Unraveling the Life Cycle of a Severe Winter Haze in Changsha with Polarization Lidar

To investigate the formation and dissipation mechanisms of severe winter haze in Changsha, this study presents a comprehensive analysis of a typical heavy haze episode from December 29 to 31, 2025, based on continuous ground-based multi-wavelength polarization lidar observations combined with near-surface PM_2.5 and meteorological data.

Lidar profiling identified a stable pollution aerosol layer of 300-500 m, closely coupled with surface pollution. The episode evolved through four distinct stages. In the initial stage (daytime, 29th), a PM_2.5 concentration of ~140 μg/m³, an extinction coefficient of ~1.8 km^-1, and a depolarization ratio of 0.17 indicated the presence of mixed aerosols dominated by relatively dry fine particles. The explosive growth stage (17:00-19:00, 29th) was critical, where under stagnant conditions with rising relative humidity (~70%), PM_2.5 surged from 134.6 to 244.2 μg/m³. The concurrent increase in the extinction coefficient to 2.3 km^-1 and a slight decrease in the depolarization ratio to 0.15 confirmed rapid pollutant accumulation in a compressed boundary layer, with newly added particles being more hygroscopic and spherical. During the mature stable stage (evening 29th to morning 31st), pollution peaked and plateaued (PM_2.5: 280-350 μg/m³). The high extinction coefficient (2.5-4.0 km^-1) and a further reduced depolarization ratio (0.11) signified fully aged aerosols dominated by hygroscopic, spherical secondary inorganic particles. In the wet scavenging stage (after 12:00, 31st), driven by precipitation and wind, PM_2.5 plummeted from 357.8 to 53.4 μg/m³ within 8 hours. Notably, the extinction coefficient temporarily peaked near 5 km^-1, and the depolarization ratio increased to 0.2, clearly capturing the scavenging signal from non-spherical raindrops.

This study delineates the complete life cycle of "stagnant accumulation—explosive growth—sustained high pollution—removal by wind and precipitation". The core finding is that the co-evolution of lidar-derived extinction coefficients and depolarization ratios visually elucidates the microphysical processes governing pollution accumulation, aerosol aging, and wet removal. It confirms that polarization lidar is an indispensable tool for dynamically discriminating aerosol phases and quantifying pollution evolution, providing crucial scientific support for understanding haze formation.