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OCR for page 202
Appendix B
Transport and
Dispersion Processes
Over the past 50 years scientists have been concerned
with the transport of materials by the atmosphere. Sub-
stances of interest have included volcanic debris, Saharan
dust, radioactive fallout, and industrial pollutants. The
type of gas or particulate matter, their physical and
chemical properties, the vigor of the atmospheric flow,
and other factors help to determine how and where the
material is finally deposited on the Earth's surface.
This appendix treats only the physical transport of
materials in the atmosphere. The effects of chemical
transformation and scavenging by clouds and aerosols are
discussed in detail in other appendixes.
The term transport encompasses the processes by which
a substance or quantity is carried past a fixed point or
across a fixed plane. In the atmosphere, the substances
or quantities of interest include air parcels, gaseous
impurities, suspended particles, and moisture (Huschke
1959).
CLASSIFICATION OF TRANSPORT PHENOMENA
Atmospheric motion and transport phenomena are extremely
complex in both horizontal and vertical dimensions, with
thermal layering, shear turbulence, convection, variation
of boundary characteristics, and so on. Because of these
complexities, meteorologists have devised an ordering of
the various atmospheric phenomena. One way to approach
the ordering is on the basis of spatial scale. After
release, a given material diffuses during transport,
coming under the influence of larger-scale motions as it
moves farther from the source. To classify transport
behavior, four scales have been defined: local, mesa,
202
OCR for page 203
203
synoptic, and global. The local scale is defined as
being on the order of the vertical dimension of the
planetary boundary layer within which pollutants are
typically emitted. This dimension is the order of a
kilometer, and the time scale on which phenomena take
place on this dimension is on the order of tens of
minutes. The next largest spatial scale is the mesoscale,
which extends up to several hundred kilometers and has an
associated time scale of the order of a day (about the
time needed for a mean horizontal transport of several
hundred kilometers). Mesoscale effects include the
diurnal variability of the planetary boundary layer and,
therefore, the dynamics of plumes. In the mesoscale, an
individual plume from a power plant or urban complex of
sources loses its identity by mixing with other plumes or
by diluting into the background. The synoptic scale is
on the order of 1000 km, with transport times of about 1
to 5 days. The hemispheric or global scale reflects
intercontinental transport with times on the order of a
week. The term "long-range transport" commonly refers to
transport on the synoptic and global scales.
The prevailing winds in the lower troposphere transport
and disperse atmospheric pollutants. A combination of the
rotation of the Earth (Coriolis effect) and the existence
of synoptic-scale pressure gradients in the atmosphere
maintain the planetary or geostrophic winds. A number of
perturbances near the Earth's surface, such as surface
roughness, heat, and moisture fluxes, influence the local
winds. The perturbed layer, called the planetary boundary
layer (PBL), is of variable height ranging typically up
to 3 km. Because most atmospheric pollutants are released
in this layer, study of the PBL is vital to understanding
local or mesocale transport as opposed to synoptic or
global transport.
LOCAL AND MESOSCALE SPORT
Mesoscale transport is usually confined to the planetary
boundary layer or the lowest 3 km. Embedded in this
region and closest to the ground is a highly dynamic
layer termed the mixing layer. Here the local effects of
mechanical and thermal turbulence can predominate. It is
called the mixing layer because within it atmospheric
turbulence very effectively and quickly mixes and dilutes
any concentrated release of mass, momentum, or heat. In
other parts of the atmosphere, dilution may be slow. The
OCR for page 204
204
mixing layer typically undergoes a diurnal cycle rising
to heights of 1 to 2 km in the day and lowering to 100 to
300 m at night. Thermal convection dominates in the day,
Because
and small-scale mechanical turbulence at night.
of the efficient mixing during the day, pollutants are
quickly moved to all areas of the mixing layer including
the ground. On the other hand, elevated releases at night
may be above the shallow mixing layer and can be trans-
ported independently.
In general, mesoscale mean winds dominate plume trans-
port, but, depending on the strength of local turbulent
eddies, the plume may also be spread horizontally and
vertically. Another factor affecting pollutant transport
is wind shear. Since thermal convection and mechanical
drag of the ground diminishes with height, the geostrophic
balance of forces varies with height and is maintained by
increasing wind speed and veering in wind direction.
Thus, vertically adjacent layers of air move at different
speeds in different directions (shear).
Wind shear may
cause dispersion and dilution of atmospheric pollutants
and becomes increasingly important as the range of
transport increases.
In addition to this general picture of local and
mesoscale transport, there are significant diurnal and
seasonal variations in the boundary layer that affect
transport on these two scales. Figure B.1 shows the
different patterns for winter and summer. The major
feature to notice is that for both periods there is a
very stable nocturnal layer that extends to 300 m.
However, during the daytime, mixing heights are much
greater in summer than in winter when an elevated daytime
inversion hinders vertical mixing.
Another factor to be considered on the mesoscale is
the vertical profile of the horizontal wind speed.
Diurnal and seasonal variations in the profile are
affected by the vigor of the synoptic-scale flow. Winter
is a period of frontal passages, whereas in summer weak
anticyclonic systems tend to prevail (Figure B.2).
From the above discussion, it can be seen that the
mesoscale transport and dilution of a given pollutant
depend on whether its source is elevated or on the
ground. For example, while most of the SO2 is emitted
from elevated point sources, NOX emissions are more
evenly distributed between elevated and ground-level
sources. Thus on the average, elevated releases spend
more of their mesoscale transport time decoupled from the
ground, while near-ground releases maintain continuous
OCR for page 205
205
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2500
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St. Louis 1976
JUL \:
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Wind speed (m/s)
FIGURE B.2 Monthly average diurnal and seasonal variations of the vertical profiles
of wind speed near St. Louis, Missoun, based on 1976 data. SOURCE: N. Gillani,
Washington University, St. Louis, Missoun, personal communication (1982).
ground contact.
This fact has a direct relation to the
importance of diurnal and seasonal dry deposition and to
some degree on the wet-deposition patterns.
While the main emphasis in acid deposition has been on
the long-range transport of pollutants to remote areas,
consideration of mesoscale transport and dispersion of
pollutants of varying source types under varying flow
conditions have an important bearing on how much of the
emissions become available for long range transport and
in what form. Important mesoscale factors such as release
height and diurnal and seasonal variabilities must not be
neglected in long-range transport modeling.
SYNOPTIC- OR CONTItIENTAL-SCALE TRANSPORT
Synoptic transport of pollutants, especially acids and
acid precursors, has been one of the major thrusts in
acid deposition research. Numerous models have been
OCR for page 207
207
devised to investigate the long-range transport (up to
1000 km) and are reviewed in Chapter 5 of this report.
Each of the models simulates the transport, transforma-
tion, and deposition of a given substance (sulfur
compounds in this case). The main transport module in
the models uses the synoptic winds, which are measured in
the vertical dimension every 12 h by balloon soundings.
The network of balloon soundings produces, unfortunately,
sparser data coverage than the precipitation chemistry
measurements over eastern North America. Considering the
spacing of upper-air measurements, it is optimistic to
expect the knowledge of the direction of the prevailing
wind at an arbitrary location in space and time to be
known to better than 5° about the "actual" adverting
wind. When one calculates forward or back trajectories
from these winds, there is an uncertainty in the cross-
wind direction of 15 to 20 percent of the trajectory
length for every timestop forward or backward in time.
One would hope that such uncertainties and errors would
cancel out when trajectories are calculated over many
days and a climatology is established.
There are several key factors that determine the
transmission of pollutants on the synoptic scale
specifically over the North American continent. Already
mentioned is the wind field. Clear patterns can be seen
from a summary of the 1975-1977 data (R. Husar, Washington
University, St. Louis, MO, personal communication, 1982).
Conclusions are that (1) the general flow is from west to
east with an important component northward from the Gulf
of Mexico, (2) winter and fall have the highest speeds,
(3) the southeastern United States is within a region of
low mean velocity during late spring and summer, and (4)
the Midwest exhibits very strong shear during summer and
spring (Figure B.3).
It is important to note that winds above 1 to 2 km are
not always important in the transport of surface releases,
depending on the mixing depth. Also, well-mixed aged
pollutants in the nocturnal stable layers may not always
be re-entrained into the mixing layer the next morning.
Contours of mixed depths (Figure B.4) provide some insight
into the gross interaction of adverting winds and the
depth of the mixing layer. However, synoptic temporal
and spatial scales of interaction may be at least as
important as the seasonal averages in determining the net
transport of emissions. It is important to note that
some of the well-mixed aged pollutants will ride over the
OCR for page 208
208
<1
~ \a ~ ~
~ ~ 1 0 m/s
~5
I{ ~ _ ~ ~ ~* ~ ~
( ~ if. ''A ;~, ~ ~ _ - -
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FIGURE B.3 Averages for 1975-1977 of winds in the layers 0-500, 500-1000, 1000-
2000, and 2000-3000 m agl for the 000 and 1200 GMT soundings. Lower-level winds
generally lie to the left and are of lower speed. a, January through March; b, April
through June; c, July through September; and d, October through December.
SOURCE: R. Husar, Washington University, St. Louis, Missoun, personal communi-
cation (1982~.
daytime mixed layer when moving either from south to
north or from west to east owing to lowered mixed depths
along the trajectory. Parameterization of the vertical
structure in the models is important for simulation of
continental-scale transport over several days and
thousands of kilometers.
Other vertical motions must be taken into account in
long-range transport, although these are difficult to
simulate properly. Vertical motions are important, for
example, in transmission of pollutants across major
physical barriers (for example, the Rocky Mountains),
along warm and cold fronts, and near simple convective
cells or clusters of cells. Also the vigor of motion of
OCR for page 209
209
W ~ NT E R
~ r:
F:,-. .
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^"" 2 '2,~
~,... ,,O, :,¢;',. .. f=N
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'~. - ~::
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~r = ~-~ c' E3 0. 3-0. 6
ia ~0,6-0.9
~ hRH 0,5 ~ O. 9-I . 2
~ rus o ~,~ ~· > 1 . 2
N5 ~ SURE 0. 8S
SUMMER
km (AGL)
:~, E]12-1s
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~ END ~ 37 ~ ~ 2. 1
\~ ~ 5URC 1.55
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[U5 I.Z.
SU8C ~ . 2S
FIGURE B.4 Contour plots of maximum afternoon mixing depths by season, indicat-
ing qualitative patterns only. a, January through March; b, April through June; c,
July through September; and d, October through December. SOURCE: Holzworth
(1972) and Portelli (1977).
OCR for page 210
210
both cyclonic and anticyclonic systems can have an impact
on accumulation of emissions. Korshover (1976) has
pointed out that the south central United States is
particularly subject to stagnating anticyclones, leading
to lower ventilation of local and adverted emissions.
Another factor critical to long-range transport is
precipitation, which removes pollutants in a sporadic
way. Trajectories from a source to a receptor will not
establish the total mass transported if the air mass is
likely to experience precipitation along the way. This
removal depends on the type, intensity, and frequency of
the precipitation. At present, the precipitation removal
process is difficult to quantify over long transport
paths.
Recently another important factor has been pointed out
by Draxler and Taylor (1982). The authors showed that
the spreading of emissions is dominated by the action of
vertical wind shear acting in combination with the
diurnal cycle of daytime mixing and nighttime layering
of the atmosphere. Further work on the importance of
this factor is being pursued.
In understanding the synoptic-scale transport, one
should not lose sight of the fact that both local and
mesoscale influences are important in continental
transport. Thus to model the regional transport, the
mesoscale must be adequately parameterized even if not
explicity nested with that scale's simulation.
HEMI SPHERIC OR GLOBAL TRANSPORT
Besides the fact that hemispheric transport involves
greater distances and times than regional transport,
there are important differences between the two scales.
One fact is that the bulk of the global transport takes
place over water. Because of the small changes in oceanic
surface temperature, the planetary boundary layer over
the oceans is relatively constant. Besides this, the
oceans can be considered a homogeneous surface over large
areas. Thus there are broad stretches of strong atmo-
spheric inversions over cold water and other well-mixed
regions over relatively warm water. One can expect that
pollution within the boundary layer is subject to dry
removal and that pollution that has been transported
above the boundary layer will remain there until removed
by precipitation processes or by large-scale subsidence.
OCR for page 211
211
The study of the movement of acidic and preacidic
material from sources in North America to other receptor
regions in the northern hemisphere has been undertaken in
several cases. However, because of the lack of chemical
and meteorological data over large stretches of the ocean,
only crude estimates of this transport can be made. For
example, the high acidity found in precipitation on the
island of Hawaii could be partially explained by long-
range transport from the west, where Japan would be the
major source (Miller and Yoshigana 1981, Dittenhoefer
1982). In this study, a single trajectory model was
useful in evaluating the transport patterns.
Another area of interest is the contribution of North
American sources to Arctic haze. This issue has been
raised more in reference to visibility or the modification
of the radiation balance, since the Canadian and U.S.
Arctic areas are deserts (100 mm per annum) with little
wet deposition. The major transport path from eastern
North America is a track around Greenland. Concentrations
of pollutant aerosols in the Arctic show a definite
wintertime peak when removal mechanisms are most inactive.
Rahn and MoCaffrey (1980) indicate that residence times
of aerosol particles in the Anti ~ ranch from ~ hm
weeks in the winter.
.
The transport of materials across the Atlantic has also
been a topic of interest though not firmly established.
Early estimates were made that North American contribution
to sulfate in rain in Norway could be important. More
recent studies in Bermuda indicate that trans-Atlantic
transport of acid precursors is important to the acidity
of precipitation on the island (Jickells et al. 1982).
Further studies of this transport are being continued
under a joint U.S.-CanadarBermuda effort.
Recent studies of precipitation in remote areas of
both the northern and southern hemisphere have shown the
acidity of rain to be on the average lower than pa 5.0
(Galloway et al. 1982). The degree to which natural
sources or long-range transport of man-made pollutants
contribute to this remote acidity in precipitation
remains to be seen. However, trajectory calculations to
estimate the transport on a global scale will be a useful
tool in such research.
CONCLUSIONS
Though the transport of materials in the atmosphere has
been studied for a number of years, there is still much
OCR for page 212
212
that can be learned in applying this knowledge to the
acid deposition problem. By and large, the vigor of the
atmosphere in both the horizontal and vertical rules
where the final deposition of a given pollutant will be.
ACKNOWLEDGMENTS
The committee thanks N. Gillani, C. Patterson, and R.
Husar for their help in preparing this appendix.
REFERENCES
Dittenhoefer, A.C. 1982. The effects of sulfate and
non-sulfate particles on light scattering at the Mauna
Loa Observatory. Water, Air and Soil Pollut.
18:129-154.
Draxler, R.R., and A.D. Taylor 1982. Horizontal
dispersion parameters for long-range transport
modeling. J. Atmos. Meteorol. 21:367-372.
Galloway, J.N., G.E. Likens, W.C. Keene, and J.M. Miller
1982. The composition of precipitation in remote areas
of the world. J. Geophys. Res. 87:8771-8786.
Holzworth, G.C. 1972. Mixing heights, wind speeds, and
potential for urban air pollution throughout the
contiguous United States. U.S. EPA AP-101.
Huschke, R.E. (ed.) 1959. Glossary of Meteorology. Boston,
Mass.: American Meteorological Society, p. 638.
Jickells, T., A. Knap, T. Church, J. Galloway, and J.
Miller 1982. Acid rain in Bermuda. Nature 297:55-57.
Korshover, J. 1976. Climatology of stagnating
anticyclones east of the Rocky Mountains, 1936-75.
NOAA Technical Memorandum ERL APL-55, 26 pp.
Miller, J.M., and A.M. Yoshigana 1981. The pH of Hawaiian
precipitation. A preliminary report. Geophys. Res.
Lett. 8:779-782.
Portelli, R.V. 1977. Mixing heights, wind speeds and
ventilation coefficients for Canada. Environment
Canada, Atmospheric Environment Service,
Climatological Studies Number 31, UDC: 551.554.
Rahn, K.A., and R.J. McCaffrey 1980. On the origin and
transport of the winter Arctic aerosol. Ann. N.Y.
Acad. Sci. 308:486-503.
OCR for page 213
Appendix
Atmospheric Deposition
Processes
1. INTRODUCTION
In this appendix we present an overview of current
scientific understanding about deposition phenomena, with
the objectives of identifying key literature sources on
this subject and providing the reader with the technical
basis necessary for effective evaluation of the available
literature. There are several important features of this
subject, which should be noted at the outset. First, the
ultimate deposition processes of interest are the end
products of a complex sequence of atmospheric phenomena
(cf. Figure 2.1). Deposition processes tend to reflect
these preceding events strongly. Much of the material
presented in this appendix therefore necessarily deals
with the predeposition processes, which may act as
important rate-influencing steps in the overall
source-deposition sequence.
A second important feature of initial interest is the
relative difference in states of our current understanding
of wet- and dry-deposition phenomena. Wet deposition is
comparatively simple to measure. As a consequence there
exists a substantial and growing base of data on wet
deposition from a variety of networks and field studies.
Precipitation processes tend to be rather complicated,
however, and currently a high level of uncertainty exists
regarding their mathematical characterization.
Dry deposition, on the other hand, tends to be
extremely difficult to measure, and the corresponding
data set is relatively meager. Partly because of this
fact most mathematical characterizations of dry-
deposition processes have been quite simple in form. The
tendency toward simplicity in most mathematical char-
acterizations of dry deposition should not be taken to
213
Representative terms from entire chapter:
mixing layer
