Atmospheric Vinyl Alcohol to Acetaldehyde Tautomerization Revisited

The atmospheric oxidation of vinyl alcohol (VA) produced by photoisomerization of acetaldehyde (AA) is thought to be a source of formic acid (FA). Nevertheless, a recent theoretical study predicted a high rate coefficient k₁(298 K) of ≈10^–14 cm³ molecule^–1 s^–1 for the FA-catalyzed tautomerization reaction of VA back into AA, which suggests that FA buffers its own production from VA. However, the unusually high frequency factor implied by that study prompted us to reinvestigate reaction . On the basis of a high-level ab initio potential energy profile, we first established that transition state theory is applicable, and derived a k₁(298 K) of only ≈2 × 10^–20 cm³ molecule^–1 s^–1, concluding that the reaction is negligible. Instead, we propose and rationalize another important VA sink: its uptake by aqueous aerosol and cloud droplets followed by fast liquid-phase tautomerization to AA; global modeling puts the average lifetime by this sink at a few hours, similar to oxidation by OH.