The high viscosity may also have an impact on ice-cloud nucleation. Some studies have suggested that glassy organic aerosol can act as heterogeneous nuclei for ice clouds. The forest fire BBOA in this study was only highly viscous up to 50% RH at room temperature, but viscosity also increases as temperatures get colder. Hence, the RH threshold at which cracking occurs will be extended to higher RH values at higher, colder altitudes. In the upper troposphere, where temperatures and RH values are often low, 134 BBOA might often be in an amorphous solid (i.e. glassy) state. Consistent with this speculation, previous studies have observed ice nucleation in smoke plumes in the free troposphere. 135 However, the nucleation of ice by glassy organic aerosol is still an active area of debate with a recent study suggesting that ice nucleation by glassy organic aerosol may be less important than previously suggested. Studies that focus on ice nucleation by glassy BBOA are needed to resolve the importance of glassy BBOA for ice cloud formation in the upper troposphere.