Renoxification via Reduction of Inorganic Nitrate by Photoacids in Aerosols

Abstract Particulate nitrate (pNO3–) present in biomass burning aerosols has been found to efficiently release reactive nitrogen oxides (NOy) in a process called renoxification. Despite the importance this process has in promoting ozone formation in the lower atmosphere, mechanisms responsible for highly efficient renoxification from pNO3– are not well understood. Herein, we show that renoxification from pNO3– is enhanced in the presence of catechol and related phenols, which act as strong reductants that directly reduce NO3– to HONO and NO when they absorb UV–visible light. Aerosol chamber studies using nebulized solutions of gallic acid and nitrate showed that 23 times more NOy was formed with gallic acid in comparison to pNO3– photolysis in the absence of gallic acid. Renoxification was found to be pH-dependent, with HONO formation also occurring at pH 8, which is higher than the pKa of HONO, suggesting phenols are also acting as photoacids in this system. Our findings provide insights into the role of phenol-containing aerosols in contributing to the control of NOy budgets in biomass burning plumes and bring us closer to more accurately representing pNO3– renoxification in atmospheric chemistry models.