Abstract
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.
Highlights
Organic acids play a key role in the troposphere, contributing to atmospheric aqueous-phase chemistry, aerosol formation, and precipitation acidity
The majority of atmospheric organic acids are believed to be produced via photochemical oxidation of biogenic and anthropogenic volatile organic compounds (VOCs)
One potential source of FA is the photo-tautomerization of acetaldehyde (CH3CHO, AC) to its enol form, vinyl alcohol (CH2=CHOH, Vinyl alcohol (VA))
Summary
Organic acids play a key role in the troposphere, contributing to atmospheric aqueous-phase chemistry, aerosol formation, and precipitation acidity. Millet[10] evaluated the global importance of the keto-enol photo-tautomerization hypothesis, and concluded that this new AC phototautomerization mechanism constitutes ~15% of global FA sources, but suggested its impact is limited because FA itself catalyzes the reverse tautomerization, VA → AC, effectively buffering its own production[21].
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