Biomass burning events emit large amounts of primary BBOA and precursors of secondary BBOA. After emission, the primary BBOA can partially evaporate and secondary BBOA can form on and in the primary BBOA. A highly viscous BBOA, as observed here, could limit the rate of evaporation of primary BBOA and the formation rate of secondary BBOA. A recent modelling study that investigated the evolution of BBOA by evaporation of primary BBOA and the formation of secondary BBOA assumed the BBOA was liquid-like with a diffusion coefficient of 10 -10 m 2 s -1 . 107 Additional modelling studies are needed to determine the effect of highly viscous BBOA on the evolution of BBOA in the atmosphere and the resulting impact on predicted BBOA mass and composition.