UC Irvine

It has been shown though numerous field studies, laboratory measurements, and the occasional modeling study that NO3 radical oxidation of monoterpenes is a significant, though often overlooked, source of secondary organic aerosol (SOA). However, this generalization is complicated by the fact that while most abundantly emitted monoterpenes (e.g. β-pinene, Δ-carene, and limonene) have moderate-to-high SOA yields with NO3 radical, the most abundantly emitted monoterpene (α-pinene) has a negligible SOA yield with NO3. As a result, the contribution of NO3 chemistry to the global SOA budget relies strongly on regional variability in vegetation and is therefore quite difficult to parameterize into models. In this work we investigate how particles form and grow from monoterpene + NO3 chemistry. SOA originates from gas-phase oxidized organics and therefore a major focus of this work is mechanism development of reaction pathways not previously characterized for this system. In chapter 2, we use quantum chemical calculations to determine the fate of first-generation alkyl and alkoxy radicals for five cyclic monoterpenes. The fate of these radical intermediates determine whether these species are able to undergo additional radical propagation reactions that would lead to highly oxidized products or if the radicals terminate at relatively low oxidation states. We find that in spite of structural similarities between many of the cyclic monoterpenes studied, they all favored different combinations of alkyl and alkoxy radical pathways, likely playing a role in the large variability of SOA yields observed from these precursors. In chapter 3, we continue using quantum chemical methods to explore the fates of second-generation peroxy and alkoxy radicals for a single monoterpene, Δ-carene. At this stage of the reaction mechanism, intermediates are already somewhat highly oxidized and these species are more likely to contribute (directly) to SOA than those discussed in the previous chapter. Here we computed rate constants for a variety of unimolecular reactions and compared those rate constants to estimated bimolecular rates. We found that both unimolecular and bimolecular reactions play a significant role in radical propagation (or termination) from second-generation peroxy radicals. Additionally, the derived mechanism was compared to NO3- CIMS data from Δ-carene + NO3 chamber experiments and we were able to identify products consistent with the dominant reaction pathways in the computational mechanism. Finally, in chapter 4 we analyze ambient observations of nanoparticle composition collected using the Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) in the boreal forest Hyytiälä, Finland). We identify two abundant ions in this dataset with a striking diurnal cycle, peaking sharply at night, that are consistent with oxidized monoterpenes, though with quite low oxidation states. We postulate that these ions originate from first-generation organonitrates from monoterpene + NO3 chemistry. We then assess the formation kinetics and partitioning thermodynamics using supporting, coincident measurements and find that the kinetics and partitioning of the hypothesized organonitrates are consistent with the TDCIMS observations, but the magnitude of these species in the particle-phase remains uncertain.