Abstract: 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, ∆-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 the details of 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. Using quantum chemical methods, we begin by probing an early oxidation step: alkyl and alkoxy radical fates for several of the most abundantly emitted monoterpenes, determining if and how their mechanisms might diverge from each other, leading to the large variability in SOA yields reported in literature. We then survey later generation unimolecular oxidation pathways for a single monoterpene, ∆-carene, to characterize the ways in which these precursors can form highly oxidized, condensable species in the gas phase, and we compare our theoretical mechanism to observations from chamber experiments probing this chemistry. Finally, we explore ambient nanoparticle composition observations, finding evidence of contributions from NO3 + monoterpene chemistry, and we assess the gas-to-particle conversion mechanism that is likely responsible for these observations. Overall, we find numerous mechanistic pathways that produce highly oxidized, low volatility compounds from these precursors in the gas-phase, and we observe even less oxidized species in ambient nanoparticles, justifying the important role this chemistry is expected to play in SOA formation in the atmosphere.
Friday, December 18, 2020 - 2:30pm