Results of remote measurements and characterization of biomass burning aerosols observed in the low troposphere over Sofia, Bulgaria, are presented and discussed. Measurements are accomplished by using two-wavelength elastic-scatter lidar, operating at 1064 nm and 532 nm. The aerosols are identified as to be consisted mainly of aged smoke of wildfires raging in the USA in the last third of July 2012. The long-range transport of the smoke aerosols, taking place from 24 July to 6 August 2012, is determined to be driven by the Northern hemisphere Polar jet stream. Spatial distribution of the observed aerosols is displayed by retrieving averaged vertical profiles of the aerosol backscatter coefficients. The temporal evolution of the aerosol layers during the period of measurement is shown by height-time coordinate colormaps of range-corrected lidar data. In order to characterize qualitatively the size range of the aerosol particles, the vertical profile of the backscatter-related Ǻngström exponent (BAE) is also retrieved. As an accent of the work, distributions of BAE corresponding to distinguished aerosol layers, as well as the overall one, are obtained and analyzed, representing qualitative counterparts of the real particle size distributions. In the case of the fire smoke layer, BAE values vary in the range 1.0-1.3, indicating processes of considerable aggregation of the finest particle size mods during the aging period. The reliability of the results and conclusions concerning the fire smoke BAE distributions and their evolution are indirectly validated by the obtained typical distribution ranges of the observed urban- and water aerosols.