Size-ratio-dependent enhancement of ultrafine aerosol coagulation by van der Waals and image potentials

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Brownian coagulation of ultrafine nanoparticles is a key process in both atmospheric particle growth and aerosol synthesis. Accurate characterization of coagulation rates among ultrafine particles is essential for understanding particle evolution in these systems. Interparticle forces such as van der Waals (VDW) potential among neutral particles and image potential between neutral and charged particles can enhance coagulation compared to hard-sphere model. While several enhancement models account for these interactions, improvements are needed to incorporate size-ratio-dependent VDW and image interactions. Furthermore, experimental data remains limited, particularly for unequal-size particle pairs. In this study, we performed coagulation experiments in a chamber using the box-decay method for unequal-size nanoparticles, with both particle populations monodisperse before entering the chamber. Coagulation coefficients were measured for both neutral–neutral and charge–neutral particle pairs to isolate the effects of VDW and image interactions. All cases fall within the transition regime, and the H(KnD) model accurately predicts neutral–neutral coagulation when VDW enhancement factors are applied based on particle size ratio and Hamaker constants. Charge further enhances coagulation in the charge–neutral case where smaller particles are singly charged, and the enhancement due to the image potential depends on the finite sizes of both particles. When the charged particles are significantly smaller compared to their coagulating pair, H(KnD) model with simplified image potential formula is still applicable. Overall, this work can improve the quantitative modeling of ultrafine nanoparticle growth and loss through coagulation in the atmosphere, improving predictions of aerosol evolution from new particle formation events to cloud-condensation-nuclei formation and associated environmental impacts.