Online

Fischer, Kevin B.

Chemistry

2021

Ph. D.

Airborne particulate matter consists of small particles suspended in the air and is a ubiquitous component of the Earth's atmosphere. These particles, known as aerosols, broadly affect both human health and the global climate. Secondary organic aerosol (SOA), a subset of atmospheric aerosol, are produced by the gas phase oxidation of volatile organic compounds (VOCs) originating from anthropogenic and biogenic sources. Of particular interest are a sub class of biogenic VOCs released by stressed plants, green leaf volatiles (GLVs), which are susceptible to oxidation via ozonolysis and form SOA. While important strides have been made in better understanding SOA, many of the fundamental yet complex chemical and physical processes occurring at the molecular level that govern SOA formation, growth, and aging are still poorly understood, and many SOA precursors have yet to be identified. Elucidating the physical and phase state of SOA is especially important, as it can provide insight into SOA formation, growth, and broader atmospheric impacts. Here, an electrical low pressure impactor (ELPI) was utilized to probe SOA phase via particle bounce measurements, which were reported between 0 (liquid) and 1 (solid). 1-octen-3-ol (OTL) was identified as a new SOA precursor from sugarcane plant emissions, suggesting it is a potentially significant source of regional SOA. Upon ozonolysis, it produced a suite of oxygenated, low volatility products that partitioned to the particle phase to form SOA exhibiting significant particle bounce (0.6 -- 0.8), indicating high particle viscosity. Further questions prompted investigations into SOA mass loading (CSOA) as it pertains to SOA phase, where SOA bounce was found to be strongly dependent on CSOA for numerous GLVs. Particle bounce dropped by at least 0.25 when CSOA was increased to beyond 10 [mu]g m-3, suggesting enhanced condensation of lower volatility products onto pre-existing SOA at higher CSOA. Furthermore, results imply caution should be exercised when extrapolating chamber results to atmospheric scenarios under high CSOA. Relative humidity (RH) affects physical water uptake and subsequent viscosity changes of SOA. ELPI instrument performance while sampling aerosol from a high RH environment has not been scrutinized previously. Results showed that technical specifications inherent in the current ELPI design, which relies on a set of cascade impactors with both decreasing downstream pressure and impaction stage RH, can prevent accurate particle bounce measurements for aerosol sampled from a high RH (> 90%) environment. The lower RH environments found within the lower impaction stages likely allowed for sufficient drying, enabling particle bounce and thus an instrumental artefact. Still, this method proved useful in determining the phase transition RH for [alpha]-pinene (a GLV) derived SOA, at 37% - 44% RH, and can be utilized for other relevant GLVs.