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
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