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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.

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slide1
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

gas phase clusters and aerosol particles

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)

aerosols in the atmosphere
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.

application of computational chemistry to atmospheric chemistry
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

conformational sampling using molecular dynamics
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

slide7

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)

slide8

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.
slide9

Thermodynamics of (H2O/NH4+/H2SO4)(H2O)n Clusters

Cluster growth is substantially easier for ionic cores than neutral ones.

slide10

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

slide11

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