Photo-tautomerization of acetaldehyde as a photochemical source of formic acid in the troposphere

• Published in: Nature communications 2018-07, Vol.9 (1), p.2584-7, Article 2584
• Main Authors: Shaw, Miranda F., Sztáray, Bálint, Whalley, Lisa K., Heard, Dwayne E., Millet, Dylan B., Jordan, Meredith J. T., Osborn, David L., Kable, Scott H.
• Format: Article
• Language: English
• Subjects: 119/118; 639/638/440/949; 704/172/169/824; Acetaldehyde; Acid production; Acid rain; Acidity; Acids; Aerosols; Alcohol; Alcohols; Atmospheric chemistry; Atmospheric models; Chemical precipitation; Chemical transport; excited states; Formic acid; Humanities and Social Sciences; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Kinetics; multidisciplinary; Oceans; Organic acids; Organic chemistry; Oxidation; Photochemicals; Rainfall; Reaction kinetics; Science; Science (multidisciplinary); Three dimensional models; Troposphere
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-018-04824-2

Organic acids play a key role in the troposphere, contributing to atmospheric aqueous-phase chemistry, aerosol formation, and precipitation acidity. Atmospheric models currently account for less than half the observed, globally averaged formic acid loading. Here we report that acetaldehyde photo-tautomerizes to vinyl alcohol under atmospherically relevant pressures of nitrogen, in the actinic wavelength range, λ  = 300–330 nm, with measured quantum yields of 2–25%. Recent theoretical kinetics studies show hydroxyl-initiated oxidation of vinyl alcohol produces formic acid. Adding these pathways to an atmospheric chemistry box model (Master Chemical Mechanism) demonstrates increased formic acid concentrations by a factor of ~1.7 in the polluted troposphere and a factor of ~3 under pristine conditions. Incorporating this mechanism into the GEOS-Chem 3D global chemical transport model reveals an estimated 7% contribution to worldwide formic acid production, with up to 60% of the total modeled formic acid production over oceans arising from photo-tautomerization. The concentration of formic acid in Earth’s atmosphere is under-predicted by atmospheric models. Here the authors show that acetaldehyde photo-tautomerizes to vinyl alcohol under tropospheric conditions, with subsequent oxidation via OH radicals supplying up to 60% of total modeled formic acid production over oceans.