Communication à un colloque
Résumé : 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 chemistry model allows us to estimate that this novel pathway could provide a source of formic acid 2-4 times the known sources combined. We show that this pathway can bring the model predictions close to remote-sensing measurements. The pathway we discovered leads to an increase of the acidity of cloud and rain especially over the continents. These results are an advancement towards consistent mechanisms for a more realistic representation of organic carbon oxidation in the atmosphere. The oxidation framework we present here is also valid for higher carbonyl compounds and can account for the large atmospheric source of more complex organic acids which influence aerosol growth and cloud formation.

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