United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
'V',
Research and Development
EPA/600/S3-86/035 Sept. 1986
Project Summary
Oxygen-18 Study of S02
Oxidation in Rainwater by
Peroxides
Ben D. Holt and Romesh Kumar
This study was conducted to exam-
ine the usefulness of oxygen isotope
ratio measurements in assessment of
the importance of peroxide oxidation of
SO2 to SOj- (hence to acid rain) in the
atmosphere. A new analytical method
was developed by which the oxygen
isotope ratio in peroxide (ppb range) in
rainwater could be determined. Sup-
plies of H2O2 of differing isotope ratios
were then generated, and, using these
supplies to oxidize SO2 to SO*', a com-
prehensive isotopic relationship was
established among H2O2, H2O, and
SO?-. With this relationship, the iso-
topic quality of sulfate that might be
formed in the atmosphere from
aqueous-phase oxidation by peroxide
was calculated. By comparing calcu-
lated to measured isotopic data for sul-
fates in rainwater, and assuming that
this sulfate was formed by either metal
(or carbon) catalyzed oxidation or by
oxidation by peroxides, it was con-
cluded that 40% or more of atmos-
pheric sutfates in the summer rains in
the northeastern U.S. are formed by
peroxide oxidation.
This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Considerable interest has been
shown in the possibility of using oxygen
isotopy to elucidate the role of H202 in
the oxidation of SO2 to sulfates in the
atmosphere. Aqueous-phase oxidation
of S02 to H2SO4 probably accounts for a
major fraction of the observed SOj" in
the precipitation occurring in the north-
eastern United States. The key reactants
responsible for this oxidation are not
well known, although a large number of
possible catalysts and oxidants exist in
the atmosphere, including carbon, tran-
sition metal ions, hydroxyl and organic
free radicals, hydrogen peroxide (in-
cluding organic peroxides), and ozone.
Of these, the last two are the only ones
believed to be present in sufficient
quantity to produce the observed
amounts of SOj" in wet deposition. It
has been postulated that the atmos-
pheric oxidation of S02 is limited by the
availability of one or both of these oxi-
dants, rather than by the availability of
SO2 itself. This is the so-called phe-
nomenon of nonlinearity between the
concentration (and, by implication, the
emission) of S02 in the atmosphere and
the conversion of this SO2 to H2S04. If
such is indeed the case, it is important
to determine if peroxides have a pivotal
role in acid formation and deposition
from the atmosphere.
Oxygen isotopic studies are useful in
distinguishing between the different ox-
idation mechanisms effective in the at-
mosphere. We have previously shown,
by laboratory simulation of several dif-
ferent atmospheric reaction sequences,
that the oxygen isotope ratio in the
product SOl" is uniquely related to the
reaction pathway followed in its forma-
tion. The present work was undertaken
to determine if atmospheric peroxide is
responsible for significant oxidation of
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S02 to sulfate and if it limits the
aqueous-phase formation of sulfuric
acid. The results of such studies can
have significant implications for energy
technology, particularly if they indicate
that, because of the nonlinearity in SO2
conversion discussed above, it may be
more important to reduce the ambient
concentrations of H2O2 than of S02.
In the 1981 JASON Committee Report
to the U.S. Department of Energy, some
recommendations for further research
were based, at least in part, on our ear-
lier work on S02 oxidation by laboratory
reagent-grade H202. According to re-
sults of our earlier work, the 5180 [devia-
tion in parts per thousand (%o) of the
180/160 ratio of the sample from that of
the standard reference material. Stand-
ard Mean Ocean Water (SMOW)] of sul-
fates produced by H202 oxidation were
significantly lower than the 8180 of sul-
fates found in rainwater. However, the
818O of the reagent-grade H202 used in
those experiments was not known (al-
though it has been found, in our current
work, to be very different isotopically
from atmospheric peroxides). It became
evident that there was a need for iso-
topic analysis of H2O2 in dilute solutions
and for a methodology in which the
8180 values of H202, H20, and SOfj~ in
rainwater could be compared to assess
the importance of H202 in the formation
of sulfate-constituted acid rain.
Approach
The plan of the investigation was to
develop a method for the determination
of the 8180 of H2O2 in dilute aqueous
solutions (simulating rainwater); to pre-
pare solutions of H202 of various 18O
enrichments; to use the freshly pre-
pared solutions of H202 to oxidize S02
to S0%~ for evaluation of the relation-
ship between 818Oso2~ and 818OH2o2;
and to apply this relationship to the
measured 8180H2o2. 8180Soz-. and
818OH2o in precipation water, for assess-
ment of the importance of H202 in the
atmospheric transformation of S02 to
sulfate.
Results
A new analytical method was suc-
cessfully developed for the determina-
tion of the 8180 of H202 (ppb range) in
rainwater. The procedure that was fi-
nally adopted after many developmen-
tal experiments follows. Rainwater, col-
lected in 25-L plastic bottles by four
funnels (inverted plastic skylights, each
of 1 m2 catchment, connected to the
bottles by plastic hose), was acidified
and kept refrigerated until the time of
analysis for effective prevention of per-
oxide decomposition. A 20-L sample of
the rainwater was degassed of dis-
solved air by a process of combined
evacuation, ultrasonic agitation, and he-
lium sparge (VUS). The dissolved per-
oxide was then oxidized to 02 by
KMn04; the 02 was removed from the
water sample by the VUS treatment and
conducted through a bed of charcoal
and platinum, where it was quantita-
tively converted to C02. The C02, con-
taining the same oxygen atoms as the
peroxide from which it was derived,
was analyzed mass spectrometrically
for its 8180. Measurement of the
amount of C02 also yielded information
on the concentration of peroxide in the
atmosphere.
Since there was no commercial
source of H2O2 solutions of various 18O
enrichments, solutions were prepared
in the laboratory. Using a high-voltage
method, water vapors of various 8180
were subjected to a corona discharge
that caused the formation of OH radi-
cals; the OH radicals were condensed
on a cold surface, where they combined
to form H2O2; the H202 was diluted with
water and preserved by refrigeration.
These stock solutions of H202 of dif-
ferent 8180 values were then used to ox-
idize S02 to SO2,". A linear relationship
was established between the 8180 of the
sulfate and the 8180 of the H202. This
relationship, combined with one we had
previously determined between the
818Oso2- and 8180H20' gave a compre-
hensive one:
818OS02- = 0.57 S18OH202 + 0.43 S'SO^o + 8.4%.
This relationship could then be used to
calculate the 8180So2-, when both
8180H2o and 818OH2o2 were known.
The relationship was used to com-
pare calculated values to measured val-
ues of S^Oso2- in rainwater samples, in
which all trtree isotopic qualities,
6180H2o2, 8180H2o. and 8180S02-, were
experimentally determined. It was
found that the measured 8180so2- in
rainwater samples from four collection
sites (Argonne, IL; Research Triangle
Park, NC; Whiteface Mountain, NY; and
Dearborn, Ml), in the late spring and
summer of 1985, ranged consistently
between the corresponding calculated
values for peroxide oxidation and for
metal-catalyzed aqueous oxidation, in-
dicating that the sulfate in the rainwater
was a mixture of sulfates formed by the
two mechanisms.
Conclusions
Relative deviations of the measured
S^Oso2- values from the correspond-
ing calculated values of the two compet-
ing mechanisms (peroxide oxidation
and metal-catalyzed aqueous oxidation)
indicated that at least 40%, and proba-
bly more, of the sulfate was formed by
peroxide oxidation. Thus, the isotopic
data obtained by our investigation indi-
cate that peroxide oxidation of SO2 is a
major source of sulfate in rainwater
and, therefore, that investigations of the
origin and possible means of control of
atmospheric peroxides are of prime im-
portance.
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B. D. Holt and R. Kumar are with the Argonne National Laboratory, Argonne, IL
60439.
Marc/a C. Dodge is the EPA Project Officer (see below).
The complete report, entitled "Oxygen-18 Study ofSOz Oxidation in Rainwater by
Peroxides," (Order No. PB 86-216751 /A S; Cost: $9.95, subject to change) will
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:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
I °"2''*6')|L f •
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