oEPA
United States
Environmental Protection
Agency
Research and Development
Environmental Sciences Research EPA-600/2-80-087
Laboratory May 1980
Research Triangle Park NC 27711
In 1IDIMR¥
ffHTAL FnOTTCr.TTOTj
Development of a
Portable Device to
Collect Sulfuric
Acid Aerosol
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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 nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-80-087
May 1980
DEVELOPMENT OF A PORTABLE DEVICE TO
COLLECT SULFURIC ACID AEROSOL
Final Report
by
Herbert C. Miller, David W. Mason
and William J. Barrett
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
Contract No. 68-02-2468
Project Officer
Kenneth J. Krost
Atmospheric Chemistry and Physics Division
Environmental Science 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 commercial products consititute endorsement or
recommendation for use.
ii
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ABSTRACT
A quantitative, interference-free method for collecting
sulfuric acid aerosol on a filter was developed and field tested.
Since previous research found that severe losses of sulfuric
acid were caused by ammonia, ambient particulate material, and
other interferents, a method was needed that converts sulfuric
acid to a stable derivative for subsequent analysis. Methods
evaluated for direct fixation of sulfuric acid aerosol were not
found to be selective. Therefore, a sampling method was investi-
gated that uses a combination of selective volatilization of
the sulfuric acid, prefiltration of particulate interferents,
and derivatization of the vaporized acid on an alkali-impregnated
filter. Research on the volatilization, prefiltration, and de-
rivatization technique, and the development and field evaluation
of a prototype sampler based on this technique are described.
This report was submitted in fulfillment of Contract No.
68-02-2468 by Southern Research Institute under the sponsorship
of the U.S. Environmental Protection Agency. This report covers
the period from September 1976 to September 1978.
iii
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CONTENTS
Abstract
Figures
Tables
1. Introduction .................... 1
2. Conclusions and Recommendations .......... 3
3. Summary of Results ................. 5
Results of Interference Studies with
Sulfuric Acid Collected on Filters ..... 5
Results of Experiments Collecting Sulfuric
Acid as a Derivative of Perimidylam-
monium Bromide (PDA-Br) ........... 6
Results of Experiments in Which Sulfuric
Acid was Collected after Volatilization
and Pref iltration .............. 6
Development of a Prototype Sampler for
Sulfuric Acid Aerosol ............ 7
Results of Field Evaluation of the
Prototype Sampler .............. 7
4. Experimental Apparatus and Procedures for
Collection of Sulfuric Acid Aerosol by
Conventional Methods ............... 9
Sulfuric Acid Aerosol Generator ........ 9
Analytical Methods .............. 18
Filter Materials ............... 22
5. Interference Studies Associated with the
Collection of Sulfuric Acid Aerosol ....... 29
Collection of Sulfuric Acid Aerosol ...... 29
Interference from Ammonium Salts of
Sulfuric Acid ................ 35
Interference from Ambient Particulate
Material — Analysis Corrected for
Interference from Ammonium Salts ...... 35
6. Experimental Apparatus and Procedures for
Collection of Sulfuric Acid Aerosol by
Derivatization Methods .............. 40
Analytical Methods .............. 40
Derivatization with Perimidylanunonium
Bromide ................... 46
Fixation of Sulfuric Acid Aerosol With
Versapor Filters .............. 49
v
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CONTENTS
Volatilization, Prefiltration, and Deriva-
tization of Sulfuric Acid Aerosol 51
Preliminary Apparatus with a Sample Flow
Rate of 3 1/min 51
Modified Apparatus with a Sample Flow
Rate of 14 1/min 56
Development of Prototype Sampler for
Sulfuric Acid Aerosol 65
Interference Studies 65
Assembly of the Prototype Sampling
Apparatus 72
Final Laboratory Evaluation of Prototype
Sampling System 76
Field Evaluation of Prototype Sampler for Sulfuric
Acid Aerosol 79
VI
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FIGURES
Number Page
1 Schematic of sulfuric acid aerosol generator . 10
2 Plot of slope of cumulative number distribution
vs. particle diameter for H-jSO,, generator . . 15
3 Plot of cumulative volume (mass) distribution
vs. particle diameter for H2SOi» generator . . 16
4 Plot of slope of cumulative volume distribution
vs. particle diameter for H2SO^ generator . . 17
5 Flame photometric detector apparatus for
H2SOU analysis 20
6 FPD calibration for H2SC\ deposited on filters
and volatilized at 200°C 23
7 Calibration of modified FPD for H2SO1| deposited
on filters and volatilized for analysis at
200°C 24
8 Calibration of modified FPD for H2SO^ deposited
on filters and volatilized for analysis at
150°C 25
9 Remodified flame photometric detector apparatus
for H2SOi» and PDA-SOu analysis 41
10 FPD calibration for PDA-SO^ decomposed at 450°C 43
11 Ion chromatograph calibration with standard
solutions of Na2SO1| in H2O 45
12 Apparatus for volatilization, prefiltration,
and derivatization of sulfuric acid aerosol . 53
13 Apparatus for removal of interfering gases
and volatilization, prefiltration, and
derivatization of sulfuric acid aerosol ... 67
Vll
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FIGURES
Number Page
14 Apparatus for the removal of basic and acid
interfering gases and the volatilization,
prefiltration, and derivatization of sulfuric
acid aerosol 73
15 Prototype apparatus for volatilization, pre-
filtration, and derivatization of sulfuric
acid aerosol 74
16 Gas diffusion denuders 75
17 Teflon filter holder 75
18 Ambient roadsite sampling with prototype
sampling system 75
19 Schematic of Teflon filter holder containing
the Teflon prefilter and the alkali-impreg-
nated Teflon collection filter 77
20 Hourly average concentrations of total sulfate
and sulfuric acid on six days beside a city
street 81
21 Remote sampling at interstate highway 82
22 Ion chromatographic analyses of collected
samples 82
23 Hourly concentrations of total sulfate and
sulfuric acid on August 23, 1978, at an
interstate highway 85
24 Hourly average concentrations of total sulfate
and sulfuric acid on three days at an inter-
state highway 86
viii
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TABLES
Number Page
1 Aerosol Generator Standard Operating Parameters 12
2 Sulfuric Acid Collected from the Generator as a
Function of Sampling Time 12
3 Determination of S02 Concentration in the HjSO^
Aerosol Generator with the West-Gaeke Method 14
4 Data for the Calibration of the FPD Apparatus . 22
5 Comparison of Parallel Fluoropore and Mitex
Filters in Sampling Sulfuric Acid Aerosol . . 26
6 Effect of Predeposited Fe203 upon Sulfuric Acid
Collected on Mitex Filters 30
7 Effect of Predeposited PbO upon Sulfuric Acid
Collected on Mitex Filters 31
8 Effect of Predeposited Ambient Particulate
Material upon Sulfuric Acid Collected on
Mitex Filters 33
9 Effect of Ambient Particulate Material on FPD
Measurement of Sulfuric Acid Predeposited on
Mitex Filters 34
10 Interference from Ammonium Salts of Sulfuric
Acid in FPD Analysis 36
11 Results of FPD Measurement of Sulfuric Acid
Collected on Mitex Filters following Collec-
tion on Ambient Particulate Material .... 37
12 Results of FPD Measurements of Sulfuric Acid
Collected on Mitex Filters prior to Collec-
tion of Ambient Particulate Material .... 39
13 Preliminary Calibration Data for Ion Chromato-
graphic Determination of Sulfate 44
IX
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TABLES
Number Page
14 Derivatization of Sulfuric Acid Aerosol with
PDA-Br in the Presence of Ambient Particulate
Material 48
15 Evaluation of Selectivity of PDA-Br with
Aerosols of Sulfate Salts 50
16 Passage of Sulfuric Acid through the Heated
Glass Sampling Probe 52
17 Passage of Sulfuric Acid through the Heated
Teflon-lined Sampling Probe 54
18 Passage of Volatilized Sulfuric Acid Aerosol
through Mitex Filters as a Function of
Temperature 55
19 Retention of Sulfate Salt Aerosols by a Heated
Mitex Filter 55
20 Derivatization Efficiency of Alkali-impregnated
Filters for Sulfuric Acid Vapor at 110°C . . 57
21 Volatilization Efficiency of the Heated Probe
at a Temperature of 120°C and a Flow Rate
of 14 1/min 59
22 Prefiltration Efficiency of Mitex Filters at a
Temperature of 130°C and a Flow Rate of
14 1/min 59
23 Efficiency of Sulfuric Acid Derivatization by
Alkali-impregnated Teflon Filters at a Flow
Rate of 14 1/min 60
24 Efficiency of Sampling Method for Sulfuric
Acid Aerosol 64
25 Sulfur Dioxide Removal by Gas Diffusion Denuder 68
26 Sulfur Dioxide Removal by Gas Diffusion Denuder 69
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TABLES
Number Page
27 Passage of Sulfuric Acid Aerosol through Gas
Diffusion Denuder 70
28 Passage of Sulfuric Acid Aerosol through the
Dehumidifier 71
29 Study of Interference from Ambient Particulate
Material 72
30 Efficiency of Prototype Sampling Apparatus for
Various Concentrations of Sulfuric Acid
Aerosol 78
31 Ambient Sulfuric Acid Aerosol Determinations
beside a City Street 78
32 Ambient Sulfuric Acid Aerosol Determinations
from Remote Interstate Highway Sampling . 84
XI
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SECTION L
INTRODUCTION
The increasing use of sulfur-bearing coal for the produc-
tion of electrical energy, the generation of significant quanti-
ties of sulfuric acid aerosol by catalytic converters in auto-
mobiles, and the appearance of new information on the irritant
effects of sulfuric acid and sulfates are factors that have com-
bined to intensify interest in the measurement of ambient sul-
furic acid aerosol. The emission of sulfuric acid by automobiles
is a problem of special interest because of the fairly high con-
centrations found in the immediate vicinity of roadways and the
small size of the sulfuric acid particles generated.
In a series of measurements made with catalyst-equipped
automobiles at the General Motors Test Track in October, 1975,
it was found that most of the sulfur in gasoline was oxidized
to sulfur trioxide and appeared as sulfuric acid aerosol near
the test site. Concentrations in the range of 3 to 6 yg/m3 were
found. The sulfuric acid was apparently neutralized or partially
neutralized by ambient ammonia within a fairly short time and
within a fairly short distance of travel. Nevertheless, the
possible harmful effect on the health of persons in the vicinity
of heavily traveled roadways continues to be a matter of serious
concern.
The potentially harmful health effects stem from certain
unique properties of sulfuric acid aerosol. Sulfuric acid is
the most severe bronchial and lung irritant among the various
species of sulfur compounds that can occur in the atmosphere.
The particles derived from automobiles have a mass mean diameter
of only a few hundredths of a micrometer and therefore can pene-
trate deeply into the lung and be deposited in the alveoli.
These small particles are relatively highly concentrated in terms
of the molarity of the sulfuric acid and are therefore stronger
irritants than particles that have grown in size over a period
of time by coagulation and adsorption of water vapor.
In order to assess the health effects potential of sulfuric
acid having the properties described, it is necessary to have
sampling and analytical methods that will measure sulfuric acid
concentrations without interference or artifacts arising from
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the presence of other substances in the sampled atmosphere or
from various environmental factors such as relative humidity
or temperature. Because many interfering reactions are possible,
especially during the collection of a sample, the accurate de-
termination of sulfuric acid aerosol is a difficult problem that
is just now beginning to be adequately solved.
Although sampling cannot be arbitrarily isolated from the
method of analysis subsequently used to determine the species
sought, there are certain difficulties peculiar to sampling
sulfuric acid aerosol. In general terms, these difficulties
may be said to result from interactions of sulfuric acid with
other particulate and gaseous constituents of the ambient atmo-
sphere or the sample and from the simultaneous presence in the
atmosphere of sulfate salts and sulfur dioxide. Reactions of
sulfuric acid with copollutants have been shown to occur on
filters, the collection medium that is generally best suited
for sampling atmospheric particulate material. The oxidation
of sulfur dioxide to sulfate also occurs on filters under certain
conditions. These phenomena—and others that may be expected
to occur—often result in failure of attempts to measure ambient
concentrations of sulfuric acid because some or all of the acid
is lost during the sampling process. Furthermore, methods of
analysis that involve dissolving the sulfuric acid along with
other soluble constituents of the sample introduce errors re-
sulting from reactions that occur in the solvent medium; a sam-
pling method that will permit subsequent processing and analysis
of the sample without loss (or addition) of sulfuric acid is
therefore desirable.
Work on this contract was concerned principally with pro-
viding a sampling and analytical method that will avoid errors
resulting from degradation of sulfuric acid during or after its
collection on a filter. During the early months of the contract
period, efforts were made to evaluate the collection of sulfuric •
acid aerosol and effects of interferents using flame photometric
detection. However, it was later concluded that the fixation
of sulfuric acid as a stable, nonreactive product is clearly
a desirable approach, and may well provide the only means of
avoiding interference effects.
This Final Report describes the techniques used to gener-
ate the sulfuric acid aerosol, the analytical methods used to
measure sulfuric acid, and the prototype apparatus and procedures
for derivatization of sulfuric acid both as the aerosol and as
the prefiltered vapor. Further, it gives the results of labora-
tory experiments designed to determine the effects of various
potential interferents on the collection of sulfuric acid, the
results of field sampling with a prototype sampler, and a summary
of the conclusions reached.
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SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
The collection of sulfuric acid aerosol on Teflon filters,
as a first step in the measurement of ambient sulfuric acid con-
centrations, is subject to interference by alkaline gases and
by particulate material in the air sample. Substantial losses
of sulfuric acid are caused by gaseous ammonia and ambient par-
ticulate material from an urban environment. Since the deter-
mination of atmospheric sulfuric acid aerosol collected on a
filter over a period of time is subject to error from interfering
reactions that may occur during sampling, the need is evident
for stabilization of sulfuric acid during sampling. Direct
methods of fixation of sulfuric acid aerosol were not found to
be selective; it appeared that a better method of sampling would
involve selective volatilization of the sulfuric acid and filtra-
tion of the ambient particulate interferents prior to derivatiza-
tion of the vapor.
A prototype sampler—based on the concept of volatiliza-
tion, prefiltration, and derivatization of sulfuric acid—was
constructed and demonstrated to be of practical value for deter-
mination of ambient sulfuric acid aerosols in both laboratory
and field studies. This methodology in conjunction with sulfate
determinations by ion chromatography has the following advantages
• Minimal losses of sulfuric acid during sampling—less
than 15%.
• Good specificity—freedom frcm the usual sources of inter-
ference in determinations of sulfuric acid, i.e., other
sulfur species.
• Good sensitivity—with a conservative limit of detection
of 0.5 yg of sulfuric acid per cubic meter of ambient
air.
• Excellent stability of collected samples for 1C analysis
in the laboratory at a later date.
The following recommendations would provide a logical ex-
tension of this work:
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• Further improve the conditions and apparatus for volatili-
zation and prefiltration of sulfuric acid, emphasizing
the effects of volatilization temperature, sampling flow
rate, and nature of exposed surfaces within the sampling
device.
• Conduct a systematic evaluation of other derivatizing
agents and filter media impregnated with the derivati-
zing agents to achieve optimal fixation of the volati-
lized sulfuric acid so that the derivative can be readily
identified and quantitated.
• Investigate in depth the potential for interference from
ambient gaseous compounds and ambient particulate material,
especially various size aerosols of ammonium sulfate
salts, during sampling of sulfuric acid aerosol with
the combined volatilization, prefiltration, and derivati-
zation methodology.
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SECTION 3
SUMMARY OF RESULTS
The most significant results of the experimental studies
conducted during this contract are summarized in the following
paragraphs. Details are given in the latter sections.
RESULTS OF INTERFERENCE STUDIES WITH SULFURIC ACID COLLECTED
ON FILTERS
• Sulfuric acid aerosol was collected on Teflon filters
with prederosited Fe203. Average recoveries of sulfuric
acid samples were 18% for "high" levels and 79% for "low"
levels of Fe203, as measured by flame photometric detec-
tion (FPD) at 200°C.
• Sulfuric acid aerosol was collected on Teflon filters
with predeposited PbO. Average recoveries of sulfuric
acid samples were 19% for "high" levels and 66% for "low"
levels of PbO as measured by FPD at 200°C.
• Ambient particulate material was collected on Teflon
filters for periods of 3 and 8 hr at 6 1/min; the filters
were then used to collect sulfuric acid aerosol. For
both levels of particulate loading the losses of the
acid approached 90% as measured by FPD with volatiliza-
tion at 200°C.
• Sulfuric acid aerosol was collected on Teflon filters;
the filters were then used to sample ambient particulate
material for 1 hr at 14 1/min. The average loss of sul-
furic acid was 93% as measured by FPD at 150°C.
• Aerosols of ammonium sulfate salts were found to be a
source of positive interference in the determination
of sulfuric acid by FPD with volatilization at 200°C.
Lowering the FPD volatilization temperature to 150°C
reduced the contribution from the ammonium salts but
also adversely affected the sensitivity of detection
of sulfuric acid.
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RESULTS OF EXPERIMENTS COLLECTING SULFURIC ACID AS A DERIVATIVE
OF PERIMIDYLAMMONIUM BROMIDE (PDA-Br)
• Approximately 100 yg of ambient particulate material
was collected on Teflon filters impregnated with the
PDA-Br reagent. The filters were used to collect sul-
fur ic acid aerosol and then analyzed by FPD with thermal
decomposition at 450°C. The results suggested that
sulfuric acid was completely recovered.
• Aerosols of the ammonium sulfate salts were collected
on Teflon filters impregnated with PDA-Br and analyzed
by FPD at 200 and 450°C. No response was found at 200°C,
but the response found at 450°C indicated that PDA-Br
was not selective for sulfuric acid since it appeared
that the ammonium salts were also derivatized.
RESULTS OF EXPERIMENTS IN WHICH SULFURIC ACID WAS COLLECTED AFTER
VOLATILIZATION AND PREFILTRATION
• Sulfuric acid aerosol was volatilized at a flow rate
of 3 1/min in a heated sampling probe at 110°C. The
vapors were then passed through a Teflon filter heated
to 120°C. At least 90% of the resulting vapor passed
the heated filter. The barium chloranilate (BCA) method
was used for determination of sulfuric acid as sulfate.
• Ammonium sulfate aerosols were sampled under the same
conditions with the heated sampling probe. BCA analysis
showed that an average of 85% of the ammonium salts were
retained by the heated (120°C) prefilter.
• Volatilized and prefiltered sulfuric acid was passed
through filters impregnated with PDA-Br, sodium carbo-
nate, or sodium hydroxide at 3 1/min. Very little of
the sulfuric acid vapor was collected by the PDA-Br
filters but an average of 60% was fixed by the sodium
carbonate filters and 80% by the sodium hydroxide filters.
• An increase in the flow rate to 14 1/min was achieved
with a new probe. The overall efficiency of the volatili-
zation and prefiltration was lowered to 70% from a pre-
vious overall efficiency of 80% at 3 1/min. The efficiency
was later increased to 80% by increasing the volatiliza-
tion temperature to 130°C.
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The amount of sulfuric acid derivatized after volatili-
zation and prefiltration at 14 1/min and 130°C was in-
creased to greater than 90% using a disk-membrane Teflon
filter impregnated with approximately 60 mg of sodium
hydroxide. The filters were extracted ultrasonically
and the sulfate concentration determined by ion chro-
ma tography.
Ammonium sulfate aerosols were sampled with the heated
sampling probe of 14 1/min and 130°C. Ion chromatographic
analysis showed that greater than 99% of the ammonium
sulfate salts were retained by the prefilter.
DEVELOPMENT OF A PROTOTYPE SAMPLER FOR SULFURIC ACID AEROSOL
• The sampling apparatus for sulfuric acid aerosol based
on the volatilization, prefiltration, and derivatization
methodology was incorporated into a prototype sampler.
• A gas diffusion denuder was used to remove more than
99% of the sulfur oxide gases from the sampled airstream
since these gases could be converted to sulfate by the
alkali-impregnated filter.
• A second denuder was used to remove like amounts of
ammonia from the sampled airstream to prevent the re-
action of ammonia with the volatilized sulfuric acid
to form sulfate salts which could be removed by the
prefilter.
• A dehumidifier was necessary to prevent the build-up
of water vapor in the two denuders.
• The overall efficiency of the prototype sampler collect-
ing laboratory-generated sulfuric acid aerosol at low
concentrations was 83%. The minimum detectable level
of the prototype sampler was approximately 0.5
RESULTS OF FIELD EVALUATION OF THE PROTOTYPE SAMPLER
• The prototype sampler was operated at two local roadside
sites in the Birmingham, Alabama, area. Two control
samples were collected with each sample to determine
the amount of total sulfate and sulfates less than 2 wm
in diameter present at each sampling site.
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Approximately 20% of the nonsized sulfate collected be-
side a city street with an average traffic flow of ap-
proximately 600 cars per hour was found to be sulfuric
acid aerosol.
Approximately 30% of the nonsized sulfate collected be-
side an interstate highway with an average traffic flow
of 8,000 to 10,000 cars per hour was found to be sulfuric
acid aerosol.
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SECTION 4
EXPERIMENTAL APPARATUS AND PROCEDURES FOR
COLLECTION OF SULFURIC ACID AEROSOL BY
CONVENTIONAL METHODS
SULFURIC ACID AEROSOL GENERATOR
Operating Parameters
A modified version of the generator described by Thomas
e_t al. * was used for laboratory studies with sulfuric acid aerosol.
The use of this system, which was constructed and used in earlier
work at Southern Research Institute (under EPA Contract No. 68-
02-2234) , was continued because it was shown to have certain
attributes that were considered to be desirable for the purposes
of this project. In particular, no other generator system could
be expected to produce predominantly submicron particles, while
simultaneously permitting flexibility for the addition of poten-
tial gaseous and particulate interferents directly to the aerosol.
It also allowed easy adjustment of concentrations, particle size,
flow rates, and other parameters.
The generator is illustrated in Figure 1. Its principal
component was a Beckman No. 4020 atomizer-burner situated at
the base of a 1.22-m by 99-mm i.d.Pyrex chimney. The fuel gas
was hydrogen and the oxidizer was a mixture of oxygen and argon.
A dilute solution of sulfuric acid was aspirated into the flame
from a beaker placed underneath the burner. The ends of two
13-mm i.d. glass probes were located close together about 10
cm below the top of the chimney. One of these probes had a side
arm closed with a silicone septum through which gases could be
added at known flow rates. The probes were connected to stain-
less steel holders for 47-mm filters or impinger-bubblers which
were followed by 6-1/min critical orifices and a carbon-vane
vacuum pump. One side of this dual sampling system was always
used as the reference for determination of the concentration
of the sulfuric acid aerosol, while the other side was used for
the introduction of interferent gases or for holding an impregnated
*Thomas, R.L., V. Dharmarajan, and P.W. West. Convenient Method
for Generation of Sulfuric Acid Aerosol, Environ. Sci. Technol.,
8(10):930-935, 1974
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1
EXHAUST
FILTER
HOLDER
OPTIONAL
CONFIGURATIONS
SAMPLE PROBE
SEPTUM
REFERENCE
PROBE
I
->
,->
rr
CRITICAL
ORIFICE
FILTER HOLDERS
80%
ISOPROPANOL
IMPINGER
TO
PUMP
CRITICAL
ORIFICE
PYREX
CHIMNEY
FLAME' I , , I
SULFURIC
ACID
SOLUTION
SAME
OPTIONAL
CONFIGURATIONS
Figure 1. Schematic of sulfuric acid aerosol generator
(not to scale).
10
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filter or a filter containing predeposited particulate material.
The excess aerosol from the top of the chimney was exhausted
through a hood.
The basic operating parameters for the aerosol generator
and their typical values are shown in Table 1. All of these
parameters were varied experimentally in order to find the opti-
mum set of conditions. Data indicating typical amounts of sul-
furic acid aerosol collected during operation of the laboratory
generator are given in Table 2.
Aerosol samples usually containing 20 to 60 ug of sulfuric
acid were collected on 47-mm Mitex filter disks in the experimen-
tal work. Smaller disks (ca. 6.3 mm) were punched from the ex-
perimental filters for analysis in the useful range of the flame
photometric detector (FPD). Each small disk represented about
3.3% of the effective area of the sample that is collected on
a 47-mm disk. Each FPD analysis reported for the filters was
calculated to give the total on the 47-mm filter. Triplicate
samples were taken from each filter by punching three 6.3-mm
disks equidistantly from the center. The "A" and "B" samples
in Table 2 were obtained simultaneously with a parallel sampling
arrangement. These data showed that while a 10-min sample col-
lected approximately one-half of the sulfuric acid that was ob-
tained with the 20-min sample, the 5-min sample collected some-
what less than one-fourth. These results suggested that a small,
relatively constant loss of sulfuric acid probably occurred in
the handling of samples prior to analysis. This loss was probably
less than 10% of the 20-min samples but may have been as much
as 50% of the 5-min samples. The nature of this loss of sulfuric
acid is uncertain but may be due to reaction of the acid with
the ambient ammonia present in the laboratory. Levels of
ambient ammonia were previously determined to be typically on
the order of 4 mg/m3 (see p. 38 of this report).
Background Contamination
Nitrogen Dioxide Concentration—
Measurements of the concentration of nitrogen dioxide in
the effluent of the sulfuric acid aerosol generator were made
previously under different operating parameters for the gener-
ator. At that time, argon was not used for dilution of the
hydrogen-oxygen flame and effluent temperatures were substan-
tially higher (ca. 100°C) Levels of nitrogen dioxide were found
to be as high as 6 mg/m under these conditions and attempts
were subsequently made to minimize this concentration. Addition
of argon to the flame in conjunction with lower pressures of
hydrogen and oxygen resulted in lower effluent temperatures of
approximately 30 to 35°C and a lower nitrogen dioxide concen-
tration of 0.11 mg/m3.
11
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TABLE 1. AEROSOL GENERATOR STANDARD
OPERATING PARAMETERS
• Feed solution—4 x 10"3 N H2SOH
• Positive burner pressures—Ha, 1.8 kPa (13.5 mmHg)
02, 18 kPa (135 mmHg)
Ar, 120 kPa (900 mmHg)
• Aspiration rate—ca. 2 ml/min
• Aerosol concentration—ca. 500 Mg/ma
• Sampling rate—6 1/min
• Sampling time—5 to 20 min
• Typical sample size—30 to 60 yg H2SOH
• Probe temperature—30 to 40°C
TABLE 2. SULFURIC ACID COLLECTED FROM THE
GENERATOR AS A FUNCTION OF SAMPLING TIME
Sampling
time •, min
20
20
10
5
H2SO* found on parallel
47-mm filters , ygj*
A
54
60
32
8
B
59
52
26
8
* Each value is the average of triplicate analyses with the FPD
technique using the square roots of the measured peak heights
on a relative scale to determine the amount of HsSOu. These
data exhibited a relative standard deviation of typically less
than ca. 10 to 20%. The significance of the "A" and "B" samples
is explained on the next page.
12
-------
The conditions for the operation of the aerosol generator
during these measurements were the same as those given in Table 1.
The nigrogen dioxide concentration was determined by a method
based on the Griess-Saltzman reaction as described in Intersociety
Committee Method 42601-01-68T.
The ambient level of nitrogen dioxide was measured simul-
taneously with that of the generator and was determined to be
about 0.05 mg/m3 or approximately one-half of the level present
in the effluent of the generator. Therefore, the nitrogen diox-
ide concentration in the generator effluent was judged to be
low enough under current operating parameters for our work.
Sulfur Dioxide Content—
Previous attempts to use the West-Gaeke method (Intersociety
Committee Method 42401-01-69T) to determine the concentration
of sulfur dioxide in the effluent of the aerosol generator were
not successful.* However, because different operating parameters
were later used for the aerosol generator, the measurements were
repeated. The results of these experiments are shown in Table
3. Essentially, these results were identical to those obtained
earlier; that is, no sulfur dioxide could be detected. Moreover,
as before, spikes of sulfur dioxide added to the tetrachloro-
mercurate absorbing solution either before or after sampling
the generator effluent could not be recovered. Thus, it appeared
that the tetrachloromercurate-sulfur dioxide complex was decom-
posed by some component of the effluent produced by the aerosol
generator. Due to the fact that sulfuric acid was eliminated
as the cause of the interference (second entry in Table 3), no
further explanation for this phenomenon can be offered. However,
based on these results and those of previous work, it can be
concluded that sulfur dioxide is not stable in the presence of
the gaseous combustion products of the flame and is probably
oxidized rapidly to sulfate.
Particle Distribution
Estimates of the particle-size and particle-mass distribu-
tions of the aerosol produced by the sulfuric acid generator
were made with a Model 3030 Thermo-Systems Electrical Aerosol
Size Analyzer (EAA) and a modified Climet Particle Analyzer.
These measurements were made with the generator operating under
the conditions given in Table 1. The results of these measure-
ments are presented graphically in Figures 2 through 4.
*Barrett, W.J., H.C. Miller, J.E. Smith, Jr., and C.H. Gwin.
Southern Research Institute. Development of a Portable Device
to Collect Sulfuric Acid Aerosol. EPA-600/2-77-027, U.S. Environ-
mental Protection Agency, Research Triangle Park, North Carolina,
1977. pp 15-16.
13
-------
TABLE 3. DETERMINATION OF S02 CONCENTRATION IN THE
H2SOU AEROSOL GENERATOR WITH THE WEST-GAEKE METHOD
SO
2 foundy yg
Standard, 20 yg of S02. (as Na2S03) 20
Standard, 20 yg of SO2 (as Na2S03) and 59 yg
of H2SOi, (standard aqueous solution) 23
Sample collected from H2SOi» aerosol generator* 0.4
Sample collected from H2SOi, aerosol generator
with spike of 20 yg of SO2 (as Na2SO3) added
to bubbler solution after sampling* 0.4
Sample collected from H2SOi» aerosol generator
with spike of 20 yg of SO2 (as Na2SO3) added
to bubbler solution before sampling* 0.8
* Samples were collected for 20 min at a flow rate of 6 1/min.
Figure 2, which is a plot of the slope of the cumulative
number distribution as a function of particle diameter, indicates
that the majority of the aerosol particles had a diameter of
less than 0.1 ym. On the other hand, it can be ascertained from
Figures 3 and 4, which are plots of the cumulative volume dis-
tribution and the slope of the cumulative volume distribution,
respectively, both as a function of particle diameter, that ap-
proximately 80% of the mass of aerosol particles was accounted
for by those having a diameter between 0.2 and 1.5 ym.
It was found in an earlier evaluation of the particle dis-
tribution that more than 98% of both total mass and number of
particles was associated with those smaller than 0.3 urn. These
measurements were made during the time when operating conditions
for the generator were such that the effluent temperature was
in excess of 80°C. A comparison of the more recent results with
those obtained earlier suggested that the overall particle dis-
tribution of the output of the aerosol generator was a sensitive
function of the flame temperature. As a result, steps were taken
to regulate accurately the fuel (hydrogen) pressure of the atomizer-
burner in order to control the characteristics of the effluent
aerosol.
Although the sulfuric acid generator does not produce an
"ultrafine" aerosol with the current operating parameters, it
appears that with appropriate adjustment of fuel, oxidant, and
14
-------
o
0.
10.0
Figure 2. Plot of slope of cumulative number distribution vs. particle
diameter for HySQq generator.
15
-------
600
500
n
o
™ 400
o
X
UJ
o
ui
O
300
200
100
0.01
O EAA
A CLIMET
DIAMETER, Mm
Figure 3. Plot of cumulative volume (mass) distribution vs. particle
diameter for HgSOq generator.
-------
1000
n
o
CM
" 100
X
Q
10
1.0
0.01
OEAA
ACLIMET
I
0.1 1.0
DIAMETER, /im
10.0
Figure 4. Plot of slope of cumulative volume distribution vs. particle
diameter for HySQq generator.
17
-------
diluent gas pressures the output can be varied to produce an
aerosol of a given distribution over a reasonably broad range
of particle sizes. However, the generator should be operated
within constraints necessary to avoid high concentrations of
potential interferents, such as nitrogen dioxide, and excessive
effluent temperatures, which can promote vaporization of the
rulfuric acid aerosol.
It is probable that the relative humidity (RH) of the gen-
erator effluent is an important factor in establishing a given
particle distribution. The RH of the effluent is/ of course,
dependent on the partial pressure of the water vapor, which is
relatively constant, and the temperature, which can be varied
over a relatively wide range (e.g_., 30 to 150°C) by changing
the pressure of the burner fuel. At low RH (high effluent tem-
peratures) the particle distribution appears to favor the smaller
particles; however, at higher RH (lower effluent temperatures)
the distribution is dominated by larger particles. Thus, it
appears that the sulfuric acid particles grow more rapidly in
the high RH environment during their brief residence time
(<1 sec) in the generator than under lower RH conditions.
ANALYTICAL METHODS
The principal analytical techniques used in this work were
based on the barium chloranilate (BCA) method for the determina-
tion of total sulfate and the flame photometric detection (FPD)
of a volatilized sulfur species.
The procedures described in the literature were modified
somewhat to meet the needs of this project. Detailed protocols
for these methods are described in the following paragraphs.
Barium Chloranilate Method*
For the determination of sulfate by the barium chloranilate
method, a filter sample was placed in a 125-ml screw-capped
Erlenmeyer flask containing about 25 mg of bariun chloranilate
and 10 ml of 80% isopropanol. The sample was then mixed for
30 min on a rotary shaker, centrifuged, and the absorbance of
the resulting supernatant solution measured at 310 nm in a 1-cm
fused-silica cell with a Beckman Model DU spectrophotometer.
An 80% isopropanol solution was used in the reference cell.
A linear calibration curve was prepared with known quantities
*Schafer, H.N.S. An Improved Spectrophotometric Method for the
Determination of Sulfate with Barium Chloranilate as Applied to
Coal Ash and Related Materials. Anal. Chem., 3_9 (14:1719-1726,
1967.
18
-------
of standard sulfuric acid solutions that were treated in the
same manner as experimental samples. The useful range of the
method was 10 to 50 yg of sulfuric acid per filter without dilu-
tion of the sample.
Flame Photometric Detection of Volatilized Sulfuric Acid
Much of the work under the previous EPA contract (No. 68-
02-2234) was intended to study the effect of selected interferent
materials on the stability of sulfuric acid collected on filter
media. Early in that study, analyses for sulfuric acid were
conducted by selective extraction of the acid with benzaldehyde
followed by a barium-Thorin microtitration of sulfate. However,
in the later stages of the investigation, an FPD apparatus was
constructed and successfully used to measure sulfuric acid.
With this methodology the sulfuric acid was thermally volatilized
from the filter medium and quantitated with the FPD. Analyses
were performed more rapidly with this technique and the limit
of detection was significantly lower than that of the "wet"
methods employed earlier. One disadvantage of the FPD apparatus,
however, was that the results obtained with it often possessed
poor reproducibility.
Apparatus—
Early in the current contract (No. 68-02-2468) efforts
were directed toward optimization of the FPD technique as it
is applied to our program. First, in order to simplify the
laboratory apparatus, the Model FPD 100AT flame photometric
detector was removed from the Meloy Laboratories Model SA 160-
2 Sulfur Gas Analyzer. This move facilitated the use of much
shorter Teflon transfer lines from the volatilization chamber
to the heated detector block. High voltage for the photomulti-
plier tube (PMT) of the FPD was provided by a Kepco Model ABC1500-
M power supply, and the PMT current was measured with a Keithley
Model 414A Picoammeter and an appropriate strip-chart recorder.
The sample chamber for thermal volatilization was also
modified to improve reproducibility and to simplify the necessary
manipulations during analysis. The glass chamber that was used
previously was replaced with a Teflon-lined glass tube with
significantly smaller surface area and "dead volume". A shorter
Teflon transfer line was also made possible by replacing the
aluminum heater block with a small tube furnace for maintaining
the temperature of the sample chamber. The sample purging system
was also modified to provide better control of the airflow neces-
sary to sweep the sulfuric acid vapors from the sample chamber
to the FPD. A schematic diagram of the FPD apparatus is shown
in Figure 5.
19
-------
£
6
CRITICAL
ORIFICE
TEFLON
SAMPLE
CHAMBER
200° C
VENT
FILTER
DISC
FPD
BLOCK
200° C
PMT
FLOWMETERS
SAMPLE
PURGE VALVE
HV SOURCE
PICOAMMETER
RECORDER
Figure 5. Flame photometric detector apparatus for H2SO^ analysis.
20
-------
Calibration with Known Amounts of Sulfuric Acid—
Several experiments were conducted to determine the practi-
cal working range for the quantitation of sulfuric acid with
the FPD apparatus. Data for a calibration curve were obtained
by measuring the peak height of the detector response as a func-
tion of the amount of sulfuric acid added to Mitex filters from
standard solutions in isopropyl alcohol. In theory, the response
of the FPD to a volatile sulfur species is proportional to the
square of the sulfur mass concentration in the hydrogen-rich
flame. As a test of this relationship and for purposes of cali-
bration, the logarithms of the peak heights were plotted versus
the logarithms of the corresponding amounts of sulfuric acid
present in the standard samples. The theoretical slope of this
plot should be 2, and a value of 2.09 was found experimentally.
The linearity of this log-log calibration covered a useful range
of approximately 0.1 to 2 ug of sulfuric acid per sample. Also,
a relative standard deviation of 5.6% was obtained for 16 repli-
cate analyses that were performed over a period of 2 weeks on
standard samples at the 2-ug level. These results demonstrated
that adequate sensitivity and reproducibility could be obtained
with the FPD device.
For practical purposes, calibration curves were constructed
from plots of the square root of detector response (peak height)
versus micrograms of sulfuric acid. The advantage of this plot
was that the calibration was linear in the amount of sulfuric
acid. Typical calibration data are given in Table 4. These
data are also shown graphically in Figure 6.
For purposes of calibration in early experiments the square
root of the peak height of the FPD response provided a satis-
factory linear correlation with the amount of sulfuric acid.
However, a more rigorous treatment was later performed by linear-
ization and integration of the sulfur-FPD peaks. The linearizer
incorporated into the FPD apparatus electronically obtained the
square root of the FPD signal and manual planimetry provided
integration of the linearized peaks.
The method of calibration for the FPD varied during differ-
ent phases of the project. The method used for calibration will
be noted in the tables that give data from FPD determinations.
Calibration of the improved FPD apparatus with known amounts
of sulfuric acid on Mitex filters produced the curves shown in
Figures 7 and 8. The volatilization temperature used to obtain
the data in Figure 7 was 200°C. The linear range of the calibra-
tion was found to be 2 to 20 yg of sulfuric acid per 47-mm filter
disk (an accurately cut portion of the 47-mm filter was used
for the analysis). The volatilization temperature for the data
in Figure 8 was 150°C, and the linear range at this temperature
21
-------
TABLE 4. DATA FOR THE CALIBRATION OF THE FPD APPARATUS
Ha SO i* added,* Square root of FPD response, t
yg arbitrary units
2.05 17.0
17.6
17.9
1.23 10.4
10.8
10.2
0.82 7.1
7.0
6.9
0.33 2.0
2.2
2.3
* Standard solutions of sulfuric acid in isopropyl alcohol were
applied to Mitex filters (6.3-mm disks) with microburet.
t These data are the square roots of the measured peak heights
on a relative scale.
was 4 to 15 yg of sulfuric acid per 47-iran filter. The slight
reduction in sensitivity was probably due to less efficient
volatilization of the sulfuric acid from the Mitex filter and
some adsorption on surfaces at the lower temperature. Poor re-
producibility was also observed in the data for the calibration
curve at 150 °C for amounts less than 1 yg. The reproducibility
at lower levels was later improved with further instrument
modifications (see p. 40 of this report).
FILTER MATERIALS
Mitex and Fluoropore are two types of Teflon (PTFE) filters
that have been used for the collection of sulfuric acid aerosol.
During previous contract work several measurements demonstrated
that Mitex filters with a 5-ym pore size rating were 99% effi-
cient in the collection of sulfuric acid aerosols with particle
sizes in the range of 0.3-to 0.005 pro. However, because Fluoro-
pore filters are frequently used by other workers for the col-
lection of sulfuric acid aerosol, a comparison was made by sam-
pling the laboratory generator simultaneously with Mitex and
Fluoropore filters in parallel.
22
-------
1.0 1.5
AMOUNT OF H2SO4, fjLg
2.0
2.5
Figure 6. FPD calibration for H^SO4 deposited on filters and
volatilized at 200°C.
23
-------
c
19
15
CO
O
a.
CO
Q
Q.
LL
Q
01
<
10
Q
<
O
UJ
N
I
I
0.0 0.1 0.2 0.3 0.4
AMOUNT OF H2SO4 , us
0.5
0.6
Figure 7. Calibration of modified FPD for H^SQj deposited on filters
and volatilized for analysis at 200PC.
24
-------
13
u)
*•
§
e-
(o
*^
!5
-------
Filter Evaluation with FPD Determinations
Mitex filters with a 5-ym pore size rating and Fluoro-
pore filters (1-ym pore size) that are manufactured without
the polyethylene backing were used simultaneously to collect
svlfuric acid from the generator. Analyses of both filters
were conducted with the FPD apparatus. The results of the com-
parison are given in Table 5. The greater pressure drop across
the Fluoropore filter resulted in a lower flow rate; consequently,
the amount of sulfuric acid collected was less than Fluoropore
than for Mitex. However, the calculated results for the sulfuric
acid concentration in the generator effluent were essentially
identical for the two filters within experimental error.
Table 5. COMPARISON OF PARALLEL FLUOROPORE AND MITEX
FILTERS IN SAMPLING SULFURIC ACID AEROSOL*
Filter
material
Mitex
Fluoropore
Flow rate,
1/min
6.0
5.2
H2SOi» found, t
yg
61.4
55.4
H2SOi» concn ,
yg/m3
513
533
* Sampling time was 20 min.
t Each result is the average of triplicate analyses. Relative
standard deviation was typically less than ca. 10 to 20%.
These results suggested that neither filter offers any
particular advantage over the other for the collection of sul-
furic acid from the aerosol generator. However, it is conceiv-
able that Fluoropore filters may be more efficient in the col-
lection of the aerosol if the operating conditions of the gene-
rator were adjusted to give a higher proportion of "ultrafine"
particles.
These results were based on FPD analysis of filters with
volatilization at 200°C. Also, these filters had collected about
60 yg of sulfuric acid from the aerosol generator, and at that
sulfuric acid level the results obtained with both filters were
essentially the same. On the other hand, it had been observed
earlier for smaller amounts of sulfuric acid aerosol (6.3., less
than about 20 yg per 47-mm disk) that the recovery upon analysis
was somewhat less than expected for Mitex filters. It appeared
that a small, constant amount of sulfuric acid (ca. 5 ug) was
not recovered from the Mitex filters. This amount was not signi-
ficant at the 60-yg level and fell within the uncertainity of
26
-------
the FPD analyses, but at the 10- to 20-pg level the loss was
substantial (25 to 50%) .
In subsequent work this phenomenon was found to be more
pronounced for Fluoropore filters than for Mitex. For example,
collection of about 15 pg of sulfuric acid aerosol on Mitex
filters (measured independently on a parallel filter by the BCA
method for total sulfate) followed immediately by FPD analysis
with volatilization at 200°C gave a result of only 8 yg or about
50% recovery. On the other hand, only about 6 yg of the acid
was found by FPD analysis of a Fluoropore filter that had col-
lected about 33 yg of aerosol (approximately 20% recovery).
These FPD results reported for the comparison of Mitex
and Fluoropore filters were based on volatilization at 200°C;
however, the recoveries were even lower at 150°C—the volatili-
zation temperature necessary to minimize FPD interferences from
the ammonium salts of sulfuric acid. For example, FPD determina-
tion of 36 yg of sulfuric acid aerosol collected on Fluoropore
gave 12 pg at 200°C (about 35% recovery) and 3 yg at 150°C (about
10% recovery). With Mitex the results were somewhat better:
for 19 yg of the aerosol, FPD determination at 200°C gave 14 M9
(about 70% recovery) and at 150°C the result was 12 yg (about
65% recovery). Further, it should be noted that the determina-
tions for each filter at both temperatures were referred to in-
dividual calibration curves. These curves were prepared from
FPD responses, at the corresponding temperatures, to known spikes
of sulfuric acid added to the appropriate filters from a solu-
tion in isopropanol.
Filter Evaluation with BCA Determinations
It was interesting also that the same phenomenon as de-
scribed for FPD occurred with the BCA determinations of sulfate
when the filters were not agitated ultrasonically during extrac-
tion into 80% isopropanol. That is, at the 70-Mg level the
recoveries were about 90% without ultrasonic extraction for both
Mitex and Fluoropore filters spiked with known amounts of sul-
furic acid, but at the 10-pg level the recoveries were only about
70% for Mitex and 50% for Fluoropore. However, with ultrasonic
extraction the low level recoveries for both filters approached
100%. These results suggested that the sulfuric acid may have
diffused into the interior pores of the filters where it was
difficult to remove by extraction or thermal volatilization.
The phenomenon appears to involve a constant amount of sulfuric
acid and the situation seems to be worse with Fluoropore filters.
The use of Teflon membrane filters may have reduced, the severity
of this effect, but these were not evaluated in this study due
to the need for high sampling flow rates (ca. 14 1/min).
27
-------
In conclusion, neither the Mitex nor Fluoropore filter
appears to be entirely adequate for the thermal volatilization-
FPD determination of sulfuric acid. However, the results tend
to favor Mitex filters because of the substantially higher re-
coveries that were found relative to Fluoropore. In any event,
these results further indicate problems in the conventional
volatilization FPD analysis of sulfuric acid.
28
-------
SECTION 5
INTERFERENCE STUDIES ASSOCIATED WITH THE
COLLECTION OF SULFURIC ACID AEROSOL
COLLECTION OF SULFURIC ACID AEROSOL
The effect of selected interferent materials upon the
stability of sulfuric acid collected on Teflon filter disks
studied with quantitation of the acid with the FPD apparatus.
The interferents were chosen from those representative of
a roadside environment and those which have the potential
to react with sulfuric acid. This study essentially concluded
the interference studies that were begun under the previous
EPA contract (No. 68-02-2234) concerning sulfuric acid aerosol.
Interference from
Experiments were conducted to measure the effect of
Fe203 on the stability of sulfuric acid collected on a Mitex
filter with analysis by the FPD method. A similar study
was performed in previous work with analyses by the benz-
aldehyde extraction technique (specific for sulfuric acid)
and insignificant losses of sulfuric acid were found. However,
results of experiments with analyses by FPD indicated that
substantial losses of acid occurred for the "high" levels
of predeposited Fe203 as opposed to somewhat more moderate
losses for the "low" level. These data are given in Table
6. The average recoveries of sulfuric acid sampled as
aerosol following deposition of Fe2O3 were 18% for the "high"
level and 79% for the "low" level. The FezOs particulate
was sampled from an aerosol produced by a DeVilbiss powder
blower; no effort was made to characterize the size distribu-
tion of this aerosol. The two levels of Fe203 were obtained
by sampling the aerosol ten times longer for the "high"
level than for the "low" level.
Recoveries of spikes of sulfuric acid (in isopropanol
solution) added to filters also depended upon the level
of predeposited Fe2O3. Average recoveries of the spikes
were- 54% for the "high" level and 106% for the "low" level.
Although the recoveries of spikes were adequate at the "low"
level of Fe203, the greater losses of sulfuric acid for
both spiked and aerosol samples at the "high" level could
have resulted from further reaction of the residual acid
29
-------
with the additional Fe203 on the filter at the elevated tempera-
ture (ca. 200°C) of the FPD volatilization chamber. Accordingly,
it is possible that the lower recoveries found with FPD analyses
as opposed to those obtained earlier with the benzaldehyde
extraction technique resulted from reaction of sulfuric acid
with Fe203 during volatization in the FPD. However, the fact
tnat the recoveries for the aerosol samples were lower than
those of the spiked samples suggested that additional losses
of acid might have occurred during aerosol sampling.
TABLE 6. EFFECT OF PREDEPOSITED Fe203 UPON SULFURIC
ACID COLLECTED ON MITEX FILTERS
Fe203
added , mg
HzSO!*
added , *
yg
H2SO.,
spike ,t
yg
H2SO.»
found , §
yg
Recovery ,
%
High levels
0.3 none
0.6 none
0.3 (0.3)# 40
0.5 45
0.3 52
Low levels
(47)
none
63
none
none
none
34
7.2 (44)
6.3
10.8
54
18 (96)
14
21
< 0 . 1 none
<0.1 none
<0.1 62
<0.1 64
<0.1 64
none
63
none
none
none
<0 . 1
67
44
54
53
—
106
71
84
82
* The amount of sulfuric acid added was determined by analysis
of a parallel filter exposed only to the sulfuric acid
aerosol.
t The spike was added to the filter exposed to FezOa from a
standard solution of sulfuric acid in isopropyl alcohol.
§ Each result is the average of triplicate FPD determinations
calculated from the square roots of the peak heights. Rela-
tive standard deviation was typically less than ca. 10 to 20%.
# The numbers in parentheses are the results of similar experi-
ments in which the analyses for sulfuric acid were conducted
with the benzaldehyde extraction technique.
30
-------
Interference from PbO
The evaluation of PbO as a cause of interference during
the collection of sulfuric acid aerosol was conducted in the
same manner as described earlier for FeaOa. The results of
these experiments are given in Table 7. Although there was
a relatively large variability in replicate values for recovery
of acid in the presence of PbO at the "high" level, it was
evident that substantial losses of acid occurred. The same
trend was observed with PbO as with FezOa in that the losses
of acid at the "low" level of interferent were significantly
less than at the "high" level. These results cannot be com-
pared with previous data because PbO was not included in inter-
ference studies conducted with the benzaldehyde extraction
technique.
TABLE 7. EFFECT OF PREDEPOSITED PbO UPON SULFURIC
ACID COLLECTED ON MITEX FILTERS
PbO added,
mg
High levels
0.5
0.5
0.7
0.7
H2SOi,
added, yg*
none
59
62
60
found, ygt
0.1
3.9
20
12
Recovery ,
6
32
20
Low levels
0.1 none 0.1
0.1 60 40 67
0.1 58 38 66
0.1 57 37 65
* The amount of sulfuric acid added was determined by analysis
of a parallel filter exposed only to the sulfuric acid
aerosol.
t Each result is the average of triplicate FPD determinations
calculated from the square roots of the peak heights. Rela-
tive standard deviation was typically less than ca. 10 to 20%.
31
-------
Interference from Ambient Particulate Material
The effect of ambient particulate material (APM) on the
stability of sulfuric acid collected on a filter was studied in
previous work with analyses by the benzaldehyde extraction tech-
nique. At that time, losses of sulfuric acid approaching 70%
were found for collection of the acid aerosol on filters pre-
viously exposed to APM. Because the effect of ambient particulate
is important and was studied with FPD analyses only semiquantita-
tively in our earlier work, this potential interferent was evaluated
again with the improved, more nearly quantitative FPD technique.
The APM was collected on 47-mm Mitex filters beside a busy
roadway in Birmingham. Because typical sampling times were several
hundred min for collection of APM and only 15 min for collection of
laboratory-generated sulfuric acid aerosol, the aerosol could not
be collected simultaneously. Rather we chose to study the effects
of collection of sulfuric acid "early" and "late" in the ambient
sampling period.
The results of the FPD determinations of sulfuric acid
added to the filters from the aerosol generator are given in
Table 8. The recoveries of sulfuric acid were determined for
two levels of APM—the "high" level representing an 8-h sampling
period and the "low" level, a 3-h sample. These data demonstrated,
again, the severe losses of sulfuric acid that can occur in the
presence of ambient particulate on a filter disk. For both the
"high" and "low" levels of particulate, the losses of the acid
approached 90%, which was somewhat more than determined by the
wet methods of analysis that were used earlier. The greater
losses of sulfuric acid that were determined by FPD analyses could
have resulted from further reaction of the residual acid with the
particulate on the filter at the elevated temperature (ca. 200°C)
of the FPD volatilization chamber. In any event, the addition of
a spike of sulfuric acid in isopropyl alcohol to a filter exposed
to APM also resulted in a low recovery of the acid (ca. 18%).
On the other hand, detectable FPD responses were routinely
observed for filters containing only the APM. The response could
possibly be due to a sulfur species other than sulfuric acid;
however, the concentration of ambient sulfuric acid calculated
from these data was approximately 1 to 2 yg/m3. These data are
also in approximate agreement with earlier results obtained with
wet methods.
Experiments were also conducted in which the order of
collection of APM and sulfuric acid was reversed. First, known
amounts of sulfuric acid from the aerosol generator were deposited
on Mitex filters, and then APM was collected, either in the labor-
atory or near a street with heavy traffic. The experimental con-
ditions and the results of the FPD analyses are presented in
Table 9. The recoveries of sulfuric acid were much greater than
when APM was deposited first.
32
-------
Essentially all of the sulfuric acid was recovered from the
filters exposed at the roadside and about two-thirds was re-
covered from the filters exposed in the laboratory.
TABLE 8. EFFECT OF PREDEPOSITED AMBIENT PARTICULATE
MATERIAL UPON SULFURIC ACID COLLECTED
ON MITEX FILTERS
Ambient
particulate
added , mg
added,*
yg
H2SOH
spike , t
HjSO.,
found , §
Recovery ,
0.3 (0.3)#
0.3
0.1 (2.0)
0.2
0.3
none
none
68 (29)
54
65
none
63
none
none
none
5.3
11.1
7.1
5.2
7.6
(4)
(10)
18
10 (34)
10
12
LOW
none
none
3.9
<0.1 60
<0.1 58
<0.1 57
none
none
none
6.4
8.5
5.4
11
15
10
* The amount of sulfuric acid added was determined by analysis
of a parallel filter exposed only to the sulfuric acid
aerosol.
t The spike was added to the filter exposed to ambient partic-
ulate material from a standard solution of sulfuric acid in
isopropyl alcohol.
§ Each result is the average of triplicate FPD determinations
calculated from the square roots of the peak heights. Rela-
tive standard deviation was typically less than ca. 10 to 20%
# The numbers in parentheses are the results of similar experi-
ments in which the analyses for sulfuric acid were conducted
with the benzaldehyde extraction technique.
33
-------
TABLE 9. EFFECT OF AMBIENT PARTICULATE MATERIAL
ON FPD MEASUREMENT OF SULFURIC ACID
PREDEPOSITED ON MITEX FILTERS
Sampl ing
time , *
added, t
H^SO,,
found, §
Recovery , t
rnin
Roadside atmosphere
5
10
20
40
60
180
45
45
46
41
41
0
45
44
46
40
34
4
100
98
100
98
83
Laboratory atmosphere
40
180
300
360
300
55
58
57
57
0
36
39
39
42
6
65
67
68
74
~m
* Sampling rate was 6 1/min at a level of APM of about 100
ug/m3.
t The amount of sulfuric acid added was determined by analysis
of a parallel filter exposed only to the sulfuric acid
aerosol.
§ Each result of the average of triplicate FPD determinations
calculated from the square roots of the peak heights. Rela-
tive standard deviation was typically less than ca. 10 to 20%.
The temperature of the volatilization chamber was 200°C.
# Explanations of the trends of these recoveries and those
reported in Table 8 are set forth in the following discus-
sion.
The two most important factors likely to influence the
results of these experiments are neutralization of the acid
by ambient ammonia and reaction of the collected sulfuric acid
with alkaline particulate materials on the filter. It was
expected that when the sulfuric acid was deposited first,
neutralization by ambient ammonia during subsequent collection
of the APM would cause loss of the acid. Based on an estimate
of the amount of ammonia sampled simultaneously with the APM,
partial losses of sulfuric acid were expected for sampling
times under 80 min and total losses for sampling times substan-
tially over 80 min. The results, however, were not consistent
with this expectation. If reaction of the predeposited acid
with ammonia did occur, then it appeared that positive inter-
34
-------
ference was caused by decomposition of the resulting ammonium
salts at 200°C. Another conceivable explanation of the high
recoveries is that neutralization by ammonia did not occur,
at least not completely and that the predeposited sulfuric
acid diffused into the interior pores of the filter where it
could not react with alkaline particulate material.
INTERFERENCE FROM AMMONIUM SALTS OF SULFURIC ACID
Several uncertainties resulted from the experiments de-
scribed above in which sulfuric acid deposited on Mitex filters
was exposed to ambient particulate material (APM). The princi-
pal uncertainties involved recoveries of sulfuric acid that
were higher than expected based on the measured concentration
of ambient ammonia that was sampled simultaneously with the
ambient particulate material. These results suggested the
possibility of positive interference from ammonium salts in
the FPD analysis at 200°C. Subsequently, a study of the inter-
ferences from ammonium sulfate and ammonium bisulfate was per-
formed by applying either aqueous or alcoholic solutions of
the salts to Mitex filters followed by FPD analysis. The results
of these experiments are given in Table 10. Although these data
are not comprehensive and are of limited significance, it appeared
that the FPD response was dependent on the temperature of the
volatilization chamber. Both ammonium salts gave positive
responses at the 200°C volatilization temperature but no signifi-
cant response was observed at 150°C with either. Because of these
findings, the evaluation of APM as a possible interferent in the
collection of sulfuric acid aerosol was repeated with attention to
the temperature of volatilization in the FPD analysis.
INTERFERENCE FROM AMBIENT PARTICULATE MATERIAL—ANALYSIS
CORRECTED FOR INTERFERENCE FROM AMMONIUM SALTS
The evaluation of APM as an interference in the collection
of sulfuric acid aerosol was continued with experiments in which
a cyclone was used in ambient sampling to eliminate particles
greater than about 2 ym in diameter. A description of the cyclone
is given in a report prepared by Southern Research Institute for
the Environmental Protection Agency (EPA 650/2-74-102-a, August
1975). Our work in the past had not included size fractionation
of APM collected for interference studies. However, by elimin-
ating the larger particles the evaluation of interference due to
APM becomes more realistic in relation to some of the state-of-
the-art devices that have been developed for sampling analysis
of ambient aerosols of sulfuric acid. Also, in these experiments
the temperature of the volatilization chamber was adjusted to
150°C to reduce interference from ammonium salts.
35
-------
TABLE 10. INTERFERENCE FROM AMMONIUM SALTS
_ OF SULFURIC ACID IN FPD ANALYSIS _
,. FPD response as II2SOi» , pg*
Ammonium salt - - - 2 —
added, Ug _ 200°C _ 150°C _
0.32t 0.12 0
0.325 0.08 0
>100# 1.14 0
(NH4) 2 SO i,
0.32t
0.34§
»100#
0.09
0.09
0.34
0
0
0.1
* The values are the averages of triplicate FPD determinations
calculated from the square roots of the peak heights. Rela-
tive standard deviation was typically less than ca. 10 to 20%.
t A standard solution of the salt in 80% isopropyl alcohol was
applied to Mitex filters.
§ A standard solution of the salt in water was applied to Mitex
filters.
# A small crystal of the ammonium salt was placed on the filter.
Predeposition of Particulate Material
Table 11 gives the results of FPD measurement of sulfuric
acid aerosol collected from the laboratory generator on Mitex
filters after sampling APM. Because of the complex nature of
the interaction of APM and sulfuric acid aerosol during sampling,
these data collectively possessed poor reproducibility. However,
the precision of the individual measurements on a given sample
was satisfactory (typically less than 10 to 20% RSD).
From the data obtained with the cyclone it appeared that
the extent of reaction of sulfuric acid with APM was a function
of the concentration of total ambient particulates. In these
experiments, higher recoveries of sulfuric acid resulted when
ambient sampling was done at the lower concentration of APM.
Sampling of APM without the cyclone did not appear to reduce
the recovery of the sulfuric acid solution compared with that
observed for experiments with the cyclone. In any event,
these results indicated that trends observed in previous
studies of interference from APM with the FPD determination
and the benzaldehyde extraction technique are valid.
36
-------
TABLE 11. RESULTS OF FPD MEASUREMENT OF SULFURIC ACID
COLLECTED ON MITEX FILTERS FOLLOWING COLLECTION
ON AMBIENT PARTICULATE MATERIAL
_____
aerosol
Ambient particulate
added, yg*
Without cyclone
-90 (109 yg/m3)**
~90
-90
-90
-90
-90
With cyclone tt
<50 (68 ug/m3)
<50
<50
<50
<50
<50
<90 (109 yg/m3)
<90
<90
<90
<90
<90
added ,
jjgt
none
none
none
10.1
7.0
9.2
none
none
none
15.4
11.2
12.2
none
none
none
8.3
11.5
9.4
spike,
none
13.7
13.7
none
none
none
none
13.7
13.7
none
none
none
none
13.7
13.7
none
none
none
found ,
0.6
4.0
4.0
2.0
1.6
2.2
0
11.4
8.3
5.9
4.8
6.4
0.5
7.1
5.6
1.3
1.9
1.6
Recovery ,
.
29
29
20
23
24
83
61
38
43
52
-
52
41
16
9
17
* The sampling rate was 14.1 1/min for 1 hr.
t Analysis of a parallel filter exposed only to the sulfuric
acid aerosol was used to determine the amount of sulfuric
acid added.
§ The spike was added from a standard solution of sulfuric
acid in isopropyl alcohol.
ft Each result is the average of triplicate FPD determinations
calculated from the square roots of the peak heights. Rela-
tive standard deviation was typically less than ca. 10 to
The temperature of the volatilization chamber was 150 L.
** The numbers in the parentheses are the total ambient particu
late levels during sampling.
tt An uncertain fraction of the particulate material in the air
sampled was removed by the cyclone.
37
-------
Predeposition of Sulfuric Acid
Table 12 gives the results of FPD measurements of sulfuric
acid when the acid aerosol was collected before sampling APM.
Calculations based on a measured concentration of ambient ammo-
nia of 3.6 mg/m3 indicated that sufficient ammonia was present
ir the 0.83 m3 of sampled air to neutralize all of the predepos-
ited sulfuric acid. The results of the FPD analyses for this
experiment agreed with this assumption due to the low recoveries
of sulfuric acid found with the 150°C volatilization temperature.
Thus, the higher recoveries reported earlier from identical
experiments, except for volatilization interference by ammonium
salts that were formed on the filter during the sampling of
APM. Furthermore, the use of the cyclone for size discrimina-
tion of the APM would not be expected to alter these results.
CONCLUSION
Although the severity of the interference from APM in the
collection of sulfuric acid aerosol appeared to be dependent
upon the level of ambient particulate, the interference was
generally sufficient in any event to complicate the accurate
measurement of the concentration of sulfuric acid aerosol by
FPD. Of equal importance is the additional interference due to
ambient ammonia that is sampled simultaneously with the sulfuric
acid.
38
-------
TABLE 12. RESULTS OF FPD MEASUREMENTS OF SULFURIC ACID
COLLECTED ON MITEX FILTERS PRIOR TO COLLECTION
OF AMBIENT PARTICULATE MATERIAL*
Ambient particulate
added, ygt
aerosol
added,
ug§
spike,
yg#
H2SO.»
found,
1 I fT^t "if
Recovery,
%
35 (45 ug/m3)tt
none
none
35
35
35
35
35
35
none
none
13.1
8.9
9.6
10.7
13.7
13.7
none
none
none
none
6.5
6.2
0.6
0.8
0.6
0.8
47
45
5
9
6
7
* Size discrimination of APM was not used for these experi-
ments .
t The sampling rate was 14.1 1/min for 1 hr.
§ Analysis of a parallel filter exposed only to the sulfuric
acid aerosol was used to determine the amount of sulfuric
acid added.
# The spike was added from a standard solution of sulfuric
acid in isopropyl alcohol.
** Each result is the average of triplicate FPD determinations
calculated from the square roots of the peak heights. Rela-
tive standard deviation was typically less than ca. 10 to 20%
The temperature of the volatilization chamber was 150°C.
tt The number in parentheses is the total ambient particulate
level during sampling.
39
-------
SECTION 6
EXPERIMENTAL APPARATUS AND PROCEDURES FOR COLLECTION
OF SULFURIC ACID AEROSOL BY DERIVATIZATION METHODS
In light of the results of the interference studies with
the conventional collection methods, it became apparent that
it would be necessary to collect the sulfuric acid aerosol as
a stabilized derivative. The first derivatization technique
investigated was based on the reaction of sulfuric acid aerosol
with perimidylanunonium bromide (PDA-Br). Later, experiments
were performed to determine if sulfuric acid aerosol could be
volatilized during sampling and solid interferents such as am-
monium sulfates of lower volatility removed by filtration of
the heated airstream prior to derivatization of the volatilized
sulfuric acid.
ANALTYICAL METHODS
The principle analytical techniques used in developing
the derivatization methods were based on the flame photometric
detection (FPD) of a volatilized sulfur species, the barium
chloranilate (BCA) method for the determination of total sulfate,
and ion chromatography (1C) for the determination of total sul-
fate in the presence of ions which interfere with the BCA and
FPD methods.
Modified Flame Photometric Detector Apparatus
Modifications to the FPD Apparatus—
Modifications were made to the FPD apparatus described
in Section 4. The major change was the addition to the apparatus
of the second volatilization chamber. Figure 9 depicts the ap-
paratus in its final stage of development. The use of two sepa-
rate chambers allowed the determination of sulfuric acid at vola-
tilization temperatures from 150 to 200°C and the perimidylam-
monium sulfate (PDA-SOJ derivative at its decomposition tempera-
ture of 450°C. Teflon tubing was used to replace all metal
transfer lines from the volatilization chambers to the FPD block.
40
-------
SAMPLE
PURGE
VALVE
CRITICAL
ORIFICE.
5 ml/min
TEFLON
TRANSFER
LINES
FPD
BLOCK.
200°C
TEFLON
SAMPLE
CHAMBER. 150°C
RECORDER
INTEGRATOR
GLASS
SAMPLE
CHAMBER
450°C
FLOWMETERS
CRITICAL
ORIFICE,
220 ml/min
NEEDLE VALVE
AIR
Figure 9. Remodified flame photometric detector apparatus for H2S04
and PDA-SO4 analysis.
41
-------
Calibration of the FPD with PDA-SO,,--
Following modifications of the FPD apparatus to allow vola-
tilization at 450°C, calibration data were obtained for thermal
decomposition of the perimidylammonium sulfate (PDA-SO.J deriva-
tive of sulfuric acid. To obtain these data, standard solutions
of sulfuric acid in isopropanol were added to Mitex filters im-
t,regnated with PDA-Br and the resulting derivative (PDA-SO,,)
was decomposed at 450°C in the FPD. The calibration curve for
PDA-SO,, is shown in Figure 10. In subsequent experiments, sam-
ples were collected from the sulfuric acid aerosol generator
with PDA-Br filters; the results of the FPD analyses of the
filters gave the expected concentration of sulfuric acid in the
generator effluent. The sensitivity of the derivative method
was found to be essentially the same as that resulting from
conventional volatilization of sulfuric acid at 200°C.
Barium Chloranilate Method
Use of the barium chloranilate method for determining total
sulfate was limited during many of the derivatization experi-
ments. The derivatization compounds used interfered positively
with the BCA method by giving erroneous high results. The BCA
method is also pH sensitive and could not be used in the presence
of strong alkali.
Ion Chromatography
Due to the substantial degree of interference with the
BCA method, a new method of determining sulfate was investigated.
The ion chromatographic (1C) method for determining water soluble
ions first described by Small et al* eliminated the interference
problems that had complicated the BCA method. The 1C method
also provided greater sensitivity for sulfate than the BCA or
FPD methods. After a Model 10 Ion Chromatograph had been pur-
chased from the Dionex Corporation (Sunnyvale, CA), experiments
were conducted to optimize the operational parameters for the
determination of sulfate.
Operational Parameters—
The instrumental parameters used in the initial experi-
mentation with the 1C for anion determinations were the follow-
ing :
• Analytical column—3-mm i.d. by 500-mm anion separator.
• Suppressor column—6-mm i.d. by 250-mm anion suppressor.
*Small, H., T.S. Stevens, and W.C. Bauman. Novel Ion Exchange
Chromatographic Method Using Conductimetric Detection, Anal.
Chem., 471801-1809, 1975.
42
-------
cc
<
cc
H
ffi
CC
LU
oo
111
cc
o
Q.
U.
0
HI
<
CC
CJ
HI
O
Ul
N
£
<
UJ
0.2 0.4
AMOUNT OF
0.6
0.8
fig
Figure 10. FPD calibration for PDA-SO4 decomposed at 45CPC.
43
-------
• Eluent—0.003 M NaHCO3/0.0024 M Na2CO3.
• Eluent flow rate—138 ml/hr.
• Sample volume—100-yl loop.
With these parameters the elution time for sulfate was
approximately 14 min. This time was later reduced to 7 min by
increasing the flow rate of the eluent to 230 ml/hr and keeping
the other parameters constant. Calibration data for sulfate
obtained with the parameters listed above are given in Table
13 and are shown graphically in Figure 11.
TABLE 13. PRELIMINARY CALIBRATION DATA FOR ION
CHROMATOGRAPHIC DETERMINATION OF SULFATE
Sulfate taken,* ppm Relative peak areat
338 650
655
169 310
315
68 Hi
110
34 51
51
17 28
28
6-8 10.7
10.7
3.4 4.1
4.3
0.7 1.1
1.2
* Standard solutions of NajSO^in distilled-
deionized H2O.
t These data are relative peak areas in
arbitrary units.
44
-------
100 200
AMOUNT OF SO49", ppm
300
Figure 11. Ion chromatograph calibration with standard solutions of
in
45
-------
In order to minimize interferences from organic materials
and suspended particulates in sulfate determinations and to ex-
tend the life of the analytical column, a precolumn supplied
by Dionex was added to the 1C. Also, the use of a shorter sepa-
rator column to decrease the analysis time and a larger (500-yl)
sample loop to increase the sensitivity for sulfate were investi-
gated but proved impractical for our purposes.
With a relative standard deviation of less than 2%, the
reproducibility of replicate determinations with the 1C was
judged to be very good. Furthermore, sulfate was determined
successfully in samples containing relatively high concentrations
of substances (e.g../ chloride and nitrate) that interfered with
analytical methods used in earlier work.
Disregarding interferences, it was calculated that as little
as 0.1 pg of sulfuric acid per 47-mm filter disk can be deter-
mined with extraction with ultrasonic agitation and quantitation
with the 1C. Results of preliminary experiments indicate, how-
ever, that a significant amount of sulfate (ca. 0.5 yg) can be
extracted from blank Teflon filters. It is conceivable that
the magnitude and variability of the sulfate blank on the filter
materials may ultimately impose the lower limit upon the deter-
mination of sulfuric acid with the 1C technique.
Several sets of instrumental operating parameters for the
determination of sulfate with the 1C were used during the develop-
ment of the final prototype sampling system. These changes in
operating parameters will be noted where appropriate in the text
that follows.
DERIVATIZATION WITH PERIMIDYLAMMONIUM BROMIDE
In light of the results of interference studies using con-
ventional collection methodologies, it became apparent that the
stabilization of sulfuric acid during sampling would be necessary.
This approach was investigated with a derivatization technique
based on the reaction of sulfuric acid with perimidylammonium
bromide (PDA-Br) first adapted to this problem by P.W. West and
his associates.* Because this methodology was reported by the
authors in their early studies to be selective for sulfuric acid,
the reaction was studied as a means for derivatization of sulfuric
acid aerosol in our work.
With this methodology samples are collected on filters
impregnated with PDA-Br. Ideally, only the sulfuric acid
aerosol reacts with the PDA-Br salt to form the insoluble sul-
fate before the acid can be neutralized or otherwise destroyed by
* Thomas, R. L.; V. Dharmarajan, G. L. Lundquist, and P. W. West.
Measurements of Sulfuric Acid Aerosol, Sulfur Trioxide, and the
Total Sulfate Content of the Ambient Air. Anal. Chem. 4_8(4) :
639-641, 1976.
46
-------
other reactions. For quantitation of the PDA derivative, PDA-SO,,
is decomposed at 450°C for FPD measurement of a volatilized sul-
fur species.
With regard to the speed of the derivatization reaction,
preliminary work indicated that the fixation process was very
rapid. For example, when Mitex filters impregnated with PDA-
Br were used to collect sulfuric acid aerosol and were quickly
analyzed (within 1 min of sampling) by FPD with volatilization
at either 200 or 450°C, the expected FPD response was obtained
at 450°C, but no response was observed at 200°C. This result
indicated that the sulfuric acid aerosol reacted quickly and
completely to form the more refractory PDA-SO,,.
Interference from Ambient Particulate Material
In further experiments ambient particulate material was
evaluated as a cause of interference in the derivatization pro-
cess during sample collection. In these experiments, Mitex fil-
ters impregnated with the PDA-Br reagent and plain filters (not
impregnated) were used to sample the ambient atmosphere. Deter-
mination of the derivatized sulfuric acid was performed with
the FPD apparatus with decomposition of the derivative at 450°C.
The results of these experiments are given in Table 14.
Approximately 100 \ig of ambient particulate material (<2 pm)
was collected on each filter. Known amounts of sulfuric
acid aerosol from the generator were then deposited on the filters
and analyzed by the FPD method with volatilization at 450°C.
The amounts of sulfuric acid found were estimated from separate
calibration curves (one for PDA-SC\ and one for H2SC\). Because
the slope of the calibration curve for sulfuric acid at 450°C
is greater than for PDA-S(\, it appeared that a given amount
of acid yields a smaller FPD signal when it is converted to the
PDA-SO,, derivative.
Although earlier experiments had shown that an attenuated FPD
response is obtained when nonimpregnated filters exposed to ambient
particulate material and sulfuric acid are heated at 150 to 200°c,
these results indicated complete recovery of sulfuric acid at
450°C (thermal decomposition of the reaction product of sulfuric
acid and ambient particulate material). Complete recovery was
also obtained from the PDA-impregnated filters. Thus it could
not be concluded from these results that PDA-SO,, was formed selec-
tively in the presence of APM. The results merely showed that
whatever the products of the reactions on the filters may be,
the acid is recoverable from treated or untreated filters at
450°C, but not at lower temperatures. However, the smaller FPD
responses for PDA-Br filters (see FPD responses listed in Table
14) were initially taken as a favorable indication that the
47
-------
TABLE 14. DERIVATIZATION OF SULFURIC ACID AEROSOL WITH PDA-Br
IN THE PRESENCE OF AMBIENT PARTICULATE MATERIAL
H2SO., Equivalent FPD response,
aerosol H2SO4 arbitrary
added, yg* found , ygt units§
Mitex filters
impregnated with
PDA-Br -1- APM#
None
28
6
32
75
350
Plain Mitex None 10 150
filters + APM# 32 36 600
* FPD analysis of the appropriate parallel filter exposed only
to sulfuric acid aerosol served to determine the amount
added.
t Each result is the average of triplicate determinations cal-
culated from linearized integrated FPD responses. Relative
standard deviation was typically less than ca. 10 to 20%. The
FPD volatilization temperature was 450°C.
§ Separate calibration curves were prepared for impregnated
filters.
# The amount of APM on each filter disk was about 100 yg (particle
size, <2 ym).
PDA-SOi, derivative may have been formed preferentially on the im-
pregnated filters.
It should be pointed out that the ammonium sulfate salts
present in APM would also contribute to the FPD response at 450°C
regardless of whether they were fixed by the PDA-Br reagent or
not. The contribution from sulfate salts in APM can be seen
in the FPD responses listed in Table 14 for the filters to which
no sulfuric acid aerosol was added. Also, the data for the
filters with added sulfuric acid reflect this interference in
that the analytical results for equivalent sulfuric acid exceeded
the amount added.
On the other hand, if the sulfate salts were not fixed
by the derivatizing reagent, it is possible that they could be
selectively removed by volatilization at temperatures below the
decomposition temperature of PDA-SO,,. Several experiments were
conducted to investigate the possibility of prevolatilization
of interfering sulfate salts, but the preliminary results were
48
-------
inconclusive. However, before proceeding further with this ap-
proach it was decided that the most important experiments were
those that would establish whether or not the PDA-Br reagent
fixed the ammonium salts of sulfuric acid.
Interference from Ammonium Sulfate Salts
Simultaneous fixation of ammonium sulfate salts and sul-
furic acid constitutes a severe limitation on any methodology
that depends upon selective derivatization of sulfuric acid
aerosol. The selectivity of the PDA-Br reagent for sulfuric
acid was evaluated in a relatively simple series of experiments
with aerosols of ammonium bisulfate and ammonium sulfate.
Aerosols of these sulfate salts were collected simulta-
neously on plain and reagent-impregnated Mitex filters in a
manner that was identical to experiments with sulfuric acid
aerosols. The sulfate aerosols were generated in a laboratory
apparatus by nebulizing dilute aqueous solutions of the appropri-
ate ammonium salt with a Beckman No. 4020 atomizer-burner. This
is the same device that is used for generation of the sulfuric
acid aerosol; however, only argon (at a pressure of 140 kPa
(20 psig)) was used to aspirate the feed solutions and the flame
was not employed for generation of the sulfate salt aerosols.
In order to determine if the sulfate salts were fixed on
the PDA-Br filters, FPD determinations were made on both plain
and impregnated filters with a volatilization temperature of
200°C. At this temperature large FPD responses were observed
for ammonium sulfate and ammonium bisulfate aerosols collected
on plain Mitex filters. Thus, if the salts were not fixed on
the PDA-Br filters, large FPD responses would be expected. On
the other hand, if the sulfate salts were completely fixed and
formed PDA-SO,, in the same manner as sulfuric acid aerosol, then
no FPD response would be observed at 200°c due to the higher
decomposition temperature of the PDA-SOH derivative. The results
of these experiments are shown in Table 15. These data indicate
that both ammonium salts were partially fixed by topochemical
reaction with PDA-Br. The estimated degree of fixation is about
80%. Thus, it was concluded that the reaction of PDA-Br to form
the PDA-SO,, derivative is not selective for sulfuric acid.
FIXATION OF SULFURIC ACID AEROSOL WITH VERSAPOR FILTERS
In earlier work at the Institute it was shown that sulfuric
acid aerosol reacted with Versapor filters (Gelman Instrument
Company) to form an insoluble product that could not be deter-
mined by methods depending upon extraction and that was probably
49
-------
TABLE 15. EVALUATION OF SELECTIVITY OF PDA-Br
WITH AEROSOLS OF SULFATE SALTS
FPD response, Sulfate
aerosol aerosol arbitrary derivatized,
added, yg* added, yg* unitst, § %
Mitex with
PDA-Br
Plain Mitex
_
81
_
81
94
—
94
"
275
250
1400
4700
80
95
_
™
* The output of the aerosol generator was measured by deter-
mining sulfate with the barium chloranilate method and
ammonium ion with the indophenol method.
t Volatilization temperature of the FPD apparatus was 200°C.
§ FPD responses were linearized and integrated.
a sulfate. The filter is characterized by the manufacturer as
a glass-fiber medium with an epoxy binder.
In experiments with sulfuric acid aerosol it was found
that the reaction product formed on Versapor filters was not
decomposed to a volatile sulfur species at either 200 or 450°C.
This result was evidenced by negligible FPD responses to filters
exposed to sulfuric acid and analyzed in the FPD apparatus.
Although not amenable to thermal volatilization-FPD analysis,
the filter could be useful if it could be shown not to react
with sulfate salts.
However, in experiments identical to those described in
the previous section for PDA-Br filters it was demonstrated that
the Versapor filters also fixed the ammonium sulfate salts as
well as sulfuric acid by virtue of the fact that no FPD response
was seen with the salts collected on these filters. Because
Versapor filters did not appear to offer any advantage over PDA-
Br impregnated filters for fixing sulfuric acid aerosol, work
with these filters was discontinued.
50
-------
SECTION 7
VOLATILIZATION, PREFILTRATION, AND DERIVATIZATION
OF SULFURIC ACID AEROSOL
After it was demonstrated that PDA-Br filters fixed aero-
sols of sulfate salts as well as sulfuric acid aerosols, experi-
ments were performed to determine if sulfuric acid aerosol
could be volatilized during sampling and sulfates of lower
volatility removed by filtration of the heated air stream.
PRELIMINARY APPARATUS WITH A SAMPLE FLOW RATE OF 3 L/MIN
To facilitate the thermal volatilization of sulfuric
acid aerosol, a 30-cm section of 12-mm (i.d.) glass sampling
probe was wrapped with a resistance heating tape. At a flow
rate of 3 1/min, the aerosol sample from the laboratory generator
could be heated in this manner from about 35°C to as high
as 200°C. The sulfuric acid, thus vaporized, could be passed
through a heated filter designed to remove particulate inter-
ferents and then onto a filter where fixation of the acid
vapor could occur.
Passage of Sulfuric Acid through Sampling Probes and Filters
In preliminary experiments, the heated probe and a heated
filter holder with a Mitex filter were run simultaneously
with an identical reference sampling line that was not heated.
The airstream was cooled in a 35-cm section of tubing following
the filter. It then passed into an impinger-bubbler containing
80% isopropanol to collect vapors and recondensed sulfuric
acid aerosol. Following the sampling period, any sulfuric
acid condensed on internal surfaces behind the filters was
rinsed into the bubblers. Because of the nature of the bubbler
samples and the higher levels of sulfuric acid used in these
preliminary experiments, it proved to be practical to use
the barium chloranilate method for these analyses.
Preliminary results indicated that although only 30%
of the total sample of sulfuric acid aerosol (relative to
the reference side) could be accounted for, over 70% of this
amount passed through the filter at 130°c. The recovery was
only slightly increased by changing from a stainless steel
filter holder to one constructed of Teflon.
51
-------
These results suggested that the losses of sulfuric acid
probably occurred in the heated zone of the glass sampling
probe. This possibility was investigated further by deter-
mining the amount of sulfuric acid passing through the heated
probe as a function of temperature. In these experiments
the sulfuric acid was collected in impinger-bubblers following
both the heated and reference probes. The results of these
experiments are shown in Table 16. These data showed that
for temperatures sufficient to volatilize an appreciable
portion of the sulfuric acid aerosol (ca. 90 to 130°C), most
of the resulting vapor was then lost to the walls of the heated
glass probe.
TABLE 16. PASSAGE OF SULFURIC ACID THROUGH
THE HEATED GLASS SAMPLING PROBE
Temperature of
heated sample
airstream, °C
76
82
84
108
129
133
200
HiSOi,
sampled ,
yg*
122
106
103
111
117
100
115
HzSO^
found ,
ugt
129
108
105
68
31
29
33
Recovery ,
%
106
102
102
61
26
29
29
* These are the amounts of sulfuric acid that passed through
the unheated reference probe (ca. 30°C) run simultaneously
with the heated probe. Analyses for H2SOif were performed
by the barium chloranilate method.
t These are the amounts of sulfuric acid that passed through
the heated glass probe.
In an effort to improve the passage of sulfuric acid
in the heated probe, a Teflon lining was inserted in the heated
zone to reduce the reactivity of the inner surface. This
thermal volatilization apparatus is shown schematically in
Figure 12. An identical series of experiments was then per-
formed to evaluate this modification. The results are given
in Table 17. In contrast, these data showed that sulfuric
acid vapor was passed efficiently through the heated zone
with the Teflon lining. In further experiments with this
apparatus the amount of sulfuric acid vapor passing through
the Mitex prefilter was determined as a function of the
52
-------
TEFLON
FILTER
HOLDER
HEATING
MANTLE
SECOND BACK-UP
FILTER
\
47-MM REAGENT-IMPREGNATED
BACK-UP FILTER
TEFLON SCREEN
47-MM PREFILTER
TO PUMP
3 l/min
CO
STAINLESS STEEL
FILTER HOLDER
17 CM
10 MM I.D. TEFLON LINE
HEATING ZONE
H2SO4
AEROSOL
30 CM
Figure 12. Apparatus for volatilization, prefiltration, and derivatization
of sulfuric acid aerosol.
-------
TABLE 17. PASSAGE OF SULFURIC ACID THROUGH THE
HEATED TEFLON-LINED SAMPLING PROBE
Temperature of
heated sample
air stream, °C
82
102
184
204
HzSOi*
sampled,
Mg*
90
78
69
47
K2SO»
found ,
ygt
96
71
56
35
Recovery,
%
107
91
81
75
* These are the amounts of sulfuric acid that passed through
the unheated reference probe (ca. 30°C) run simultaneously
with the heated probe; analyses for H2SOi» were performed by
the barium chloranilate method.
t These are the amounts of sulfuric acid that passed through
the heated Teflon-lined probe.
volatilization temperature. The results of these experiments
are given in Table 18. These data indicated that sulfuric
acid aerosol was volatilized efficiently at 110°C and that
at least 90% of the resulting vapor passed the initial Mitex
filter (heated to about 120°C).
Interference from Ammonium Sulfate Salts
After establishing that sulfuric acid aerosol could be
efficiently volatilized and prefiltered at 110°C, experiments
were conducted to determine the extent of interference, if
any, from aerosols of ammonium sulfate salts under identical
conditions. The results of these experiments are given in
Table 19. These data indicated that prefiltration of the
heated sample air stream effectively eliminated the sulfate
salts. Therefore, it appeared to be feasible with this tech-
nique to eliminate interference from these particulates in
the determination of sulfuric acid aerosol.
Derivatization of Sulfuric Acid Vapor with PDA-Br
In order to stabilize a sample collected in the field,
we originally attempted the use of a back-up filter impregnated
with PDA-Br (in a separate, unheated filter holder) to fix
sulfuric acid aerosol that recondensed following prefiltration
54
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TABLE 18. PASSAGE OF VOLATILIZED SULFURIC ACID AEROSOL
THROUGH MITEX FILTERS AS A FUNCTION OF TEMPERATURE
Volatiliza-
tion
temperature ,
°C
85
100
105
110
Amount of
aerosol
sampled, yg*
83
45
94
141
H2SO,
> found, yg
on passing
prefilter prefiltert
43
12
10
10
40
33
84
131
Portion of
H2SO,,
passing
prefilter, %
48
73
89
93
* The amount of aerosol sampled was determined with a parallel
unheated sampling probe. All analyses for HzSO^ were per-
formed by the barium chloranilate method.
t Sulfuric acid vapor passing the prefilter was determined with
a back-up impinger-bubbler containing 80% isopropanol.
TABLE 19. RETENTION OF SULFATE SALT AEROSOLS
BY A HEATED MITEX FILTER
Aerosol*
NtUHSOt,
(NHH) zSOn
Amount
sampled ,
ygt
24
48
Amount found, yg
on
prefilter
16
45
passing
prefilteri
5
<4
Portion of
salt retained,
76
>90
* Temperature of the volatilization zone was 110°C.
t The amount of aerosol sampled was determined with a parallel
unheated sampling probe. Analyses were performed by the
barium chloranilate method for sulfate.
§ The amount of interferent slipping the prefilter was deter-
mined with a back-up filter followed by an impinger-
bubbler containing 80% isopropanol.
55.
-------
of the vapor. However, in a series of experiments it was
consistently observed that most of the vapor condensed on
cool surfaces after prefiltration without forming an aerosol
that could be collected on the impregnated filter. Therefore,
it was of interest to determine if the vapor itself could
be fixed by a PDA-Br filter. If so, the reagent-impregnated
filter could be mounted behind the prefilter (with an appropri-
ate spacer) in the same heated filter holder.
Accordingly, PDA-Br filters were used to sample sulfuric
acid vapor at about 110°C and were analyzed by FPD to determine
their retention of the vapor as PDA-SCU. in these experiments
it was found that less than 5% of the vapor was derivatized.
These results suggested that any fixation reaction relying
upon the formation of an insoluble sulfate derivative "(e.g_.,
PDA-SOi, or BaSOO may not occur rapidly enough with sulTuric
acid vapor to be useful. It appeared to be likely that the
o,nly suitable type of fixation scheme for sulfuric acid vapor
would involve an acid-base reaction. Therefore, we began
to evaluate the effectiveness of back-up filters impregnated
with alkaline substances, such as NaaCOs and NaOHr for re-
action with the vapor at elevated temperatures. The prelimi-
nary results were favorable.
Derivatization of Volatilized Sulfuric Acid Aerosol with Alkali
After it was demonstrated that sulfuric acid vapor was
not efficiently derivatized by filters impregnated with
perimidylammonium bromide (PDA-Br), several experiments were
conducted with Mitex filters impregnated with alkaline solids,
such as sodium carbonate or sodium hydroxide. It was antici-
pated that the reaction of sulfuric acid vapor passing the
prefilter would be very rapid on the back-up filter impregnated
with the strongly alkaline material. Results of experiments
in which sulfuric acid aerosol was sampled with the combined
volatilization, prefiltration, and derivatization technique
are given in Table 20. These data showed that sulfuric acid
vapor was adequately fixed on Mitex filters impregnated with
sodium hydroxide. A comparable level of sodium carbonate
was somewhat less efficient in fixing the vapor. Although
the alkali-impregnated filters were much more efficient than
PDA-Br filters for derivatization of sulfuric acid vapor,
a significant amount of the acid did slip the impregnated
filters.
MODIFIED APPARATUS WITH A SAMPLE FLOW RATE OF 14-L/MIN
After it was established that sulfuric acid aerosol could
be efficiently volatilized and then prefiltered at approximately
110°C and that more than 75% of interfering sulfate salts
56
-------
were removed by the prefilter after passage of the aerosols
through the heated probe at temperatures as high as 130°C,
modifications were made so that the sampling rate of the
apparatus could be increased from 3 to 14 1/min to allow
shorter collection periods for ambient levels of sulfuric
acid aerosol.
TABLE 20. DERIVATIZATION EFFICIENCY OF
ALKALI-IMPREGNATED FILTERS FOR
SULFURIC ACID VAPOR AT 110°C
Impregnated.
alkali*
H2SOi,
aerosol
sampled,
ugt
H2SOi» found., yg
on
prefilter
on im-
pregnated slipping
back-up back-up
filter filter!
Vapor
collection
efficiency
of back-up
filter, %*
Na2CO3
low level of 58
impregnation
high level of 76
impregnation
12
14
57
32
10
30
85
NaOH
low level of
impregnation
60
53
106
12
<4
23
40
43
66
9
10
17
82
81
80
* The alkali was impregnated onto Mitex filters from saturated
alcoholic solutions .
f The amount of aerosol sampled was determined with a parallel,
unheated sample probe. All analyses were performed by the
barium chloranilate method for sulfate and reported as equiva-
lent
§ Slippage was determined with a secondary back-up filter
(reagent-impregnated) followed by an impinger-bubbler con-
taining 80% isopropanol.
# The collection efficiency is relative to the amount of H2SOi,
vapor to which the impregnated filter was actually exposed.
57
-------
New Sampling Probe
A larger sampling probe was constructed from a 1.2-m by
12.5-mm i.d. brass outer tube with a longer 1.35-mm by 9-mm
i.d. Teflon liner. Both ends of the Teflon tube were threaded
so that filter holders or other devices could be attached
directly. To effect the thermal volatilization of sulfuric
,cid aerosol, an 80-cm section of the Teflon-lined brass tube
was wrapped with a resistance heating tape and then insulated.
A thermocouple was attached to the surface of the probe to
monitor the temperature of the heated zone so that overheating
of the Teflon liner could be avoided. A second thermocouple
was inserted into the Teflon filter holder behind the impreg-
nated collection filter. This thermocouple was used to record
the actual temperature of the heated airstream during sample
collection.
To evaluate the modifications to the sampling apparatus,
the heated probe (120°C) and the heated filter holder (130°C)
were operated simultaneously with a reference sampling line
that was not heated. Results of preliminary experiments with
sulfuric acid aerosol are given in Table 21. These data in-
dicated that an average of about 85% of the volatilized sul-
furic acid successfully passed through the heated zone of the
new probe. The data presented in Table 22 show further that
an average of about 84% of the sulfuric acid vapor from the
heated zone passed through the Mitex prefilter. On the basis
of the data from both tables an overall efficiency of about
70% was indicated for the combined volatilization and prefil-
tration process. In earlier experiments the overall efficiency
for the volatilization and prefiltration at a flow rate of
3 1/min was found to be in excess of 80%. Nevertheless, the
modified sampling system appeared to be adequate.
Perivatization of Volatilized Sulfuric Acid Aerosol
with Alkali at Increased Flow Rate"
With regard to the fixation or derivatization of sulfuric
acid on an alkali-impregnated filter (following the prefilter),
preliminary results with the new probe, shown in Table 23,
suggested that the efficiency was significantly lower at the
increased flow rate. Low efficiency was indicated by slippage
of a substantial portion of the acid vapor past the impregnated
filter into a back-up impinger-bubbler. Initial efforts to
improve the efficiency by increasing the level of impregnated
alkali were only partially successful. Furthermore, inter-
ferences with the analytical method precluded conclusive evalu-
ation of the effect of increased impregnation of alkali.
It appeared that the face velocity through the impregnated
47-mm filters may have been too great at the 14-1/min flow
rate to allow efficient reaction of the acid vapors with the
58
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TABLE 2]. VOLATILIZATION EFFICIENCY OF THE HEATED PROBE
AT A TEMPERATURE OF 120°C AND A FLOW RATE OF 14 1/MIN
Amount
aerosol
Of H2SOU
sampled, yg*
152
141
143
117
Amount of fUSOi*
found , y g f
130
125
99
112
Recovery
86
89
70
96
* Determined by barium chloranilate analysis of the filter
from the reference probe.
t Determined by barium chloranilate analysis of the impinger-
bubbler catch following the heated probe.
TABLE 22. PREFILTRATION EFFICIENCY OF MITEX FILTERS
AT A TEMPERATURE OF 130°C AND
A FLOW RATE OF 14 1/MIN
Amount of H2SOi»
vapor sampled,
yg*
94
104
Amount of H2SOi» found, yg Portion of
on passing t^SOij passing
prefilter prefiltert prefilter, %
17 75 82
16 88 85
* This is the amount of volatilized sulfuric acid aerosol from
the heated zone of the sampling probe. All HaSOn analyses
were performed by the barium chloranilate method.
t The sulfuric acid vapor that passed through the prefilter
was determined by analysis of an alkali-impregnated filter
and a back-up impinger-bubbler.
59
-------
TABLE 23. EFFICIENCY OF SULFURIC ACID DERIVATIZATION
BY ALKALI-IMPREGNATED TEFLON FILTERS AT A
FLOW RATE OF 14 1/MIN
Amount of Amount of HeSOit found,* yg Efficiency
HaSO^ on passing of im-
aerosol on impregnated impregnated pregnated
sampled, yg prefilter filter filter filter, %
29 8 9 12 43
104 16 28 60 32
60 20 <2 38 <5
50 13 25 12 68
* Determinations of sulfate were made by the barium
chloranilate method. H2SOi» passing the impregnated filters
was collected in an impinger-bubbler containing 80%
isopropanol.
impregnated alkali. In other experiments it was found that
higher volatilization temperatures (ca. 150°C) also decreased
the efficiency of derivatization with the impregnated filters.
Because the higher flow rate is desirable for shorter samp-
ling periods with ambient sulfuric acid aerosols, an effort
was made to increase the efficiency by first increasing the
level of impregnation of alkali and then by using thicker Teflon
filters for impregnation. Unfortunately, evaluation of the
results of these experiments was complicated by a substantial
degree of interference with the barium chloranilate (BCA) method
for sulfate. The interference was apparently due to increased
levels of chloride that resulted from the neutralization with
hydrochloric acid of excess alkali extracted from the impreg-
nated filters. However, preliminary results based on BCA
analyses of the back-up bubblers containing the isopropanol
indicated that somewhat less sulfuric acid slipped the thicker
alkali-impregnated filters.
To obtain a more meaningful evaluation of the modifications
to the sampling system, it was necessary to develop a new method
for determining sulfate extracted from the alkali-impregnated
filters. The major problem was due to substantial interference
with the barium chloranilate method that was previously used
to measure sulfate fixed on the impregnated filters. Because
of this interference problem, a study of the feasibility of
determining derivatized sulfuric acid in a filter extract with
ion chromatography (1C) was begun. In addition to offering
60
-------
greater sensitivity for sulfate than the BCA method, the 1C
method eliminated the interference problems that had complicated
previous experiments.
Modified Extraction and Determination Procedures for
Sulfate on Impregnated Filters
After operational parameters were established for the deter-
mination of sulfate with the 1C, experiments were performed
to study the extraction of sulfate from alkali-impregnated Teflon
filters. In previous work with impregnated filters, isopropanol
(IPA), rather than water, was used for extraction of sulfate
due to the hydrophobic nature of Teflon. Although IPA readily
wets the Teflon filters and efficiently extracts sulfate, an
organic solvent was not expected to be compatible with direct
analysis by 1C. This conclusion was substantiated in a series
of experiments in which sulfate was determined in aqueous solu-
tions containing a small amount of IPA. These conditions ap-
proximated the process of initial wetting of the Teflon filter
with a small volume of IPA and subsequent dissolution of the
extracted sulfate and IPA with aqueous extraction. It was
found, however, that solutions containing as little as 5% of
IPA caused deleterious swelling of the ion-exchange resin in
the analytical column of the 1C.
In order to eliminate the need for an organic solvent to
wet the filters, a method of aqueous extraction in which the
Teflon filters are submerged in water in small vials was found
to be very efficient when performed with ultrasonic agitation.
However, because a substantial amount of excess NaOH was extrac-
ted from alkali-impregnated filters, only a limited number
(ca. five) of sulfate determinations could be performed with
the 1C before it was necessary to regenerate the suppressor
column. Neutralization of the excess alkali with HCi prior
to analysis of the filter extracts appeared to extend the re-
generation interval, although a problem was encountered with
poor resolution of the resulting large chloride and small sul-
fate peaks. Subsequently, it was found that adequate resolution
could be obtained with the use of two anion separator (analyti-
cal) columns in series. With a flow rate of 184 ml/hr and with
3-mm i.d. by 500-mm and 3-mm i.d. by 250-mm columns installed
in the 1C, a retention time of about 14 min was observed for
sulfate. This arrangement facilitated 1C determination of
sulfate extracted with water from alkali-impregnated filters
following neutralization of the excess alkali in the extracts.
Experiments were subsequently conducted to investigate
the feasibility of direct analysis of the aqueous extract by
1C without neutralization of the excess alkali. Therefore,
several parameters were modified to eliminate the need for prior
neutralization. The amount of KOH impregnated on each Teflon
filter (temporarily used as a substitute for NaOH) was reduced
61
-------
by one-half to approximately 60 mg/filter. Also, the volume
of water used for extraction was increased to 4 ml/filter.
Theses changes effectively reduced the concentration of KOH
present in the extract by 75%.
After the above changes had been made, calibration data
for sulfate were generated with alkali-impregnated filters
spiked with known amounts of standard sulfuric acid solutions.
The spiked filters were extracted with water by ultrasonic
agitation for 10 min in 4-ml glass vials sealed with Teflon-
lined closures. The 1C parameters used for analysis of these
extracts were the following:
Analytical column—3 mm i.d. by 500-mm anion separator
Supressor column--6-mm i.d. by 250-mm anion suppressor
Eluent—0.003M NaHC03/0.0024 M Na2C03
Sample volume—100-yl loop
With these parameters, the elution time for sulfate was approxi-
mately 12 min.
Perivatization of Volatilized Sulfuric Acid Aerosol with
Alkali at Increased Flow Rate as Measured by Ion Chromatogratony
After a quantitative method for the extraction and deter-
mination of sulfate from the alkali-impregnated filters had
been established, experiments were continued in the evaluation
of the combined volatilization, prefiltration, and derivatiza-
tion methodology for sulfuric acid aerosol. Specifically,
emphasis was placed on improving the efficiency of the alkali-
impregnated Teflon filters for the collection of the volatilized
sulfuric acid aerosol. As stated, previous evaluations of the
efficiency of Teflon filters impregnated with large amounts
(=50 mg/ml) of NaOH for the collection of volatilized sulfuric
acid were inconclusive.
In further experiments Mitex filters were replaced with
a new Teflon filter purchased from Schleicher and Schuell (TE-
35 disk-membrane). These filters consist of a Teflon membrane
similar to Fluoropore supported by a coarse Teflon-fiber mat.
In the current filter sequence for the combined sampling method-
ology, a Mitex prefilter is followed by a second Mitex filter
that serves as a spacer. These filters precede the NaOH or
KOH-impregnated TE-36 Teflon filter, which is oriented with
the heavy felt-like side toward the airstream. Because the
impregnated filter and spacer are in contact, both are extracted
for the sulfate determination.
After calibration data were generated, sulfuric acid aero-
sol samples were collected by the combined volatilization, pre-
filtration, and derivatization methodology. Parallel samples
were collected (including the unheated reference probe) at
62
-------
14 1/min for 10 min with the temperature of the airstream of
the heated probe maintained at about 110°C (measured behind the
filter) . In these experiments the heating mantle that was used
previously for the filter holder was not needed to maintain the
elevated temperature.
The results of these experiments are given in Table 24.
These data indicated that an average of about 90% of the sul-
fur ic acid aerosol that was sampled was found on the filter
media (primarily on the impregnated filter with a small portion
on the prefilter). Therefore, the slip through the impregnated
filters could have been no more than 10% of the total sulfuric
acid that was sampled. However, as suggested by previous ex-
periments, the sulfuric acid that was not collected on the
filters (i.e., 10%) was probably lost in the heated zone of
the probe and never reached the prefilter or impregnated filter.
These derivatization efficiency results can be compared to a
value of 43% that was found earlier (Table 23) for the same
sampling flow rate of 14 1/min. This increase in efficiency
was probably due to the greater surface area and more effective
level of impregnation afforded by the thicker felt-like Teflon
filter (S & S TE-36).
63
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TABLE 24. EFFICIENCY OF SAMPLING METHOD
FOR SULFURIC ACID AEROSOL
Amount of
HaSO^ aerosol
added,* yg
10.7
23.9
25.0
32.0
28.0
36.0
Amount of
found, t
On
pref ilter
2.0
4.4
4.0
3.8
4.4
5.0
ug
On im-
pregnated
filter
6.8
18.8
19.0
26.2
22.4
28.6
Collection
efficiency
of im-
pregnated
f ilter, § %
78
97
91
93
95
92
Portion of
total
f^SO,, on
impregnated
filter,* %
64
76
76
82
82
80
* The amount of aerosol added was determined with a parallel,
unheated probe with an impregnated filter. All analyses
were performed by 1C.
t Samples were collected at 110°C at 14 1/min for 10 minutes.
§ The collection efficiency is based on the maximum amount of
H,SOU reaching the impregnated filters (e_.£. /
[28.6/(36.0 - 5.0)] x 100% = 92%).
# These figures relate the amount of H2SOH found on the im-
pregnated filter as compared to the total H2SO^ sampled
(e.g_. , [28.6/36.0] x 100% = 80%).
64
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SECTION 8
DEVELOPMENT OF PROTOTYPE SAMPLER FOR
SULFURIC ACID AEROSOL
The sampling apparatus for sulfuric acid aerosol described
at the conclusion of Section 7 was considered to have properties
appropriate for its incorporation into a prototype sampler for
sulfuric acid aerosol. It was realized, however, that steps
would have to be taken to deal effectively with various sources
of interference in the sampler. Thus, various interference
studies as described below were undertaken in an effort to de-
velop the changes in design that might be needed in the final
design of the prototype sampler.
INTERFERENCE STUDIES
Studies were needed to determine the level of interference
from gaseous copollutants that would not be removed by prefiL-
tration. Sulfur oxide gases (specifically, sulfur dioxide) cbuld
be converted to sulfate by the alkali-impregnated filter. Basic
gases such as ammonia could potentially react with the volati-
lized sulfuric acid aerosol to form sulfate salts which could
be removed by the prefilter. Furthermore, APM collected on the
prefilter might react with volatilized sulfuric acid and act
as an additional cause of interference
Interference from Sulfur Dioxide
In the experiments with sulfur dioxide, the gas was intro-
duced into the sampling probe with a syringe pump while room
air was sampled under normal operating conditions of 110°C and
14 1/min. Determinations of sulfur dioxide were performed with
the West-Gaeke method, in which Greenburg-Smith impingers con-
taining the tetrachloromercurate reagent solution were placed
in the sampling line after the impregnated filters. When a
sulfur dioxide concentration of 75 ug/m3 was sampled through
the impregnated filters at either 25 or 110°C, about 75% of the
sulfur dioxide was found in the back-up bubblers. The remaining
25% of the sulfur dioxide was collected on the impregnated filters,
and when the filters were analyzed by 1C, sulfate equivalent
to a sulfuric acid aerosol concentration of 10 pm/iir was found.
65
-------
Therefore, based on the results of these experiments, sulfur
dioxide appeared to be a significant cause of interference with
the method in its then-existing configuration.
The interference from sulfur dioxide was anticipated and
two approaches that could possibly eliminate the problem were
considered. One method involved low-temperature aqueous extrac-
tion of sulfate in the presence of sulfite on the impregnated
collection filter. This method has been used successfully by
other workers to demonstrate that very little sulfite exists
with sulfate in ambient particulates. This approach depends,
of course, upon sulfur dioxide reacting on the back-up filter
to form sulfite with little or no oxidation to sulfate. However,
because of the high alkalinity of the collection filter and the
elevated temperature during sampling, sulfur dioxide collected
on the impregnated filter was evidently converted to sulfate
prior to extraction. Thus, this method was not promising.
The other method involved the use of an acid gas "denuder"
to remove the sulfur dioxide prior to the thermal volatilization
of the sulfuric acid aerosol. This denuder would simply be an
extension of the existing sampling line to include a portion
of tubing coated with an alkaline solid (e.g_., NaOH) . During
sampling, acidic gases would quickly diffuse to the walls of
the tubing and react with the alkaline material, eliminating
the interference. Because sulfuric acid aerosol diffuses much
more slowly, it would not react and would pass on to the col-
lection device unattenuated.
Experiments were therefore conducted to evaluate the fea-
sibility of using a gas diffusion denuder device in the sampling
line for selective removal of sulfur dioxide. It was calculated
from diffusion theory that at a flow rate of 14 1/min a single
5-mm i.d. by 800-cm tube coated with sufficient reactive alkaline
material would remove more than 99.9% of the sulfur dioxide.
However, for practical application of the gas diffusion denuder,
a parallel arrangement of sixteen 5-mm i.d. by 50-cm tubes took
the place of the single longer tube. This device is similar
in concept to a denuder used by R.K. Stevens* and coworkers at
the Environmental Protection Agency in Research Triangle Park,
North Carolina, for removal of ammonia. The sampling apparatus,
including the denuder, for the volatilization, prefiltration,
and derivatization of sulfuric acid aerosol is shown schemati-
cally in Figure 13.
*Stevens, R.K., T.G. Dzubay, G. Russworm, and D. Rickel. Sam-
pling and Analysis of Atmospheric Sulfates and Related Species,
Atmos. Environ., 12(1-3):55-68 (1978).
66
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ON
-J
AMBIENT
AIR INTAKE
TEFLON FILTER
HOLDER
GAS DIFFUSION DENUDER
HEATING ZONE
1 m l m vi \ u n \ in rvTi \mmunmr
9-mm i.d. TEFLON TUBE
THERMOCOUPLE
1. MITEX PREFILTER
2. MITEX SPACER
3. IMPREGNATED FILTER
(S&S TE 36-0.5 TEFLON)
80-cm -
50-cm
16 PARALLEL 5-mmi.d.
GLASS TUBES
CYCLONE
2-pm CUT-OFF
Figure 13. Apparatus for removal of interfering gases and volatilization,
prefiltration, and derivatization of sulfuric acid aerosol.
-------
For evaluation of the effectiveness of the denuder, experi-
ments were conducted with a single tube from the sixteen parallel
tubes at an equivalent flow rate of 0.88 1/min. Sulfur dioxide
was introduced into a mixing chamber with a syringe pump and
diluted with room air. This mixture (sulfur dioxide concentra-
tion, 2.5 to 25 mg/m3) was then sampled before and after passing
'.hrough the denuder tube to determine the amount of sulfur di-
oxide removed by different alkaline materials coated from aqueous
solutions onto the walls of the denuder tube. The effluent
sulfur dioxide was collected in bubblers containing a 3% hydrogen
peroxide solution and the sulfate that resulted was determined
by ion chromatography.
The results of experiments with an uncoated denuder tube,
one coated with sodium hydroxide, and another with sodium carbon-
ate are shown in Table 25. Sulfuric acid aerosol (0.5 to 1 ym
MMD) was also sampled through the denuder coated with sodium
hydroxide to determine if any losses occurred. When a volume
of aerosol containing 13.6 ug of sulfuric acid was sampled, the
amount found in the effluent from the denuder tube was also 13.6
Mg, indicating that no loss occurred.
TABLE 25. SULFUR DIOXIDE REMOVAL BY GAS DIFFUSION DENUDER
Denuder
coating
None
Na2C03
NaOH
*
S02 added,
mg/m3
23.7
20.8
15.7
17.1
19.1
25.6
2.5
7.9
23.5
SO 2 passing denuder
tube,* mg/m3
23.7
20.9
0.3
0.09
0.08
0.08
<0.003*
<0.003|
<0.003t
SO 2 removal
efficiency, %
0
0
98.2
99.5
99.6
99.7
>99.0
>99.6
>99.9
* Sulfur dioxide was measured by determination of sulfate
in the peroxide bubblers with ion chromatography (1C).
t Limit of detection for this series of experiments
was about 0.003 mg/m3 of sulfur dioxide.
68
-------
Thus, it was determined that the calculated efficiency for
removal of sulfur dioxide (>99.9%) was obtained with the denuder
coated with sodium hydroxide and that the sulfuric acid aerosol
passed through this denuder with essentially no attenuation (100%
recovery). The efficiency of a denuder coated with sodium car-
bonate was also measured because of the conversion of the hydrox-
ide to carbonate that was expected to occur with ambient carbon
dioxide. It was subsequently determined that conversion to car-
bonate would not cause a problem since the denuder coated with
sodium carbonate operated with essentially the same efficiency
(>99.5%) as the denuder coated with sodium hydroxide. Although
these experiments were performed with inordinately large concen-
trations of sulfur dioxide, these results were taken as a favor-
able indication that the denuder would eliminate interference
from ambient levels of sulfur dioxide.
Upon receipt of a gas diffusion denuder manufactured by
University Research Glassware (Carrboro, N.C.), a method of
coating the 16-parallel glass tubes was developed. A device
was constructed to rotate the denuder while heated air was being
drawn through the denuder to speed evaporation of the coating
solutions. Sodium hydroxide was the substance applied for re-
moving sulfur dioxide.
Experiments were conducted to determine the efficiency of
the acid gas denuder for removal of sulfur dioxide. The results
of these experiments are given in Table 26. Experiments were
also conducted to determine the loss, if any, of sulfuric acid
aerosol passing through the acid gas denudes:. These data are
given in Table 27. The results of the experiments with the acid
gas denuder show that 99% of the sampled sulfur dioxide was
removed while the sulfuric acid aerosol passed unattenuated.
TABLE 26. SULFUR DIOXIDE REMOVAL BY GAS DIFFUSION DENUDER
S02 added*'"I" S02 passing denuder S02 removal
mg/m3 tube,* mg/m3 Efficiency, %
34.5
34.5
0.34
0.38
99.0
98.9
* Sulfur dioxide was measured by determination of sulfate
in hydrogen peroxide (3%) bubblers with ion chromato-
graphy (1C).
t SO2 was metered into a 14 1/min airstream with a
syringe pump.
69
-------
TABLE 27. PASSAGE OF SULFURIC ACID AEROSOL THROUGH GAS
DIFFUSION DENUDER
HjjSO^ aerosol H2SOH aerosol passing
added, pg*'^ denuder, ug''"
12.2 12.5
11.3 11.1
H2SOU attenuated
by denuder, %
0
1
* Determined with parallel reference probe.
t Sulfuric acid aerosol was sampled at 14 1/min for ten
minutes. All analyses were done by 1C.
Denuder for Ammonia
Interference from ammonia and other basic gases was pre-
cluded by the use of a denuder for these gases as well as for
sulfur dioxide. A gas diffusion denuder for basic gases similar
to the one for sulfur dioxide was fabricated by coating the walls
of a second denuder with phosphoric acid rather than sodium
hydroxide. The ammonia denuder performed with similarly favor-
able efficiencies as the acid gas denuder.
Dehumidifier Ahead of the Denuders
Because sodium hydroxide is hygroscopic, the removal of
some water vapor from the sampled airstream by the acid gas de-
nuder was expected. However, during only a 1-hr sampling period,
an amount of water vapor was removed from a cubic meter of ambient
air (relative humidity greater than 85%) that was sufficient
to wash more than half the coatings from both the sodium hydroxide
and phosphoric acid-coated denuders. To eliminate this problem,
a method of dehumidifying the sampled airstream prior to passing
through the denuders was developed.
The diffusion dehumidifier consists of an 11.5-cm i.d. by
80-cm clear acrylic tube mounted coaxially with a 20-mm i.d.
by 80-cm porous stainless steel tube. The space between the
two tubes is filled with approximately 5 kg of an indicating
silica gel desiccant. The porous center tube allows the aerosol
to pass through the dehumidifier while the water vapor diffuses
into the silica gel, where it is adsorbed. The original silica
gel desiccant functioned efficiently without needing to be re-
placed or regenerated after 50 hr of operation.
70
-------
No measurements were made to determine the actual reduction
in the humidity of the sampled airstream after passing through
the dehumidifier, but the reduction was sufficient to eliminate
the buildup of water in the denuders while sampling at relative
humidities greater than 95%. On the other hand, experiments
were conducted to determine the loss of sulfuric acid aerosol,
if any, upon its passing through the dehumidifier. The results
of these experiments are given in Table 28. These data show
that the quantity of sulfuric acid aerosol was unattenuated by
the dehumidifier.
TABLE 28. PASSAGE OF SULFURIC ACID AEROSOL THROUGH
THE DEHUMIDIFIER
jSO^ aerosol H2SOM aerosol passing H2SO,, aerosol passing
added, pg*»t dehumidif ier, pgt dehumidif ier, %
175.6
124.5
200.0
108.5
120.4
171.1
129.1
180.0
109.0
121.6
98
104
90
100
101
* Determined with parallel reference probe
Sulfuric acid aerosol was sampled at 14 1/min for 15 rain,
All analyses were done by ion chromatography.
Interference from APM
Experiments were also conducted to investigate the possi-
bility of interference from APM deposited on the prefilter during
sampling. Size-fractionated APM (<2 Mm) was collected for 1
hr through a cyclone onto two Mitex filters in parallel lines.
A Teflon overlay was used with each filter during sampling so
that the APM was deposited on only one-half of each filter.
With this arrangement of two parallel half-filters and a combined
flow rate of 14 1/min, a face velocity was maintained through
each filter that was consistent with normal collection conditions.
Laboratory generated sulfuric acid aerosol was then sampled
at 14 1/min and 120°C using the Mitex prefilters preloaded with
APM. Because half of each filter was clean or unexposed, each
filter served as its own reference. Each prefilter and the cor-
responding impregnated filter were then cut in half and analyzed
individually for sulfate by 1C. The results of these experiments
are given in Table 29. These data indicate that the relatively
71
-------
TABLE 29. STUDY OF INTERFERENCE FROM AMBIENT
PARTICIPATE MATERIAL
H2SO^ found on
Type prefilter impregnated
Run used filter, yg
1 Unexposed 92.0
APM preloaded 88.2
2 Unexposed 34.4
APM preloaded 37.2
3
Unexposed
APM preloaded
45.6
46.2
small amount of sized-fractionated APM deposited on the prefilter
during the maximum 1-hr sampling period did not interfere signi-
ficantly with the determination of sulfuric acid aerosol.
ASSEMBLY OF THE PROTOTYPE SAMPLING APPARATUS
After the feasibility of the combined volatilization, pre-
filtr.Ttion, and derivatization methodology together with the
removal of gaseous interferents using gas diffusion denuders
had been demonstrated, a prototype sampler was constructed.
The prototype sampler is shown schematically in Figure 14. The
prototype sampler is mounted on an aluminum flex-frame lattice
attached to a 2-m tall open-type relay rack. Casters added to
both the bottom and side of the support rack allow the apparatus
to be moved easily in either a vertical or horizontal position.
A photograph of the complete prototype sampler and supporting
framework is shown in Figure 15.
When the apparatus is in use, ambient air enters via a
3.2-mm i.d. stainless steel tube covered with a weather cap.
(The sample is collected 1-m above ground level.) The sample
is then passed through a cyclone to remove all atmospheric par-
ticulate matter p\PM) larger than 2 pm in diameter. This lowers
the buildup of APM on the prefilter during sampling. After the
humidity is lowered in the dehumidifier, the sample passes through
the two gas diffusion denuders. The denuders are shown in Figure
16. The first denuder is coated with phosphoric acid for re-
72
-------
9-mm i.d. TEFLON /
U-TUBE WITH
BEND
1(1
SULFUR DIOXIDE
GAS DIFFUSION
DENUDER
^. n nn
AMMONIA
GAS DIFFUSION
DENUDER
1 1.5-cm i.d.
SILICA
GEL —• —
DEHUMIDIFIER
20-mm i.d.
POROUS STAINLESS
WEATHERCAP ^
3.2-mm i.d. JJ,
STAINLESS Tf
STEEL TUBE J I
r
50
cm
2.7!
50
cm
t
E
1
r1
CYCLnwc ^>-
2-nm CUTOFF \
•
s
j
!
Pr
A
,
rm
fc
y
o
or
24
J
•err
?
j
ft
\
\
fy
\
v
V
\
~s
^
\
K
\
\
g
O
/
JA
g
Sv
1
9-mm i.d. TEFLON TUBE
HEATING
ZONE
\ TEFLON ) 1. MITEX PREFILTER
1 ', u" ( 2. MITEX SPACER
uni ncp I 3. ALKALI IMPREGNATED
HOLDER J TEFLON F|LTER
- — THERMOCOUPLE
~ te-TO PUMP
Figure 14. Apparatus for the removal of basic and acid interfering gases
and the volatilization, prefiltration, and derivatization of
sulfuric acid aerosol.
73
-------
UR D,
i/il I USION
AMMONIA
GAS DIFFUSION
DKNUDER
'>N-I.INED
HEATING ZONE
TEMPERATURE
CONTROLLF.R
Figure 15. Prototype apparatus for volatilization, prefiltration, and
derivatization of sulfuric acid aerosol.
74
-------
Figure 16. Gas diffusion denuders.
Figure 17. Teflon filter holder.
"**!
Figure 18. Ambient roadsite sampling with prototype
sampling system.
75
-------
moving ammonia and other alkaline gases, and the second is coated
with sodium hydroxide for removing sulfur dioxide and other
acidic gases.
After the larger APM and gaseous interferents have been
removed, the airstream is then heated to 130°C in the 9-mm i.d.
uy 80-cm Teflon-lined heating zone. The heated sample in which
the sulfuric acid aerosol has been selectively volatilized then
passes into a Teflon filter holder containing a series of three
Teflon filters. The Teflon filter holder is shown in Figure
17 and schematically in Figure 19. The first filter is a Mitex
prefilter to collect the APM and unvolatilized sulfate salts.
The third filter is the alkali-impregnated Teflon fiber-membrane
filter for derivatizing the sulfuric acid vapor. The middle
filter is a separating filter to keep the alkali from the pre-
filter and is analyzed with the impregnated filter. A thermo-
couple is located below the collection filter to allow regulation
of the volatilization temperature at the point of collection.
FINAL LABORATORY EVALUATION OF PROTOTYPE SAMPLING SYSTEM
Experiments were conducted in which the concentration of
the laboratory-generated sulfuric acid aerosol sampled by the
prototype sampling system was gradually reduced toward antici-
pated ambient levels. Four concentration levels of sulfuric
acid aerosol were generated; the average amounts sampled for
the different levels were 121, 55, 19, and 7.4 yg/filter. The
collection times for the different levels were 5, 10, 15 and
20 min, respectively. The results of these experiments are given
in Table 30.
These data show that the percentage of sulfuric acid aerosol
collected on the impregnated filter and the percentage of sul-
furic acid aerosol collected on both filters increased as the
amount of aerosol sampled was decreased. This apparent increase
in sampling efficiency is probably due to the increase in sam-
pling time rather than the amount of sulfuric acid aerosol sam-
pled. Each sampling interval requires a short equilibration
period to stabilize the volatilization temperature at 130°C.
As the sampling time is increased from 5 to 20 min, the equili-
bration period becomes less significant. At a 1-hr sampling
time for ambient samples, this initial warmup effect should be-
come insignificant.
At the lowest level of sulfuric acid aerosol (7.4 Mg/filter),
an average of 83% of the total sulfuric acid aerosol sampled
was collected on the prefilter and impregnated filter. Of the
total amount collected, an average of 86% was found on the im-
pregnated filter. However, these results are based on a labora-
tory generated sample concentration of 26 ug/m3. Better results
76
-------
HEATING ZONE
TEFLON FILTER
HOLDER
47-mm MITEX PREFILTER
47-mm MITEX SPACER
47-mm ALKALI-
IMPREGNATED TEFLON
FILTER
SEPTUM
THERMOCOUPLE
TO PUMP
Figure 19. Schematic of Teflon filter holder containing the Teflon
pre filter and the alkali-impregnated Teflon collection filter.
77
-------
TABLE 30. EFFICIENCY OF PROTOTYPE SAMPLING APPARATUS FOR
VARIOUS CONCENTRATIONS OF SULFURIC ACID AEROSOL
Amount of H2SO4
aerosol sampled,*
Amount of H2SO^ found,t
pg (% of total HaSO..)
On
On impregnated
prefilter, filter,
\ig (%) \ig (%)
Average
Average
33.5
76.9
54.6
55.1
55.0
17.4
23.8
19.6
15.0
19.0
3.0
26.6
9.9
19.8
7.1
7.1
11.0
(54)
(41)
(50)
1551
(50)
(57)
(83)
(36)
(47)
(56)
Portion of
total H2SOH
collected,
Average
120.3
109.1
101.7
152.4
120.9
21.7
22.4
27.2
46.3
29.4
(17)
(21)
(27)
(31)
(24)
59.4
47.0
34.3
65.1
51.5
(49)
(43)
(34)
(43)
(42)
67
64
61
74
66
90
60
57
61
67
68
86
66
71
73
Average
7
7
8
7
.0
.2
.0
.4
0.
0.
0.
0.
84
87
85
85
(12)
(12)
(11)
(12)
5.
6.
4.
5.
40
05
10
18
(77)
(84)
(54)
(72)
89
96
65
84
The amount of aerosol sampled was determined with a parallel,
unheated sampling probe. All analyses were performed by
ion chromatography.
t Sulfuric acid aerosol was sampled at a flow rate of 14 1/min
and volatilized at 130°C.
may be obtained when the lower ambient levels that are antici-
pated are sampled. The minimum detectable level of the prototype
sampling system appears to be approximately 0.5 pg/m3.
78
-------
SECTION 9
FIELD EVALUATION OF PROTOTYPE SAMPLER FOR
SULFURIC ACID AEROSOL
After the laboratory experiments were completed, ambient
sampling was done with the prototype sampler at two local road-
side sites in the Birmingham, Alabama, area. A series of 1-hr
samples were first collected for 6 days beside a city street
during the period from August 6 to August 18, 1978. The sampler
is shown at this location in Figure 18 on page 100. The average
traffic flow during sampling was approximately 600 cars per hr.
The sampler was located approximately 3 m from the street. Two
control samples were collected with each sample to determine
the amount of total sulfate and sulfate aerosol less than 2 ym
in diameter present at the sampling site. The control data are
reasonably consistent with the data obtained from the prototype
sampling system. The data from these samplings are listed in
Table 31. A graphic comparison of the amount of total sulfate
and sulfuric acid aerosol present beside the city street is shown
in Figure 20. Approximately 20% of the nonsized sulfate col-
lected beside the city street was found to be sulfuric acid
aerosol. The average amounts of total ambient sulfate and am-
bient sulfuric acid aerosol collected were 1.5 and 0.3 pg/m3,
respectively.
After the sampling beside the city street was completed,
a field test was conducted next to a major highway. The proto-
type sampling system was set up in a parking deck adjacent to
an elevated (8 m) six-lane interstate highway on the north side
of Birmingham, Alabama. The prototype sampler is shown collect-
ing samples at the interstate highway site in Figure 21. The
samples were collected 8 m from the near side and 1.5 m above
the interstate highway. The angle of the incline from the center
of the interstate highway to the sampling system was approxi-
mately 5°. Samples of 1-hr duration were collected for 12 hr
on August 21, 22, and 23, 1978. On these dates the local at-
mospheric particulate counts were 94, 108, and 97 yg/m3, respec-
tively, with a temperature low of 21°C and a high of 34.5°C on
each of the test dates. On August 21 and 23 there was a slight
breeze of 3 to 5 km/hr crossing the interstate highway toward
the sampling system. On August 22, however, the wind direction
79
-------
TABLE 31.
AMBIENT SULFURIC ACID AEROSOL DETERMINATIONS
BESIDE A CITY STREET
Prototype samples*
Control samples*
Date Time
collected started
8-9-78 9:30
10:30
11:30
8-14-78 8:30
9: 30
10: 30
11:30
12:30
1: 30
8-15-78 8:30
9:30
10:30
11:30
12:30
1:30
2:30
8-16-78 8:30
9:30
10:30
11:30
12:30
1:30
8-17-78 9:30
10:30
11:30
12:30
1:30
2:30
8-18-78 9:30
10:30
11:30
12:30
1:30
SO,," found
on prefilter,
ng/m3
0. 32
0.11
0.41
0.89
1.20
0.90
1.41
0.55
0.60
0. 36
0.40
0.49
0.45
0.54
0.54
0.81
0.64
0.32
0.79
0.64
0.48
0.00
1.27
1.11
0.64
2.06
0.79
0.48
0.64
0.32
0.79
0.48
0.79
H2SOV found
on impregnated
filter,1" yg/m3
0.23
0.19
0.07
4.12
0.40
0.90
1.70
1.80
2.31
0.14
0.28
0.19
0.15
0.10
0.22
0.10
0.00
0.00
0.00
0.24
0.00
0.00
0.00
0.00
0.48
0.00
0.00
0.00
0.00
0.00
1.29
0.16
0.00
Total SO,,-
found on
prefilter and
impregnated
filter, wg/m'
0.55
0.30
0.48
5.01
1.60
1.80
3.11
2.36
2.91
0.50
0.68
0.68
0.50
0.64
0.76
0.91
0.64
0.32
0.79
0.88
0.48
0.00
1.27
1.11
1.12
2.06
0.79
0.48
0.64
0.32
2.08
0.64
0.79
Total SO,,= less
than 2-iim
diameter, S pg/m3
0.29
0.26
0.68
1.49
1.19
1.69
3.07
0.39
0.64
0.57
0.63
0.63
0.52
0.63
0.72
0.74
0.64
0.48
0.95
0.64
0.48
0.00
1.11
1.11
0.95
1.27
0.79
0.64
0.64
0.32
2.06
0.64
0.79
Total
SO..V
ijg/m3
0.14
0.11
0.16
3.71
3.82
7.64
8.10
0.63
1.39
0.71
0.89
0.99
1.07
0.83
0.94
0.81
0.48
0.79
1.43
0.64
0.48
0.32
1.59
1.27
2.54
1.27
0.64
1.11
0.95
1.11
2.86
0.64
1.90
* All analyses by ion chromatography.
t Volatilization temperature was 130°C. All sampling was done at a flow rate of 14 1/min.
§ Determined with separate control filter with a 2-pm diameter cutoff cyclone.
I Determined with separate open-face filter.
80
-------
AVERAGE TOTAL SULFATE
AVERAGE SULFURIC ACID
STARTING TIME OF ONE HOUR SAMPLING
Figure 20. Hourly average concentrations of total sulfate and sulfuric
acid on six days beside a city street.
81
-------
Figure 21. Remote sampling at interstate highway.
Figure 22. Ion chromatographic analyses of collected samples.
82
-------
and velocity were variable during most of the sampling period.
The average traffic flow passing the sampling site as determined
from government traffic counts was approximately 8,000 to 10,000
cars per hour.
As before, two control samples were collected with each
sulfuric acid aerosol sample. All samples collected were brought
back to the laboratory in individual glass vials for analysis
by 1C (Figure 22.) The data from these samplings are given in
Table 32. Figure 23 shows a graphic comparison of the concen-
trations of total sulfate and sulfuric acid aerosols collected
hourly on August 23, 1978, at the interstate highway. Figure
24 is a graphic comparison of the average concentrations of total
sulfate and sulfuric acid aerosols by time of day collected from
the 3 days of field testing at the interstate highway location.
Approximately 30% of the nonsized sulfate collected beside the
interstate highway was found to be sulfuric acid aerosol. The
average amounts of total ambient sulfate and ambient sulfuric
acid aerosol collected were 4.4 and 1.4 pg/m3, respectively.
83
-------
TABLE 32. AMBIENT SULFURIC ACID AEROSOL DETERMINATIONS
FROM REMOTE INTERSTATE HIGHWAY SAMPLING
Date Time
collected started
8-21-78 6:00
7:00
8:00
9:00
10:00
11:00
12:00
1:00
2:00
3:00
4:00
5:00
8-22-78 6:00
7:00
8:00
9:00
10:00
11:00
12:00
1:00
2:00
3:00
4:00
5:00
8-23-78 6:00
7:00
8:00
9:00
10:00
11:00
12:00
1:00
2:00
3:00
4:00
5:00
SO,," found
on prefilter,
ug/m
1.46
1.72
1.32
1.85
0.93
0.66
0.79
1.06
1.06
0.93
1.19
0.79
0.79
0.48
1.11
0.64
0.48
0.56
0.64
0.64
0.48
0.48
0.48
0.64
0.64
0.95
1.27
1.43
0.64
0.95
0.64
0.64
0.48
2.06
1.27
0.64
HjSO,, found
on impregnated
filter,1' (jg/m
3.10
4.29
0.71
3.33
0.00
1.91
2.38
3.10
1.67
1.19
1.90
1.19
1.43
1.43
1.43
1.43
3.57
3.10
0.00
2.70
0.32
0.00
2.70
1.58
0.00
0.00
0.00
3.57
1.67
0.00
2.38
0.00
0.00
1.90
0.00
0.00
Total S0»=
found on
prefilter and
impregnated
filter, wg/m
4.56
6.01
2.03
5.18
0.93
2.57
3.17
4.16
2.73
2.11
3.09
1.98
2.22
1.91
2.53
2.07
4.05
3.66
0.64
3.34
0.80
0.48
3.18
2.22
0.64
0.95
1.27
5.00
2.31
0.95
3.02
0.64
2.38
2.06
1.27
0.64
Control samples*
Total SO,, less
than 2-ym
diameter,s pg/ro
4.37
6.22
2.12
5.03
0.93
2.65
2.91
4.10
0.53
2.12
3.31
1.98
2.06
1.90
2.43
2.22
0.48
0.48
0.79
0.32
0.79
0.48
0.48
2.06
0.48
0.64
1.
,27
,24
38
0.95
3.02
0.64
2.38
1.94
1.27
0.64
0.93
14.29
3.02
3.90
6.98
0.64
2.54
1.27
0.48
0.79
0.48
0.48
3.02
0.64
0.32
2.06
6.35
5.08
2.22
4.44
2.54
3.65
8.41
2.38
3.02
* All analyses by ion chromatography.
t Volatilization temperature was 130'C. All sampling was done at a flow rate of 14 1/min
§ Determined with separate control filter with a 2-um diameter cutoff cyclone.
* Determined with separate open-face filter.
84
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0
E
3
u.
_1
co
K
o
UJ
UJ
oc
UJ
SULFURIC ACID
AM AM
Figure 23.
AM AM AM AM NOON PM PM PM
STARTING TIME OF ONE HOUR SAMPLING
PM PM
Hourly concentrations of total sulfate and sulfuric acid
on August 23, 1978, at an interstate highway.
85
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CO
^
2
Q
o
o
E
D
CO
DC
O
CO
_i
<
g
UJ
O
DC
LU
>
AVERAGE TOTAL SULFATE
AVERAGE SULFURIC ACID
8 9 10 11 12 1 2 3
AM AM AM AM NOON PM PM PM
STARTING TIME OF ONE HOUR SAMPLING
4
PM
5
PM
Figure 24. Hourly average concentrations of total sulfate and sulfuric
acid on three days at an interstate highway.
86
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4. TITLE AND SUBTITLE
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Southern Research Institute
2000 Ninth Avenue South
Birmingham, Alabama 35205
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/2-80-037
3. RECIPIENT'S ACCESSION NO.
DEVELOPMENT OF A PORTABLE DEVICE
TO COLLECT SULFURIC ACID AEROSOL
Final Report
5. REPORT DATE
May 1980
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
Herbert C. Miller, David W. Mason, and
William J. Barrett
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
10. PROGRAM ELEMENT NO.
1AA601 CA-28 (FY^78)
11. CONTRACT/GRANT NO.
68-02-2468
13. TYPE OF REPORT AND PERIOD COVERED
Final 9/76-9/78
14. SPONSORING AGENCY CODE
EPA/60Q/09
15. SUPPLEMENTARY NOTES
Previous Related Reports:
EPA-600/2-77-027, February 1977
EPA-600/2-78-060, March 1978
16. ABSTRACT
A quantitative, interference-free method for collecting sulfuric acid aerosol on a
filter was developed and field tested. Since previous research found that severe
losses of sulfuric acid were caused by ammonia, ambient particulate material, and
other interferents, a method was needed that converts sulfuric acid to a stable
derivative for subsequent analysis. Methods evaluated for direct fixation of
sulfuric acid aerosol were not found to be selective. Therefore, a sampling
method was investigated that uses a combination of selective volatilization of
the sulfuric acid, prefiltration of particulate interferents, and derivatization
of the vaporized acid on an alkali-impregnated filter. Research on the volati-
lization, prefiltration, and derivatization technique, and the development and
field evaluation of a prototype sampler based on this technique are described.
17
a.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
*Air Pollution
*Sulfuric Acid
*Aerosol
Collecting Methods
*Filters
Tests
b.lDENTIFIERS/OPEN ENDED TERMS
COS AT I Held/Group
13B
07B
07D
14B
ITR'BIJTION STATEMCNT
RELEASE TO PUBLIC
19 SECURITY CLASS /This Report)
UNCLASSIFIED
f~20 SECURITY CLASS I'fhix page)
I UNCLASSIFIED
21. NO OF PAGES
99
22. PRICE
SPA Torn! 2220-! ',FUv. 4-77) P--.C/IOUS EDITION i s OBSOLETE
87
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