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
 What are aerosols?
Suspended particles in the atm.
 Why study them?
To understand their role in atmospheric
chemistry2
 Direct radiative forcing
Pre-critical clusters;
not experimentally detectable
 Indirect effect
(serve as cloud condensation nuclei)

1 Sipila,
( Radius of
Size regimes
 Experimental detection limit
(rp ~ 3 nm)1
 Critical cluster size
(rp ~ 3-100 nm)
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
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
CLUSTERS
Chemistry
o
Computer
Science
Biology
Physics
o
o
o …
Math
Computational
Chemistry
Development and application
of physical and chemical
principles to interesting
problems using computers
NATURE OF STUDY
o
o
o
Atmospheric
Chemistry
(H2O)n=1-10
(NH4+)(H2O)n=1-10
(H2SO4)(H2O)n=1-6
o
The growth of molecular
clusters and atmospheric
aerosols
Structures and energies
Thermodynamics of formation
Abundances at ambient
conditions
Mechanisms of growth to
aerosols
Conformational Sampling using Molecular Dynamics
Molecular dynamics – applying Newton’s equations to classical
molecular mechanics potential
TIP3P (H2O)8 simulation
Heated to 200K
Schematic of potential
energy surface
http://gold.cchem.berkeley.edu/research_path.html
Quantum Mechanical Energy Minima - (H2O)2-6,8
Thermodynamics of Water Cluster Growth
Thermodynamic quantities are Boltzmann/ensemble averaged over all low energy conformers
Total Growth
n(H2O)
Stepwise Growth
On the basis of chemical thermodynamics,
the stepwise growth of water clusters is not
favorable at ambient conditions.
Thermodynamics of Water Cluster Growth
Water clusters grow only at low temperatures
(supercooled) or if the vapor phase is
substantially supersaturated (S >> 1).
[H2O] ~
17
3
10 /cm
[(H2O)2] ~
at RH=100% at STP.
12
3
10 /cm
at RH=100% at STP.
[(H2O)n] are even more rare.
Thermodynamics of
+
(H2O/NH4 /H2SO4)(H2O)n
Clusters
Cluster growth is substantially easier for ionic cores than neutral
ones.
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: 159167
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
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