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.
Silva et al. (Fri,) studied this question.