ENVIRONMENTAL
PR6TECTION
AGENCY
EPA-600/3-76-088
August 1976 Ecological Resea
UBRARY
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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 tecnnology 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 Socio-economic 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-600/3-76-088
August 1976
CONTINUOUS MEASUREMENT OF
SULFUR IN SUBMICROMETRIC AEROSOLS
BY
Jack L. Durham and William E. Wilson
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
Research Triangle Park, NC 27711
and
E. Baker Bailey
Northrop Services, Inc.
Research Triangle Park, NC 27711
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
RESEARCH TRIANGLE PARK, NC 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 publication.
Mention of trade names or commercial products does not constitute endorsement
or recommendation for use.
ii
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ABSTRACT
A method is described for measuring continuously the total sulfur
in submicrometric aerosols suspended in air containing sulfur dioxide. The
aerocolloid is passed through a tube coated internally with lead
dioxide. The gaseous sulfur dioxide diffuses to the surface of the
tube and reacts irreversibly to form lead sulfate. The aerosol is not
significantly removed in the tube. The total sulfur in the aerosol
is determined by a hydrogen-air flame photometric detector.
A sulfur balance has been demonstrated for the sulfur dioxide-ozone-
olefin reaction system, which produces aerosols containing sulfur.
111
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CONTENTS
Abstract iii
I Introduction 1
II Summary 3
III Conclusions 3
IV Recommendations 4
V Experimental 5
VI Results and Discussion 7
References 15
v
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SECTION I
INTRODUCTION
For the study of sulfur dioxide reaction systems, the estimation
of sulfur in aerosols, such as sulfuric acid droplets, may be made
by taking the difference between the responses of a sulfur flame photo-
metric detector (FPD) analyzer to filtered and unfiltered sample air
(1). The filtered air yields only a response for sulfur dioxide, but
the unfiltered air yields a response for sulfur dioxide plus aerosol
sulfur. Presumably, the difference in responses is an estimate of
the aerosol sulfur concentration. An appreciation of the difficulties
encountered by employing this method can be gained by examining its
application to the measurement of sulfuric acid in the aerosol phase
in the presence of sulfur dioxide at concentrations likely to be encountered
in power plant plumes and laboratory chamber experiments. For example,
if the sulfur dioxide concentration is 2660 yg/m (1 ppm), and the
sulfate concentration is 150 yg/m3 (100 yg/m3 or 0.038 ppm expressed
as equivalent to sulfur dioxide), the difference in the linearized
FPD responses between the unfiltered and filtered sample air would
be about 3.7%. For most laboratory and atmospheric measurements, such
a low signal-to-noise ratio is not acceptable. If a FPD sulfur analyzer
is to be employed, the signal-to-noise ratio for measuring aerosol
phase sulfur can be increased only by selectively decreasing the concentra-
tion of sulfur dioxide (and other gases present that contain sulfur).
The difference in diffusivities is a physical parameter that may
be conveniently exploited in separating gases from aerosols with diam-
eters greater than about 0.01 ym. The diffusion coefficient of sulfur
dioxide is 0.13 cm2/sec (2), whereas the diffusion coefficients of
0.01 to 1 ym diameter aerosols range from 5.2 x 10"1* to 2.7 x 10~7
cm2/sec. If a dilute mixture of sulfur dioxide and sulfuric acid aerosol
flows laminarly through a tube whose wall is a perfect sink for sulfur
dioxide, at the exit the concentration of each species (gases and
aerosols) being depleted by diffusion to the wall is given by the
Gormley-Kennedy (3) equation:
C. = C . {0.819 exp (-3.657 M.) + 0.097 exp (-22.3 M.) + 0.033 exp (-57 M.)}
i 01 i i i
where i = the diffusing species
C. = the exiting concentration
C . = the entering concentration
01 *
M. = irD.L/Q
i i ' *
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D. = the diffusion coefficient of species i
L = the length of the tube
Q = the volume flow rate
The use of a "diffusion denuder" for selectively removing sulfur dioxide
in the presence of sulfuric acid has been mentioned previously by Crider
et al (4), but operational parameters and performance were not reported.
These workers showed that cupric ion intensifies the sulfur chemilumi-
nescence. Fish and Durham (2) have shown that the removal of sulfur
dioxide by a diffusion denuder coated with lead dioxide can be described
by the Gormley-Kennedy equation.
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SECTION II
SUMMARY
A sulfur dioxide "diffusion-denuder" was used to measure continuously
the concentration of sulfur in the aerosol phase for the sulfur dioxide-
ozone-propylene-water vapor reaction system.
The disappearance of sulfur dioxide was quantitatively matched
by the appearance of sulfur in the aerosol phase.
SECTION III
CONCLUSIONS
I It appears that for the reaction conditions studied, the sulfur
dioxide diffusion denuder permitted direct observation of aerosol sulfur
concentration. For the three cases observed, the sum of the sulfur
FPD response for sulfur dioxide concentration and the sulfur FPD response
for the aerosol sulfur concentration was approximately constant (a < 3%)
over the duration of the reaction period, demonstrating that a sulfur
balance has been made. This behavior indicates that the depletion
of sulfur dioxide occurred by conversion to aerosols, probably in the
form of sulfuric acid.
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SECTION IV
RECOMMENDATIONS
1. The sulfur dioxide "dif fusion-denuder" should be used to measure
submicron aerosol sulfur (sulfate) continuously in smog chamber
experiments. Caution should be exercised in using this technique
for aerosols that contain metal cations, which may modify the
chemiluminescence in the FPD.
2. The sensitivity of the sulfur FPD (^10ug/m3) needs to be improved
in order to allow the sulfur dioxide "dif fusion-denuder" technique
to be used for continuous submicron aerosol sulfate concentration
measurements in urban atmospheres.
3 . Development work should begin to develop techniques for using
"dif fusion-denuders" to remove reactive gases (sulfur dioxide,
ammonia, ozone, nitrogen dioxide, hydrocarbons, etc.) before
they contact filters in aerosol samplers. The "dif fusion-denuder"
technique offers a means of reducing to an insignificant level
gaseous chemical reactions with the filter sample.
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SECTION V
EXPERIMENTAL
For this study, diffusion denuders were constructed from tubes
8 mm in diameter and 20 cm long. The inside wall of a diffusion denuder
was made a perfect sink for sulfur dioxide by coating it with lead
dioxide. The lead dioxide coating was applied by introducing a thick
slurry of lead dioxide mixed with isopropyl alcohol into the tube. The
tube was held horizontally and rotated about its axis, and warm air was
passed through the tube until the lead dioxide coating was dry. The
lead dioxide coating may also be prepared as described by Durham et al (5).
In sampling the air mixture given as an example in the Introduction,
for which [S02] = 2660 yg/m3 and [Sa] = 100 yg/m3 (expressed as equivalent
to SO2) , the diffusion denuder at a flow rate of 3.33 cm3/sec will reduce
[862] to 0.24 yg/m3. If the size of the aerosol is between 0.01 ym and
1 ym, the decrease in [Sa] by diffusion to the wall in the denuder is on
the order of 2% or less. The ratio [Sa]/[SC>2] at the entrance of the
diffusion denuder is 0.038, but is 410 at the exit. For this set of
conditions, the ratio has been increased by a factor of about 10\ mainly
by a reduction in the sulfur dioxide concentration. Aerosol smaller than
0.01 ym may be depleted by diffusion to the tube wall, and aerosol larger
than 1 ym may be depleted by sedimentation and impaction.
The dark gas phase reaction of propylene, ozone and sulfur dioxide
to produce aerosol was employed in an attempt to obtain a sulfur balance
for reactants and products. The experimental system has been described
in detail by McNelis (6). The reaction took place in a 400-liter bag
constructed from Dupont "Tedlar" PVF film. The surface-to-volume ratio
was approximately 7.8 m~ . The "Tedlar" bag was covered with an alumi-
nized polyester bag, which served as an optical filter to prevent ultra-
violet radiation from entering the reactor. During the experiments,
continuous measurements were made of the ozone concentration. Propylene
concentration measurements were made approximately at 10-minute intervals.
The sulfur dioxide concentration measurement was made continuously with
a sulfur FPD analyzer that had a Teflon filter (pore size: 0.45 ym)
inserted in its Teflon sample inlet line. The production of submicron
aerosol containing sulfur (Sa) was monitored continuously by using a
sulfur FPD analyzer that had only a diffusion-denuder tube inserted in
its sample inlet line. The sample line with diffusion-denuder tube was
attached to the bottom of the bag and operated in the vertical position.
The sulfur analyzer was modified such that the exit line from the diffusion-
denuder was connected directly to the air inlet of the hydrogen flame
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burner of the flame photometric detector. Bypassing the valves and roto-
meter in the air-flow control section of the analyzer is necessary
because the loss of aerosol at these points would be significant. In
order to make this direct connection, a 180 degree bend in the sampling
line was required, which may introduce turbulence in the sampling line
and result in aerosol deposition. However, this loss is not as great
as that for the aerosol sample passing through the air-flow control
section of the analyzer. The optimum configuration would have been to
mount the sulfur analyzer above the reactor and aspirate the sample through
a vertical line (containing the diffusion-denuder tube) with no bends.
Laboratory space considerations precluded the use of such a configuration
for this investigation. Aerosol size distributions were made during the
reaction by using an electrical mobility analyzer. The commercially-
available instruments used in these experiments are listed in Table I.
TABLE I. COMMERCIAL INSTRUMENTS USED IN THESE EXPERIMENTS.
Parameter Instruments
Propylene Tracer, Inc., Model MT150 Gas Chromatograph;
Poropak Q column
Ozone Meloy Laboratories, Inc., Model OA350
Ozone Analyzer
Sulfur dioxide Meloy Laboratories, Inc., Model SA 185-2
Flame Photometric Detector Sulfur Analyzer
(with Teflon particulate filter)
Sulfur in aerosol (S ) Meloy Laboratories, Inc., Model SA 185-2
Flame Photometric Detector Sulfur Analyzer
(air sample line modified; see text)
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SECTION VI
RESULTS AND DISCUSSION
The performance of the diffusion-denuder was tested for three
different mixtures of ozone, propylene and sulfur dioxide. The
relative humidity was 16% and the temperature was 31°C for these
mixtures. The initial reactant concentrations for Run A were:
[SO2] = 1700 yg/m3 (0.5 ppm), [C3H5] = 8000 yg/m3 (4 ppm), and [03] =
2000 yg/m3 (1 ppm). In Figure 1, the disappearance curves for the
reactants and the appearance curve for aerosol sulfur [Sa] are plotted.
Although the aerosol sulfur is probably sulfuric acid, [sa] is plotted
in the figures in gravimetric units equivalent to S02. It can be seen
in Figure 1 that the [S02] depletion rate is approximately matched by
an increase in [Sa]. Since the concentrations of S02 and Sa are in
compatible units, the sum [SO2] + [Sa] should be constant if a sulfur
balance has been made. For Run A, the initial sulfur dioxide concen-
tration was 1700 yg/m3. During the 60 minutes of reaction that were
monitored, the average of the sum of [S02J + [Sa] was 1670 yg/m3, with
a standard deviation (a) of 2%.
Particle size distributions were made during the reaction period
and were converted to volume distributions, which are shown in Figure 2.
These data indicate that generally the aerosol volume below 0.07 ym
was not significant during the reaction period, especially after the
first 7 minutes. Size distributions were not obtained for diameters
greater than 1 ym; thus, it is not known what the volume fraction was
that had diameter greater than 1 ym. From the small variation (a = 2%)
of the sum of the sulfur dioxide and aerosol sulfur concentrations during
the reaction period, it appears that loss of aerosol sulfur in the
sampling line was small. Most likely, the fractional aerosol volume
with diameter greater than about 2 ym was insignificant. The small
variation in [802] + [Sa] could reasonably be due to deficiencies in
the performance characteristics of the sulfur FPD, such as a 95% response
time of about 10 minutes to a change of a factor of 10 in the sulfur
concentration.
The initial reactant concentrations for Run B were: [SO2] =
1660 yg/m3; [03] = 3000 yg/m3; and [C3H6] = 8200 yg/m3. The decrease
in reactant concentrations and the appearance of the reactant product
Sa for Run B are plotted in Figure 3. This reaction mixture had the
same concentration of sulfur dioxide as for Run A, but had about 50%
more ozone and 30% more propylene. Inadvertently, the concentration
of Sa was not recorded for the first 24 minutes of the reaction, but
for the periods for which data were recorded, the behavior exhibited
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30
TIME, minutes
Figure 1. Variation of concentration with time for Run A.
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1500
1000
ro
u
en
o
500
O 7 minutes
030
A45
• 60
0.05
0.1
0.3
0.5
1.0
DIAMETER (Dp),
Figure 2. Development of the volume distribution with time for Run A.
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z
o
o
z
o
u
30
TIME, minutes
60
Figure 3. Variation of concentration with time for Run B.
10
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by the reactants and aerosol sulfur product is similar to that of Run A.
The aerosol volume distribution, shown in Figure 4, is not as wide as
that of Run A. A mode exists at about 0.4 ym, and it appears that most
of the aerosol volume was below 1 ym. For Run C, the initial reactant
concentrations were: [S02] = 830 yg/m3; [03] = 2000 yg/m3; and [C3H6] =
5280 yg/m3. This mixture had the same initial concentration of ozone
as Run A, but 50% less sulfur dioxide and 20% less propylene. The
behavior exhibited by the reactants and aerosol sulfur product Sa for
Run C is shown in Figure 5 and is also similar to that of Run A. The
aerosol volume distributions for Run C, shown in Figure 6, are shifted
toward smaller sizes with respect to those of Run A. The distributions
for Run C indicate that the aerosol volume is insignificant below 0.05 ym.
After the reaction time of 30 minutes, a mode appeared at about 0.3 ym,
and due to the small change in the sum [SO2] + [sa], it is unlikely that
significant aerosol volume existed for aerosol with diameters greater than
several micrometers. For Runs B and C, the consumption of sulfur dioxide
is matched by the appearance of aerosol sulfur, similar to the observa-
tion for Run A, within the expected performance limitations of the sulfur
FPD. The sums [SO2] + [Sa] for Runs B and C are plotted in Figures 3 and
5, respectively. The standard deviation of [S02] + [S_J for Run B is 2%
and for Run C is 3%. The initial values for the sulfur dioxide concentra-
tion, average values for [S02] + [Sa], and standard deviations for the
three Runs are summarized in Table II.
TABLE II. COMPARISON OF AVERAGE VALUE OF [SO2] + [S-J WITH
INITIAL VALUE OF [SO2].
Average of Standard
Initial [302], pg/m3 [S02] + [Sa] Deviation, %
Run A 1660 1670 2
Run B 1660 1670 2
Run C 830 810 3
11
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1500
1000
PO
E
u
PO
500
0.05
O 5 minutes
015
A 30
• 60
0.1
0.3
0.5
1.0
DIAMETER (Op), pun
Figure 4. Development of the volume distribution with time for Run B.
12
-------
30
TIME, minutes
60
Figure 5. Variation of concentration with time for Run C.
13
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600
500
400
CO
E
u
CO
a 300
200
100
O 5 minutes
030
A 45
• 60
0.05
0.1
0.3
0.5
1.0
DIAMETER (Dp),
Figure 6. Development of the volume distribution with time for Run C.
14
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REFERENCES
1. Crider, W.L. (1965) "Hydrogen Flame Emission Spectrophotometry
in Monitoring Air for Sulfur Dioxide and Sulfuric Acid," Anal.
Chem., 37_, 1770-1773.
2. Fish, B.R. and J.L. Durham (1971) "Diffusion Coefficient of Sulfur
Dioxide in Air," Environ. Letters, 2_, 13-21.
3. Gormley, P. and M. Kennedy (1949) "Diffusion from a Stream Flowing
Through a Cylindrical Tube," Proc. Royal Irish Acad., 52A, 163-169.
4. Crider, W.L., N.P. Barkley, M.J. Knott, and R.W. Slater, Jr. (1969)
"Hydrogen Flame Chemiluminescence Detector for Sulfate in Aqueous
Solutions," Anal. Chem. Acta, 47, 237-241.
5. Durham, J.L., J. Wagman, B.R. Fish, and F.G. Seeley (1972)
"Radiochemical Analysis of Sulfur-35 Dioxide Adsorbed on Lead
Dioxide," Anal. Letters, 5_, 469-478.
6. McNelis, D.N. (1974) "Aerosol Formation From Gas-Phase Reactions
of Ozone and Olefin in the Presence of Sulfur Dioxide," U.S.
Environmental Protection Agency, Report No. EPA-650/4-74-034.
15
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing}
REPORT NO.
EPA-600/3-76-088
3. RECIPIENT'S ACCESSIOf*NO.
4. TITLE AND SUBTITLE
CONTINUOUS MEASUREMENT OF SULFUR IN
SUBMICROMETRIC AEROSOLS
5. REPORT DATE
August 1976
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Jack L. Durham, William E. Wilson,
and E. Baker Bailey
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Environmental Sciences Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
10. PROGRAM ELEMENT NO.
1AA001
11 CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
In-house. 6/74-6/75
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A method is described for measuring continuously the total sulfur in
submicrometric aerosols suspended in air containing sulfur dioxide. The aero-
colloid is passed through a tube coated internally with lead dioxide. The
gaseous sulfur dioxide diffuses to the surface of the tube and reacts
irreversibly to form lead sulfate. The aerosol is not significantly removed
in the tube. The total sulfur in the aerosol is determined by a hydrogen-air
flame photometric detector.
A sulfur balance has been demonstrated for the sulfur dioxide-ozone
olefine reaction system, which produces aerosols containing sulfur.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
* Air pollution
* Aerosols
* Sulfur
Sulfur dioxide
Flame photometry
13B
07D
07B
14B
13. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
UNCLASSIFIED
21 . NO. OF PAGES
22
20. SECURITY CLASS (This page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
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