Secondary organic aerosol (SOA) makes up a substantial portion of airborne particles. SOA particles are known to significantly impact air quality and climate, and to severely and negatively impact health. A comprehensive understanding of physical and chemical processes governing particle formation and growth is needed to accurately predict particle effects.
This thesis examines growth processes of organic particles by quantifying the incorporation of molecular probes (or tracers) into SOA particles from the ozonolysis of α-pinene, prepared both with and without cyclohexane as an OH radical scavenger. The incorporation into model organic solids with varying polarity and functionalities is also examined. The tracers serve to probe properties such as (1) initial surface interactions occurring between the gas and solid phase, (2) magnitude of the partitioning of gases into particles or solid films, and (3) diffusivity changes during uptake and evaporation. This work focuses predominantly on three gaseous organic nitrates (RONO2, 2-ethylhexyl nitrate, β-hydroxyhexyl nitrate, and β-hydroxypropyl nitrate) as tracers, with some experiments using two gas phase organic nitriles (nonanenitrile and valeronitrile) for comparison.
The first approach used attenuated total reflectance (ATR) FTIR to quantify initial uptake coefficients and bulk partition coefficients into thin films of triacontane (TC), cis-pinonic acid (PA), poly(ethylene adipate) di-hydroxy terminated (PEA), and impacted SOA particles both with and without an OH scavenger. The gas phase tracer was introduced over the thin films in a flow of clean dry air. The characteristic IR peaks of the organic nitrates were followed over time until equilibrium was established. There was no measurable uptake onto TC for all of the organic nitrates, but uptake into films of PA, PEA and SOA was significant. The subsequent desorption by flowing clean air over the film showed the uptake was reversible and non-reactive. Quantum chemical calculations provided binding energies and insight into the intermolecular interactions involved in uptake. Kinetic modeling of the diffusion throughout the films showed that the RONO2 acted as a plasticizer, with the diffusion coefficients increasing as more RONO2 was taken up. Upon desorption of the RONO2 from the surface layer, a ‘crusting’ effect was observed, hindering the diffusion from the underlying layers. These studies demonstrate how uptake and evaporation of gases can lead to varying viscosity throughout particles.
The second approach followed the incorporation of the tracers in the flow reactor during particle formation and growth. The incorporation was quantified using ATR-FTIR and high resolution time-of-flight aerosol mass spectrometry. The precursor concentrations were varied to examine how the particle diameter and organic mass loading affect the incorporation of the organic nitrates. The amount of a given RONO2 taken up into the particles relative to its gas phase concentration within the flow reactor was larger than the amount taken up into the thin films. The results of these studies indicate that simultaneous incorporation of RONO¬2 and low volatility ozonolysis products into highly viscous particles may result in gas partitioning that is in excess of that expected for equilibrium partitioning. This supports a new mechanism of particle growth in the atmosphere, an irreversible "burying mechanism". These two systems show that growth of organic particles in air may not always be predicted via equilibrium considerations.