Analytical Chemistry Seminar: Peter H. McMurry
Jointly sponsored by the Department of Chemistry and Biochemistry, CIRES, and the Environmental Program
Chemical Nucleation in the Atmosphere: Recent Discoveries Enabled by Instrument Development
Peter H. McMurry - Department of Mechanical Engineering, University of Minnesota
Observations throughout the atmosphere have shown that nucleation from the gas phase occurs every few days and that nucleation rates are correlated with collision rates of sulfuric acid vapor molecules. Our research team, which includes the groups led by Drs. Fred Eisele, Jim Smith and David Hanson, have developed three new instruments to study atmospheric nucleation: the Cluster Chemical Ionization Mass Spectrometer (Cluster CIMS), for measuring the concentrations of neutral molecular clusters formed by nucleation (1-10 ppqv sensitivity); the DEG Scanning Mobility Particle Spectrometer (DEG SMPS), for measuring the number distributions of freshly nucleated particles as small as 1 nm; and the Ambient Pressure Proton Transfer Mass Spectrometer (AmPMS), for measuring concentrations of basic organic and inorganic gases that react with and stabilize sulfuric acid-containing clusters (1pptv sensitivity).
Together, these instruments allow measurements of precursor vapor concentrations and the complete particle number distribution down to 1 molecule. This information is providing us with a new understanding of the physical and chemical processes that lead to nucleation.
Atmospheric observations and laboratory studies have confirmed that nucleation occurs due to a sequence of acid-base reactions that form stable clusters that subsequently grow by the further uptake of organic and inorganic compounds. Evaporation of sulfuric acid from clusters that contain 1 to 3 sulfuric acid molecules is the primary bottleneck to nucleation. Our measurements have provided estimates of these evaporation rates, leading to a simple analytic expression for nucleation rates that is in reasonable agreement with observations.
The seminar will describe the bases for these conclusions.