Environmental Engineering Reference
In-Depth Information
From the equation for [S(VI)], we can obtain (see Seinfeld, 1986)
SO
2
4
and
HSO
4
[S
(
VI
)
]
1
+
H
+
/
K
s4
[S
(
VI
)
]
1
+
K
s4
/
H
+
≈
≈
with
K
s4
=
0.012 mol/L.
The rates of conversion to S(VI) by H
2
O
2
and O
3
are given in Table 6.9. Hence
d [S
(
VI
)
]
d
t
[S
(
VI
)
]
t
=
1min
=
[S
(
VI
)
]
t
=
0
+
t
Δ
t
.
t
−Δ
We then obtain a new value for [HSO
4
] and [SO
2
4
]. Substituting this in the elec-
troneutrality equation, we get the new [H
+
], and hence the new pH. Continuing in this
manner to obtain [S(VI)] at different times, we can follow the changes in pH as more of
S(IV) is converted to S(VI) by H
2
O
2
and O
3
. This is provided by Seinfeld and Pandis
(2006) (Figure 6.37). In 60 min, the pH decreased to 5.3 from its initial value of 6.1.
The assumption of an open system is questionable. In a given cloud volume, it is
unlikely that the partial pressures of the different species will remain constant over
the duration of the reaction. In such a case, one should consider the changes in par-
tial pressures of NH
3
and HNO
3
. A detailed account of this aspect of atmospheric
reaction modeling is given by Seinfeld and Pandis (1998) and is beyond the scope of
this topic.
Although SO
2
oxidation in the gas phase to form H
2
SO
4
(aerosol) is a linear
function of OH
•
concentration, it indirectly depends on the concentration of NO
x
in the atmosphere as well. As seen above, the oxidation in the aqueous droplet
also depends on the concentration of other oxidants such as H
2
O
2
and O
3
. It can
be shown that if the calculations in the above example are repeated with a differ-
ent concentration of HNO
3
, the level of NO
3
in the aqueous phase will influence
[S(VI)] in the droplet. The conversion of NO
x
to HNO
3
within the droplet is primar-
ily a function of the photochemical reactions in the gas phase, and hence responds
directly to changes in NO
x
levels in the atmosphere. The atmospheric moisture con-
tent is obviously an important factor in deciding the fraction of S(IV) converted to
S(VI). With increasing moisture content, the fraction oxidized also increases. Thus
a cloud with a large moisture content will have high acidity due to a large concen-
tration of S(VI). In conclusion, acid rain, which is mostly a regional problem, is
a complex process that depends on the prevailing local conditions, particularly the
levels of other species present in the atmosphere. Models exist to predict the acid-
ity to be expected in precipitation if sources, and their strengths are identified with
confidence.
In our analysis thus far, we have only considered reactions within aqueous droplets
in the atmosphere. However, this may not always be the controlling resistance for
conversion of S(IV) to S(VI). The evolution of acidity in atmospheric precipitation
(rain or fog) depends on factors such as diffusion of species toward the droplet from
the air and reaction within the droplet.
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