Changes of optical properties of two carbonaceous nanoparticles upon their coalescence: Computations at the atomistic level

Once emitted into the atmosphere, soot nanoparticles undergo ageing processes during which they may collide and form aggregates, possibly modifying their physico-chemical properties. Here, we especially focus on their optical properties and the Dynamic Atomic Dipole Interaction (DADI) approach has been used to calculate the refractive index and the mass absorption cross section of two carbonaceous nanoparticles modeling soot. Three typical situations have been considered: separated particles, particles just in contact, interpenetrated particles. Molecular dynamics simulations based on a reactive interatomic interaction potential have been used to get the atomic coordinates in the three configurations considered and the graphite parametrization has been used for defining the polarizability tensors of the carbon atoms. The results of the DADI calculations show that the interpenetration of the nanoparticles actually modifies both their refractive index and their mass absorption cross section with respect to well-separated particles. Because the corresponding changes are shown to be tightly bound to local modifications of the close neighborhood of the carbon atoms, the present study emphasizes that an as precise as possible description of the coalescence process at the atomic scale is required to better quantify the influence of aggregation processes on the optical properties of carbonaceous nanoparticles. In addition, calculations at the atomic scale via the DADI method allow the computation of refractive indices taking into account the local modifications of the particles, which are shown to be then usable as inputs in macroscopic models, such as T-matrix. This would certainly help at better assessing, e.g., the actual impact of soot on climate.