Spontaneous H2O2 formation at the interfaces of salt-containing aqueous droplets: A mechanistic study Maria Angelakia, , Yoan Carreira Mendes Da Silvaa, Sébastien Perriera and Christian Georgea, aUniv Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON, F-69626, Villeurbanne, France Key words: Air – water interface, Spontaneous chemistry, OH and H2O2 formation, Mechanistic investigation Abstract: Sea-salt aerosol particles are considered to be a major source of biogenic atmospheric particles in marine environments. They play a key role in the atmosphere as they can affect cloud albedo and the Earth’s radiation budget. Sea spray aerosols mainly consist of inorganic matter, but they can also contain organics in various mixing ratios. They represent a large fraction of interfaces in the atmosphere, since their size can range between sub-micron to super-micron, depending on their composition and the ageing processes. There is now much evidence that OH and H2O2 can be spontaneously formed at the air – water interface of aqueous droplets due to the presence of a strong electric field (~109 V m−1). OH– anion has been suggested that it partially exist as an ion pair (OHe-), which may undergo charge separation in the presence of this electric field. This can lead to OH radical and electron production, via OH– ⇌ OH. + e–, while H2O2 can be formed via subsequent reactions. In this study, we investigated the interfacial production of H2O2 in salt-containing droplets. We performed measurements on droplets generated by the nebulization of Na2SO4, NaCl, NaBr and NaI bulk solutions, as their ions are the most in marine aerosols. We investigated how the presence of different halides in aerosols could influence the spontaneous H2O2 formation and also their potent involvement in the total mechanistic scheme. The effect of bulk concentration on H2O2 production was also explored. Furthermore, experiments at different bulk pH levels were also carried out, so as to assess the effect of the acidity/alkalinity on oxidants formation. Finally, to enhance our understanding in the reactions that participate in the H2O2 production, we performed a mechanistic investigation. The role of O2 to the overall process was also examined. All the experiments provide evidence that H2O2 is spontaneously formed at the air–water interface of salt-containing aqueous droplets. NaBr droplets, led to higher H2O2 production probably due to the ability of Br– to generate solvated electrons and Br radicals in the presence of a strong electric field, like OH–. O2 was found to play a key role in the H2O2 production, as it shifts the equilibrium of the main reaction OH– ⇌ OH. + e– to the right, promoting the OH and electrons formation. Our mechanistic investigation revealed that H2O2 formation involves different and complex reaction pathways that should be further examined via theoretical calculations. Results from this study are expected to significantly improve our insights on the interfacial processes that occur in atmospheric droplets and on the atmospheric multiphase oxidation chemistry.