Atmospheric Nanoparticles: Formation and Physicochemical Properties
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Atmospheric nanoparticles, defined as particles with spherical equivalent diameters smaller than 50 nm, are either directly emitted from combustion sources, or are formed in the atmosphere by a process called nucleation. They then quickly grow by the condensation of gas monomers or clusters, or by coagulation with other particles, to become a critical participant in a number of important atmospheric processes such as heterogeneous chemistry, cloud formation, precipitation, and the scatter and absorption of solar radiation. An understanding of nanoparticles has increased dramatically in recent years because of significant advances in instrumentation to detect, size, and determined their chemical composition. Recent observations now show us that atmospheric nanoparticles are ubiquitous: Measurements carried out in city centers, isolated islands and forests, and the remote troposphere have never failed to encounter periods characterized by concentrations of up to 106 nanoparticles/cm3. Theoretical advances in nucleation and growth, at present, have not made the same sort of progress as observation, but this is an area of rapid progress.
The goal of this article is to provide an overview of the role that nanoparticles play in the atmosphere, the observational and theoretical tools currently being employed in their study, and recent observations that continue to direct progress in understanding their formation and fate. Although much is known about these transient particles, researchers appear to be at the cusp of huge scientific advances that will make it possible to predict their formation and behavior in the atmosphere.