Formation, Abundance, and Evolution of Molecular Products in α-Pinene and β-Pinene Secondary Organic Aerosol

Citation

Kenseth, Christopher M. (2022) Formation, Abundance, and Evolution of Molecular Products in α-Pinene and β-Pinene Secondary Organic Aerosol. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/bj1b-1441. https://resolver.caltech.edu/CaltechTHESIS:03242022-211905773

Abstract

The atmospheric oxidation of α-pinene and β-pinene (C₁₀H₁₆), emitted in appreciable quantities from forested regions (~85 Tg y^–1), contributes significantly to the global burden of secondary organic aerosol (SOA), a substantial component (15–80% by mass) of atmospheric fine particulate matter (PM_2.5), which exerts large but uncertain effects on climate as well as adverse impacts on air quality and human health. Deciphering the molecular composition, and in turn formation and aging mechanisms, of α-pinene and β-pinene SOA is essential to reducing uncertainty in assessment of their environmental and health impacts. However, molecular characterization of α-pinene and β-pinene SOA is significantly hindered by their chemical complexity. In this work, we constrain the formation, abundance, and evolution of molecular products in SOA derived from ozonolysis and photooxidation of α-pinene and β-pinene using a combination of laboratory experiments, liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS), and organic synthesis. Through detailed MS/MS analysis, coupled with ¹³C isotopic labeling and OH scavenging, we identify a suite of dimeric compounds (C_15–19H_24–32O_5–11) formed from synergistic O₃ + OH oxidation of β-pinene (i.e., accretion of O₃- and OH-derived products/intermediates). Informed by these structural analyses, together with ¹⁸O isotopic labeling and H/D exchange, we synthesize the first authentic standards of several major dimer esters identified in SOA from ozonolysis of α-pinene and β-pinene and elucidate their formation mechanism from targeted environmental chamber experiments. Additionally, we synthesize a series of pinene-derived carboxylic acid and dimer ester homologues and find that the ESI efficiencies of the dimer esters are 19–36 times higher than that of commercial cis-pinonic acid, a common quantification surrogate. Finally, we investigate the aqueous (photo)chemistry (kinetics, products, and mechanisms) of the carboxylic acid and dimer ester homologues at cloudwater-relevant concentrations as a function of pH.

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