Shan‐Hu Lee

New particle formation (NPF) has been observed at various locations, but NPF does not occur in isoprene‐dominant forests. Recent laboratory studies were conducted to understand the role of isoprene in biogenic NPF, and these studies show that isoprene can suppress biogenic NPF, with contradicting theories. To reconcile these discrepancies, we conducted flow tube experiments of biogenic nucleation under a wide range of isoprene over monoterpene carbon ratios ( R ) and oxidant conditions (OH vs. ozone). Our results show isoprene either suppresses or enhances biogenic NPF, depending on R and oxidation regimes, demonstrating the synergetic effects of isoprene and HO x (OH and HO 2 ) on biogenic NPF. Whereas the suppression of NPF by isoprene is due to the product suppression effects of monoterpene dimers (C20), RO 2 + HO 2 termination reactions also play important roles in suppressing the dimer formation, another likely process to suppress NPF in the atmosphere.