAR (satellite aerosol retrieval) has been applied to compare the satellite AOD products with those based on the
Raman lidar and MAX-DOAS measurements. It allows complete monitoring of atmospheric aerosols' vertical profiles
for better estimation of their radiative effects on atmospheric environment and climate change.
In inelastic Raman scattering the scattered signal consists of radiation that has undergone a frequency shift which is
characteristic for the stationary energy states of an irradiated molecule. Nowadays, Raman and fluorescence
spectroscopy is commonly used in chemistry. Information on the radiation that results from transition between the
vibrational energy states of the excited molecules, respectively, is specific to the chemical bonds and symmetry of
molecules. This radiation therefore provides unique information regarding the irradiated molecule according to which the
molecular species can be identified. Raman spectroscopy represents a particularly powerful tool for laser remote sensing
because it allows us to both identify and quantify the trace constituent relative to the major constituents of a mixture.
In this paper we present a multi-channel spectrometric lidar system which allows us to measure Raman and fluorescence
spectrums that give us information on chemical signatures characteristic for chemical components of aerosol particles
and pollutions. In the following, we describe the methodology, the system and we show experimental results.
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 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.
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.
Multiwavelength Raman lidar observations have matured into a powerful tool for the vertical resolved characterization of optical and microphysical properties of atmospheric aerosol particles. Raman lidars that operate with laser pulses at three wavelengths are the minimum requirement for a comprehensive particle characterization. Parameters that are derived with such systems are particle backscatter and extinction coefficients, and particle extinction-to-backscatter (lidar) ratios. Effective radius and complex refractive index can be derived with inversion algorithms. In the past ten years we carried out regular observations over Leipzig, Germany, with multiwavelength Raman lidar. We could
establish a time series of important aerosol properties. For instance, we find that pollution layers are present in the free troposphere in more than 30% of our observations in each year. These layers result from long-range transport of, e.g., forest-fire smoke from North America and Siberia, anthropogenic pollution from North America, Arctic haze from North polar areas, and mineral dust from the Sahara. Observations were also carried out with our mobile six-wavelength Raman lidar during several international field campaigns since 1997. Those data allow us to establish a first comprehensive overview on the vertical distribution of optical and microphysical particle properties in different areas of the world.
We have explored a multi (dual))-wavelength Raman-lidar observation of tropospheric aerosols and clouds at Tokyo University of Marine Science and Technology (TUMST) in Tokyo, Japan since the February of 2002. Here we present the results on lofted Asian dust and Siberian forest-fire smoke plumes occured in the spring of 2003. The Asian-dust layer shows that the particle depolarization ratio (PDR) at 532 nm is higher than -20%, and the extinction-to-backscatter ratio (lidar ratio) at 355 nm is ~49 sr, which is close to that at 532 nm of ~43 sr. On the contrary, the smoke layers show that the PDR is lower than ~10% or less, and the lidar ratio at 355 nm is ~40 sr, which is considerably lower that that at 532 nm of ~65 sr. We also applied an inversion algorithm for the smoke case. The effective radius and the single-scattering albedo were found as ~0.22μm and ~0.95 at 532 nm. These microphysical results are consistent with the ones retrieved from the collocated skyradiometer analysis. We also estimated the aerosol radiative forcing for the smoke case using the radiative transfer code with these height-resolved microphysical properties.
The transportable scanning six-wavelength eleven-channel aerosol lidar of the Insitute for Tropospheric Research represents the most powerful tool for a comprehensive characterization of atmospheric particles with lidar. Particle backscatter coefficients are determined at 6 wavelengths between 355 and 1064 nm. Particle extinction coefficients are determined at 355 and 532 nm. The instrument makes use of the elastic backscatter, Raman lidar, and scanning lidar technique. The physical particle parameters including the single-scattering albedo are retrieved from the optical data with an inversion scheme based on Tikhonov's inversion with regularization. The optical and physical parameter allow to perform radiative impact studies on the basis of lidar observations. The system was successfully operated in the Aerosol Characterization Experiment 2 (ACE 2) and the Indian Ocean Experiment (INDOEX). A measurement example taken from the Lindenberg Aerosol Characterization Experiment 98 (LACE 98) exemplifies the potential of this instrument.
A transportable scanning multiwavelength lidar has been installed for the independent and simultaneous determination of the particle backscatter coefficient at 6 wavelengths between 355 and 1064 nm and of the particle extinction coefficient at 355 and 532 nm. The physical particle parameters including the complex refractive index are retrieved from the optical data by an inversion scheme based on the Tikhonov's regularization technique. The optical and physical parameter sets serve as input in radiative transfer calculations to estimate the radiative forcing of the particles at the top of the atmosphere and at the surface. Quite different particle properties could be observed during the Aerosol Characterization Experiment (Portugal, 1997), the Lindenberger Aerosol Experiment (Germany, 1998) and the Indian Ocean Experiment (Maldives, 1999-2000). We present measurement examples which demonstrates this approach of comprehensive aerosol characterization.
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