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.

生物质燃烧有机气溶胶（BBOA），其中包含棕色碳色团，在大气化学和气候强迫中发挥着至关重要的作用。然而，在不同环境条件下，蒸发对BBOA挥发性及粘度的影响尚缺乏深入了解。本研究聚焦于由木材裂解排放物生成的实验室制备的BBOA代理物的分子表征。初始混合物“裂解油（PO1）”逐渐蒸发，以产生更浓缩的混合物（PO1.33、PO2和PO3），其体积缩减因子分别为1.33、2和3。采用温度程序脱附结合实时直接分析及高分辨率质谱（TPD-DART-HRMS）对化学组分及挥发性进行了研究。此新颖方法量化了单一物种的饱和蒸汽压和焓，从而构建挥发性基础集分布并量化气体-颗粒分配。粘度估计值，经戳流实验验证，显示出随蒸发过程的显著增加，减缓了颗粒相扩散并延长了平衡时间。这些发现表明，在生物质燃烧排放物老化过程中，高粘度焦油球颗粒的形成与半挥发性成分的蒸发密切相关。本研究强调了蒸发过程在塑造BBOA性质中的重要性，并强调了将此类因素纳入大气模型以更好地预测BBOA老化及其环境影响之必要性。