6.7.2 Sulphate Aerosol
Early one-dimensional box-model estimates of the radiative forcing (e.g., Charlson
et al., 1992) using simplified expressions for radiative forcing have been superseded
by global calculations using prescribed aerosol concentrations from chemical
transport models (CTMs). These studies use either three-dimensional observed
fields of for example, clouds, relative humidity and surface reflectance (e.g.,
Kiehl and Briegleb, 1993; Myhre et al., 1998c), or GCM generated fields (e.g.,
Boucher and Anderson, 1995; Haywood et al., 1997a) together with the prescribed
aerosol distributions from CTMs and detailed radiative transfer codes in calculating
the radiative forcing. A growing number of studies perform both the chemical
production, transformation, and transportation of aerosols and the radiative
forcing calculations (see Chapter 5) with the advantage
of correlating predicted aerosol distributions precisely with fields determining
aerosol production and deposition such as clouds (e.g., Penner et al., 1998b).
Table 6.4 summarises estimates of the radiative forcing
from global modelling studies.
| Table 6.4: The global and annual mean direct radiative
forcing (DRF) for the period from pre-industrial (1750) to present day (2000)
due to sulphate aerosols from different global studies. The anthropogenic
column burden of sulphate and the source of the sulphate data are also shown
together with the normalised radiative forcing. “Frac” indicates
a cloud scheme with fractional grid box cloud amount, and “On/off”
indicates that a grid box becomes overcast once a certain relative humidity
threshold is reached. An asterisk indicates that the maximum hygroscopic
growth of the aerosols was restricted to a relative humidity of 90%. The
ratio of the Northern to the Southern Hemisphere (NH/SH) DRF and the ratio
of the mean radiative forcing over land to the mean radiative forcing over
oceans is also shown. NA indicates data is not available. “Langner
and Rodhe (1991)s” indicates that the slow oxidation case was used
in the calculations. |
 |
|
Study
|
DRF (Wm-2)
|
Column burden (mgm-2)
|
Normalised DRF (Wg-1)
|
Cloud scheme
|
DRF NH/SH
|
DRF land/ocean
|
Source of sulphate data
|
|
| Ghan et al. (2001a) |
-0.44
|
4.0
|
-110
|
On/off
|
5.2
|
1.9
|
Ghan et al. (2001a) |
| Jacobson (2001) |
-0.32
|
2.55
|
-125
|
Frac
|
2.7
|
1.2
|
Jacobson (2001) |
| Boucher and Anderson (1995) |
-0.29
|
2.32
|
-125
|
Frac
|
4.3
|
3.4
|
Langner and Rodhe (1991) |
| Graf et al. (1997) |
-0.26
|
1.70
|
-153
|
Frac
|
2.0
|
NA
|
Graf et al. (1997) |
| Feichter et al. (1997) |
-0.35
|
2.23
|
-157
|
Frac
|
4.2
|
1.4
|
Feichter et al. (1997) |
| Kiehl and Briegleb (1993) |
-0.28
|
1.76
|
-159
|
Frac
|
3.3
|
NA
|
Langner and Rodhe (1991)s |
| Iversen et al. (2000) |
-0.41
|
2.40
|
-167
|
Frac
|
4.1
|
NA
|
Iversen et al. (2000) |
| Myhre et al. (1998c) |
-0.32
|
1.90
|
-169
|
Frac
|
6.9
|
NA
|
Restad et al. (1998) |
| van Dorland et al. (1997) |
-0.36
|
2.11
|
-171
|
Frac
|
5.0
|
NA
|
van Dorland et al. (1997) |
| Koch et al. (1999) |
-0.68
|
3.3
|
-200
|
Frac
|
NA
|
NA
|
Koch et al. (1999) |
| Kiehl and Rodhe (1995) |
-0.66
|
3.23
|
-204
|
Frac
|
NA
|
NA
|
Pham et al. (1995) |
| |
-0.29
|
1.76
|
-165
|
Frac
|
NA
|
NA
|
Langner and Rodhe (1991)s |
| Chuang et al. (1997) |
-0.43
|
2.10
|
-205
|
On/off*
|
4.7
|
2.4
|
Chuang et al. (1997) |
| Haywood et al. (1997a) |
-0.38
|
1.76
|
-215
|
Frac
|
4.0
|
NA
|
Langner and Rodhe (1991)s |
| Hansen et al. (1998) |
-0.28
|
1.14
|
-246
|
Frac
|
NA
|
NA
|
Chin and Jacob (1996) |
| Kiehl et al. (2000) |
-0.56
|
2.23
|
-251
|
Frac
|
2.7
|
1.3
|
Kiehl et al. (2000) |
| Haywood and Ramaswamy (1998) |
-0.63
|
1.76
|
-358
|
On/off
|
3.6
|
2.6
|
Langner and Rodhe (1991)s |
| |
-0.82
|
1.76
|
-460
|
On/off
|
5.8
|
2.7
|
Kasibhatla et al. (1997) |
| Penner et al. (1998b) and Grant et al. (1999) |
-0.81
|
1.82
|
-445
|
On/off
|
4.5
|
2.3
|
Penner et al. (1998b) |
|
The calculated global mean radiative forcing ranges from -0.26 to -0.82 Wm-2,
although most lie in the range -0.26 to -0.4 Wm-2. The spatial distribution
of the forcings is similar in the studies showing strongest radiative forcings
over industrial regions of the Northern Hemisphere although the ratio of the
annual mean Northern Hemisphere/Southern Hemisphere radiative forcing varies
from 2.0 (Graf et al., 1997) to 6.9 (Myhre et al., 1998c) (see Section
6.14.2 for further details). The ratio of the annual mean radiative forcing
over land to that over ocean also varies considerably, ranging from 1.3 (Kiehl
et al., 2000) to 3.4 (Boucher and Anderson, 1995). The direct radiative forcing
(DRF) is strongest in the Northern Hemisphere summer when the insolation is
the highest although different seasonal cycles of the sulphate burden from the
chemical transport models result in maximum global mean radiative forcings ranging
from May to August (e.g., Haywood and Ramaswamy, 1998), the ratio of the June-July-August/December-January-February
radiative forcing being estimated to lie in the range less than 2 (e.g., van
Dorland et al., 1997) to > 5 (e.g., Penner et al., 1998b; Grant et al., 1999)
with a mean of approximately 3.3. The range of uncertainty in the radiative
forcings can be isolated from the uncertainties in the simulated sulphate loadings
by considering the range in the normalised radiative forcing i.e., the radiative
forcing per unit mass of sulphate aerosol (e.g., Nemesure et al., 1995; Pilinis
et al., 1995). Table 6.4 shows that this is substantial,
indicating that differences in the radiative forcing are not due solely to different
mass loading.
Figure 6.3: The normalised radiative forcing for sulphate aerosol
from the intercomparison study of Boucher et al. (1998). A log-normal distribution
with a geometric mean diameter of 0.170 µm and a geometric standard
deviation of 1.105 together with an aerosol optical depth of 0.20 at 0.55
µm and a Lambertian surface reflectance of 0.15 was assumed. For details
of the acronyms and the radiation codes used in the calculations see Boucher
et al. (1998). |
The optical parameters for sulphate aerosol in each of
the global studies vary. Although the single scattering albedo of pure sulphate
and sulphate mixed with water is close to unity throughout most of the solar
spectrum, some of the studies (e.g., Feichter et al., 1997; van Dorland et al.,
1997; Hansen et al., 1998) include some absorption. Charlson et al. (1999) show
considerable variation in the specific extinction coefficient used in different
studies, particularly when accounting for relative humidity effects. The treatment
of the effects of relative humidity and clouds appear to be particularly important
in determining the radiative forcing. The studies of Haywood and Ramaswamy (1998),
Penner et al. (1998b) and Grant et al. (1999) produce normalised radiative forcings
a factor of two to three higher than the other studies. Both Haywood and Ramaswamy
(1998) and Penner et al. (1998b) acknowledge that their use of on/off cloud
schemes where cloud fills an entire grid box once a threshold relative humidity
is exceeded may lead to strong radiative forcings due to strong non-linear relative
humidity effects. Chuang et al. (1997) use an on/off cloud scheme and report
a radiative forcing lower than these two studies, but the hygroscopic growth
is rather suppressed above a relative humidity of 90%. The use of monthly mean
relative humidity fields in some of the calculations leads to lower radiative
forcings as temporal variations in relative humidity and associated non-linear
effects are not accounted for (e.g., Kiehl and Briegleb, 1993; Myhre et al.,
1998c). Kiehl et al. (2000) improve the treatment of relative humidity compared
to Kiehl and Briegleb (1993) and Kiehl and Rodhe (1995) by improving the relative
humidity dependence of the aerosol optical properties and by using interactive
GCM relative humidities rather than monthly mean ECMWF analyses, resulting in
a larger normalised radiative forcing. Ghan et al. (2001a) perform a sensitivity
study and find that application of GCM relative humidities changes the direct
radiative forcing by -0.2 Wm-2 compared to the use of ECMWF analyses
because the frequency of occurrence of relative humidities over 90% is higher
when using GCM relative humidities. Haywood and Ramaswamy’s (1998) GCM
study indicates a stronger radiative forcing when sulphate resides near the
surface because the relative humidity and subsequent hygroscopic growth of sulphate
particles is higher. GCM sensitivity studies (Boucher and Anderson, 1995) and
column calculations (Nemesure et al., 1995) show that the radiative forcing
is a strong function of relative humidity but relatively insensitive to chemical
composition. Ghan et al. (2001a) suggest a number of reasons for the relatively
low DRF from their study, including the relatively large fraction of anthropogenic
sulphate in winter when the insolation is lowest, and the fact that sulphate
aerosols are smaller in this study and therefore have smaller scattering efficiencies
(see also Boucher and Anderson, 1995). The contribution to the global forcing
from cloudy regions is predicted to be 4% (Haywood et al., 1997a), 11% (Haywood
and Ramaswamy, 1998), 22% (Boucher and Anderson, 1995) and 27% (Myhre et al.,
1998c) and hence remains uncertain. However, it should be noted that all of
these studies suggest the majority of the global annual mean direct radiative
forcing due to sulphate occurs in cloud-free regions. The global mean long-wave
radiative forcing has been estimated to be less than 0.01 Wm-2 and
is insignificant (Haywood et al., 1997a).
Boucher and Anderson (1995) investigate the effects of different size distributions,
finding a 20 to 30% variation in the radiative forcing for reasonable size distributions.
Nemesure et al. (1995) and Boucher et al. (1998) find a much larger sensitivity
to the assumed size distribution as sulphate is modelled by much narrower size
distributions. Column radiative forcing calculations by fifteen radiative transfer
codes of varying complexity (Boucher et al., 1998) show that, for well constrained
input data, differences in the computed radiative forcing when clouds are excluded
are relatively modest at approximately 20% (see Figure 6.3).
This indicates that uncertainties in the input parameters and in the implementation
of the radiative transfer codes and the inclusion of clouds lead to the large
spread in estimates as suggested by Penner et al. (1994).
Additional column calculations show a weakened radiative forcing when the cloud
optical depth is much greater than the aerosol optical depth (Haywood and Shine,
1997; Liao and Seinfeld, 1998) and show that the forcing is insensitive to the
relative vertical position of the cloud and aerosol. Haywood et al. (1997b,
1998b) and Ghan and Easter (1998) used a cloud resolving model to investigate
effects of sub-grid scale variations in relative humidity and cloud. For their
case study, the optical depth and radiative forcing in a GCM sized grid box
were underestimated by 60%. Effects of sub-grid scale variations in relative
humidity and cloud on a global scale have not been rigorously investigated.
Until differences in estimates of radiative forcing due to sulphate aerosol
can be reconciled, a radiative forcing of -0.4 Wm-2 with a range
of -0.2 to -0.8 Wm-2 is retained. This estimate is based on the range
of radiative forcings provided by the model estimates shown in Table
6.4. It is not possible to perform standard statistical procedures because
of the limited number of studies and the fact that the resulting DRFs are not
normally distributed. However, these results are broadly consistent with the
estimates of the uncertainties derived in Chapter 5 (see
Sections 5.4.2, and Sections 6.7.8).
