New particle formation from isoprene under upper-tropospheric conditions

Aircraft observations have revealed ubiquitous new particle formation in the tropical upper troposphere over the Amazon^1,2 and the Atlantic and Pacific oceans^3,4. Although the vapours involved remain unknown, recent satellite observations have revealed surprisingly high night-time isoprene mixing ratios of up to 1 part per billion by volume (ppbv) in the tropical upper troposphere⁵. Here, in experiments performed with the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we report new particle formation initiated by the reaction of hydroxyl radicals with isoprene at upper-tropospheric temperatures of -30 °C and -50 °C. We find that isoprene-oxygenated organic molecules (IP-OOM) nucleate at concentrations found in the upper troposphere, without requiring any more vapours. Moreover, the nucleation rates are enhanced 100-fold by extremely low concentrations of sulfuric acid or iodine oxoacids above 10⁵ cm^-3, reaching rates around 30 cm^-3 s^-1 at acid concentrations of 10⁶ cm^-3. Our measurements show that nucleation involves sequential addition of IP-OOM, together with zero or one acid molecule in the embryonic molecular clusters. IP-OOM also drive rapid particle growth at 3-60 nm h^-1. We find that rapid nucleation and growth rates persist in the presence of NO_x at upper-tropospheric concentrations from lightning. Our laboratory measurements show that isoprene emitted by rainforests may drive rapid new particle formation in extensive regions of the tropical upper troposphere^1,2, resulting in tens of thousands of particles per cubic centimetre.