Ångström coefficients calculated from aerosol optical depth data obtained over Sofia, Bulgaria

One year (October 2006 – October 2007) continuous measurements of the aerosol optical depth (AOD) during the development of planetary boundary layer (followed by lidar) were carried out by a sun photometer Microtops II over the city of Sofia, Bulgaria. In order to asses aerosols microphysical properties, the Angstrom coefficients α and β are retrieved from the spectral AOD data using the Volz method. Daily behavior of AOD, Angstrom exponent α and turbidity parameter β as well as their monthly mean variations are presented. Variations in the monthly mean AOD at λ = 500 nm, α and β (computed from the wavelength pair 500/1020 nm) had similar behavior. The mean values of the AOD, α and β for the whole period (October 2006 –October 2007) are found to be 0.28±0.07, 1.21±0.17 and 0.11±0.03, respectively, as highest values of the AOD (0.42±0.21), α (1.62±0.53) and β (0.17±0.10) were obtained in August 2007. The lowest monthly mean AOD, α and β were 0.21±0.09 (in January and February 2007), 1.03±0.42 (in April 2007) and 0.09±0.04 (in January and October 2007), respectively. In addition, Angstrom coefficients α and β are calculated from the wavelength pair 500/675 nm and comparison is done between the results obtained from the two wavelength pairs. The above mentioned values reveal low to moderate turbidity of the atmosphere and dominance of fine-mode aerosols over the city of Sofia associated with heavy vehicular traffic and local anthropogenic pollution sources.