Isoprene nitrates drive new particle formation in Amazon's upper troposphere.

New particle formation (NPF) in the tropical upper troposphere is a globally important source of atmospheric aerosols. It is known to occur over the Amazon basin, but the nucleation mechanism and chemical precursors have yet to be identified. Here we present comprehensive in situ aircraft measurements showing that extremely low-volatile oxidation products of isoprene, particularly certain organonitrates, drive NPF in the Amazonian upper troposphere.