EPA-600/3-77-027
March 1977
Ecological Research Series
LABORATORY MEASUREMENT OF SULFUR
DIOXIDE DEPOSITION VELOCITIES
Environmental Sciences Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ECOLOGICAL RESEARCH series. This series
describes research on the effects of pollution on humans, plant and animal
species, and materials. Problems are assessed for their long- and short-term
influences. Investigations include formation, transport, and pathway studies to
determine the fate of pollutants and their effects. This work provides the technical
basis for setting standards to minimize undesirable changes in living organisms
in the aquatic, terrestrial, and atmospheric environments.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/3-77-027
March 1977
LABORATORY MEASUREMENT OF SULFUR DIOXIDE DEPOSITION VELOCITIES
by
H. S. Judeikis and T. B. Stewart
The Aerospace Corporation
El Segundo, California 90245
Grant No. R-802687-02
Project Officer
Jack L. Durham
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISCLAIMER
This report has been reviewed by the Environmental Sciences Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. Approval does not signify that the contents necessarily reflect the
views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commerical products constitute endorsement or
recommendation for use.
ii
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ABSTRACT
Measurements of sulfur dioxide deposition velocities have been carried
out in the laboratory with the use of a cylindrical flow reaction. Analysis
of data from these experiments was performed with models that specifically
account for diffusive transport in the system. Consequently, the resulting
deposition velocities were independent of diffusion processes and represented
the maximum removal rates that would be encountered in the.environment under
turbulent atmospheric conditions. The measured values ranged from 0.04 cm/
sec for asphalt to 2.5 cm/sec for cement, and were independent of sulfur
dioxide and oxygen concentrations as well as relative humidity and total
pressure.
Prolonged exposure to sulfur dioxide eventually destroyed the ability of
the various solids to remove this species. Overall capacities increased
significantly at moderate relative humidities, yielding values of 0.4 to
2.8 grams of sulfur dioxide per square meter of solid in moist systems.
Several experiments indicated that the reactivity of a solid subjected to
prolonged sulfur dioxide exposures could be restored by washing the surface
with distilled water or exposing the spent solid to ammonia. Some implica-
tions of these findings relative to the environment are discussed.
This report was submitted in fulfillment of Grant No. R-802687-02 by The
Aerospace Corporation under the sponsorship of the U.S. Environmental
Protection Agency. This report covers a period from November 1973 to
February 1976, and work was completed as of February 1976.
iii
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CONTENTS
Abstract ill
Acknowledgement vi
1. Introduction 1
2. Experimental 3
Apparatus 3
Materials 5
Data Analysis 5
3. Results 9
4. Discussion 16
References 18
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ACKNOWLEDGMENTS
The authors thank Dr. Jack L. Durham, Environmental Sciences Research
Laboratory, Environmental Research Center, Research Triangle Park, North
Carolina, for many helpful comments, and J.E. Foster for performing many of
the laboratory experiments.
vi
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SECTION 1
INTRODUCTION
Deposition velocities of pollutant gases are used extensively in
calculating atmospheric budgets for these species (1-2). Both field and
laboratory measurements of these quantities have been made. Field measure-
ments are generally carried out by one of two methods for determination of
deposition velocities. The first involves simultaneous measurements of wind
velocity, temperature, and pollutant gas concentration profiles above the
surface (3-7). The vertical atmospheric diffusity K(z) is estimated from the
former two quantities, and the deposition velocities V are calculated from
O
F = -K(z) dc = V c (1)
dz g
relating the downward flux (F) of the pollutant gas to K(z) and the concen-
tration gradient. The concentration c is measured at some fixed height above
(but near) the surface.
The second method, which is also used extensively in the laboratory, is
based on total uptake of sulfur dioxide (8-12) and sulfur-35 dioxide labelled
sulfur dioxide is frequently used (13-19). In the latter case, the total
uptake of sulfur-35 dioxide is measured as well as its concentration just
above the surface. The deposition velocity is then readily calculated from
Equation (1). In some cases, flow systems are also used for laboratory
measurements (20-24).
Measured deposition velocities typically range from a few tenths of a
centimeter per second or less to several centimeters per second (18). Sub-
stantial variations in the magnitude of the deposition velocity determined
at a given field site or for a given material in the laboratory are common
(3,4,5,7). These variations may be partly related to surface changes that
are dependent on environmental conditions. For example, sulfur dioxide
uptake by leaves is largely controlled by the stomata (25) . The opening and
closing of the stomata depend on a number of environmental factors, e.g.,
daylight, relative humidity, and season. In the laboratory, deposition veloc-
ities, in many cases, represent values that are limited by mass transport to
the surface. Questions then arise concerning the limits of deposition
velocities imposed by physical and chemical processes related to the actual
removal of the pollutant gas at the surface.
In this report, a method is presented for laboratory measurement of
deposition velocities independent of mass transport phenomena together with
experimental results for sulfur dioxide removal on several environmental
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surfaces. The values obtained in this manner represent the maximum deposition
velocities that would be encountered in the open atmosphere, particularly
when turbulent mixing is sufficiently high to remove mass transport limita-
tions.
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SECTION 2
EXPERIMENTAL
APPARATUS
A block diagram of the apparatus used in these experiments is shown in
Figure 1. This system, which is basically a cylindrical flow reactor, is
similar to systems previously described by Hedgpeth et al. (27), and by
Stewart and Judeikis (28). The major difference between the present system
and those previously described is the method of analyzing gases flowing
through the reactor. Additional changes consisted of replacing system com-
ponents that were found to be reactive toward sulfur dioxide. Virtually all
components in the final version of the modified system consisted of pyrex
glass, 316 stainless steel, and teflon-coated aluminum.
In the system shown in Figure 1, a carrier gas stream was initially split
into two streams; one of the streams passed through a humidifier, where it
was saturated with water vapor. The two streams were subsequently recombined.
(The ratio of flow rates of the split streams determined the relative (humi-
dity of the carrier gas.) The carrier gas stream was then mixed with a small
amount of nitrogen that contained traces of sulfur dioxide and the mixture
was fed into the cylindrical flow reactor (2.5 cm radius) that contained a
concentric pyrex cylinder (2.1 cm radius) coated with the solid of interest.
(The choice of a cylinder for a substrate was not unique, and other geome-
tries, such as parallel plates, could have been used.) The latter cylinder
was coated by preparing a slurry of the solid of interest, coating the blank
pyrex cylinder (outside the reactor), and permitting the coating to air dry
and then to dry overnight in a vacuum in the tubular reactor.
Reaction of sulfur dioxide with the coated walls led to a concentration
gradient for sulfur dioxide along the axial (as well as radial) directions.
(In the absence of a solid coating, there was no change in the sulfur dio-
xide concentration on passage through the reactor.) In order to measure the
axial concentration gradient, the gas mixture in the reaction chamber was
sampled by means of a set of small probes (connected through a 16-port rotary
valve to mass spectrometer) whose intakes were centered along the axis of
the coated cylinder. The probes were nominally 0.15 cm outer diameter and
0.08 cm inner diameter. Flow through the sampling system was sufficiently
slow so that the flow pattern in the reaction chamber was not disturbed, but
yet it was sufficiently fast so that transit time through the sampling system
was minimal (^ 3 to 4 sec).
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PRESSURE
GAUGE
FLOWMETER
VALVE
CARRIER GAS
1 .VALVE
p*PUMP
'VALVE
VALVE
PRESSURE GAUGE
REACTION CHAMBER
FLOWMETER
—GAS SAMPLING PROBES
PUMP
MASS SPECTROMETER (gas analysis)
VALVE
CARRIER GAS
HUMIDIFIER
GAS MIXER
-CARRIER GAS
Figure 1. Block diagram of cylindrical reactor with sampling probes.
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Typical operating conditions used were pressures of 10 to 700 Torr1,
flow velocities of 1 to 30 cm3/sec (average linear velocities of 0.05 to
1.5 cm/sec), and ambient temperatures (Reynolds numbers < 50). Sub-ambient
pressures were frequently required in order to measure non-diffusion-limited
deposition velocities. (This point is discussed in more detail in the
following sections.) Flow rates were selected to give a sufficiently high
axial sulfur dioxide concentration gradient in order to permit accurate
measurements of this quantity.
Gases sampled through the probes were analyzed by using a mass spectrom-
eter. The sensitivity of the mass spectrometer for sulfur dioxide detection
was ^ 0.3 ppm. Consequently, experiments were conducted with initial sulfur
dioxide concentrations of >_ 3 ppm. In addition, high concentrations of
oxygen in the reaction mixture tended to oxidize the filaments in the mass
spectrometer. For this reason, oxygen concentrations were limited to "\» 10%
or less.
MATERIALS
Solids investigated in this study included commerical formulations of
cement, ready mix cement (cement with sand and gravel), asphalt, and exterior
stucco. In the cases of cement and exterior stucco, samples from two
different sources of each material were used. Other materials studied were
soil samples of sandy loam and adobe clay taken from the Los Angeles area.
These materials were generally sifted through a screen to eliminate particles
>1 mm in diameter. Water-based slurries of these materials were used in
preparing the coated pyrex cylinders (except for asphalt, where a trichlor-
oethylene slurry was used). Consequently, the cement, ready mix cement, and
exterior stucco were cured during the process of preparing the coatings.
Surface roughnesses were typically < VL mm.
Gases used were reagent grade gases obtained from Matheson and were used
as received. Two specially prepared mixtures were used for sulfur dioxide
and oxygen in order to obtain the desired concentrations of these gases in
the reaction mixture. These were 1000 ppm SO- in N and 20% 0 in N . In
addition, distilled water was used for humidifying gas mixtures.
DATA ANALYSIS
Mass transport in a cylindrical flow tube, under conditions of nontur-
bulent flow and at steady state, has been described by Walker (29), and
Stewart and Judeikis (28) (and their cited references) as:
9c + 8zc \ - V f-^ = 0 (2)
x 8 x
1 To convert from Torr to newton/meter2, multiply by 1.333 22 E + 02,
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subject to the boundary conditions
c = CQ at r, x = 0
|^ =0atr=0,x
and
- D - = i k c at r = R, x > 0.
or r
k = (RT/2TTM)3*
(4)
where r = radial coordinate
x = axial coordinate
c = concentration of the reacting species
c, = initial concentration
D = diffusion coefficient of the reacting gas in the
mixture
V = linear gas flow velocity in the axial direction
X
k = molecular velocity of the reacting species in the
radial direction
R = cylinder radius
k c = the gas-solid collision frequency
<(> = reactivity, the fraction of collisions that lead to
removal of the reacting species from the gas phase*
R = the gas constant
T = the absolute temperature
M = the molecular weight of the diffusing gas
In this report, binary diffusion coefficients are calculated for sulfur
dioxide in nitrogen by using expressions given by Present (30) . The pre-
sence of oxygen or water vapor in the reaction mixture would lead to dif-
fusion coefficients slightly different than the calculated values. The
uncertainties that arise from these differences are less than those from
other sources. Equation (3) expresses the condition that the diffusion of
the reacting species to the walls is equal to its removal by heterogeneous
reaction. (Actually, c', the concentration at one mean free path away from
first-order or pseudo-first-order processes, it can be shown that 4> is
actually composed of a collection of constants, including the sticking
coefficient, as well as the rate constants for adsorption, desorption, and
surface reaction (32).
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the walls, should be used in Equation (3) in place of c (31, 32). However,
except for $ -1, the two are essentially equal.)
The deposition velocity V~ is related to <|» , as is shown by comparing
Equations (1) and (3). Equating the right sides of these equations yields
V = <(>k . (5)
Thus, the deposition velocity over a given material can be obtained from
laboratory determinations of <|> values. Note also that the depostion veloci-
ties determined in this manner correspond to values at one mean free path
above the surface.
Solution of Equation (2) is generally accomplished by making several
simplifying assumptions. One is the assumption of plug flow (V = constant).
The solution in this case is (28,29) X
2J_a. f- B.x
C0 1=1 a± fl + 62a
where Jn(a. r/R) and J1 (a.) are Bessel functions of the first kind,
\.—°— <7>
Q _ £l
Pi " 2D
and a. is the i root of
J0(a.) = 6aiJ1(ai). (9)
For laminar flow, expressed as
Vx = ^average'1 ' * V) > <10>
solutions to Equation (2) have been obtained (for equivalent heat transfer
problems by Sideman, Luss, and Peck (33) and their cited references) where
axial diffusion can be neglected (D 2c/ X2.»0) . (Criteria necessary for this
assumption were delineated in an analogous heat transfer problem by Singh
(34). In general, the conditions for which these solutions apply in this
study are of limited use in the determination of value for because reac-
tions tend to become diffusion-limited under experimental conditions where
axial diffusion can be neglected, particularly for high reactivities.
Examples of this are illustrated below. Consequently, numerical solutions of
Equation (2), with laminar flow, were required for the cases of interest
here. These were obtained by using a modification of the method of finite
differences (35).
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The geometry of the present system is such that laminar flow is not
fully developed at the entry to the coated cylinder. This generally presents
no problem, however, because, under most experimental conditions, either the
plug or laminar flow models adequately describe the experimental results and
yield <|> values that agree to within a few to 20%. (Values derived from the
plug flow model are always lower than those derived from the laminar flow
model.)
The major discrepancy between values derived from the two models
occurs at high pressures (^ 700 Torr) and high reactivities ( >10~1+).
Under these conditions, sulfur dioxide removal tends to become diffusion-
limited. Although these conditions are avoided in most experiments (see
Section 3), they do provide an opportunity to distinguish between the two
models (since the solutions become independent of ). In such cases,
generally, sulfur dioxide concentration gradients calculated from the plug
flow model are more consistent with experimentally measured values. Conse-
quently, the plug flow model was used for the analysis of data reported here.
As noted earlier, any deviations from this model would result in slightly
higher values (by as much as 20%) for (or V ) than are reported in Section
3. 8
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SECTION 3
RESULTS
Values of ((> derived from a number of measurements of sulfur dioxide
removal over various solids are given in Table 1 together with deposition
velocities calculated from Equation (5) for a temperature of 250C.2 The
values given represent averages from 3 to 6 experiments on each material
investigated. In the cases of cement and exterior stucco, data on the
material from different sources are reported individually.
TABLE 1. EXPERIMENTAL RESULTS FOR S02 REMOVAL
Material
p
Cement-I
Ready mix cementa
Exterior stucco-Ia
Cement-II
Exterior stucco-II
Adobe clay soil
Sandy loam soil
Asphalt
*
3.2 X 10~4
2.6 X 10~4
2.3 X 10~4
L
2.0 X 10
-4
1.1 X 10
8.4 X 10~5
8.3 X 10~5
-6
5.1 X 10
V
g»
cm/sec
2.5
2.0
1.8
1.6
0.86
0.66
0.65
0.04
3Cured
Values determined from consecutive measurements of sulfur dioxide
concentration gradients on a given sample usually agreed to within 20 to
30%. Variations in values from sample to sample of the same material
(for an equivalent sulfur dioxide exposure) were comparable. Overall, the
2 To convert from °C to °K, add 273.15.
t, = t + 273.15
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probable errors for the values given in Table 1 are about 30%.
The values reported in Table 1 were generally found to be independent of
sulfur dioxide concentrations over variations of one to two orders of mag-
nitude. (The minimum partial pressure of sulfur dioxide used in these
experiments was ^ 0.15 m Torr.) Representative data illustrating this point
for exterior stucco-I are shown in column A of Table 2. (Here, as in the
other data in Table 2, these comparisons were made in sequential runs on the
same sample of a given material in order to minimize uncertainties that arise
from sample to sample variations.) Thus, sulfur dioxide removal over these
solids follows first-order kinetics.
We also examined reactivities as a function of oxygen concentration and
relative humidity and found to be independent of these parameters to with-
in experimental error. Representative results from these experiments over
ready-mix cement and sandy loam soil are given in Table 2, columns B and C,
respectively. For oxygen, problems with oxidation of the mass spectrometer
filaments limited us to the use of oxygen concentrations < ^ 10%. However,
even with these limitations, the oxygen concentration exceeded that of
sulfur dioxide by factors ranging from ^ 103 to 10^ (except, of course, for
experiments conducted in the absence of oxygen).
For materials with reactivities of 'v 10 ^ or greater, measurements made
at atmospheric pressure yielded sulfur dioxide concentration gradients near
the diffusion-limited value. Moreover, values derived from such measure-
ments were subject to large uncertainties* The reason for this is illustra-
ted in Figure 2A, which shows concentration profiles that were calculated
for different values of at 1 atm total pressure.3 It is shown that a
reactivity > 10 3 results in a diffusion-limited sulfur dioxide concentration
gradient, whereas the gradient for $ = 10 ** <|> differs by only 10% from the
diffusion-limited gradient.
Consequently, experimental conditions were altered for those materials
with reactivities near 10 **. Although, in principle, several parameters
could be varied, in practice, the most sensitive and easiest to vary was
the total pressure. The effects of reducing the total pressure can be seen
by comparing Figures 2A and 2B. In the latter case (for 0.1 atm total
pressure), the concentration gradients differ by approximately a factor of
two for reactivities of 10 3 and 10 **.
Since sub-ambient pressures were frequently used in these experiments,
the effects of total pressure on measured reactivities were studied. In
general, values were found to be independent of total pressure, to within
experimental uncertainties, for pressures ranging from ^ 50 to 500 Torr.
This is illustrated in Table 2, column D^ for sulfur dioxide removal over
sandy loam soil.
To convert from atmosphere to newton/meter , multiply by 1.013 250*
E + 05.
10
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TABLE 2. REACTIVITIES AS A FUNCTION OF S0? AND 0 CONCENTRATIONS,
RELATIVE HUMIDITY, AND TOTAL PRESSURE
Pressure,
Parameter Varied
A. SO- concentration
B. 0? concentration
C. Relative humidity
D. Total pressure
Material Total 02
b
Exterior stucco-I 55
55
Ready mix cement 58
58
Sandy loam soil 100
100
Sandy loam soil 50
400
2.
2.
0.
6.
4.
4.
0.
0.
6
6
0
2
4
3
0
0
Torr
so2-
1.
13.
1.
1.
3.
4.
4.
4.
10-
1
2
6
6
7
2
2
8
» RHC
28
28
57
57
0
100
50
50
2.
2.
2.
2.
6.
5.
8.
7.
*
4 x
2 x
0 x
4 x
1 x
9 x
3 x
4 x
10
10
10
10
10
10
10
10
-4
-4
-4
-4
-5
-5
-5
-5
a 3
Flow rates were nominally 10 cm /sec.
bCured
CRelative Humidity %
11
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0
Figure 2. Calculated SC>2 concentration gradients. Gradients calculated for total pressures
of 1.0 (A) and 0.1 (B) atm. In both cases, R = 2. 1 cm, T = 25°C, and
V = 1 cm/sec.
x
-------
w
(Hj
15
Xlcm)
Figure 3. Measured SO? concentration gradients as a function of time
(SO2 exposure). Experimental parameters for SC>2
removal over adobe clay soil: P(total) = 300 Torr;
P(O2) = 19 Torr; P(SO2) = 22 m Torr; T = 24°C;
Vx = 0.5 cm/sec. Gradients after exposure to SO2 of
3.6 min (A), 2.7 hr (O), and 7. 7 hr (D), or 0.009, 0.39,
and 1. 1 g SO2/m2 of solid surface, respectively. Data
points are from experimental measurements. Solid curves
are calculated for 0 = 1.0 X 10-4 (A), 1.2 X 10'5 (O), and
2.2 X 10-6
13
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We also attempted to analyze solids after reaction for sulfate formed.
Wet chemical methods were used. These efforts were largely unsuccessful
because of interferences by various species present in the unexposed samples.
However, in a related study (to be reported on later) on sulfur dioxide by
metal oxides and other materials, wet chemical and photoelectron spectro-
scopy methods indicate a near quantitative conversion of sulfur dioxide to
sulfate. (Similar results were found by Seim (9) after exposing various
soils to sulfur dioxide.)
The reactivities and deposition velocities reported above are for sul-
fur dioxide removal over freshly prepared coatings. With time (sulfur
dioxide exposure), these reactivities diminish as the capacity to remove
sulfur dioxide is expended. This poisoning effect is shown in Figure 3 for
adobe clay soil. In general, this type of behavior was noted with all of the
solids investigated in this study.
The capacities for sulfur dioxide removal can be determined from experi-
ments such as that illustrated in Figure 3. Values obtained for several of
the solids listed in Table 1 (the adobe clay and sandy loam soils ready-mix
cement, and exterior stucco-I) range from 0.04 to 0.6 grams of sulfur
dioxide per square meter of solid surface for dry reaction mixtures, and
0.4 to 2.8 grams of sulfur dioxide per square meter of solid surface for
humidified reaction mixtures (50 to 95% relative humidity). Typically,
capacities were a factor of 3 to 10 higher for humidified reaction mixtures
compared with dry mixtures. A sufficient number of experiments to measure
capacities were not conducted to determine if the absolute value of the
relative humidity affects capacities. The limited data available, however,
indicate that the capacity for sulfur dioxide removal from humidified reac-
tion mixtures does not depend on relative humidity so long as the latter is
> ^ 30 to 40%. Other than the relative humidity, parameters such as the
sulfur dioxide oxygen concentrations and the total pressure did not appear
to have any significant effect on capacities for sulfur dioxide.
Although the experimental results indicate only a limited capacity for
sulfur dioxide removal by the ground-level surfaces examined here, several
possibilities exist for continued removal in the open atmosphere. For
example, rain could wash away soluble sulfates (or other products) , which
would rejuvenate the surfaces for further sulfur dioxide uptake (8). Several
authors (18,24) have suggested that sulfur dioxide removal may be pH-limited,
e.g., sulfuric acid being formed from sulfur dioxide taken up by the surface,
with the reaction gradually decreasing as the acid concentration builds up.
Interaction with atmospheric ammonia could diminish such an effect. Of course,
sulfates are nutrients for plant growth, and sulfates formed on soils could
be removed by this process.
In order to examine these possibilities, several additional experiments
were carried out. In one experiment, a sample of ready mix cement was ex-
posed to sulfur dioxide at 95% relative humidity until the capacity of this
material for sulfur dioxide removal was completely expended. (It was general-
ly found that sulfur dioxide removal was an irreversible process. Thus,
termination of sulfur dioxide exposures and evacuation of solid samples did
not result in any desorption of sulfur dioxide restoration of the ability of
14
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the solid to remove sulfur dioxide). The coated cylinder was then removed
from the reactor, and the coating was rinsed with distilled water and allowed
to air dry. The coated cylinder was then replaced into the reactor, dried
overnight in vacuum, and subsequently re-exposed to sulfur dioxide at 95%
relative humidity. Experimental measurements indicated a complete restora-
tion of the ability of ready-mix cement to remove sulfur dioxide.
In another experiment, adobe clay soil was exposed to sulfur dioxide in
a dry reaction mixture until completely poisoned. The gas mixture was then
humidified (95% relative humidity). The result was a complete restoration of
the reactivity toward sulfur dioxide removal.
The effects of ammonia were investigated in an experiment with a sample
sandy loam soil. The sample was exposed to sulfur dioxide (95% relative
humidity) until completely poisoned. The sulfur dioxide exposure was then
terminated, and the sample was exposed to ammonia (the total ammonia ex-
posure was only ^ 20% of the sulfur dioxide exposure required to initially
poison the sample). After exposure to ammonia, the system was purged with
nitrogen and then re-exposed to sulfur dioxide. The result, again, was a
complete restoration of the activity of the sandy loam soil toward sulfur
dioxide.
15
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SECTION 4
DISCUSSION
In the analysis of data obtained from these experiments, we specifically
account for transport-related phenomena. Thus, the deposition velocities
given in Table 1 represent values that are limited only by the adsorption
and chemical processes leading to sulfur dioxide removal from the gas phase.
These values, then, represent the maximum deposition velocities that would
be encountered over the materials listed in Table 1 under turbulent atmos-
pheric conditions.
_»»
Experimentally, for materials with reactivities > 10 , such as exterior
stucco or cement, it was necessary to conduct experiments at sub-ambient
pressures, in order to qbtain n,on-dif fusion-limited reactivities. Although
such conditions deviate from the ambient atmosphere, the results are more
applicable than those that would be obtained from experiments conducted at
atmospheric pressures. The reason for this is that diffusivities for
sulfur diexide in our experiments, which were conducted under nonturbulent
conditions, are ^ 103 fo 105- (37,38). Thus, a process that would be diffu-
sion-limited in our laboratory experiments, would not be likely to be limited
by transport to the surface in the open atmosphere, but rather by the
adsorption and chemical processes responsible for uptake.
An a.dded feature of the type of experiment reported here is the ability
to measure changes, in deposition velocities with time (sulfur dioxide exposure).
In a number of measurements reported in the literature, materials are exposed
for a fixed period of time and determined total sulfur dioxide uptake. Such
measurements can only give an average value for the deposition velocity, the
magnitude of which depends on the degree of poisoning of the solid under
study.
It is instructive to compare our results with other data reported in the
literature for related materials. In a study conducted on seven European
soils in a system in which a fan was used to mix the air above the soil,
Payrisset and Beilke (24) reported deposition velocities, of 0.19 to 0.60 cm/
sec. They also observed first-order kinetics for sulfur dioxide removal
and evidence of poisoning and measured a slight dependence of removal rates
on relative humidity. In an additional study on five soils from the mid-
western United States, Seim (9) measured average deposition velocities of
0.2 cm/sec. He also reported that deposition velocities were relatively
independent of sulfur dioxide concentrations (first-order kinetics) and
moisture levels.
16
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Measurements of deposition velocities over building materials, notably
limestone, have been reported in the literature (8,13,21). Reported values
range from 0.03 to 0.3 cm/sec, which is considerably lower than the values
we found for cements and stuccos. However, Braun and Wilson (8) measured
the sulfur content of limestone exposed to atmospheric sulfur dioxide to be
2.4 to 2.6 g/m2, which compares favorably with the higher capacities for
sulfur dioxide we have measured, in humidified reaction mixtures.
Several interesting possibilities are suggested from the deposition
velocities and capacities for sulfur dioxide uptake measured here and in
other laboratories. If we assume an average deposition velocity of 1 cm/
sec and an atmospheric sulfur dioxide concentration of 0.1 ppm, from Equation
(1) , a deposition rate of 2.6 X 10 6 g/m2 sec can be calculated. If we
further assume a capacity of 2.5 grams of sulfur dioxide per square meter
of solid surface, we conclude that the ability of a solid surface to remove
sulfur dioxide from the atmosphere will be expended in 11 days, in the
absence of any processes such as precipitation that might act to rejuvenate
the surface activity for sulfur dioxide removal. In an urban area such as
Los Angeles, where midsummer precipitation is negligible, this could lead
to higher sulfur dioxide concentrations than would otherwise be experienced.
Of course, this type of calculation and conclusion is greatly oversimplified
for a number of reasons.
Other variables, such as surface roughness, total areas, and source
strengths, enter into application of the data in Table 1 to the environment.
Generally, our samples had surface roughnesses < ^ 1 mm. Surface roughnesses
in the environment are usually greater than this, in some cases by large
factors. Thus, in the environment, the actual surface area available for
uptake could be significantly greater than that available in our reactor. Of
course, vegetation would have a very high ratio of actual to ground-level
surface areas.
In addition, several possibilities for rejuvenating poisoned surfaces
were indicated above. The few experiments conducted during this study to
explore these possibilities supported those suggestions. Thus, in the open
atmosphere, uptake of sulfur dioxide might be determined by the balance of
rates of surface poisoning and rejunvenation of the active surface.
17
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2. Kellogg, W. W., R. D. Cadle, E. R, Allen, A. L. Lazrus, and E. A. Martell.
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4. Garland, J. A., D. H. F. Atkins, C. J. Readings, and S. J. Caughey.
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10. Hill, C. A. Vegetation: A Sink for Atmospheric Pollutants. J. Air
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11. Abeles,. F. B., L. E. Craker, L. E. Forrence, and G. R. Leather. Fate of
Air Pollutants: Removal of Ethylene, Sulfur Dioxide, and Nitrogen
Dioxide by Soil. Science, 173: 914-916, 1971.
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12. Cox, R. A., and S. A. Penkett. Effect of Relative Humidity on the
Disappearance of Ozone and Sulphur Dioxide in Contained Systems. Atmos.
Environ., 6: 365-368, 1972.
13. Spedding, D. J. Sulphur Dioxide Uptake by Limestone. Atmos. Environ.,
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14. Spedding, D. J., and R. P. Rowland. Sorption of Sulphur Dioxide by
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17. Spedding, D. J. The Sorption of Sulphur Dioxide by Indoor Surfaces.
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27. Hedgpeth, H., H. S. Judeikis, S. Siegel, and T. Stewart. Cylindrical
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20
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/3-77-027
2.
3. RECIPIENT'S ACCESSIOf»NO.
4. TITLE AND SUBTITLE
LABORATORY MEASUREMENT OF SULFUR DIOXIDE
DEPOSITION VELOCITIES
5. REPORT DATE
March 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
H.S. Judeikis and T.B. Stewart
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
The Aerospace Corporation
P.O. Box 92957
Los Angeles, California 90245
10. PROGRAM ELEMENT NO.
1AA603 (1AA008)
11. CONTRACT/GRANT NO.
Grant No. R802687
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory - RTF, NC
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
13. TYPE OF REPORT AND PERIOD COVERED
Interim 11/73-11/76
14. SPONSORING AGENCY CODE
.EPA/600/09
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Measurements of sulfur dioxide deposition velocities have been carried out in
the laboratory with the use of a cylindrical flow reaction. Analysis of data from
these experiments was performed with models that specifically account for diffusive
transport in the system. Consequently, the resulting deposition velocities were
independent of diffusion processes and represented the maximum removal rates that
would be encountered in the environment under turbulent atmospheric conditions.
The measured values ranged from 0.04 cm/sec for asphalt to 2.5 cm/sec for cement,
and were independent of sulfur dioxide and oxygen concentrations as well as relative
humidity and total pressure. Prolonged exposure to sulfur dioxide eventually
destroyed the ability of the various solids to remove this species. Overall
capacities increased significantly at moderate relative humidities, yielding values
of 0.4 to 2.8 grams of sulfur dioxide per square meter of solid in moist systems.
Several experiments indicated that the reactivity of a solid subjected to prolonged
sulfur dioxide exposures could be restored by washing the surface with distilled
water or exposing the spent solid to ammonia. Some implications of these findings
relative to the environment are discussed.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
*Air pollution
*Sulfur dioxide
*Deposition
-'Velocity measurement
*Tests
13B
07B
14B
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
27
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
21
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