Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 29
~ Atmospheric Processes
Physical and chemical processes in the atmosphere deter-
mine the fates of emissions of precursor gases and hence
the exposures of primary receptors to pollutants. In
this chapter we review current understanding of these
atmospheric processes in light of the need to characterize
relationships between emissions and deposition. For
convenience, we consider these atmospheric processes as
occurring in a sequence of clearly defined steps (Figure
2.1). The separate processes are as follows:
I. Transport and mixing
II. Chemical reactions in the homogeneous gas phase
III.
IV.
V.
(dry reaction)
Dry deposition
Attachment
Chemical reactions in the homogeneous aqueous
phase (wet reaction)
VI. Wet deposition
Heterogeneous chemical processes may occur between gases
and liquids adsorbed on solid surfaces, although these
are generally considered to be less important in the
development of acid deposition than the homogeneous
processes. We therefore do not consider heterogeneous
processes in this report.
Each of the separate processes takes a certain amount
of time; the sum of the processing times along any
particular pathway is the source-receptor transport time
for the pollutant along that pathway. The processing
times are extremely variable, depending strongly on
meteorological processes, ambient conditions, and the
presence and concentrations of various chemical species.
Many of the steps are reversible, so that itinerant
29
OCR for page 30
30
· UNREACTED POLLUTANT
O REACTED POLLUTANT
OCONDENSED WATER
f
_ ~WET
15E ~ _m REACTION
A1TA9HMENT~
~-_
TRANSPORT ~ /
AND MIX~
~ _
art
DRY
- REACTIONS
m- ". ~
DRY
OPPOSITION
PRECIPITATION; !. \'.'\ \
FIGURE 2.1 Atmosphenc pathways leading to acid deposition.
pollutant molecules may undergo repeated cycling with
corresponding lengthening of the effective processing
times.
TRANSPORT AND MIXING
The transport of pollution by normal atmospheric advec-
tion and mixing is a vitally important influence on
deposition phenomena, and it works both directly and
indirectly. Transport phenomena directly determine where
the pollution goes before it is deposited and therefore
affect the atmospheric residence time of pollutant
materials. Dry deposition, for example, is often limited
by the speed at which the atmosphere can vertically
transport pollution to the proximity of the surface.
Transport can indirectly affect pollutant deposition
in a number of ways. Transport processes, for example,
bring pollution into contact with storm systems, where
precipitation scavenging occurs. Transport also can
introduce pollutants into environments more (or less)
conducive to transformation chemistry. This complex of
interactions links transport with the other processes
shown in Figure 2.1.
OCR for page 31
31
It has been the usual practice to divide atmospheric
transport processes into two categories. The first,
usually termed advection, pertains to the net motion of a
parcel of air as it drifts with the mean wind. The
second category, diffusion, pertains to the intermixing
of the parcel with its surroundings. Historically the
distinction between atmospheric advection and diffusion
has not been totally clear. Quite often, for example,
atmospheric transport models incorporate diffusionlike
terms to account for time-averaging of meandering plumes,
when in fact the physical processes described have little
to do with actual intermixing of materials. Similar
treatments often arise in transport models using grids to
approximate the desired solutions numerically. Advection
processes occurring on scales smaller than the grid
spacing escape resolution by the system and thus are
often lumped in terms of pseudo-diffusion processes (see
Appendix B).
Such approximations are often inescapable. They do,
however, contribute significantly to the uncertainty in
our ability to model atmospheric pollution, and they
obscure the meaning of diffusion in such processes. It
is therefore important to remember that advection and
mixing are indeed distinct transport phenomena that can
lead to different behavior of parcels of polluted air.
The distances associated with pollution transport
obviously depend strongly on how long the pollutant
resides in the atmosphere and thus is available for
action by the advection-diffusion process. In this
context it is important to note that atmospheric residence
times for typical power plant pollutants {sulfur com-
pounds, for example) are rather uniformly distributed;
some pollutant molecules are deposited from the atmosphere
relatively quickly and thus at locations near the source,
whereas others are deposited more slowly and thus much
farther away. On the basis of the best current estimates,
it is not unusual for the transport distance of a given
pollutant molecule to be of the order of hundreds or even
a thousand kilometers. It also is not unusual, however,
for a molecule to be deposited close to the source. From
this one can conclude that while long-range transport
processes certainly are important, shorter-range
phenomena are occurring as well.
Another factor that must be taken into account in
assessing transport is the height at which pollutants are
released into the atmosphere. One approach to local air-
quality problems has been to increase the height of
OCR for page 32
32
stacks in accordance with the notion that the higher the
point of release the less the pollutants would affect the
surrounding area. This approach to pollution control was
applied to large new plants where taller stacks were con-
structed. At older plants, relatively short stacks were
replaced with taller ones. A number of factors, such as
meteorology and terrain, influence how the height of an
individual stack affects dispersal of a given pollutant,
so it is difficult to evaluate the effectiveness of tall
stacks for dispersal in general.
Recently Koerber (1982) studied a set of 62 coal-fired
power plants in the Ohio River Valley. He developed a
measure of the potential for long-range transport that
involved physical stack height, plume rise, and mixing
height. Figure 2.2 shows the temporal trend in Koerber's
parameter between 1950 and 1980. The implication of this
and other work is that stack heights must be taken into
account when assessing source-receptor relationships
involving long-range transport.
Because of cumulative uncertainties, the trajectories
and times associated with long-range transport are much
more difficult to estimate than their shorter-range
counterparts.
Early, very crude attempts to simulate
long-range phenomena simply employed local wind roses and
straight trajectories from the sources in question. The
obvious deficiencies associated with this approach
prompted further efforts to develop curved-trajectory
simulations, which were driven by conceptualized, time-
evolving wind fields.
The curved-trajectory approaches, while representing a
major advancement over their straight-line predecessors,
suffered from two major disadvantages. The first of
these was that the data from which the wind fields were
derived were usually extremely sparse in both time and
space--a problem that becomes particularly severe under
complex meteorological conditions involving fronts and
storm systems. Although a variety of sophisticated
interpolation techniques has been advanced subsequently
to offset this problem, the poor coverage of meteoro-
logical data in both space and time remains particularly
troublesome.
The second major problem associated with these types
of trajectory approaches is caused by mass motions of air
vertically and vertical wind shear, i.e., the dependence
of wind speed and direction on altitude. Early trajectory
simulations, based on constant-altitude wind fields, soon
were replaced by layer-averaged or constant-pressure
OCR for page 33
33
o
0.3~
. _
~ O 0.2
Q) A
en ~
~ ~ 0.1
.=
a)
~ 0
Weighted by
SO2 emission
/_'
_
1 _
Weighted by I/
MW rating l,/
117
_;
If
1950 1955 1960 1965 1970 1975 1980
Year
FIGURE 2.2 Trend in long-range transport potential for 62 sources in the Ohio River
Valley. SOURCE: Koerber (1982~.
surface versions to overcome this disadvantage par-
tially. On the basis of thermodynamic arguments, it is
expected that vertical motions of air parcels should
adhere rather closely to constant-entropy surfaces in the
atmosphere, and from this a few ~isentropic~ trajectory
simulations have evolved as well. However, vertical
motions caused by the heat released or absorbed during
cloud formation are not taken into account by either
method.
Although curved-trajectory simulations can produce
rather reliable results in simple meteorological situa-
tions, they are fraught with uncertainty when conditions
become complex, such as near frontal systems. Some idea
of this uncertainty may be gained from Figure 2.3, which
shows the results of two different calculations of a
trajectory under the same conditions in the vicinity of a
frontal storm. One calculation (solid curves) uses the
assumption of isentropic transport, while the other
(dashed curves) employs isobaric transport. After 24
hours, the calculated positions of the two hypothetical
air parcels are several hundred kilometers apart (also
see Chapter 3 on the treatment of transport and mixing in
models as well as Appendix B).
OCR for page 34
34
1""'
,,, mu;
\~\,
,' ~ ~ ~N ,}
, · ~ ~ ~
FIGURE 2.3 Calculated plume trajectories in the vicinity of frontal systems for
24 hours after release. Solid plumes were calculated on the assumption of isentropic
vertical motion; dashed plumes were calculated on the assumption of isobaric motion.
Shading indicates area of precipitation. SOURCE: Adapted from Davis and Wendell
(1977~.
The difficulties associated with wind shears and
vertical motions could be largely overcome if the
vertical motions were indeed known. Several weather
prediction models produce such information, albeit
prognostic in nature, rather coarse in scale, and
dedicated more to upper regions of the troposphere than
to the planetary boundary layer. In addition, a few
mesoscale dynamic models currently exist that can supply
initial estimates, at least, of information of this
type. The application of such techniques for pollution
trajectory simulation has been comparatively limited
owing to the complexity of the modeling process and the
expense of the simulations. A summary of several of the
trajectory simulation techniques discussed here appears
in Chapter 3 (Table 3.2). They have been applied to form
composite regional pollution models.
A major factor contributing to the uncertainty in
long-range trajectory simulations stems directly from our
current inability to measure long-range transport.
Several balloon studies have been attempted, but they
have been less than satisfactory because of technical
difficulties, the balloons' supposed inability to track
OCR for page 35
35
vertical motions exactly, and statistical problems
associated with tagging a stochastic system with too few
units. Chemical tracers have not been particularly
successful to date owing to detection difficulties over
large distance scales. Tracer techniques are evolving
rapidly, however, and it is not unreasonable to expect
some highly significant results to emerge from experiments
with tracers in the next 5 years. For example, during
the summer of 1983 six releases of tracers are to be made
from Ohio and Ontario under the Cross Appalachian Tracer
Experiment (CAPTEX). A wide arc of measurement sites
will be set up over 600 km downwind of the releases.
This experiment will be the first step in a long-range
tracer program.
An important feature of the long-range transport of
air pollutants is that the plumes from individual sources
may become so dilute and so thoroughly mixed far downwind
of major source areas that the attribution of specific
parcels of polluted air to specific sources is imprac-
tical. In these cases, the sources contribute pollutants
to air masses that may be considered to be entrained in
synoptic-scale meteorological systems. The classic
example of mixing occurs in large stagnant air masses
that occur most frequently in summer in the eastern
United States (see Chapter 4). The motion of air masses
on the synoptic scale may be important for understanding
acid deposition in areas remote from major source regions.
The average flows across North America are shown in
Figure 2.4, which illustrates that the region in which
acid deposition is currently thought to be an environ-
mental problem is also a region of intense interaction
between tropical marine and arctic air masses.
CHEMICAL TRANSFORMATION
During transport through the atmosphere, SO2, NOX,
hydrocarbons, and their oxidation products participate in
complex chemical reactions that transform the primary
pollutants into sulfates and nitrates. The transformation
processes are important because, as we discuss later,
deposition of the primary pollutants and that of their
transformation products are governed by different
processes.
There are many chemical pathways through which SO2
and NOX in the atmosphere can be transformed "oxidized)
into sulfate and nitrate compounds, including homogeneous
OCR for page 36
36
Hi\
r~
, .S.trono~` _ ~ w~ . ~- WN
\
\
~ ~..,w'~'
~,\\~
,~
/
\
\.
\
~ ~ ~< v )Arctic a~rstrearri~ \ ~~
~ r
N.~..\ \N
a'
-
I ~ ~ \ ~
westerli-~l ~ ~ ~ ~
W~
:,Wopim a Mr~/~,'.'~
20~.
~ TOW'
. _
.
-con
FIGURE 2.4 Surface flows across North America, illustrating the area of complex
entrainment and mixing of air masses in the eastern portion of the continent.
SOURCE: Bryson and Hare (1974~.
processes that take place
droplets or heterogeneous
the surfaces of particles
indicates the pathways by
transformed into gaseous
in the gas phase and in liquid
processes that take place on
or droplets. Figure 2.5
which SO2 and NOx are
and aqueous-phase acids. Field
studies indicate that the relative importance of gas- and
liquid-phase reactions depends on meteorological
conditions, such as the presence of clouds, relative
humidity, intensity of solar radiation, and the presence
and concentrations of other pollutants.
A comprehensive review of homogeneous gas- and
solution-phase atmospheric chemistry associated with acid
deposition is presented in Appendix A. The appendix
OCR for page 37
37
o
~n
I
~n ~
._ ._
, .O
o '
3
CD
r~
o
I
O Ce,
I
_
~ '
._ ~
~_
~ o -
o ~ ' O
·- x Q -
o o
=5 o
X ~ ~ ~
O I I O
ase4d sea
o
z
I
.=
.O
. _
z
4e
-
cn
/! ,
-
O _
I O
_ _
C
a,
._ ~
X
o X
o
Q ~
C ~o
O ~n
· _ lD ~· _
~o ~
_ . _
.-
X
O I O C:
O O _
I ~ O
I I" ~r
Z I
- Z Z O
~ ~ - Z
4-~L ~o~ -
4 - 4 -
3 ~ ~ s
.
~n-
._ ~._
~ V} C ~
E ,c E ~
._ ._ ._ 4 -
c c c 'c
0 0 0
E ,c E `~,
E ~ E ~
<: ~ s~
c~
o
s
C ~
_
c
~ 0
x
0 ~
~ 4 -
c
,o
4 -
ca
o o
0. N
~ I
c
~ -
S 11
_
o ._
. _ ~
~ _
~ C
t0 e
~ C
~ _
o~a ~u!e~ 'slaldolp pnolo
ase~d snoanb~
I
ct
._
c'
ce
c'
·_.
_ c~
~0
O ~
~ ct
c
-
o'~ <°n
c/) I
I -
o
c
o
c~
CO O
Z
_ ,_
,,, V'~
Q ~ o
~._
~ C ~
._,
+ C . -
- o
~ - C
C
o `+
·- c
T
C o
4- 0
~ C 4-
O ·- C
V)
C ~ 4 -
C) .O
E o 4
. ~ ~
o . ~ ,,,
o
V} o o
I I
cr
O
o
.~,
Ct
.
~3
Ct
3
c~
OCR for page 38
38
includes detailed descriptions of alternative oxidation
pathways and analyses of reaction rates. General
descriptions and conclusions, drawn from this material,
are presented below.
Homogeneous Gas-Phase Reactions
SO; and NOX have been observed in the atmosphere to be
oxidized through homogeneous gas-phase reactions at rates
of a few and 20 to 30 percent/in, respectively (Step II in
Figure 2.1). The observed rates cannot be explained by
direct oxidation by atmospheric oxygen, reactions that
occur too slowly for typical concentrations of pollutants
and, in the case of SO2, in the absence of catalysts.
Similarly, although there are direct pathways to the
formation of sulfuric and nitric acids beginning with
absorption of solar radiation by SO2 and NC2, respec-
tively, these processes also appear to be unimportant
under typical conditions in the troposphere.
According to current understanding, most of the
As-phase chemistry in the lower atmosphere that results
in oxidation of S02, nitric oxide (NO), and nitrogen
dioxide (NC>) entails reactions with a variety of
highly reactive intermediate s -excited molecules, atoms,
and free radicals (neutral fragments of stable
molecules)--that are generated in reactions initiated by
the absorption of solar radiation by trace gases. The
most important of the intermediates for gas phase
oxidation appears to be the hydroxy radical, HO.
The hydroxy radical can be formed in the troposphere
by a number of reactions. ~ ~~ ~
A common process begins wltn
dissociation of NCk by absorption of sunlight, which
forms a highly reactive oxygen atom that combines quickly
with a diatomic oxygen molecule to form the triatomic
oxygen molecule, ozone (O3). Ozone may be photodis-
sociated, yielding an electronically excited diatomic
molecule of oxygen and an electronically excited oxygen
atom, O(1D), which reacts readily with a water molecule
to form HO. The hydroxy radical, unlike many radicals
that are fragments of complex molecules containing
carbon, does not react readily with molecular oxygen; HO
survives in the atmosphere to react with most impurity
gases, such as hydrocarbons, aldehydes, NO, NO2, SO2,
and carbon monoxide (CO). Reactions between HO and
several impurity gases produce additional classes of
reactive transient species, which, in turn, react with
OCR for page 39
39
atmospheric constituents to form additional reactive
species. For example, reactions of HO with CO and
hydrocarbons produce peroxy radicals; peroxy radicals
react rapidly with NO to form NO2 and alkoxy, acyloxy,
and other HO radicals.
The net result of all of these interactions is a large
number of chemical pathways for oxidation of SO2 and
NOx to sulfuric acid (H2SO4) and nitric acid (HNO3),
respectively, many of which depend initially on the
formation of HO. A sequence of these reactions can be
constructed in which a single HO radical may oxidize CO,
hydrocarbon, or aldehyde, followed by oxidation of NO to
NO2 accompanied by production of additional HO radicals.
Repeated cycling of the sequence results in continued
oxidation of NO to NO2 and relatively constant concen-
trations of HO.
There are a number of gaseous-phase chemical reactions
between SO2 and reactive transient species that may
lead to formation of H2SO4; these reactions, along
with currently accepted values for the reaction rates,
are listed in Appendix A. While many of the rate con-
stants are known only with an uncertainty of 50 percent,
it appears as if the most important reaction is that
between SO2 and HO, yielding HOSO2.
TV 1clence i s goon
that this reaction ultimately leads to the generation of
sulfuric acid, and a number of pathways for this subse-
quent reaction have been explored. Which of these
pathways is most important is still unknown, but it is
likely that the oxidation of SO2 by HO is a chain-
propagating reaction.
The principal agents for oxidizing NO to NO2 are
ozone and peroxy radicals, whereas NO2 is oxidized to
HNO3 by a well-characterized reaction with HO (Appendix
A).
According to current understanding, then, the rates at
which sulfuric and nitric acids are formed in homogeneous
gas-phase reactions depend on ambient concentrations of
the hydroxy radical. Direct measurement of HO in the
atmosphere is difficult, but both theoretical and
experimental estimates are available from which to
estimate rates of conversion from SO2 and NO2 to
H2SO4 and HNO3, respectively. Using the rate
constants listed in Appendix A, we find that for high
concentrations of HO--characteristic of polluted summer
sunny skies--SC will be converted to H2SO4 by
reaction with HO at a daily averaged rate of about 0.7
percent/in (16.4 percent per 24-h period), whereas NOx
OCR for page 44
44
both nitric and sulfuric acids were formed rapidly in the
cloud, although the oxidizing agent remained unidentified
because of weaknesses in the analytical methods.
Data from another experiment are now available showing
an appreciable rate of conversion of SO2 to H2SO4
at night in clouds over coastal waters, indicating an
oxidation process other than reaction with the hydroxy
radical, which is present in significant concentrations
only in daytime.
The importance in atmospheric chemistry of aqueous-
phase processes taking place in clouds is illustrated
theoretically in Figure 2.7, which gives the results of
calculations that combine homogeneous gas-phase chemistry
with the current picture of aqueous reactions (Environ-
mental Research & Technology, Inc., and MEP, Inc. 1982).
The figure shows the progress of oxidation in clear air
(beginning with NO and NO2 concentrations of 10 ppb,
concentration of reactive hydrocarbon vapors of 200 ppb,
SO2 concentration of 5 ppb, and SO4 concentration of
2 ug/m3) and the effects of introducing a cloud with
1 g/m3 of liquid water at 1400 h. In theory the inser-
tion of cloud water causes dramatic decreases in atmo-
spheric concentrations of H2O2, HNO3, SO2, and SO4. The
behavior of NO, NO2, O3 and peroxyacetylnitrate (PAN) was
not strongly influenced by the presence of cloud water.
The example demonstrates that clouds have the potential
to dominate chemical interactions involving water-soluble
or water-scavengable constituents. Field experiments are
required to determine if this dramatic effect actually
occurs in the atmosphere.
DEPOSITION
Dry Deposition
The term dry deposition is used to denote a variety of
processes by which pollutant gases and aerosol particles
reach the Earth's surface, including the surfaces of both
living and inanimate objects on the ground (vegetation
and buildings, for example). The processes depend on
concentrations of the pollutants and small-scale meteoro-
logical effects near the surface as well as on the
characteristics of the receiving surface.
Superficially, dry deposition seems to be almost
trivially simple in comparison with other aspects of the
relationships between emissions and deposition. Dry
OCR for page 45
45
Q
-
-
Q 120 _
-
z
O 80 _
of
UJ
of
8
40 _
12 _
81-~02
NO \ \
4 - \ \
01 II ~ l\ ,
810 12 14 16
TIME (hours)
o3/
O ~1 1 1 1
8 10 12 14 16
TIME (hours)
12 _
o
lo:
8 _
4 _
of
UJ
of o
O Q
Ca) V
-
Q 6 _
Q
O ~ _
/ I
HNO3/
/ 1
I,~ I 111
10 12 14
TIME (hours)
D
16
8 10 12 14
TIME (hours)
Z=4 2
O O Z O
8 10 12 14 16 O 8 10 12 14 16
TIME (hours) TIME (hours)
FIGURE 2.7 Theoretical calculations of gas and aerosol concentrations as a function
of time for gas-phase reactions only (solid line) and with the introduction of cloud
water (dashed line) at 1400 hours. SOURCE: Environmental Research & Technology,
Inc., and MEP, Inc. (1982~.
deposition takes place at the Earth's surface and thus is
inactive in the volume of the atmosphere in which
chemical transformation and processes leading to wet
deposition occur.
In fact, however, dry deposition is incompletely
understood. Uncertainties in dry deposition may be an
important source of error in today's regional modeling
efforts.
OCR for page 46
46
There are several reasons for the current uncertainties
in understanding dry deposition (Appendix C). The first
is that dry-deposition rates are extremely difficult to
measure. Although a number of possible techniques exist
(Hicks et al. 1981) and considerable effort has been
devoted to developing appropriate methods for measuring
fluxes to surfaces, the base of high-quality data is
still distressingly small. Furthermore, the more reliable
data that do exist tend to have been obtained under
experimentally convenient conditions (for example, high
pollutant concentrations, uniform terrain) and thus
reflect only a small subset of the potentially important
environmental conditions.
A second reason for uncertainty in dry-deposition
rates is a consequence of the complexity of the physical
processes in the atmosphere. As indicated in Figure 2.8,
several mechanisms convey pollutants to the surface, and
it is often not clear which processes dominate under when
conditions. Especially important in this regard are the
near-surface mechanisms for aerosol particles, such as
inertial impact, phoresis, and electrical effects. Uncer-
tainties in this area are currently substantial,
especially for deposition to surfaces of vegetation.
The third reason arises from uncertainties In the
characteristics of the substrate on which materials are
deposited. Contrary to the superficial view that dry
deposition is ourelv a surface phenomenon, phenomena both
at and in the substrate can Play a role in determining
the deposition flux.
It is well known, for example, that
stomata! openings on leaf surfaces influence the deposi-
tion of eases such as SON and ozone. Soils and
building materials have been shown to "saturate" with
depositing gases. Re-emission of sulfur compounds from
plant surfaces has been detected. All of these results
render the concept of a simple boundary condition
approach to dry deposition somewhat questionable; the
corresponding uncertainties are again large.
These difficulties combine to give a number of widely
varying estimates for the temporal and spatial scales of
dry deposition of specific pollutants. As a rule of
thumb, for sulfur and nitrogen compounds at least, dry
deposition is taken on the average to be about as effec-
tive as wet deposition in pollutant removal. About one
third of sulfur emissions is transported out of the
continent. Thus roughly one third of northeastern
emissions is assumed to be dry-deposited on the North
American continent (see the section in Chapter 3 on
material balance).
OCR for page 47
47
4,
o
[ ' ---
C)
._
a)
o
Cat
o _
._ ~
~ o
· _ U)
~ C
~i.
E
~ .
1- ·_
_' Ct5
.O o
~ ._
V,
C' ~
~ An.
U.! ·-
_ ~
a,
C'--
o o \
Cow ~
-
o ~ o
in . _ ~
~ 4 -
Cal o
~C ~
~ CO CO
_ ._
~ C ~
Cal
~ o ~
o ~ o
me
~ 30~JUnS 3~00V lH913H
. 1 I c
1 it)
ix,
N
z
,,0
Hi:
J
o
:^
U'
3
Cal
Go
OCR for page 48
48
There are also studies, however, that obtain a scale
length for dry deposition in excess of 104 km for some
species (Slinn 1983), strongly suggesting interaction
with global circulation patterns. This work is in
concordance with observations of deposition in Greenland
and the Arctic, as well as the general haze buildup in
the northern hemisphere. Until the extent of such
long-range transport is more thoroughly understood, the
modeling of dry deposition is likely to remain highly
uncertain.
Wet Deposition
The term wet deposition encompasses all processes by
which atmospheric pollutants are transported to the
Earth's surface in one of the many forms of precipita-
tion: rain, snow, or fog, for example. Wet deposition
therefore involves attachment of pollutants to atmo-
spheric water and includes chemical reactions in the
aqueous phase as well as the precipitation process
itself. Aqueous phase chemical processes (step V in
Figure 2.1) have been discussed previously; here we
address only the physical processes by which pollutants
first become attached to water droplets and then are
deposited in wet form (also see Appendix C).
A rough indication of the significance of wet deposi-
tion on a continental scale can be obtained from a map of
annual precipitation in the United States (Figure 2.9).
From the distribution, one would expect that wet depo-
sition would be an important contribution to total
deposition in the East and in the Pacific Northwest. In
regions with frequent precipitation, wet deposition also
becomes relatively more important than dry deposition far
away from sources, where SO2 is depleted and sulfate
particles are a significant fraction of the atmospheri
sulfur burden. This also appears to be the case in
remote areas of southeastern Canada.
Attachment Processes
CThe physical processes by which pollutants become
attached to droplets and other falling hydrometeors such
as ice crystals (step IV in Figure 2.1) have been the
subject of extensive research, and a number of technical
OCR for page 49
OCR for page 50
50
reviews of current knowledge in this area are available
(see, for example, Slinn 1983).
The most important attachment process under most
inrcloud conditions is undoubtedly nucleation. Nuclea-
tion is a kinetic process in which water molecules
condense from the vapor phase onto a suitable surface.
Dust and pollutant aerosol particles provide such surfaces
in the air. The result is a cloud of droplets (or ice
crystals) containing the pollutant. The droplets may
grow by the same process (condensation) or may lose water
by evaporation.
The tendency of water vapor to condense on aerosol
particles depends on the characteristics of the particles
and the degree of saturation of the air with water vapor.
As a consequence the aerosol and associated cloud par-
ticles compete for water molecules. Some particles will
capture water with high efficiency and grow substantially
in size. Others will acquire only small amounts of water,
whereas still others will remain essentially "dry"
elements. In addition, some particles may be effective
for nucleation of ice crystals, whereas others will be
active only for the formation of liquid water. The
nucleating capability of a particular aerosol particle is
determined by its size, its morphological characteristics,
and its chemical composition. Acid-forming particles, by
their very nature, are chemically competitive for water
vapor and thus tend to participate actively as condensa-
tion nuclei for liquid water. This attribute enhances
their propensity to become scavenged early in storms and
has a significant effect on the nature of the acid-
precipitation formation process.
Diffusional attachment, as its name implies, results
from diffusion of the pollutant molecule or particle
through the air to the surface of a water droplet. The
process may be effective in the case of both suspended
cloud elements and falling hydrometeors. It depends
chiefly on the magnitude of the molecular (or Brownian)
diffusivity of the pollutant; because diffusivity is
inversely related to particle size, this mechanism is
less important for larger particles. For practical
purposes, diffusional attachment can be ignored for
particles with radii of more than a few tenths of a
micrometer.
The motion of a molecule or particle to the surface of
a water droplet by diffusion depends on the gradient in
the concentration of the pollutant in the vicinity of the
surface. Thus, if the cloud or precipitation droplet can
OCR for page 51
51
accommodate the influx of pollutant readily (for example,
the pollutant is highly soluble in water), it will
effectively depopulate the adjacent air, thus making a
steep concentration gradient and encouraging further
diffusion to the droplet.
particle "bounce off" or low gas solubility) the droplet
cannot accommodate the pollutant, further diffusion to
the droplet will be discouraged. If the cloud or
precipitation droplet supplies the pollutant to the local
air through an outgassing mechanism, the concentration
gradient will be reversed and diffusion will carry the
pollutant away from the droplet. In general, diffusional
attachment processes are sufficiently well understood to
allow their mathematical description with reasonable
accuracy.
Inertial attachment arises by virtue of the facts that
pollution particles and scavenging droplets are constantly
in motion and that both have finite volume and mass. The
most important example of inertial attachment is the
impaction of aerosols by falling hydrometeors. In this
case, the hydrometeor falls under the influence of
gravity, sweeping out a volume in space. Collisions
occur between the falling hydrometeors and some aerosol
particles, resulting in attachment.
The effectiveness of impaction depends on the size of
both the aerosol particle and the hydrometeor; mathe-
matical formulas exist to estimate the magnitudes of
these processes. Impaction generally becomes unimportant
for aerosols less than a few micrometers in size. In
this context it is interesting to note that a two-stage
capture mechanism can exist, in which a small aerosol
first grows through nucleation to form a larger droplet
that is then captured by inertial attachment. This
two-stage process, called accretion, is an essential
factor in the generation of precipitation in clouds and
has been postulated as an important mechanism in
scavenging pollutants below clouds.
A second example of inertial attachment is turbulent
collision. In this case, the particles and scavenging
elements, subjected to a turbulent field, collide because
of dissimilar dynamic responses to velocity fluctuations
~ "~ 1 ~__1 _: _
If for some reason (such as
-
~ ..~ _~ ~_ ~. This scavenging mechanism is thought
to be of secondary importance and has received compara-
tively little attention in the literature, although some
recent theoretical analyses have suggested it to be
significant for droplets and particles of specific sizes.
OCR for page 52
52
10°
10-1
in
-
LL
us 10 2
LL
a:
10-3 _
10-4
10-3
\
~G reenf ield gap
\
I
it
Diffusional \ - _
Attachment `` _ ~~'
Inertial
Attachment
-
1o-2 lo-l 1.0 10
RADIUS OF AEROSOL PARTICLE (,um)
FIGURE 2.10 Theoretical scavenging efficiency of a falling raindrop of diameter
0.31 mm as a function of aerosol particle size. SOURCE: Adapted from Pruppacher
and Klett (1978~.
Although the diffusional and inertial attachment
processes are efficient for capturing very fine and very
coarse particles, respectively, neither mechanism is
effective for particles in the range of 0.1 to 5 Am.
The resulting minimum in capture efficiency as a function
of particle size, shown schematically in Figure 2.10, is
known as the Greenfield gap.
Depending on circumstances, there are several
additional attachment mechanisms (including accretion via
the two-stage nucleation-impaction mechanism mentioned
earlier) that can operate in the Greenfield gap. The
processes include turbulent deposition, electrical
attraction, and phoretic effects (see Appendix C for
details). As indicated by the dashed lines in Figure
2.10, these mechanisms can significantly relieve the
Greenfield effect under appropriate circumstances
(Appendix C).
OCR for page 53
53
From this discussion, it should be evident that the
aggregate of possible attachment processes comprises a
complex system that is difficult to characterize mathe-
matically. This complexity, combined with the processes
of formation and delivery of precipitation that occur
both consecutively and concurrently, provides a major
source of uncertainty in current models of regional
pollution transport and deposition.
REFERENCES
Bryson, R.A., and F.K. Hare. 1974. Climates of North
America. World Survey of Climatology, Vol. 11. New
York: Elsevier Scientific Publishing Company.
Davis, W.E., and L.L. Wendell. 1977. Some Effects of
Isentropic Vertical Motion Simulation in a Regional
Scale Quasi-Lagrangian Air Quality Model. BNWL-2100 PT
3. Richland, Wash.: Battelle Pacific Northwest
Laboratories.
Environmental Research & Technology, Inc., and MEP, Inc.
1982. Models for Long Range and Mesoscale Transport
and Deposition of Atmospheric Pollutants. Phase I:
Modeling System Design. Report SYMAP-101. Toronto,
Ontario: Ontario Ministry of Environment.
GCA Corporation. 1981. Acid Rain Information Book. Final
Report. DOE/EP-0018. Prepared for the U.S. Department
of Energy. Springfield, Va.: National Technical
Information Service.
Hicks, B.B., M.L. Wesely, and J.L. Durham. 1981. Critique
of methods to measure dry deposition; concise summary
of workshop. Presented at the 1981 National Meeting of
the American Chemical Society, Atlanta. Ann Arbor:
Ann Arbor Scientific Publications.
Koerber, W.M. 1982. Trends in SO2 emissions and
associated release height for Ohio River Valley Power
Plants. In Proceedings of the 75th Annual Meeting of
the Air Pollution Control Association, paper 82-10.5,
New Orleans. Pittsburgh: Air Pollution Control
Association.
Martin, L.R. 1983. Kinetic studies of sulfite oxidation
in aqueous solutions. In Acid Precipitation:
NO, and NO2 Oxidation Mechanisms:
Considerations. Ann Arbor, Mich.:
Scientific Publications. In press.
Atmospheric
Ann Arbor
SO2,
OCR for page 54
54
Pruppacher, H.R., and J.D. Klett. 1978e Microphysics of
Clouds and Precipitation. Boston: D. Reidel
Publishing Company.
Sling, WeGeNe 1983e Precipitation scavenging. In
Atmospheric Sciences and Power Production. D.
Randerson, ad. Washington, D.C.: UeSe Department of
EnergY,-In press.
Representative terms from entire chapter:
cloud water
