Quantification and Mechanistic Investigation of the Spontaneous HO Generation at the Interfaces of Salt-Containing Aqueous Droplets.

There is now much evidence that OH radicals and HO are spontaneously generated at the air-water interface of atmospheric aerosols. Here, we investigated the effect of halide anions (Cl, Br, I), which are abundant in marine aerosols, on this HO production. Droplets were generated via nebulization of water solutions containing NaSO, NaCl, NaBr, and NaI containing solutions, and HO was monitored as a function of the salt concentration under atmospheric relevant conditions. The interfacial OH radical formation was also investigated by adding terephthalic acid (TA) to our salt solutions, and the product of its reaction with OH, hydroxy terephthalic acid (TAOH), was monitored. Finally, a mechanistic investigation was performed to examine the reactions participating in HO production, and their respective contributions were quantified. Our results showed that only Br contributes to the interfacial HO formation, promoting the production by acting as an electron donor, while NaSO and NaCl stabilized the droplets by only reducing their evaporation. TAOH was observed in the collected droplets and, for the first time, directly in the particle phase by means of online fluorescence spectroscopy, confirming the interfacial OH production. A mechanistic study suggests that HO is formed by both OH and HO self-recombination, as well as HO reaction with H atoms. This work is expected to enhance our understanding of interfacial processes and assess their impact on climate, air quality, and health.