Observations of Biomass Burning Smoke Transported from Southeast Asia Using a Multi-Wavelength Polarization Raman Lidar

Biomass burning smoke (BBS) emitted from Southeast Asia can be transported through regional atmospheric circulation, exerting significant impacts on air quality and climate over southwestern China. However, continuous observations of the vertical structures and optical properties of transported BBS remain limited. This study used one year of continuous multi-wavelength polarization Raman lidar observations at Mengzi (23.37°N, 103.38°E) in Yunnan, China, to identify transboundary BBS transport events and characterize their optical properties. Eight transboundary BBS events originating from Southeast Asia were identified using a comprehensive data processing framework involving aerosol extinction coefficient, backscatter coefficient, lidar ratio, particle linear depolarization ratio (PDR), and Ångström exponent. The transported smoke layers were mainly distributed below 4 km, exhibiting significantly enhanced aerosol extinction coefficients of 0.22–0.32 km−1 (compared with 0.02–0.06 km−1 for background aerosols). The 532 nm lidar ratios of the smoke layers (49.0–68.3 sr) were higher than those of the background aerosols (31.6–39.0 sr). The particle linear depolarization ratios ranged from 0.050 to 0.097 at 532 nm and from 0.14 to 0.19 at 355 nm, revealing a pronounced wavelength dependence. These values indicate measurable particle depolarization but do not independently establish particle morphology or the degree of smoke aging. Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) backward trajectory analysis together with fire hotspot observations revealed that this smoke mainly originated from biomass burning regions over the Indochina Peninsula and was transported to Yunnan across northern Myanmar, Laos, and northern Vietnam. This study provides important observational evidence of transport pathways, vertical distribution, and optical properties of transboundary BBS aerosols transported from Southeast Asia to southwest China.