Physical and radiative properties of coated nanoparticle aggregates

Coated aggregate nanoparticles are ubiquitous in nature and present a transformative opportunity for nanotechnology. Their unique coatings influence various fields, including climate change, energy, and medicine. However, the impact of coated nanoparticles on climate remains poorly quantified, highlighting the need for a more comprehensive characterisation of the bare nanoparticles and liquid phase. This dissertation introduces a microphysics model (CA2M-OpenCOAT) for generating, visualising, and quantifying realistic coated nanoaggregates. The model adjusts particle size, fractality, polydispersity, and overlap for black carbon (BC) aggregates, aligning them with actual combustion emissions. Artificial liquid-phase growth is incorporated as a proxy for condensational processes. Three growth pathways are explored: capillary condensation in low-saturation environments and extended uniform and non-uniform coatings at higher saturations. Capillary condensation leads to aggregate pore filling via pendular rings, modelled with an ensemble of minimal surfaces, while non-uniform coatings require advanced interface-capturing methods. These growth processes induce significant changes in particle properties, including mass, electrical mobility, surface area, and optical cross-sections. When applied to atmospheric BC aging, the model provides new insights into the climate impact of these short-lived climate forcers. The absorption enhancement of non-uniformly lightly-coated BC is less pronounced than that of laboratory-generated particles and core-shell models, aligning more closely with field measurements. This highlights the influence of BC core and coating morphologies on absorption efficiency and proposes a resolution to discrepancies between laboratory and field data. A mass absorption cross-section parametrisation is developed to emphasise the effects of non-uniform coatings at low coating fractions, extending to cases where BC is fully encapsulated in droplets. The resulting direct radiative forcing is computed in a global model, yielding an average value of 0.18 W/m2 , lower than scenarios with uniformly coated BC aggregates or coated spheres. Additionally, the morphology of coated BC affects its atmospheric removal, with dry deposition simulations showing a significantly extended lifetime compared to spherical models, adversely altering air quality and atmospheric burden. This dissertation also proposes a novel online technique for directly measuring nanoparti cle volume and density. A dedicated growth apparatus is developed to coat particles with an oily liquid and form spherical droplets of controlled size. This allows for precise tracking of changes in mass and mobility, enabling the recovery of volume, porosity, and skeletal density without prior knowledge of material density. Successful recovery is demonstrated for various sample morphologies and compositions, including polymer spheres, powders, and silver and BC aggregates. The unique growth dynamics are identified for each sample, revealing key findings on cluster formation, pore filling, and aggregate restructuring. Coating mechanisms for BC remain poorly understood, yet they are commonly assumed in global models and experimental data inversion. This dissertation proposes a novel experimental strategy to unravel the contributions of condensation and coagulation of BC with a secondary organics surrogate. Both the radiative and transport properties are affected by these mechanisms. Absorption enhancement is minor for coagulated BC, as light interaction between the attached droplet and BC core is limited. In contrast, a lensing effect dominates for condensed BC and a high enhancement plateau is reached after a critical coating fraction. The dynamic shape factor, critical for particle transport, is also investigated. For coagulated BC, its non-monotonic evolution is driven by the attached structure morphology, while for condensed BC the shape factor decreases to one at spherical encapsulation. Finally, new empirical parametrisations of the mass-absorption cross-section for both mechanisms are proposed for aerosol modelling applications. In conclusion, this dissertation advances the understanding of coated nanoparticles, focusing on the detrimental effects of atmospheric coated BC particles and leveraging nanoparticle-coating interactions for advanced aerosol metrology.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Jourdain, Cyprien

Advisor dc:contributor.advisor

• Dedoussi, Irene

• aerosol
• climate
• energy
• experiments
• nanoparticles
• optics
• simulation

Licence

• https://www.repository.cam.ac.uk/bitstreams/ba436aee-185f-4dee-961b-75f5a059dbdd/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.120933

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/388708