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Journal ArticleDOI

H2O2 levels in rainwater collected in south Florida and the Bahama Islands

20 Jun 1982-Journal of Geophysical Research (John Wiley & Sons, Ltd)-Vol. 87, pp 5015-5017

Abstract: Measurements of H2O2 in rainwater collected in Miami, Florida, and the Bahama Islands area indicate the presence of H2O2 concentration levels ranging from 100,000 to 700,000 M No systematic trends in H2O2 concentration were observed during an individual storm, in marked contrast to the behavior of other anions for example, NO3(-), SO4(-2), and Cl(-) The data suggest that a substantial fraction of the H2O2 found in precipitation is generated by aqueous-phase reactions within the cloudwater rather than via rainout and washout of gaseous H2O2

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Title
H
2
O
2
levels in rainwater collected in south Florida and the Bahama Islands
Permalink
https://escholarship.org/uc/item/96b26222
Journal
Journal of Geophysical Research, 87(C7)
ISSN
0148-0227
Authors
Zika, R.
Saltzman, E.
Chameides, W. L
et al.
Publication Date
1982
DOI
10.1029/JC087iC07p05015
Copyright Information
This work is made available under the terms of a Creative Commons Attribution License,
availalbe at https://creativecommons.org/licenses/by/4.0/
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 87, NO. C7, PAGES 5015-5017, JUNE 20, 1982
H202 Levels in Rainwater Collected in South Florida and the Bahama Islands
R. ZIKA AND E. SALTZMAN
School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida 33149
W. L. CHAMEIDES AND D. D. DAVIS
School of Geophysical Sciences, Georgia Institute of Technology, Atlanta, Georgia 30332
Measurements of H202 in rainwater collected in Miami, Florida, and the Bahama Islands area
indicate the presenc e of H202 concentration levels ranging from (1-7) x 10 -5 M. No systematic trends
in H202 concentration were observed during an individual storm, in marked contrast to the behavior of
other anions (e.g., [NO3-], [SO4=], and [C1-]). The data suggest that a substantial fraction of the H202
found in precipitation is generated by aqueous-phase reactions within the cloudwater rather than via
rainout and washout of gaseous H202.
INTRODUCTION
Gaseous H202 is recognized to be a key component in the
photochemistry of the earth's lower atmosphere, and, as a
result, there is much interest in understanding the chemical
and physical processes that determine its abundance. It is
presently believed that atmospheric H202 is generated ex-
clusively by gas-phase photochemical reactions. In the re-
mote trophosphere, the primary gas-phase photochemical
H202 source is via reaction (1)
HO2 + HO2 '-> H202 + 02 (1)
while H202 may be removed by photolysis
H202 + he--> 2OH (2)
reaction with OH
H202 + OH--> HO2 + H20 (3)
or by heterogeneous loss processes such as rainout and
washout [cf Levy, 1973].
On the basis of this mechanism, current photochemical
model calculations predict lower and mid-tropospheric H202
levels of the order of 1 ppbv (cf Chameides and Tan, 198t;
Logan et al., 1981]. In the urban or polluted atmosphere,
significantly higher H202 concentrations are believed to
occur as a result of the oxidation of reactive hydrocarbons
[cf Bufalini et al., 1972]. To test the accuracy of these
predictions, attempts have been made to measure the gas-
phase levels of H20 2 by using wet-chemical techniques [½f
Kok et al., 1978]. Unfortunately, because H202 is itself
generated in solution by a series of complex reactions when
air is bubbled through water, it appears that this technique
must be considered unreliable [Zika and Saltzman, 1982]. As
a result, model-predicted H202 concentrations still remain
unconfirmed by direct atmospheric observations.
Because the reaction of HSO3- with H202 dissolved in
cloudwater may be a major pathway by which atmospheric
SO2 is converted to SO4 = [Penkerr et al., 1979; Moller, 1980],
there is currently great interest in characterizing H202
Copyright 1982 by the American Geophysical Union.
Paper number 2C0193.
0148-0227/82/002C-0193502.00
concentrations levels in cloudwater and rainwater. In this
regard, it is commonly believed that the H202 in cloudwater
arises from the incorporation into the droplets of gaseous
H202 present in the air within and surrounding the cloud. In
fact, the measurements of Kok [1980], who found [H202]
levels ranging from about 1 x 10 -6 M to 3 x 10 -5 M in
rainwater collected in southern California are not inconsis-
tent with this premise. For instance note that the complete
removal of 1 ppbv of H202 in a cloud containing 1 g of liquid
H20 m -3 would result in a iH202] level in the cloudwater of
---10 -5 M. However, in vie. 6f the very limited number of
rainwater H202 measureffi6fit s and the absence of reliable
gas-phase H202 measUi;ements, we view this agreement
between measured and observed [H202] as qualitative rather
than quantitative. Many more measurements of H202 in
rainwater under a variety of conditions are needed to quanti-
tatively test our understanding of the processes that control
atmospheric H202 levels in both the gas and the aqueous
phases. Toward this end we report here on recent field
measurements of the concentrations of H202 in rainwater
gathered in southern Florida and around the Bahama Is-
lands; to the best of our knowledge these measurements are
the first to be carried out in maritime air. Interestingly, these
measurements appear to suggest that the chemistry of H202
in cloudwater is considerably more complex than had been
previously assumed.
EXPERIMENTAL
Rainwater samples were collected and analyzed for H202
concentration by using the fluorescence technique described
by Perschke and Broda [1961] and Zika and Zelmer [1982].
The method involves the addition of a known amount of
scopoletin (6-methoxy-7-hydroxy-l,2-benzopyrene) to a pH
7.0 buffered sample. Subsequent addition of a horseradish
peroxidase (HRP) phenol mixture catalyzes the oxidation of
the s½opoletin by H202, thereby reducing the fluorescence of
the sample. The H202 content of the sample is thus deter-
mined from the difference in fluorescence of the sample
measured before and after addition of HRP. Calibration
curves were obtained by analyzing a series of solutions of
known peroxide concentrations prepared by dilution of a
0.01 M H202 stock solution. By varying the amount of
s½opoletin, hydrogen peroxide concentrations of 2 x 10 -9 M
5015

5016 ZIKA ET AL.: BRIEF REPORT
TABLE 1. Concentration of H202 In Rainwater
ß
Time
Average Rainfall
Rate for Col-
H202 x 105, lection Interval
Moles 1- (m/cm -2 min- •)
Total
Accumulated
Rainfall, cm
Miami, Florida
Feb. 2, Noon-4:20 continuous 2.25 0.0014 0.036
sampling 2.75 0.015 0.072
2.05 0.015 0.11
2.10 0.015 0.14
1.40 0.0i5 0.18
4:32 1.80 0.015 0.22
Feb. 12, 7:00-7:10 P.M. 4:05 ......
7:10--7:20 P,M. 3.95 ......
Feb. 18, 8:30 P.M. 1.75 ......
March 13, 4:20-4:30 P.M. 4.00 ......
4:30-4:35 P.M. 2.65 ß ß ß 0.20
April 17, 2:00 P.M. 3.45 ß ß ß 0.06
May 7, 4:20 P.M. continuous 7.50 ......
sampling 4.75 ......
4.85 ......
3.15 ......
3.43 ......
5:00 P.M. 4.9 ......
6.55 P.M. 0.9 ......
2.6 ......
1.15 ß ß ß 1.90
Bahamas, 25ø20'N 77055'42" W
May 13, 11:10 A.M. 2.10 0.013 0.10
Wind Velocity,
mph
SW 15-20
NE 10-20
SE 15
WNW 10
ESE 15-20
to 1 x 10 -6 M can be accurately determined with a precision
of ñ2%. Laboratory tests have confirmed that this technique
can be used to discriminate between H202 and other oxi-
dants, such as organic peroxides [Zika and Zelmer, 1982].
No interfering oxidants were detected in this work.
The rainwater samples from the Miami area were gathered
by using a wet-dry precipitation collector modified by the
addition of a linear polyethylene funnel (radius = 13.5 cm)
and tube so that the rainwater was delivered directly into a
laboratory buret for analysis. A similar system was em-
ployed in the region of the Bahama Islands with H202
determinations carried out in a shipboard laboratory. Mea-
surements of samples obtained by injecting a known quantity
of H202 and distilled water into our precipitation collector
confirmed that our collection procedures did not introduce
spurious H202 decay or formation characteristics.
During one rainstorm (i.e., July 22) the concentration of
cations and anions in the rainwater was measured in addition
to H202. This measurement was accomplished by using
standard ion chromatography and atomic absorption proce-
dures.
RESULTS AND DISCUSSION
Measured concentrations of H202, [H20:], in rainwater
samples coliected in Miami, Florida, and the Bahama Islands
area, are indicated in Table 1 and Figure 1. We find the range
in [H202] varies from about 1.4 x 10 -5 M to 7.5 x 10 -5 M.
Since [H:O:] determined at the urban sampling sites was
consistent with that obtained from the Bahamas, it appears
that the city samples were not predominantly influenced by
pollutants and the associated products of photochemical
smog. In addition, our measurements do not give any
obvious indication of a correlation between [H20:] and wind
direction.
The [H•O2] levels reported here tend to be generally
somewhat higher than those obtained by Kok [1980] for
rainwater collected in California. Conceivably, this differ-
ence could reflect higher levels of SO2 in California relative
to those of Florida and Bahama Islands, since HSO3- (the
dissolved form of SO2) by reacting wit. h H202 in solution can
cause lower [H202] levels. It is also possible that greater
concentrations of H202 precursors were present in the air we
sampled relative to that of Kok [1980].
During three rainstorms, the rainwater analysis was car-
ded out continuously to obtain the concentration of dis-
solved H202 as a function of time during the course of each
storm. As indicated in Table 1 and Figure 1, for the storms of
February 2 and July 22, 1981, which occurred during the
midday hours, H202 levels remained fairy constant as a
function of time. By contrast, however, H202 was found to
decline significantly as a functioa•of time during the storm of
May 7, 1981, which occurred in the early evening. Given the
limited data, it is not yet possible to discern if this difference
in the H202 temporal trends is related to the time of day that
the storm occurred or to another parameter.
An interesting facet of our data is the striking difference
between the variation in the measured [H202] as a function
of time during the course of the storm on July 22 compared
with those of [NO3-], [SO4=], and [C1-] (see Figure 1). In
the case of [NO3-] and excess [SO4=], the concentration
levels decreased monotonically be a factor of 2-4 over the
rainstorm's lifetime. (Note that excess [SO4 =] is defined as
total dissolved [SO4 =] minus the component due to the
marine aerosol, where [SO4 =] due to the marine aerosol is
scaled from the observed [Na +] level.) On the other hand,
[C1-] exhibited a more complicated pattern, an initial de-
crease, then a sharp increase, coinciding with a second pulse
in rainfall intensity, and then a precipitous decline. In
contrast to these species, [H202] levels illustrated in Figure 1
gave no indication of a downward trend with time and in fact
tended to remain fairly constant over the entire course of the
storm.

ZIKA ET AL.' BRIEF REPORT 5017
m 0.O3-
-• E O.02-
z
i I I
0-
5' -• I
O. •'• ' I
5-
I I
,5- '
-- I I I I
,O. -
ß .i -
-- I I I I
01' , '
0 20 40
TIME (MINUTES)
Fig. 1. Observed levels of [H202], [NO3-], excess [SO4=],
[CI-], and rainfall rate during the storm of July 22, 1981, in Miami,
Florida, as a function of time. The onset of the storm at t = 0
corresponds to 12:50 P.M. EDT and the vertical arrow at approxi-
mately t = 16 min indicates the brief presence of sunshine during the
storm. (Note that excess [SO4 =] is derived from the difference
between total observed [SO4 =] and 0.06 times observed [Na+], see
text.)
The difference in the variation of [C1-] compared with
those of [NO3-] and excess [SO4 =] indicates that different
mechanisms were responsible for incorporating these spe-
cies into our rainwater samples. While C1- most likely
originated from marine aerosols scavenged from the atmo-
sphere by cloud and rain droplets, it is probable that the
NO3- and excess SO4 = present in the rainwater we analyzed
was derived in large part from the scavenging of gaseous
species. Given the high solubility of H202, we would expect
that if the H202 in our rain samples had been derived from
the dissolution of gaseous H202, then [H202] would have
exhibited a trend similar to that observed for [NO3-] and
excess [SO4=]. However, the difference between the time
patterns of [H202] and these latter two species in Figure 1
appears to suggest that the H202 dissolved in the rain of July
22 was derived from a mechanism different from the mecha-
nism which caused the presence of NO3- and excess SO4 =.
One interpretation of our data is that while NO3- and excess
SO4 = was derived from the dissolution of gaseous species, a
significant fraction of the H202 in our samples had been
generated in the cloudwater as a result of aqueous-phase
chemical reactions such as those recently proposed by
Chameides and Davis [1982]. The results of Zika and
Saltzman [1982] and Heikes et al. [1982] documenting the
aqueous-phase generation of H202 when air is bubbled
through water is not inconsistent with the above conclusion.
Because of the very limited amount of data presented
here, our hypothesis concerning the chemical generation of
aqueous-phase H202 must be considered, at this time, to be
highly speculative. Nevertheless, our data does appear to
suggest that the processes which control the levels of H202
in precipitation may be considerably more complex than had
been previously assumed. We believe therefore that further
laboratory studies of the aqueous-phase chemistry of H202
as well as direct measurements of H202 levels in cloudwater
and in the gas phase are needed to more accurately establish
the budget of H202 in precipitation.
Acknowledgment. The authors wish to thank the following agen-
cies for supporting this work: The National Science Foundation
through grants OCE 78-25628 and ATM 79-09239 (to R.Z.), the
National Aeronautics and Space Administration through grant NAG
1-85 (to W.L.C.), and the Office of Naval Research through grant
NOOO 14-80-C-0042.
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1972.
Chameides, W. L., The photochemical role of tropospheric nitrogen
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Chameides, W. L., and A. Tan, The two-dimensional diagnostic
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Chameides, W. L., and D. D. Davis, The free radical chemistry of
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Zika, R., and E. Saltzman, Production of H202 in the aqueous phase
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Zika, R. G., and P. Zelmer, An evaluation of the HRP-Scopoletin
method for the measurement of hydrogen peroxide in natural
waters, submitted to Anal. Chem., 1982.
(Received December 21, 1981'
revised February 4, 1982;
accepted February 5, 1982.)
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