6.8.3.1 Carbonaceous aerosols

In this section, carbonaceous aerosols refer to the mixture (internal or external) of OC and BC aerosols. Carbonaceous aerosols (and in particular biomass burning aerosols) are efficient cloud condensation nuclei (see Chapter 5 and e.g., Novakov and Penner, 1993; Novakov and Corrigan, 1996). There have been few GCM studies estimating the indirect forcing from carbonaceous aerosols. Penner et al. (1996) reported a range of forcing from -2.5 to -4.5 Wm^-2 (not included in Table 6.6), which is probably an overestimate because of neglect of other aerosol types such as dust and sea salt and underestimated natural emissions of organic aerosols. In a new set of simulations using an updated model accounting for dust and sea salt aerosols, Chuang et al. (2000b) obtained a forcing of –1.51 Wm^-2 for the first indirect effect from carbonaceous aerosols (–0.52 and –1.16 Wm^-2 for fossil fuel and biomass burning aerosols, respectively). This estimate includes the effect of black carbon absorption in cloud droplets. Lohmann et al. (2000) predicted a radiative impact for the combined effect (i.e., first and second effects) of -1.3 and -0.9 Wm^-2 for externally and internally mixed carbonaceous aerosols, respectively. These estimates do not include the effects of secondary organic aerosols, nor the effects of absorption of solar radiation by black carbon within the cloud.

Kaufman and Nakajima (1993) used AVHRR data to analyse bright warm clouds over Brazil during the biomass burning season. They found a decrease in the cloud reflectance when the smoke optical thickness increased. Kaufman and Fraser (1997) used a similar approach to observe thinner and less reflective clouds. They showed that smoke from biomass burning aerosols reduced the cloud droplet size and increased the cloud reflectance for smoke optical depth up to 0.8. They estimated the indirect radiative forcing by smoke to be -2 Wm^-2 over this region for the three months when biomass burning takes place, which would suggest a much smaller global average. However, using a combination of satellite observations and a global chemistry model, Remer et al. (1999) estimated that 50% of the cumulative biomass burning aerosol indirect forcing occurs for smoke optical depth smaller than 0.1, that is well away from the source regions.

6.8.3.2 Combination of sulphate and carbonaceous aerosols

Lohmann et al. (2000) found that the radiative impact of sulphate and carbonaceous aerosols considered simultaneously (-1.5 and -1.1 Wm^-2 for the externally and internally mixed assumptions, respectively) is comparable to the sum of the radiative impacts calculated separately (see Table 6.6). Chuang et al. (2000b) reached a similar conclusion in their experiments where they considered only the first indirect effect, with a total radiative forcing of –1.85 Wm^-2. It is noteworthy, however, that in these two studies, the predicted radiative forcings for sulphate aerosols are on the low side. This indicates that it is not wise to add the radiative forcings due to sulphate and carbonaceous aerosols obtained separately from two different models. In fact, most of the GCM studies of the indirect aerosol effect used sulphate as a surrogate for the total anthropogenic fraction of the aerosol (e.g., Boucher and Lohmann, 1995; Feichter et al., 1997; Lohmann and Feichter, 1997). In this case the computed forcings incorporate the effects of other aerosol types which have a similar spatial distribution to sulphate aerosols, such as nitrate aerosols or carbonaceous aerosols from fossil fuel combustion. It will not include, however, the effects of biomass burning aerosols which have a different spatial distribution from sulphate aerosols.

Another issue is the potential for light-absorbing aerosols to increase in-cloud absorption of solar radiation – and correspondingly decrease the cloud albedo – when incorporated inside cloud droplets. Twohy et al. (1989) concluded from measurements off the coast of California and from simple radiative calculations that the observed levels of soot would not lead to a significant impact on the cloud albedo. Chylek et al. (1996a) estimated an upper bound for increased absorption of solar radiation of 1 to 3 Wm^-2 (global and annual average) for a black carbon concentration of 0.5 mgm-3. Considering the modelled atmospheric concentrations of soot (Chapter 5 and Sections 6.7.3 and 6.7.5) and the fact that only a fraction of the soot is incorporated in the cloud droplets, the effect is probably smaller by one to two orders of magnitude. Heintzenberg and Wendisch (1996) showed that the decrease in radiative forcing due to a decrease in soot concentrations with increasing distances from the pollution sources could be compensated by a concurrent increase in the fraction of soot which is incorporated in the cloud droplets. Only one GCM study to date has considered the in-cloud absorption of soot. Chuang et al. (2000b) estimated a radiative forcing for in-cloud BC of +0.07 Wm^-2 for the soot concentrations predicted by their model and using an effective medium approximation. More studies are needed to confirm these results.

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