Inorganic Salts Modulate Spontaneous Interfacial H2O2 Formation in Surfactant-Containing Atmospheric Droplets.

Hydrogen peroxide (H2O2) forms spontaneously at the air-water interface of atmospheric droplets, yet how this process is regulated in atmospherically relevant droplets containing surfactants remains unresolved. Here, by the gas-nebulization method, we generated droplets containing nonanoic acid (NA), a ubiquitous surface-active fatty acid in atmospheric aerosols, and systematically examined its influence on interfacial H2O2 formation. Increasing the NA concentration slightly suppressed H2O2 production primarily via scavenging its precursor •OH. Notably, as different inorganic salts were added to surfactant-containing droplets, H2O2 production displayed a distinct "V-shaped" trend with the salt concentration. This salt-induced "V-shaped" dependence closely mirrors surface tension variation reported for bulk surfactant solutions. Ab initio molecular dynamics simulations revealed that salt-induced deeper anchoring of the NA headgroup at the H2O2 minimum may enable optimal packing of NA, thereby increasing the interfacial NA concentration, enhancing •OH scavenging and suppressing H2O2 formation. Similar concentration-driven suppression is also observed for two charged surfactants, while salt responses diverge, with "V-shaped" behavior occurring only for headgroups capable of hydrogen bonding with interfacial water. These findings demonstrate that spontaneous H2O2 formation at droplet interfaces is governed by the coupled effects of inorganic salts and surfactants, which commonly coexist in atmospheric aerosol droplets.