Molecular Insights\ninto Gas–Particle Partitioning\nand Viscosity of Atmospheric Brown Carbon

Biomass burning organic aerosol (BBOA), containing brown\ncarbon\nchromophores, plays a critical role in atmospheric chemistry and climate\nforcing. However, the effects of evaporation on BBOA volatility and\nviscosity under different environmental conditions remain poorly understood.\nThis study focuses on the molecular characterization of laboratory-generated\nBBOA proxies from wood pyrolysis emissions. The initial mixture, “pyrolysis\noil (PO<sub>1</sub>)”, was progressively evaporated to produce\nmore concentrated mixtures (PO<sub>1.33</sub>, PO<sub>2</sub>, and\nPO<sub>3</sub>) with volume reduction factors of 1.33, 2, and 3, respectively.\nChemical speciation and volatility were investigated using temperature-programmed\ndesorption combined with direct analysis in real-time ionization and\nhigh-resolution mass spectrometry (TPD-DART-HRMS). This novel approach\nquantified saturation vapor pressures and enthalpies of individual\nspecies, enabling the construction of volatility basis set distributions\nand the quantification of gas–particle partitioning. Viscosity\nestimates, validated by poke-flow experiments, showed a significant\nincrease with evaporation, slowing particle-phase diffusion and extending\nequilibration times. These findings suggest that highly viscous tar\nball particles in aged biomass burning emissions form as semivolatile\ncomponents evaporate. The study highlights the importance of evaporation\nprocesses in shaping BBOA properties, underscoring the need to incorporate\nthese factors into atmospheric models for better predictions of BBOA\naging and its environmental impact.