AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Insights from Cluster Thermodynamics: Atmospheric Conditions that Promote Nucleation for a Variety of Neutral and Ionic Systems
KARL D. FROYD, National Oceanic & Atmospheric Administration
Abstract Number: 576 Working Group: Aerosol Nucleation: From Clusters to Nanoparticles
Abstract In this presentation we summarize the viability of several atmospheric chemical systems to generate atmospheric nanoparticles from molecular clusters. Positive ion, negative ion, and neutral cluster systems comprised of sulfuric acid and water, with and without ammonia, are considered. Step-wise association reaction energies are determined starting from bare monomers and continuing up through stable nanoparticles. For the smallest clusters, where thermodynamics most strongly influence the nucleation potential, association reaction energies are determined using experimental measurements and selected quantum chemical calculations from literature. Large, bulk-like nanoparticle energies are calculated using classical theory. Energies for the intermediate transitional clusters are determined by connecting the two approaches with globally consistent functions. The growth of molecular clusters into nanoparticles becomes thermodynamically favorable when the free energy surface contains a growth trajectory that has little or no uphill step, i.e., no energetic barrier to nucleation. Barrier heights, critical cluster compositions, and the overall potential for nucleation vary enormously between systems and as a function of precursor concentrations and temperature. Most of the binary and ternary mechanisms considered here achieve barrierless growth under conditions relevant to some region of the troposphere. The effect of amines and organic species are estimated based on the limited available cluster stabilities. Gaps and uncertainties in the thermodynamic database are addressed, and the need for future experimental and theoretical studies is discussed.