Acid Deposition - Air Pollution and Global Warming: History, Science, and Solutions - page 227

Whereas this is the dominant mechanism by which
S(IV) produces S(VI) in aerosol particles, particularly
when the relative humidity is below 70 percent, a second
mechanism more rapidly produces S(VI) from S(IV) in
cloud drops and raindrops.

Table 10.1. Names and formulae of S(IV) and S(VI)
species

S(IV) Family S(VI) Family

Chemical Chemical Chemical Chemical

name formula name formula

10.3.2. Aqueous-Phase Oxidation of S(IV)
The second conversion process of S(IV) to S(VI)
involves (1) dissolution of SO 2 (g) into liquid water
drops to produce SO 2 (aq), (2) in-drop conversion of
SO 2 (aq) to H 2 SO 3 (aq) and dissociation of H 2 SO 3 (aq) to
HSO 3 − and SO 3 2 − , and (3) in-drop oxidation of HSO 3 −
and SO 3 2 − to SO 4 2 − .The dissolution process (Step 1)
is represented by the reversible reaction

Sulfur dioxide SO 2 (g,aq)

Sulfurous acid H 2 SO 3 (aq) Sulfuric acid H 2 SO 4 (g,aq)

Bisulfite ion HSO 3 − Bisulfate ion HSO 4 −

Sulfite ion SO 3 2 − Sulfate ion SO 4 2 −

Canada; among western, northern, and eastern Euro-
pean countries; and among Asian countries.
Sulfur dioxide and sulfuric acid are but two of sev-
eral sulfur-containing species in the air. Table 10.1 lists
some additional species. The species are conveniently
divided into two families, the S(IV) and S(VI) fami-
lies ,inwhich the IV and the VI represent the oxidation
states (

SO 2 (g)
Sulfur
dioxide
gas

SO 2 (aq)
Dissolved
sulfur
dioxide

(10.10)

because SO 2 (aq) can evaporate as well. The formation
and dissociation of sulfurous acid [H 2 SO 3 (aq)] (Step 2)
occurs by

6, respectively) of the members of
the respective families. Thus, S(VI) members are more
oxidized than are S(IV) members. Because sulfur diox-
ide is in the S(IV) family and sulfuric acid is in the
S(VI) family, the oxidation of gas-phase sulfur dioxide
to aqueous-phase sulfuric acid represents a conversion
from the S(IV) family to the S(VI) family. This conver-
sion occurs along two pathways, described next.

+ 4 and +

H +
Hydrogen
ion

HSO 3
Bisulfite
ion

+ +

SO 2 (aq)
Dissolved
sulfur
dioxide

H 2 O(aq)
Liquid
water

H 2 SO 3 (aq)
Sulfurous
acid

2H +
Hydrogen
ion

2 −

+

SO (10.11)

3
Sulfite
ion

Step 3 involves the irreversible conversion of the S(IV)
family (primarily HSO 3 − and SO 3 2 − )totheS(VI) fam-
ily (primarily SO 4 2 − ). At pH levels of 6 or less, the most
important reaction converting S(IV) to S(VI) is
HSO 3 −
Bisulfite
ion

10.3.1. Gas-Phase Oxidation of S(IV)
The first conversion mechanism of S(IV) to S(VI) in-
volves (1) gas-phase oxidation of SO 2 (g) to H 2 SO 4 (g),
(2) condensation of H 2 SO 4 (g) and water vapor onto
aerosol particles or cloud drops to produce an
H 2 SO 4 (aq)-H 2 O(aq) solution, and (3) dissociation of
H 2 SO 4 (aq) to SO 4 2 − in the solution. The gas-phase
chemical conversion process (Step 1) is

H + → SO 2 −

4
Sulfate
ion

2H + + H 2 O 2 (aq)

Dissolved
hydrogen
peroxide

+ + H 2 O(aq) +

(10.12)

This reaction is written in terms of HSO 3 − and SO 4 2 −
because at pHs of 2 to 6, most S(IV) exists as HSO 3 −
and most S(VI) exists as SO 4 2 − .
At pH levels greater than 6, which occur only in cloud
drops that contain basic substances, such as ammonium
or sodium, the most important reaction converting S(IV)
to S(VI) is
SO 2 3
Sulfite
ion

+ O 2 (g)

H O 2 (g)

OH(g)

+ , M + H 2 O( g)

→ H 2 SO 4 (g)
Sulfuric
acid
(10.9)

→ HS O 3 (g)
Bisulfite

SO 2 (g)
Sulfur
dioxide

↓→ SO 3 (g)
Sulfur
trioxide

Because sulfuric acid has a low SVP (Section 5.3.2.1),
nearly all H 2 SO 4 (g) produced by Reaction 10.9 con-
denses onto aerosol particle or cloud drop surfaces
(Step 2). At typical pHs of aerosol particles and cloud
drops, nearly all condensed H 2 SO 4 (aq) dissociates to
SO 4 2 − by Reaction 10.6 (Step 3). The dissociation
releases two protons, decreasing pH and increasing
acidity.

SO 2 4
Sulfate
ion

+ O 3 (aq)

Dissolved
ozone

→ + O 2 (aq)

Dissolved
oxygen

(10.13)

This reaction is written in terms of SO 3 2 − and SO 4 2 −
because the HSO 3 − +

O 3 (aq) reaction is relatively slow,
and at pH levels greater than 6, most S(VI) exists as
SO 4 2 − .