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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 work(5). Supplementary data from: two meteorological stations; HYSPLIT back trajectory model(4); BSC-DREAM8b dust model(9); 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.