George C. Shields Department of Chemistry, Office of the Dean of Arts and Sciences,

1. Modeling the Growth of Clusters and Aerosols from First Principles: How do We Understand Feedback Systems in a Warming Climate? George C. Shields Department of Chemistry, Office of the Dean of Arts and Sciences, Bucknell University, Lewisburg, PA 17837

2. Gas Phase Clusters and Aerosol Particles Gas Phase Clusters and Aerosol Particles • What are aerosols? Suspended particles in the atm. • Why study them? To understand their role in atmospheric chemistry2 • Direct radiative forcing • Indirect effect (serve as cloud condensation nuclei) • Size regimes • Experimental detection limit (rp ~ 3 nm)1 • Critical cluster size (rp ~ 3-100 nm) Pre-critical clusters; not experimentally detectable 1 Sipila, M., et al. (2010). Science, 327(5970), 1243–1246 2 Penner, JC, Ed. (2001) Aerosols, their Direct and Indirect Effects. Climate Change 2001. 289-348 3Curtius, J et al. (2006) Space Science Reviews. 125: 159-167 ( Radius of particle)

3. Aerosols in the Atmosphere LOSU = Level of Scientific Understanding Our understanding of aerosol creation and growth and its impact on the atmosphere is very limited Climate Change 2007: Synthesis Report. IPCC.

4. Application of Computational Chemistry to Atmospheric Chemistry Development and application of physical and chemical principles to interesting problems using computers Computational Chemistry Atmospheric Chemistry The growth of molecular clusters and atmospheric aerosols

5. Conformational Sampling using Molecular Dynamics Molecular dynamics – applying Newton’s equations to classical molecular mechanics potential Schematic of potential energy surface http://gold.cchem.berkeley.edu/research_path.html TIP3P (H2O)8 simulation Heated to 200K

6. Quantum Mechanical Energy Minima - (H2O)2-6,8

7. Thermodynamics of Water Cluster Growth Thermodynamic quantities are Boltzmann/ensemble averaged over all low energy conformers Total Growth Stepwise Growth On the basis of chemical thermodynamics, the stepwise growth of water clusters is not favorable at ambient conditions. n(H2O)

8. Thermodynamics of Water Cluster Growth • Water clusters grow only at low temperatures (supercooled) or if the vapor phase is substantially supersaturated (S >> 1). • [H2O] ~ 1017/cm3 at RH=100% at STP. • [(H2O)2] ~ 1012/cm3 at RH=100% at STP. • [(H2O)n] are even more rare.

9. Thermodynamics of (H2O/NH4+/H2SO4)(H2O)n Clusters Cluster growth is substantially easier for ionic cores than neutral ones.

10. Mechanism for Aerosol Growth Radii NH4+(H2O)5 < 0.4 nm H2SO4(H2O)4 < 0.5 nm Initial stages of growth involve nucleation of NH4+(H2O)n<5 H2SO4(H2O)n<4 Curtius, J et al. (2006) Space Science Reviews. 125: 159-167

11. Conclusions • A combined classical molecular dynamics sampling and high level quantum mechanical methodology has been used to identify low energy gas phase clusters of atmospheric interest. • Growth of water clusters is thermodynamically unfavorable at ambient conditions. • NH4+(H2O)n gets readily hydrated with peak abundance at n=4 in a closed H2O-NH4+ system at STP and RH=100%. • H2SO4(H2O)n also grow to n=4, with a peak abundance at n=2. • Initial stages of aerosol growth must involve • NH4+(H2O)n<6 • H2SO4(H2O)n<5

12. NSF RUINSF MRINSF TeraGridMERCURY Consortium Thank You!