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OCR for page 183
Wet and Dry Surface Deposition
of Air Pollutants and Their Modeling
BRUCE B. HICKS
The net rate of delivery of trace gases to receptor surfaces is largely determined
by the chemical affinity of surface materials for the gas in question. If molecules
of the gas are captured efficiently or react quickly upon contact with the
surface, then high surface flux densities can be expected. Large particles are
deposited by gravitational settling and by inertial impaction; the efficiency of
their capture depends on their shape and the structure of the surface at the
point of impact. Small, submicron particles have difficulty penetrating the
quasilaminar air layer adjacent to smooth surfaces, but once they contact the
surface they are efficiently retained by van der Waals forces. All particles are
susceptible to electrostatic forces that will encourage deposition if either the
particles or the receptor surfaces carry an electrical charge. The presence of
temperature and humidity gradients near the surface can also promote or
hinder the deposition of particles. Most of these matters have been investigated
in studies of deposition to relatively uniform surfaces of pipes or plates in
wind tunnels. Extrapolation to the real-world case of complicated surface shapes
is sufficiently uncertain that quantitative statements cannot be made. The
role of rainfall and other kinds of atmospheric precipitation is equally com-
plicated. Current ecological concern about the acidity of rain has focused
attention on adverse effects associated with precipitation chemistry, but it
must be recognized that rainfall provides a natural cleansing mechanism in
many instances. In highly polluted areas, it is possible that the major effect
of rainfall will be to remove some previously deposited pollutants from exposed
surfaces and promote the subsequent deposition of soluble gases and small
particles to those areas j such as crevices) that remain moist.
Bruce B. Hicks is Director, NOAA Atmospheric Turbulence and Diffusion Laboratory,
Oak Ridge, Tend.
This work was supported in part by the Multistate Atmospheric Power Production
Pollution Study and sponsored by the U.S. Environmental Protection Agency.
183
OCR for page 184
84
CONSERVATION OF HISTORIC STONE BUILDINGS
There has been considerable recent work on the transfer of air pollu-
tants to receptor surfaces. Much of this work has been associated with
concern about potential ecological effects of the increases in atmo-
spheric sulfur loading expected to accompany an increase in the use
of coal as an energy source. Current fears about acid rain have con-
centrated attention on chemical deposition by precipitation, but there
is a continuing awareness that dry deposition processes are capable of
delivering similar quantities of material even to areas fairly distant
from pollution sources. With near sources, such as within cities where
pollution levels are high, we must expect dry mechanisms to deliver
at least as much material to exposed surfaces as wet, especially when
the surfaces in question are sloping or are somehow protected from
the direct impact of precipitation.
The results of modern research on ecological factors associated with
chemical deposition are not usually transferable to the case of stone
weathering because the ecological work places strong emphasis on
matters related to biology. However, a small component of these stud-
ies seeks to identify and formulate the mechanisms that control the
rates of deposition of airborne pollutants. This work combines theo-
retical and laboratory research with field investigations of pollutant
fluxes to provide a comprehensive understanding of the processes that
determine the dry fluxes of many trace gases and small particles to
uniform, natural surfaces. In the present context of deposition to stone-
work, the recent ecologically oriented work allows us to reconsider
some of the formulations developed in earlier chemical engineering
studies of the deposition to flat plates and to the surfaces of pipes.
Likewise, recent work on the chemistry of rainfall has tended to
concentrate on its acidic properties and their possible changes with
both time and space, since these factors are of definite ecological im-
portance. These studies have provided greatly improved understanding
of the processes that combine to produce polluted rain and have given
workers a much better fee] for the natural variability of precipitation
chemistry. But before discussing details of the wet and dry deposition
processes that are capable of delivering pollutants to exposed stone-
work, it is useful to consider the mechanisms that contribute to de-
terioration and hence to identify the specific deposition phenomena
that are likely to be most important. The mechanisms are:
.
· Physical Mechanisms The presence of water at the surface is
known to be a key factor in promoting the fracturing and erosion of
stone. Water penetrates pores and cracks and causes mechanical stresses
OCR for page 185
Surface Deposition of Air Pollutants Affecting Decay
both by freezing and by the hydration and subsequent crystallization
of salts.~.23.4
· Chemical Mechamsms Some deposited chemical agents wiD react
with stone surfaces. Sulfur compounds have been indicted as the most
critical factors in this regard, 5 mainly because they are often acidic
and can have high concentrations in city and suburban air; but nitrogen
compounds should be considered as well. Fluxes of trace gases (e.g.,
sulfur dioxideJ can be high, especially when promoted by biological
activity like that mentioned below. Dissolution by chemical reaction
with contaminants contained in precipitation is one of the most fa-
miliar eroding processes, particularly in the case of carbonaceous stone.
Details of the chemical reactions involved are well documented.6
· Biological Mechanisms Many different biological factors have
been shown to be important. Growths of lichens, mosses, algae, mold,
fungi, and bacteria are capable of promoting at least surface deterio-
ration.7 Some bacteria can synthesize sulfuric for nitric) acid from
airborne sulfur dioxide (or nitrogen oxides). Guano contains phosphoric
acid, which can also cause considerable damage.3
185
In light of the above comments, it appears desirable to focus present
attention on the deposition of sulfur dioxide and small (potentially
acidic) particles, on the condensation of water at the surface and at
already deposited particles, and on the characteristics of pollutants
. . . . .
c .e. 1verec . in prec1p1tat1on.
DRY DEPOSITION
A pollutant in air near a surface will be transported to the immediate
vicinity of the surface by average winds and turbulence. This process
is usually rapid; only at night, when conditions become very calm,
can pollutant uptake rates be limited by Tow turbulence. As a pollutant
approaches the surface, molecular (or Brownian, in the case of particles)
diffusion becomes increasingly important. Brownian diffusivity can be
so low, however, that aerosol particles have difficulty penetrating the
quasilaminar layer adjacent to the surface. Once a pollutant particle
or molecule contacts the surface, it is not necessarily captured (al-
though van der Waals forces are usually considered sufficient to capture
particles).8 Thus, there is a surface resistance that quantifies the ab-
sorption of trace gases or the retention of particles at the surface. Once
material is deposited, chemical reactions can impose further variability
on the overall uptake characteristics and are likely to be especially
OCR for page 186
186
CONSERVATION OF HISTORIC STONE BUILDINGS
important if the surface is wet. The entire process of particle deposition
will be modified if-particles carry an electrostatic charge.
Much of what we know about nongravitational deposition of gases
and small particles to surfaces follows from studies of transfer to the
walls of pipes from fluids flowing through them. These studies have
shown, for example, that the transfer coefficient associated with dif-
fusion through the quasilaminar layer in contact with smooth surfaces
can be conveniently formulated in tenets of the diffusivity of the quan-
tity in question {in most literature, nondimensionalized as the Schmidt
number, Sc = v/D, where v is viscosity and D is the pollutant diffu-
sivity) and the friction velocity, u*. The conductance, or transfer ve-
locity across the quasilaminar layer is proportional to u*; the constant
of proportionality is usually written as a quantity B that is then directly
dependent on Sc. Figure 1 shows the results of several experiments
which indicate that B ~Sc-2'3.
The similanty between deposition to flat smooth surfaces in con-
10-3
m
·~`
10-4
\
~ O
~0
\ O
10-5~ 1 1 1 1 1 1 111 1 1 1 1
1o2
o
103
104
Sc
105
FIGURE 1 Variation of the surface boundary layer property B (see equation 1J with
Schmidt number for transfer to smooth flat surfaces {after Lewellen and Sheng, 19801.
Data are derived from Harriot and Hamilton {1965; open circlesI, Hubbard and Lightfoot
{1966; triangles), and Mizushina et al. {1971; solid circlesI.
OCR for page 187
Surface Deposition of Air Pollutants Affecting Decay
187
trolled circumstances and to stone surfaces in natural conditions may
be somewhat limited. However, in wind tunnel studies they appear to
be in general agreement. Moller and Schumann, for example, find close
to a SC-2/3 dependence in the case of small-particle transfer to water
surfaces in wind tunnels.9 Figure 2 presents a familiar set of wind
tunnel observations of the deposition velocity Ivy = F/C, where F is
the flux density and C is the airborne concentration of the pollutant)
to horizontal, flat surfaces, as a function of particle size. For particles
sufficiently small that gravitational settling is insignificant, these-re-
sults are dominated by near-surface, quasilaminar behavior such as
that seen in Figure 1. Indeed, the line drawn on the left in Figure 2
has a slope compatible with that in Figure 1. The corresponding expres-
SlOn IS:
Vd = u*B = A u*Sc-2'3,
(1)
where A is a constant. There are, however, inconsistencies between
the sets of results that cause some loss of confidence in generalizing
expressions of this kind. First, it appears that the precise value of the
exponent is not known. While studies of trace gas transferal and particles) ~
agree in the relevance of a - 2/3 power law relationship, a survey by
Brutsaert indicates exponents ranging between -0.4 and -o.8.~2
Second, the value of the numerical constant A in equation 1 appears
quite uncertain. The line drawn through the data of Figure 1 corre-
sponds to A ~ 0.06, yet agreement with the small-particle data of
Figure 2 seems to require A ~ 0.6. These values span the result rec-
ommended by Wesely and Hicks for the case of sup dioxide fluxes
to fibrous, vegetated surfaces.~3 They suggest relations equivalent to
A ~ 0.2, as was indicated by earlier experiments conducted by Shep-
herd. 14
Regardless of the uncertainties about the detailed formulation of
deposition through the quasflaminar layer, it is clear that large particles
will penetrate it less easily than smaller ones, unless influenced by
gravity or some other process (such as inertial impaction. Figure 2
refers to the special case of smooth horizontal surfaces. At the right
hand side of the diagram, observations conform with the predictions
of Stokes-law settling, although with some enhancement because of
inertial impaction.8 Generalization of these results to sloping surfaces
is not a trivial exercise, although calculations based on horizontally
projected areas might provide acceptable estimates of gravitational
settling rates in some situations. The contribution by inertial impac-
OCR for page 188
188
C,,
C:
o
it
o
CL
LL
CONSERVATION OF HISTORIC STONE BUILDINGS
10
0.1
0.01
0.001
100 _
O X:/
A_ 0! i.
~ ~X++~!
- ~~:
_ +/X
1 1 1
1 1 1 1 1 1 1 1 1
0.01 0.1 1 10 100
PARTICLE DIAMETER (,um)
FIGURE 2 Particle-size dependence of the deposition velocity to relatively smooth
horizontal- surfaces. The line at the left represents a Schmidt-number relationship like
that of Figure 1. At the right is the expected Stokes-law relationship, with a curve drawn
by eye to draw attention to the enhancement due to inertial impaction. Crosses indicate
results concerning aerodynamically smooth surfaces: vertical crosses to filter paper
{slough, 1973J and diagonal crosses to glass {Liu and Agarwal, 1974J. Dots apply to
artificially roughened surfaces (slough, 1973J. The remaining data refer to water surfaces,
for friction velocities of 40 cm/s {Moller and Schumann, 1970; inverted triangles J and
11, 44, and 117 cm/s {Sehmel and Sutter, 1974; triangles, squares and circlesJ.
OCR for page 189
Surface Deposition of Air Pollutants Affecting Decay
tion also cannot be easily calculated, since it will be influenced greatly
by local flow distortion and microscale roughness characteristics.
Recent work has shown that most aerosol acidity is associated with
small particles of the "accumulation" size range, mostly between 0.2
and 1.0 ,um diameter. In the context of the dry deposition of acidic
particles to smooth surfaces, therefore, gravitational settling and in-
ertial impaction become less important concerns than diffusive trans-
port and surface retention. Diffusion has been considered above; near-
surface transport is conveniently formulated in terms of the Schmidt
number. If particles are efficiently captured on contact with the sur-
faces, then available information indicates that equation 1 provides a
way to evaluate how deposition will vary with particle size.
The above arguments concerning larninar layers and their effect on
the transfer of particulate pollutants are relevant when the surface
involved is homogeneous and smooth. However, when it is roughened,
the barrier presented by the quasilaminar layer is likely to be pene-
trated, and transfer rates might be considerably enhanced. In particular,
large values of vat might be expected when surface discontinuities and
sharp irregularities occur with characteristic scales greater than the
scale thickness of the diffusive layer. The depth of this layer is usually
assumed to be determined by viscosity and the friction velocity; cias-
sical studies of flow over sand in wind tunnels indicate layer thick-
nesses of the order of 50 Am in moderate velocities twind speed of a
few m sod. Surface roughness elements of this characteristic size should
therefore be suspected as sites for preferred deposition, especially if
they are sharp and irregular.
189
PHORETIC EFFECTS
Transfer of both gases and particles can be influenced by evaporation
from a wetted surface or by condensation. The role of condensation is
of special interest, since as moisture is deposited at the surface, there
is a mean flow of air to replace the condensed vapor, and net deposition
velocities of all pollutants are increased accordingly. The magnitude
of this Stefan flow, Its' is readily calculable as:
VS = (ma/mW) · (`E/p),
1,2)
where E is the evaporation rate {in kg m-2s-~), p is air density, ma is
the molecular weight of air, and mw is that of water. At standard
temperature and pressure, the overall deposition velocity is increased
OCR for page 190
190
CONSERVATION OF HISTORIC STONE BUILDINGS
by about 0.005 cm s-i for every 100 W m-2 of latent heat transferred
by condensation. The maximum rate of dewfall to vegetated surfaces
is known to be about 45 W m-2 Equivalent to about 0.07 mm h- i j2i,
so that the increase in vet is unlikely to exceed 0.003 cm so-. Even
lower values should apply in the case of statues and monuments, but
these might stfl] be of similar magnitude to the values predicted for
the transfer of small particles to smooth surfaces, as considered in
Figure 2.
In daytime, evaporation from wet surfaces will tend to protect them
from pollutant deposition. When liquid water is present at the surface,
evaporation rates are controlled by the availability of heat, primarily
from insolation, and can easily be more than an order of magnitude
greater then the condensation rates considered above. When evapo-
ration is proceeding strongly, Stefan flow might provide a barrier against
the deposition of those pollutants {especially small particles) having
deposition velocities normally less than about 0.03 cm s- i. Such strong
evaporation rates are not infrequent, but wet stone surfaces will be
rapidly dried and the effect cannot persist for Tong periods.
Stefan flow affects particles and gases alike; however, there are re-
lated mechanisms that act primarily on particles. These phoretic forces
result from the response of particles to the impaction of air and water
molecules in the presence of temperature and humidity gradients. The
effect is to drive particles toward cold or evaporating surfaces.
Frie~ander shows that the thermophoretic velocity increment can
be expressed as:
v~~ -0.3 (v/~ VT,
(3)
where T is absolute temperature. The negative sign indicates the coun-
terflux direction of the imposed motion (away from warm surfaces),
and the constant 0.3 is actually a slight function of particle size (0.33
for 0.1 ,um, 0.28 for 0.3 ~m, and 0.16 for 1.0 ~m). It is informative to
introduce the sensible-heat flux H and to rewrite equation (3) as
via ~ - 0.3 Pr H/(p cpT)
{4)
where Pr is the Prandt! number, v/DT, analogous to the Schmidt num-
ber in equation 1; DT is thermal diffusivity, p is air density, and cp is
the specific heat of air at constant pressure. Equation 4 indicates an
additional velocity increment amounting to about 0.0065 cm s- ~ for
every 100 W m-2 of sensible-heat flux. At night, H rarely exceeds this
value and is typically - 10 to - 20 W m-2, so that even though dep-
OCR for page 191
Surface Deposition of Air Pollutants Affecting Decay 191
osition is enhanced, it is only by a small amount. In daytime, however,
heat fluxes con exceed 500 W m-2, imposing a considerable barrier
against particle deposition.
A similar phoretic force is exerted on particles by water molecules
Effusing past them. Whereas the Steen velocity considered above was
a consequence of a mean displacement of the gas by evaporating water
molecules, ~ffusiophoresis is a result of a net flux of water molecules
past the particle in question and of a flux of heavier air molecules to
replace them. The resulting velocity is sometimes combined with and
often confused with the Stefan flow described earlier. The detailed
investigation presented by Goldsmith and May shows ~ffusiophoresis
to be far less important than either Stefan flow or thermophoresis in
the circumstances of interest here.22
A word of caution seems appropriate at this stage. Many of the
equations given above assume the existence of a laminar layer in con-
tact with the surface, a situation that appears highly unlikely undess
the surface under consideration is remarkably smooth and uniform
Which may well be so when a stone surfaceiis new and polished).
Thus, we should avoid the use of these relations to quantify deposition
with precision, but instead should employ them to identify critical
properties and to determine the circumstances in which deposition
will be greatest.
WET DEPOSITION
Recent interest in the quality of precipitation {especially its acidity)
has resulted in a wealth of data on the chemical composition of rain
and snow. Rain fall itself is a highly variable quantity that is known
to conform quite closely to a Tog-normal frequency distribution. Its
chemical characteristics are even more varied, but analyses of data
obtained by the Multistate Atmospheric Power Production Pollution
Study (MAP3S) show that pollutant concentrations in precipitation events
also tend to follow the expected log-normal distribution.23 Table 1
summarizes some of the results of the MAP3S survey, and combines
them with data derived from sampling conducted by the Department
of Energy Environmental Measurements Laboratory (DoE/EMt).24 The
intent is to extend the MAP3S conclusions to suburban and city areas;
the MAP3S sites are carefully selected to be well away from such areas
and are intended to be indicative of regional-scale characteristics rather
than the local variations that are likely to be mainly of interest here.
Hydrogen, sulfate, and nitrate ion concentrations are selected for
consideration in Table 1. The rural sites in the northeastern United
OCR for page 192
192
CONSERVATION OF HISTORIC STONE BUILDINGS
TABLE 1 Long-Term Mean Concentrations of Hydrogen, Sulfate,
and Nitrate Ions in Precipitation Collected at Selected Sites
H + SO4- NO3
pH C . SC C SC C SC
Rural sites
Chester, N.Ja 4.1 75 73 36
Ithaca, N.y.b 4.1 81 1.9 60 2.2 32 2.0
State College, pate 4.1 79 2.2 62 2.4 39 2.2
Charlottesville, Va.b 4.1 72 2.1 61 2.2 28 2.3
Miami, Ohiob 4.2 65 1.9 65 1.8 29 1.9
Urbana, [lib 4.4 43 3.5 71 1.8 30 1.8
Beaverton, Oreg.a 5.5 3 20 7
Urban and city sites
Argonne, Hl.a 4.6 25 105 32
New York, N.y.a 3.9 130 150 40
NOTE: Concentrations are in microeqliivalents per liter. C is the mean concentration,
and sc is the appropriate standard deviation. A log-normal distribution is assumed, so
that Sc applies to the normal distribution of a variable x = in C.
a Data from Feely and Larsen t1979~.
b Data from the MAPLES network {MAPLES, 1981~.
States appear to yield similar results: Hydrogen and sulfate ion con-
centrations are in the range 60 to 80 microequivalents per liter, End
nitrate is 30 to 40. The Chester site of the DOE/EML network is well
away from upwind sources and does indeed provide results that agree
with the MAP3S data in the Northeast; hence the two data sets appear
to be compatible, even though DOE/EML provides monthly averages
only, whereas MAP3S concentrates on events. The Urbana data show
evidence that midwestern rural precipitation chemistry is somewhat
different from that farther east; while sulfate and nitrate concentrations
appear much the same, hydrogen ion concentrations seem to be sub-
stantially lower. The Beaverton, Oregon, data extend this trend to the
West Coast, where the precipitation is cleaner in all aspects.
The New York City data were obtained at a central rooftop location.
Hydrogen ion and sulfate concentrations are about `double the values
typical of the Northeast as a whole, but nitrate concentrations are
relatively unaffected. The Argonne, Illinois, data were obtained at a
suburban site about 40 km upwind of downtown Chicago, but in an
area influenced by considerable industrial activity. The Argonne nitrate
values seem unaffected, but the sulfate concentrations are intermediate
between regionally characteristic values and the New York City max-
OCR for page 193
Surface Deposition of Air Pollutants Affecting Decay
193
imum. Hydrogen ion concentrations at Argonne seem strangely low,
but are suspected of being influenced by soil-derived material capable
of neutralizing the more acidic constituents. The Urbana data are thought
to be similarly affected, but clearly to a lesser extent.
The MAPaS data provide excellent evidence that the Tog-normal fre-
quency distribution usually associated with the precipitation process
itself also provides a good representation of the chemical concentration
data. The standard deviations listed in Table 1 show remarkable uni-
formity. With the sole exception of the Urbana hydrogen ion data,
values range between 1.8 and 2.4, and the average is 2.0. Thus the
shape of the frequency distribution is fairly constant and can therefore
be extended with some confidence to the case of cities. In this way it
is possible to estimate the probability of encountering exceedingly
acidic ra~nfaD when only its long-term average characteristics are known,
and it is similarly possible to estimate the frequency distributions of
concentrations of different chemical species. For central New York
City, for example, Table 1 indicates that the probability of any single
rainfall event producing a pH less than 3.0 is about 15 percent.
It might be noted that the standard deviations in Table 1 indicate a
fairly constant spread of pH values at every site except Urbana. In
general the standard deviation of the pH of event precipitation is about
0.9 (1.5 at Urbanal. Since pH depends on the logarithm of the hydrogen
ion concentration, the frequency distribution of event pH values will
be close to Gaussian.
It is evident in Table 1 that even polluted rain has very low ionic
strengths. A complication that is not evident in the table is that the
greatest ionic strengths tend to be associated with the smallest rain-
falis, so that high concentrations need not necessarily suggest large
doses of chemical contaminants. Moreover, the probability of an ex-
tremely acidic event appears to be quite low, even in city and suburban
environments. These considerations combine to suggest that deteri-
oration by the action of chemicals in precipitation might not be as
important as that caused by the hydration and mobilization of surface
materials already deposited. In some situations the net washing effect
might even be beneficial. It is certainly clear that local variations of
precipitation chemistry can be large and that generalizing is bound to
be dangerous.
AIR CONCENTRATIONS
The mechanisms that deposit pollutants constitute only the final step
in a chain of events that transport and transform them between sources
OCR for page 194
94
CONSERVATION OF HISTORIC STONE BUILDINGS
and receptors. The mechanisms that combine to result in the dry
deposition of gaseous pollutants are likely to be most effective when
the air is turbulent (i.e., mainly in daytime) and when the surface is
moist and therefore acting as a good sink for soluble gases. For particles,
dry deposition is mainly limited by transport very near the surface and
may well be greatest when water is condensing. But none of these
mechanisms will result in a large flux of pollutants unless sufficient
concentrations of the pollutant are accessible in air near the surface.
Many variables contribute to the diurnal cycle in concentration of
any air pollutant. The highest concentrations can occur at night in
some cases and during the daytime in others. Variability of this kind
is greatly influenced by the spatial distribution and height of emission
sources. It is also greatly influenced by whether relatively undiluted
pollutant plumes can be carried near the surface by atmospheric mixing
(fumigation) or by some local interference with flow patterns, such as
might be caused by a large building, a hill, or rows of trees. Thus there
is no general consensus that sulfur dioxide, for example, will peak in
the daytime, although an early morning peak associated with fumi-
gation is frequently observed.
Superimposed on any regular cycle of this kind will be a random
variability whose magnitude and frequency distribution will vary greatly
with time and location. The temporal variability in air pollution con-
centrations is a well-known feature that emphasizes the need to obtain
relevant data by experiment whenever specific areas of interest can be
identified. The need is amplified by the additional uncertainty asso-
ciated with spatial (differences, especially within cities or in areas af-
fected by local traffic. Furthermore, it is evident that more detail is
required than is obtained in most pollution monitoring programs, since
the deposition processes vary with the time of day. It would be difficult
to interpret daily averaged concentration data, whereas averaged daily
cycles and frequency distributions would be quite informative.
CONCLUSIONS
Both wet and dry deposition of polluters can cause significant dete-
rioration of exposed stonework. Wet deposition imposes sudden but
infrequent doses of pollutants, most of which will be in dilute solution.
Concentrations will vary widely both in time and in space, but as a
rule of thumb the pH will be roughly normally distributed, with a
standard deviation of about 1.~. It is obviously possible to protect
exposed surfaces from the direct effect of precipitation, but it is not
immediately clear that the use of shelters will generally be beneficial.
OCR for page 195
Surface Deposition of Air Po11u tan ts Affecting Decay
195
Dry deposition is a slower but more continuous process than wet
deposition, and it is always possible that incident precipitation will
wash off material previously deposited by dry processes. However, in
cold weather the mechanical effects associated with repeated freezing
and thawing of water are likely to overwhelm all other factors.
Both dry and wet fluxes will be greatest when air concentrations of
pollutants are high. Although the relationship between air concentra-
tions snot the chemical composition of precipitation is exceedingly
complicated, rates of deposition by dry mechanisms are intimately
related to air quality in the immediate vicinity of receptor surfaces.
Regarding dry deposition to exposed stonework, the following addi-
tional points seem clear:
· In daytime, particle fluxes will be greatest to the coolest parts of
exposed surfaces.
· Both particle and gas fluxes will be increased when condensation
is taking place at the surface and decreased when evaporation occurs.
· If the surface is wet, impinging particles will have a better chance
of adhering, and soluble trace gases wfl] be more readily captured.
· The chemical nature of the surface is important; if rates of reaction
with deposited pollutants are rapid, then surfaces can act as nearly
perfect sinks.
· Biological factors can influence uptake rates by modifying the
ability of the surface to capture and bind pollutants.
· The texture of the surface is important. Rough surfaces will pro-
vide better deposition substrates than smoother surfaces and will per-
rnit easier transport of pollutants across the near-surface quasflaminar
layer.
Finally, it should be emphasized that the present state of knowledge
regarding pollutant uptake by surfaces of any kind is rather rudimen-
tary. Nevertheless, important processes can be identified with some
confidence. While the rates of uptake cannot be predicted at all closely,
the circumstances under which the greatest fluxes occur can be de-
tem~ined. In particular, some surface properties that are likely to cause
locally enhanced deposition can be identified, and hence areas that are
potentially at risk can be singled out.
REFERENCES
1. Winkler, E.M., and E.J. Wilhelm. 1970. Saltburst by hydration pressures in archi-
tectural stone in urban atmosphere, Geol. Soc. Am Bull. 81:57~572.
OCR for page 196
196
CONSERVATION OF HISTORIC STONE BUILDINGS
2. Winkler, E.M., and P.C. Singer. 1972. Crystallization pressure of salts in stone and
concrete, Geol. Soc. Am. Bull. 83: 3509-3514.
3. Fassina, V. 1978. A survey of air pollution and deterioration of stonework in Venice,
Atmos. Environ . 12:220~221 1.
4. Gauri, K.L. 1978. The preservation of stone, Sci. Am. 238: 196-202.
5. Yocom, J.E., and R.O. McCaldin. 1968. Effects of air pollution on materials and
the economy. In Air Pollution (A.C. Stern, ea.) Academic Press: New York.
6. Keller, W.D. 1977. Progress and problems in rock weathering related to stone decay
In Engineenng Geology Case Histories Number 11. Geological Society of America.
7. Torraca, G. 1977. Brick, adobe, stone, and architectural ceramics: Deterioration
process and conservation practices, reprinted in Papers in Atmosphenc Science {A.
Mestitz and O. Vittori, eds.) Consiglio Nazionale delle Richereche: Italy.
8. Friedlander, S.K. 1977. Smoke, Dust and Haze Fundamentals of Aerosol Behavior.
John Wiley: New York.
9. Moller, U. and G. Schumann. 1970: Mechanisms of transport from the atmosphere
to the Earth's surface, J. Geophys. Res. 75: 3013~019.
10. Deacon, E.L. 1977. Gas transfer to and across an air-water interface, Tellus 29:
363-374.
11. Lewellen, W.S., and Y.P. Sheng. 1980. Modeling of Dry Deposition of SO2 and
Sulfate Aerosols. Electric Power Research Institute Report EA-1452.
12. Brutsaert, W. 1975. The Roughness length for water vapor, sensible heat, and
other scalars, I. Atmos. Sci. 32: 202~2031.
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Representative terms from entire chapter:
stone buildings
