Environmental Engineering Reference
In-Depth Information
TABLE 4.6
Properties of Atmospheric Particles and Droplets
Liquid Water
Typical
Nature of
Surface Area,
Content,
Atmospheric
S
T
(m
2
/m
3
)
L (m
2
/m
3
Droplet
Size (
μ
m)
ofair)
Lifetime,
τ
10
−
2
-10
10
−
3
10
−
11
-10
−
10
Aerosols
−
1
×
4-7 days
10
−
4
10
−
8
-5
10
−
7
Fog droplets
1-10
−
8
×
5
×
×
3h
10-10
2
10
−
1
10
−
7
-10
−
6
Cloud drops
−
2
×
7h
10
2
-10
3
−
5
×
10
−
4
10
−
7
-10
−
6
Raindrops
3-15 min
10
3
-10
5
Snowflakes
−
0.3
15-50 min
Source:
From Seinfeld, J.H. 1986.
AtmosphericChemistryandPhysicsofAirPollution
. NewYork: Wiley;
Gill P.S., Graedel, T.E., and Wechsler, C.J. 1983.
Reviews of Geophysics and Space Physics
21, 903-920; and Graedel, T.E. and Crutzen, P.C. 1993.
Atmospheric Change: An Earth System
Perspective
. NewYork: W.H. Freeman Co.
Table 4.6 lists the typical properties of atmospheric droplets and particles. Typical
aerosol and fog droplets are smaller than 10
m in diameter. Cloud drops, raindrops,
and snowflakes have larger diameters. The surface areas of most particles are in
the range of 10
−
4
-10
−
1
m
2
/m
3
, except for snowflakes that have large surface areas.
Small particles (e.g., aerosols and fog) have relatively large atmospheric lifetimes
compared with large particles. Large particles settle out faster through sedimentation
and gravity settling. Small particles (colloids) are kept in suspension by frequent
collisions resulting from their Brownian motion. Large particles (raindrops, cloud
drops, and snowflakes) have high liquid water content than aerosols and fog. The
water content of aerosols depends largely on the relative humidity of air.
Many organic compounds (volatile or semi-volatile) as well as inorganic com-
poundsaretransportedoverlongdistancesandappearevenintheremotearcticregions
via dispersion through the troposphere. Compounds volatilize from their sources in
the temperate and tropical regions and are transported through the atmosphere to the
oceans and polar caps. Pollutants exist both in the gaseous form (
μ
≡
G) and bound to
particulates (
P). A large fraction of the material is transported in the gaseous form,
which exchange directly with the earth and oceans via
dry deposition
; the flux being
J
Dry(G)
. Gaseous materials are also solubilized and absorbed by hydrometeors and
scavenged to the earth. This is called
wet deposition
. Materials bound to aerosols in
the atmosphere are also scavenged by hydrometeors, which is also called wet depo-
sition. The total flux due to
wet deposition
is
J
Wet(G+P)
. The aerosols grow in size
and settle to the earth by
dry deposition
and the flux is
J
Dry(P)
. A schematic of these
exchange processes is given in Figure 4.5. The total flux is
≡
J
Total
=
J
Wet
+
J
Dry
=
J
Wet
(
G
+
P
)
+
J
Dry
(
P
)
+
J
Dry
(
G
)
.
(4.36)
4.2.1 W
ET
D
EPOSITION OF
V
APOR
S
PECIES
The concentration of a gaseous species
i
, scavenged by a hydrometeor at equilibrium,
is given by the ratio of concentration per m
3
of water to the concentration per m
3
of
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