6.7.3 Fossil Fuel Black Carbon Aerosol
Since the SAR there have been a number of more refined threedimensional global
model estimates of the radiative forcing due to black carbon (BC) aerosol from
fossil fuel burning which have superseded calculations using a simple expression
for the radiative forcing where the contribution from cloudy regions was not
included (e.g., Chylek and Wong, 1995; Haywood and Shine, 1995). These estimates
now include the contribution to the total radiative forcing from areas where
BC exists either above or within clouds, although the treatment of BC within
clouds remains crude. Table 6.5 includes recent global
annual mean estimates of the radiative forcing due to BC aerosol from fossil
fuels. Haywood et al. (1997a) and Myhre et al. (1998c) assumed that fossil fuel
BC was directly proportional to the mass of sulphate from Langner and Rodhe
(1991) and Restad et al. (1998), respectively, by applying a 7.5% mass scaling
(equivalent to a global mean burden of approximately 0.13 mgm^{2} to
0.14 mgm^{2}). Global annual mean radiative forcings of +0.20 Wm^{2}
and +0.16 Wm^{2} are calculated for external mixtures. Both studies
suggest that BC internally mixed with sulphate will exert a stronger forcing
although the method of mixing was fairly crude in both of these studies. These
estimates of the radiative forcing were performed before global modelling studies
for BC were generally available.
Table 6.5: The global and annual mean direct radiative
forcing for the period from preindustrial (1750) to present day (2000)
due to black carbon (BC) and organic carbon (OC) aerosols from different
global studies. The anthropogenic column burden of BC and OC is shown, where
appropriate, together with the normalised direct radiative forcing (DRF). 

Aerosol

Author and type of study

Mixing or optical parameters

DRF (Wm^{2})

Column burden (mgm^{2})

Normalised DRF (Wg^{1})

Remarks


Fossil fuel BC

Haywood et al. (1997a)

External

+0.20

0.13

1525

GCM study. 7.5% mass scaling of BC to SO_{4} assumed. SO_{4}
from Langner and Rodhe (1991) (slow oxidation case). 


Internal with sulphate

+0.36


2770

Internal mixing approximated by volume weighting the refractive indices
of BC and SO_{4}. 

Myhre et al. (1998c)

External

+0.16

0.14

1123

Threedimensional study using global climatologies for cloud, surface
reflectance etc.
7.5% mass scaling of BC to SO_{4} assumed. SO_{4} from Restad
et al. (1998). 


Internal with sulphate

+0.42


3000

Internal mixing approximated by volume weighting the refractive indices
of BC and SO_{4} 

Penner et al. (1998b) and Grant et al. (1999)

Internal with fossil fuel OC

+0.20

0.16

1287

BC modelled using chemical transport model and GCM. 

Cooke et al. (1999)

External

+0.17

0.14

1210

BC modelled using chemical transport model and GCM. 

Haywood and Ramaswamy (1998)

External mixture

+0.2

0.13

1500

Threedimensional GCM study using Cooke and Wilson (1996) BC data scaled
to Liousse et al. (1996) total BC mass. 50% of the BC mass assumed to be
from fossil fuels. 

Fossil fuel OC

Penner et al. (1998b)

Internal/external with fossil fuel BC

0.04 to 0.24

0.7

60 to 340

OC modelled using chemical transport model and GCM. Weakest estimate corresponds
to internal mixing with BC, strongest estimate corresponds to external mixing
with BC. 

Cooke et al. (1999)

External mixture

0.02

0.34

70

OC modelled using chemical transport model and GCM. May be more negative
due to effects of
RH and assumption of partial absorption of OC. 

Myhre et al. (2001)

External mixture

0.09

0.66

135

Threedimensional study using global climatologies for cloud, surface
reflectance etc. OC scaled linearly to SO_{4}. SO_{4} from
Restad et al. (1998). 

Biomass burning (BC+OC)

Hobbs et al. (1997)

Optical parameters of biomass smoke

0.3

3.7

80

Uses simplified expression from Chylek and Wong (1995). Neglects radiative
forcing from cloudy areas. Other parameters including estimated column burden
from Penner et al. (1992). 

Iacobellis et al. (1999)

Optical parameters of biomass smoke

0.74

3.5

210

Threedimensional chemical transport model and GCM. Simplified expressions
also examined. 

Penner et al. (1998b) and Grant et al. (1999)

Internal/external mixture of OC and BC

0.14 to 0.23

1.76

80 to 120

Threedimensional chemical transport model and GCM using biomass optical
parameters modelled from two observational studies. 

Fossil fuel and biomass burning BC

Haywood and Ramaswamy (1998)

External mixture

+0.4

0.27

1500

Threedimensional GCM study using Cooke and Wilson (1996) BC data scaled
to Liousse et al. (1996) total BC mass. 

Hansen et al. (1998)

Observed single scattering albedo

+0.27

NA

NA

Adjustment of modelled single scattering albedo from 1.0 to 0.920.95
to account for the absorption properties of BC. 

Jacobson (2001)

Internal with BC core.

+0.54

0.45

1200

GCM study using data from Cooke and Wilson (1996) scaled by a factor of
0.85. 

Fossil fuel and biomass burning OC

Hansen et al. (1998)

External mixture

0.41

NA

NA

Threedimensional GCM study using Liousse et al. (1996) OC data. OC modelled
as scattering. 
Jacobson (2001)

Internal with BC core.

0.04 to 0.06

1.8

17

OC treated as shell with BC core. Strongest forcing for nonabsorbing
OC. 

Penner et al. (1998b) and Grant et al. (1999) used a chemical transport model
in conjunction with a GCM to estimate a global column burden of BC from fossil
fuel emissions of 0.16 mgm^{2} and a radiative forcing of +0.20 Wm^{2}
for an external mixture. Cooke et al. (1999) estimated the global burden of
optically active BC aerosol from fossil fuel burning from a 1° by 1°
inventory of emissions to be 0.14 mgm^{2} (note the good agreement
with the assumed column burdens of Haywood et al. (1997a) and Myhre et al. (1998c))
and a subsequent radiative forcing of +0.17 Wm^{2}. Haywood and Ramaswamy
(1998) estimated the BC radiative forcing due to fossil fuel and biomass burning
to be approximately +0.4 Wm^{2} (see also Section
6.7.5), approximately half of which (+0.2 Wm^{2}) is due to fossil
fuel sources. Jacobson (2000, 2001) modelled BC from both fossil fuels and biomass
burning sources and investigated the effects of different mixing schemes, finding
a direct radiative forcing of +0.54 Wm^{2} and a normalised direct
radiative forcing of 1,200 Wg^{1} when BC was modelled as an absorbing
spherical core. These studies suggest that the global mean radiative forcing
due to fossil fuel BC is strongest in June/July/August owing to the larger insolation
coupled with higher atmospheric concentrations in the Northern Hemisphere. From
these studies the normalised radiative forcing for an external mixture of BC
aerosol appears better defined than for sulphate aerosol ranging from +1,123
Wg^{1} to +1,525 Wg^{1}. However, all these studies used the
same size distribution and exclude effects of relative humidity, thus the modelled
specific extinction is independent of the relative humidity at approximately
10 m^{2}g^{1} at 0.55 µm. Observational studies show
a wide range of specific extinction coefficients, a_{sp}, (see Section
5.1.2 and Table 5.1) of approximately 5 to 20
m^{2}g^{1} at 0.55 µm, thus the uncertainty in the associated
radiative forcing is likely to be higher than the global model results suggest.
Additionally, if BC were modelled as an internal mixture, Haywood et al. (1997a)
and Myhre et al. (1998c) suggest the degree of absorption may be considerably
enhanced, the radiative forcing being estimated as +0.36 Wm^{2} and
+0.44 Wm^{2}, respectively. Both of these studies use relatively simple
effective medium mixing rules for determining the composite refractive index
of internally mixed BC with sulphate and water which may overestimate the degree
of absorption (e.g., Jacobson, 2000). Detailed scattering studies including
a randomly positioned black carbon sphere in a scattering droplet show that
the absorption is relatively well represented by effective medium approximations
(Chylek et al., 1996b). Column studies by Haywood and Shine (1997) and Liao
and Seinfeld (1998) and the global studies by Haywood and Ramaswamy (1998) and
Penner et al. (1998b) suggest that the radiative forcing due to BC will be enhanced
if BC exists within or above the cloud, but reduced if the BC is below the cloud;
thus the vertical profile of BC aerosol must be determined from observations
and modelled accurately.
On the basis of the calculations summarised above, the estimate of the global
mean radiative forcing for BC aerosols from fossil fuels is revised to +0.2
Wm^{2} (from +0.1 Wm^{2}) with a range +0.1 to +0.4 Wm^{2}.
The significant contribution to the global annual mean radiative forcing from
cloudy regions is the main reason for the increase in the radiative forcing
since the SAR. Uncertainties in determining the radiative forcing lie in modelling
the mixing of BC with hygroscopic aerosols and the subsequent wet deposition
processes which consequently influence the modelled atmospheric lifetime and
burden of BC aerosol. Additionally, mixing of BC with other aerosol types and
cloud droplets influences the optical parameters.
