To estimate the zonally-averaged viscosity of the two phases of a BBOA particle in the troposphere, we implemented the parameterizations for temperature-and RH-dependent viscosity for each phase into the mean annual tropospheric RH and temperature profiles from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2, (75)). The underlying distribution of the RH and temperature as a function of altitude and latitude are reported in Fig. . Shown in Figure are the zonally-averaged viscosities resulting from this implementation. Based on these zonallyaveraged viscosities, the hydrophilic phase is mostly liquid-like (<10 2 Pa s) below an altitude of 2 km, mostly in a semi-solid state between 2 and 9 km and in a glassy state above that (Fig. ). Conversely, the hydrophobic phase is mostly semi-solid below 2 km, a semi-solid or glassy state between 2 and 9 km, and glassy above that (Fig. ).