In this work some features are revealed, simultaneously by LIDAR and contact facilities, of the aerosol pollutions near high-traffic sites in Sofia City. Such features of interest are the extinction coefficient of the aerosol ensembles and their mass concentration, as well as the shapes and sizes and the structure of the aerosol particles and their chemical composition and biological content. They outline possible negative effects of the aerosol pollutants on the human health. The experiments are conducted in the spring and summer, in different sites of interest in Sofia City. The results about the aerosol mass concentration-to-extinction calibration of the LIDAR constant, obtained in the spring and in the late spring and the summer, are consistent with each other and with the combined spring-and-summer result. Using electron microscopy, various aerosol samples are visualized resembling mineral dust particles, pollen, soot, and even microorganisms. The results from the analysis of the images obtained would serve as guidelines of further chemical and microbiological studies.
Atmospheric aerosol is known to considerably influence the Earth’s radiative budget and to make an impact on air quality. The influence of aerosols strongly depends on their spatial distribution and optical properties. The aerosol has natural and anthropogenic origin. Aerosol types can be also classified according to their size, sources or geographical origin (desert, continental, marine etc.). Mineral dust is one of the natural aerosols presented in the atmosphere. Its main source is the Sahara desert region. Saharan aerosol layers are frequently observed in Europe by means of active and passive remote sensing devices, especially in the frame of EARLINET and ACTRIS 3, 5, 6, 7, 8, 9. In this paper, observations of vertical distribution of aerosols and assessment of their optical properties will be presented. Two-year (2013-2014) complex measurements were carried out by a ceilometer CHM-15k (Jenoptic) and two lidars in an urban area located in a mountain valley (Sofia, Bulgaria)1. The ceilometer works 24 hours in automatic mode. Part of the results is compared with results obtained by lidars operating in photon counting modes for specific periods of simultaneous work5. Supplementary data from: two meteorological stations; HYSPLIT back trajectory model4; BSCDREAM8b dust model9; and the database of atmospheric radio sounding profiles from Department of Atmospheric Engineering of Wyoming University (USA) are also used in the analysis of the obtained results.
The distribution of aerosol load in the atmosphere due to two forest fires near Sofia (the capital city of Bulgaria) was studied using two aerosol lidars which operated at 510.6 nm and 1064 nm. Experimental data is presented as 2D-heatmaps of the evolution of attenuated backscatter coefficient profiles and mean profile of the aerosol backscatter coefficient, calculated for each lidar observation. Backscatter related Angstrom exponent was used as a criterion in particle size estimation of detected smoke layers. Calculated minimal values at altitudes where the aerosol layer was observed corresponded to predominant fraction of coarse aerosol. Dust-transport forecast maps and calculations of backward trajectories were employed to make conclusions about aerosol’s origin. They confirmed the local transport of smoke aerosol over the city and lidar station. DREAM forecast maps predicted neither cloud cover, nor Saharan load in the air above Sofia on the days of measurements. The results of lidar observations are discussed in conjunction with meteorological situation, aiming to better explain the reason for the observed aerosol stratification. The data of regular radio sounding of the atmosphere showed a characteristic behavior with small differences of the values between the air temperature and dew-point temperature profiles at aerosol smoke layer altitude. So the resulting stratification revealed the existence of atmospheric layers with aerosol trapping properties.
We present a lidar measurement of atmospheric extinction coefficient. The measurement is performed by inversion of the backscatter lidar signal at wavelengths 3’000nm and 3’500nm. The inversion of the backscatter lidar signal was performed with constant extinction-to-backscatter ration values of 104 and exponential factor 0.1.
A procedure to approach synthesized profile of lidar returns from hypothetical Rayleigh atmosphere to real lidar data is described in this work. The procedure is used in preprocessing of lidar data to determine the magnitude of background noise which should be removed before starting inversion processing (by Klett, or Fernald algorithm) to retrieve atmospheric backscatter or extinction profiles. Some applications of Rayleigh-fit procedure on real lidar data are discussed. After removing background noise from the lidar data, atmospheric backscatter profiles at two nearby laser wavelengths 510.6 nm and 578.2 nm (CuBr-vapor laser) are calculated. The ratio of these two backscatter profiles is used as justification of the results of Rayleigh-fit procedure for background noise removal.
Aerosols, clouds and aerosol-cloud interactions are recognized as the key factors influencing the climate. Clouds are the primary modulators of the Earth’s radiative budget. This paper focuses on the detection of high-altitude aerosol layers in the troposphere over mid-latitude lidar station in Sofia, Bulgaria. They are situated in the height-region 6 km÷16 km, with thickness in the range 0.2 km÷5 km and have varying optical characteristics. On the basis of the general utilized classification of the Cirrus clouds, high values of the calculated atmospheric backscatter coefficient and Angströmexponent estimation results we conclude that the registered strongly scattered aerosol layers are Cirrus clouds. Lidar measurements are performed with an aerosol lidar, equipped with Nd:YAG laser at wavelengths 532 nm and 1064 nm. Mainly, lidar data are presented in terms of vertical atmospheric backscatter coefficient profiles. We also include 2Dcolormap in height-time coordinates build on the basis of so called range corrected signals. It shows in general changes of the aerosol stratification over the lidar station during the measurement period. We employed HYSPLIT backward trajectories and DREAM forecasts to analyze the lidar profile outlines and characterize the events during which Cirrus cloud samples were observed. So was remarked that most of the results were obtained during Saharan dust long-way transport over the city of Sofia. Reported experimental examples are extracted from regular lidar investigations of the atmosphere within the frame of European project EARLINET.
In this work we present results of lidar remote sensing of aerosol layers in the atmosphere above Sofia during an episode of Sahara dust transport, 02-07 April, 2012. The investigations were made using two lidar systems, one equipped with a CuBr-vapor laser, emitting at wavelength 510.6 nm, and a second one - with Nd:YAG laser, at wavelengths 1064 nm and 532 nm. The results of lidar measurements are presented in terms of vertical atmospheric backscatter coefficient profiles and color maps of the aerosol stratification evolution. The involved into discussions ceilometer data (CHM 15k ceilometer) and satellite data from CALIPSO lidar, enhance the synergy of observations. Conclusion about atmospheric aerosol’s origin was made upon analyses of the information of weather-forecast maps provided by the Forecast system of Barcelona Supercomputing Centre, which are accessible via Internet. Additional information was provided by calculations of the backward air mass trajectories, using online software of NOAA about HYSPLIT model. The comparison between the data from the two lidars and the ceilometer showed similar behavior of aerosol layers development in the atmosphere above Sofia. All information about aerosol layers origin, their altitude above ground, persistence during lidar observations, confirmed the conclusion of observation of a long-distance Sahara dust transport beyond Balkans and Sofia. An interesting completion of CALIPSO lidar and ground based lidars results of measurement is presented in case of thick opaque cloud layer in the atmosphere, which slices the path of lidar sensing in both directions.
Second half of April and beginning of May 2010, were remembered by a big trouble in the airplane traffic over Europe,
due to the eruption of the volcano Eyjafjallajokull in Iceland. The volcanic ash propagated quickly in the atmosphere
traversing most of European countries. Its trajectories were forecasted and observed by many meteorological stations to
prevent unintended consequences of the airplane transport
The lidar stations of the European lidar network EARLYNET-ASOS [1] performed a large campaign of measurements to
identify the position, the height above ground level (AGL) and the thickness of the volcanic aerosols transported in the
air. It was an appreciable work to update meteorological forecasting and to study volcanic distribution directions, power
and sedimentation in continental scale. As partner in EARLINET-ASOS project, Sofia lidar station performed
measurements of the atmospheric aerosol profiling which results where quickly presented on the WEB-page of the
Institute of Electronics - BAS [2]. A more detailed discussion and comments concerning only Sofia-lidar measurements
of the volcanic dust layers observed over the town we present in this work.
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.
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.
In this work we developed a method for estimation of the ratio of aerosol to molecular backscattering coefficients in
lidar sounding of the atmosphere. We presented our first results of such analysis of lidar measurement carried out in June-
July 2008, using lidar system with a CuBr-laser and two receiving channels. The laser emits simultaneously two closely
disposed wavelengths λ1=510.6 nm and λ2=578.2 nm of similar output powers. The spectral distance between these
wavelengths provides well distinguished molecular scattering signal. From the other side the both wavelengths are too close
to accept (in the first order of approximation) a similarity in the aerosol scattering conditions. Both lidar channels have very
similar technical parameters (as overlapping functions, photon detector sensitivity, optical transmission, etc.) and thus,
provide similar output signal levels for both wavelengths. The preliminary results from the analysis of experimental data
demonstrated the opportunity to detect and to process well distinguished lidar signals in clear atmosphere. The operational
heights exceed 9 km at accumulation time 30 min, that is typical for the adopted integration time in European lidar network
EARLINET.
In this work, a combined Raman-elastic backscatter lidar is described. It is based on a Q-switched powerful frequencydoubled
Nd:YAG laser (output pulse power: up to 1 J at 1064 nm; up to 100 mJ at 532 nm; pulse duration 15 ns FWHM;
repetition rate 2 Hz). A Cassegrain telescope (35 cm diameter, 200 cm focal length) collects backscattered radiation from
atmospheric particles and molecules. Lidar's spectral receiving module consists of three cannels. The first two channels
separate and detect elastic-backscattered lidar signals at laser wavelengths 1064 nm and 532 nm, respectively. In the
third lidar channel, a radiation at a wavelength of 607 nm is selected and detected, resulting from Raman backscattering
of laser second harmonic (532 nm) by atmospheric nitrogen molecules. Experimental profiles of the aerosol
backscattering coefficient demonstrating measurement abilities of the system are shown and discussed. Particularly,
attention is paid to profiles containing signals from high-altitude cirrus clouds, low-altitude stratus clouds, hazy
atmospheric areas and Saharan dust over the city of Sofia.
Image data-handling technique using a Statistical Approach for measuring the velocity of inhomogeneities drifting in the
atmosphere is suggested. Velocity measurements of imaged cloud fields have been conducted by gathering time-spatial
realizations at equal time intervals. Properly selected data are used to develop the temporary instabilities for fixed dots of
the field. The coordinates of the minimum value for every temporary instabilities are fixed. The cross-correlation
function of two imaged cloud fields round these coordinates is calculated. The modules and the direction of the velocity
vector are sets by the position of the global correlation maximum.
Within the framework of the project "EARLINET-ASOS (European Aerosol Research Lidar Network - Advanced
Sustainable Observation System)" Contract No 025991 of European Commission's Framework Program 6 (EC FP6)
(http://www.earlinet.org), transportation of Saharan dust over Europe has been observed in the sky over Sofia, during the
last week of May, 2008. This phenomena, predicted by the Atmospheric Modeling and Weather Forecasting Group of
National Technical University of Athens (NTUA) (http://forecast.uoa.gr/) and the Forecast system of Barcelona
Supercomputing Center (BSC) (http://www.bsc.es/projects/ earthscience/DREAM), was interesting with its relatively
long-time existence and large scale of propagation over the continent. In this work, we present the results and some
comments of the measurements of the atmospheric backscatter coefficient made in Sofia, using aerosol lidar with CuBrvapor
laser.
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.
In this work the results of an experimental assessment of the operation accuracy of three lidar systems in Sofia City are
presented. The investigation was carried out performing an inter-comparison of profiles of the atmospheric aerosol
backscattering coefficient simultaneously measured by the lidars. All three lidar systems are involved in regular
measurements within the frame of the EARLINET-ASOS (European Aerosol Research Lidar Network: Advanced
Sustainable Observation System) European project. Twenty-four lidar stations, distributed over 16 countries in Europe
are associated in this network. The main objective of the project is the establishment of a large statistical database of the
aerosol distribution on the European scale. In order for the data to be reliable, preliminary tests of the quality of work of
the individual lidar systems should be performed. That was the reason to perform the present study. The experiments on
simultaneous operation of the lidar systems were carried out according to the fixed EARLINET-ASOS schedule starting
in April 2006. The mean difference in the aerosol backscatter for all profiles obtained simultaneously and the standard
deviation of the differences were calculated and compared.
Statistical image processing methods are widely used for analyzing various random processes. One of these methods is the correlation method, which is applicable in the remote sensing for measuring the transport velocity of aerosol fields and determination of their statistical characteristics such as averaged lifetime, size, orientation, etc. In this work, an image data processing technique based on the temporary instabilities method is described, enabling velocity measurements of statistically homogeneous objects. The ability of the proposed technique for accomplishing simultaneous velocity measurements of different inhomogeneities is demonstrated experimentally. The main drift velocity vector is set by the position of the global correlation maximum, wich is obtained for different couples of recorded images.
A novel method for acquisition and preprocessing of intensive, highly dynamic lidar signals has been developed and tested with computer simulations. The method is based on the time-resolved counting of photon streams by convolving of photoevents into decayed oscillations, linear analog to digital sampling and deconvolution. It is effective simultaneously in analog, overlapped and photon counting regimes, saving the dynamics and the temporal resolution of lidar data for more than four orders (104) of the illumination magnitude.
This paper presents a study of the distribution of aerosol extinction coefficient and its time and spatial variations during the morning breeze transition period. The experiment was carried out near the town of Akhtopol with a groundbased aerosol lidar. The results show an existence of characteristic extinction distribution (aerosol stratification) for each stage. In the beginning of the period, when in the lowest 200 m there is still an off-shore breeze, an aerosol layer with high extinction exists at heights of about 150 - 200 m above the sea surface. As the sunheating gets stronger this layer is gradually destroyed. At the end of the period, when the on- shore breeze is already established, in the lowest 200 m, a layer of low extinction is observed at height of about 200 m above the sea surface. A coincidence has been revealed between the heights of the wind changes and location of layers where the extinction differs from the one of the surrounding air. Besides the change in breeze pattern, the change in the relative humidity also affects the observed time and spatial variations in aerosol extinction.
The image quality of the laser sounding of distant objects is limited mainly by the resolution of the optical system 'a telescopic objective -- a turbulent layer.' Fried formulated a theoretical expression concerning the system's resolution, but he assumed a diffraction-limited objective. Our goal here is to increase the estimation accuracy of the resolution of lidar optical systems, using the measured in advance resolution of the receiving objective. We show the possibility to substitute the maximum angular frequency of a diffraction- limited objective, in the expression of the optical transfer function, by the measured angular frequency of the actual objective. We propose a method, based on measuring the common resolution of the system 'a lidar telescope -- a turbulent layer,' for estimating the structural constant Cn2 of the turbulence. We have also calculated numerically the resolution of such a system for one-kilometer turbulent layer.
KEYWORDS: LIDAR, Atmospheric modeling, Backscatter, Scattering, Aerosols, Signal to noise ratio, Error analysis, Signal attenuation, Environmental sensing, Lead
The inversion of lidar returns, i.e. the determination of the extinction or backscattering profile, is a classical example of an incorrect mathematical task. If one assumes single scattering approximation and known relation between both the extinction (alpha) (z) and backscattering (beta) (z) coefficients, then the remaining problem deals with the selection of the boundary value. Many authors agree that this is the key operation that determines the accuracy of the lidar data inversion. The utilization of the Klett's method requires estimating of the boundary value near the end of the sounding trace. Two common problems accompany this process. First is the very low value of the signal-to-noise ratio and the second is the lack of any kind of a priori information. In our previous work we investigated the proposed by us modification of the well-known slope method. It permits boundary value estimation if an isolated peak is present in the lidar return. Here we discuss the possibility of inverting the lidar return in the vicinity of an S-function extremum.
Observations of the horizontal distribution of the aerosol extinction coefficient within the atmospheric marine boundary layer are presented. The extinction coefficient profiles are calculated using Klett's inversion method. The variations in aerosol optical properties caused by the underlying surface are studied. Constant-elevation cross sections of extinction distribution along different azimuths over land and sea surfaces ((sigma) equals 0.23 divided by 0.14 km-1 and (sigma) equals 0.2 - 0.29 km-1 over land and sea respectively) are presented and analyzed.
The slope method is widely recognized for handling of lidar signals in homogeneous atmosphere. We propose and investigate a modification of this method that is aimed to evaluation of the extinction coefficient in the center of an isolated aerosol layer. Thus obtained point estimate may be used further as a boundary value in other analytical inversion methods.
The feasibility is estimated of an approach for measuring the atmospheric wind velocity by spatial filtration of moving turbulence-induced cross-section speckle structure of a propagating laser beam. It is shown that there are some optimum relations between the atmospheric conditions (turbulence intensity, transversal wind velocity) and the experimental arrangement parameters (characteristics of the laser beam, receiving optical system, spatial filter, beam propagation distance etc.), that ensure a maximum measuring sensitivity.
The paper continues our previous investigations, based on the backscattering coefficient distribution, of the optical properties of aerosol in a coastal zone. The material presents some results of an elastic-backscattering lidar experiment carried out in the Bulgarian Black Sea coastal area in September 1992. The lidar experimental results are presented as 2-D images of the cross-sections of the variation of the extinction as a function of the height and the distance along the sounded path at different elevations. The value of the volume extinction coefficient is calculated according to Klett's inversion method, based on single elastic scattering of the laser emission. Along with the lidar measurements a conventionally measured meteorological parameters are presented. The experimental data considered in the paper demonstrates the lidar sensitivity to underlying surface influence over the optical characteristics of the aerosol.
The transport of air pollution from local sources (industrial stacks) is connected with the motion of air masses. We report an investigation of the air quality in the region of Silistra (Bulgaria)-Kalarash (Romania), characterized by the possibility of transborder transfer of air pollutants. We applied a combined technique of lidar sounding and mapping of the aerosol field and conventional sampling of the contaminating harmful substances at stationary stations. The concentrations of pollutants above the limit value were measured. We identified and localized the unknown emission souce of the organochlorine compounds and proved the transborder origin of the pollution in the town of Silistra.
The clouds play a significant role in the atmosphere particularly in the determination of the radiant balance. Since their type, structure and height considerable influence the transmission, absorption and re-radiation of the IR radiation they are of great importance for the local meteorology and the global climatology. In small spatial scales (micro- and meso- ones) the clouds considerable change in space and time. The ECLIPS (Experimental Cloud Lidar Pilot Study) program in which our lidar group also takes part is well described. In general, the main purpose is a ground-based lidar observation of the clouds simultaneously with the NOAA 10 and NOAA 11 meteorological satellites overpassing the same areas to be performed. Essential lidar data are the recorded profiles from which the information about the clouds height, optical depth and vertical extinction can be derived. The experimental data required for the ECLIPS Phase II measurements were recorded during the period from 21 May to O9 July 1991 at 30 observations by 3 hours each i.e. 1 hour before and 1 hour after the NOAA 10 and NOAA 11 satellites overpasses. The lidar data are completed with certain meteorological information obtained by the conventional means. The presented experiment was conducted using triple- beam aerosol meteorological lidar developed at the Institute of Electronics of Bulgarian Academy of Sciences, which are well described previously.
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