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
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