Gas- and Particle-Phase Products and Their Mechanisms of Formation from the Reaction of Δ-3-Carene with NO3 Radicals.

Monoterpenes are a major component of the large quantities of biogenic volatile organic compounds that are emitted to the atmosphere each year. They have a variety of structures, which influences their subsequent reactions with OH radicals, O3, or NO3 radicals and the tendency for these reactions to form secondary organic aerosol (SOA). Here we report the results of an environmental chamber study of the reaction of Δ-3-carene, an abundant unsaturated C10 bicyclic monoterpene, with NO3 radicals, a major nighttime oxidant. Gas- and particle-phase reaction products were analyzed in real time and offline by using mass spectrometry, gas and liquid chromatography, infrared spectroscopy, and derivatization-spectrophotometric methods. The results were used to identify and quantify functional groups and molecular products and to develop gas- and particle-phase reaction mechanisms to explain their formation. Identified gas-phase products were all first-generation ring-retaining and ring-opened compounds (ten C10 and one C9 monomers) with 2-4 functional groups and one C20 dinitrooxydialkyl peroxide dimer. Upon partitioning to the particle phase, the monomers reacted further to form oligomers consisting almost entirely of C20 acetal and hemiacetal dimers, with those formed from a hydroxynitrate and hydroxycarbonyl nitrate comprising more than 50% of the SOA mass. The SOA contained an average of 0.94, 0.71, 0.15, 0.11, 0.16, 0.13, and 7.80 nitrate, carbonyl, hydroxyl, carboxyl, ester, peroxide, and methylene groups per C10 monomer and was formed with a mass yield of 56%. These results have important similarities and differences to those obtained from a previous similar study of the reaction of ß-pinene and yield new insights into the effects of monoterpene structure on gas- and particle-phase reactions that can lead to the formation of a large variety of multifunctional products and significant amounts of SOA.