Nucleation: Formation of Stable Condensed Phase

1
Nucleation Formation of Stable Condensed Phase
Homogeneous Homomolecular H2O(g) ?H2O(l)
Not relevant to atmosphere
Homogeneous Heteromolecular nH2O(g)
mH2SO4(g) ? (H2O)n(H2SO4)m
Appears to be important in atmosphere
Heterogeneous Homomolecular nH2O(g) Xs/l ?
(H2O)nXs/l
Certainly happens (clouds)
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New Particle Formation in Atlanta
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Kinetics of Cluster Formation
kri1
i
i1
i2
i-2
i-1
i-3
Because N1 gtgtNi
Describes time rate of change of cluster i as
system adjusts to some initial perturbation and
approaches steady state
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Steady State Cluster Flux
0
At steady state, the concentration of cluster i
no longer changes with time. But, there is a
steady state flux of molecules from one cluster
to another as the system approaches equilibrium
kfi-1Ni-1 - kriNi kfiNi - kri1Ni1 J
J describes the net rate of formation of any
cluster size and hence, for S gt 1, it is the
nucleation rate
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Question
What is J, once equilibrium has been achieved?
Does it make sense to calculate a nucleation rate
by assuming the cluster distribution is at
equilibrium?
Which is larger, kfi or kfi1?
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Forward and Reverse Rate Constants
Forward Rate Constant reaction of monomer with
cluster i
A Ai ? Ai1
From kinetic theory Rate proportional to
collision frequency
Reverse Rate Constant evaporation from cluster i
More challenging, but should only depend on T and
ri Connect to Kelvin Equation
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Thermodynamics of Cluster Formation

• Recall Kelvin Equation Derivation
• Obtained from examining free energy change
  associated with increasing size of arbitrary
  particle

Slt1
Sgt1
iA ?Ai
?G
Ri
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Critical Radii and Numbers
S2 ? dynes cm-1 vlx1023 cm3 molec-1 Ri Ang i
H2O 72 3 15 482
Acetone 23 12.3 20 265
Ethanol 22 9.7 15 147
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Binary Nucleation
is now a surface
is now a saddle point where na and nb are such
that
H2SO4-H2O
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New Particle Formation in the Atmosphere
Wide-spread phenomenon
Observed in continental and marine boundary
layers, forested regions, polluted urban areas,
and cloud outflow
Regional and frequent
Tend to occur over 100s of km, with a frequency
of 5 40 of days.
Photochemical in nature
Events tend to be in morning to midday suggesting
a photochemical process with possible influence
from boundary layer dynamics
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Impacts of New Particle Formation
Formation events tend to increase aerosol number
concentrations by factors of 2-10.
Newly formed particles (lt10nm) tend to grow into
accumulation mode particles (100 nm) at a rate of
1-20 nm/hr (fast).
Accumulation mode particles act as cloud
condensation nuclei such that new particle
formation may impact cloud cover and direct
scattering of solar radiation.
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Nuclei vs Measured New Particles
A typical stable nuclei will have a radius lt 1nm
Size measurements are limited to particles with r
gt 3nm
Thus significant post-nucleation growth will have
occurred before measurement
What formed the nuclei?
What contributed to growth?
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When Will New Particle Formation Be Observed?
Formation of stable clusters
Rapid growth of nuclei to observable size with
slow loss of nucleated particles
i
monomer
condensational growth to observable sizes
condensation sink
coagulation loss
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Existing Aerosol Limits Observation of New
Particle Formation
Nucleation Rate will scale with N1
khet?Available surface area
Net production of observable new particles
Pobs Condensational Growth Rate Coagulation
Sink
Area of nuclei relative to area of preexisting
aerosol important for Pobsgt0
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The McMurry Number
When coagulation of nuclei with preexisting
aerosol dominates their condensational growth,
new particle formation will not be observed even
though nucleation may be occurring.
L McMurry number
L 1 equal condensing vapor molecules lost to
preexisting aerosols as contribute to nuclei
growth
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Questions
When should new particle formation be observed,
when Lgt1 or Llt1?
How do we interpret the lt10nm particles that
appear when Lgt1?
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What are the key players to nucleation and growth?
H2SO4-NH3-H2O Binary or Ternary nucleation
H2SO4-Organic acid complexes
Zhang, et al. 2004
Location dependent?
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Mass Spectrometer to Measure New Particle
Composition
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Whats in those new particles?
Mass spectrometry of new particles suggests H2SO4
and NH3 are most important constituents. No
organics were observed.
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Hygroscopicity and Volatility Apparatus
size select
If hygroscopic will grow with humidification
humidify or volatilize
If volatile will shrink with heat
resize
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Hygroscopicity of New Particles
These new particles should take up water like AS
Volatility measurements also suggest no
significant organic component. OC volatile _at_
100oC Sulfates involatile
Consistent with GF for small ammoniated sulfates
measured in lab