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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 onRand oxidation regimes, demonstrating the synergetic effects of isoprene and HO_x(OH and HO₂) on biogenic NPF. Whereas the suppression of NPF by isoprene is due to the product suppression effects of monoterpene dimers (C20), RO₂ + HO₂termination reactions also play important roles in suppressing the dimer formation, another likely process to suppress NPF in the atmosphere.

 

Abstract. New particle formation (NPF) consists of two steps: nucleation andsubsequent growth. At present, chemical and physical mechanisms that governthese two processes are not well understood. Here, we report initial resultsobtained from the TANGENT (Tandem Aerosol Nucleation and Growth EnvironmentTube) experiments. The TANGENT apparatus enables us to study these twoprocesses independently. The present study focuses on the effects oftemperature on sulfuric acid nucleation and further growth. Our results showthat lower temperatures enhance both the nucleation and growth rate.However, under temperatures below 268 K the effects of temperature on thenucleation rate become less significant and the nucleation rate becomes lessdependent on relative humidity, indicating that particle formation in the conditions of ourflow tube takes place via barrierless nucleation at lower temperatures. Wealso examined the growth of newly formed particles under differingtemperature conditions for nucleation and further growth. Our results showthat newly nucleated clusters formed at low temperatures can indeed surviveevaporation and grow in a warmer environment in the presence of SO2 andozone and potentially other contaminant vapors. These results implythat some heterogeneous reactions involving nanoparticles affect nucleationand growth of newly formed particles. 

New particle formation (NPF) represents the first step in the complex processes leading to formation of cloud condensation nuclei. Newly formed nanoparticles affect human health, air quality, weather, and climate. This review provides a brief history, synthesizes recent significant progresses, and outlines the challenges and future directions for research relevant to NPF. New developments include the emergence of state‐of‐the‐art instruments that measure prenucleation clusters and newly nucleated nanoparticles down to about 1 nm; systematic laboratory studies of multicomponent nucleation systems, including collaborative experiments conducted in the Cosmics Leaving Outdoor Droplets chamber at CERN; observations of NPF in different types of forests, extremely polluted urban locations, coastal sites, polar regions, and high‐elevation sites; and improved nucleation theories and parameterizations to account for NPF in atmospheric models. The challenges include the lack of understanding of the fundamental chemical mechanisms responsible for aerosol nucleation and growth under diverse environments, the effects of SO₂and NO_xon NPF, and the contribution of anthropogenic organic compounds to NPF. It is also critical to develop instruments that can detect chemical composition of particles from 3 to 20 nm and improve parameterizations to represent NPF over a wide range of atmospheric conditions of chemical precursor, temperature, and humidity.