Interdroplet Electrostatic Interactions Contribute to Dark Oxidation in Nighttime Sea Sprays

Abstract

Nighttime oxidative processing in the marine boundary layer remains poorly understood because conventional photochemical oxidant sources are strongly diminished after sunset. Here we show that freshly emitted sea spray exhibits enhanced oxidative activity under dark conditions, with oxidative signals consistently exceeding those of the corresponding source seawater at three coastal sites in China. Controlled laboratory experiments further demonstrate that salinity strongly suppresses oxidation in isolated droplets, whereas interacting droplet populations retain pronounced hydrogen peroxide and hydroxyl radical formation, indicating that the oxidative behavior of real sea sprays cannot be fully explained by single-droplet action. Numerical simulations, high-speed imaging, photon-emission measurements, and flow-controlled chamber experiments support a mechanism in which droplet interactions create localized electrostatic environments that promote hydroxyl radical and hydrogen peroxide formation and are linked to measurable changes in gas-phase oxidants under controlled spray conditions. Nighttime observations from three coastal stations show that elevated wave activity coincides with higher ozone and lower NOx and SO2 during hours when photochemical oxidant production is minimized, consistent with sea-spray-associated oxidative processing. These results identify droplet interactions as a previously underappreciated contributor to dark oxidation in sea sprays.