important size range for aerosol optical properties.
Efficient aerosol scattering types are sulfates and
sea-salt, while efficient absorption types are black
carbon and dust constituents such as hematite.
One of the more important parameters associated with the direct radiative effect is aerosol
optical depth (AOD) which describes the extinction of radiation as it propagates through the
atmosphere:
I=I 0 ¼ e
Àt
and
t ¼ s ext H,
where s ext is the extinction coefficient of radiation
propagating through an aerosol layer of height H.
In terms of radiative forcing, an aerosol layer
can either cause a negative or positive sign change
to planetary albedo, leading to a cooling or
warming effect. o is the key parameter, along
with the albedo of the underlying surface, that
determines whether an aerosol layer leads to
heating or cooling. Over dark surfaces like the
ocean, and regardless of the degree of absorption,
albedo will be increased primarily due to the
upscatter of incoming radiation. In contrast, over
bright surfaces such as snow and desert surfaces,
an absorbing component will reduce the amount
of surface-reflected radiation radiated back out to
space. An additional important factor related to
absorbing aerosol layers is that they warm as they
absorb radiation, leading to increased atmospheric
stability and reduced convection. This effect, it
will be seen later, can have important implications
for cloud formation.
The global distribution of AOD for total aerosol types and individually for black carbon, particulate organic carbon, sulfate, dust, and sea-salt
is shown in Fig. 14. The global average AOD,
derived from the medium of the 10 AEROCOM
models, is0.12 (compared to 0.137 derived from
satellites and sunphotometers) [57]. Peak AOD
values of >0.6 are encountered over Northwest
Africa and the Asian continent, and to a lesser
degree, over Europe, the USA, and South America. The lowest AOD values of ~0.01–0.02 occur
over Antarctica. Black carbon is seen to contribute
0.0036 to the global AOD, with hot spots over
Europe and China where black carbon contributes
0.05 to AOD. Dust contributes 0.025 to global
AOD with peak contributions of 0.3 arising over
Northwest Africa, and 0.2 of Asia. Particulate
Organic Matter contributes 0.017 to global
AOD, with hot spots in midwest Africa (from
biomass burning), South America, some regions
in Europe and China contributing AOD greater
than 2. Sulfate contributes 0.032 to the global
AOD, with Europe, east Asia, and east USA contributing to hot spots with AOD about 0.3. Seasalt contributes 0.033 to the global AOD, with
values up to 0.2 seen in some oceanic regions
were wind speeds are generally high (i.e.,
midlatitudes).
The AEROCOM models predict [76] that, for
anthropogenic aerosols, a harmonized estimate
for the aerosol direct radiative forcing under allsky conditions, on a global annual basis, is
À0.22 Wm
À2 , ranging from +0.04 to
À0.41 Wm
À2 , with a standard deviation of
Æ0.16 Wm
À2 . Anthropogenic nitrate, secondary
organic aerosol, and dust were not, however,
included in the estimate. By comparison, the
IPCC AR4 [100] reported a direct aerosol radiative effect best estimate of À0.5 W m
À2 with an
uncertainty of À0.9 to À0.1 W m
À2
. The global
annual mean aerosol surface radiative forcing is
illustrated in Fig. 15 where it can be seen in the
regions with highest aerosol loadings, negative
forcings as high as À10 W m
À2 are encountered.
More recent estimates, using a combination of
satellite data with a global model [77] suggested
a value of À0.65 W m
À2 . The diversity in model
estimates stems more from different model
approaches to transport processes, removal mechanisms, and microphysics treatment rather than
aerosol emission inventories.
Indirect Effect
There are a number of indirect aerosol radiative
effects whereby aerosol fields modify the cloud
microphysics, structure, or lifetime, all of which
have albedo impacts. The first indirect effect
(typically associated with “warm” or water clouds
of fixed liquid water content), results from an
264
Aerosol in Global Atmosphere
Efficient aerosol scattering types are sulfates and
sea-salt, while efficient absorption types are black
carbon and dust constituents such as hematite.
One of the more important parameters associated with the direct radiative effect is aerosol
optical depth (AOD) which describes the extinction of radiation as it propagates through the
atmosphere:
I=I 0 ¼ e
Àt
and
t ¼ s ext H,
where s ext is the extinction coefficient of radiation
propagating through an aerosol layer of height H.
In terms of radiative forcing, an aerosol layer
can either cause a negative or positive sign change
to planetary albedo, leading to a cooling or
warming effect. o is the key parameter, along
with the albedo of the underlying surface, that
determines whether an aerosol layer leads to
heating or cooling. Over dark surfaces like the
ocean, and regardless of the degree of absorption,
albedo will be increased primarily due to the
upscatter of incoming radiation. In contrast, over
bright surfaces such as snow and desert surfaces,
an absorbing component will reduce the amount
of surface-reflected radiation radiated back out to
space. An additional important factor related to
absorbing aerosol layers is that they warm as they
absorb radiation, leading to increased atmospheric
stability and reduced convection. This effect, it
will be seen later, can have important implications
for cloud formation.
The global distribution of AOD for total aerosol types and individually for black carbon, particulate organic carbon, sulfate, dust, and sea-salt
is shown in Fig. 14. The global average AOD,
derived from the medium of the 10 AEROCOM
models, is0.12 (compared to 0.137 derived from
satellites and sunphotometers) [57]. Peak AOD
values of >0.6 are encountered over Northwest
Africa and the Asian continent, and to a lesser
degree, over Europe, the USA, and South America. The lowest AOD values of ~0.01–0.02 occur
over Antarctica. Black carbon is seen to contribute
0.0036 to the global AOD, with hot spots over
Europe and China where black carbon contributes
0.05 to AOD. Dust contributes 0.025 to global
AOD with peak contributions of 0.3 arising over
Northwest Africa, and 0.2 of Asia. Particulate
Organic Matter contributes 0.017 to global
AOD, with hot spots in midwest Africa (from
biomass burning), South America, some regions
in Europe and China contributing AOD greater
than 2. Sulfate contributes 0.032 to the global
AOD, with Europe, east Asia, and east USA contributing to hot spots with AOD about 0.3. Seasalt contributes 0.033 to the global AOD, with
values up to 0.2 seen in some oceanic regions
were wind speeds are generally high (i.e.,
midlatitudes).
The AEROCOM models predict [76] that, for
anthropogenic aerosols, a harmonized estimate
for the aerosol direct radiative forcing under allsky conditions, on a global annual basis, is
À0.22 Wm
À2 , ranging from +0.04 to
À0.41 Wm
À2 , with a standard deviation of
Æ0.16 Wm
À2 . Anthropogenic nitrate, secondary
organic aerosol, and dust were not, however,
included in the estimate. By comparison, the
IPCC AR4 [100] reported a direct aerosol radiative effect best estimate of À0.5 W m
À2 with an
uncertainty of À0.9 to À0.1 W m
À2
. The global
annual mean aerosol surface radiative forcing is
illustrated in Fig. 15 where it can be seen in the
regions with highest aerosol loadings, negative
forcings as high as À10 W m
À2 are encountered.
More recent estimates, using a combination of
satellite data with a global model [77] suggested
a value of À0.65 W m
À2 . The diversity in model
estimates stems more from different model
approaches to transport processes, removal mechanisms, and microphysics treatment rather than
aerosol emission inventories.
Indirect Effect
There are a number of indirect aerosol radiative
effects whereby aerosol fields modify the cloud
microphysics, structure, or lifetime, all of which
have albedo impacts. The first indirect effect
(typically associated with “warm” or water clouds
of fixed liquid water content), results from an
264
Aerosol in Global Atmosphere