OH-initiated Photooxidation of Gas-phase Monoterpene-derived Hydroxy Nitrates
NGA LEE NG, Yuchen Wang, Masayuki Takeuchi,
Georgia Institute of Technology Abstract Number: 129
Working Group: Aerosol Chemistry
AbstractMonoterpene-derived hydroxy nitrates (MT-HNs) are first-generation products formed from the oxidation of monoterpenes by OH or NO
3 radicals. The subsequent chemical fates of MT-HNs can determine their roles in acting as a permanent sink or temporary reservoir of NO
x, which can further impact NO
x recycling and O
3 formation. However, research on subsequent reaction rates and mechanisms of atmospherically relevant MT-HNs is limited. In this work, we investigated the gas-phase OH-initiated photooxidation of three synthetic HNs derived from α-pinene, β-pinene, and limonene through chamber experiments. The OH reaction rate constants at 297 K measured relative to cyclohexane for these three MT-HNs are in the range of 5.73–11.1 10
-11 cm
3 molecule
−1 s
-1. We also found that the photooxidation of MT-HNs can lead to aerosol formation with high SOA yields. To probe the effects of peroxy radical (RO
2) fate on aerosol formation, “RO
2+NO dominant” and “RO
2+RO
2/HO
2 dominant” experiments are performed. The SOA yields in the “RO
2+NO dominant” experiments (~3-60%) are much lower than “RO
2+RO
2/HO
2 dominant” experiments (~80-150%) with similar initial MT-HN concentrations, which can be explained by the products formed via the RO
2+RO
2/HO
2 pathway are less volatile than those formed via the RO
2+NO route. Photooxidation mechanisms in both gas and particle phases were proposed based on major products identified by high-resolution time-of-flight chemical-ionization mass spectrometer (HR-ToF-CIMS) coupled with the filter inlet for gases and aerosols (FIGAERO). The major products without nitrooxy groups were only identified in “RO
2+RO
2/HO
2 dominant” experiments. We proposed that these compounds were mainly formed via H-abstraction or H-shift, followed by epoxide formation with the loss of NO
2. Results from this study provide the fundamental data to evaluate the contributions of OH-initiated photooxidation MT-HNs as NO
x reservoirs/sinks, and their possible influence on NO
x recycling and SOA formation.