Long-sought atmospheric intermediate detected

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Using a high-sensitivity spectroscopic technique, researchers have detected an elusive chemical species in a common atmospheric reaction—the formation of carbon dioxide via the combination of the engine combustion by-product carbon monoxide and hydroxyl radicals naturally formed from water (Science 2016, DOI: 10.1126/science.aag1862). The findings provide a more detailed understanding of the reaction, allowing for improved modeling of atmospheric processes that contribute to air pollution and climate change. Forty-three years ago, Ian W. M. Smith and Reinhard Zellner at the University of Cambridge proposed that the reaction proceeds through an excited hydrocarboxyl radical intermediate, HOCO•* (J. Chem. Soc. Faraday Trans. 2 1973, DOI: 10.1039/f29736901617). They noted that collisions with other molecules could deactivate HOCO•* to form HOCO•, which can then participate in other reactions. But HOCO• is so reactive and fleeting that, in all this time, no technique used to study the reaction has had the requisite sensitivity to detect HOCO•