Researchers have long puzzled over the origin of some 90 megatons of organic acids measured in the troposphere, the region of the atmosphere closest to Earth. Scientists in Australia may have now figured out the acids’ source. A team led by the University of Sydney’s Meredith J. T. Jordan and Scott H. Kable propose that acetaldehyde is first converted to vinyl alcohol in a tautomerization reaction driven by sunlight. Vinyl alcohol is a known precursor to organic acids, such as formic acid. So the researchers argue, on the basis of theoretical and experimental analysis, that vinyl alcohol may thus be the source of the troposphere’s unexplained organic acid content, which current climate models underestimate by a factor of two (Science, DOI: 10.1126/science.1220712). The group points out that until now, troposphere models have not included formation of enols (of which vinyl alcohol is one) in their calculations. As researchers get a better handle on these reactions under complex atmospheric chemistry conditions, modelers may be wise to include them in climate calculations.