At least seven orders of magnitude viscosity variability in biomass burning organic aerosol from smoldering eucalyptus smoke

Uncontrolled wildfires in Australian eucalyptus forests emit large amounts of smoke, primarily composed of biomass burning organic aerosol (BBOA). Although BBOA viscosity has been studied for other fuels, it remains uncharacterized for eucalyptus. In this work, we generated BBOA by smoldering eucalyptus leaves and wood in a tube furnace and determined viscosities using optical microscopy, transmission electron microscopy, and rectangular fluorescence recovery after photobleaching. Early-stage burning of eucalyptus leaves produced non-hygroscopic tar balls with viscosities exceeding 8×10^10 Pa s. In contrast, late-stage leaf burning and both early- and late-stage wood burning produced hygroscopic BBOA with viscosities below 3×10^3 Pa s—over seven orders of magnitude lower. These results show that BBOA viscosity is strongly influenced by both fuel type and burn stage, factors that should be considered in atmospheric models. Importantly, our findings demonstrate that smoldering eucalyptus leaves can directly produce tar balls without requiring atmospheric processing. These particles may act as ice-nucleating agents in mixed-phase and cirrus clouds. We further show that BBOA viscosity can strongly affect the atmospheric lifetime of brown carbon in eucalyptus smoke, potentially extending it by up to four orders of magnitude. This has important implications for evaluating the climate impact of eucalyptus wildfire emissions.