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.