Effects of Nitrogen Oxides on the Production of Reactive Oxygen Species and Environmentally Persistent Free Radicals from α-Pinene and Naphthalene Secondary Organic Aerosols.

Reactive oxygen species (ROS) and environmentally persistent free radicals (EPFR) play an important role in chemical transformation of atmospheric aerosols and adverse aerosol health effects. This study investigated the effects of nitrogen oxides (NO) during photooxidation of α-pinene and naphthalene on the EPFR content and ROS formation from secondary organic aerosols (SOA). Electron paramagnetic resonance (EPR) spectroscopy was applied to quantify EPFR content and ROS formation. While no EPFR were detected in α-pinene SOA, we found that naphthalene SOA contained about 0.7 pmol μg of EPFR, and NO has little influence on EPFR concentrations and oxidative potential. α-Pinene and naphthalene SOA generated under low NO conditions form OH radicals and superoxide in the aqueous phase, which was lowered substantially by 50-80% for SOA generated under high NO conditions. High-resolution mass spectrometry analysis showed the substantial formation of nitroaromatics and organic nitrates in a high NO environment. The modeling results using the GECKO-A model that simulates explicit gas-phase chemistry and the radical 2D-VBS model that treats autoxidation predicted reduced formation of hydroperoxides and enhanced formation of organic nitrates under high NO due to the reactions of peroxy radicals with NO instead of their reactions with HO. Consistently, the presence of NO resulted in the decrease of peroxide contents and oxidative potential of α-pinene SOA.