Specific Ion Chemistry at the Air-Water Interface of Nitrite/Nitrate-Containing Droplets

An increasing number of studies report on the spontaneous production of OH radicals and H₂O₂ at the air/water interface of aqueous droplets. However, none of these studies have investigated OH production in droplets containing nitrate (NO₃^-) or nitrite (NO₂^-) anions. Those two ions, in particular NO₃^-, play a key role in atmospheric chemistry. The goal of this work was to study the spontaneous production of H₂O₂ in water droplets containing these two anions under dark conditions. Compared to ions that we have already studied (i.e., Cl^- and Br^-), H₂O₂ production in droplets containing NO₂^- or NO₃^- can be either enhanced or inhibited depending on the NO₂^- or NO₃^- concentrations. These findings suggest that, below a certain concentration, NO₂^- and NO₃^- enhance the H₂O₂ production due to their ability to disrupt the water structure at the interface, where at larger concentrations, a specific interfacial NO₂^- or NO₃^- chemistry starts to be important, producing gas-phase species such as HONO and NO₂. By monitoring H₂O₂ and several other gas-phase products, we propose a mechanism to explain this specific interfacial chemistry of NO₂^- and NO₃^-. This chemistry may not be significant during day time due the photolysis of these two anions, but it can have an important impact during night time.