UC Irvine

Atmospheric aerosol particles affect climate, air quality, and human health. A major source of ultrafine particles is new particle formation (NPF), in which gas-phase precursors react to form molecular clusters that can grow into particles. Although particle number and size distributions are routinely measured during NPF, the molecular composition of newly formed and growing particles remains difficult to characterize, especially in chemically complex urban-coastal environments. This thesis investigates ultrafine particle composition using thermal desorption chemical ionization mass spectrometry (TDCIMS), with measurements spanning controlled laboratory chamber experiments, outdoor chamber experiments, and ambient field observations.In Chapter 2, particles formed from α-pinene ozonolysis are studied in a controlled chamber experiment to develop a method for interpreting complex TDCIMS thermograms. Positive matrix factorization (PMF) was applied to ultrafine particle TDCIMS measurements to determine whether thermal desorption profiles could be separated into chemically meaningful factors. The analysis resolved six volatility-related factors and one thermal decomposition factor, demonstrating that PMF can organize overlapping TDCIMS ion signals into chemically interpretable classes.In Chapter 3, this PMF-TDCIMS framework is applied to particles formed from ambient precursor gases inside the Captive Aerosol Growth and Evolution (CAGE) chamber during the TRacking Aerosol Convection interactions ExpeRiment - Ultrafine aerosol Formation and Impacts (TRACER-UFI) campaign in Houston. This outdoor chamber provided a partially constrained environment between laboratory experiments and fully ambient sampling. A recurring five-factor structure was resolved across selected NPF days, including a sulfate-related factor and additional factors comprising sulfur-containing, nitrogen-containing, and oxygenated organic ions. The resulting particles were chemically multicomponent, with composition varying as atmospheric conditions changed.In Chapter 4, ambient NPF events during TRACER-UFI are investigated using TDCIMS measurements of size-selected 30 nm particles combined with aerosol, meteorological, and transport analyses. NPF events showed enhanced sulfur-containing classes, including SOx, CHOS, and CHONS, relative to inactive periods. Condensation sink values before NPF onset were comparable to those during inactive periods, indicating that particle sink alone did not control NPF occurrence. Instead, NPF was associated with favorable source and precursor conditions linked to a southwestern transport corridor. This thesis advances molecular-level interpretation of ultrafine particle formation and growth across increasing atmospheric complexity.