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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.

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 λ 1 =510.6 nm and λ 2 =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.

For a (0.5–0.6)m long He−Ne laser tube, single longitudinal mode 632.8 nm generation is obtained, using a higher gas-mixture pressure and application of an axial magnetic field. An efficient and very simple mode selection is obtained by gas-mixture pressure increase only. A multimode power to single-mode power conversion coefficientk=0.75 is obtained by increasing the tube pressure. A further increase tok=0.85 is possible with optimum magnetic field and gas pressure.