UV-light-induced water condensation in air and the role of hydrogen peroxide

Abstract

We report the UV-light-induced formation of water aerosol/droplets in air based on laser-light scattering measurements, differential mobility analyses and cavity ring-down laser spectroscopy (CRDS), and propose a mechanism. It was shown that UV-light (185 nm) irradiation of air within a soap bubble rapidly produced water droplets under supersaturated conditions. Multiple UV irradiation cycles were demonstrated to increase particle sizes in a controlled reaction chamber. We propose that the reaction mechanism is initiated by oxygen (O 2) photodissociation and ozone (O3) formation and further photodissociation of O3 leads to sequential dark reactions, generating hydrogen peroxide (H2O2) as a hygroscopic stable product. Exciting excess O3 in wet air with mid-UV radiations (254 nm) was also demonstrated to produce water particles, indicating that this series of reactions can be started at an intermediate stage using a longer wavelength of UV light. For the first time, it was demonstrated that H 2O2 in air is a precursor of the initial nucleation of water with two different aerosol formation schemes at 263K: a) from a mixture of H2O2 and water vapor in dark and b) from a mixture of O3 and water vapor by mid-UV (266 nm) light irradiation. These results indicate that H2O2 can capture water molecules and form water particles. © 2014 The Chemical Society of Japan.