Environmental Engineering Reference
In-Depth Information
TABLE 6.8
Typical Constituents of Rainwater in the Northeastern United States
Rainwater Concentration Collected
Average Concentration in the
Ion
in Ithaca, NY, on July 11, 1975 (
μ
M)
Northeast United States (
μ
M)
Sulfate
57
28
±
4
Nitrate
44
26
±
5
Ammonium
29
16
±
5
pH
3.84
4.14
±
0.07
Source:
From Park, D.H. 1980.
Science
208, 1143-1145; Galloway, J.N., Likens, G.E., and
Edgerton, E.S. 1976.
Science
, 194, 722-724.
10
−
3.5
atm,
At a constant partial pressure of carbon dioxide,
P
CO
2
=
350 ppm
=
(
1
/K
aw
) P
CO
2
=
(
10
−
1.5
)(
10
−
3.5
)
10
−
5
M. Hence [H
+
]=
we have [H
2
CO
3
]=
=
10
−
11.4
M. Thus, the pH of water is 5.7, which is
1.3 units less than neutral. Natural rainwater is therefore slightly acidic. When strong
acids are present that lower the pH to below 4.3, the above reaction is driven to the
left and carbonic acid makes no contribution to acidity. The components of strong
acids are derived from SO
2
4
and NO
3
.
Gaseous SO
2
in the atmosphere dissolves in atmospheric moisture to form sulfu-
ric acid and contributes to sulfate in atmospheric particles. SO
2
arises mainly from
anthropogenic emissions (burning of fossil fuels). The wet deposition of sulfate by
rainfall occurs near industrial locations that emit SO
2
to the air. The emission of
total S to the atmosphere from natural sources is estimated to be
(
10
−
6.4
)(
10
−
5
)
K
c1
[H
2
CO
3
]=
=
10
13
g/y, of
∼
8.4
×
10
13
g/y is attributed to gaseous SO
2
. Episodic events such as volcanic
eruptions also contribute to the global emission of S compounds to the atmosphere;
they disrupt the quasi-steady-state conditions that exist in the atmosphere.
Let us estimate what sources contribute most to H
+
in the atmosphere. If only CO
2
is the contributing factor toward acidity, the pH of rainfall on earth will be 5.7. If the
average annual rainfall is 70 cm/y and the surface area of the earth is 7
which
∼
1.5
×
10
18
cm
2
,
×
this gives a total deposition of [H
+
]=
(
10
−
5.7
)(
10
−
3
)(
70
)(
7
10
18
)
×
=
1.2
×
10
12
mol/y. The total acidity from all natural sources is
10
12
mol/y. In com-
parison, the anthropogenic sources due to fossil fuel burning, automobile, and other
industrial sources contribute NO
x
and SO
2
, which lead to
≈
8
×
10
12
mol of [H
+
]
per year. It is, therefore, clear that human activities are contributing to the acidity of
our atmosphere. Over geologic time our atmosphere has been acidic, but has increased
only slightly in acidity. Natural alkalinity resulting from NH
3
tends to neutralize the
aciditytosomeextent.ThetotalNH
3
emissionis
∼
7.4
×
10
12
mol/y,andhenceareduc-
≈
3
×
10
12
mol of [H
+
] per year can be attributed to reaction with NH
3
. This
tion of
≈
3
×
10
12
mol [H
+
] that are being annually added to the atmosphere.
Gaseous SO
2
is photochemically activated in the atmosphere and subsequently
oxidized to SO
3
, which reacts further with water to form H
2
SO
4
:
still leaves
≈
12
×
hv
−−−−→
SO
2
,
SO
2
SO
2
+
O
2
−→
SO
3
+
O,
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