pH Affects the Spontaneous Formation of H 2 O 2 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. Na 2 SO 4 , NaCl, and NaBr bulk solutions, at the range of pH 4 to 9.5, were nebulized, using ultra high-purity N 2 /O 2 (80%/20%), and H 2 O 2 was measured in the collected droplets. All of the experiments were performed in T = 292 ± 1 K and humidity levels of 90 ± 2%. For Na 2 SO 4 and NaCl, the H 2 O 2 concentration was increased by ∼40% under alkaline conditions, suggesting that OH - enriched environments promote its production. When CO 2 was added in the ultrapure air, H 2 O 2 was observed to be lower at higher pH. This suggests that dissolved CO 2 can initiate reactions with OH radicals and electrons, impacting the interfacial H 2 O 2 production. H 2 O 2 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.