Modeling the Growth of Clusters and Aerosols from First

Principles: How do We Understand Feedback

Systems in a Warming Climate?

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

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. 289-3483 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 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)

Gas Phase Clusters and Aerosol ParticlesGas Phase Clusters and Aerosol Particles

Pre-critical clusters; not experimentally

detectable

( Radius of particle)

Climate Change 2007: Synthesis Report. IPCC.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

Our understanding

of aerosol creation and

growth and its impact on the atmosphere is

very limited

LOSU = Level of Scientific Understanding

Application of Computational Chemistry to Atmospheric Chemistry

Atmospheric Chemistry

Atmospheric Chemistry

The growth of molecular clusters and atmospheric aerosols

Computational Chemistry

Computational Chemistry

Development and application of physical and chemical principles to interesting

problems using computers

CLUSTERS

o (H2O)n=1-10

o (NH4+)(H2O)n=1-10

o (H2SO4)(H2O)n=1-6

o …

NATURE OF STUDY

o Structures and energieso Thermodynamics of

formationo Abundances at ambient

conditionso Mechanisms of growth to

aerosols

Conformational Sampling using Molecular Dynamics

Molecular dynamics – applying Newton’s equations to classical molecular mechanics potential

TIP3P (H2O)8 simulationHeated to 200K

Schematic of potential energy surface

http://gold.cchem.berkeley.edu/research_path.html

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

Total Growth

On the basis of chemical thermodynamics, the stepwise growth

of water clusters is not favorable at ambient conditions.

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(H2O)

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.

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

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

Mechanism for Aerosol Growth

Initial stages of growth involve nucleation of

NH4+(H2O)n<5

H2SO4(H2O)n<4

Initial stages of growth involve nucleation of

NH4+(H2O)n<5

H2SO4(H2O)n<4

Radii

NH4+(H2O)5 < 0.4 nm

H2SO4(H2O)4 < 0.5 nm

Radii

NH4+(H2O)5 < 0.4 nm

H2SO4(H2O)4 < 0.5 nm

Curtius, J et al. (2006) Space Science Reviews. 125: 159-167Curtius, J et al. (2006) Space Science Reviews. 125: 159-167

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

Conclusions

NSF RUINSF MRI

NSF TeraGridMERCURY Consortium

Thank You!