Доклади на Българската академия на науките Comptes rendus de l’Acad´ emie bulgare des Sciences Tome 63, No 8, 2010 GEOPHYSIQUE Physique de l’atmosph` ere LIDAR, CEILOMETER AND SUN PHOTOMETER INVESTIGATION OF THE AEROSOL OPTICAL CHARACTERISTICS IN THE TROPOSPHERE OVER SOFIA, BULGARIA Tsvetina Evgenieva, Bo L. B. Wiman * , Nikolay Kolev, Evgeni Donev ** , Danko Ivanov ** , Ventsislav Danchovski ** , Doyno Petkov *** , Vera Grigorieva, Ivan Kolev (Submitted by Corresponding Member P.I.Y. Velinov on April 21, 2009 ) Abstract The paper presents the results from planetary boundary layer (PBL) height and aerosol optical depth (AOD) measurements carried out in two dif- ferent experimental sites in Sofia as well as from three-point measurements of aerosol number concentration. The main aim of the present investigation is to determine optical and microphysical characteristics of the atmospheric aerosol in three points of the valley and their variation during the PBL formation over urban area, park zone and mountain site. Four instruments (lidar, ceilometer, aerosol particle counter and sun photometer) were used in this study. The experimental AOD data obtained at λ = 500nm gave values in the range from 0.22 to 0.41 in case of cloud-free skies and up to around 0.8 under This work was carried out within the frame of the following contracts: “City versus Mountain Tropospheric Ozone: The Sofia-Plana region in an air quality and ecological sustainability per- spective” (Contract DO 02-127/08). Aerosol particle counter measurements were partly funded by the Kalmar University, Faculty of Natural Science and Technology. 1191