Based on Fig. , the viscosity of phenolic BB-SOA is strongly dependent on both RH and temperature. Under dry conditions, phenolic BB-SOA is highly viscous at room temperature (~10 Pa s) and becomes a glass (viscosity greater than 10 12 Pa s) when the temperature is only slightly below room temperature (< 280 K). The viscosity of the BB-SOA particles observed here has important implications for heterogeneous chemical reactions and interactions with other compounds co-emitted during biomass burning. An important species co-emitted during biomass burning is BB-POA, which often contains organic molecules that strongly absorb solar radiation, termed brown carbon. This brown carbon component is important for the radiative properties of biomass emissions, i.e.