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Based on a novel combination of approaches and instruments, this article presents campaign-based results from atmospheric boundary layer (ABL) height and aerosol optical depth (AOD) measurements carried out at two different experimental sites in Sofia, as well as from three-point measurements of aerosol number concentrations. Several instruments (lidar (developed by the IE), ceilometer, aerosol particle counter, sun photometer and meteorological sensors) were used in this study. Based on joint interpretation of the instruments' data we assess the influence of the atmospheric aerosol in the planetary boundary layer and the significant influence of aerosol layers and high clouds on AOD values. Measurements of AOD in the city basin gave values in the range 0.22–0.41 for cloud-free skies, and up to around 0.8 under partly cloudy conditions. The information obtained during the two campaigns indicates that aerosol particle concentrations were lower in park areas than along heavy-traffic thoroughfares in the city, but higher than in the mountain area. In conclusion, our study demonstrates the potential of employing a broad array of instruments for the study of boundary layer and aerosol over large, valley-situated and heavily urbanized city areas. [ABSTRACT FROM AUTHOR]

A series of campaigns involving a systematic investigation of the atmosphere over an urban area of Sofia city were carried out. A European Aerosol Research Lidar Network (EARLINET) scanning aerosol lidar, a spectroradiometer, a standard sun photometer and a ground meteorological station were used in the observations. Multiple aerosol layers of variable thickness (200-600 m) were observed systematically in the planetary boundary layer (PBL) over the study area and the experimental data were compared with theoretical data. A study of the optical characteristics of the atmospheric aerosol, including the extinction coefficient, aerosol optical depth (AOD) and Angstrom parameters α and β, was performed and their variations followed during the convective boundary layer (CBL) formation. Values of the AOD obtained using the different instruments during simultaneous measurements were compared. Preliminary results show that the AOD values recorded by the sun photometer and those calculated on the basis of the spectroradiometer data are higher than those retrieved from the lidar data. Determination of the atmospheric optical depth and extinction coefficient using a ground-based spectral instrument is a relatively simple and inexpensive method of monitoring the total aerosol content in the atmosphere as well as the air quality over the region. [ABSTRACT FROM AUTHOR]