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$^{1–4}$. It is known to occur over the Amazon basin, but the nucleation mechanism and chemical precursors have yet to be identified$^2$. 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. The organonitrates originate from OH-initiated oxidation of isoprene from forest emissions in the presence of nitrogen oxides from lightning. Nucleation bursts start about 2 h after sunrise in the outflow of nocturnal deep convection, producing high aerosol concentrations of more than 50,000 particles cm$^{-3}$. We report measurements of characteristic diurnal cycles of precursor gases and particles. Our observations show that the interplay between biogenic isoprene, deep tropical convection with associated lightning, oxidation photochemistry and the low ambient temperature uniquely promotes NPF. The particles grow over time, undergo long-range transport and descend through subsidence to the lower troposphere, in which they can serve as cloud condensation nuclei (CCN) that influence the Earth’s hydrological cycle, radiation budget and climate$^{1, 4-8}$. ... mehrIsoprene is emitted in large quantities by vegetation such as broadleafed trees and it has the largest source strength of all biogenic volatile organic compounds (VOCs). Globally, emissions amount to about 500–600 Tg a$^{−1}$ , representing more than half of the total biogenic VOC emissions, with tropical South America contributing an estimated 163 Tg a $^{−1}$ (refs. 9,10). Isoprene is primarily emitted into the atmosphere during daylight hours and is typically converted within 1–2 h into oxygenated VOCs, mostly through reaction with hydroxyl radicals (OH) $^{11,12}$ . Isoprene mixing ratios in the continental boundary layer of the Amazon basin range between 1 and 20 ppbv, with a diurnal cycle peaking in the afternoon$^{13}$. Although isoprene-derived oxygenated organic molecules (IP-OOMs) are known to contribute substantially to the production of secondary organic aerosol mass by means of condensation onto pre-existing articles$^{14,15}$, they are unable to form new particles in the boundary layer and are even thought to inhibit NPF from monoterpenes $^{16,17}$.