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OCR for page 94
The Role of Modeling in Unclerstanding
6 Tropospheric Chemical Processes
BY R. DICKINSON AND S. LTU
PRINCIPLES OF MODELING
We have just discussed the various processes entering
into tropospheric chemical cycles. These processes are
studied individually to better understand them and to
improve our descriptions of the quantitative relation-
ships between the different variables entering into a
process. These relationships are never exact representa-
tions of reality. They are subject to continuing improve-
ment using better measurements and new insights.
These mathematical relationships between various
components of a process are referred to as "process
models. "
It is useful to distinguish between those variables of a
process system that are internal to that system, that is,
calculable from the process model, and those that are
external, that is, prescribed from observation. In partic-
ular, the linkages to other processes are external varia-
bles in the formulation of an individual process.
If all the linked processes of the tropospheric chemical
system are considered together with their linkages in-
cluded as internal variables, we have a "system model. "
A system model still contains some external parameters,
but if the model is sufficiently comprehensive, such pa-
rameters can be reliably prescribed for current condi-
tions.
The system model has two basic functions. First, be-
cause it has maximized the number of variables it calcu-
lates, it provides a good opportunity to carry out exten
94
sive comparisons between the model calculated versus
observed variables of the system; such comparisons help
identify weaknesses in the individual process models.
Second, it allows projections as to the future state of the
tropospheric chemical system as various external pa-
rameters change with time. Of special interest in this
context are external changes imposed by human activ-
ity, but also of interest is long-period natural variability
in external conditions.
Because of the complexity of system models, they are
generally integrated by means of computer programs.
One important aspect of such integration is the develop-
ment and use of numerically accurate procedures for
solving the differential equations that are used to define
the process models and, hence, system models. Tropo-
spheric chemistry shares with meteorology a concern for
a wide range of interacting scales, beginning on the scale
of individual microeddies, e.g., within a smoke plume
from a power plant, and ending in the global scale.
Satisfactory parameterizations of the role of smaller
scale processes should be one of the objectives in devel-
oping and improving system models of global tropo-
spheric chemistry.
EXISTING MODELS
The generality and detail possible in a complete sys-
tem model are limited by difficulties of interpretation as
OCR for page 95
THE ROLE OF MODELING
a result of its extreme inherent complexity and the large
demands placed on computer and programming re-
sources. Up to now, the necessary staff and institutional
support to pursue a complete system model of tropo-
spheric chemistry have not been developed. Further-
more, there have been considerable uncertainties in the
individual processes. Consequently, the research tools
that have been used in tropospheric chemistry studies
for synthesis and interpretation lie between the concept
of a process model and a complete system model. We
shall refer to these tools simply as tropospheric chemistry
models. There has been developed a wide range ofthese
models, with their content depending on the interests
and objectives of their developers as well as their access
to computing resources. Basically, what distinguishes
the models now used from a complete system model is
that they attempt to model accurately only some of the
processes of the system, the others being included only
in a simplified or ad hoc fashion.
Existing models can be best classified by the issues
they address. Usually these models consist of two major
parts: the chemistry and the transport. Depending on
the characteristics of the subject being studied, each
model employs different degrees of sophistication in the
treatments of chemistry and transport. At one extreme
are meteorologically oriented models that obtain the
motions ofthe atmosphere and temperature structure as
three-dimensional time-varying fields by solution of the
continuum equations of hydrodynamics and therrnody-
namics in response to realistic boundary conditions;
however, these models have until recently approximated
tropospheric chemistry by ignoring all species except
water. Some studies are now under way using the winds
generated by some regional and global meteorological
models to provide transport for simplified chemical
models. At the other extreme are the one-dimensional or
box chemical models. As a consequence of their ex-
tremely complex processes of chemical species transfor-
mations, their transport consists essentially of empiri-
cally derived time scales for movement of species from
one box or level of the model to another.
At the current state of development, one ofthe promi-
nently distinguishing features of chemical models is
their dimensionality. Thus there are zero-, one-, two-,
and three-dimensional models. The zero-dimensional
box models simulate laboratory chemical reaction mea-
surements such as in smog chambers. Reactants in the
reaction chamber are assumed to be completely mixed
so that transport can be neglected. Chemistry is treated
in detail by including all relevant elementary reactions.
Usually Gear's code with small integration time steps is
used to study the time-dependent behaviors of all reac-
tants. Multidimensional models can be viewed as alarge
number of zero-dimensional models, coupled together
95
by transport and radiation submodels, and each differ-
ent because of different transport source-sinks and dif-
fering environmental conditions. In the modeling of the
global tropospheric photochemistry, emphasis has been
on the problems of 03, CO, CO2 (or carbon cycle),
nitrogen cycle, and sulfur cycle. Box models and one-,
two-, and three-dimensional models have been devel-
oped to study the natural chemistry and possible effects
of anthropogenic activities on these species. Because of
the computer resource requirements for three-dimen-
sional modeling, full-scale chemistry has not been in-
cluded. On the other hand, simpler models with full-
scale chemistry usually do not successfully parameterize
the important transport processes and hydrological cy-
cle ofthe atmosphere.
Besides dimensionality, treatment of model time
structure is also notable. For example, is the model
steady state or capable of following transient changes?
How does it treat diurnal and seasonal variations? Some
models only calculate fast chemical transformations but
prescribe as external, slowly changing species. Such
models avoid the need for transport submodels because
transport is primarily important for determining the
distribution of the slowly changing species. Other
models prescribe the species with fast chemistry, in par-
ticular OH, as external, and concentrate on the interac-
lion among source, sinks, and transport of slow species.
One important distinction with regard to model
objective is the difference between climatological and
event models. This distinction arises because ofthe large
day-to-day variability of meteorological processes,
including transport. Thus a detailed case study of the
processes of tropospheric chemistry over several days or
less must recognize the actual transport occurring over
that interval, either by explicit measurement of it over
the interval, or by measuring enough initial meteorolog-
ical data to allow integration of a weather forecast model
for the time and space domains of interest. On the other
hand, if a study is more concerned with the average
behavior of the atmosphere as described by means and
higher statistical moments, then there is less demand on
temporal accuracy in providing the meteorological
transport terms. The most effective tools in this instance
are the general circulation models that obtain from first
principles the statistical properties of the atmosphere by
direct numerical simulation. That is, they calculate day-
to-day weather variations over a long period oftime that
do not correspond to any particular time period but are
supposed to have the same statistics as actual weather
systems. In other words, they model the climate of the
atmosphere system.
Much of the work on three-dimensional modeling of
tropospheric chemistry up to now has been on the urban
and, more recently, regional scale. The chemical models
OCR for page 96
96
developed for these studies should be considered in
developing models for global tropospheric chemistry.
Dispersion models have been developed to study the
dispersion of air pollutants from power plants as well as
cities. These models are concerned with urban-scale
transport but usually ignore chemical transformations
of air pollutants. Models that do include air pollution
chemistry but employ simpler transport pararneteriza-
tions are called air quality models. Most of these models
are developed for metropolitan and suburban areas;
occasionally regions as large as the eastern United States
have been included.
Because air quality models emphasize the oxidant
problem, the chemistry usually includes that of 03,
NOx, and hydrocarbons. Studies using smog chambers
have led to the development of detailed mechanisms for
specific hydrocarbons. Unfortunately, the chemistry in
these mechanisms is far too extensive to be incorporated
into air quality computer models. In order to circum-
vent this problem, at least two approaches have been
utilized. These involved "lumping" the hydrocarbons
by classes and using a generalized reaction mechanism
for these classes, or by using a carbon-bond approach,
which partitions the chemical species on the basis of the
similarity of their chemical bonding. These chemical
models have been tested against and tuned to a variety of
smog chamber data. Usually good agreement is
achieved between measured and predicted concentra-
tion-time profiles for all measured species. When these
reaction mechanisms are incorporated into air quality
models and compared to field measurements, the agree-
ment becomes much poorer. Discrepancies could be due
to poor transport pararneterizations, but there is little
doubt that lack of understanding of the chemistry in the
real atmosphere also contributes to the discrepancies. In
particular, the chemistry of aged and diluted air pollu-
tants may be poorly understood because it cannot be
effectively tested in smog chambers. Furthermore, het-
erogeneous reactions are either not included or treated
by oversimplified parameterizations.
Acid deposition models are used to study wet and dry
deposition of acid material such as sulfur and nitrogen
compounds. Acid deposition models have been devel-
oped for Europe and the eastern half of North America.
The major objective of these models is to establish the
source-receptor relationship of acid deposition. So far,
very little chemistry is included in the acid deposition
models. Constant, linear conversion rates of SO2 to SO4-
and NO2 to NO3 have been used. Wet scavenging and
dry deposition are assumed to be independent of cloud
types or topography. Most of the modeling effort is
focused on the development ofthe meteorological aspect
of the model. There is a clear need to incorporate into
these models the full-scale fundamental chemistry and
PART II ASSESSMENTS OF CURRENT UNDERSTANDING
kinetics involved in the transformation of sulfur and
nitrogen compounds.
In conclusion, as the field of tropospheric chemistry
matures, the various kinds of models should tend to
converge more toward ideal system models. This
occurs, on the one hand, as modelers learn to treat more
elaborate model systems and, on the other hand, as they
are able to understand the error implied by various con-
venient approximations and hencejustify these approxi-
mations when their implied error is acceptable. Global
modelers and regional modelers should collaborate
on those aspects of their models that are of common
interest.
MODELING IN SUPPORT OF THE PROPOSED
RESEARCH PROGRAMS
We discuss here the modeling programs required to
provide guidance to and help synthesize the results ofthe
research programs proposed in Part I ofthis report. The
programs in biological sources, photochemical proc-
esses, and removal processes require the development of
submodels describing the individual processes involved.
These would serve three purposes: (1) help to under-
stand better the individual process, (2) help to extrapo-
late from individual observational sites to regional and
global averages, and (3) provide submodels to be used in
a comprehensive three-dimensional meteorological
model coupled to global tropospheric chemistry. By con-
trast, the global distributions and long-range transport
program would be used to help validate the overall per-
formance of the chemical aspects of comprehensive
three-dimensional models of tropospheric chemical
processes. As submodels are developed for the various
subprograms, they will be incorporated into the com-
prehensive models.
Biological and Surface Source Models
The models required to support the biological and
surface source subprogram fall into three categories: (1)
global empirical models, (2) mechanistic models of bio-
logical processes, and (3) micrometeorological and oce-
anic models of surface transport processes.
The observational efforts in the biological source sub-
prograrn will provide measurements at individual field
sites. Initial exploratory efforts will identify the ecologi-
cal communities that provide significant emissions, but
as a second stage, it will be necessary to obtain sufficient
observations to determine annual average emissions at
various sites. Variability with environmental parame-
ters such as temperature, solar radiation, and moisture
will also be obtained. However, due to the great variety
and small-scale structure of biological systems, it will
OCR for page 97
THE ROLE OF MODELING
always be very difficult to collect sufficient data to permit
straightforward numerical averaging to establish
regional and global average emissions. Rather, more
sophisticated approaches will be required to interpolate
and extrapolate the available observations to all the non-
sampled areas. Exactly the same problem occurs in
summarizing other types of data obtained from ecologi-
cal communities. Ecologists, in particular, have been
forced to resort to empirical procedures for obtaining
such parameters as net primary productivity and bio-
mass carbon (e. g., Table 5. 1) from a ratherlimited num-
ber of field sites (less than 100~. One systematic proce-
dure has been to correlate ecological data with readily
available climatic parameters pertaining to the sampled
sites, in particular rainfall and temperature. The corre-
lations so obtained are used to transform global maps of
climatic parameters into global maps of ecological
parameters. Such a procedure will be used to develop
empirical models (i.e., maps) for the global distributions
of the various measured biological emissions.
Complementary to the development of global distri-
butions of biological emissions will be the development
of models of the detailed biological mechanisms and
processes responsible for the measured emissions. These
will range from models of soil or oceanic biochemical
processes to models of whole leaf physiology. Their
development will require intensive collaboration with
experts in other biological and chemical areas outside
the atmospheric chemistry community. These efforts
will, however, differ from current and past modeling in
these other disciplines in the following aspects. First,
they will be focused on the processes responsible for
providing atmospheric emissions. Because these emis-
sions have for the most part been recognized only
recently, or in some cases not yet, the other disciplines
have only begun to consider how such emissions could
be provided from their existing submodels. Second, this
modeling effort will be focused on the whole biological
-
system (plant-soil-microorganisms, etc.) as it interacts
with the atmospheric environment. Because of the great
complexity of the processes involved, a model of the
whole biological system will undoubtedly require sim-
plifications in the descriptions of biochemical processes
and the treatments of differences between species of
organisms.
Modeling the effects of soil microorganisms would
require modeling the environment where the processes
occur. For example, the question of methane production
requires a model of the diffusion of CO2, H2, and CH4
from the production site to the atmosphere to address
the question as to whether increasinglevels of CO2 could
increase methane production.
Boundary layer and surface transport models are
required to describe the movement between ocean or
97
land surfaces and the atmosphere. In the case of oceanic
processes, such models require consideration of oceanic
as well as atmospheric boundary layers and the effects at
the ocean interface of wave breaking, i.e., the move-
ment of air bubbles on the ocean side and spray droplets
on the atmospheric side.
The transfer of gases from and to a surface generally
involves near-stationary diffusion-like transport proc-
esses that are represented in terms of effective resistances
or conductances. That is, if ca represents the concentra-
tion of a gas in the atmospheric mixed layer, and this gas
is maintained at some concentration cs at some surface,
then the rate of flux of the species to the mixed layer from
the surface is modeled as given by (cs-ca)/r`, where rat is
the total resistance of the diffusion processes between the
atmospheric mixed layer and the surface.
The most thoroughly studied gas transfer process, for
example, is that of water vapor from soil and foliage.
The water vapor concentration next to the mesophyllic
cells inside a leaf is that of saturation at the temperature
ofthe leaf. To reach the mixed layer, the water must pass
through leaf stomata, the leaf boundary layer, the leaf
canopy, and a roughness sublayer above the canopy
before reaching the atmospheric mixed layer. Each of
these barriers is modeled by one or more resistance in
series or parallel. This description in terms of resistance,
although somewhat simplistic, provides the maximum
level of detail that can be practicably matched to models
of atmospheric transport above the mixed layer and
validated by micrometeorological observations.
In most cases, the surface boundary conditions are
not as easily modeled as that of water vapor. Surface
boundary conditions for species of interest would be one
of the practical outputs of the modeling in the biological
source subprogram. For example, rather detailed
models are now available for leaf photosynthesis that
provide the concentrations of CO2 within the leaf cavity.
Plants exert physiological control over water losses
through stomata! closure; the stomata! resistance is sig-
nificant not only for leaf exchange of water and CO2 but
also for SO2, NO2, 03, and NH3, and is modeled in
terms of soil moisture and root resistances. Detailed
boundary conditions for other gases, in particular SO2,
NO2, 03, and NH3, and biologically emitted sources
need to be established.
Field programs, together with continuation of labora-
tory (i.e., wind tunnel) studies should provide the data
needed to develop and refine models of oceanic gas
transfer processes, in particular for those species where
surface boundary layers within the ocean provide addi-
tional resistance to their flux between ocean and
atmosphere.
The micrometeorological processes of gaseous and
particulate exchange within complex vegetated cano
OCR for page 98
98
pies are still poorly understood in detail, and need to be
further studied through combined observational and
theoretical approaches to generate improved models to
be used in modeling gas and particle exchange.
A special effort should also be made to develop pa-
rameterizations for the suspension into the atmosphere
of continental soil aerosols, so that generation of these
aerosols can be simulated in meteorological models.
The Monin-Obukhov similarity theory gives an ade-
quate basis for modeling fluxes through the surface
mixed layer over horizontally homogeneous terrain. Its
adequacy over more complex terrain, for example, with
only sparse trees, is still an open question. This inade-
quate understanding of the micrometeorology above
complex terrain introduces uncertainty into the model-
ing of gaseous and particulate fluxes between the atmo-
sphere and these surfaces, and so into the average re-
gional and global fluxes.
Modeling Noncyclic Transformation and Removal
Processes In Clouds
Clouds and precipitation play important roles in the
removal, transport, and transformation of species in
element cycles. For instance, wet removal is probably
one of the most effective sinks for nitrogen and sulfur
compounds. Important species such as SO2, N2O5, and
perhaps NO3 may go through fast aqueous transforma-
tion in cloud droplets. Furthermore, cloud convection
may be an efficient vertical transport mechanism for
trace gases and aerosol particles.
In order to evaluate these processes quantitatively, a
cloud-removal model should include detailed treat-
ments of the physical and chemical mechanisms
involved. The cloud model should be a submodel of a
meteorological model that includes self-consistent and
realistic treatments of heat, moisture, and momentum
transports. Physical aspects of the model include the
parameterization of radiation, condensation and evapo-
ration, and stochastic coalescence and breakup. Chemi-
cal aspects of the model include both homogeneous
gas-phase and liquid-phase reactions as well as heteroge-
neous reactions. In the clean atmosphere, chemical spe-
cies treated within the cloud model should include at
least 03, odd-nitrogen species, hydrogen radicals,
H2O2, sulfur species, CO, and CH4 and its oxidation
products. In the polluted atmosphere, nonmethane
hydrocarbons and their oxidation products, metals such
as Mn and Fe, and graphitic carbon should also be
considered.
Many fundamental parameters of gas-to-particle
reactions and kinetics need to be studied in the labora-
tory and by modeling. For instance, sticking coefficients
for above-mentioned gases on various types of aerosols
PART II ASSESSMENTS OF CURRENT UNDERSTANDING
under atmospheric conditions need to be measured and
investigated theoretically.
Currently, cloud models exist that include various
degrees of sophistication in treating the physical proc-
esses mentioned above. Their incorporation into a self-
consistent meteorological mesoscale model is in prog-
ress. On the other hand, photochemical cloud-removal
modeling is at a rudimental stage. Some advances have
been made recently, due primarily to the worldwide
interest in the acid deposition problem. Because these
processes are subscale for global models and sometimes
for regional models, proper parameterization of their
effects will be a crucial step in modeling these processes.
A program that consists of well-coordinated field and
laboratory measurements and model studies is needed
to gain an adequate understanding ofthe cloud-removal
processes. Each important mechanism involved, both
physical and chemical, needs to be tested through itera-
tive intercomparisons among modeling, field, and labo-
ratory studies. Only then can a realistic cloud-removal
model be developed based on the parameterization of
these mechanisms.
Modeling Fast-Photochemical Cycles and
Transformations
Some of the outstanding problems confronting the
modeling of fast-photochemical transformations are (1 )
the prediction of the concentrations of OH and HO2 in
the ambient atmosphere and their dependence on the
concentrations of NOx and hydrocarbons; (2) the bud-
get of 03; (3) the mechanisms and rates of the oxidation
of NOx and SO2 to nitrate and sulfate, involving both
homogeneous and heterogeneous reactions; and (4)
the nature and rates of fast transformations involving
atmospheric hydrocarbons and their oxidation prod-
ucts. Unlike the modeling of cloud-removal processes,
mathematical techniques of modeling fast gas-phase
photochemical transformations are well developed.
Computer models dealing with fast-photochemical
transformations exist for polluted urban air as well as for
clean background troposphere. The major deficiency in
the understanding of fast-photochemical transforma-
tions lies in the lack of laboratory and field data. For
instance, concentrations of key species such as OH,
HO2, and H2O2 have not been reliably measured, and
kinetic data of many key reaction rate constants have not
been determined under tropospheric conditions.
Recent developments in field and laboratory meas-
urement techniques have made it possible to measure
some of the above-mentioned key species and reaction
rate constants. Further major advances in the under-
standing of fast-photochemical transformations can be
OCR for page 99
THE ROLE OF MODELING
made by coordinating modeling with the field and labo-
ratory studies.
Heterogeneous processes may play some important
roles in the fast photochemistry. As discussed in the pre-
vious section, major advances in measurement technol-
ogy and theoretical treatment are needed in this area.
Modeling Global Distributions and Long-Range
Transport with a Three-Dimensional Meteorological
Model
A three-dimensional model of tropospheric chemical
processes linked to meteorological and climatic proc-
esses or, in brief, a Tropospheric Chemistry System
Model (TCSM) is an important tool for the overall syn-
thesis and theoretical guidance of the GTCP. A recom-
mended institutional framework for the development
and operation of such a TCSM is given in the next
subsection. Here we outline some ofthe research studies
that one or more such TCSMs would carry out in sup-
port of the global distributions and long-range transport
subprogram and, more generally, for modeling explora-
tion of the tropospheric chemical system. Two classes of
studies would be carried out with TCSMs. First would
be studies intended to validate and possibly develop the
ability of models to simulate long-range transport and
global distributions and variability of long-lived chemi-
cal species. These studies would compare model results
with the data sets obtained through the global distribu-
tion and long-range transport subprogram of GTCP
and, if neccessary, develop the model improvements
required for satisfactory validation. Most of the model-
ing studies would be carried out in a climatological
framework, but one or more detailed event studies
would be performed in conjunction with intensive pe-
riods of field data collection. Such studies could also be
carried out with more simple chemistry as needed to
simulate the sources and sinks of the long-lived species.
The second class of studies would emphasize the sim-
ulation of medium-lived species. At present, the sources
and sinks of these species are not sufficiently well known
for such studies to be used to test model transports.
Rather, these studies, assuming adequate model trans-
port submodels, would explore the role of meteorologi-
cal processes in determining the spatial distributions
and temporal variability of the medium-lived species.
Such modeling studies, for example, could address the
question as to how important are continental pollution
sources of sulfur and odd nitrogen for atmospheric dis
. . .
try buttons at remote sites.
The exploration of such questions would help im-
prove interpretation ofthe data on many ofthe species to
be monitored in the global distribution and long-range
transport subprogram. Medium-lived species of special
99
current interest include NO-NO2-HNO3, NH3, SO2,
sulfate-nitrate aerosol, and continental soil aerosol. The
soil aerosol is of interest not only because of its optical-
radiative effects, but also as a source of Ca, which can
raise the pH of droplet aerosols.
Institutional Framework for Development and
Application of a Three-Dimensional Tropospheric
Chemistry System Model (TCSM)
The study of tropospheric chemistry, as it has devel-
oped over the past decade, has largely been in an explor-
atory phase of study with emphasis primarily on the
development of new instruments and concomitant pio-
neering measurements of previously unseen species. It
is the thesis of our report that tropospheric chemistry as
,. . . . . . .
a dISCIp Ine Is ripe to become a more mature science wit.
large-scale field programs devoted to the systematic col-
lection of required data. Essential to the successful appli-
cation of these data to advance scientific understanding
are not only submodels of the various processes studied
but also system models of the overall tropospheric chem-
ical system. Such models necessarily indude the mete-
orological processes that transport and in other ways
interact with the chemical species. The meteorological
processes are best provided through versions of atmo-
spheric general circulation models (GCMs) that have
been especially designed to satisfactorily provide not
only large-scale tracer transports in the free atmosphere
but also transport through the planetary boundary layer
across the tropopause and through cloud processes.
These models also require physically based cloud sub-
models, a good description of land surfaces, and ade-
quate treatment of the solar radiation driving tropo-
spheric photochemistry.
The three-dimensional distribution of chemical spe-
cies should be represented with spatial and temporal
resolution comparable to that of the meteorological vari-
ables. The distribution of these species is determined by
meteorological transport and source and removal proc-
esses and also by wet and dry chemical transformations.
It is recommended that one or more research groups
be established, building on current modeling strengths,
to develop such models in the time frame of the Global
Tropospheric Chemistry Program. These groups
should be strongly committed to carrying out the sys-
tems modeling studies required by the Global Tropo-
spheric Chemistry Program, as well as efforts in devel-
oping the critical submodels required for successful
application of the TCSM. They should contain exper-
tise in both chemical and meteorological modeling and
maintain close contacts with the observational subpro-
grams of the Global Tropospheric Chemistry Program.
The computer and programming resources necessary
OCR for page 100
100
for successfully carrying out the task should be made
available.
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Representative terms from entire chapter:
tropospheric chemical
