United States
 Environmental Protection
 Agency
 Environmental Sciences Research
 Laboratory                  ~  "
 Research Triangle Park NC 27711
 Research and Development
EPA-600/S3-82-042  Oct. 1982
 Project  Summary
Determination of Dry
Deposition  Rates for Ozone

Elmer Robinson, Brian Lamb, and M. P. Chockalingam
  Measurements were taken of the
rates of loss of Os from the atmosphere
onto the earth's surface under non-
precipitation conditions. This is com-
monly called "dry deposition" and the
critical parameter  is  usually the
"deposition velocity"  Vd. Other in-
vestigators have determined that the
general magnitude of Va for O3  is
about 0.6 cm/s. The major objective
of the present research was to measure
ozone Vd and  other meteorological
parameters  over  a  wide  range of
ambient conditions and to relate the
values of Vd to commonly measured or
estimated meteorological parameters.
In this way a calculation procedure for
ozone Vd cou Id be developed for use i n
meteorological pollutant transport
models.
  The micrometeorological profile
method for estimating Vd was used in
this research program. Measurements
were made over a soybean field in
Illinois, a  grain and grass field in
Pullman, WA, and a hardwood forest
area in south central  Pennsylvania.
The measurements over the soybean
and grain fields show  that Vd has a
strong diurnal cycle and is correlated
with a lateral gustiness parameter, a
u, where og is the standard deviation of
wind direction and u is the average
wind speed. The model is of the form
Vd = A(OQ u)b  where the value of A is
between 0.2 and 0.4 and b is between
0.7 and 1.3. The measurements of Vd
over the Pennsylvania  forest canopy
did  not follow this  model,  but the
reasons are not clearly understood.
However,  there were  some notable
meteorological features of the experi-
mental period. In particular, there was
little diurnal variation in turbulence
over  the  forest;  conditions  were
generally turbulent  over the forest
canopy during the day, as might be
expected, but also, unexpectedly, at
night. Values of Vd over the forest area
were  generally larger by a factor of
two or  more compared to the Vd
measurements over the crop surfaces.
  This Project Summary was developed
by  EPA's  Environmental  Sciences
Research 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

  Air pollutants are scavenged from the
atmosphere  largely by adsorption or
reaction  at  the earth's  surface.  The
effects of scavenging mechanisms
usually  have  not been considered
important factors  in  plume diffusion
modeling because the models have
been applied to relatively short distances;
thus, the time periods being considered
would have been too short for deposition
to have had a significant impact on the
system. However, diffusion and trans-
port are now being modeled over much
greater distances and covering longer
time periods. Under such conditions it is
no longer prudent to  ignore the  sca-
venging phase of the transport model.
  The concept of deposition velocity (Vd)
was first described in a 1953 paper by A.
C. Chamberlain and  R.  C. Chadwick
from the Atomic Energy Research

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Establishment,  Harwell, Great Britain,
as a  means of explaining the loss of
material  from  the atmosphere to the
underlying  surface when the material
was too small to be covered adequately
by Stokes' law. By definition, Vd is the
coefficient  that relates air concentra-
tion,  C, to the deposition rate,  D:
              D = VdC
where the deposition rate, D, is essen-
tially the downward flux of the pollutant
(fjg cm "V1); and the concentration of
pollutant,  C,  can  be expressed  in
//g/cm3. Hence, the  dimensions of Vd
must  have the units cm/s in order for
the expression to balance. Since these
are the dimensions of velocity, it was
natural to label the coefficient deposition
velocity.
  Further  study has shown that the
surface loss is equal to the downward
flux to the surface,  f, which  can  be
expressed  in  terms of  well  known
micrometeorological variables and the
vertical concentration profile.
  The primary objective of the experi-
mental program was to measure the dry
deposition  velocity of Oa.  A second
objective was to relate the deposition
velocity for Oa to a wind or a gustiness
parameter,  such as ad, for inclusion in
Gaussian-type plume  models. The
profile method was selected  as the Vd
estimation technique.
  The field measurements were con-
ducted in several major phases at three
locations. In July and September, 1977
measurements were made at Robinson,
IL. This was followed by a program in
Pullman, WA from April to July, 1978
and near Lancaster, PA in July-August,
1979. The underlying surfaces at these
three  locations  were different.  In
Illinois,  the measurements involved a
soybean field at two stages in its growth
cycle; in Washington, the surface was
grain  and grass. The measurements in
Pennsylvania were made over the top of
a deciduous forest with  a 20-m high
canopy. The measurements  produced
an extensive set of dry deposition and
meteorological data that can be applied
to modeling programs.

Procedure
  The field system consisted of four
major components: (1) an instrumenta-
tion tower of either a scaffold type or a
taller  antenna type; (2) the meteorolog-
ical sensor system; (3) the air sampling
system; and (4) a field facility to house
the instrumentation.
  The Robinson IL field study began
July 15, 1977 with the installation of
meteorological and air sampling instru-
ments adjacent to the Robinson airport,
and  continued to  July 31, 1977. A
similar experiment was conducted at
the same site from  September  5 to
September  15,  1977.  The scaffold
instrumentation tower was erected at
the eastern  edge of  a soybean field.
Because flat agricultural lands extend-
ed several km upwind to the west, winds
blowing from the directions of south
through west to  north were generally
applicable to profile experiments; this
was  especially true for winds from the
sector  enclosed between  225° and
360°. The most  frequent winds were
from westerly directions satisfying the
experimental requirements that  the
wind and pollutant flow be stabilized
over an  "infinite  uniform"  surface
before reaching the tower location and
profile sensors.
  Commercial steel  scaffold compo-
nents were used to construct  the
scaffold instrumentation tower. Mete-
orological sensors weref ixed at levels of
approximately 25, 75, 200, and 500 cm
above the crop surface.  The meteoro-
logical sensors and air sampling intakes
were located about 2 m from  the
structure in the direction of the prevail-
ing wind. The observation levels and the
types of observations on the tower are
presented in Table  1.
  The second phase  of the study was
conducted between April 1 and August
7, 1978 on a site approximately 4.8 km
northwest of Pullman, Wa. At this site it
was  necessary to  erect the scaffold
instrumentation  tower  on the gentle
slope of a hill which forms a shoulder of
a shallow valley  in the midst of rolling
hills. The prevailing wind was from the
southwest along  the axis of the valley;
                   this satisfied in a reasonable manner
                   the requirements that the windflow be
                   stabilized  over an  "infinite uniform"
                   surface before  reaching  the  instru-
                   mentation.  Although low hills with
                   gentle slopes rising about  30 m above
                   the valley axis were located about 0.8
                   km southwest of the tower, the general
                   terrain conformed   more or less to a
                   uniform formation  which  ensured a
                   stable fetch of wind for a distance of
                   approximately 8 km  to 16 km. The crop
                   surface was a mixed growth of grain,
                   grass, and weeds with an average crop
                   height of approximately 30 cm  at the
                   beginning of the April  1978  experi-
                   ments. The same scaffold tower instru-
                   mentation profile given in Table 1 was
                   used for these experiments.
                     The Pennsylvania forest site was
                   located on Tower  Mountain between
                   Lancaster and York. The tower was set
                   up at the edge  of a  predominantly oak
                   forest with an estimated canopy height
                   of approximately 20 m. To  the west, in
                   the direction of the prevailing wind, the
                   forest surface was relatively uniform for
                   about 2 km.  The ground was cleared
                   and leveled at  the  edge of the forest
                   where the tower was located. A 34-m
                   antenna tower  was assembled on the
                   ground  with  the  instruments  and
                   sampling  lines in  place. It  was then
                   tilted  into position using a gin pole and
                   winch with the supporting guy cables
                   used to maintain a stable attitude during
                   the lift. The  profile extended approx-
                   imately 12m  above the  top of the
                   canopy. The observation   levels  and
                   types of observations made at the forest
                   site are presented in Figure 1.
                     At  all  three  experimental sites  air
                   samples  for O3 analysis were  drawn
                   from six levels above the canopy surface
Table 1.    Scaffold Tower Instrumentation Profile
Observation Level     Profile Height*            Type of Observation
        1
       2
Crop Surface fapprox.J
0.25m


0.75 m


2.00m

5.00 m
Air Sample Inlet
Air Sample Inlet, Wind Speed,
(WSJ, Temperature, (TEMP.),
Dew Point (DP)
Air Sample Inlet, Wind Speed,
Wind Direction, fWDJ, Direction
Sigma, la*). Temperature
Air Sample Inlet, Wind Speed,
Temperature
Air Sample Inlet, Wind Speed,
Wind Direction, Direction Sigma,
Temperature, Dew Point
*Profile heights are given in relation to the top of the underlying vegetation.

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        34.0

        32.0-
    §

   I

    I!
    o
   •Q
        25.8-
       22.6-

       20.9-
       20.0-
  7
  1
  7
  7
WS,WD,Oa. Oe
-A-WS,Temp,O3

     WS,WD,Temp,03.0e

"7"'\WSJem~p~0
Figure 1.  Antenna tower instrumentation for forest profile studies,
          Pennsylvania,  1979.
using 0.5 in O.D. (0.25 in, I.D.) Teflon
tubing through a solenoid valve assem-
bly into  a common  manifold  leading
from the tower to the mobile laboratory.
A  pump  inside the mobile laboratory
moved the sample air stream through
the common manifold and the pollutant
analyzer  obtained its sample from this
manifold. By electrically sequencing
and opening one valve at a time, it was
possible to draw an air sample  into the
common   manifold successively  from
each  of  the six sampling ports.  Each
valve was kept open for 5 minutes. Line
loss of Oa through the  manifold was
determined  and  suitable corrections
were applied to the measured concen-
trations.
                             Results and Discussion

                             Crop Sites
                               The  experimental  data  from  the
                             Washington and Illinois sites for ozone
                             Vd values apply to a height of about 1 m
                             above the crop surface and  show a
                             general  correspondence between the
                             three experimental periods. At night,
                             values of Vd were often close to zero
                             except  when  the  nighttime  winds
                             maintained some turbulence and pre-
                             vented   the  formation  of  a  strong
                             radiation inversion. The typical daytime
                             Vd value of approximately 0.6 cm/s is
                             similar  to recent values published by
                             investigators in England and the United
                             States.  The  nighttime Vd range of 0.1
cm/s to 0.3 cm/s, under situations
when  the surface-based nocturnal
inversion  did  not  occur, agrees  well
with the range reported from measure-
ments in England.
  The trend of variation of the ground
level Os  concentration  generally
exhibits a  diurnal  cycle with clearly
defined  daytime maxima and nighttime
minima  at both crop sites.  This  is a
normal situation and has been reported
by other investigators. It is attributed to
the transport of Os within and below the
nocturnal  inversion and destruction by
deposition onto the underlying surface.
The  morning increase of Os concen-
tration  is  attributed  primarily to
downward mixing of air from aloft when
the radiation inversion is  dissipated.  In
some  situations,  photochemical  Oa
formation  could play a role in morning
concentration patterns.
  At the Illinois and Washington sites,
the  vegetation  characteristics were
generally uniform for several hundred
meters in the direction of the prevailing
wind. There is no  evidence  that the
terrain features adversely affected the
profile  measurements.   The 03  dry
deposition  velocity  and Os flux were
calculated using only profiles recorded
during the time periods when the wind
fetch was from a suitable sector.
  The recorded profiles of wind speed,
temperature  and  Os  concentrations
were considered for an averaging period
of 30 min. A graphical procedure  was
followed in obtaining the average profile
for Os concentration during the 3 min
averaging period. A similarity between
the transfer of heat  and the transfer of
pollutant was  assumed in  relation to
ozone Vd and ozone flux estimates. The
Os concentrations  in these cropland
experiments   were   indicative  of
"natural"   or  background  conditions
rather than urban areas photochemis-
try.  This  was  perhaps  a  significant
difference  compared  to the  forest
experiment.
  The field data show a general corre-
spondence between the three experi-
mental periods and the two sites. Daytime
values of Vd averaged  0.6 cm/s with a
standard deviation  of 0.17 cm/s. At
night Vd values were in the general
range of less than 0.1 cm/s to 0.3 cm/s,
with some higher  values when the
nighttime winds were especially strong.
  The daytime average Os flux deter-
mined from the cropland field study data
is around 6 x 1011  molecules cm"2 s"1,
and the  nighttime flux is about 1.5 x 10"
molecules cm"2 s"1.  The Os flux reported
                                                                                    * U S. GOVERNMENT PRINTING OFFICE 1062-559-017/0834

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    by other investigators is in the general
    range of 1 x 10" molecules crrf2s~1 to 6
    x 10" molecules cm~2 s"1.
     From  the basic considerations of the
    micrometeorology of the boundary layer,
    we would expect  Vd to be generally
    related to the product G of lateral  wind
    gustiness, erg ,  and the  mean wind, u.
    From data at the two sites,  acceptable
    statistical relationships between ozone
    Vd and G were obtained as follows:

    Robinson, IL:
     July, 1977: Vd = 0.0339(G)084
     September, 1977: Vd =0.0241 (G)078
    Pullman, WA:
     April/May, 1978: Vd = 0.0406(G)076
    where G = OQ u (deg m s 1).
    Although there  are some differences
    between these three power law regres-
    sions that may be due  to differences in
    the  surfaces,  they  are still generally
    similar and have been  combined into a
    single power law regression:
             Vd =0.0322(G)079
    This expression could be used to derive
    values of Vd from estimates of og and u
    for a variety  of  cropland surfaces and
    sites.

    Forest Site
      The forest experiment produced data
    that were not as easily interpreted  as
    the  Illinois and  Washington crop site
    data.  The top of the forest canopy
    apparently did not develop a strong
    diurnal pattern of nighttime stability and
    daytime turbulence, and there was a
    great deal of scatter in the values of all
    the calculated parameters. On the basis
    of median values of the various param-
        eters,  the  boundary layer  over the
        canopy was determined to be charac-
        teristically unstable and turbulent.
        Median values of Vd were about 1 cm/s
        with  no significant diurnal cycle
        Although the gustiness parameter, erg
        U, had an apparent diurnal cycle with
        maximum  median values in the late
        afternoon,  nighttime median values
        were also relatively large The correlation
        between Vd and erg u was much lower
        than for  the crop experiments. It is
        believed that the Vd estimates showed
        relatively large fluctuations because the
        Os profiles were not in equilibrium with
        the canopy  surface.  This  probably
        resulted from local formation  and
        scavenging processes due to the  influ-
        ences  of  local  and  regional photo-
        chemical air pollutants. It appears that
        the model developed  from the low crop
        experiments underestimates values of
Vd for the forest site by 50 percent or
more.

Recommendations
  The application  of these results to
transport model  calculations  was a
principal objective of this research and it
is  believed that the results described
above  provide reasonable  process
toward this goal. For large-scale trans-
port studies,  and  in most other model
studies, it will be necessary to estimate
values of erg  and u from the prevailing
meteorological conditions. Values of u
can be obtained directly from  the
observed or  forecast wind field.  Esti-
mates of erg  can  be obtained from an
estimate of surface layer stability and
the relationships between stability and
erg. The stability classes  can  be deter-
mined from general weather conditions
using generally accepted  techniques.
          Elmer Robinson, Brian Lamb, and M. P. Chockalingam are with Washington
            State University, Pullman, WA 99164.
          W. A. Lonneman is the EPA Project Officer (see below).
          The complete report, entitled  "Determination of Dry Deposition Rates for
            Ozone," (Order No. PB 82-258 989; Cost: $10.50, 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
                Postage and
                Fees Paid
                Environmental
                Protection
                Agency
                EPA 335
Official Business
Penalty for Private Use $300
            0000329
                                       46Ehcl

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