Ultrafine aerosol particles: Long-range interactions, aggregation kinetics and structure

A theoretical and computational study of ultrafine aerosol particle aggregation including the long-range van der Waals interaction force is presented. Previously, studies of aggregation have not rigorously incorporated the effects of particle interactions. The significance of this work lies in the use of physically motivated interaction potentials in calculations of aggregation. In the first part of this study, a highly accurate approximation is developed whereby, for the first time, the van der Waals energy can be calculated for any geometry. In the aggregation process considered here, the geometry of interest is an irregular aggregate of adhering, spherical primary particles and an approaching primary particle (monomer). The effect of retardation of the long-range energy is also incorporated. In the second part of this study, the effect of these retarded, long-range van der Waals interactions, particle transport and ambient pressure and temperature on aggregate-monomer collision rate constants and aggregate structure are investigated by performing molecular dynamics simulation calculations. Glassy carbon is chosen as the prototype material for the simulations.; In general, the aggregates grown with the interaction potential tend to have relatively open structures, with few branches, while the aggregates grown without the potential tend to be more compact and branched. Further, the interaction potential results in enhancements in the collision rate constants over the corresponding geometric rate constants. The effects are smaller in the transition regime than in the free molecular regime. Simulations performed with the non-retarded and the retarded interaction potential show that the percentage of relatively open aggregates, and the magnitude of the collision rate constants are greater in the latter case than in the former. An increase in temperature resulted in a collapse of aggregate structure and decrease in collision rate constants. The effects are more pronounced in the free molecular than in the transition regime. No significant difference was observed in the structure of the aggregates or in the aggregate-monomer collision rate constants as a result of changing the pressure of the simulations from 760 mm to 3040 mm.