Atmospheric Environment 35 (2001) 6219–6225
The characterization of hydrogen ion concentration in
sequential cumulative rainwater
Luo Wanqing*
Environmental and Sanitary Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606-8501 Japan
Received 15 June 2000; received in revised form 22 March 2001; accepted 18 April 2001
Abstract
From March to June 1996, eight rain events were sampled sequentially on time and volume basis in Uji, Kyoto,
Japan. pH was measured by a pH meter. The concentrations (mmol l1) of chloride ion (Cl), nitrate ion (NO
3 ), sulfate
+
+
+
2+
ion (SO2
) and magnesium ion
4 ), sodium ion (Na ), ammonium ion (NH4 ), potassium ion (K ), calcium ion (Ca
2
+
+
+
2+
(Mg2+) were measured by ion chromatography. It is found that Cl, NO
and Mg2+
3 , SO4 , Na , NH4 , K , Ca
concentrations in sequential cumulative rainwater decreased with rain duration. However, hydrogen ion (H+)
concentration (derived from pH value) showed approximately an increasing trend with rain duration. The factors
influencing H+ concentration as well as the H+ concentration varying pattern for sequential cumulative rainwater were
discussed. r 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Acid factors; Alkali factors; Rainwater chemistry; Ionic balance; Weak acids
1. Introduction
Atmospheric aerosol particles and gases play a
major role in the chemistry of rainwater by in-cloud
and below-cloud scavenging processes. As a result, the
following chemical species are typically found in
rainwater: ammonium, potassium, sodium, calcium,
magnesium, hydrogen, sulfate, chloride, nitrate, sulfite,
hydrogen sulfate, bicarbonate, and carbonate ions
(Hobbs, 1993; Liljestrand and Morgan, 1978; Pruppacher and Klett, 1980). Among these chemical species,
hydrogen ion concentration (or pH) is very important
for acid rain assessment (Brennan and Peden, 1987;
Charlson and Rodhe, 1982; Finlayson-Pitts and Pitts, Jr,
1986; Sanusi et al., 1996; Seinfeld, 1986; Warneck, 1988).
However, compared with other chemical species,
*Corresponding author. Present address: 203, 9-5, 3CHO,
Akasakadai, Sakai, Osaka 590-0144, Japan. Fax: +81-722-93249.
E-mail address: [email protected] (L. Wanqing).
hydrogen ion concentration often expressed different
characteristics. For example, Ravichandran and
Padmanabhamurty (1994) observed that during
monsoon although cations and anions concentrations decreased considerably, the hydrogen ion
concentration increased with the increase of
precipitation amounts. The experimental results
from Durana et al. (1992), Michael and Thomas
(1983), Seymour and Stout (1983), and Tuncel and
Ungor (1996) showed that a varying pattern of hydrogen
ion concentration in sequential rainwater were different
from other ions.
In this study, rainwater was collected sequentially
on time and volume basis for a period as short
as possible in order to maintain a constant meteorological condition. The purpose of this study is (1) to
identify detailed chemical species concentrations
profiles of sequential cumulative rainwater on
temporal variations; (2) to analyze factors influencing
hydrogen ion concentration in sequential cumulative
rainwater.
1352-2310/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved.
PII: S 1 3 5 2 - 2 3 1 0 ( 0 1 ) 0 0 2 3 5 - 7
6220
L. Wanqing / Atmospheric Environment 35 (2001) 6219–6225
2. Methods and materials
2.1. Sampling
Rainwater was collected manually and sequentially on
time and volume basis (Lim et al., 1991), on 15, 21, 29
March, 16, 19 April and June 24–26 1996, respectively.
The collector is located, approximately, 20 m above the
ground on the roof of the five-floor building of the
Graduate School of Energy Science, Kyoto University,
in Uji, Japan. The collector is composed of a 4-funnel
made from poly-4-methyl-1-pentene and a funnel support. Each funnel (0.3 m in diameter) is connected to a
plastic tube. A plastic cover is suspended over the 4funnel to prevent contamination. Before each sampling,
the collector and accessories were rinsed with deionized
water until the electric conductivity of the final rinse is
zero. The plastic cover was removed immediately at the
onset of each rain. Rainwater was channeled by the
plastic tubes during rain, and then gathered in a 50 ml
test tube. The test tube was changed and capped when
more than 20 ml was collected. The sampling time,
sampling duration as well as the volume of each sample
were recorded. Here, for the event on 26 June 1996, since
rain intensity is high (over 10 mm h1), rainwater was
gathered sequentially in a 300 ml beaker and only a 2funnel was used, Fig. 1 shows the sampling time and
sampling duration for each event as well as each
sequential rainwater sample.
2.2. Analysis
Once each sampling was completed, all the samples
were transferred to the Laboratory of Socio-Environ-
mental Energy Science which is just under the roof.
Upon returning to the laboratory, pH was measured
immediately by a portable digital pH meter (HORIBA
COMPACT pH METER B-112). After the pH meter was
calibrated with two standard buffer solutions (HORIBA
pH 7 and pH 4), and the contact part was washed with
the rainwater sample, the pH of each sample was
measured continuously at least two times, until a stable
reading was obtained. Usually, the amount consumed
for each sample measurement was approximately 5 ml.
After pH was analyzed, 10 ml fractions of each sample
was drawn, and filtered through a 0.2 mm pore size
cellulose acetate membrane filter into a clean 50 ml
polyethylene sample bottle. Before filtering, the glass
filter holder (including the stainless steel connector) and
the sample bottles were rinsed with deionized water,
until the electric conductivity of the final rinse is zero.
After each rainwater sample was filtered, chemical
+
+
2+
2
+
species Cl, NO
and
3 , SO4 , Na , NH4 , K , Ca
2+
Mg
concentrations in the extract were measured by
ion chromatography. The ion chromatograph instrument is composed of CDD-6A, LC-10AD, DGU-4A
and CTO-4A, made by Simazu Chemical Instruments
Company in Japan. The columns equipped are Shim
pack IC-A2 for determining anions F, Cl, NO
2 , Br ,
2
NO3 , SO4 and Shim-pack IC-C3 for determining
+
2+
cations Na+, NH+
and Ca2+. Approxi4 , K , Mg
mately, 5 ml (including 3 ml for washing the sample loop
and the injector) was required for each sample
measurement. Usually, within 24 h after sampling, the
rainwater measurements were completed. The unused
portions of the samples were stored in a refrigerator at
41C. The data were converted to mmol l1 for statistical
analysis. The overall precision was 76%.
2.3. Calculation
2.3.1. Calculation of hydrogen ion concentration
For each sample, hydrogen ion concentration [H+] is
calculated by [H+]=10pH. Actually, the [H+] here is
the proton activity aH+=r[H+]. Since rainwater is a
sufficiently dilute solution, its coefficient r is close to
unity. Hydrogen ions in aqueous solution always occur
in the form of H3O+, the notation H+ is merely a
convention (Warneck, 1988).
Fig. 1. Sampling time and sampling duration for each sequential rainwater sample as well as rain events that occurred on 15,
21, 29 March, 16, 19 April and 24–26 June 1996, in Kyoto,
Japan.
2.3.2. The calculation of chemical species concentrations
in sequential cumulative rainwater
+
+
2+
2
+
H+, Cl, NO
or
3 , SO4 , Na , NH4 , K , Ca
2+
Mg concentration in sequential cumulative rainwater
is calculated by the following equation:
P
ci vi
Cxi ¼ Pi
;
ð1Þ
i vi
+
+
2
+
where, Cxi is H+, Cl, NO
3 , SO4 , Na , NH4 , K ,
2+
2+
1
Ca
or Mg
concentration (mmol l ) in the ith
L. Wanqing / Atmospheric Environment 35 (2001) 6219–6225
sequential cumulative rainwater; ci is H+, Cl, NO
3,
+
+
+
2+
2+
SO2
,
Na
,
NH
,
K
,
Ca
or
Mg
concentration
4
4
(mmol l1) in the ith sequential rainwater sample; vi is the
precipitation amount (mm) of the ith sequential rainwater sample.
2.3.3. The calculation of bicarbonate and carbonate ions
concentrations
There are two sources of bicarbonate (HCO
3 ) and
carbonate (CO2
3 ) ions in rainwater. One is the atmospheric carbon dioxide (CO2) that dissolves in rainwater
to form bicarbonate ion (HCO
3 ) and carbonate ion
+
(CO2
3 ) as well as H . The other is the atmospheric
aerosol particles of calcite (CaCO3) reacting as strong
bases with H+ to give HCO
3 . These reactions occur
according to the following equations:
CO2 þH2 O"CO2 H2 O;
KH ¼ 3:4102 mol l1 atm1
ðat 298 KÞ;
CO2 H2 O"Hþ þ HCO
3;
K1 ¼ 4:6107 mol l1
ðat 298 KÞ;
ð3Þ
3. Results and discussion
3.1. The characterization of H+ concentration in
sequential cumulative rainwater
Fig. 2 shows the concentration varying patterns of
2+
+
, K+ and
Ca2+, Na+, Cl, SO2
4 , NH4 , NO3 , Mg
+
H
in sequential cumulative rainwater with rain
duration for the event on 21 March 1996. It is clear
2+
+
that Ca2+, Na+, Cl, SO2
and
4 , NH4 , NO3 , Mg
+
K concentrations decreased with rain duration; however, H+ concentration showed an increasing trend.
Furthermore, for the total eight events, Fig. 3 shows the
2
concentration varying patterns of Cl, NO
3 , SO4 ,
+
+
2+
2+
+
+
Na , NH4 , K , Ca , Mg
and H of sequential
cumulative rainwater with rain duration. It is seen that
there exists an inverse concentration varying trend
between H+ and the other chemical species. This
phenomenon is similar to that mentioned by Ahmed
et al. (1990) and Ravichandran and Padmanabhamurty
(1994). In order to interpret the H+ concentration
characterization, the following discusses the factors
influencing H+ concentration in sequential cumulative
rainwater.
3.2. Factors influencing H+ concentration
2
þ
HCO
3 "H þ CO3 ;
K2 ¼ 4:51011 lmol l1
ð2Þ
6221
ðat 298 KÞ;
ð4Þ
where KH is Henry’s coefficient; K1 is CO2 H2O
dissolution coefficient; and K2 is HCO
3 dissolution
coefficient. Then the equilibrium relations are
KH PCO2 ¼ ½CO2 H2 O;
ð5Þ
K1 ½CO2 H2 O ¼ ½Hþ ½HCO
3 ;
ð6Þ
2
þ
K2 ½HCO
3 ¼ ½H ½CO3 ;
ð7Þ
3.2.1. Acid factors and alkali factors
From the historical perspective of acid rain studies,
the most well known rainwater acidification mechanism
is SO2 and NOx emitted into the atmosphere being
oxidized to H2SO4 and HNO3 through both gas and
aqueous phase processes. HCl also play a role in
contributing to rainwater acidity. In addition, weak
where PCO2 is the atmospheric CO2 partial pressure.
2
Thus, in rainwater the HCO
3 and CO3 concentrations
following from the dissolution of CO2 as well as
carbonate aerosol particles is
þ
þ
½HCO
3 ¼ ½CO2 H2 OK1 =½H ¼ KH pco2 K1 =½H ¼ 5:5=½Hþ ðmmol l1 Þ;
ð8Þ
þ 2
þ 2
½CO2
3 ¼ ½CO2 H2 OK1 K2 =½H ¼ KH pco2 K1 K2 =½H ¼ 0:00025=½Hþ 2 ðmmol l1 Þ;
ð9Þ
where the atmospheric CO2 concentration is assumed to
be equal to 350 ppm. Since CO2
3 concentration is very
low, generally, it is neglected (Losno et al., 1991;
Warneck, 1988).
Fig. 2. The variations of calcium, sodium, chloride, sulfate,
ammonium, nitrate, magnesium, potassium, and hydrogen ions
concentrations in sequential cumulative rainwater with rain
duration for the events that occurred on 21 March 1996, in Uji,
Kyoto, Japan.
6222
L. Wanqing / Atmospheric Environment 35 (2001) 6219–6225
Fig. 3. The variations of hydrogen, chloride, nitrate, sulfate, ammonium, potassium, sodium, calcium, magnesium ions concentrations
in sequential cumulative rainwater with rain duration, for rain events that occured on 15, 21, 29 March, 16, 19 April and 24–26 June
1996, in Kyoto, Japan.
acids, such as H2CO3 from dissolved atmospheric CO2
in rainwater, and organic acids (formic acid, acetic acid,
etc) contribute to rainwater acidity (Brimblecombe,
1996; Khwaja and Husain, 1990; Lijestrand and
Morgan, 1978; Saylor et al., 1992; Schlesinger, 1991;
Seinfeld, 1986; Warneck, 1988). In this paper, anions
2
NO
and HCO
3 , SO4 , Cl
3 are proposed as acid
factors.
On the other hand, alkaline compounds containing
Na+, K+, Ca2+ and Mg2+ can neutralize strong and
weak acids in rainwater. The major fraction of alkali
source is the aerosol particles originated from the soil
(Samara et al., 1992; Saylor et al., 1992; Schlesinger,
1991; Tuncel and Ungor, 1996). Atmospheric ammonia
derived from a variety of sources is also an important
alkali source (ApSimon et al., 1987). In addition, a few
amines will serve to neutralize acids (Brimblecombe,
1996; Lijestrand and Morgan, 1978; Schlesinger, 1991;
Suzuki et al., 1997). In this paper, cations Na+, NH+
4 ,
K+, Ca2+ and Mg2+ are suggested as alkali factors.
H+ concentration in rainwater is the result of the
interaction of acids and alkalis. The acids containing
acid factors act as donors of H+, and the alkalis
containing alkali factors act as acceptors of H+. In
order to examine the influences of these acid and alkali
factors to H+ concentration in rainwater, Table 1 gives
2
the correlation coefficients of H+ to Cl, NO
3 , SO4 as
2
well as their sum e.g. [Cl + NO3 + 2SO4 ] in
concentration. On the other hand, Table 2 gives the
+
correlation coefficients of H+ to Na+, NH+
4 , K ,
+
2+
2+
+
Ca , Mg
as well as their sum e.g. [Na +NH4 +
K++Ca2++Mg2+] in concentration. Table 1 showed
+
that, except SO2
4 for the event on 26 June 1996, H had
no positive correlation with the acid factors as well as
their sum in concentration. However, in Tables 1 and 2,
there exist, simultaneously, negative correlations between H+ and other chemical species in concentration,
which implies that the acid and alkali factors incorporated into rainwater, probably not as acids, but as slats
except for the rain event on June 26, 1996. Since in Table
2, H+ had good negative correlations with Mg2+, Ca2+,
Na+, K+ and NH+
4 in concentration, which indicates
that these alkali factors played a dominating role in
influencing H+ concentration in sequential cumulative
rainwater.
3.2.2. The difference between the sum of alkali factors
and the sum of acid factors in concentration
Furthermore, in order to interpret the H+ concentration varying pattern in sequential cumulative rainwater,
6223
L. Wanqing / Atmospheric Environment 35 (2001) 6219–6225
Fig. 3 (Continued)
Table 1
The correlation coefficients of hydrogen ion to chloride, nitrate and sulfate ions, as well as their sum in concentration for sequential
cumulative rainwater sampled on 15, 21, 29 March 16, 19, April 24–26 and June 1996, respectively, in Uji, Kyoto, Japan
[H+]
15 March
1996
21 March
1996
29 March
1996
16 April
1996
19 April
1996
24 June
1996
25 June
1996
26 June
1996
[Cl]
[NO
3]
[SO2
4 ]
2
[Cl-+NO
3 +2SO4 ]
0.26
0.21
0.22
0.23
0.60
0.69
0.66
0.65
0.79
0.00
0.29
0.50
0.52
0.14
0.25
0.27
0.90
0.83
0.92
0.89
0.87
0.09
0.00
0.14
0.34
0.88
0.90
0.81
0.02
0.23
0.66
0.27
the difference between the sum of alkali factors and the
sum of acid factors in concentration was calculated, e.g.
+
2+
The difference=[Na++NH+
+2Mg2+]
4 +K +2Ca
2
[Cl +NO3 +2SO4 +HCO3 ]
Fig. 4 shows the difference varying pattern and the
H+ varying pattern in concentration with rain duration,
for the events on 29 March and 16 and 19 April. 1996. It
is found that there exists an inverse varying trend
6224
L. Wanqing / Atmospheric Environment 35 (2001) 6219–6225
Table 2
The correlation coefficients of hydrogen ion to sodium, ammonium, potassium, calcium and magnesium ions, as well as their sum in
concentration for sequential cumulative rainwater sampled on 15, 21, 29 March, 16, 19 April and 24, 25, 26 June 1996, respectively, in
Uji, Kyoto, Japan
[H+]
15 March
1996
21 March
1996
29 March
1996
16 April
1996
19 April
1996
24 June
1996
25 June
1996
26 June
1996
[Na+]
[NH+
4 ]
[K+]
[Ca2+]
[Mg2+]
+
2
2+
[Na++NH+
]
4 +K +2Ca +2Mg
0.24
0.26
0.26
0.65
0.09
0.09
0.62
0.68
0.62
0.75
0.66
0.71
0.82
0.23
0.41
0.74
0.81
0.66
0.59
0.39
0.42
0.54
0.68
0.50
0.92
0.43
0.87
0.91
0.88
0.92
0.75
0.11
0.77
0.05
0.72
0.09
0.23
0.91
0.23
0.99
0.99
0.97
0.60
0.50
0.51
0.61
0.80
0.06
tion exists, e.g. when the difference decreased in
concentration with rain duration, the H+ concentration
showed an increasing trend. Therefore, the difference
between the sum of alkali factors and the sum of acid
factors in concentration is a dominating factor for
influencing the H+ concentration varying pattern in
sequential cumulative rainwater.
On the other hand, the total acidity consists of both
free protons (from strong acids, such as H2SO4, HNO3
and HCl) and non-dissociated protons (weak acid, such
as acetic acid and carbonic acid). In this study, the
difference between the sum of cations and the sum of
anions in concentration indicates that there are anions
deficit exists. These undetermined anions probably are
organic acidic (formic, acetic, etc.) anions. Therefore, it
is possible that weak acids or organic acids such as acetic
acid, etc. may influence H+ concentration. However, in
this study, due to the limitation of experimental
facilities, it was not possible to determine these weak
acids or organic acids such as formic acid and acetic
acid, etc. whatever weak acids effect H+ concentration
as well as H+ concentration varying pattern in
sequential cumulative rainwater needs further research.
4. Conclusions
Fig. 4. The varying patterns of hydrogen ion and the difference
between the sum of sodium, ammonium, potassium, calcium
and magnesium ions and the sum of chloride, nitrate and
sulfate ions in concentration with rain duration, as well as their
correlations for the rain events that occured on 29 March and
16 and 19 April 1996, in Uji, Kyoto, Japan.
between the difference and the H+ in concentration. The
correlation coefficients indicate that a good negative
correlation between the difference and H+ concentra-
(1) For almost all events, the concentrations of
+
+
2
+
chemical species Cl, NO
3 , SO4 , Na , NH4 , K ,
Ca2+ and Mg2+ decreased with rain duration, H+
concentration showed an increasing trend. It implies
that even for the same event, the chemical species
concentrations in rainwater will vary greatly due to the
change of sampling time and sampling duration. (2) H+
concentration in sequential cumulative rainwater was
not controlled by the strong acids containing Cl, NO
3
2+
and SO2
,
4 . However, the compounds containing Mg
Ca2+, Na+, K+ and NH+
were
the
dominating
factors
4
influencing H+ concentration in sequential cumulative
rainwater. The difference between the sum of Na+,
+
2+
NH+
and Mg2+ and the sum of Cl, NO
4 , K , Ca
3,
2
SO4 and HCO3 was a main factor effecting the H+
L. Wanqing / Atmospheric Environment 35 (2001) 6219–6225
concentration varying pattern with rain duration. (3)
Weak acids or organic acids, such as carbonic acid,
formic acid and acetic acid, etc. probably play a role in
influencing the H+ concentration as well as H+
concentration varying pattern in sequential cumulative
rainwater. (4) In order to obtain ionic balance rainwater,
it is necessary to develop a more complete analytical
method to determine rainwater, such as the determination of organic acidic anions, bicarbonate and carbonate
ions concentrations, etc.
Acknowledgements
The author thanks Prof. M. Kasahara, Associate
Prof., S. Tohno, Assistant Prof., K. Yamamoto, and the
students in the Laboratory of Socio-Environmental
Energy Science, Graduate School of Energy Science,
Kyoto University, as well as Prof., Y. Maeda, Associate
Prof., H. Bando, Assistant Prof., K. Takanaka and the
students in the Environmental Chemistry Laboratory,
Graduate School of Engineering, Osaka Prefecture
University for their advice and helps. The author also
appreciates Rotary Yoneyama Memorial Foundation,
the Association of International Education of Japan,
and the Foundation of International Soroptimist for
their generosities.
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