In this work, we generated BBOA from smoldering pine in a controlled laboratory furnace. We studied the phase-behavior of the sampled primary BBOA using fluorescence microscopy and showed that BBOA particles comprise two phases (an outer hydrophobic phase and an inner hydrophilic phase) for a wide range of RH values relevant for the atmosphere. Using a photobleaching method, we showed that that the two phases possess very different RH-dependent viscosities. Using these results, we implement a kinetic multi-layer model of gas-particle interactions (KM-GAP) to demonstrate how liquid-liquid phase separation and viscosity of the BBOA can significantly affect the lifetime of BrC in the atmosphere and to highlight the need to include the phase behavior and viscosity of BBOA in calculations of the Earth's radiative balance.