Examining the characteristics of aerosols: a statistical analysis based on a decade of lidar and photometer observations at the Eastern border of ACTRIS

Abstract. A decade-long (2015–2024) analysis of aerosol properties was conducted at RADO (Romanian Atmospheric 3D Observatory)-Bucharest station in Romania, a key atmospheric observational site at the Eastern border of the Aerosol, Clouds and Trace gases Research Infrastructure (ACTRIS). This study aims to characterize the optical and microphysical properties of aerosols, classify predominant aerosol types, and investigate their seasonal variability and transport pathways based on long-term multiwavelength Raman lidar and sun/sky/lunar photometer measurements. Results indicate a dominance of fine-mode aerosols, with an average Aerosol Optical Depth (AOD) of ∼ 0.2 and Ångström Exponent (AE) values between 1.5 and 2.0, highlighting pollution-driven aerosol regimes. Seasonal variations were observed, with continental aerosols prevailing in winter, dust transport peaking in spring (altitudes of 2–8 km), and biomass-burning aerosols increasing during summer. Analysis of 408 aerosol layers using the NATALI (Neural network Aerosol Typing Algorithm based on LIdar data) identified complex aerosol mixtures, with 63 high-resolution cases revealing a predominance of “dust polluted” and “continental smoke” types. In the lower troposphere, the extinction-related Ångström exponent shows a narrow mono-modal distribution with the peak at 0.9, indicating predominantly medium-sized particles, whereas in the high troposphere it becomes bi-modal, reflecting alternating occurrences of small and large particles. Lidar ratio values have a distribution peak at around 48–49 sr in both altitude regions, but their spread is much wider in the lower troposphere – revealing frequent layers of highly absorbing aerosols – while lofted layers in the high troposphere exhibit a narrower range typical of moderately absorbing particles. Distinct differences between fresh and aged biomass-burning aerosols (smoke) are identified through their altitude, depolarization, Ångström exponent, and lidar-ratio characteristics, demonstrating microphysical change during transport. FLEXPART (FLEXible PARTicle dispersion model) retro-plume simulations provided insights into aerosol source regions and transport patterns showing contributions from local emissions, long-range transported desert dust, and biomass burning events from Europe and North America. These findings emphasize the persistent influence of regional pollution and transported aerosols on air quality and climate. The integration of ground-based remote sensing and advanced retrieval algorithms like NATALI provides a robust framework for aerosol characterization, enhancing climate models and air quality assessments.