Aromatic Photooxidation, A New Source of Atmospheric Acidity

Formic acid (HCOOH), one of the most important and ubiquitous organic acids in the Earth’s atmosphere, contributes substantially to atmospheric acidity and affects pH-dependent reactions in the aqueous phase. However, based on the current mechanistic understanding, even the most advanced chemical models significantly underestimate the HCOOH concentrations when compared to ambient observations at both ground-level and high altitude, thus underrating its atmospheric impact. Here we reveal new chemical pathways to HCOOH formation from reactions of both O3 and OH with ketene-enols, which are important and up-to-now undiscovered intermediates produced in the photo-oxidation of aromatics and furans. We highlight that the estimated yields of HCOOH from ketene-enol oxidation are up to 60% in polluted urban areas, and great than 30% even in the continental background. Our theoretical calculations are further supported by the chamber experiment and the modeling study. Considering that aromatic compounds are highly reactive and contribute ca. 10 % to global non-methane hydrocarbon emissions and 20 % in the urban areas, the new oxidation pathways presented here might help to narrow the budget gap of HCOOH and other small organic acids, and can be relevant in any environment with high aromatic emissions, including urban areas and biomass burning plumes.