We report the lidar observations of a prolonged unusual event of atmospheric aerosol load extending up to about 15 km above the city of Sofia, Bulgaria. Based on the results obtained and air-transport modeling data, we show that the observed diffused aerosol layers consisted predominantly of Saharan mineral dust partly mixed with other aerosol types. We present retrieved time-averaged vertical profiles of the aerosol backscatter coefficient (at 532 and 1064 nm) in order to display the aerosol density height distribution, whereas the aerosol layering temporal dynamics is illustrated by color-coded maps of range-corrected lidar signals. We also calculated the profiles of the backscatter-related Ångström exponent (BAE), which showed the domination of moderately coarse aerosol particle modes. Further, frequency count distributions of the BAE occurrences were obtained and analyzed statistically. Particular attention was paid to the revealed BAE distribution skewness, in view of better identifying and characterizing the aerosol size and composition changes. The results obtained enabled us to argue that the atmosphere above Sofia in the period considered had been strongly affected by intrusion of Saharan dust. Desert aerosols had perturbed or covered over the top of the atmospheric boundary layer and filled up the free troposphere, considerably influencing the local meteorological conditions. We also ascertained that aged volcanic aerosols had been likely present and detected at tropopause altitudes.
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