2. 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

3. 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 3 Curtius, J et al. (2006) Space Science Reviews. 125: 159-167  What are aerosols? Suspended particles in the atm.  Why study them? To understand their role in atmospheric chemistry 2  Direct radiative forcing  Indirect effect (serve as cloud condensation nuclei)  Size regimes  Experimental detection limit (r p ~ 3 nm) 1  Critical cluster size (r p ~ 3-100 nm) Gas Phase Clusters and Aerosol Particles Pre-critical clusters; not experimentally detectable ( Radius of particle)

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

5. Application of Computational Chemistry to Atmospheric Chemistry Chemistry Biology Physics Math Computer Science Atmospheric Chemistry The growth of molecular clusters and atmospheric aerosols Computational Chemistry Development and application of physical and chemical principles to interesting problems using computers CLUSTERS o (H 2 O) n=1-10 o (NH 4 + )(H 2 O) n=1-10 o (H 2 SO 4 )(H 2 O) n=1-6 o … NATURE OF STUDY o Structures and energies o Thermodynamics of formation o Abundances at ambient conditions o Mechanisms of growth to aerosols

6. Conformational Sampling using Molecular Dynamics Molecular dynamics – applying Newton’s equations to classical molecular mechanics potential TIP3P (H 2 O) 8 simulation Heated to 200K Schematic of potential energy surface http://gold.cchem.berkeley.edu/research_path.html

7. Quantum Mechanical Energy Minima - (H 2 O) 2-6,8

8. Total Growth On the basis of chemical thermodynamics, the stepwise growth of water clusters is not favorable at ambient conditions. Thermodynamics of Water Cluster Growth Thermodynamic quantities are Boltzmann/ensemble averaged over all low energy conformers Stepwise Growth n(H 2 O)

9. Thermodynamics of Water Cluster Growth  Water clusters grow only at low temperatures (supercooled) or if the vapor phase is substantially supersaturated (S >> 1).  [H 2 O] ~ 10 17 /cm 3 at RH=100% at STP.  [(H 2 O) 2 ] ~ 10 12 /cm 3 at RH=100% at STP.  [(H 2 O) n ] are even more rare.

10. Cluster growth is substantially easier for ionic cores than neutral ones. Thermodynamics of (H 2 O/NH 4 + /H 2 SO 4 )(H 2 O) n Clusters

11. Mechanism for Aerosol Growth Initial stages of growth involve nucleation of NH 4 + (H 2 O) n<5 H 2 SO 4 (H 2 O) n<4 Initial stages of growth involve nucleation of NH 4 + (H 2 O) n<5 H 2 SO 4 (H 2 O) n<4 Radii NH 4 + (H 2 O) 5 < 0.4 nm H 2 SO 4 (H 2 O) 4 < 0.5 nm Radii NH 4 + (H 2 O) 5 < 0.4 nm H 2 SO 4 (H 2 O) 4 < 0.5 nm Curtius, J et al. (2006) Space Science Reviews. 125: 159- 167

12.  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.  NH 4 + (H 2 O) n gets readily hydrated with peak abundance at n=4 in a closed H 2 O-NH 4 + system at STP and RH=100%.  H 2 SO 4 (H 2 O) n also grow to n=4, with a peak abundance at n=2.  Initial stages of aerosol growth must involve  NH 4 + (H 2 O) n<6  H 2 SO 4 (H 2 O) n<5 Conclusions

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