Molecular Insights into Gas–Particle Partitioning and Viscosity of Atmospheric Brown Carbon

Biomass burning organic aerosol (BBOA), containing brown
carbon
chromophores, plays a critical role in atmospheric chemistry and climate
forcing. However, the effects of evaporation on BBOA volatility and
viscosity under different environmental conditions remain poorly understood.
This study focuses on the molecular characterization of laboratory-generated
BBOA proxies from wood pyrolysis emissions. The initial mixture, “pyrolysis
oil (PO1)”, was progressively evaporated to produce
more concentrated mixtures (PO1.33, PO2, and
PO3) with volume reduction factors of 1.33, 2, and 3, respectively.
Chemical speciation and volatility were investigated using temperature-programmed
desorption combined with direct analysis in real-time ionization and
high-resolution mass spectrometry (TPD-DART-HRMS). This novel approach
quantified saturation vapor pressures and enthalpies of individual
species, enabling the construction of volatility basis set distributions
and the quantification of gas–particle partitioning. Viscosity
estimates, validated by poke-flow experiments, showed a significant
increase with evaporation, slowing particle-phase diffusion and extending
equilibration times. These findings suggest that highly viscous tar
ball particles in aged biomass burning emissions form as semivolatile
components evaporate. The study highlights the importance of evaporation
processes in shaping BBOA properties, underscoring the need to incorporate
these factors into atmospheric models for better predictions of BBOA
aging and its environmental impact.