Van der Waals and Image Potential Enhanced Coagulation in Unequal-sized Ultrafine Particles
BRANDON BOREN, Deepak Sapkota, Yuechen Qiao, Hui Ouyang, The University of Texas at Dallas
Abstract Number: 346
Working Group: Aerosol Physics
Abstract
Coagulation in ultrafine particles is a fundamental process that impacts particle growth and evolution in atmospheric aerosol systems. Accurate estimation of coagulation rates is essential for predicting particle size distribution and concentration in the atmosphere. Van der Waals (VDW) potential among neutral particles and image potential among neutral-charge pairs are common interactions that can enhance nanoparticle coagulation rates, with the degree of enhancement depending on particle size. However, experimental studies on the coagulation of unequal-sized particles with VDW and/or image potential effects are limited. In this study, two separate differential mobility analyzer (DMA) lines were used to generate monodisperse ultrafine particles from sodium chloride (NaCl), ammonium sulfate ((NH₄)₂SO₄), and potassium chloride (KCl) solutions via atomization. One DMA produced smaller particles, while the other generated larger ones. These particles were introduced into a 0.5 m³ coagulation chamber, and a scanning mobility particle sizer (SMPS) was employed to measure the evolution of particle size distributions in the chamber. Coagulation rates were derived from the decay of smaller particles in the presence of larger ones. To measure VDW-enhanced coagulation rates, larger charged particles were removed before entering the chamber. For image-enhanced coagulation measurements, the charge-to-neutral ratio was adjusted. The measured VDW enhanced coagulation rates closely followed the predictions of the H(KnD) model and exhibited a dependence on the particle size ratio of the coagulating pairs. Assuming equal-size enhancement factors may lead to overestimation of coagulation rates, particularly for sub-10 nm particles. Image potentials were found to enhance coagulation more than VDW forces. Further studies are needed to fully quantify the effect of image potential on coagulation, especially for particles smaller than 10 nm.