Unusual wintertime transport of Saharan dust to Sofia, Bulgaria, detected by lidar

We report on lidar detection and characterization of Saharan dust in the atmosphere above Sofia, Bulgaria, during the months of February in the years 2017 and 2018. Presented are results of aerosol measurements performed by using twochannel lidar based on a frequency-doubled pulsed Nd:YAG laser emitting at 1064 nm and 532 nm. We studied and characterized optical, geometrical, and dynamical properties of the registered aerosol fields. Vertical profiles of the atmospheric backscatter coefficient and color-coded diagrams in height-time coordinates based on range-corrected lidar signals are obtained in order to analyze the aerosol/dust layer density distribution and dynamics. Based on the retrieved backscatter profiles at the two lidar wavelengths, the corresponding vertical profiles of the backscatter-related Ǻngstrom exponents were calculated in order to characterize qualitatively the dominant particle-size fractions in the aerosol/dust layers observed. The lidar results obtained were combined with air-transport modelling and forecasting data in order to derive conclusions concerning the aerosol’s origin and type. It is shown that on the days of lidar observations meteorological conditions in Sofia region, as well as the vertical structure and aerosol composition of the near surface atmospheric layers, have been significantly influenced by the Saharan dust intrusions. Analyzing the aerosol Angstrom exponent, an anomalous height distribution of the dust particle size fractions is ascertained and discussed. The results obtained confirm the tendency of increasing frequency of transporting considerable amounts of warm air masses from North Africa to Europe during the winter, indicating seasonal deviations in the air-intercontinental circulation systems. They also illustrate the potential of using lidar approaches to help monitor ongoing processes of local, regional, and global climate changes.