Formation of secondary organic aerosol due to terpenoid ozonolysis in ventilated settings

Somayeh Youssefi

doi:10.17918/etd-6405

The average American spends 18 hours indoors for every hour spent outdoors. Therefore, the quality of air indoors is important and can impact human health. The ozonolysis of monoterpenes impacts indoor pollutant exposure because those reactions generate secondary organic aerosols (SOA), which are condensed phase airborne particulate matter. Ozone (O₃) typically infiltrates indoors with outdoor air, and monoterpenes (C₁₀H₁₆) are unsaturated hydrocarbons emitted from consumer products, such as air fresheners and cleaning agents. Organic aerosol mass formation owing to terpene oxidation can be parameterized with aerosol mass fraction (AMF). The AMF is the ratio of the produced SOA mass to the terpene mass that is oxidized, and it is not constant and increases concurrent with more organic aerosol being available. Prior to this work, prediction of indoor-formed SOA was limited in accuracy because indoor models assumed a constant AMF. As such, the first main objective of this work was to develop an improved indoor formation model that could account for varying AMFs, which was validated with field and laboratory measurements in the literature. Furthermore, current available AMF data in the literature were from atmospheric studies and were measured mostly in unventilated smog chambers for ozone-excess conditions, which is not realistic in most indoor settings. Therefore, the second main objective of this work was to determine the impact of the building air exchange rate (hP-1P), which is the volume normalized airflow through a space, on the AMF of SOA formed due to monoterpene ozonolysis. To do so, two series of experiments were performed with limonene and [alpha]-pinene in a chamber at different air exchange rates (AER) and at realistic concentrations to study the AER and initial reactants' concentrations on SOA formation and the AMF. Limonene ozonolysis AMFs ranged from 0.026 to 0.47, and [alpha]-pinene AMFs ranged from 0.071 to 0.25. Results indicated that as AER increased, the AMF strongly decreased for limonene, but for [alpha]-pinene the impact was in the opposite direction and weaker. Also, for limonene ozonolysis, the ratio of ozone-limonene initial concentrations affected SOA formation positively. These differences arise due to molecular structural differences: Limonene has two double bonds, and secondary ozone chemistry with the remaining exocyclic bond in the SOA phase is the driving factor; [alpha]-pinene only has one, and resulting AER impacts are due to removal of concentrations and competing loss effects. Moreover, limonene has a greater potential to influence indoor SOA concentrations than [alpha]-pinene. Finally, the first and second objectives focused only on aerosol mass formation, but experiments revealed differences in the resulting aerosol size distributions and number formation. For instance, the peak number concentration was decreased for both limonene and [alpha]-pinene ozonolysis as the AER increased. It is due to the fact that exchange of air with outdoors shortens residence time of reactants and continuous removal of indoor air causes a non-equilibrium condition between the gaseous and the particle phases. In the third and final objective of this dissertation, I developed a model to predict the size distribution evolution, which can be used in the future to explore the drivers of the evolution of the SOA size distribution indoors.

Formation of secondary organic aerosol due to terpenoid ozonolysis in ventilated settings

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Formation of secondary organic aerosol due to terpenoid ozonolysis in ventilated settings

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