Anthropogenic organic aerosols are a by-product of fossil fuel and biomass combustion and they consist of many complex chemical compounds and are released either as primary aerosol particles or as volatile organic gases (see Chapter 5). Studies that investigate the radiative forcing due to organic carbon (OC) from fossil fuels are included in Table 6.5. Penner et al. (1998b) and Grant et al. (1999) found a DRF of +0.16 Wm-2 when modelling the direct radiative forcing due to an internal mixture of fossil fuel BC and OC, and +0.2Wm-2 when modelling the radiative forcing due to externally mixed fossil fuel BC. From these results, an annual global mean radiative forcing of -0.04 Wm-2 for fossil fuel OC from a global mean burden of approximately 0.7 mgm-2 may be derived. However, if OC were modelled as an external mixture with BC and/or if the effects of relative humidity are included, the radiative forcing due to OC from fossil fuels is likely to be more negative, thus this represents an approximate weakest limit. An alternative method for calculating the DRF due to fossil fuel OC from the results of Penner et al. (1998b) is to note that the absorption is approximately doubled when BC is modelled as an internal mixture rather than an external mixture (e.g., Haywood et al., 1997a). Thus, for an external mixture of fossil fuel OC a radiative forcing of -0.24 Wm-2 may be more appropriate. Cooke et al. (1999) performed GCM calculations for externally mixed fossil fuel OC, finding a radiative forcing of -0.02 Wm-2 from a global mean burden of 0.34 mgm-2. Myhre et al. (2001) scale the atmospheric concentrations of fossil fuel OC to modelled sulphate aerosol concentrations and include the effects of relative humidity to estimate a radiative forcing of -0.09 Wm-2 from a global mean burden of 0.66 mgm-2. Thus, modelling estimates suggest that the normalised radiative forcing for OC is in the range -60 to -340 Wg-1, which is smaller in magnitude than that due to BC due to the larger specific extinction coefficient for BC and the fact that BC may exert a significant radiative forcing in cloudy regions. Cooke et al. (1999) assume that OC is partially absorbing with a modelled single scattering albedo of approximately 0.97 at a wavelength of 0.55 µm. Hansen et al. (1998) use a three-dimensional GCM and the OC distribution from Liousse et al. (1996) and estimate the radiative forcing due to combined fossil fuel and biomass sources to be -0.41 Wm-2. The approximate fraction of the atmospheric burden of the fossil fuel component may be estimated from the emission inventory of Liousse et al. (1996) who estimate that fossil fuels contribute 38% to the total OC emissions. Thus the radiative forcing due to fossil fuel OC may be inferred to be approximately -0.16 Wm-2. This may constitute an approximate upper estimate as the majority of fossil fuel OC occurs over mid-latitude land areas where the surface reflectance is generally higher and insolation is lower than in the equatorial regions where biomass burning is the major source of OC. From these calculations, the radiative forcing due to fossil fuel OC is estimated to be -0.10 Wm-2. The uncertainty associated with this estimate is necessarily high due to the limited number of detailed studies and is estimated to be at least a factor of three.
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