United States
Environmental Protection
Agency
Environmental Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-84-041 Apr. 1984
Project Summary
Regional Acid Deposition: Models
and Physical Processes
This report presents the results of a
10-month study on the current status of
research on fundamental concepts and
physical processes relevant to regional
acid deposition modeling. The role of
models in environmental assessment is
described first. This is followed by a
review of existing models in a chapter
designed to establish a reference
framework for the bulk of the report.
Most, if not all, of the principal concepts
in model construction and evaluation
are discussed. After extensive discus-
sions of state-of-the-art regional me-
teorological modeling and the chemistry
of acid generation in the troposphere are
presented, the discussion focuses on
the development of a new generation of
acid deposition models. Based on the
topics reviewed, the desirable features
of a comprehensive model are described
with emphasis on components needing
great improvement or omitted in pre-
sent models. These features include
emissions data, detailed acid rain
chemistry, cloud processes, dry deposi-
tion, model validation, and sensitivity
analysis.
This Project Summary was developed
by EPA's Environmental Sciences Re-
search Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Although the acid rain phenomenon has
been recognized for the past 100 years, im-
portant features of the acid rain problem are
new: (a) quantitative questions are perceived
that must be answered to fully understand
the essential chemical and meteorological
processes, and (b) mathematical models and
field- and laboratory-measured programs are
available to investigate these questions.
Similarly, now available are reasonably
logical and mature formulations of relation-
ships between ecological systems (and
physical structures) and acid deposition that
can be investigated quantitatively. Also, as
noted above, public awareness of the poten-
tial effects and probable causes of acid rain
is new, as is the understanding that some
kinds of pollution traverse political
boundaries.
The bulk of the report examines the full
range of meteorological and chemical pro-
cesses that are involved in the overall
phenomenon that is, the production and
deposition of acidified rain, snow, fog, and
mist, and the dry deposition of acid anhy-
drides over important inhabited regions, such
as the east central United States and
Canada. Particular attention is given to
issues in the study of acid rain through
mathematical models. Although the scien-
tific questions dictate the kinds of field
measurements, laboratory experiments, and
model development to be undertaken (all of
which are necessary), particular interest is
in how to develop and employ credible
models. A credible model is defined as one
built on basic physical and chemical pro-
cesses that can test hypotheses and guide
the design and assessment of field measure-
ment programs; its ultimate use is for
predicting acid deposition rates and source-
receptor relationships and for reliably
estimating the effects of emission control
strategies.
The Physical Picture
To understand and model the acid rain
phenomenon one must recognize a wide
range of physical and chemical processes
and their interactions. Briefly, these are (a)
emissions of materials that cause and regu-
late acidity in precipitation and deposition.
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(b) meteorological motions that transport
and dilute the emitted substances laterally
and vertically, (c) the various physical and
chemical transformations that alter the
physical phase and chemical properties (e.g.,
valence or oxidation state) of the emitted
substances, and (d) the meteorological fac-
tors and surface adhesiveness that lead to
deposition of the transformed substances.
Less recognized than the above processes
are those properties of the Earth's surface
that control the rate of uptake of dry
materials (e.g., gaseous S02 and/or airborne
particles).
Because the principal acids in precipitation
are sulfuric (H2SO4) and nitric (HN03), most
concern is with sulfur and nitrogen emis-
sions. However, the hydrocarbons and their
oxidation products are important reactants
in the chemistry which ultimately leads to
HN03 and H2S04. Estimates of anthropo-
genic emissions of SO2 (mostly from coal-
and oil-burning electrical power plants and
metal-smelting plants) and of NOX (mostly
NO and N02 from high temperature combus-
tion processes, including those in auto and
truck engines and power plants) are rea-
sonably reliable for the world's industrialized
countries. Much less credible, but probably
less important, are estimates of natural emis-
sions of organic sulfur gases and of natural
NOX compounds. Natural sources of
gaseous NH3 and particulate NH4+, gaseous
hydrocarbons, airborne mineral dusts, and
lightning-produced NOX must also be
estimated reliably. Minor contributions to
precipitation acidity from HCI and organic
acids are often negligible.
Whether the key emissions are an-
thropogenic or natural, they are injected in-
to the atmosphere at or near the Earth's
surface, usually within the planetary bound-
ary layer. Accordingly, boundary layer
meteorology is at the core of the acid rain
problem. The physics of turbulence and con-
vection, diurnal variation in surface heating,
terrain geometry, and surface and boundary
layer hydrology exert strong control over the
initial dispersion of the emitted substances.
Further, during the time these substances
spend in the boundary layer, their physical
environment (e.g., temperature, pressure,
humidity, available sunlight) and proximity
to surfaces and to other pollutants such as
aerosol particles control the rate and type of
chemical transformations that occur. These
chemical transformations differ markedly
from those that normally occur above the
boundary layer in the free troposphere.
Perhaps only one important acid precursor
or regulator, NOX from lightning, does not
begin its atmospheric life in the boundary
layer, although background tropospheric
ozone is central to all tropospheric chemistry.
In dirty or clean air, in the boundary layer
and above, chemicals react with each other.
The precise rates and types of reactions de-
pend strongly on the local pressure,
temperature, available sunlight (both direct
and scattered), the presence of liquid and
vapor H20, and on the production of
photochemical oxidants (like ozone and
peroxyacetylnitrate co-reactants). The
discussion is organized into categories of
homogeneous reactions (gaseous and liquid)
and heterogeneous processes and by prin-
cipal categories of chemical species. Key
considerations include the exact rates of
transformation (oxidation) of S02 and NOX
into H2SO4 and HNO3, respectively, the
major pathways of transformation, and the
essential controlling agents.
In an oxidizing atmosphere such as that
of the Earth, the oxidation of S02 and IMOX
to H2S04 and HN03 is inescapable, given
enough time in the atmosphere. In
characterizing regional acid deposition,
however, one must determine what fraction
of a region's total emissions is oxidized and
deposited within the region and what frac-
tion of the total is transported long distance
(at high altitudes, for example) for eventual
deposition onto territories hundreds or
thousands of kilometers from the sources.
That is, a credible description and model of
this physical system must include quan-
titative treatment of material transport and
transformation above the boundary layer.
Similarly, the factors that limit the rate of sur-
face deposition and uptake of gases (dry
deposition) must be treated quantitatively.
These factors include nearground tur-
bulence, the condition and type of the sur-
face (e.g., vegetation, soils), and the
chemical stickiness and reactivity of the rele-
vant substances on the surfaces.
Existing Models and
Components of Models
Two distinct types of models used are
defined, described, and compared for stu-
dying long-range transport of air pollutants:
Eulerian grid models and Lagrangian trajec-
tory models. Also, because of different goals
and problems facing air pollution
meteorologists and chemists, it has been
useful to develop and employ distinctly dif-
ferent models for air quality modeling (AOM)
and acid deposition modeling (ADM). For
ADM, it is concluded that the three-
dimensional nature of the problem and the
importance of simulating with adequate
generality specific source distributions and
eventual control strategies require an
Eulerian framework.
Although ADM has improved the
understanding of the acid rain problem, a
number of phenomena have not yet been
fully treated largely because the ADM field
is relatively new. Reasonably well-based
treatments of each phenomenon have been
attempted, but the best available
mathematical parameterizations have not
been coupled within one model. Individual
models tend to be strong in one respect, but
very weak in others. A number of funda-
mental weaknesses that are widespread, or
even ubiquitous, can be mentioned. For ex-
ample, existing acid deposition models do
not allow for mixing of pollutants above the
boundary layer. Similarly, no recognition is
given to different types of precipitation (rain,
snow, dew, etc.) or to the temperature and
pH that characterize precipitation scaveng-
ing and acid formation. No cloud-chemical
processes have been considered, nor have
fundamental (or elementary) chemical reac-
tions been treated with sufficient detail. In-
stead, linear overall transformation rates
have been employed. (For example, the con-
version rate of S02 to S04= has been set
equal to x% per hour without regard to
mechanisms or controlling factors, although
seasonal dependence of x is sometimes per-
mitted.) No published model has included
reasonably complete chemical reaction
schemes, and nitrogen oxides are usually
omitted entirely. Similarly, dry deposition of
pollutants has been simulated with fixed
deposition velocities; and dependences on
winds, surface topography, moisture, and
vegetation types have been ignored. Sub-
grid-scale inhomogeneities in emissions,
transport, chemical reaction types and rates,
and deposition have not been included. Few
data and the use of nonmechanistic model
parameterizations have led to more model
"tuning" than is desirable. Consequently,
much verification of ADM work is required.
The ADM and regional meteorological
modeling (RMM) fields have a longer history
and a greater data base than acid deposition
modeling. Fortunately, AQM and RMM
techniques and results are valuable for ADM
development. For example, the experience
and results of AQM researchers in dealing
with large numbers of chemical reactions can
be tapped. Schemes to classify and to
reduce systematically the numbers of in-
dependent chemical reaction equations of-
fer help to ADM. Also, methods of
incorporating emissions into air quality
models and the AQM emissions data base
itself are largely applicable in ADM.
The relative maturity and quantitative
nature of the RMM field can be of enormous
benefit to those developing more general and
realistic acid deposition models. Thus, over-
view of regional meteorological models is
presented, history of their goals, methods,
and capabilities is outlined; and the principal
components of these models are identified.
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Briefly, these are the mathematical or
numerical aspects and the more physical
features. In the former category, we review
the essential features of the spatial grids in
these models, the various numerical
methods employed to solve the governing
partial differential equations, the lateral
boundary conditions, and the overriding
need for adequate data analysis and data in-
itialization. Each of these RMM components,
as well as the main purposes of RMM, is
closely related and applicable to the task at
hand in ADM, namely to model accurately
the' dispersion and transport of pollutants.
Similarly, physical aspects of regional
meteorological models that have been im-
proved and tested will benefit ADM develop-
ment. These physical aspects include the
transport of heat, moisture and momentum
at the Earth's surface, in the planetary
boundary layer, and free troposphere, and
the energy sources and sinks that govern the
transports. Also included are phase changes
of water and the interaction of radiation with
clouds and the surface. Because these
physical phenomena occur on many
disparate spatial scales, including scales
shorter than a model's grid spacing, param-
eterizations are necessary — for example, to
relate the cumulative effect of subgrid-scale
phenomena on the fluid flow to the model-
resolvable scales of motion. We review
parameterizations of surface processes, of
planetary boundary layer processes, of con-
densation and evaporation processes, and of
radiative effects of layered clouds in current
models; and we identify strong indicators of
profitable research.
Another important consideration in the
field of RMM that will be directly useful in
ADM is that of objective measures of model
skill, i.e., the accuracy of model predictions.
Several standard quantitative measures of
forecast skill are reviewed and also sum-
marize the state of the art of RMM's to
forecast (precipitation, for example). RMM
methods will serve as good preliminary
guides for measuring objectively the ability
of acid deposition models to forecast deposi-
tion patterns (say, annual totals) or deposi-
tion amounts in distinct events.
Because of the great potential for transfer-
ring methods and parameterizations from
RMM to ADM, components of the former
models have been reviewed in some detail.
First, the need for objective analysis is
recognized — irregularly spaced initial
meteorological data must be transformed to
provide initial conditions on a model grid.
The techniques, quality, computational
costs, and history of objective analysis
methods are summarized, and several case
studies are discussed. The related need for
data initialization is discussed in similar detail.
General physical considerations, mathe-
matical analysis, and experience with
meteorological models can indicate gen-
eral spectral and transient characteristics
of data-caused noise. In specific applica-
tions (e.g., for a specific regional topog-
raphy and synoptic situation), RMM pro-
vides sound theory and practical experi-
ence to guide the choice of initialization
procedure. Accordingly, unneeded compu-
tational costs can be avoided. Because
boundary conditions must be specified to
solve differential equations, principal
techniques used in RMM (spatial damp-
ing (or sponge) conditions, wave-radiation
conditions) and bounded derivative schemes
are reviewed with respect to various ADM
applications. Numerical methods and
mathematical principles for objective
analysis, data initialization, and boundary
conditions are also reviewed. Once again,
the available general theory plus the ex-
perience of RMM researchers constitute a
well-based foundation for ADM
development.
On a more physical side, the essential
RMM components mentioned above, sur-
face physics and effects, planetary boundary
layer physics and effects, and the thermo-
dynamic and radiative physics and effects of
clouds and precipitation are also reviewed.
The methods and problems extent in the field
of RMM are very close to those that will
prevail in ADM.
The Chemistry of Acid
Generation in the Troposphere
The chemical phenomena and reaction
sets in existing acid deposition models are
far from complete for many reasons. An in-
adequate understanding of long-range
transport of pollutants, whose importance
has only recently been perceived by the
public and its agencies, has prompted much
ADM work to focus on the meteorological
aspects of transboundary transport. Also in-
complete and confusing until recently has
been mechanistic information on the actual
chemical processes that transform S02 into
sulfuric acid and N0x into nitric acid. Fur-
ther, the chemistry of acid generation is more
complicated than that of regional chemical
oxidants; the former involves gas-phase and
aqueous reactions, while the latter is due to
gas-phase reactions alone.
Accordingly, the discussions and review
of the chemistry of acid generation are
focused at first on the essential chemistry
itself rather than the chemistry in the existing
ADM's. The main categories of the review
are gas-phase reactions, aqueous-phase and
heterogeneous processes, and photodis-
sociative processes. Any credible ADM must
be based upon reaction mechanisms as op-
posed to depending completely on^param-
eterizations of overall reaction or
transformation rates.
For example, this simple parameterization
in widespread use is inherently linear: The
rate of production of SO4= is proportional
to the gaseous S02 concentration. In reality,
the supply of chemicals that oxidize S02 to
S04= might be limited in certain locations,
and little or no SO«= production could take
place even when large amounts of S02 are
available. Similarly, the S02-to-S04= conver-
sion rate probably depends on the exact
species that is accompanying the oxidation
so that the rate x is not constant but varies
with time. Analogous fundamental con-
siderations apply to the conversion of NOx
to nitric acid, to the production of
photochemical oxidants like ozone and per-
oxyacetylnitrate, and to the production of
S02 from biogenic organic sulfides, for
example.
The main goal of the very detailed presen-
tations is to identify from available research
results the principal elementary reaction
mechanisms and key species in the gas-
phase, aqueous-phase, and heterogeneous
reactions that cause and control acid genera-
tion. From a complex and encyclopedic list
of chemicals and reactions, a smaller, more
concise list of chemical variables and pro-
cesses must be distilled to develop a trac-
table and useful ADM. From fundamental
principles, laboratory data or photochemistry
and kinetics, laboratory simulations of com-
plex systems and field data, we can explain
the essence of acid generation. These
shortened lists of species and processes
(elementary reactions when possible) will re-
quire further testing, such as zero-
dimensional sensitivity calculations. In some
cases, such as gas-phase species (i.e.,
hydrocarbons), the representative categories
have been ground in AQM research
previously, so only refinements will be
needed for ADM development. In cases such
as for solution-phase chemistry, achieving
conciseness in the reaction list while still
simulating the essential features and rateg of
reactions has not been accomplished, partly
because the role of in-cloud chemistry in
generating acids has been appreciated only
recently.
Certain clear indications of how to proceed
in ADM development do appear in the
course of our review. For example, because
all gas-phase processes that lead to S02 oxi-
dation are initiated by the gas-phase OH
radical (in daylight, of course), the minimal
reaction set for ADM most embody the ma-
jor processes that control OH concentra-
tions. Similarly, because of its role in NOX
chemistry and because it is a major source
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of OH, tropospheric O3 must be calculated
accurately. In the liquid phase, it will be
necessary to simulate behavior of 03, H202,
OH, H02, N03, and probably 02- and N205.
Fortunately, the fundamental data necessary
in ADM development are forthcoming or are
largely available already.
Acid Deposition Model
Development and Testing
Many of the issues that can arise in the
design of a comprehensive model, i.e., one
which includes coupled meteorology and
chemistry, are discussed. The key
meteorological and chemical processes that
are identified earlier are stated more con-
cisely, and certain other phenomena and
practical considerations are introduced into
the discussion. For example, the apparent
importance of dry deposition of acidic gases
and particles, the available methods for its
measurement, the controlling physics and
chemistry, and how ADM might treat dry
deposition are discussed. Questions and
facts are introduced concerning surface
emissions of pollutants and natural sources
of acid precursors and of those species that
regulate acid generation. Other general
features, components, and questions in
ADM development are also reviewed and
summarized, including model resolution,
subgrid-scale processes and how to begin to
treat them, mathematical and numerical
techniques for large comprehensive models,
special considerations coupling the laws of
chemistry and physics, and issues in model
validation and sensitivity analysis.
This Project Summary was authored by staff of The NCAR Acid Deposition
Modeling Project, National Center for Atmospheric Research, Boulder, CO
80307
K. L. Demerjian is the EPA Project Officer (see below).
The complete report, entitled "Regional Acid Deposition: Models and Physical
Processes, "(Order No. PB84-115997; Cost: $29.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
U S GOVERNMENT PRINTING OFFICE, 1984—759-015/7665
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
£30
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