Environmental Engineering Reference
In-Depth Information
The exact transformation mechanisms of the precursors to acidic products is still being de-
bated. Evidently, there are two routes of transformation: the “gas-phase” and “aqueous-phase”
mechanisms. In the gas phase mechanism the following reactions seem to occur:
SO
2
+
OH
→
HSO
3
(9.16)
HSO
3
+
OH
→
H
2
SO
4
(9.17)
and
NO
2
+
OH
→
HNO
3
(9.18)
The acids may directly deposit on land and water as gaseous molecules, or adhere onto ambient
aerosols, and then deposit in the dry particulate form. The acid molecules and aerosols may be
scavenged by falling hydrometeors and then be deposited in the wet form. This is called “wash-out.”
In the aqueous phase mechanism the precursors are first incorporated into cloud drops, a
process called “rain-out,” followed by reactions with oxidants normally found in cloud drops,
namely, hydrogen peroxide (H
2
O
2
), and ozone (O
3
). The distinction between the two mechanisms
is of some importance, because the gas-phase mechanism would indicate that the acids are formed
in a linear proportion to SO
2
and NO
2
concentrations in the air, whereas in the aqueous phase
mechanism the proportion may not be linear. If a nonlinear relationship prevailed, one would not
expect that
acid deposition rates
were directly proportional to
acid precursor emission rates
. Trend
analyses of acid deposition show that the total amount of deposition integrated over a large region
and over a season or year is nearly linearly proportional to the total amount of emissions of the
precursors integrated over the same region and time. This does not necessarily mean that all the
transformation occurs in the gas phase, but that the aqueous phase transformation is also nearly
proportional to the precursors concentration in the cloud drop.
Because the sulfate ion (SO
2
4
) is bivalent and the nitrate ion (NO
3
) is monovalent, if only
these two ions were present in equal molar concentrations, two-thirds of the hydrogen ions (H
+
)
would come from sulfuric acid while one-third would come from nitric acid. However, precipitation
also contains other cations and anions; thus, the proper ion balance equation is
H
+
=
a
SO
2
4
b
NO
3
c
Cl
−
+
d
HCO
3
e
CO
2
3
f
NH
4
g
Ca
2
+
−
h
Mg
2
+
−
i
Na
+
+
+
+
−
−
(9.19)
where
a
,
b
i
are factors weighting the respective ion concentrations, and the square brackets
have been omitted. Some of the ions are from man-made sources, whereas others are natural (e.g.,
sea salt, carbonic acid, and ions from earth crustal matter). In Eastern North America (ENA), an
approximate empirical ion balance equation appears to hold
7
:
,...,
H
+
(
SO
2
4
NO
3
.
±
.
)
+
(
.
±
.
)
1
63
0
1
0
95
0
1
(9.20)
For equimolar concentrations of SO
2
4
and NO
3
, about 63% of the hydrogen ions are due to sulfate
ions and 37% are due to nitrate ions.
In ENA, in the 1970s to 1980s, typical SO
2
4
and NO
3
concentrations in precipitation were in
the range 15-25 micromoles per liter, each. Using (9.20), the average pH of precipitation can be
calculated in the range 4.2-4.4. Figure 9.9(a) shows measurements of the 1986 annual average pH
7
Golomb, D., 1983.
Atmos. Environ.,
17,
1380-1383.
