Fig. 5. Schematic of new particle formation from isoprene in the upper troposphere.
Isoprene from forests is efficiently transported at night by deep convective clouds into the upper troposphere. During daylight, the isoprene accumulated overnight, together with daytime-convected isoprene, reacts with hydroxyl radicals and NOx from lightning to produce IP-OOM. The IP-OOM combine with trace ambient acids to produce high particle-number concentrations at cold temperatures below −30 °C. The newly formed particles grow rapidly over several hours to days while following the descending air masses. This mechanism may provide an extensive source of CCN for shallow continental and marine clouds, which strongly influence Earth’s radiative balance.