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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