Ultraviolet light-induced water-droplet formation from wet ambient air

We report the formation of water droplets by irradiating wet ambient air with deep UV light. The light sources were either a continuous low-pressure mercury lamp or pulsed ArF laser, which both emit light shorter than 200 nm. Water droplets were produced in reaction vessels under different temperature, relative humidity, and moisture-supply conditions. The particles grew as large as about 0.2 mm. The suggested mechanism is discussed with the photo-dissociations of oxygen and successively formed ozone, and further dark reactions giving hydrogen peroxide as a seeding nucleus. Observed concentrations of intermediates were well explained by simulating the proposed chemical reactions. A possible application to artificial rain is briefly described.