Evaporation Rates from Molecular Clusters formed by Nucleation in the Atmosphere

Mark Emery (ME), advisors: Peter McMurry (ME), Jeff Roberts (Chem)

Accurate predictions of nucleation rates is one of the great challenges in atmospheric science. Nucleation rates are determined by the forward and reverse rates of reactions on molecular clusters. When forward rates are sufficiently in excess of reverse rates, then new particle formation can occur.  Forward rates can be calculated with reasonable confidence using existing theories, but reverse rates are often highly uncertain. My research will involve an experimental determination of reverse reaction rates from molecular clusters.  I plan to work with chemical systems thought to be important in the atmosphere, including sulfuric acid, water, ammonia, and possibly certain organic vapors.  I will carry out measurements on clusters that contain between 1 and 10 sulfuric acid molecules, which have diameters of 0.55 to 1.19nm.  Both laboratory and field experiments are planned.

Evaporation rates will be inferred from measurements of molecular cluster distributions.  These measurements will be done using a specially designed chemical ionization mass spectrometer (CIMS).  My first responsibility will be to develop a technique that involves the use of a high-resolution differential mobility analyzer and an electrospray to produce clusters of known mass and concentration.  These clusters will be used to quantify the response of the CIMS system, so as to enable measurements of both mass and concentration.  We will then apply the CIMS system to measurements in the atmosphere and in a well-characterized laboratory reactor.