Geoscience Reference
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collide is low; thus, such particle collisions usually
result in coalescence (Pruppacher and Klett, 1997).
Asecond important mechanism causing coagulation
is
gravitational collection
.When two particles of dif-
ferent size fall, the larger one may catch up and col-
lide with the smaller one. The kinetic energy of the
larger particle is higher, increasing the chance that col-
lision will result in a bounce-off rather than a coales-
cence; thus, not all collisions during gravitational col-
lection result in coalescence. Gravitational collection is
an important mechanism for producing raindrops.
Va n der Waals forces
are weak dipole-dipole attrac-
tions caused by brief, local charge fluctuations in nonpo-
lar molecules having no net charge. That is, uncharged
particles experience random charge fluctuations that
cause one part of the particle to experience a brief pos-
itive charge and the other part to experience a brief
negative charge, so that the particle still exhibits no net
charge. When a particle experiencing a brief charge fluc-
tuation approaches another, the first induces a charge of
the opposite sign on the closest end of the second par-
ticle. The opposite charge between the two particles
causes an attraction, enhancing the rate of coagulation
between the particles. Van der Waals forces enhance
the rate of coagulation of small particles, particularly
of particles smaller than 50 nm in diameter.
Brownian motion affects coagulation significantly
when at least one of two colliding particles is small.
When both particles are large (but not exactly the same
size), gravitational collection is the dominant coagula-
tion process. For small nanoparticles (
10
6
100
10
5
Initial
80
10
4
60
10
3
After 1 day
10
2
40
10
1
After
1 day
20
Initial
10
0
10
-1
0
0.01
0.1
1
10
Particle diameter (D,
ยต
m)
Figure 5.10.
Modeled change in aerosol number and
volume concentrations at Claremont over a 24-h
period when coagulation alone was considered.
Number concentration is shown in red; volume
concentration, in blue. From Jacobson (1997).
80 to 90 percent of silicate particles over the equatorial
Pacific Ocean between Ecuador and Hawaii contained
sea spray constituents. Murphy et al. (1998) discovered
that almost all aerosol particles larger than 0.13
m
in the boundary layer in a remote South Pacific site
contained sea spray components. Posfai et al. (1999)
found that almost all soot particles in the North Atlantic
contained sulfate. The internal mixing of aerosols by
coagulation is supported by model simulations that
show that on a global scale, about half the increase
in size of soot particles following their emissions may
be due to coagulation with nonsoot particles, such as
sulfate, organic matter, sea spray, and soil, whereas the
rest may be due to growth processes (Jacobson, 2001b).
Thus, although coagulation does not affect the number
concentration of large particles very much, it does affect
the composition and mixing state of particles of all sizes
(Jacobson, 2002b).
15 nm in diam-
eter), van der Waals forces can increase the rate of
coagulation compared with Brownian motion alone by
afactor of five or more.
Outside of clouds, small aerosol particles are affected
more by coagulation than are large aerosol particles
because noncloudy air contains many more small par-
ticles than large particles, and coagulation rates depend
alot on particle number. In urban regions, coagulation
affects the number concentration of aerosol particles
primarily smaller than 0.2
<
5.3.2. Growth Processes
Coagulation is a process that involves only particles,
whereas condensation/evaporation; vapor deposition/
sublimation; dissolution, dissociation, and hydration;
and gas-aerosol chemical reaction are gas-to-particle
conversion processes. These processes are discussed in
the following subsections.
m(200 nm) in size over the
course of a day. Figure 5.10 shows results from a model
calculation of the change in the number and volume
concentration of particles in polluted urban air over a
24-hour period. Whereas the number concentration of
small particles was affected, changes in the volume con-
centration size distribution were affected less.
Over the ocean, coagulation is an important mecha-
nism by which sea spray drops become internally mixed
with other aerosol constituents, such as soil dust par-
ticles. Andreae et al. (1986), for example, found that
5.3.2.1. Condensation/Evaporation
Condensation and evaporation occur only after homo-
geneous or heterogeneous nucleation. On a nucleated
liquid surface, gas molecules continuously
condense
(change state from gas to liquid) and liquid molecules
continuously
evaporate
(change
state
from
liquid
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