Multi-Instrumental Evidence of the 2025 Absorbing Aerosol Perturbation at the RADO-Bucharest Observatory

This paper presents a multi-parameter characterisation of the atmospheric composition at the RADO-Bucharest observatory, a regional WMO-GAW and ACTRIS facility in southeastern Europe, by anchoring recent observations within a multi-annual baseline (2015–2024). This paper utilises data from multi-wavelength active remote sensing, high-resolution near-surface speciation, and modelling to evaluate complex urban and transboundary processes across different aerosol and clouds regimes. The year 2025 was marked by an atmospheric perturbation, which was characterised by a quantifiable departure from the decadal climatology. Using Z-score analysis, this perturbation was identified as a seasonal decoupling: an atypical reversal in the vertical particle-size distribution occurred in the free troposphere between July and October, while a transition toward a high-absorption aerosol regime was recorded near the surface during the winter months. These shifts are quantified by a significant drop in the columnar Single Scattering Albedo and a systemic increase in high-troposphere Lidar ratios exceeding 70 ± 12 sr, indicating the presence of advected combustion products aloft. At the surface level, chemical speciation measurements recorded an overall increase in wintertime particulate mass concentrations alongside elevated levels of More-Oxidized Oxygenated Organic Aerosol (MOOOA) compared to previous years, reflecting an intensified accumulation of aged, processed emissions during the January–February period. Furthermore, this paper documents cloud vertical structure and phase occurrence, highlighting a persistent seasonal stratification. The application of unified inversion frameworks is demonstrated through case studies of smoke and mineral dust, applying the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) algorithm to retrieve vertically distributed aerosol microphysics through lidar–photometer integration. Maintained under rigorous quality assurance protocols, these results demonstrate the value of continuous, multi-instrumental profiling to quantify transboundary perturbations and improve regional energy budget representations.