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

The importance of nitrate radicals $\left({\text{NO}}_3\right)$ as a nighttime atmospheric oxidant is well-established. For decades, laboratory studies of multiphase ${\text{NO}}_3$ chemistry have used the same methods - either ${\text{NO}}_2+{\text{O}}_3$ reactions or ${\text{N}}_2{\text{O}}_5$ thermal decomposition - to generate ${\text{NO}}_3$ as it occurs in the atmosphere. These methods, however, come with limitations, especially for ${\text{N}}_2{\text{O}}_5$ , which must be produced and stored under cold and dry conditions until use. Recently, we developed a new photolytic source of gas-phase ${\text{NO}}_3$ by irradiating aerated aqueous solutions of ceric ammonium nitrate and nitric acid. In this study, we adapted the method to maintain stable ${\text{NO}}_3$ concentrations for over 24 hr. 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 ${\text{NO}}_3$ oxidation of volatile organic compounds (VOCs) emitted by biogenic sources (isoprene, $\beta$ -pinene, limonene and $\beta$ -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 ${\text{NO}}_3$ sources. Maximum SOA yields obtained in our studies ranged from 0.02 ( $\text{isoprene}/{\text{NO}}_3$ ) to 0.96 ( $\beta$ -caryophyllene/ ${\text{NO}}_3$ ). The highest SOA oxygen-to-carbon ratios (O/C) ranged from 0.48 ( $\beta$ -caryophyllene/ ${\text{NO}}_3$ ) to 1.61 ( $\text{syringol}/{\text{NO}}_3$ ). Additionally, we characterized novel condensed-phase oxidation products from $\text{syringol}/{\text{NO}}_3$ reactions. Overall, the use of irradiated aqueous cerium nitrate as a source of gas-phase ${\text{NO}}_3$ may enable more widespread studies of ${\text{NO}}_3$ -initiated oxidative aging, which has been less explored compared to hydroxyl radical chemistry.