Observational and Computational Studies of Atmospheric Particle Formation

Aerosols are a ubiquitous component of the atmosphere, playing pivotal roles in air quality and climate. This thesis explores the way these particles come to be, and their roles in these atmospheric processes. Aerosols form from a variety of anthropogenic and biogenic activities, processes which are very prominent in urban settings. In Los Angeles, the last decade of research has been dominated by the role of summertime secondary organic aerosol (SOA) in contributing to particulate matter (PM). Here, we make observations in the equinox seasons and in the winter and detail the formation of atmospheric aerosols in these seasons. Using aerosol mass spectrometry, we demonstrate that ammonium nitrate persists as one of the dominant secondary aerosol components despite dramatic reductions in nitrogen oxide (NOₓ) emissions. Further, we show that this ammonium nitrate is not measured by routine air quality measurements, biasing regulatory PM2.5 measurements. In the wintertime, similar techniques demonstrate that primary organic aerosol, as opposed to secondary, is an important component of the PM2.5, contrary to the prevailing narratives that SOA dominates the aerosol mass. At global scales, the role of these aerosols in cloud formation and climate processes is of primary interest. While a variety of physicochemical properties of aerosols are important in the formation of cloud droplets, we focus here on the specific process of organic surface-partitioning. It has been suggested that in phase-separated aerosol, organic-rich surface layers can depress the surface tension of the particles, lowering their barrier to activate into cloud droplets. We assess this propensity for surface tension depression in two SOA systems, α-pinene and β-caryophyllene. Synergizing laboratory measurements, a thermodynamic model, and field data, it is shown that surface-active organics in these SOA systems can impact their hygroscopicity, though perhaps not sufficiently to warrant inclusion of these processes in global-scale models.