Viscosity and phase state of wildfire smoke particles in the stratosphere from pyrocumulonimbus events: an initial assessment

Large wildfires periodically inject massive amounts of biomass burning organic aerosol (BBOA) into the stratosphere (~15 km ≤ altitude ≤ 50 km) via pyrocumulonimbus (pyroCb) events. These aerosols may remain aloft for many months and engage in multiphase chemistry that contributes to the destruction of stratospheric ozone. To predict this chemistry, it is essential to understand the viscosity and phase state of BBOA in the stratosphere. In this study, we use laboratory data and a thermodynamic model to evaluate these properties. Our findings indicate that unaged and aged wildfire smoke particles may be in a glassy state (viscosity 10^12 Pa s) for certain conditions in the stratosphere, particularly when the H2SO4-to-BBOA mass ratio in the aerosol particles is ≲ 0.33. Lidar observations of wildfire smoke in the stratosphere are consistent with these findings. In such a state, bulk reactions will be inhibited, and multiphase chemistry will be limited to the particle surfaces. The glassy state may also nucleate crystalline polar stratospheric clouds, potentially exacerbating stratospheric ozone depletion. Even after prolonged aging and a H2SO4-to-BBOA mass ratio of 1 (considered a likely upper limit for pyroCb smoke in the lower stratosphere with an aging time of ≲ 1 year), the viscosities may reach 10^6 to 10^11 Pa s for specific temperatures and relative humidities. These high viscosities should be considered when describing the stratospheric chemistry of wildfire smoke particles. We also identify critical areas where future research is needed to better constrain the viscosity and phase state of BBOA in the stratosphere; and thus, its impact on stratospheric ozone.