CorvaHis En v,ronmenta!
Research Laaoraiory
n 97330
tates
Environmental Protect;o
A.\D DRY CEPGSITIOrt A
SYNfpSIS CONTAINIr-i." ESTIMATES C
HEPC5ITION v'ELCCITIES FC?;
::'^T AND TRACE GASES I« Yr
AT^SPHERE
CER1.-037
April 1977
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950R77028
WET AND DRY DEPOSITION - A
SYNOPSIS CONTAINING ESTIMATES OF
DEPOSITION VELOCITIES FOR SOME
POLLUTANT AND TRACE GASES IN THE
ATMOSPHERE
CERL-037
April 1977
by
Ernest W. Peterson, Ph.D.*
Research Meterologist
Terrestrial Ecology Branch
Ecological Effects Research Division
Corvallis Environmental Research Laboratory
U.S. Environmental Protection Agency
*0n Assignment from the National Oceanic and Atmospheric Administration
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Introduction
The initial point at which most air pollutants become of concern to
human health, and ecology is when removal from the atmosphere occurs at
the earth's surface. Some pollutants such as nitrogen oxides and hydro-
carbons undergo chemical reactions while in the atmosphere and are
removed in a different form than when they were first emitted. The
impact of air pollutants on terrestrial ecosystems, however, is a
function of the quantitative and qualitative deposition at the earth's
surface. The purpose of this report is to summarize, analyze, and
present a synopsis of what is presently known about the rates at which
some common pollutants are removed from the atmosphere.
Locally, the concentration of a pollutant in the atmosphere is
determined primarily by the rate of horizontal dispersion and is roughly
proportional to the inverse of the wind speed. On continental and
global scales, the concentration is directly related to removal pro-
cesses at the surface; thus wet and dry deposition are key factors in
the mass budgets of air pollutants on these scales. An analysis of the
relative importance of horizontal dispersion and surface deposition is
presented in the appendix.
The information presented in this report allows one to estimate the
rate of removal by the earth's surface of a variety of pollutants, both
globally and locally, by sea, soil, and vegetation surface types, given
the pollutant concentrations in the atmosphere. Figure 1 shows the
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and
PAN
CON-
DENSATION
NUCLEI
DRY DEPOSITION
DRY DEPOSITION
RAINOUT
Fioure 1: Removal and interaction of some common pollutant and trace gases in the atmosphere.
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removal mechanisms and interactions among some common pollutants and
trace gases and Table 1 presents estimates of deposition velocities from
which removal rates can be determined given the concentration of these
components in the atmosphere.
The report also contains an annotated bibliography of the relevant
literature on this subject and which is the source of most of the
material contained herein.
Deposition Velocity
It has been found that the rate of pollutant removal from the
atmosphere by the surface is roughly proportional to the concentration
of the pollutant in the atmosphere near the surface, i.e.:
F =
1 (1)
(typical units)
F2 1
is the removal rate (flux) (g cm sec )
5
T£ is a characteristic concentration (g cm" )
and (JT is a constant of proportionality (cm sec" )
The proportionality constant, IT" , has units of velocity and for this
reason it is called a deposition velocity since it relates the rate of
deposition or removal to the concentration. Rearranging (1) we can
define the deposition velocity as:
(2)
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TABLE 1
WET AND DRY DEPOSITION
GAS
SO?
NO?
NH3
HZS
Oa
HF
PAN
C?H,,
NO
CO
CH,,
HyO
CC1,,
CC1 -,F
Mel
(Me,)S
** TT*
cm/ sec
o
100
10
10"1
7
0
100
10
10
IO'1
<10"'
IO"2
10"2
TO"2
IO"3
IO'3
10'3
10'3
GLOBAL
(1) (2)
£r X t
cm/sec ug/m3 yr
MORE REACTIVE COMPO
0 n
100
10
10' >
7
IO"6
0
?
7
io"7
10'7
IO'7
io"6
io"6
7
io'5
10"5
100
100
10
IO'1
0
10
10"1
?
0
100
10
LESS REACTIVE
IO2
IO3
IO2
?
7
7
7
IO"2
10'2
10'1
?
10'2
IO"2
10"2
IO"1
10'1
LOCAL
cm/ sec
sea soil
JNDS
*
*
IO'1
?
*
*
7
7
<10'»
COMPOUNDS
0
10
0
100
100
100
100
100
10
IO'2
IO"2
10'
10"3
7
io'3
io"3
*
*
*
?
*
*
7
7
io"1
10'1
?
<10'2
7
7
7
7
flora
*
*
*
?
*
*
7
io'1
0
7
-------
For a given pollutant, surface, and level of atmospheric turbu-
lence, I/ , is often relatively constant over a wide range of concen-
trations while the removal rate.r » is not. It is thus useful for
estimating the removal rate when the concentration is known. Because
the assumption of proportionality between removal rate and concentrations
is usually a crude approximation there is sometimes considerable varia-
bility in the values of the deposition velocity determined experimen-
tally. Moreover because of the great variability in environmental
factors, the deposition velocity should be expected to be somewhat
dependent on the particular situation in which it is determined. For
these reasons the deposition velocities reported here are presented only
to within a factor of ten, although for a particular well-defined
situation it is possible to determine a more precise relationship bet-
ween the removal rate and the concentration. In general, however, one
can only expect to estimate removal rates to within about one order of
magnitude. With the deposition velocities presented in Table 1, using
equation (1), removal rates can be estimated for any given concentration
of each of several common pollutant gases under a variety of conditions.
Dry Deposition
Pollutants are removed from the atmosphere by contacting the
earth's surface. Their transport to the surface is a result of air
turbulence and in the case of particulate pollutants or particles to
which gaseous pollutants have been sorbed, gravitational setting. They
can also be removed by rainfall; this process is called wet deposition
or rainout. In the absence of precipitation, the pollutant is said to
be dry deposited on the surface if the pollutant, upon reaching the
5
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surface, is sorbed by the materials of the surface, i.e., the sea, soil,
and vegetation.
For gases, dry deposition takes place upon sorption by the surface
(in the case of re-emission of the gas by the surface, dry deposition
will bo considered as the net deposition, the sorption minus the re-
emission). For particles, dry deposition does not take place until the
pollutant material in the particle is taken up by the surface. This is
often the rate-limiting process for the dry deposition of particles.
For although the particles may be carried or fall out of the atmosphere
rather rapidly, the actual transfer and sorption of the pollutant from
the particle to the surface can be rather slow, slower than the dry
deposition rate of many gases. Prior to sorption or incorporation the
particle is subject to resuspension by the wind.
Dry deposition can be rate-limited by atmospheric or surface
processes. The transport of a pollutant gas to the ground from the
atmosphere is determined by the level of turbulence in the atmosphere.
On a windy, sunny day the atmosphere is highly turbulent while on calm,
clear nights turbulence may be nonexistent. Over a forest or other very
rough surface the deposition velocity under highly turbulent conditions
can be 10 cm/sec or more while under nonturbulent (or stable) conditions
the deposition velocity may be less than 0.1 cm/sec.
Once the pollutant has reached the surface, the rate of uptake is a
complex function of the type of pollutant, biological and chemical state
of the surface, dryness of the surface, etc. These factors themselves
-------
are complex functions of climate, season, weather, time of day, soil
structure, ecostructure and a host of other variables. For very reac-
tive pollutants such as sulfur dioxide, nitrogen dioxide, ammonia,
ozone, and hydrogen fluoride, the surface sorbs the pollutants essen-
tially as rapidly as the atmosphere can deliver them, i. e. the de-
position velocities of highly reactive pollutants are rate-limited by
atmospheric turbulence. Thus the deposition velocities for these
pollutants vary between about 0.1 cm/sec to 10 cm/sec, typically about 1
cm/sec.
For less reactive pollutants, the surface cannot remove the pol-
lutant as fast as it is delivered, and therefore the deposition veloci-
ties for these materials are rate-limited by the surface. Deposition
velocities determined by surface processes vary widely and cannot be
easily generalized. For instance, the noble gases such as argon es-
sentially do not react at all with the surface and thus have deposition
velocities which are practically zero while other relatively slowly
reacting pollutants such as carbon monoxide have deposition velocities
approaching 0.1 cm/sec.
Table 1 gives estimates of dry deposition velocities for various
pollutant and trace gases. These estimates are based on reports of
actual measurements and modelling estimates found in the literature (see
annotated bibliography). Dry deposition velocities over sea, soil, and
vegetation as well as a global average deposition velocity for each
compound are presented.
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Wet Deposition or Rainout
Certain pollutant gases, notably sulfur dioxide, nitrogen dioxide,
and ammonia which are readily water soluble, are taken up in the process
of raindrop formation and deposited on the surface by the rain (or
snow). The wet deposition velocity, t,, is related to a quantity,
, called the washout ratio by
where is the precipitation rate.
Table 1 presents values of the washout ratio for various gases.
The global average wet deposition velocity for a precipitation rate of
100 cm/year is also given. A comparison of the global average wet and
dry deposition velocities for SO^, NOp, and NH3 shows that wet and dry
deposition are on the average of about equal importance in removing
these gases from the atmosphere. However, locally large quantities of
these gases can be removed during a rain storm; wet deposition during
rainy weather can be far more important than dry deposition.
Residence time
The typical time that a pollutant remains in the troposphere is
determined by the rate of removal or emission, the concentration, and
the depth through which it is mixed. An effective tropospheric mixing
depth for estimating residence time in the lower atmosphere is about 5
km . The relation between removal rate and concentration is contained
8
-------
in the deposition velocity. The residence time, £" , can be estimated
by:
- rk-
(4)
where f\ is the tropospheric mixing depth. It is seen that the re-
sidence time is inversely proportional to the deposition velocity, i.e.
pollutants which are removed at a relatively slow rate remain in the
atmosphere a long time while those which are removed at a rapid rate
have short residence times. Thus, for a reactive gas such as sulfur
dioxide, the residence time is only a few days while a less reactive gas
such as carbon monoxide can remain in the atmosphere for months.
Table 1 presents estimated global average tropospheric residence
times for various pollutants. Pollutants which manage to be carried
into the stratosphere can remain much longer because the mixing between
stratosphere and the troposphere is quite slow relative to the mixing in
the troposphere itself.
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ANNOTATED BIBLIOGRAPHY
1. Abeles, F.B., L.E. Craker, I.E. Forrence, and G.R. Leather. 1971.
Fate of air pollutants: removal of ethylene, sulfur dioxide
and nitrogen dioxide by soil. Science. 193:914-916.
Paper supports hypothesis that soil is a sink for some air
pollutants.
2. Abeles, F.B. and H.E. Heggestad. 1973. Ethylene: an urban air
pollutant, vh Air Poll. Cont. Asso. 23:517-521.
3. Babich, H. and G. Stotzky. 1974. Air pollution and microbial
ecology. CRC Critical Review in Environmental Control. 353-
421.
Review article on interaction of microbes and air pollutants.
4. Bennett, J.H. and A. Clyde Hill. 1973. Absorption of gaseous air
pollutants by a standardized plant canopy, vh Air. Pol 1.
Cont. Asso. 123:203-206.
Wind tunnel study. Dry deposition velocities can be deduced.
5. Bennett, J.H. and A.C. Hill. 1975. Interactions of air pollutants
with canopies of vegetation. J^n_ Responses of Plants to Air
Pollution (J.B. Hudd and T.T. Kozlowski, eds.), Academic
Press, 273-306.
General descriptive review of theory of plant uptake of air
pollutants.
6. Bidwell, R.G.S. and D.E. Fraser. 1972. Carbon monoxide uptake and
metabolism by leaves. Canadian J. Botany. 50:1435-1439.
Laboratory study. Deposition velocities can be deduced.
7. Bonn, Hinrich L. 1972. Soil absorption of air pollutants. J_.
Environ. Quality. 1:372-377.
Review article. Some deposition velocities given.
8. Bromfield, A.R. 1972. Absorption of atmospheric sulphur by
mustard (Sinapsis alia) grown in a glass house. J. Agric.
Sci. 78:343-344.
Laboratory study. Dry deposition rate determined from measurements.
9. Calder, K.L. 1972. Absorption of ammonia from atmospheric plumes
by natural water surfaces. Water, Air, and Soil Ppllut.
1:375-380.
Model estimates.
10
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10. Chilcote, D.O. 1975. The role of plant in removal of air pol-
lutants. In: The Role of Plants in Environmental Purification,
Environmental Health Sciences Center, OSU, Corvallis, Oregon.
23-29.
Short review paper.
11. Dannevik, W.P., S. Frisella, L. Granat, and R.B. Husar. 1976. S02
deposition measurements in the St. Louis region. In: Third
Symposium on Atmospheric Turbulence, Diffusion, and Air
Quality, Preprint volume, American Meteorological Society,
506-511.
Deposition velocities determined.
12. Davies, T.D. 1976. Precipitation scavenging of sulphur dioxide in
an industrial area. Atmos. Environ. 10:879-890.
Observes that wind speed correlates better than washout with
pollutant concentration in an industrial area.
13. Davison, A.W. A.M. Rand and W.E. Betts. 1973. Measurement of
atmospheric fluoride concentrations in urban areas. Environ.
Pollut. 5:23-33.
14. DeCormis, L., 1968. Some aspects of sulphur absorption by plants
subjected to an atmosphere containing SO-, (translated from
French). APTIC Ref. No. TR-0916. L
Laboratory study.
15. DeSanto, R.S., W.H. Smith, J.A. Miller, W.P. McMillen and K.A.
McGregor. 1976. Open Space as an Air Resource Management
Measure, Vol. II Sink Factors, COMSIS Corporation, draft
report to USEPA, Contract No. 68-02-2350.
Contains extensive bibliography on removal of air pollutants
by plant, soil, and water surfaces.
16. Dovland, H. and A. Eliassen. 1976. Dry deposition on a snow
surface. Atmos. Environ. 10:783-785.
Field study. Deposition velocities for SOp and Pb determined.
17. Droppo, J.G., D.W. Glover, O.B. Abbey, C.W. Spicer, and J. Cooper.
1976. Measurement of Dry Deposition of Fossil Fuel Plant
Pollutants. Report to USEPA, Contract No. 38-02-1947, EPA-
600/4-076-056, 124 pp.
Field measurements and modelling studies. Deposition veloci-
ties determined.
18. Droppo, J.G. 1976. Dry removal of air pollutants by vegetative
canopies. Preprint Volume Fourth National Conference on Fire
and Forest Meteorology, St. Louis, MO, Nov. 16-18, 1976.
Summary from various sources of dry deposition rates.
11
-------
19. Faller, N. 1972. Absorption of sulphur dioxide by tobacco plants
differently supplied with sulphate. IAEA Proc. Set 292, 51-
58.
20. Fowler, D. 1976. Uptake of S02 by crops and soil. School of
Agricultural Science, Nottingham University, Scotland. Ab-
stract of thesis.
Field measurements. Deposition velocities determined.
21. Fowler, D. and M.H. Unsworth. 1974. Dry deposition of sulphur
dioxide on wheat. Nature. 249:389-390.
Field measurements of dry deposition velocities on wheat
including diurnal variations.
22. Garland, J.A. 1974. Dry deposition of SOp and other gases. In:
Atmospheric-Surface Exchange of Particulate and Gaseous Pol-
lutants, ERDA Symposium Series 38, NTIS No. Conf-740921, 212-
227.
35
Field study. S used as tracer. Deposition velocities
determined for S02 and 0-,.
23. Garland, J.A., D.H.F. Atkins, C.J. Readings and S.J. Caughey.
1974. Deposition of gaseous sulphur dioxide to the ground.
Atmos. Environ. 8:75-79.
Deposition velocities determined from field measurements.
24. Godzik, S. 1972. Comparative investigations on the uptake of
sulphur dioxide from the atmosphere, (translated from Ger-
man), APTIC Ref. No. TR652-73.
Laboratory study.
25. Guderian, R. 197]. Effect of nutrient supply on the absorption of
sulfur dioxide from the air and on plant susceptibility.
(translated from German), APTIC Ref. No. TR1512.
26. Hanawalt, R.B. 1969. Environmental factors influencing the sorp-
tion of atmospheric ammonia by soils. Soil Sci. Soc. Amer.
Proc. 33:231-234.
Laboratory study. Wind tunnel.
27. Heck, W.W., J.A. Dunning, I.J. Hindawi. 1965. Interactions of
environmental factors on the sensitivity of plants to air
pollution. J_. Air Poll. Cont. Asso. 15:511-515.
Field and laboratory measurements.
28. Heichel, G.H. 1973. Removal of carbon monoxide by field and
forest soils. J_. Environ. Quality. 2:419-423.
Laboratory study.
12
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29. Hicks, B.B. 1974. Some micrometeorological aspects of pollutant
deposition rates near the surface. In: Atmosphere-Surface
Exchange of Particulate and Gaseous Pollutants, ERDA Symposium
Series 38 NTIS No. CONF-740291, 434-449.
Theoretical computations for atmosphere limited deposition
velocities.
30. Hill, A.C. 1971. Vegetation: A sink for atmospheric pollutants.
J,. Air Poll. Cont. As so. 21:341-346.
Review article.
31. Hill, A.C. and E.M. Chamberlain, Jr. 1974. The removal of water
soluble gases from the atmosphere by vegetation. In: Atmos-
pheric-Surface Exchange of Particulate and Gaseous Pollutants,
ERDA Symposium Series 38, NTIS No. CONF-740921, 153-170.
Wind tunnel study. Deposition rates for several pollutants
onto alfalfa determined.
32. Hutchinson, G.L., R.S. Millington, and D.B. Peters. 1972. Atmospheric
Ammonia. Science 175:771-772.
Deposition velocities can be deduced.
33. Ingersoll, R.B. 1972. The capacity of the soil as a natural sink
for carbon monoxide. Final Report, SRI Project LSU-1380,
Stanford Research Institute, EPA-650/2-73-043, 38 pp.
Deposition velocities can be deduced. Static chamber measure-
ments in field.
34. Inman, R.E., R.B. Ingersoll, and E.A. Levy. 197]. Soil: a nat-
ural sink for carbon monoxide. Science. 172:1229-1231.
35. Israel, G.W. 1974. Deposition velocity of gaseous fluorides on
alfalfa. Atmos. Environ. 8:1329-1330.
36. Jaffe, L.S. 1973. Carbon monoxide in the biosphere: sources,
distribution, and concentrations. J. Geophysical Res.
78:5293-5305.
Review article. Includes temporal variations in concentration.
37. Jensen, K.F. 1973. Absorption, translocation, and soil deposition
of aerial pollutants by vegetation. Proc. Soil Conserv. Soc.
Assoc. 28:26-29. :
Some deposition velocities reported.
13
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38. Jensen, K.F. and T.T. Kozlowski. 1975. Absorption and trans-
location of sulfur dioxide by seedlings of four forest tree
species. J_. Environ. Qual. 4:379-382.
39. Judeikis, H.S. and T.B. Stewart. 1976. Laboratory measurement of
S02 deposition velocities on selected building materials and
soils. Atmos. Environ. 10:769-776.
40. Junge, C. 1972. The cycle of atmospheric gases - natural and
manmade. Quart. J_. Roy. Meteor. Soc. 98:711-729.
Review article.
41. Kabel, R.L. 1976. Natural removal of gaseous pollutants. In:
Third Symposium on Atmospheric Turbulence, Diffusion, and Air
Quality. Preprint Volume, Oct. 1976. American Meteorology
Society. 1:488-495.
Global removal rates. Deposition velocities can be deduced.
42. Kabel, R.L., O'Dell, R.A., M. Taheeri, and D.D. Davis. 1976. A
preliminary model of gaseous pollutant uptake by vegetation.
CAES Pub. No. 455-76. Center for Air Environment Studies,
Penn. State U.
43. Liss, P.S. and P.G. Slater. 1974. Flux of gases across the air-
sea interface. Nature. 247:181-184.
Model estimates of deposition velocities.
44. MacClean, D.C. and R.E. Schneider. 1973. Fluoride accumulation by
forage: continuous vs. intermittent exposures to hydrogen
fluoride. J_. Environ. Quality. 2:501-503.
45. Malo, B.A. and E.R. Purvis. 1964. Soil absorption of atmospheric
ammonia. Soil Science 97:242-247.
Laboratory and field measurements, including ammonia content
of precipitation.
46. Martin, A. and F.R. Barber. 1971. Some measurements of loss of
atmospheric sulphur dioxide near foliage. Atmos. Environ.
5:345-352.
Field measurements of uptake of SOp by a hedge. Deposition
velocities not determined.
47. Mason, B.J. 1971. The Physics of Clouds, 2nc[ ed_., Oxford Univ.
Press, 671 pp.
General text on atmospheric cloud physics.
48. Materna, J. and R. Kohout. 1969. Uptake of sulphur dioxide into
the leaves of some tree species. Communicationes Instuti
Forestalis Czechoslovenia. 6:37-47.
14
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49. McConnell, J.C. 1973. Atmospheric ammonia. J_. Geophys. Res.
78:7813-7821.
Theoretical discussion of ammonia budget. Estimate of global
deposition velocity for NCL given.
50. McMahon, T.A., P.J. Denison, and R. Fleming. 1976. A long-distance
air pollution transportation model incorporating washout and
dry deposition components. Atmos. Environ. 10:751-761.
Contains some estimates of deposition velocities and washout
coefficients.
51. Murphy, B.D. 1976. Deposition of S02 on ground cover. In:
Third Symposium on Atmospheric Turbulence, Diffusion, and Air
Quality, preprint volume, American Meteorology Society, 500-
505.
Model study.
52. Nikolaevskii, V.S. 1971. Some regularities in absorption of
sulfur dioxide by woody plants. (Translated from Russian).
APTIC Ref. No. TR 19-75.
53. Owers, M.J. and A.Q. Powell. 1974. Deposition velocity of sulphur
dioxide on land and water surfaces using a S tracer method.
Atmos. Environ. 8:63-67.
Deposition velocities for S02 determined from field observations.
' 54. Rasmussen, R.A. 1972. What do the hydrocarbons from trees con-
tribute to air pollution? J. Air. Poll. Cont. Asso. 22:537-
543.
55. Rasmussen, K.H., M. Taheri, and R.L. Kabel. 1975. Global emissions
and natural processes for removal of gaseous pollutants.
Water, Air & Soil Pollut. 4:33-64.
Global deposition velocities can be deduced.
56. Roberts, B. 1974. Foliar sorption of atmospheric sulphur dioxide
by woody plants. Environ. Pollut. 7:133-140.
Laboratory study.
57. Roberts, B.R. and C.R. Krause. 1976. Changes in ambient S02 by
rhododendron and pyracantha. HortScience 11:111-112.
Laboratory study.
58. Robinson, E. and R.C. Robbins. 1972. Emissions, concentrations,
and. fate of gaseous atmospheric pollutants. _[n: Air Pol-
lution Control (W. Strauss, Ed.), Part 2, Wiley - Interscience,
1-93.
o
59. Ruhling, A. and G. Tyler. 1973. Heavy metal deposition in Scan-
dinavia. Wate_r, Air, and_ SoJJ_ PoVhrt. 2;445-455.
15
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Observed deposition patterns of heavy metals in a Scandinavia
moss.
60. Saito, T. 1974. Absorption of sulfur dioxide in leaves as affected
by light and wind. J_. Japan. Soc. Air Poll. 9:1-4.
Wind tunnel study. Variation in uptake influenced by en-
vironmental factors. Deposition velocities determined, which
seem lower than those for atmosphere.
61. Seiler, W. and C. Junge. 1970. Carbon monoxide in the atmosphere.
0.. Geophysical Res. 75:2217-2226.
Study of global distribution.
62. Shepard, J.G. 1974. Measurements of the direct deposition of
sulphur dioxide onto grass and water by the profile method.
Atmos. Environ. 6:69-74.
Deposition velocities determined from field measurements.
63. Slinn, W.G.N. 1974. Precipitation scavenging: some problems,
approximate solutions and suggestions for future research.
Proceeding of UAEC Sponsored Symposium "Precipitation Scaven-
ging - 1974," in press.
Theoretical review of precipitation scavenging.
64. Slinn, W.G.N. 1976a. Formulation and solution of the diffusion-
deposition-resuspension problem. Atmos. Environ. 10:763-768.
Theoretical paper.
65. Slinn, W.G.N. 1976b. Some approximations for the wet and dry
removal of particles and gases from the atmosphere. J_. Air,
Water, and Soil Pollut. 7.
Review of theoretical and semi-empirical approach to wet and
dry deposition.
66. Slinn, W.G.N. 1976c. Natural atmospheric cleansing processes.
Preprint Volume, Fourth National Conference on Fire and Forest
Meteorology, St. Louis, Missouri, Nov. 16-18, 1976.
Residence times defined, and contributions from wet and dry
removal of trace gases and particles.
67. Slinn, W.G.N., L. Hasse, B.B. Hicks, A.W. Hogan, D. Lai, P.S. Liss,
K.O. Munnich, G.A. Sehmel, 0. Vittori, 1977. Some aspects of
the transfer of atmospheric trace constituents past the air-
sea interface. Atmos. Environ., to be published.
Overall review of wet and dry removal processes. Numerical
values presented for washout ratios and depositions velocities.
16
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68. Spedding, D.J. 1969. Uptake of sulphur dioxide by barley leaves
at low sulphur dioxide concentrations. Nature. 224:1229-
1231.
Laboratory study. Deposition velocities determined and
related to relative humidity for barley and various inorganic
surfaces.
69. Spedding, D.J. 1972. Sulphur dioxide absorption by sea water.
Atmos. Environ. 6:583-586.
Laboratory study. Deposition velocity over sea water deter-
mined under turbulent atmospheric conditions.
70. Sflderlund, R. and B.H. Svenssen. 1976. The global nitrogen cycle.
Ecol. Bull. (Stockholm) 22:23-73.
Review article.
71. Terraglia, P.P. and R.M. Manganelli. 1966. The influence of
moisture on the absorption of atmospheric sulfur dioxide by
soil. Int. J. Air and Water Poll. 10:783-791.
Laboratory study.
72. Terraglio, P.P. and R.M. Manganelli. 1967. The absorption of
atmospheric sulfur dioxide by water solutions. J_. Air Poll.
Cont. Asso. 17:403-406.
Laboratory study. Details of absorption by water solutions.
No deposition velocities determined.
73. Turner, N.C., S. Rich, and P.E. Waggoner. 1973. Removal of ozone
by soil. J.. Environ. Quality. 2:259-264.
Field and laboratory measurements. Deposition velocities can
be determined from reported resistances.
74. Turner, N.C., P.E. Waggoner, and S. Rich. 1974. Removal of ozone
from the atmosphere by soil and vegetation. Nature 250:486-
489.
Field observations and mathematical simulations. Depositions
velocities can be determined.
75. Unsworth, M.H. and D. Fowler. 1974. Field measurements of sulphur
dioxide fluxes to wheat. In: Atmosphere Surface Exchange of
Particulate and Gaseous Pollutants. ERDA Symposium Series 38,
NTIS No. CONF-740921. 342-353.
Deposition velocities determined.
76. Weinstock, B. and H. Niki. 1972. Carbon monoxide balance in
nature. Science 176:290-292.
17
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77. Whelpdale, D.M. and R.W. Shaw. ]974. Sulphur dioxide removal by
turbulent transfer over grass, snow, and water surfaces.
Tell us 26:]96-205.
Field study. Deposition velocities determined.
18
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APPENDIX
A Note Concerning the Relative Importance
of Deposition versus Transport
in Affecting Air Pollutant Concentrations
in an Industrial Area
March 1977
Ernest W. Peterson, Ph.D.*
Research Meteorologist
Terrestrial Ecology Branch
Ecological Effects Research Division
Corvallis Environmental Research Laboratory
U. S. Environmental Protection Agency
*0n assignment from the National Oceanic
and Atmospheric Administration
to be published in
Atmospheric Environment
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This note will help clarify the roles played by deposition (wet and
dry), and by transport by the mean wind in the removal of pollutants
from the air in an industrial region. It explains why some results
reported in recent literature, for example Davies (1976), should not
have been unexpected. (Davies attempted to relate the wet deposition of
sulfur dioxide in Sheffield, England, to "ground-level atmospheric SC^
concentrations and to various meteorological parameters" but observed
that the washout of pollutants by rainfall had only a minor effect on
the concentration when compared with effects caused by variations in
wind speed.) A simple, heuristic box model indicating factors affecting
spatial-mean pollutant concentrations in industrial air will illustrate
this point.
Consider an industrialized region with a spatial-mean source flux
of pollutants,Q (mass per unit time per unit area), and a uniform wind
blowing through it (with a mean speed ofl^). For simplicity, assume
that the region and flow are homogeneous in the crosswind direction so
that we can use a two-dimensional model. Figure 1 illustrates the
situation and the model.
It can be shown that the relative rate of change with time of the
local spatial-mean pollutant concentration is described by the following
equation:
(1)
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Uhx0
r\i
0
- dx
v
A> at
D D D D
n ODD
vd*X
Figure 1: Diagram of an isolated industrial region illustrating the components of a simple
box model for air pollutant concentration (see text for definitions). . .
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where
is the spatial-mean concentration of pollutants
in the area,
the depth through whi_ch the pollutants are
assumed to be thoroughly mixed,
is the length of the region,
is the pollutant concentration at the upwind
edge of the area,
and (7""1S tne pollutant deposition velocity (assumed
uniform).
The first term on the right side of Equation (1) is the rate of
pollutant input into the region from local sources, scaled by the local
spatial-mean concentration and mixing depth. Its value is high for
heavily industrialized regions and low where there are few sources of
pollutants. The second term represents the removal rate of pollutants
by wind transport out of the region; the last term represents the re-
moval rate by deposition onto the ground.
Obviously, if the pollutant concentration remains nearly constant,
the source term must be of the same order of magnitude as at least one
of the removal terms, i.e. the rate of input must nearly equal the rate
of removal. We will therefore concern ourselves with the relative
magnitudes of the last two terms. The ratio of the deposition term to
the transport term is:
(2)
When this ratio is 0 [10] or larger, deposition is the key removal
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process and changes in deposition velocity will noticeably affect the
pollutant concentration; if the ratio is 0 [10 ] or less, transport is
the main removal mechanism and changes in wind speed, not deposition
velocity, will create the greater effect on concentration.
First, consider the significance of the factor^l""«) . If the
industrial region is isolated, i.e. the air entering the region is
relatively fresh so that V\ is small when compared to jC , then the
value of the factor is nearly unity and the removal of pollutants by
transport could be important. However, if the air entering the region
is itself polluted, i.e. is of the same order of magnitude as
then the wind brings pollutants into the region as well as removing them
so that its net effect as a removal mechanism is diminished. In this
case deposition becomes the dominant means of removal, the wind moving
the pollutants through the region but not contributing to concentration
reduction. For an isolated industrial region the relevant ratio is
thus:
U^_
(3)
For the atmosphere, the mixing height, h, is generally of order 1
kilometer. For very light winds (order 0.5 m/sec or 1 knot), Table 1
gives the value of Ratio (3) for various values of deposition velocity
and size of the industrial area.
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Linear Dimension of Industrial Region
A (km)
1
1
10
100
0.01
0.1
1
10
0.1
1
10
100
1
10
100
Deposition
Velocity
LT (cm/sec)
Table 1. Ratio of deposition removal to transport removal for
very light winds.
A deposition velocity of 1 cm/sec is representative of dry depo-
sition of S02 (whose dry deposition rate is higher than that of most
other pollutants) while a deposition velocity of 100 cm/sec is represen-
tative of wet deposition of sulfates for rainfall of somewhat more than
10 mm/hr (see e.g. McMahon, Denison and Fleming, 1976). As Table 1
shows, removal by deposition is more important than removal by transport
only for very large industrial regions and rather high dry deposition
rates or for moderately large industrial areas in rainy weather, and
this only for exceedingly light winds. Table 2 shows the same ratio of
deposition removal to transport removal for moderate winds (order 5m/sec
or 10 knots).
Linear Dimension of Industrial Region
J (km)
1 10 100
Deposition
Velocity
\J~ (cm/sec)
1
10
100
0.001
0.01
0.1
0.01
0.1
1
0.1
1
10
Table 2. Ratio of deposition removal to transport removal for moderate
winds.
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As Table 2 shows, only during rainy weather in very large indus-
trialized areas is deposition more important than transport in the
removal of pollutants, if the winds are moderate or greater.
These arguments demonstrate that in most industrial regions one
should expect that the winds are a more important factor in the removal
of air pollutants than is deposition on to the ground. Therefore var-
iation in wind speed will usually have a greater effect on pollutant
concentration than variations in deposition velocity. Removal of pol-
lutants by deposition becomes comparable in magnitude to transport only
when considering large regions (greater than 100 kilometer in extent).
Thus the results reported by Davies (1976) could have been anticipated.
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References
1. Davis, T. D. (1976) Precipitation scavenging of sulphur dioxide in
an industrial area. Atmospheric Environment, 10, 879-890.
2. McMahon, T. A., P. J. Denison, and R. Fleming. (1976) A long-
distance air pollution transportation model incorporating
washout and dry deposition components. Atmospheric Environment,
10, 751-761.
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