Geoscience Reference
In-Depth Information
Newly nucleated aerosol particles usually contain
sulfate ions and water, although they may also con-
tain ammonium ions.
Biomass, biofuel, and fossil fuel combustion pro-
duce primarily small accumulation mode particles,
but coagulation and gas-to-particle conversion move
these particles to the middle and high accumulation
modes. Coagulation also moves some particles to the
coarse mode.
Metals that evaporate during industrial emissions
recondense, primarily onto accumulation mode soot
particles and coarse mode fly ash particles. The metal
emitted in greatest abundance is usually iron.
Sea spray and soil particles are primarily in the coarse
mode.
When sulfuric acid condenses, it usually condenses
onto accumulation mode particles because these
aerosol particles have more surface area, when aver-
aged over all particles in the mode, than do nucleation
or coarse mode particles.
Once in accumulation mode particles, sulfuric acid
dissociates primarily to sulfate ions [SO
4
2
−
]. To
maintain charge balance, ammonia gas [NH
3
(g)] dis-
solves and dissociates in such particles, producing
ammonium ions [NH
4
+
], the major cation in accumu-
lation mode particles. Thus, ammonium and sulfate
ions often coexist in accumulation mode particles.
Because sulfuric acid has a lower SVP and a greater
solubility than does nitric acid, nitric acid is inhibited
from entering those accumulation mode particles that
already contain sulfuric acid.
Nitric acid tends to dissolve in coarse mode particles
that contain cations (and have a high pH). It displaces
the chloride ion in sea spray drops to hydrochloric
acid gas and the carbonate ion in soil dust particles
to carbon dioxide gas during sea spray acidification
and soil dust acidification, respectively. Sulfuric acid
also displaces chloride ions and carbonate ions during
acidification.
Table 5.6 summarizes the predominant components
and their sources in the nucleation, accumulation, and
coarse particle modes.
a liquid cloud drop or ice crystal and the drop or crystal
coagulates with other cloud drops or crystals to become
rain or graupel, which falls to the surface, removing
the aerosol particle inclusion.
Washout
occurs when
growing or falling precipitation particles coagulate with
aerosol particles that are either interstitially between
cloud drops or in the clear sky below clouds that the
rain passes through. When the precipitation falls to the
surface, it brings the aerosol particles with it. Together,
rainout and washout are the most important mecha-
nisms removing aerosol particles globally (Jacobson,
2010b). Because rain clouds occur only in the tropo-
sphere, rainout is not a process by which stratospheric
particles are removed. However, rainout is an effective
removal process for volcanic particles.
Sedimentation
is the sinking of particles to lower
altitudes by their own weight against the force of drag
imposed by the air.
Dry deposition
is a process by
which gases and aerosol particles are carried by molec-
ular diffusion; turbulent diffusion; or winds to the sur-
face of trees, grass, rocks, the ocean, buildings, or roads,
and then rest on, bond to, or react with the surface.
Sedimentation and dry deposition are important
removal processes for very large particles over short
periods and small particles over long periods, but not
so important relative to rainout or washout for small
particles over short periods. Table 5.4 indicates that
particles less than 0.5
mindiameter stay in the air sev-
eral years before sedimenting even 1 km by their own
weight. For these and smaller particles, sedimentation is
only a long-term removal process. Gases also sediment,
but their weights are so small that their sedimentation
velocities are negligible. A typical gas molecule has a
diameter of 0.5 to 1 nm. Such diameters result in gases
falling only 1 to 3 km per 10,000 years.
If small particles are near the ground, dry deposition
can usually remove them more efficiently than can sed-
imentation. Dry deposition is more efficient for remov-
ing particles than gases because particles are heavier
than are gases. As such, particles fall and tend to stay on
asurface more readily than do gases, unless wind speeds
are high. Gases, especially if they are chemically unre-
active, are more likely to be resuspended into the air.
5.5. Aerosol Particle Morphology
and Shape
The morphologies (structures) and shapes of aerosol
particles vary with composition. The older an aerosol
particle, the greater the number of layers and attach-
ments the particle is likely to have. If the aerosol particle
5.4. Summary of the Composition
of Aerosol Particles
The composition of aerosol particles varies with particle
size and location. Some generalities about composition
are summarized as follows:
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