A Framework for Quantifying the Size and Fractal Dimension of Compacting Soot Particles

Black carbon (BC) is a strongly absorbing component of atmospheric aerosols that has a significant warming effect. BC particles are emitted from combustion sources as open-structured fractal aggregates. After emission, BC is often compacted due to capillary condensation of semivolatile vapors to form coatings. The addition of coatings influences the size and radiative properties of BC, but representing these details in radiative transfer models is computationally difficult and often neglected. Laboratory studies have measured BC restructuring during coating but rarely provide information on changes in particle shape. Here, we combine laboratory measurements of BC compaction with detailed restructuring models to develop a framework for predicting the size and shape of BC as a function of coating volume ratio, a property already tracked in large-scale atmospheric models. The framework predicts the mobility diameter and fractal dimension of BC particles as a function of coating volume throughout compaction with root-mean-squared error (RMSE) values less than 6.8 and 4.3%, respectively. These properties are predicted for both the coated particle and the BC core. Our proposed framework will enable a more complete representation of the evolving size and shape of BC throughout its atmospheric lifetime, thereby improving model accuracy at a low computational cost.