UC Irvine

A major fraction of secondary organic aerosol (SOA) in the atmosphere are generated from oxidation of volatile organic compounds (VOCs) emitted from pine trees. Because α-pinene and limonene typically dominate the monoterpene emission profile in forested areas, they have commonly been used as representative monoterpenes in laboratory studies investigating biogenic SOA fundamental properties. However, pine trees emit a wide range of terpenes with different reactivities and structures including numerous isomers of monoterpenes (C10H16) and sesquiterpenes (C15H24), drastically altering the resulting properties of SOA particles such as chemical composition and viscosity. Viscosity is an important physical property of SOA and can lead to much slower diffusion rates within the particles, impacting particle growth and evaporation, gas-particle partitioning, and the ability of SOA particles to act as nuclei for liquid cloud droplets or ice particles. The goal of this thesis is to compare chemical composition and viscosity of SOA particles generated from single terpenes as well as from a mixture of terpenes emitted by pine trees. This work will elucidate how single-terpene SOA compares to real biogenic SOA chemical composition and viscosity, and thus determine if a single-terpene SOA is a good proxy system to model biogenic SOA. A second goal of this work is to identify the influence plant stress has on chemical composition and viscosity of SOA. This is important because plant stress (ex. insect-herbivory) leads to an increase in the emission rates of sesquiterpenes from pine trees. Due to climate change, plants are expected to endure longer periods of stress. However, the impact this change in VOC profile has on SOA properties compared to SOA from healthy trees is not well characterized. Chapter 2 investigated the humidity-dependent viscosity of SOA from ozonolysis of β-caryophyllene, which is the most abundant sesquiterpene emitted by pine trees. In this study, we measured viscosity as a function of RH using a poke-flow technique and measured chemical composition using nano-desorption electrospray ionization mass spectrometry. We found that at an RH of 0 and 48%, the viscosity was between 6.9 × 105 and 2.4 × 107 Pa s, and between 1.3 × 103 and 5.6 × 104 Pa s, respectively. Based on these viscosities and the fractional Stokes–Einstein equation, we found that mixing times of organics within 200 nm β-caryophyllene SOA are fast (<1 h) for RH and temperatures typically found in the planetary boundary layer. Chapter 3, focused on characterizing the molecular composition, viscosity, and liquid–liquid phase separation (LLPS) for SOA derived from synthetic mixtures of terpenes representing emission profiles for Scots pine trees under healthy and aphid-herbivory stress conditions. This work revealed that at 40% and 50% RH, stressed plant SOA had the highest viscosity, followed by healthy plant SOA and then α-pinene SOA in descending order. The stressed plant SOA had increased abundance of higher molecular weight species, reflecting a greater fraction of sesquiterpenes in the stressed VOC mixture compared to the healthy plant VOC mixture. These findings suggest that plant stress influences the physicochemical properties of biogenic SOA. Furthermore, a complex mixture of VOCs resulted in a higher SOA viscosity compared to SOA generated from α-pinene alone, highlighting the importance of studying properties of SOA generated from more realistic multi-component VOC mixtures. As a result of the high viscosity, the mixing time of organic molecules within a 200 nm SOA particle emitted by stressed or healthy tree takes hours, even, under conditions of ambient relative humidity <40%, which is greater than the mixing times for α-pinene SOA. As a follow-up study, we investigated the viscosity of real Canary Island pine secondary organic aerosol to confirm our previous results from synthetic mixtures of tree SOA, using real SOA samples (Chapter 4). We found that SOA generated from real healthy and aphid-stressed pine trees closely matches the viscosity results we found previously where aphid-stressed pine tree SOA consistently had higher SOA viscosity compared to healthy pine tree SOA over all relative humidities investigated. This increased viscosity for stressed SOA was attributed to an increase in terpenes (oxygenated monoterpenes and sesquiterpenes) emitted as a result of aphid-herbivory. Overall, this work highlights the need for using mixtures of terpenes to study the fundamental properties of biogenic SOA and how plant stress leads to even more viscous SOA compared to healthy pine tree SOA.