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.