Generation of reactive oxygen species in water droplets levitated in air

Water droplets exceeding 100 µm in diameter are commonly found in natural aerosols, where the surfaces of these droplets often acquire charges owing to contact and friction with the surrounding air and particulates, influencing various atmospheric processes. This study reports the generation of reactive oxygen species (ROS) in millimeter-sized droplets levitated in air. The friction between the droplets and air results in positive charge accumulation on the droplet surface, which leads to ROS production from the water while smaller, negatively charged droplets escape from the surface. The concentration of ROS increases with droplet suspension time. Under electrostatic fields, such as those present in cumulonimbus clouds, corona discharge occurs on the droplet surface, further enhancing ROS formation. These findings offer a new mechanistic explanation for the presence of ROS in raindrops, arising from chemical processes at the charged air-water interface of atmospheric droplets.