The fresh, unmistakable scent of a pine forest comes from a medley of chemicals produced by its trees. Researchers have now, for the first time, accurately determined the chemical structure of one of these compounds in its gas phase, a molecule called alpha-pinene, which is also produced by tropical forests. The new analysis can help scientists better detect and understand how alpha-pinene—and potentially other similar molecules—react with other gases in the atmosphere, a process which produces pollutants and particles called aerosols that affect health and climate.