Environmental Sciences Department Seminar

Atmospheric observations made during the past decade have shown that new particles are frequently formed by nucleation from the gas phase. The number of particles formed can be much greater than the number of preexisting particles, and freshly nucleated particles typically grow to sizes of 10-100 nm during the course of a day. Furthermore, nucleation often occurs over widespread areas in locations including urban areas, the continental boundary layer, the outflows of convective clouds, and coastal zones. Therefore, nucleation may be an important global source of cloud condensation nuclei, and may play an important role in regulating climate. In this lecture, observations of particle production and growth rates in various locations will be summarized. These observations were made possible by the recent availability of instruments to measure concentrations of certain gas phase precursors (e.g., H2SO4, NH3), and size distributions of particles as small as 3 nm and ions as small as 0.5 nm. Valuable insights have also been obtained from techniques that have recently been developed to measure the composition and properties of freshly nucleated particles. These new measurement techniques include the thermal desorption chemical ionization mass spectrometer to measure the composition of sub-10 nm particles and the Nano-Tandem Differential Mobility Analyzer to study their hygroscopicity and volatility. This lecture will summarize what these new analytical capabilities have taught us about atmospheric nucleation. A simple criterion that determines whether or not observable new particle formation occurs on a given day will also be described.