Simultaneous Reduction and Oxidation of NO ₂ on Water Microdroplets Provides Previously Unknown Pathways to the Formation of HONO and HNO ₃

HONO and HNO₃ are important atmospheric species, yet their formation pathways in the atmosphere remain uncertain. In this study, we report the simultaneous formation of HONO and HNO₃ at the air-water interface of water microdroplets in the presence of trace amounts of NO₂. Water microdroplets were generated either in ambient air or in a glovebox where the gas content could be precisely controlled. The products generated by the microdroplets and surrounding gas were directly analyzed with mass spectrometers. Our results indicated that NO₂ gas could be reduced by an electron to form NO₂^-, which was further protonated to yield HONO, or could be oxidized by a hydroxyl radical (·OH) to form HNO₃. Electrons and ·OH were produced by a strong electric field that facilitated OH^- dissociation at the air-water interface of the microdroplets. Tandem mass spectrometry, isotope labeling, and chemical ionization methods confirmed the formation of neutral HONO and HNO₃. Our results provide a previously unknown pathway for the formation of HONO and HNO₃ in the troposphere.