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