Lidar observations indicate that fresh smoke particles in the stratosphere exhibit a high depolarization ratio, suggesting the presence of non-spherical particles. This elevated depolarization could signal that BBOA is in a glassy or highly viscous state in the stratosphere. In newly emitted smoke plumes, BBOA particles can coagulate into fractal aggregates. If BBOA is in a glassy or highly viscous state, the material will not flow, maintaining its fractal structure after coagulation, which would contribute to a high depolarization ratio. Additionally, black carbon particles from combustion sources initially have a fractal geometry. When coated with a low-viscosity material, the fractal geometry can become more compact and spherical, with a low depolarization ratio. However, if black carbon is coated with BBOA in a glassy or highly viscous state, black carbon can maintain its fractal geometry, and the depolarization ratio can stay high. Nevertheless, interpretation of lidar depolarization is complicated because depolarization depends on particle size as well as the degree of nonsphericity. Lidar data also show that the depolarization ratio of smoke in the stratosphere decreases over a period of 3 -4 months, eventually approaching near-zero values (< 0.05). This trend may indicate that BBOA transition from a highly viscous state to a less viscous one. A decrease in viscosity would allow fractal aggregates of BBOA to transform into spherical particles, which typically exhibit a near-zero depolarization ratio. Similarly, a reduction in BBOA viscosity would enable fractal black carbon aggregates coated with BBOA to compact into a core-shell structure, also associated with a low depolarization ratio. However, it is important to note that glassy or highly viscous BBOA, such as tar balls, that have not coagulated into fractal aggregates in the atmosphere will possess a spherical geometry and exhibit a low depolarization ratio. Therefore, a low depolarization ratio does not definitively indicate that BBOA particles are in a non-glassy and non-viscous state.