Atmospheric fine particulate matter, or aerosol, has major implications for human health, climate, and visibility. However, our ability to forecast ambient concentrations and climate impacts is limited in part due to uncertainties in how the organic aerosol (OA) fraction forms and evolves over time. OA that is emitted directly is primary (POA), while that which forms from the oxidation of gas-phase species and the subsequent condensation of oxidized products is secondary (SOA). Here we will present the results from experiments on both classes of OA aimed at enhancing our understanding of the underlying chemistry of formation and evolution. In the first half of this talk, I will discuss the results from a series of experiments that highlight the role of molecular structure in generating (or not) OA from the gas-phase oxidation reactions of various C<sub>10 </sub>species: an alkane, alcohol, ketone, aldehyde, nitrate ester, and hydroperoxide. The most likely reaction pathways are then determined from data on the chemical composition, volatility, and yields of OA generated from each, as well as theoretical predictions of product yields from a simple kinetic model. In the second half of the talk, I will discuss our approaches to isolating reactions unique to the condensed (organic or aqueous) phase that impact the chemical composition of OA. Using photolabile alkyl iodides and alkyl nitrites, we directly generated radical intermediates formed in the oxidation of aerosol components within various OA proxies. By monitoring changes to OA composition, we were able to identify the important mechanisms involved in the condensed phase. The results from both sets of experiments enhance our ability to predict ambient aerosol concentrations and properties by identifying the specific structural criteria responsible for OA formation, and highlighting the differences in reactions occurring within the condensed (aerosol) phase.

Formation and evolution of organic aerosol: Role of precursor molecular structure

ACS Fall 2021

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