Abstract:

Aerosol particles are ubiquitous in the atmosphere and influence climate and human health. Ultrafine particles (< 100 nm in diameter) deposit deep in the respiratory tract and can translocate beyond the lungs to other organs. They dominate atmospheric particle number and, once grown to cloud condensation nuclei sizes, affect cloud formation and haze. Most of these particles are formed in the atmosphere through new particle formation, in which trace gases react to produce molecular clusters that grow into ultrafine particles. Knowing what these particles are made of is central to understanding how they form and grow, but their small size and low mass put them beyond the reach of most atmospheric instruments, particularly in chemically complex urban-coastal environments where many precursors compete.

My dissertation addresses this gap using thermal desorption chemical ionization mass spectrometry to measure ultrafine particle composition across three settings of increasing atmospheric complexity. In controlled laboratory experiments, I show that positive matrix factorization can separate overlapping thermal desorption signals into interpretable volatility-related and decomposition factors, establishing a framework for reading these measurements. Applying that framework to particles grown from real Houston air inside the Captive Aerosol Growth and Evolution (CAGE) outdoor chamber during the TRacking Aerosol Convection interactions ExpeRiment (TRACER), I find a recurring multi-factor structure that reveals chemically multicomponent particles whose composition shifts with ambient conditions. Finally, in fully ambient measurements of size-selected 30 nm particles, I show that formation events are marked by enhanced sulfur-containing chemistry and are governed less by the availability of particle sink than by precursor-rich air arriving along a southwestern transport corridor.

This work advances our molecular-level understanding of how ultrafine particles form and grow, and demonstrates an approach that carries from the controlled laboratory into the real urban atmosphere.