Phototautomerization of Acetaldehyde to Vinyl Alcohol: A Primary Process in UV-Irradiated Acetaldehyde from 295 to 335 nm

The concentrations of organic acids, key species in the
formation of secondary organic aerosols, are underestimated by atmospheric
chemistry models by a factor of ∼2. Vinyl alcohol (VA, CH2CHOH, ethenol) has been suggested as a precursor to
formic acid, but sufficient tropospheric sources of VA have not been
identified. Here, we show that VA is formed upon irradiation of neat
acetaldehyde (CH3CHO) in the actinic ultraviolet region,
between 295 and 330 nm. Besides the well-known photochemical products
CO and CH4, we infer up to a 15% quantum yield of VA at
20 Torr acetaldehyde pressure and a photolysis wavelength of 330 nm.
The experiments confirm a recent model predicting phototautomerization
of acetaldehyde to VA and imply that photolysis of small aldehydes
and ketones could provide tropospheric sources of enols sufficient
to impact organic acid budgets. We also report absolute infrared absorption
cross sections of VA.