10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Formation and Aging of Secondary Organic Aerosol from Aromatic Compounds
QI CHEN, Yong Jie Li, Xi Cheng, Yan Zheng, Keren Liao, Ying Liu, Tong Zhu, Peking University
Abstract Number: 688 Working Group: Oxidation Flow Reactor: Development, Characterization, and Application to Aerosols
Abstract Oxidation flow reactors have been widely used to study the formation and evolution of secondary organic aerosol (SOA) over time scales ranging from hours to multiple days of equivalent atmospheric exposure. We deployed a Potential Aerosol Mass (PAM) flow reactor in the laboratory to study the formation and oxidative aging of SOA from typical aromatic precursors such as benzene, toluene and naphthalene. NOx levels in the PAM chamber were controlled by N2O injection. Gas-phase precursors and oxidation products are detected by using a proton transfer reaction mass spectrometer and a nitrate-ion chemical ionization time-of-flight mass spectrometer. Non-refractory particle components were detected by using a high-performance time-of-flight aerosol mass spectrometer. Data are compared to ambient observations in Beijing. The mass spectra of highly oxygenated molecules (HOMs) produced by the photooxidation of the three types of aromatic precursors cannot resemble the ambient spectra, suggesting that the majority of HOMs may come from other sources in Beijing. Particle-phase organic nitrate contributes greatly to the SOA mass under conditions of high NOx. The mass fraction of organic nitrate in organic aerosol is highest for toluene SOA and lowest for naphthalene SOA. However, PAM-generated aromatic SOA shows much less N-containing fragments in the spectra compared to urban SOA in Beijing, implying important brown carbon sources other than aromatic oxidation. The elemental ratios of PAM-generated SOA agree well with the previous findings. As the exposure level increases, the atomic oxygen-to-carbon (O/C) ratios of benzene and toluene SOA increase and the hydrogen-to-carbon (H/C) ratios decrease. In contrast, the H/C ratios of SOA generated from naphthalene photooxidation show an opposite trend with aging, suggesting significant fragmentation.