The Greenhouse Effect and Global Warming - Air Pollution and Global Warming: History, Science, and Solutions - page 294

and cloud absorption effects become more important,
burning off clouds. In fact, at high AOD, the semidi-
rect and cloud absorption effects overpower the indi-
rect effects, decreasing COD with increasing AOD.
The shapes of the curves in Figure 12.27 are similar
to that of a boomerang, so the curves are referred to as
boomerang curves (Koren et al., 2004; Kaufman and
Koren, 2006; ten Hoeve et al., 2011).

particles (hydration; Section 5.3.2.3) increases with
increasing relative humidity, a decrease in the relative
humidity causes most aerosol particles to shrink, reduc-
ing their surface area and liquid water content. As such,
less gas, such as sulfuric acid, nitric acid, and ammonia,
condenses on or dissolves in the aerosol particles. The
resulting reduction in the size of particles increases the
penetration of solar radiation to the surface in a pos-
itive feedback referred to as the self-feedback effect
(Jacobson, 2002a).

12.4.3.7. Effect of Airborne Absorbing Particles
on Surface Albedo
Black carbon, brown carbon, and soil dust in the air
above snow, sea ice, clouds, and other reflective surfaces
not only absorb downward sunlight, but also absorb
light reflected upward from these surfaces. The warm-
ing of the air due to particle absorption in such cases
feeds back to melt snow and ice, uncovering darker
surfaces below, increasing temperatures in a positive
feedback (Jacobson, 2002a). This enhanced warming is
most efficient at high latitudes, where more snow and
sea ice exist, rather than at lower latitudes. It is also
most efficient where snow and sea ice cover are thin.

12.4.3.10. Photochemistry Effect
By intercepting UV light in the air, aerosol particles alter
photolysis rates of gases, affecting their concentrations
and the subsequent chemical production or destruction
of other gases. Because many gases absorb UV radi-
ation, changing the concentration of some gases by
changing the photolysis rates of other gases affects
temperatures. The process by which aerosol parti-
cles change photolysis coefficients, thereby affecting
temperatures, is the photochemistry effect (Jacobson,
2002a).

12.4.3.11. Particle Effect through Large-Scale
Meteorology
Aerosol particles affect local temperatures, which thus
affect local air pressures, winds, relative humidities, and
clouds. Changes in local meteorology slightly shift the
locations and magnitudes of semipermanent and ther-
mal pressure systems and jet streams. The effect of
local particles on large-scale temperatures is the parti-
cle effect through large-scale meteorology .

12.4.3.8. Snow Darkening Effect
When aerosol particles containing absorbing material
deposit onto snow and sea ice, they reduce the albedo
or reflectivity of the surface, increasing the solar heat-
ing of the surface (e.g., Hansen and Nazarenko, 2004;
Jacobson, 2004; Flanner et al., 2007). If snow or sea ice
thins or completely melts as a result, the darker surface
underneath (land or ocean) is revealed, triggering an
even greater warming of the surface in a positive feed-
back loop, referred to as the snow darkening effect .
Aerosol particles reach snow and sea ice surfaces
primarily by wet deposition (the formation of snow
and rain particles on top of aerosol particles, followed
by the deposition of snow and rain to the surface, and
the scavenging of aerosol particles by snow and rain
as they are falling; Section 5.3.3). On a global scale,
asmallfraction of particles (

12.4.3.12. Rainout Effect
When aerosol particles reduce precipitation due to the
second indirect effect, the semidirect effect, or the cloud
absorption effect, they reduce the removal of aerosol
particles by wet deposition, increasing the concentra-
tion of aerosol particles in the air. The resulting climate
effect of this process is referred to as the rainout effect
(Jacobson, 2002a).

10 percent) (Jacobson,
2010b) also fall to the surface by their own weight or
by winds driving them to the surface ( dry deposition ;
Section 5.3.3).

<

12.5. Consequences of Global Warming
Projections suggest that carbon dioxide mixing ratios
may increase from about 393 ppmv in 2011 to 730
to 1,040 ppmv in 2100. During this period, global
near-surface temperatures may increase by a mean esti-
mated range of 1.8 K to 4 K, depending on the future
emission scenario assumed (IPCC, 2007). The possible

12.4.3.9. Self-Feedback Effect
When aerosol particles are emitted, they change air tem-
perature and thus the relative humidity. For example,
warming aerosol particles decrease the relative humid-
ity. Because the uptake of liquid water by aerosol