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Comparison of Nitrate Radical Initiated Oxidation of Four Monoterpenes in a Laboratory Chamber Study to Gain Mechanistic Insight
MICHELIA DAM, Danielle C. Draper, Andrey Marsavin, Juliane L. Fry, James Smith, University of California, Irvine
Abstract Number: 230
Working Group: Aerosol Chemistry
Abstract
A wide range of SOA and organic nitrate yields have been reported for different monoterpene systems, suggesting that the oxidation mechanisms depend the specific structures of the monoterpenes and illuminating a need to further investigate these mechanisms. To address this need and gain mechanistic insight by assessing yield and composition of oxidation products in both gas and particle phase, laboratory chamber studies were conducted of nitrate radical-initiated oxidation of four monoterpenes, delta-carene, beta-pinene, alpha-pinene and alpha-thujene. Time series of gas phase oxidation products were measured using an LTOF high resolution time of flight analyzer (Tofwerk, AG) and a home-built transverse chemical ionization inlet employing two different reagent ions sensitive to different stages of the oxidation mechanism. Nanoparticle composition was measured using a thermal desorption chemical ionization mass spectrometer (TDCIMS), and organic nitrates in gas and particle phases were measured using a thermal desorption cavity ring down spectrometer (TDCRDS). New particle formation was observed for all monoterpenes except alpha-pinene. Comparisons of organic nitrate yield in gas and particle phase were made between the four monoterpene systems. Relative abundance of highly oxidized gas phase compounds generally corresponded to observed particle mass, with delta-carene and beta-pinene producing many dimers and highly oxidized species and alpha-pinene producing no dimers and few detectable highly oxidized species. In contrast, alpha-thujene exhibited a high gas phase dimer yield but low particle yield. The number and ratios of C, H, N and O atoms were used to identify classes of possible reaction products and assess the dominance of specific mechanistic pathways. These ratios varied greatly between monoterpene systems, highlighting differences in competition between unimolecular and bimolecular radical propagation and termination reaction steps, some of which lead to the formation of low volatility reaction products and particle formation.