Role of black carbon in the formation of primary organic aerosols: Insights from molecular dynamics simulations

Abstract. Many studies on the mixing state of suspended particulate matters (PM) have pointed to the role of carbon particles as nucleation seeds in the formation of atmospheric aerosols. However, the underlying physicochemical mechanisms remain unclear, particularly concerning the involvement of volatile organic compounds (VOCs) at the primary stage of clustering. Here we gain insights into those microscopic formation mechanisms through molecular dynamics simulations of the physisorption of gaseous organic molecules on the surface of a carbon nanoparticle (NP). Six different organic species are selected among the VOCs dominating the atmospheric pollutants of several megacities, to interact with an onion-shell NP that mimics the primary soot particle. We consider organic molecules at various densities on the surface of a NP, as well as the same molecules in a freestanding configuration without any NP.

The molecular clusters formed on the NP are found to be energetically more stable than those formed in the gas phase for all the six organic compounds. This points to a catalytic role of black carbon in the primary formation of aerosols from VOCs. Morphology analysis reveals different manners of clustering of aromatic and aliphatic compounds, which lead to different values of the binding energy and thus different thermal stability. Simulation results also suggest a layer-by-layer formation process of aerosol PM, consistent with previous transmission electron microscopy observations. These results shed light on the microscopic mechanisms of the primary formation of aerosol particulate matters, and are correlated with a variety of experimental measurements.