United States
Environmental Protection
Agency
Atmospheric Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-86/037 Sept. 1986
SEPA Project Summary
A Dry Deposition Module for
Regional Acid Deposition
C. M. Sheih, M. L. Wesely, and C. J. Walcek
Methods to compute surface dry dep-
osition velocities for sulfur dioxide, sul-
fate, ozone, NO plus NO2, and nitric acid
vapor over much of the North American
continent have been developed for use
with atmospheric numerical models of
long-range transport and deposition.
The resulting dry deposition module,
actually a FORTRAN subroutine and a
landuse map, has been designed for use
with Eulerian models but can also pro-
duce maps and averages of deposition
velocities for other types of models.
The module provides much of the data
required to compute deposition veloc-
ities: a computerized landuse map, sur-
face roughnesses keyed to landuse
type and season, and similarly keyed
surface resistances of pollutant uptake.
The landuse map has basic grid cells
with dimensions of 1/4 degree longi-
tude by 1/6 degree latitude, over the re-
gion from 52 to 134 degrees west longi-
tude and 24 to 55 degrees north
latitude. External input data must
specify geographical location, season,
and height at which deposition velocity
estimates are to be made, as well as
provide values of atmospheric parame-
ters such as solar irradiation, wind
speed, atmospheric stability, and
boundary-layer mixing height. These
parameters are usually average values
for gridded areas defined by the Eule-
rian model. A fairly general dry deposi-
tion module has been produced as well
as a module adapted specifically for the
Regional Acid Deposition Model being
developed at the National Center for At-
mospheric Research.
This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Dry deposition results in substantial
removal of trace substances from the
atmosphere and accounts for a large
portion of the sulfur and nitrogen com-
pounds delivered to the surface. Atmos-
pheric numerical models of long-range
transport and deposition of such sub-
stances must take into account dry dep-
osition. This is the case, for example, in
Regional Acid Deposition Model
(RADM) being developed by the Na-
tional Center for Atmospheric Research
for the National Acid Precipitation As-
sessment Program. The rates of deposi-
tion are obtained by the expedient of
multiplying pollutant concentrations by
deposition velocities. To improve esti-
mation of the deposition velocities, it is
necessary to incorporate the latest re-
search findings and compile the neces-
sary formulae and data. For RADM, the
results of such work are applied in the
form of a dry deposition module, which
includes a FORTRAN subroutine and a
landuse map for the continental United
States and its surrounding regions. In
addition to sulfur dioxide and sulfate
particles addressed in earlier efforts, 03,
NO plus NO2, and NH03 are included in
the present study.
Many variables control the dry depo-
sition velocities of airborne chemical
substances. Each chemical species has
distinct chemical and physical proper-
ties that strongly affect uptake at the
surface. Correspondingly, each type of
surface has its own influential set of
physical, chemical, and biological char-
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acteristics. Indeed, deposition velocities
vary widely depending on chemical
species, surface type, season, and time
of day. For a region as large as the con-
tiguous United States and surrounding
areas in Canada, Mexico, and bordering
seas, development of an appropriate
dry deposition module requires some
balance between identification of the
details of processes that control dry
deposition and the computational time
available to estimate deposition veloc-
ities over large areas in small time
steps. In the present study, we address
a simplified scheme to compute dry
deposition velocities averaged over sur-
face grids with typical side dimensions
of tens and kilometers. Considerable
generalization of results from research
on dry deposition is made in order to
achieve a working dry deposition
module.
Procedures
The deposition velocity vg for a gas at
height z over land is computed as
vg = ku.[ln(z/z0) + 2(Dh/DgP3
+ ku.rg - i|»cr1 , (1)
where k is the von Karman constant, u,
is the friction velocity, z0 is the surface
roughness scale length, Dh and Dg are
the molecular diffusivities for heat and
the gas of interest, respectively, <|ic is the
stability correction function for the gas,
rg is the surface resistance to uptake of
the gas. The deposition velocities of sul-
fate particles over water and land are
computed from
vp = ku.[ln(z/z0) + ku.rp
(2)
where rp is the surface resistance for
particle uptake.
To determine deposition velocities
with these equations, the parameters
that must be specified are surface
roughess, molecular diffusivity, stability
correction function, friction velocity,
and surface resistance. Molecular diffu-
sivities are provided within the module,
while friction velocity and atmospheric
stability are input variables. The surface
roughnesses and the surface resis-
tances for SO2 and 03 are provided by
lookup tables that specify landuse types
and seasonal categories. The landuse
types are classified as follows:
1. urban land,
2. agriculture land,
3. range land,
4. deciduous forest,
5. coniferous forest,
6. mixed forest including wetland,
7. water,
8. barren land,
9. non-forested wetland,
A. mixed agricultural and range land,
and
B. rocky open areas occupied by low
growing shrubs.
The seasonal categories are:
1. midsummer,
2. autumn,
3. late autumn,
4. winter, and
5. transitional spring.
The landuse data used in the present
study cover an area from 52 to 134 de-
grees west longitude and 24 to 55 de-
grees north latitude. The area is divided
into a matrix of 328 x 186 (longitude by
latitude) grid cells with increments of
1/4 and 1/6 degree longitude and lati-
tude, respectively. Data for each grid
cell contain longitude and latitude coor-
dinates of the cell and the percentages
of the areas used by the 11 landuse
types in the cell. A sample landuse map
of the most prevalent landuse type in
each one-degree square is shown in Fig-
ure 1.
The surface resistance tables for S02
and 03 contain values partitioned ac-
cording to solar irradiation during the
daytime in order to account for the influ-
ence of variations of vegetational stom-
atal openings on surface uptake. A spe-
cial category is added at night for very
light winds when dew is likely to form
on the surface. For NO plus N02, the
surface resistances are computed on
the basis of 03 surface resistance cou-
pled with selected algorithms. The sim-
plest species to deal with is HN03 be-
cause the surface resistance is assumed
to be 0.1 s cm"1 in all cases. For sulfate
particles, the surface resistance tables
are not used. Instead, equations provide
the surface resistance as a function of
friction velocity, atmospheric stability,
and boundary-layer inversion height,
with only a minimal dependance on sur-
face roughness.
Parameters of input to the module
must include chemical species, height
at which deposition velocity is needed,
friction velocity, Monin-Obukhov length
(i.e., atmospheric stability), solar irradi-
ation, atmospheric temperature,
boundary-layer inversion height, and
air temperature and humidity.
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by alphanumeric symbols.
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Results and Discussion
Sample maps of dry deposition veloc-
ities of nitric acid vapor and sulfate par-
ticles are shown in Figures 2 and 3.
These assume summer daytime condi-
tions when solar irradiation levels are
greater than 400 W m~2 and wind
speeds are moderate, corresponding to
approximately 3.8 m s~1 at a height of
10 m above a surface with z0 equal to
3 cm. A sensible heat flux of 150 W m"2
is assumed over all land surfaces and
20 W m"2 is assumed for water sur-
faces. Nitric acid vapor and sulfate parti-
cles are chosen for illustration because
they have extremes in deposition veloc-
ity. At heights of 10 m above the north-
eastern United States during daytime
summer conditions, for example, the
average deposition velocity values are
near 3.5 cm s~1 for HN03 DUt are an
order of magnitude less for paniculate
sulfur. At night, values of deposition ve-
locity for paniculate sulfur, are typically
smaller than 0.05 cm s~1 (not shown).
RADM has successfully utilized the
dry deposition module to compute dep-
osition of S02, S04~, and HNO3 over the
eastern United States for three rainy
days in the springtime. Domain aver-
aged midday deposition velocities were
found to be 0.8 cm s~1 for S02, 0.2 cm
s"1 for sulfate, and 2.5 cm s"1 for HN03.
Most Eulerian models provide sets of
atmospheric parameters that are meant
to be averages over entire grid cells. For
example, the mesoscale meteorology
model of RADM gives one average pro-
file of wind speed, temperature, and hu-
midity per grid square of 80 by 80 km.
From the profiles, the fluxes of momen-
tum, heat, and moisture required for in-
put to the dry deposition module must
be derived in order to compute deposi-
tion velocities via Equations. (1) and (2).
In the current version of the module
used in RADM, grid-averaged fluxes are
derived from the profiles with semiem-
pirical equations designed to avoid
time-consuming iterative calculations
necessary with conventional microme-
teorological formulae.
For each grid cell in RADM, a new av-
erage friction velocity is estimated on
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map represent deposition velocities with intervals of 0.596 cm s"1; e.g., 0 and 1
represent the ranges of deposition velocities 0 to 0.596 and 0.596 to 1.192 cm s~\
respectively.
the basis of a grid-averaged wind speed
and a surface roughness computed as
the logarithmic average of values
weighted by the fraction of area covered
by each landuse type. The products of
wind speed and friction velocity are
then assumed constant over all surfaces
within the grid, at a specified height
near 40 m. This allows computation of
the local friction velocity and wind
speed above each landuse type, via the
equation of the logarithmic wind profile
and the surface roughness for each
landuse type. This provides both realis-
tic variations in local friction velocity
above each grid cell and a distribution
of wind speed that, when averaged, is
consistent with the grid-averaged wind
taken from RADM. The heat and mois-
ture fluxes important in computing the
Monin-Obukhov length are assumed
constant and equal to the grid averages,
which is a deficiency, but a small one
compared to the practice of assuming
constant friction velocity, or alterna-
tively, wind velocity.
Conclusions
The dry deposition module provides a
means of computing surface dry depo-
sition velocities of major chemical com-
pounds for numerical modeling of acid
deposition. The subroutine will produce
deposition velocities for S02, SO^, 03,
NO plus NOX, and HN03 at each grid cell
with dimension of 1/4 degree longitude
by 1/6 degree latitude, for the continen-
tal United States and surrounding
regions. It should be noted that insuffi-
cient information on surface resistances
necessarily limits the accuracy of the
computed deposition velocities. Current
knowledge of the resistances is good for
SO2, HN03 and 03, fair for SO4=, and
poor for NO and NO2. However, the sub-
routine can be easily updated to include
new results of research or modified to
include additional chemical species.
The dry deposition module is quite
easy to apply as provided. The multiple
factors that control surface resistances
for a given chemical substance over a
specified type of surface are taken into
account, but not explicitly, so that calcu-
lations of deposition velocity can be
made very efficiently. A drawback of
this approach is that considerable work,
often of a research nature, is necessary
to modify the module to include addi-
tional chemical species. Other deficien-
cies of the module include ignoring
such factors as rapid in-air chemical re-
actions, which can change the deposi-
tion velocity with height, and the effects
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Figure 3. A dry deposition velocity map of paniculate sulfur with deposition velocity intervals
of 0.0412 cm s"1
of nonuniform and hilly terrain. Also,
the temporal variations of surface resis-
tance caused by the surface being
wetted by dew or rain are not addressed
explicitly in the module, and thus must
be taken into account by other compo-
nents of the controlling numerical
model.
C. M. SheihandM. L Weselyare with Argonne NationalLaboratory, Argonne, IL
60439; C. J. Walcek is with the National Center for Atmospheric Research,
Boulder, CO 80307.
Jason K. S. Ching is the EPA Project Officer (see below).
The complete report, entitled "A Dry Deposition Module for Regional Acid
Deposition," {Order No. PB86-218 104/A S; Cost: $11.95, 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:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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