3. Results and discussion: 3.1 RH in the stratosphere Before discussing the viscosity of BBOA in the stratosphere, we first considered the RH in the stratosphere since the RH strongly impacts particle viscosity. As the RH decreases, the water activity (and hence water content) of BBOA particles decreases to maintain equilibrium with the gas phase and the viscosity increases due to the plasticizing effect of water. We calculated the RH as a function of temperature in the stratosphere at altitudes of 15 and 32 km and a water vapor mixing ratio of 15 ppmv. An altitude range of 15 -32 km corresponds to the range of altitudes where wildfire plumes have been observed in the stratosphere. A water vapor mixing ratio of 15 ppmv is consistent with the water vapour mixing ratio expected in wildfire plumes that have reached the stratosphere via pryoCb events. RH reaches extremely low levels (≲ 1% RH) under a wide range of conditions relevant to BBOA in the stratosphere. In contrast, in the troposphere, the RH is almost always greater than 10%. The low RH levels relevant for BBOA in the stratosphere, combined with the cold stratospheric temperatures, have significant implications for the viscosity of BBOA in this region as discussed below.