Atmospheric nanoparticle growth

New particle formation of liquid or solid nanoparticles from gas-phase precursors is a decisive process in Earth’s atmosphere and is considered one of the largest uncertainties in climate change predictions. Key for the climate relevance of new particle formation is the growth of freshly formed molecular clusters, as it determines the survival of these particles to cloud condensation nuclei sizes, where they can contribute to the aerosol-indirect effect. This review lays out the fundamental definitions of nanoparticle growth and addresses the rapidly emerging field of new particle formation studies with a focus on the diverse processes contributing to nanoparticle growth, explicitly comparing the latest experimental findings and their implementation in large-scale models. Atmospheric nanoparticle growth is a complex phenomenon including condensational and reactive vapor uptake, aerosol coagulation, and sink processes. It is linked to thermodynamics, cluster- and phase-transition physics. Nanoparticle growth rates measured from the evolution of the particle-size distribution describe growth as a collective phenomenon, while models often interpret them on a single-particle level and incorporate it into highly simplified size-distribution representations. Recent atmospheric observations show that sulfuric acid together with ammonia and amines, iodic acid, and oxidized organic species can contribute to nanoparticle growth, whereas most models describe the growth effects from a limited subset of this variety of condensable vapors. Atmospheric simulation chamber experiments have clarified the role of ions, intermolecular forces, the interplay of acids and bases, and the contribution of different types of organic vapors. Especially in the complex thermodynamics of organic vapor condensation, the field has had noteworthy advances over the last decade. While the experimental field has achieved significant progress in methodology and process level understanding, this has not led to a similar improvement in the description of the climate impact of nanoparticle formation in large-scale models. This review sets the basis to better align experimental and modeling studies on nanoparticle growth, giving specific guidance for future studies aiming to resolve the questions as to why the climate response in large-scale models seems to be buffered against high survival probabilities and why the global growth observations herein show surprisingly low variation.