Towards a Physically Informed Understanding of New Particle Formation and Growth in the Atmosphere

August 29, 2025

Samuel O’Donnell

Committee: Jeffrey Pierce (Advisor); Emily Fischer; Sonia Kreidenweis; Shantanu Jathar (Mechanical Engineering)

Abstract

Aerosol particles are among the most uncertain yet influential components of Earth’s radiative balance, influencing it directly through interactions with radiation and indirectly by serving as the formation sites for cloud droplets and ice crystals. Understanding the size and composition of the aerosol particles is critical for understanding their climate impacts. The majority of aerosol particles in the atmosphere form through the process of new particle formation and growth (NPF&G), the process by which new small particles form from the clustering of low-volatility vapors. These newly formed particles may grow to climatically relevant sizes through condensation of vapors or be lost through collisions with pre-existing larger particles. NPF&G events are time periods where NPF&G occurs, and the particles grow substantially over the following several hours. This dissertation examines the processes governing NPF&G in the atmosphere.

First, we use observations and a 3D chemical transport model to contrast stationary-site and Lagrangian perspectives of NPF&G events, showing that the trajectory-based analysis can resolve ambiguities in stationary aerosol size distribution observations at the Southern Great Plains (SGP) site. Second, we examine NPF&G in an urban context using the Captive Aerosol Growth and Evolution (CAGE) chamber during the TRACER-UFI campaign, combining unique chamber measurements with the SOM-TOMAS box model to elucidate the processes governing NPF&G in the chamber. Finally, we assess the global impacts of particle phase state and size-dependent SOA condensation on aerosol size distributions using GEOS-Chem–TOMAS, finding substantial changes in aerosol number concentrations and improved model to measurement agreement when phase-state-dependent condensation is included. Together, these studies provide insights into the NPF process, how particles grow, and how NPF&G shapes aerosol size distributions in different environments.