One of the key compounds emitted into the air by trees is isoprene. Once in the atmosphere, this common diene reacts with hydroxyl radicals and O2 to produce six peroxy radical isomers. Those peroxy species affect air quality through reactions with nitrogen oxides and affect climate through reactions that form organic species able to condense into sunlight-reflecting aerosol particles. A new lab study digs into the complex and dynamic chemistry of isoprene-derived species, determining a comprehensive picture of their lifetimes and reaction rates (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.6b12838). Alexander P. Teng, John D. Crounse, and Paul O. Wennberg of Caltech used an environmental chamber with mass spectrometry measurements to track yields of isomer-specific reaction products. The researchers found that the amounts of the peroxy radicals depend on their thermodynamic stability and the rate of hydrogen-shift intramolecular chemistry for two of the isomers. The results suggest that in typical atmospheric conditions β-hydroxyl peroxy radical isomers compose 95% of the radical pool, even though they are not initially formed in that proportion. In most prior laboratory studies, the peroxy radical pool reflected the nascent isomer distribution.