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

Abstract

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 O3 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: Δ-3-carene, β-pinene, α-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• + RO2• 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 ∼1 nitrate, ∼0.2–0.7 carbonyl groups, and ∼0–0.4 hydroxyl, carboxyl, ester, and peroxide groups per C10 product for all the monoterpenes. SOA mass yields were 56, 89, 48, 78, and 69% for Δ-3-carene, β-pinene, α-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.