Spontaneous formation of OH radicals and H2O2 at the air-water interface of aqueous droplets

Overview & Synopsis

In recent years, an increasing number of studies have focused on interfacial regions such as the air–water interface. This region lies midway between the two phases it separates and becomes particularly important in small systems owing to the large surface-to-volume ratio. At the air–water interface, the physical and chemical properties differ from those of both the bulk and the gas phase. As a consequence of these interfacial specificities, reactions can be accelerated relative to the bulk, and processes that are unfavourable in the bulk may occur at the interface. One example is the spontaneous formation of OH radicals and Hydrogen peroxide (H2O2) at the air-water interface of aqueous droplets. A first hypothesis is that the strong electric field at the air–water interface permits the dissociation of hydroxide ion (OH-) into OH radical and an electron. H2O2 can then be formed by recombination of two OH radicals or via an electron–oxygen pathway. This spontaneous interfacial chemistry would constitute a potential additional source of oxidants in the atmosphere. The studies presented here investigate the spontaneous formation of OH radicals and H2O2 in salts containing droplets. An experimental approach was employed. Droplets were generated inside a reactor using a nebulizer and salt solutions. Instruments were connected to the reactor to characterize both the droplets and the gas phase. An impactor system collected liquid from the droplets for offline analyses, such as H2O2 quantification. This approach allows quantification of the spontaneous formation of OH radicals and H2O2 by droplets and enables mechanistic investigation. The main results of the studies presented is that H2O2 and OH radicals are indeed produced by the droplets, and that this production depend on the ionic composition of the droplets. The observed trends follow the Hofmeister series, suggesting that interfacial solvation is the dominant driver of the reaction and not the strong interfacial electric fields. Another key finding is that droplets containing nitrite (NO2-) or nitrate ions (NO3-) can inhibit H2O2 formation due to ion-specific interfacial chemistry. This chemistry is observed under dark conditions and shows similarities to their photolysis pathways and may be relevant to night time atmospheric processes. Overall, this work provides results that helps to understands why oxidant can be produced at the interface and highlight the fact that interfacial region has a reactivity different from the bulk and can play a crucial role in atmospheric chemistry.