Gas-Phase Nitrate Radical Production Using Irradiated Ceric Ammonium Nitrate: Insights into Secondary Organic Aerosol Formation from Biogenic and Biomass Burning Precursors

Abstract

The importance of nitrate radicals (NO₃) as an atmospheric oxidant is well-established. For decades, laboratory studies of multiphase NO₃ chemistry have used the same methods – either NO₂ + O₃ reactions or N₂O₅ thermal decomposition – to generate NO₃ as it occurs in the atmosphere. These methods, however, come with limitations, especially for N₂O₅, which must be produced and stored under cold and dry conditions until its use. Recently, we developed a new photolytic source of gas-phase NO₃ by irradiating aqueous solutions of ceric ammonium nitrate and nitric acid. In this study, we adapted the method to maintain stable NO₃ concentrations for over 24 h. We applied the method in laboratory oxidation flow reactor (OFR) experiments to measure the yield and chemical composition of oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA) formed from NO₃ oxidation of volatile organic compounds (VOCs) emitted by biogenic sources (isoprene, β-pinene, limonene, and β-caryophyllene) and biomass burning sources (phenol, guaiacol, and syringol). SOA yields and elemental ratios were typically within a factor of 2 and 10%, respectively, of those obtained in studies using conventional NO₃ sources. Maximum SOA yields obtained in our studies ranged from 0.02 (isoprene/NO₃) to 0.96 (β-caryophyllene/NO₃). The highest SOA oxygen-to-carbon ratios (O/C) ranged from 0.48 (β-caryophyllene/NO₃) to 1.61 (syringol/NO₃). Additionally, we characterized novel condensed-phase oxidation products from syringol/NO₃ reactions. Overall, the use of irradiated aqueous cerium nitrate as a source of gas-phase NO₃ may enable more widespread studies of NO₃-initiated oxidative aging, which has been less explored compared to that of hydroxyl radical chemistry.