The Laser Radars Laboratory was established in 1974 as a specialized unit for development of experimental methods and systems for laser remote sensing of the stratification of the atmospheric aerosols and pollutants with high spatial and temporal resolution. Since 2002, it has been involved in regular and synchronized lidar measurements of the atmospheric aerosol within the framework of the European Aerosol Research Lidar Network (EARLINET) by using two lidar systems certified and included (as Sofia lidar station) into the structure and activities of the Network. The types of lidar measurements performed are as follows: regular climatological observations; observation of special phenomena, such as unusually high concentrations of aerosols in the troposphere (transportation of mineral dust from Sahara desert over the Mediterranean Sea to Europe, volcanic eruptions, formation of smoke layers resulting from forest or industrial fires, etc.); measurements in coordination with satellite missions within the objective of detailed comparison of ground-based and spaceborne lidar data sets over Europe.
The Laser Radars Laboratory staff has taken part in the implementation of projects within the 5th (EARLINET), 6th (EARLINET-ASOS), 7th (ACTRIS) EU FPs and Horizon2020 program (ACTRIS2 and ACTRIS PPP projects); at present, the Laboratory implements tasks of the ACTRIS – IMP project under the Horizon2020 program.
Sofia Lidar Station has at the moment two elastic backscatter lidar systems certified for the measurements under EARLINET: an aerosol lidar with a Nd:YAG-laser (1064 nm, 532 nm) (Fig. 1a) and an aerosol lidar with a CuBr-vapor laser (510.6 nm, 578.2 nm) (Fig. 1b).
Fig. 1. Lidar systems based on a Nd:YAG laser (a) and a Cu-vapor laser (b), part of Sofia lidar station.
Nd:YAG-laser-based aerosol lidar system
The lidar transmitter is a high-power solid-state Nd:YAG laser. The laser has one additional amplifying module and provides a maximum pulse energy of 1 J at the fundamental wavelength of 1064 nm and about 100 mJ at the second laser harmonic at 532 nm, with pulse a duration of 15 ns FWHM and a repetition rate of 2 Hz. The Cassegrain-type receiving optical telescope has an aperture of 35 cm and a focal length of 195 cm. The system is equipped with an opto-mechanical unit for separating the spectral components of the backscattered radiation in each working channel, the latter consisting of high-sensitivity photoelectric modules, analogue-to-digital converters, shaping and controlling electronics, and a computer system for registration, accumulation and processing of lidar signals and data. The lidar allows measurement and recording of scattered signals from distances up to 30 km (heights up to 16 km, covering the troposphere and the low stratosphere), with 15-m spatial resolution and 100-ns temporal resolution, possibility for vertical scanning in the range 0÷32º and horizontal scanning within an angle range of 50º.
Metal-vapor-laser-based aerosol lidar system
- Laser Source – laser on metal vapors ( Cu-vapors, CuBr-vapors, Cu-Au vapors), depending on the goals of the atmospheric measurements.
- Mean output power – 2 – 4 Watts, typically.
- Repetition frequency – 5 KHz ( laser on Cu-vapors), 8-10KHz- (lasers on Cu-Br and Cu-Au vapors).
- Pulse duration ~ 10ns.
- Emitted wavelengths – 510.6nm., 578.2nm (by Cu-vapors laser); 510.6nm., 578.2nm and 627nm (Cu-Au vapors laser).
- Divergence of the emitted radiation ~ 1-2 mrad, Diameter of the laser beam near the lidar ~ 10cm.
- Telescope apertures ~ 18cm with focus length ~ 100 cm.
- Photon receiver in photon counting regime (Malvern ltd), Frequency bandwidth ~ 100MHz.
- Vertical tropospheric and stratospheric (up to 20÷-30km) measurements.
- Dead zone ~ 800 m.
Fig. 2. Results of atmospheric aerosol layers studies over Sofia by both lidar systems of Sofia Lidar Station: registration of transcontinental transport of Saharan dust (a-d, f), stratospheric aerosols (19 km) from forest fires in Canada (d) and smoke aerosols from a fire close to Kremikovtsi monastery (e).