pH Affects the Spontaneous Formation of H2O2 at the Air–Water Interfaces

Recent studies have shown that the air–water interface of aqueous microdroplets is a source of OH radicals and hydrogen peroxide in the atmosphere. Several parameters such as droplet size, salt, and organic content have been suggested to play key roles in the formation of these oxidants. In this study, we focus on the effect of acidity on the spontaneous interfacial hydrogen peroxide formation of salt-containing droplets. Na2SO4, NaCl, and NaBr bulk solutions, at the range of pH 4 to 9.5, were nebulized, using ultra high-purity N2/O2 (80%/20%), and H2O2 was measured in the collected droplets. All of the experiments were performed in T = 292 ± 1 K and humidity levels of 90 ± 2%. For Na2SO4 and NaCl, the H2O2 concentration was increased by ∼40% under alkaline conditions, suggesting that OH– enriched environments promote its production. When CO2 was added in the ultrapure air, H2O2 was observed to be lower at higher pH. This suggests that dissolved CO2 can initiate reactions with OH radicals and electrons, impacting the interfacial H2O2 production. H2O2 formation in NaBr droplets did not display any dependence on the pH or the bath gas, showing that secondary reactions occur at the interface in the presence of Br–, which acts as an efficient interfacial source of electrons.