Smoldering Eucalyptus Leaves and Wood Results in Both High Viscosity Tar Balls and Low Viscosity Biomass Burning Organic Aerosol

CHANGDA WU, Evan Chartrand, Hamed Nikookar, Julia Zaks, Mei Fei Zeng, Steven Rogak, Allan K. Bertram, University of British Columbia

     Abstract Number: 268
     Working Group: Aerosol Physics

Abstract
Uncontrolled burning of eucalyptus forests in Australia is a major source of atmospheric smoke, primarily composed of biomass burning organic aerosol (BBOA). Viscosity and phase state are fundamental physical properties that impact the behavior of aerosols in the atmosphere and have not been characterized for eucalyptus BBOA. In this study, we generated BBOA in the lab by smoldering eucalyptus leaves and wood in a tube furnace, and measured viscosity and phase state using transmission electron microscopy, optical microscopy, and fluorescence recovery after photobleaching. Early-stage burning of eucalyptus leaves produced non-hygroscopic tar balls with viscosities exceeding 10⁹ Pa s. In contrast, late-stage burning produced hygroscopic BBOA with much lower viscosities (~3×10¹ Pa s), differing by over seven orders of magnitude. Eucalyptus wood produced two types of hygroscopic BBOA: early-stage particles with viscosities below 3×10³ Pa s and late-stage particles below 3×10² Pa s. We used a resistor model and our viscosity data to estimate the atmospheric lifetime of brown carbon within BBOA. The results suggest that brown carbon in tar balls from early-stage leaf burning may be resistant to whitening by ozone due to its high viscosity. In contrast, brown carbon in lower-viscosity BBOA—from later-stage leaf burning and all stages of wood burning—may react quickly with ozone (lifetime 0.4-6.7 days), leading to particle whitening and changes in their radiative properties.