Formic acid is a pervasive trace gas in the troposphere. It enhances cloud droplet activation and contributes to determining the acidity of clouds and rain. Despite many efforts, knowledge of its tropospheric budget is unsatisfactory as state-of-art models considerably underestimate its burden. Models inferring either photochemical sources or large emissions fail to reproduce the measured concentrations. This is an indication that relevant key processes still elude our understanding. In this study we present lab evidence and theoretical predictions of how formic acid is efficiently formed by oxidation of hydrated formaldehyde, methanediol, outgassing from cloud droplets. Explicit representation of these processes in a global atmospheric che...