United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S4-88/027 Sept. 1988
Project Summary
Comparison of the RADM Dry
Deposition Module with Site -
Specific Routines for Inferring
Dry Deposition
M. L. Wesely and B. M. Lesht
The computer module for calculating
dry deposition velocities in the Regional
Acid Deposition Model (RADM) has been
modified to operate with site-specific
data provided by measurement stations
such as CORE (COre Research Establish-
ment, for dry deposition) satellite sites.
Data collected during 1986 at seven
widely separated sites in the eastern
United States were used to estimate
weekly averages of deposition velocities
for SO2, 03, HN03, and S042'. Similar
calculations were made with the inferen-
tial technique that was developed at At-
mospheric Turbulence and Diffusion
Division of the National Oceanic and At-
mospheric Administration's Air Re-
sources Laboratory. Comparison of
results obtained with the two techniques
indicate that some systematic dif-
ferences exist, even when the module
uses distributions of landuse types that
match as closely as possible the observ-
ed vegitation coverages used in the in-
ferential technique. When one ignores
the systematic differences that could be
removed by minor changes in the
algorithms for computing resistances to
deposition, the relative uncertainties for
S02and O3 deposition velocities are ap-
proximately ±30%. Likewise, the relative
uncertainties corresponding to non-
systematic differences in the deposition
velocities for HN03 and SO42' are about
±30% and ±50%, respectively. Use of
the landuse map to extrapolate to areas
as large as RADM qrid cells (approx-
imately 8O km square) around the
measurement sites produces weekly
averages of deposition velocities for
SO2, O3, and SO42' that are within
±20% of those computed for the local
site and approximately ±30% for HNO3,
if one avoids landuse types such as ur-
ban and water areas that are
nonrepresentatlve and have very dif-
ferent characteristics from the measure-
ment sites. These estimates do not
represent true uncertainty or accuracy
because of unknown sources of error
possible in the input data and deposition
velocity algorithms for nonuniform
surfaces.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems 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
The dry deposition module of the
Regional Acid Deposition Model (RADM) is
used to compute the dry deposition
velocities (downward flux divided by con-
centration at a specified height) for SO2,
SO42', O3, HNO3, and other compounds.
With its computerized landuse map, the
module can provide estimates of the
deposition velocities for a given set of
meteorological and surface conditions that
are representative of an area located
anywhere in the contiguous United States
and nearby locations. As part of this proj-
ect, the module uas been modified to ac-
cept data on meteorological and surface
conditions observed at measurement sta-
tions rather than inputs computed with
-------
RADM. The purpose of these modifications
was to make site-specific estimates of dry
deposition velocities, and compare them
with results from the site-specific inferen-
tial technique that has been developed at
the Atmospheric Turbulence and Diffusion
Division (ATDD) of NOAA's Air Resources
Laboratory. Goals of the comparisons in-
clude estimating the relative uncertainties
of the two techniques, suggesting im-
provements, and examining the ability of
the modified module to provide estimates
of dry deposition for expanded areas
around the measurement sites.
The observational data used in these
analyses were collected with ATDD in-
struments during 1986 at the seven sites
listed in Table 1. The sites identified are
either CORE (COre Research Establish-
ment, for dry deposition) stations or CORE
satellite stations. The data set itself consists
of hourly averages of wind speed, the
standard deviation of the horizontal wind
direction, solar irradiation, air temperature,
relative humidity, and surface wetness from
a sensor that indicates the presence of
moisture from dewfall and rainfall. Other
data include one observation per week of
the fraction of full leaf cover, from which the
seasonal categories used in the module are
derived, and the information given in Table
1 describing the types and amounts of
vegetative species present at the local site.
The landuse distributions for the local site
are determined from the relative amounts
of these species, while the distributions for
the RADM grid squares (approximately 80
km square) encompassing each site are
derived entirely from the computerized
RADM landuse map. Although the deposi-
tion velocities (vd) are computed for every
hour, the results considered here are
averages over periods of at least one week.
Results and Conclustions
The dry deposition module of the RDAM
has been successfully adapted to use site-
specific data obtained at dry deposition sta-
tions such as the CORE satellite sites. The
module was originally written to use more
complete local micrometeorological infor-
mation than is included in the parameters
listed above, so considerable manipulation
of the coding was needed in the RADM
module in order to accept these inputs.
Perhaps the least exact formaulations in the
modified module are those that deal with
estimating atmospheric stability, but uncer-
tainties introduced by the compromised
equations are probably overshadowed by
the fact that most of the sites are located
in areas with surface properties insufficient-
ly uniform, or with terrain insufficiently flat,
to allow very precise micrometeorological
calculations. The algorithms in the modified
module that are used to compute crucial
parameters such as aerodynamic resist-
ance above the surface and friction veloci-
ty are quite similar to those used in the in-
ferential technique. Some notable differ-
ences exist, however. One is that the modi-
fied module attempts to mimic the RADM
module by computing changes in the gas-
phase resistances for surfaces identified by
landuse types different from the dominant
landuse type for the local site. At most of
the sites, this alteration produces smaller
aerodynamic resistances and larger friction
velocities averaged over the areas sur-
rounding the site, and thus increases the
deposition velocities for HNO3. The deposi-
tion velocities for other substances do not
seem to be significantly affected, because
their surface resistances are usually much
larger than the aerodynamic and gas-
phase sublayer resistances (which depend
on friction velocity).
The alogrithms used in the modified
module to compute the bulk surface resist-
ance, usually the most important term for
determining deposition velocities, are the
same as those used in the RADM module.
Some changes had to be made in the
Table 1. Site names, locations, predominant surface vegetation (within one kilometer), and landuse types assumed with the modified module
General Location
(deg lat., long.)
Site
ID
Local Surface Vegetation and Landuse Types*
(percent coverage)
Argonne National
Laboratory, IL
41.TON, 87.98 W)
Bondville
(Champaign), IL
(40.05 N, 88.37 W)
Oak Ridge, TN
(35.96 N, 84.28 W)
Panola State
Park, GA
(33.63 N, 84.18 W)
Pennsylvania
State Univ., PA
(40.72 N, 77.92 W)
West Point, NY
(41.35 N, 74.05 W)
Whiteface
Mountain, NY
(44.39 N, 73.86 W)
ARG
BON
OAK
PAN
PSU
WST
WHT
grass (SO), white oak (SO)
local site: 2(50), 4(50)
RADM square: 1(23), 2(62), 4(2), 7(13)
maize (50), soybeans (50)
local site: 2(100)
RADM square: 1(2), 2(98)
white oak (70), loblolly pine (30)
local site: 4(70), 5(30)
RADM square: 1(3), 2(30), 4(64), 7(3)
Loblolly pine (50), chestnut and red oak (50)
local site: 4(50), 5(50)
RADM square: 1(13), 2(22), 4(28), 5(6), 6(29), 7(2)
maize (30), white oak (30)
local site: 2(50), 4(50)
RADM square: 1(4), 2(20), 4(73), 5(3)
maple (60), white oak (30)
local site: 4(100)
RADM square: 1(41), 2(3), 4(28), 6(2), 7(26)
white birch (70), maple (10)
local site: 4(100)
RADM square: 1(1), 2(6), 4(28), 5(10), 6(43), 7(12)
'Landuse types specified are [1] urban land
fresh water.
[2] agricultural land, [4] deciduous forest, [5] coniferous forest, [6] mixed forest including wetland, and [7] water, both salt and
-------
parameterizations of the SO<2 sublayer
resistance in air, which strongly controls
SO42" deposition velocity, but the changes
conform with the interpretation of the field
experiments on which the parameteriza-
tions are based. The inferential technique
is configured somewhat differently and ap-
pears to overestimate the deposition veloci-
ty for SO42- in some situations. The mod-
ule, however, tends to underestimate when
the standard deviation of the horizontal
wind direction is not measured very well.
The most striking difference between the
results of the module and the inferential
technique is the much smaller estimates
made with inferential technique for SO2 and
03 deposition velocities during nonsummer
conditions. These are probably underesti-
mates caused by overly restrictive assump-
tions on the surface resistances of bare soil
and sparse vegetation during nonsummer
seasons, which could easily be adjusted by
changing the values of a few numerical
coefficients used in the inferential
technique.
While the algorithms for surface
resistance in the modified module are bas-
ed on a more complex and versatile set of
multiple surface resistances to various por-
tions of the surface than the algorithms us-
ed in the current version of the inferential
technique, the module is strongly limited by
the fact that the surface can be described
by at most 11 landuse types. The inferen-
tial technique employs description of
vegetation by species and tailors the sur-
face resistances more precisely than the
module to describe the physiological re-
sponses to environmental parameters such
as solar irradiation and temperature. An ad-
ditional feature that may be desirable is a
factor to take into account soil moisture
stress on the stomatal resistances of
healthy vegetation during the daytime,
because the current version of the inferen-
tial technique assumes no soil moisture
stress and the module is effectively tuned
to a small amount of moisture stress. As a
result, the deposition velocities of
substances such as SO2 and O3 that are
strongly influenced by stomatal resistances
are slightly larger when computed by the
inferential technique than with the modified
module.
For the weekly averages of deposition
velocities computed for 1986 at the seven
sites, the inferential technique and the
modified module produce estimates within
about ±3O% of each other for SO2 and O3,
if we neglect strong systematic differences
such as those that occur during nonsum-
mer conditions at surfaces with bare soil
or sparse vegetative canopies. Likewise,
the differences for HNO3 and SO42'
deposition velocities are usually about
±30% and ±50%, respectively. These
relative uncertainties could probably be
reduced somewhat if the suggestions made
above were adopted.
Estimates of deposition velocities over
large areas are easily computed with the
modified module by use of the appropriate
landuse distributions. Of course, if the
areas are very large, the observations of
meteorological conditions at the measure-
ment stations might not be sufficiently
representative. Some trial runs involving the
20-km squares of the landuse map cell and
the 80-km RADM cells at the seven
measurement sites show that the estimates
of weekly averaged deposition velocities for
SO2 ,O3 and SO42' are fairly sensitive to
the amounts of urban and water areas in-
cluded in landuse distributions. Aside from
this effect, the deposition velocities
caiculated for these three substances over
the RADM grid cells are usually within
±20% of those for the local site descrip-
tions. These limits should not be taken as
generally valid at sites other than the seven
examined in this study, because different
meteorological conditions and altered com-
binations of landuse types could cause
larger changes in deposition velocities. For
HNO3, minor changes in the assumed lan-
duse distributions often cause rather large
changes, easily ±30%, in deposition
velocities.
We conclude that the differences in
deposition velocity estimates caused by
scaling from local site conditions to RADM
grid cells are often slightly smaller than the
relative uncertainties of the deposition
velocities estimated with the modified
module and the inferential technique.
Hence, inferences of deposition velocities
for the local conditions at carefully chosen
sites appear to be reasonably represen-
tative of larger areas extending at least to
sizes of RADM grid squares. Nevertheless,
adjustments for changes in surface condi-
tions are clearly desirable, and extrapola-
tion to types of surfaces that have notably
different properties can be made only at the
cost of considerably greater uncertainty. By
use of a computerized landuse map or,
preferably, a more detailed description of
surface conditions, one should be able to
estimate weekly averaged dry deposition
amounts for sulfur, nitric acid, and ozone
to within about ±30% over some fairly
large areas.
The above number of 30% is not the true
uncertainty or accuracy. An assessment of
accuracy would require experiments
designed specifically for dry deposition,
because a number of factors can cause ad-
ditional uncertainty. For example, one must
determine how representative the local
meteorological measurements actually are.
Relatively low wind speeds measured at
OAK and PAN suggest that proper ex-
posure of instruments in a forested, hilly
area might be difficult. In addition,
measurements taken at sites not represen-
tative of any one surface or influenced by
local aerodynamic obstacles might not be
suitable for application of the algorithms
used in the inferential and the module ap-
proaches. Finally, these algorithms do not
directly consider the effects of surface
nonuniformities in the area considered, in-
cluding hills, isolated surface irregularities,
and edges associated with changes in
height of surface cover. The aerodynamic
resistances could be strongly altered by
surface nonuniformities, and stomatal re-
sistances could be altered by varying
degrees of shading of sunlight by hills and
other vegetation.
-------
M. L Wesely and B. M. Lesht are with Argonne National Laboratory, Argonne,
IL 60439.
Steven M. Bromberg is the EPA Project Officer (see below).
The complete report, entitled "Comparison of the RADM Dry Deposition Module
with Site-Specific Routines for Inferring Dry Deposition," (Order No. PB 88-
238 191/AS; Cost: $19.95, 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:
Environmental Monitoring Systems Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S4-88/027
0000329
AGENCY
------- |