The viscosity of the water-soluble component of BBOA (generated by extracting BBOA with water) has been measured previously and was shown to depend strongly on the RH . However, the water-soluble component neglects the low-polarity compounds in BBOA that may significantly affect the viscosity of the total aerosol. Using a parameterization and high-resolution mass spectra of BBOA, DeRieux et al. predicted that the viscosity of BBOA is strongly dependent on RH and greater than 10 4 Pa s under dry conditions . However, these results had a high degree of uncertainty and were based on the assumption that BBOA contained a single phase. In addition, researchers often assume BBOA is in equilibrium with the gas phase in chemical transport models, implying a low viscosity . No previous studies have measured the RH-dependent viscosity of the total BBOA. Furthermore, if BBOA undergoes LLPS then the viscosity of each individual phase must be measured independently.