The formation of new particles from “extremely minute quantities of matter” (John Aitken, 1911) contributes significantly to the global concentrations of atmospheric particles and cloud condensation nuclei (CCN). Through their role as CCN, these particles affect the earth’s radiation budget, so it is important to account for them in climate models. Challenges in this work involve identifying the trace species that participate in nucleation and growth, measuring their concentrations (typical mole fractions are 10-14 to 10-11), and understanding the chemical mechanisms by which particles are formed from gases and subsequently grow. This presentation will describe our progress at developing suitable measurement methods and using this new information to gain insights about the chemical and physical processes that determine nucleation and growth rates. Measurements of the complete particle number distributions during nucleation events, including molecules and neutral molecular clusters, will be described. Evidence will be shown that the smallest stable particles contain just a few (two to four) sulfuric acid molecules, and that uptake of sulfuric acid vapor for particles larger than this is collision-limited.