Products and Mechanisms of Secondary Organic Aerosol Formation from the NO3 Radical-Initiated Oxidation of Cyclic and Acyclic Monoterpenes

Biogenic sources dominate annual emissions of volatile organic compounds (VOCs) to the atmosphere. A large fraction of these are monoterpenes, which react with OH radicals, NO3 radicals, or O-3 to form oxidized products, some of which partition to particles as secondary organic aerosol (SOA). Here, we compare the results of studies of the reaction of NO3 radicals, a nighttime oxidant, with five monoterpenes: delta-3-carene, beta-pinene, alpha-pinene, limonene, and ocimene. Whereas all of these monoterpenes have the molecular formula C10H16, they differ by having 1, 2, or 3 C=C double bonds and 0, 1, or 2 rings. Experiments were conducted in an environmental chamber under conditions in which RO2 center dot + RO2 center dot reactions were dominant, and gas- and particle-phase products were analyzed using mass spectrometry, gas and liquid chromatography, infrared spectroscopy, and derivatization-spectrophotometric methods. Gas-phase products were first-generation compounds with 2-4 functional groups, whereas SOA products were mostly acetal and hemiacetal dimers formed by particle-phase accretion reactions. The large contribution of dimers formed from hydroxycarbonyl nitrate and hydroxynitrate monomers indicates that they might be used as atmospheric tracers for NO3 radical-initiated reactions of monoterpenes. Conversely, gas-phase formation of ROOR dimers was negligible. Functional group analysis of SOA indicated similar to 1 nitrate, similar to 0.2-0.7 carbonyl groups, and similar to 0-0.4 hydroxyl, carboxyl, ester, and peroxide groups per C-10 product for all the monoterpenes. SOA mass yields were 56, 89, 48, 78, and 69% for delta-3-carene, beta-pinene, alpha-pinene, limonene, and ocimene, which combined with functional group analysis gives lower-limit estimates of organic nitrate yields of 34, 56, 35, 50, and 40%. Results were used to develop reaction mechanisms to explain the formation of gas- and particle-phase products and provide improved understanding of the role of molecular structure in VOC oxidation and particle-phase accretion reactions.