The role of background ionization in the generation and evolution of ultrafine atmospheric particles is developed through modeling and data analysis. It is found that charged molecular clusters condensing around natural air ions can grow significantly faster than corresponding neutral clusters, and thus preferentially achieve stable, observable sizes. Detailed microphysical simulations of this process seem to explain recent measurements of ultrafine particle behavior, as well as the diurnal variation seen in tropospheric mobility spectra. The proposed ion-mediated nucleation mechanism leads to the production of new particles under conditions that are unfavorable for binary homogeneous nucleation, and provides a consistent explanation for a variety of tropospheric observations. ¿ 2000 American Geophysical Union