Heterogeneous Atmospheric Chemistry

277 Pages·7.27 MB·English

Geophysical Monograph 26
Heterogeneous
Atmospheric Chemistry
David R. Schryer
Editor
American GeophysicalU nion
Washington, D.C.
Published under the aegis of the AGU Geophsysical
Monograph Board: Rob Van der Voo, Chairman; Donald H.
Eckhardt, Eric J. Essene, Donald W. Forsyth, Joel S.
Levine, William I. Rose, and Ray F. Weiss, members
Library of Congress Cataloging in Publication Data
M(cid:127)in entry under title:
Heterogeneous atmospheric chemistry
( Geophysical monograph: 26 )
Includes bibliographies.
1. Atmospheric chemistry--Addresses, essays, lectures.
I. Schryer, David R. II. Series.
QC879.6.H47 1982 551.5'112 82-11451
ISBN 0-87590-051-8
ISSN 0065-8448
Copyright 1982 by the American Geophysical Union,
2000 Florida Avenue, N.W., Washington, D.C. 20009
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First Printing: 1982
Second Printing: 1985
Printed in the United States of
CONTENTS
Preface v
CLUSTERS, MICROPARTICLES, AND PARTICLES
CommonP roblems in Nucleation and Growth, Chemical Kinetics, and Catalysis
Howard Reiss
Effect of the Mechanism of Gas-to-Particle Conversion on the Evolution of Aerosol
Size Distributions John H. Seineeld and Mark Bassett
Neutral and Charged Clusters in the Atmosphere: Their Importance and Potential Role 13
in Heterogeneous Catalysis A. W. Castleman, Jr.
Role of Ions in Heteromolecular Nucleation: Free Energy Change of Hydrated Ion 28
Clusters S. H. Suck, T. S. Chen, R. W. Emmons,D . E. Hagen, and J. L. Kassner, Jr.
Structural Studies of Isolated Small Particles Using Molecular Beam Techniques 33
Sang Soo Kim and Gilbert D. Stein
ChemicalR eactions With Aerosols EgonM atijevi(cid:127) 44
Photophoretic Spectroscopy: A Search for the Composition of a Single Aerosol 50
Particle S. Arnold and M. Lewittes
glectron Beam Studies of Individual Natural and Anthropogenic Microparticles: 57
Compositions, Structures, and Surface Reactions Peter R. Buseck and John P. Bradley
Electronic Structure Theory for Small Metallic Particles R. P. Messmer 77
GAS-SOLID INTERACTIONS
The Surface Science of Heterogeneous Catalysis: Possible Applications in Atmospheric 88
Sciences G. A. Somorjai
The Removal of Atmospheric Gases by Particulate Matter Julian Heicklen 93
Reactions of Gases on Prototype Aerosol Particle Surfaces Alan C. Baldwin 99
Laboratory Measurements of Dry Deposition of Acetone Over Adobe Clay Soil 103
Henry S. Judeikis
Kinetics of Reactions Between Free Radicals and Surfaces (Aerosols) Applicable to 107
Atmospheric Chemistry Daryl D. Jech, Patrick G. Easley, and Barbara B. Krieger
Photoassisted Reactions on Doped Metal Oxide Particles J. M. White 122
Photoassisted Heterogeneous Catalysis: Definition and Hydrocarbon and Chlorocarbon 136
Oxidations David F. Ollis and Ann Lorette Pruden
Heterogeneous Catalyzed Photolysis via Photoacoustic Spectroscopy L. Robbin Martin 143
and Marilyn Wun-Fogle
Observations of Sulfate Compoundso n Filter Substrates by Means of X-Ray Diffraction 149
Briant L. Davis, L. Ronald Johnson, Robert K. Stevens, Donald F. Gatz,
and Gary J. Stensland
Atmospheric Gases on Cold Surfaces - Condensation, Thermal Desorption, and Chemical 157
Reactions R. J. Fezza and J. M. Calo
Incomplete Energy Accommodatioinn Surface-Catalyzed Reactions Bret Halpern and
Daniel E. Rosner
AQUEOUSS TUDIES
Oxidation of SO2 by NO?.a nd Air in an AqueousS uspensiono f Carbon 174
Robert S. Rogowski, David R. Schryer, Wesley R. Cofer III, Robert A. Edahl, Jr.,
and Shekhar Munavalli
Sulfur Dioxide Absorption, Oxidation, and Oxidation Inhibition in Falling Drops: 178
An Experimental/Modeling Approach Elmar R. Altwicker and Clement Kleinstreuer
Sulfur-Dioxide/Water Equilibria Between Oø and S0øC. An Examination of Data at 187
Low Concentrations Hoard G. Maahs
Theoretical Limitations on Heterogeneous Catalysis by Transition Metals in Aqueous 196
Atmospheric Aerosols Charles J. Weschler and T. E. Graedel
EVIDENCE REGARDING HETEROGENEOUS REACTIONS IN THE ATMOSPHERE
Evidence for Heterogeneous Reactions in the Atmosphere George M. ttidy 204
Soot-Catalyzed Atmospheric Reactions T. Novakov 215
The Relative Importance of Various Urban Sulfate Aerosol Production Mechanisms - 221
A Theoretical Comparison Paulette Middleton, C. S. Kiang, and Volker A. Mohnen
Importance of Heterogeneous Processes to Tropospheric Chemistry: Studies With a One- 231
Dimensional Model R. P. Turco, O. B. Toon, R. C. Whitten, R. G. Keesee, and
P. Hamill
Sulfate in the Atmospheric Boundary Layer: Concentration and Mechanisms of Formation 241
R. F. Pueschel and E. W. Barrett
Water Vapor and Temperature Dependence of Aerosol Sulfur Concentrations at Fort Wayne, 250
Indiana, October 1977 John W. Winchester and Alistair C. D. Leslie
Evidence for Aerosol Chlorine Reactivity During Filter Sampling WangM ingxing and "57
John W. Winchester
The Possible Role of Heterogeneous Aerosol Processes in the Chemistry of CH4 and CO 264
in the Troposphere Cir(cid:127)fy J. Luther and f(cid:127)oraz(cid:127)d K. Peters
Acknowledgments 2
PREFACE
In the past few years it has become increasingly clear that heterogeneous,
or multiphase, processes play an important role in the atmosphere. Unfortu-
nately the literature on the subject, although now fairly extensive, is still
rather dispersed. Furthermore, much of the expertise regarding heterogeneous
processes lies in fields not directly related to atmospheric science. There-
fore, it seemed desirable to bring together for an exchange of ideas, informa-
tion, and methodologies the various atmospheric scientists who are actively
studying heterogeneous processes as well as other researchers studying similar
processes in the context of other fields.
This was accomplished in the summer of 1981 at the Conference on Multiphase
Processes - Including Heterogeneous Catalysis - Relevant to Atmospheric
Chemistry (originally entitled Workshop/Conference on Heterogeneous Catalysis -
Its Importance to Atmospheric Chemistry). The conference was held June 29 to
July 3, 1981, at Albany, N.Y. It was sponsored by the National Science Foun-
dation and the National Aeronautics and Space Administration and was hosted by
the Atmospheric Sciences Research Center (ASRC) of the State University of New
York at Albany. The conference chairman was Volker A. Mohnen of ASRC. Over
100 researchers from a wide variety of fields attended the conference, which
brought to attention the broad scope of ongoing research on various phenomena
related to heterogeneous processes.
During the organization of the conference it was decided that, in addition
to the conference itself, a book was needed which would present, in a single
source, an effective introduction to the present state of kn6wledge of hetero-
geneous processes of relevance to the atmosphere. It was soon decided, however,
that the book should not be just a collection of conference papers. Rather,
additional papers not received in time for the conference would be included and
alltpapers would be individually refereed and edited.
This has now been accomplished and the result is the present monograph. The
majority of the papers presented herein were presented at the Albany conference,
although many of these have been revised to reflect the latest research results.
Several papers presented at the conference are, for a number of reasons, not
included in the monograph, and a few papers not presented at the conference are
presented here. The attempt has been made, within individual papers as well as
among the papers, to present a mix between review of the various fields covered
and presentation of new research results. A similar mix has been sought between
papers dealing directly with atmospheric science and papers from other fields
dealing with phenomena and/or methodologies which are potentially applicable to
atmospheric science but are not directly related to it.
It is hoped that this monograph will prove useful not only to researchers
actively engaged in the study of heterogeneous processes in the atmosphere but
also to researchers who might profitably consider such processes but who have
not done so, either for lack of knowledge of their potential importance or
because of the dispersed nature of the relevant literature. If this monograph
contributes to a broader consideration of heterogeneous processes in the study
of atmospheric chemistry then the editor's goal will have been realized.
Use of trade names or names of manufacturers in this monograph does not con-
stitute an official endorsement of such products or manufacturers, either
expressed or implied, by the sponsoring organizations.
David R. Schryer
National Aeronautics and Space Administration
Langley Research Center
Hampton, Virginia
March 1,
Clusters, Microparticles,
and
Geophysical Monograph Series Heterogeneous Atmospheric Chemistry Vol. 26
COMMONP ROBLEMS IN NUCLEATION AND GROWTH, CHEMICAL KINETICS, AND CATALYSIS
Howard Reiss
Department of Chemistry, University of California at Los Angeles, Los Angeles, California 90024
The editor of this monographh as asked me to problem. The "interfaces" between these methods
make a few remarks about nucleation and growth in of approximation have only rarely preserved
the atmosphere in the context of the goals of this continuity of concept, and this lack of con-
document. The task, it turns out, is a formidable tinuity has invariably led to confusion, some of
one. The difficulty lies in the interdisciplinary which is not even resolved today.
character of our mission and, in the end, en- For example, the earliest practitioners in the
counters the pro](cid:127)lem which has characterized the field of nucleation were experimentalists (physi-
field of nucleation and particulate formation for cal chemists, metallurgists, or engineers) who
as long as I can remember. The field seems, tended to view the process more in terms of its
always, to fall "between the cracks". From time finished product, e.g., a macroscopic fragment or
to time it has attracted the attention of such drop of the daughter phase. Reasoning then
diverse specialists as metallurgists, engineers, traveled along the path directed from the macro-
chemists, physicists, astronomers, aerodynami- scopic to the molecular level, and, through all of
cists, meteorologists, and many more. These this, the drop remained a drop until it became
individuals have brought with them such diversi- evident that one could not proceed further without
ties of languages and pyschologies of (cid:127)nnovation accounting for molecular detail. The strategy
that they have really experienced difficulty in then became one in which the drop was retained
communicating with one another. while attempts were made to graft molecular-like
Although this is frustrating it does illustrate behavior onto its continuum personality.
one cardinal point. The problem is of immense For example, the drop was now regarded as a
scientific and technological importance. A list large molecule and as such had to be described by
of those subjects to which nucleation and/or a molecular partition function. Since the calcu-
growth have contributed would have to include lation of this partition function, beginning truly
metallurgy, high energy physics, atmospheric at the molecular level, represented a very dif-
science, planetary physics and cosmochemistry, ficult task, the issue was avoided by appealing to
vulcanism, aerodynamics, turbine technology, the relation between the partition function of the
cavitation, boiling, air pollution, crystal drop and its free energy. The free energy could
growth, magnetism, and chemical processing. Now, be evaluated from the bulk thermodynamic proper-
we are asked to consider the relationship between ties of the liquid if satisfactory corrections
atmospheric particulates and catalysis. We will were made for surface free energy, a quantity that
be adding one more discipline, and another group had to be considered in view of the drop's large
of specialists, to an already top heavy collec- ratio of surface to volume. This procedure is the
tion. It therefore seems appropriate to consider well-known capillarity approximation.
some of the examples from the past in which con- But then it was noted that not all the free
fusion was generated among capable people solely energy was accounted for by this device. For
because of their diverse orientations. With such example, a drop suspended from a capillary tube
knowledge we might avoid similar difficulties does not seem to be translating (as a molecule
because of our own diverse orientations. would) throughout the laboratory. Furthermore, it
The process of nucleation involves the escape of does not seem to be rotating (as a molecule
a system from a metastable state, and, often to would). Corrections were therefore made for these
the development of a new (daughter) phase within omissions. These corrections which endeavor to
an outwardly homogeneous mother phase. The stu- take account of the fact that the necessary de-
dent of this phenomenon is therefore faced with grees of freedom were nonetheless present in the
spanning a long sequence of events which stretches drop (but not as translations and rotations) are
all the way from the molecular to the macroscopic known as "replacement free energy", and lead to
domain. In different stages of this sequence he the prediction that calculated rates of nucleation
may be forced to use different approximate methods might be in error by a factor as large as 1018.
in order to arrive at tractable solutions to the Later work took account of the fact that trans-
Copyright American Geophysical Union
Geophysical Monograph Series Heterogeneous Atmospheric Chemistry Vol. 26
REISS 3
lational degrees of freedom refer to the motion of vibrational degrees of freedom. Since the center
the center of mass of the drop and that a drop of mass is chosen consistently in this approach,
suspended from a capillary tube in the labora- the problem of replacement free energy never even
tory still had a center of mass which was able makes its debut. We are then left with another
to translate (fluctuate). So the original drop discontinuity of concept - this time originating
did have some translational free energy. at the molecular rather than the macroscopic side
Furthermore, since a liquid cannot rotate rigidly, of the conceptual interface. Other conventions
a drop cannot rotate in the conventional manner as involve confining molecules inside large spherical
a rigid body. However, the molecules in the drop regions with perfectly hard walls, the center of
still had angular momentum, even if not concerted. the sphere being located at the center of mass of
But the same nonconcerted angular momentum was the assembly of molecules contained within it.
present in the bulk liquid. Thus, very little The free energy of this "physical" cluster is then
replacement free energy was needed for the case of evaluated by Monte Carlo means. This convention
rotation. This brought the factor of predicted is more "droplike", except that in a drop the
error downf roma pproximately1 018 to about 104. center of massi s not confinedt o the geometric
Although I think that the confusion has been center so that another discontinuity of concept
resolved, it still exists in the minds of many. arises, and an adjustment has to be made.
This example is a good one because the confusion There is another kind of dichotomy which arises
was generated by the need to maintain a continuity in this field. This has to do with the distinc=
of concept at an interface between the stages of tion between equilibrium (thermodynamics) and
reasoning associated with the molecular level on kinetic processes. In recent years photochemists
the one hand and the macroscopic point of view on and more conventional chemical kineticists have
the other hand. As soon as individuals, more become interested in nucleation for a number of
accustomed to thinking about things on the molec- reasons. Among these is a strong interest in the
ular level, became involved they began to make photochemistry of atmospheric processes, especial-
progress but also generated confusion since they ly those which lead to photochemically generated
crossed the interface in the other direction, particles and pollutants. In addition, nucleation
traveling along the path from the molecular to the and growth are being used for detection and am-
macroscopic level. They also started from several plification in the study of the mechanisms of some
different points. chemical reactions. This constitutes another
A popular but erroneous starting point was the reason. Now, although most chemical kineticists
theory of so-called "mathematical" clusters which, are well schooled in the modern disciplines of
among other things, are useful for the statistical chemical rate theory, and are even well into the
mechanical treatment of imperfect gases. Unfor- subject of "selected state" chemistry made pos-
tunately, the mathematical clusters imply just sible by the advent of the laser, the molecular
what their name says; they are merely bookkeeping beam, and other modern instrumentation, they have
devices for keeping track of the details of a shown a tendency to misunderstand the kinetic
complicated mathematical development. What was problems in nucleation theory.
really needed was a treatment using "physical" In many, but not all, nucleation processes a
clusters such as the elementary aggregates of quasi-equilibrium theory does fairly well. There
molecules which are incipient fragments of the new are several good examples of this kind of theory
phase. However, it immediately became apparent within the discipline of standard chemical kinet-
that the "convention" by means of which a physical ics. One example concerns unimolecular decompo-
cluster was defined was central to the whole prob- sition. The old Lindemann mechanism first pub-
lem. For example, if one was interested in de- lished in 1922 is a case in point. In this
veloping a theory (an equation of state) for an process two molecules which we denote by A enter
imperfect gas, making use of physical clusters, into a reversible reaction as follows:
any number of conventions were suitable as long as
they were dealt with consistently, but all of them A+A++A +A (1)
made a theory far more difficult than with the
simple device of the mathematical cluster. in wbich A* represents somes ort of excited mole-
In addition, the problem of nucleation and cule, capable of undergoing the decomposition
subsequent growth is a kinetic problem, not an represented by the following reaction:
equilibrium one. In this case the physical clus-
ter had to be chosen so that it was, at once A*+B+C (2)
treatable, and yet corresponded to the fragments
of the new phase which are the products of the where B and C represent decomposition products.
nucleation process. Even today, no thoroughly To the forward reaction in equation (1) we assign
satisfactory solution to this dilemma has been the specific rate constant kl, while to the
devised. reverse reaction we assign the constant k_ 1. At
Several partial solutions have involved the use equilibrium in the reaction specified by equation
of large computers, appealing to molecular dynam- (1), we then have the following relation:
ics or Monte Carlo techniques. In one example the
kl[A]=2 k_i[A*][A] (3)
cluster is treated as a molecule having internal
Copyright American Geophysical Union
Geophysical Monograph Series Heterogeneous Atmospheric Chemistry Vol. 26
4 NUCLEATION AND GROWTH
where bracketed quantities represent concentra- pearance of the kinetics changes. The important
tions. feature of equation (9) is the appearance of the
If the reaction of equation (2) occurs rela- equilibrium constant K. This reminds us that the
tively slowly compared to the rate at which the result is made possible by having one of the steps
equilibrium of equation (3) is established, then of the overall process being virtually in equilib-
it perturbs that equilibrium only slightly. In rium.
fact, the rate associated with equation (2) can be What is not always perceived by kineticists, who
accounted for by the term often tend to regard the process of nucleation as
a kind of chain polymerization affair, is that
k2[A]* (4) mosnt ucleatinsgy stemress embthlee c aseo f
unimolecular decomposition (a case with which they
are very familiar). A cluster of a new phase is
wherek 2 is againa specific rate constant. The built up by the stepwisea ddition of moleculetso
rateo f changoef thec oncentratoiofn A * is then smallecrl usters.E acho f theses tepsis a reac-
givenb y the net rate of productionre presentebdy tion muchli ke equation( 1), and, for the most
equation (3) with a minus sign replacing the equal part, most of the early steps are in virtual
sign, fromw hichw es ubtractt he loss represented equilibrium,a gainl ike equation( 1). Eventually
by expression( 4). Thus,w e have a step is reached( at the so-called nucleuss ize)
where the cluster can "disappear", this time, not
d[A*_]-k [A2]- k [A*][A- k] 2[A] *( 5) byd ecompobsuitbti oyr na,p idglyro wiinntgoa
dt 1 -1 macroscopdicro p. Ther ate of this processw, hich
is the rate of nucleation, is basically the rate
However,t he concentrationo f A* is assumetdo be
of this last step. If the nucleus contains n
so smallt hat it reachesa quasi-steadsyt ate, and moleculesa, ndi f we denoteit by the formulaA n ,
the derivative on the left side of equation( 5) then the concentrationo f nuclei will be givenb y
can be set to zero. Then one can solve for the
concentration of A* in the resulting equation and
[An] = K'[A] n (10)
substitute it into expression (4) to obtain an
expression for the rate of appearance of the de- in which K' is simply the equilibrium constant
composition products, or, what is the same thing, characterizing the equilibrium between single
the rate of disappearance of the reactants A. molecules (monomers) A and nuclei An. This
The result is expression is the analog of equation (7), and to
2 complete a crude theory of nucleation we require
-d[A]_ - klk2[A ] the analog of expression (4). This is simply the
(6)
dt k_l[A] + k2 rate at which monomers encounter and join the
nucleus, so that it quickly grows to a macro-
scopic drop. We can express this rate as
We also note from equation (3) that an equilibrium
constant K can be defined as follows: J : k [An][A] (11)
n
K: [A_(cid:127)_[(cid:127)[_A: *=] kl
(7) where again kn is a specific rate constant and J
[A]2 [A] k_l
is the rate of nucleation. Substituting equation
(10) into equation (11) gives
so that equation (6) may be written in the form
J = k K'[A]n +l (12)
n
-d[A] k-lk2K[A]2
(8) From this we see that the rate may depend very
dt k_l[A] + k2
critically on the concentration of monomers, since
n may be a fairly large number and a slight change
In the case that the concentration or pressure in the concentration of A may lead to an enormous
of A is high, the first term in the denominator of change in J. Since the concentration of A will be
equation (8) dominates the second term to the roughly proportional to its pressure, the nu-
extent that the second may be ignored. In that cleation rate will be very sensitive to pressure
case equation (8) reduces to or so-called supersaturation. If, for example,
there are of the order of 100 molecules in the
d[A] nucleus, n will be of the order of 100, and a
dt = k2K[]A (9)
slight change in pressure will have an enormous
effect on J.
This is a first-order reaction even though the Now the quasi-equilibrium nature of the theory,
overall process is not really unimolecular, and embodied in the use of K', has prompted workers to
this famous result shows how a reaction can appear attempt to evaluate K' by estimating the standard
to be unimolecular. Of course at low pressures free energy of reaction (as with the case of any
one must use the full equation (8) and the ap- good equilibrium constant), and what better means
Copyright American Geophysical Union
Geophysical Monograph Series Heterogeneous Atmospheric Chemistry Vol. 26
REISS 5
for doing this than by evaluating the free energy becomes possible to perform "trajectory analyses",
of the drop destined to represent the nucleus. in the modern sense, for the non co l linear case,
Thus, the capillarity approximation enters the and to treat the nucleation process as an ordinary
picture, and the drop becomes the model for a chemical reaction. So here again, we have the
cluster. Unfortunately, only the drop represent- union of two diverse disciplines, and it is impor-
ing the nucleus can be considered to be in equi- tant to carry out this union in as smooth a manner
librium with the metastable phase, and the use of as possible.
a drop to represent smaller clusters becomes In this monograph we are considering the pos-
immediately suspect. However, if one truly uses sible importance of heterogeneous catalysis on
an equilibrium theory as is the case with equation atmospheric particulate surfaces. Here again we
(12), this inconsistency is avoided, since smaller are bringing together individuals from different
clusters do not have to be considered. fields who already have worked on problems, each
Unfortunately, no theory of rate can be a truly in their respective fields, which happen to be
equilibrium theory. When adjustments are made to isomorphic, but which may be couched in different
take into account contributions of nonequilibrium languages. For example, in the case of hetero-
phenomena, more problems are created because one geneous nucleation (or in the growth of partic-
must then deal with clusters having sizes smaller ulates) atoms or molecules adsorbed on a particle
than the nucleus. Incidentally, the primary are limited by steric factors, surface energies,
effect of these nonequilibrium corrections is to and processes of migration which take place on the
"perturb" the equilibrium distribution of clus- surface itself. In the field of nucleation and
ters, which for the nucleus would be represented growth of particulates, these factors have fre-
by equation (10), in view of the fact that clus- quently been considered. Perhaps one of the most
ters are being "drained" away by the process famous examples, pioneered in the Albany/
represented by equation (9). This is not an Schenectady region, is that of silver iodide which
uncommon effect in other kinetic examples; it is capable of nucleating ice crystals, presumably
occurs, for example, when one considers the pro- because, in some way, the silver iodide surface
cess of effusion of a gas through a pin hole and the ice crystal surface can be made to reg-
where the distribution of velocities can no ister. Heterogeneous nucleation is itself an
longer be considered Maxwellian near the pin hole example of catalysis. In this case the nucleus
because molecules with selected velocities are catalyzes the decay of the metastable state.
being drained out of the distribution. In fact, Cluster calculations have been relevant to both
chemical kineticists are also familiar with this fields. I have already mentioned the treatment of
process in connection with chemical rate theories. nuclei as though they were large molecules having
For example, the theory of absolute reaction rates internal degrees of vibrational freedom. Fairly
must be adjusted for the fact that activated com- ambitious computer experiments have been carried
plexes are not truly in equilibrium with reactants out in this direction. But clusters have also
simply because they are being drained away by the been used by theorists interested in exploring the
reaction whose rate one is trying to estimate. quantum mechanical aspects of catalytic surfaces.
All of this has been meant to emphasize the For example, studies have been performed on clus-
similarities between the problems which appear in ters of as many as 36 beryllium atoms. In these
the kinetics of phase transitions and more con- studies a fairly complete treatment of the elec-
ventional kinetics associated with chemical pro- trons in the cluster is performed. The surface
cesses. In spite of this, there is ample evidence properties of the cluster seem to become fairly
that many sophisticated chemical kineticists have stable even when only 26 atoms are involved.
not fully penetrated the subtleties of nucleation Alternatively, surface theorists have used the
theory. Of course the reverse is also true. strategy of embedding a cluster in a surface,
Sometimes, because of 'a concentration on thermo- treating the rest of the surface in some average
dynamics in the evaluation of K', workers seem to way (by means of boundary conditions), and learn-
think that nucleation is a branch of thermody- ing something about the electronic properties of
namics rather than kinetics. Of course this is the surface through the calculated behavior within
not true. the embedded cluster. Sometimes instead of em-
Incidentally, an interesting goal in the theory bedding the clusters the "dangling" bonds are
of nucleation is the use of modern chemical kinet- "capped" by satisfying them with hydrogen atoms.
ic theory. We are approaching the point where I could, in fact, go much further. For example,
this may be feasible. For example, it may be pos- the catalyzing of heterogeneous processes on a
sible to produce such high states of super- particular surface could itself affect the nature
saturation (e.g., by rapid expansion of a gas of the particulate, its size and configuration,
through a nozzle) so that the nucleus is a dimer. and in so doing affect the growth. There is a
This may be possible in the case of supersaturated real physical symbiosis between the fields we are
argon vapor. In that case the process represented considering and a potential intellectual symbiosis
by equation (11) would involve no more than three between the workers therein. One of the primary
molecules, the two molecules in the nucleus and objectives of this monograph is to explore these
the third molecule needed to further the process symbioses and to determine what effect they have
of condensation. With only three molecules, and on the atmospheric processes that affect all of
the knowledge of the intermolecular potential, it us.
Copyright American Geophysical Union