Ageing, Dilution and the Limits of Detection: A Canada-to-Romania Wildfire Smoke Transport Episode Observed with the RADO-Cluj ACTRIS National Facility and PACE Multi-Angle Polarimetry

Smoke from the Canadian wildfires of August 2025 reached Cluj-Napoca, Romania, on 6–9 August as a thin layer at 5–8 km. We followed the plume from its source with the multi-angle polarimeters HARP2 and SPEXone and the OCI radiometer on NASA’s PACE and characterised it at the RADO-Cluj ACTRIS National Facility with a multiwavelength depolarisation lidar, a ceilometer, AERONET photometry, GRASP inversions, and HYSPLIT trajectories. Along the air mass trajectory, the HARP2 retrievals resolved the ageing of the smoke: an optical depth decaying from 0.9 to 0.2–0.3, a layer rising to 10 km and descending to 5 km, growth of the fine-mode radius, and a single-scattering albedo consistent with brown carbon loss. At the site, the lidar ratio and particle depolarisation ratio retrieved by HARP2 (52 sr, 0.042) agreed with the lidar’s derived lidar ratio and measured depolarisation (50 sr, 0.042), whereas the three satellite optical depths differed by 0.2 around an AERONET value of 0.1 and the NATALI typing networks returned three different classes. For this faint, aged plume, intensive polarimetric and vertical quantities could no longer be identified for the aerosol by its optical depth and Ångström exponent, which motivates testing an intensive-first pairing of satellite polarimetry with ground-based lidar on further episodes.