Synergetic Multi-Instrument and Modelling Approach for the Detailed Characterisation of Rapid Transitions Between Contrasting Aerosol Regimes

This study demonstrates a synergetic multi-instrument and modelling approach for the rigorous characterisation of rapid aerosol regime transitions, applied to an exceptional atmospheric event over the RADO-Bucharest station during the July 2023 “Cerberus” heatwave. By anchoring our analysis in high-power Raman lidar profiling, integrated with sun-photometry (GRASP inversions), near-surface chemical speciation (ACSM, Aethalometer), and Lagrangian transport models (FLEXPART, HYSPLIT), we quantitatively resolve the vertical dynamics of shifting aerosol plumes. We identify a definitive two-phase evolution: a regional biomass-burning fine-mode plume advected at 2 km altitude on 12 July, abruptly displaced by a coarse-mode Saharan mineral dust intrusion at 3 km altitude on 13 July. Convective planetary boundary layer (PBL) growth drove the daytime entrainment of these lofted layers, while high-resolution Doppler wind lidar measurements confirm that nocturnal subsidence provided the mechanical forcing necessary to entrain these lofted layers during the night into the planetary boundary layer, driving near-surface severe air quality degradation. Furthermore, parameterised scenario demonstrations indicate that these aerosol layers influenced the subsequent cloud microphysical evolution. Organic smoke aerosols acted as cloud stabilisers, with simulated high droplet concentrations suppressing collision–coalescence and locking the hydrometeor population in a non-precipitating liquid state (median reff_liq ∼ 5.0 ± 1.2 μm). Conversely, the subsequent mineral dust intrusion provided potent ice nucleating particles, triggering the Wegener–Bergeron–Findeisen process. This catalysed a rapid phase transition into a deep glaciated system (reff_ice ∼ 49.7 ± 12.5 μm) and established a bimodal melting layer that culminated in widespread washout. This work underscores that high-resolution vertical profiling, specifically via Raman lidar, is indispensable for resolving transient atmospheric events and parameterising aerosol-mediated modifications to the regional hydrological cycle.