The writing of the manuscript was led by AKB with contributions from MFZ. MFZ calculated viscosities from glass transition temperatures of BBOA and produced the figures in the manuscript with contributions from NGG. AZ calculated the viscosities of sulfuric acid particles using the AIOMFAC-VISC thermodynamic model and provided direction and insight for viscosity predictions. DM and GS provided measurements of BBOA in the lower stratosphere. DM identified the observational constraints of viscosity and phase state in the stratosphere. All authors discussed and interpreted data and contributed to revising the original manuscript. Supporting Information Details of the ATom campaign, H2SO4:BBOA mass ratio figure, simulated monthly temperature figure, and BBOA viscosity figures for sensitivity analyses (DOC).