Modeling and measurement of sodium chloride and ammonium sulfate droplet growth in laminar flow

Cloud droplet growth is governed by a balance between vapor diffusion, Kelvin effect, and Raoult’s law, as expressed in K¨ohler theory. In this study, a thermodynamic droplet growth framework initially developed by Brewster and Li (2021) for radiation-induced cooling of pure water droplets is extended to include the effects of dissolved salts through an ion-number-based weighting in the Kelvin–Raoult balance. Two atmospherically relevant salts—sodium chloride (NaCl) and ammonium sulfate ( (NH4)2SO4)—are investigated at multiple concentrations in a controlled laminar-flow environment. Droplets are generated via ultrasonic nebulization, transported through a polyethylene tube under well-characterized temperature and humidity, and measured using optical particle sizing at multiple downstream positions. The model predicts the temporal evolution of droplet radius by solving the coupled mass and heat transfer equations with supersaturation determined from the modified K¨ohler equilibrium. Results show that the extended model captures the droplet size distribution’s primary mode diameter and general shape for both salts across the tested concentrations. Higher ion yield and molar mass effects for ( NH4)2SO4 lead to greater growth enhancement compared to NaCl, consistent with the Raoult term’s suppression of equilibrium vapor pressure. Discrepancies at high solute concentrations and in the large-droplet tail (> 20 μm) are attributed to neglected hydrodynamic coalescence and concentration-dependent surface tension effects. This work demonstrates that pure water thermodynamic growth models can be extended to salt droplets with modifications, providing a validated basis for parameterizing cloud and climate models.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Zhao, Zhiyu

Contributors dc:contributor

• Brewster, M. Quinn

• droplet growth

Statement dc:rights

• Copyright 2025 Zhiyu Zhao

Language dc:language

en

Handle dc:identifier

https://hdl.handle.net/2142/132627

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/132627