Georgi Kolarov

Remote monitoring of aerosol layers over Sofia during Sahara dust transport episode (April, 2012)

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.

LIDAR Atmospheric Sensing by Metal Vapor and Nd:YAG Lasers

LIDAR systems have widely been used for remote investigation of atmospheric parameters (Measures, 1984; Kovalev & Eichinger, 2004; Weitkamp, 2005). They are based on the socalled LIDAR (LIght Detection And Ranging) principle which consists in sending a laser pulse to the atmosphere and subsequent detecting of the radiation backscattered (at angle π) by atmospheric constituents or pollutants. As LIDAR is a time-of-flight technique, the return signal profile detected in the time domain contains range-resolved information about the atmospheric characteristics along the line of laser beam propagation. Advantages of the lidar measurement approaches, as compared to other available active techniques (e.g. radars), are the high spatial and temporal resolution, higher sensitivity and accuracy in sensing atmospheric particles, covering large observation areas, etc. These features make lidar systems powerful instruments for environmental measurements. At present, lidars find a variety of applications in different fields of the human activity. Along with the meteorology, atmospheric physics, and ecological monitoring, lidars are extensively used for volcanic and fire alerting, laser ranging, altimetry and bathymetry, lidar mapping and forestry, coastal morphology and hazards assessment in geology, as well as for many other applications in physics and astronomy, nuclear fusion, military, aviation, robotics, transportation, etc. There exists a variety of ground-based, air-borne and space-borne lidar systems distinguished by their types, schematics, regimes of operation, monitored parameters, constructions, etc. (Kovalev & Eichinger, 2004; Weitkamp, 2005). Among the most widely used systems are the oneor multi-wavelength aerosol lidars exploiting elastic scattering of light.

High-precision laser range measurements using convolution and deconvolution of reflected pulses

A novel high-precision pulsed ranging method is developed. It involves preliminary transformation of photon detector signals (convolution), analog-to-digital sampling, software processing by deconvolution, digital filtering, pulse shape retrieving and pulse center determination. The method is effective for arbitrary pulse durations (shorter or larger than the sampling step) and is low sensitive to the shapes of the reflected pulses. Using 20 MHz/8bits ADC, an experimental timing accuracy is achieved of approximately 600 ps (approximately 10-2 times the ADC-sampling step) for single measurements and of approximately 30 - 50 ps (approximately 10-3 - 10-4 times the sampling step) in averaging regime.

Remote Monitoring of Aerosol Layers over Sofia in the Frame of EARLINET‐ASOS Project

In this work we present some results of lidar remote sensing of aerosol layers in the atmosphere in Sofia region. The investigations were made using a lidar system equipped with a CuBr‐vapor laser with high pulse repetition of 13 kHz and receiver in photon counting mode. These measurements were performed in frame of the project European Aerosol Research Lidar Network—Advanced Sustainable Observation System (EARLINET—ASOS). For some of presented results a conclusion about atmospheric aerosol’s origins was made upon analyses of the information about the weather condition during the lidar measurements. Such information was obtained by the weather‐forecast maps provided by the Atmospheric Modeling and Weather Forecasting Group of NTUA and the Forecast system of Barcelona Supercomputing Centre and accessible via Internet. Additional information is provided by calculations of the backward air mass trajectories, using online software of NOAA about HYSPLIT model (HYbrid Single‐Particle Lagrangian Integrated Traje...

Investigation of transborder pollution by combining remote lidar sounding and stationary gas sampling

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.

LIDAR detection of forest fire smoke above Sofia

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.

Rayleigh-fit approach applied to improve the removal of background noise from lidar data

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.

Lidar observation of volcanic dust layers over Sofia

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.

Retrieving the optical time-profiles with variable resolutions using linear processing of intensive photocount streams

A novel method for high-resolution measurements of photon arrivals and and time profiling has been developed. It is acting as a TOF method, but is effective at higher (up to 100 times) count rates. The minimal tolerable interval between the adjacent pulses is determined by pulse count duration and the discriminator dead time. The method is based on ADC sampling of counts and their software processing. It provides a time accuracy comparable with the accuracy of well-known TOF-techniques as well as restoring (zooming) lidar profiles of varible resolutions after the receiving. The final resolution can be quite better than the sampling step of raw data (δT ≥ 1 ns has been demonstrated).

Possibility of measuring wind velocity by spatial filtration of laser radiation propagating through the atmosphere

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.