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

Understanding the viscosity and phase state of biomass-burning organic aerosol (BBOA) from wildfires and pyrocumulonimbus (pyroCb) events in the stratosphere is critical for predicting their role in stratospheric multiphase chemistry and ozone depletion. However, the viscosity and phase state of BBOA under stratospheric conditions, including interactions with sulfuric acid (H₂SO₄), remain largely unquantified. In this study, we combine laboratory data with a thermodynamic model to predict the viscosity and phase state of BBOA under stratospheric conditions. Our results suggest that BBOA with a H₂SO₄-to-BBOA mass ratio of 0.37—an estimated upper limit for pyroCb smoke in the lower stratosphere after two months of aging— is highly viscous and frequently exists in a glassy state. Even at a higher H₂SO₄-to-BBOA mass ratio of 0.79 – an estimated upper limit after nine months of aging – BBOA can still transition to a glassy state under certain stratospheric conditions. In the glassy state, bulk reactions are suppressed, and multiphase chemistry may be limited to the particle surfaces. We also highlight key areas for future research needed to better constrain the viscosity and phase state of BBOA in the stratosphere and its subsequent impact on ozone.