The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 291

advected long distances. Warmi ng particles, which
absorb sunlight, thus heat the air in a manner differ-
ent from greenhouse gases, which absorb the Earth's
thermal-IR radiation and reemit it to the air around
them .
Because warming particles heat the air relative to the
ground, they increase the air's stability, reducing the
vertical mixing of pollutants, energy, and other atmo-
spheric properties (Section 6.6.1.2). Because cooling
particles reflect downward solar radiation to space, they
prevent the radiation from reaching the surface, cool-
ing the surface relative to the air and increasing the
air's stability (Bergstrom and Viskanta, 1973; Acker-
man, 1977; Venkatram and Viskanta, 1977). In sum, all
aerosol particles increase the stability of the air .
An increase in the stability of the air slows near-
surface winds and increases wind speeds above the sur-
face. The reason is that wind speeds generally increase
with increasing height, starting with zero wind speed
at the surface. The more stable the air, the less that
air rises and sinks buoyantly. The rising and sinking
of air is one form of turbulence .Thelessthe verti-
cal turbulence, the less that fast winds from above the
surface mix toward the surface, slowing near-surface
winds and increasing winds above the surface. As such,
all aerosol particles reduce surface near-wind speeds
(Jacobson and Kaufman, 2006).
The reduction in near-surface wind speed due to
aerosol particles reduces the wind speed-dependent
emission rates of sea spray, soil and road dust, pollens,
spores, and gas-phase aerosol precursors. The reduc-
tion in the concentration of these particles reduces day-
time solar reflectivity and day- and nighttime thermal-
IR heating near the surface. As such, aerosol particles
affect temperatures not only directly, but also by chang-
ing the emission rates, and thus concentrations, of other
particles and gases. This feedback is referred to as the
daytime stability effect (Jacobson, 2002a).

emission rates of other aerosol particles and gases in a
manner similar to the daytime stability effect. The effect
of thermal-IR absorption by aerosol particles on surface
heating and emissions of other particles and gases, and
the resulting feedback to climate, is the smudge pot
effect (Jacobson, 2002a). Unlike the daytime stability
effect, the smudge pot effect operates during the day and
night but is relatively more important at night because
it does not compete with the daytime stability effect.

12.4.3.3. Indirect Effects
All cloud drops form on top of aerosol particles. Aerosol
particles that can potentially form cloud drops are cloud
condensation nuclei (CCN). When pollution particles
are emitted, many serve as CCN along with natural
aerosol particles. For the same total liquid water content
of a cloud, the addition of more CCN produces more
small cloud drops and less large cloud drops. A larger
number of small drops has a greater cross-sectional
area, summed among all drops, than does a smaller
number of large drops. The greater cross-sectional area
of small drops in comparison with large drops means
that adding CCN to the air increases the reflectivity of
sunlight by the cloud ( first indirect effect ), cooling the
ground during the day (Twomey, 1977).

Example 12.3
For a cloud liquid water content of 1 g m −3 ,cal-
culate the aggregate cross sectional area among
cloud drops if the water were spread over equally
sized drops with a concentration of (a) n = 1,000

drops cm −3 and (b) n 500 drops cm −3 . =

Solution
Dividing the cloud liquid water content by the
number of drops in each case gives the vol-
ume of each drop as 10 −9 10 −9

cm 3 ,respectively. The radii of spherical drops of
these volumes are r

cm 3 and 2 ×

m,
respectively. The summed cross-sectional area
among all drops in each case is, therefore, n

= 6.20 and r = 7.82

r 2 =

12.4.3.2. The Smudge Pot Effect
During the day and night, all aerosol particles trap
the Earth's thermal-IR radiation, warming the surface
air (Bergstrom and Viskanta, 1973; Zdunkowski et al.,
1976). This warming is well known to citrus growers,
who, at night, used to burn crude oil in smudge pots
to fill the air with smoke and trap thermal-IR radiation,
preventing their citrus crops from freezing. The warm-
ing of the air relative to a surface below increases the
stability of air, slowing near-surface winds and increas-
ing them aloft, reducing the wind speed-dependent

121,000 m 2 cm −3 ,respec-

tively. Thus, for the same mass of liquid water,
agreaternumber of small drops has a larger
summed cross-sectional area than does a lesser
number of larger drops.

m 2 cm −3 and 96,000

An increase in the number of small drops and a
decrease in the number of large drops due to the addition
of pollution particles also reduces the rate of drizzle in
alow cloud, thereby increasing the liquid water content