Condensing water vapor to droplets generates hydrogen peroxide

Significance Water molecules in bulk liquid are stable and inert under ambient conditions. In sharp contrast, we show that the condensation of water vapor in air to form microdroplets on cold surfaces causes the formation of hydrogen peroxide (H 2 O 2 ) which we suggest is promoted by the large, intrinsic electric field at the air–water interface. This finding provides an alternative pathway for the production of atmospheric H 2 O 2 , for example, in raindrops as well as fogs and mists. This easy means of naturally producing H 2 O 2 suggests many interesting possibilities, from the use of condensed steam for disinfection to how water microdroplets might have promoted formation of the building blocks of life in the prebiotic era.