United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-81-013 May 1981
Project Summary
Analysis System for Total
Sulfuric Acid in Ambient
Air—Development and
Preliminary Evaluation
James D. Barden
A total sulfuric acid analysis (TSAA)
system was developed and shown to
provide quantitative determinations
of sulfuric acid in air at concentrations
as low as 0.26 //g/m3. Quantitation at
•lower concentrations appears to be
possible. The general approach in the
design and development effort empha-
sized sample conditioning, rather than
detector selectivity, to provide unam-
biguous detection. Separation of the
acid from other sample components
was accomplished by stagewise con-
densation and revaporization; therefore
particulate filters were not required.
Effects of major potential interferences
(SOz,NH3, and ammonium sulfates)
apparently were eliminated by the
addition of a small amount of hydrogen
chloride to the sample.
This report was submitted in fulfill-
ment of Contract No. 68-02-2465 by
Versar Inc. under the sponsorship of
the U.S. Environmental Protection A-
gency. This report covers a period from
September 30, 1976 to April 17,
1980, and work was completed as of
April 17, 1980.
This Project Summary was developed
by EPA's Environmental Sciences
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully doc-
mented in a separate 'report of the
same title (see Project Report ordering
information at back).
Introduction
The increase in damage to properties
and crops in the United States by fall-
out effects of "acid rain" has brought
about a greater needfor instrumentation
to monitor airborne H2S04. To date, the
determination of ambient concentrations
of H2S04 is based on exposing a screen-
type filter to large quantities of air and
analyzing from the filter the total quantity
of collected SO*'2, by x-ray diffraction,
and H+, by pH measurement. Although
this method appears to be the most valid
technique currently available, it does
suffer from serious limitations. For
example, although forced passage of
large volumes of air through a screen-
type filter appears to be an efficient
method of removing H2SO« droplets
from the air, the airflow itself adversely
affects the stability of the collected
HgSO*. Due to the nature of the filtration
technique, the local concentrations of
H2SO4 and other collected particulates
on the surface of the filter are increased
by several factors (relative to their re-
spective concentrations in the ambient);
the surface of the filter thus can serve as
a site for enhanced chemical and physi-
cal reactions which can take place be-
tween the collected HzSCU and such
ambient compounds as NH3, SOz, H20,
metal oxides and metal salts.
An alternative approach to sampling
and analyzing airborne H2SO< has been
developed and a pre-prototype instru-
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ment incorporating these alternative
methods has been fabricated and pre-
liminarily tested. The design, mode of
utilization and preliminary results from
efficiency tests and interference evalua-
tions that were conducted on the pre-
prototype instrument are discussed
herein.
Analyzer System
Design and Operation
The instrumental system is based on
the use of stagewise condensation and
revaporization to separate H2S04 from
other components of an ambient air
sample. Following the separation step,
potential interferents in the sample
(NHa and S02 in particular) can be
eliminated through the action of gaseous
HCI added to the sample. Unambiguous
detection is thus effected by the sample
conditioning steps rather than detector
selectivity. A schematic of the overall
system is shown in Figure 1 . The two
major subsystems, i .e., the H2S04 sampler
and the FPD analyzer, are described
below.
H2SOA Sampler —
Sampling Mode
There are two operating modes of the
system: the sampling mode and the
analysis mode. At initiation of the
Source
Moist
Air
Source
Dilute HCI
Gas Solenoid
/ Valve
sampling mode, a sample stream is
drawn into and out of the HzS04 sam-
pler by a vacuum pump, as illustrated by
Figure 1. The method by which H2SO< is
removed from the sample stream is
based on condensation of the H2S04. As
the sample gas enters the hot inlet
transport tube of the sampler during the
sampling mode, the sample gas is
rapidly heated to a temperature exceed-
ing the dew point temperature of the
H2SO4. Within the hot transport tube
(before entering the collector tube), the
H2S04 thus is believed to be in a vapor
state. The H2S04 vapor is then con-
densed on the cold walls of the collector
tube. The remaining gas is ejected from
the sampler through the vacuum pump.
It should be noted that H2S04 is not
removed from the sample gas through
the utilization of a screen-type filter,
but, rather, is collected on the cold wall
of the tubular collector. Therefore, the
non-condensible particulates should
not be collected along with the H2S04.
In addition to segregating H2S04from
the non-condensible particulates, the
H2S04 sampler appears to be free of NH3
and S02 interferences. This is accom-
plished through the addition of HCI gas
upstream of the hot inlet transport tube
and the subsequent reaction of the
potential interferents with the HCI.
These reactions appear to be preferen-
Heating/Cooling H?SQ4
Chamber ,— Sampler
Source
Sample
Stream
Hot Inlet Heating
Transport Chamber
Tube
H2S04 ,
Collector
Tube
FPD
Recorder
Figure 1.
H2S04 Peak
Residual HCI Peak
H2SO4 Sampler and FPD Analyzer.
2
Sampling Mode
Analysis Mode
tial to reaction of the interferents with
the H2S04.
The actual mechanisms involved in
the reactions of S02 and NH3 with HCI
have not been determined, but HCI gas
will readily react with NH3 to form
particulate NH4CL which would be
ejected from the sampler during the
H2SO4 condensation step. With regard
to SC*2, there are several possible
reactions which might proceed in the
sampler. One possibility is the forma-
tion of SOCI2 which might also be
ejected as a solid particulate.
It should be pointed out that if the
H2S04 collector in the sampler were a
screen-type filter (the conventional
collector) in place of a cold condensation
surface, the elimination of SOz and NHs
interference in the analysis of H2S04 by
the introduction of HCI gas into the
sample might not occur because partic-
ulate reaction products would be col-
lected by the filter.
HzSO4 Sampler-Analysis Mode
The analysis of collected H2S04 takes
place immediately upon completion of
sampling. In the analysis mode, clean
air saturated with moisture is drawn
into the inlet of the sampler (in place of
the sample gas) and is ejected by a
vacuum pump downstream of the FPD
analyzer. In other words, the FPD analy-
zer receives all the gas swept through
the collector tube during the analysis
mode. As the moist air passes through
the sampler to the FPD analyzer, the
internal walls of the collector tube are
rapidly heated to drive the H2SO4 off the
walls of the collector tube to the FPD
analyzer for subsequent analysis.
The data representing the analysis of
H2S04 from a sample stream are in the
form of a chromatogram as shown in
Figure 2.
In operation of the H2S04 sampler and
FPD analyzer, it is observed that some
residual HCI gas will also come off the
walls of the collector tube. However, the
HCI gas comes off the walls much
earlier than H2S04 response in the
presence of HCI. In addition, the com-
ponent (NH4)2S04, if artificially intro-
duced into the front end of the H2S04
sampler, will come off the walls of the
collector tube at a later time than the
H2SO4. The HzSO4 resolution in this
case in shown in Figure 3, which is a
traced copy of an actual chromatogram. I
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Sample Stream:
10.8fig/M3 H2SO4
90 ppb SO2
Dilute HCI
HCI
^
.H2SO4
It.
024 6 8 10
Minutes into Analysis Mode
Figure 2.
Elimination of SO2
interference.
Sample Stream: 10.8/jg/M3 H2SO4
52 ppb NH3
NO HCI
Sample Volume: 31.3 Liters
Amperes Full Scale: 1.0 X 10~e AFS
10
4
aerosol on a glass fiber filter and analyz-
ing the filter for H+ concentration by pH
determination. H2S04 aerosol was pro-
duced by a nebulizer. Throughout the
comparison test the nebulizer was
operated with all conditions constant
with the exception of the concentration
of H2S04 in the nebulizer solution.
Solutions containing 1.0N H2SO4 were
used in the nebulizer to produce levels
of collected H2SO4 aerosols sufficiently
high to allow measurement of H* con-
centration on the glass fiber filters by pH
determination. The output (H2S04 con-
centration in air) of the nebulizer using
1 .ON H2SC>4 as measured by the conven-
tional method, was divided by 1000 to
provide an extrapolative comparison
with the output from a 0.0001 N H2SC>4
solution used with the new system.
On the above basis, the output of the
nebulizer during sampling asdetermined
by conventional filtration was 11.1
mg/m3. The standard deviation for 6
data points was 26.7%. The output of
the nebulizer from analysis by the
developed system was determined to be
11.8 yt/g/m3, and the standard deviation
for 8 data points was 1.8. In addition to
the comparison test, the sampler was
challenged by the nebulizer to a level as
low as 0.24 fjg/m3 H2SO4 in air, as
determined by appropriate dilution of
the nebulizer (H2S04J solution. The
analyzer measured the output to be .26
/ug/m3.
The sampler was also challenged
with significant levels of S02 and NH3
which without the presence of HCI gas
in the sample stream would have re-
sulted in extensive analytical inter-
ference. The levels of S02 and NHa in
the sample air were 90 ppb and 52 ppb,
respectively, and the H2S04 concentra-
tion were of the order of 11 ug/m3
throughout the interference tests. There
was no measurable interference by the
S02 and NH3 when HCI gas was intro-
duced into the sample stream of the
sampler/FPD analysis system.
Recommendations
Further development and evaluative
testing of the general system and tech-
nique described herein are strongly
recommended, based on the results
obtained to date. Specific recommenda-
tions include construction and optimiza-
tion of several prototype units for field
evaluation, extensive testing of the
effects of solid particulates on the
system, and design and implementation
of field evaluation experiments.
-757-012/7110
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James D. Barden is with VERSAR, Inc., Springfield, VA 22151.
Robert K. Stevens is the EPA Project Officer (see below).
The complete report, entitled "Analysis System for Total Su If uric Acid in
Ambient Air: Development and Preliminary Review, "(Order No. PB81-159 139;
Cost: $8.00, subject to change) wilt be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
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