Effects of NOx on the Formation of Reactive Oxygen Species and Environmentally Persistent Free Radicals from Biogenic and Anthropogenic Secondary Organic Aerosols

KASEY EDWARDS, Alexandra Klodt, Tommaso Galeazzo, Meredith Schervish, Jinlai Wei, Ting Fang, Bernard Aumont, Sergey Nizkorodov, Manabu Shiraiwa, University of California, Irvine

     Abstract Number: 108
     Working Group: Aerosol Chemistry

Abstract
Reactive oxygen species (ROS) such as hydrogen peroxide (H₂O₂), superoxide (O₂^-), hydroxyl radical (^·OH), and hydroperoxy radical (HO₂^·) play an important role in chemical transformation of aerosols in the atmosphere and adverse aerosol health effects. Aqueous reactions of secondary organic aerosol (SOA) compounds, such as organic hydroperoxides and alcohols, can lead to the formation of ROS. Environmentally persistent free radicals (EPFRs) with longer lifetimes, from minutes to months, are contained in SOA derived from aromatic precursors. This study investigated whether the presence of NOx during SOA formation, which is known to affect SOA chemical composition, has an impact on the EPFR content of SOA and formation of ROS from SOA. SOA produced by OH oxidation of alpha-pinene and naphthalene under low-NOx and high-NOx conditions was analyzed with electron paramagnetic resonance (EPR) spectroscopy to quantify the concentrations of ROS and EPFRs. The analysis showed that alpha-pinene and naphthalene SOA generated without NOx forms OH radicals and superoxide in the aqueous phase, which can be reduced by 50-80% for SOA generated in the presence of NOx. High resolution mass spectrometry analysis showed the formation of organic nitrates in a high NOx environment. The GECKO-A model, an automated and explicit gas-phase chemistry model, was used to compute the amounts of different functional groups present in alpha-pinene SOA. The Radical 2D-VBS model, which includes autoxidation and dimerization, was also used to compute the formation of organic hydroperoxides. Both model outputs showed that the formation of hydroperoxides would be dramatically reduced under a high NOx environment due to the reactions of peroxy radicals with NOx instead of their reactions with HO₂.