Discrete element method model of soot aggregates

TL;DR

This paper introduces a discrete element method model to simulate the restructuring of fractal soot aggregates, revealing how neck fraction influences compaction pathways and the rapid timescale of full compaction impacts atmospheric soot aging.

Contribution

The study develops a novel contact model for soot restructuring based on atomic force data, providing insights into aggregate compaction mechanisms and timescales.

Findings

Neck fraction determines local versus global compaction pathways.

Full compaction occurs within tens of nanoseconds.

Condensation rate influences restructuring and soot coating characteristics.

Abstract

Soot is a component of atmospheric aerosols that affects climate by scattering and absorbing the sunlight. Soot particles are fractal aggregates composed of elemental carbon. In the atmosphere, the aggregates acquire coatings by condensation and coagulation, resulting in significant compaction of the aggregates that changes the direct climate forcing of soot. Currently, no models exist to rigorously describe the process of soot restructuring, reducing prediction accuracy of atmospheric aerosol models. We develop a discrete element method contact model to simulate restructuring of fractal soot aggregates, represented as assemblies of spheres joined by cohesion and by sintered necks. The model is parametrized based on atomic force spectroscopy data and is used to simulate soot restructuring, showing that the fraction of necks in aggregates determines the restructuring pathway. Aggregates with fewer necks undergo local compaction, while aggregates with nearly-full necking prefer global compaction. Additionally, full compaction occurs within tens of nanoseconds, orders of magnitude faster than the time scale of soot aging through condensation. An important implication is that in atmospheric soot aggregates, the rate of condensation determines how many necks are fractured simultaneously, affecting the restructuring pathway, e.g., producing highly compact, thinly-coated soot as observed in recent studies.