Functional Group Composition of Secondary Organic Aerosol Formed from Ozonolysis of α‑Pinene Under High VOC and Autoxidation Conditions

The formation of secondary organic aerosol (SOA) from α-pinene ozonolysis has been widely studied, with a recent focus on contributions from highly oxidized multifunctional compounds (HOMs) that have been observed in laboratory and field studies. Most of what is known about the chemical composition of SOA and HOMs, however, consists of molecular formulas and limited molecular structure identification based on mass spectrometric analysis. Here, we characterized the SOA formed from α-pinene ozonolysis using derivatization-spectrophotometric methods to quantify peroxide, carbonyl, carboxyl, ester, and hydroxyl groups. Experiments were conducted over a range of α-pinene concentrations and relative humidities, including regimes in which gas-phase HOMs were detected using NO3– chemical ionization mass spectrometry. Results for experiments conducted with high concentrations of α-pinene were also compared with predictions of a model that employed the Master Chemical Mechanism and included gas-particle and gas-wall partitioning. It appears that gas-phase monomer and dimer products formed through RO2• + RO2•, RO2• + HO2, RO2• isomerization, and stabilized Criegee intermediate + carboxylic acid or water reactions contributed to SOA formation, but that in particles the aldehyde and ketone groups in these compounds were often converted to carboxyl and ester groups through Baeyer–Villiger reactions with hydroperoxides and peroxycarboxylic acids. Evidence also indicates that hydrolysis of dimers containing diacyl peroxide groups contributed to the formation of carboxyl and ester groups, that hydroxyl groups were less abundant in SOA than expected (because of minor gas-phase alkoxy radical isomerization or conversion to an undetectable acetal oligomer), and that gas-to-particle partitioning of small carbonyl compounds may have contributed to SOA