Geoscience Reference
In-Depth Information
originates from sea spray and sea water evaporation.
Over land, it is emitted anthropogenically during coal
combustion. If HCl(g) becomes supersaturated in the
gas phase (if its partial pressure exceeds its saturation
vapor pressure), HCl(g) dissolves into water-containing
particles and dissociates by the reversible process
SO 4 2- (0.66)
Na + (0.66)
NH 4 + (0.5)
Mg 2+ (0.23)
H +
+
Cl
HCl(g)
HCl(aq)
(5.6)
Hydrochloric
Dissolved
Hydrogen
Chloride
Ca 2+ (1.47)
acid gas
hydrochloric acid
ion
ion
Dissociation of HCl(aq) is complete as long as the
pH exceeds -6, which almost always occurs. The pH
of fresh sea spray drops, which are primarily in the
coarse particle mode, ranges from
Cl - (0.12)
NO 3 - (4.06)
9; however,
the pH of such drops decreases during dehydration and
sea spray acidification. Sea spray acidification ,briefly
discussed in Section 5.2.1.1, occurs when nitric acid or
sulfuric acid enters a particle and dissociates, adding H +
to the solution and forcing Cl to reassociate with H +
and evaporate as HCl(g). A net sea spray acidification
process involving nitric acid is
+
7to
+
Figure 5.13. Example of sea spray and soil dust
acidification. The pie chart shows measured mass
concentration (
gm −3 ,inparentheses) of inorganic
ions summed over particles with diameters between
about 3.3 and 6.3
monAugust 29, 1987, from 05:00
to 08:30 PST, at Riverside, California. Chloride
associated with the sodium in sea spray and
carbonate associated with the calcium in soil dust
particles were most likely displaced to the gas phase
by the addition of nitrate and sulfate to the particles.
Data from the seventh stage of eight-stage impactor
measurements by John et al. (1990).
NO 3
Cl
HNO 3 (g)
+
HCl(g)
+
(5.7)
Nitric
Chloride
Hydrochloric
Nitrate
acid gas
ion
acid gas
ion
Sea spray acidification is most severe along coastal
regions near pollution sources and can result in a deple-
tion of chloride ions from sea spray drops. Figure 5.13
shows the effect of sea spray acidification and the mea-
sured composition of aerosol particles 3.3 to 6.3
When nitric acid dissolves in sea spray drops con-
taining chloride, it displaces the chloride to the gas
phase by sea spray acidification, as shown in Reaction
5.7. Similarly, when nitric acid dissolves in soil parti-
cle solutions containing calcium carbonate and water,
it causes calcium carbonate to dissociate and the car-
bonate to reform carbon dioxide gas, which evaporates.
The process, called soil dust acidification ,isdescribed
by
min
diameter at Riverside, California, about 60 km from the
Pacific Ocean. Sodium in the particles originated from
the ocean. In clean air over the ocean, the mass ratio of
chloride to sodium is typically 1.8:1. Figure 5.13 further
shows that, over Riverside, the ratio was about 0.18:1,
one-tenth the clean air ratio. The fact that Riverside par-
ticles contained a lot of nitrate and sulfate suggests that
acidification by these ions was responsible for the near
depletion of chloride in the particles.
Ca 2 +
2NO 3
CaCO 3 (s)
+
2HNO 3 (g)
+
Calcium
Nitric
Calcium
Nitrate
carbonate
acid gas
ion
ion
5.3.2.1.4. Nitric Acid. Gas-phase nitric acid
[HNO 3 (g)] forms from the chemical oxidation of
nitrogen dioxide (Reaction 4.5). Because emitted
aerosol particles generally do not contain nitric acid,
nitric acid usually enters aerosol particles from the gas
phase. The process is
+
CO 2 (g)
+
H 2 O(aq)
Carbon
Liquid
(5.9)
dioxide gas
water
(e.g., Dentener et al., 1996; Hayami and Carmichael,
1997; Tabazadeh et al., 1998; Jacobson, 1999c). The net
result of this process is that nitrate ions build up in soil
dust particles that contain calcite. A similar result occurs
when nitric acid gas is exposed to soil dust particles
that contain magnesite [MgCO 3 (s)]. Because nitric acid
readily enters soil dust and sea spray particles during
acidification and these particles are primarily in the
NO 3
H +
HNO 3 (g)
HNO 3 (aq)
+
(5.8)
Nitric
Dissolved
Hydrogen
Nitrate
acid gas
nitric acid
ion
ion
Nitric acid is a strong acid because it dissociates when
the pH exceeds -1.
 
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