| Abstract
| - The composition of secondary organic aerosol (SOA) from the ozonolysis of C5−C8 cycloalkenes and α-pinene,as well as the effects of hydrocarbon precursor structure and particle-phase acidity on SOA formation, havebeen investigated by a series of controlled laboratory chamber experiments. A liquid chromatography−massspectrometer and an ion trap mass spectrometer are used concurrently to identify and to quantify SOAcomponents with molecular weights up to 1600 Da. Diacids, carbonyl-containing acids, diacid alkyl esters,and hydroxy diacids are the four major classes of low-molecular-weight (MW < 250 Da) components in theSOA; together they comprise 42−83% of the total SOA mass, assuming an aerosol density of 1.4 g/cm3. Inaddition, oligomers (MW > 250 Da) are found to be present in all SOA. Using surrogate standards, it isestimated that the mass fraction of oligomers in the total SOA is at least 10% for the cycloalkene systems(with six or more carbons) and well over 50% for the α-pinene system. Higher seed particle acidity is foundto lead to more rapid oligomer formation and, ultimately, to higher SOA yields. Because oligomers are observedto form even in the absence of seed particles, organic acids produced from hydrocarbon oxidation itself mayreadily promote acid catalysis and oligomer formation. The distinct effects of carbon numbers, substituentgroups, and isomeric structures of the precursor hydrocarbons on the composition and yield of SOA formedare also discussed.
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