The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 292

and fractional cloudiness of such a cloud ( second indi-
rect effect ), further cooling the surface during the day
(Albrecht, 1989; Gunn and Phillips, 1957). Thus, the
indirect effects of anthropogenic aerosol particles are
effects of their emission on clouds' reflectivity, lifetime,
and precipitation, and, consequently, surface tempera-
ture.
Different particle components activate cloud drops
to different degrees. For example, newly emitted diesel
soot particles, which contain black carbon coated by
lubricating oil, unburned fuel oil, and some sulfate
and metals, do not serve as good CCN because they
are largely hydrophobic when emitted. As they age,
though, soot particles gradually become coated with
sulfuric acid or another hygroscopic material, increas-
ing their ability to serve as a CCN. Sodium chloride par-
ticles and sulfuric acid-ammonia-water particles, how-
ever, are good CCN upon emissions or formation.

data indicate that thick clouds often have a low albedo
(Danielson et al., 1969). It has also been used to suggest
through scaling arguments that the upper limit of the
annual, global average of BC heating due to cloud
absorption may be 1 to 3 W m − 2 , depending on the
position of the BC in cloud drops (Chylek et al., 1996).
Global-scale calculations accounting for absorbing
inclusions have suggested that it is a strong contributor
to the global warming caused by BC (Jacobson, 2006,
2010b).
Figure 12.26 compares heating rates due to absorp-
tion by BC inclusions within cloud drops with heating
rates due to absorption by the same total volume of BC
intestinally between cloud drops and with heating rates
due to BC in the clear sky (outside a cloud). It indicates
that heating of the air by BC that is interstitial between
cloud drops exceeds that due to BC in clear sky. Clear
sky BC receives light primarily from direct rays of the
sun. Interstitial BC, however, receives light scattered
from many solar rays that hit cloud drops and are scat-
tered within the cloud many times, eventually hitting
the BC.
Figure 12.26 also shows that cloud heating by BC
inclusions in cloud drops exceeds heating by BC inter-
stitially between drops. The reason is that BC inclu-
sions receive not only light scattered by cloud drops,
but also light that refracts into the drop in which they
reside. Light refracted into a drop internally reflects
multiple times, increasing the chance that it will hit and
be absorbed by BC in the drop.
Particle inclusions can be represented in different
ways within a cloud drop. Figure 12.26b illustrates
results from two representations. One is the assumption
that BC exists as several randomly distributed inclu-
sions within each drop. This representation is known
as the dynamic effective medium approximation
(DEMA). The second is the assumption that BC exists
as a single core surrounded by a shell of water. This
representation is referred to as the core shell approxi-
mation (CSA) The DEMA is arguably a more physical
representation than is the CSA for treating inclusions in
clouds drops because BC can enter drops by both nucle-
ation scavenging and aerosol-hydrometeor coagulation;
thus, multiple BC inclusions usually exist in one drop.
The CSA assumes that only one BC inclusion exists
in each drop. Figure 12.26b indicates that the multiple
inclusion treatment of BC heats the cloud more than
does the single-core treatment.
The effect of absorbing aerosol inclusions within and
between cloud drops is to darken and thus heat a cloud,
causing it to burn off more rapidly. The darkening of

12.4.3.4. The Semidirect Effect
Absorption of solar radiation by large liquid water or ice
particles in a cloud increases the stability of air below
the cloud, reducing the vertical mixing of moisture from
the surface to the cloud and the relative humidity in the
cloud, thinning the cloud (Nicholls, 1984). In fact, any
decreases in the relative humidity near a cloud corre-
lates with a decrease in low cloud cover (Bretherton
et al., 1995; Klein, 1997). Similarly, absorbing aerosol
particles below, within, or above a cloud warm the air
relative to the surface, decreasing the near-cloud rela-
tive humidity and increasing atmospheric stability, both
of which reduce cloud cover (e.g., Hansen et al., 1997;
Ackerman et al., 2000). Reduced cloud cover increases
sunlight reaching the Earth's surface, warming the sur-
face in a positive feedback process called the semidirect
effect (Hansen et al., 1997).

12.4.3.5. Cloud Absorption Effects
Aerosol particles enter cloud drops in two major
ways. Cloud drops either form on top of aerosol
particles ( nucleation scavenging )orcoagulate with
aerosol particles interstitially between them ( aerosol-
hydrometeor coagulation ). Thus, absorbing aerosol
particles within clouds can be present either as inclu-
sions within cloud drops or interstitially between cloud
drops.
Effects from cloud heating due to the direct ab-
sorption of solar radiation by aerosol inclusions and
interstitial aerosol particles within a cloud are cloud
absorption effects (Jacobson, 2012). Absorption by
drop inclusions has been used to explain in part why