5.2.2.4 Carbonaceous aerosols (organic and black carbon)

Carbonaceous compounds make up a large but highly variable fraction of the atmospheric aerosol (for definitions see Glossary). Organics are the largest single component of biomass burning aerosols (Andreae et al., 1988; Cachier et al., 1995; Artaxo et al., 1998a). Measurements over the Atlantic in the haze plume from the United States indicated that aerosol organics scattered at least as much light as sulphate (Hegg et al., 1997; Novakov et al., 1997). Organics are also important constituents, perhaps even a majority, of upper-tropospheric aerosols (Murphy et al., 1998b). The presence of polar functional groups, particularly carboxylic and dicarboxylic acids, makes many of the organic compounds in aerosols water-soluble and allows them to participate in cloud droplet nucleation (Saxena et al., 1995; Saxena and Hildemann, 1996; Sempéré and Kawamura, 1996). Recent field measurements have confirmed that organic aerosols may be efficient cloud nuclei and consequently play an important role for the indirect climate effect as well (Rivera-Carpio et al., 1996).

There are significant analytical difficulties in making valid measurements of the various organic carbon species in aerosols. Large artefacts can be produced by both adsorption of organics from the gas phase onto aerosol collection media, as well as evaporation of volatile organics from aerosol samples (Appel et al., 1983; Turpin et al., 1994; McMurry et al., 1996). The magnitude of these artefacts can be comparable to the amount of organic aerosol in unpolluted locations. Progress has been made on minimising and correcting for these artefacts through several techniques: diffusion denuders to remove gas phase organics (Eatough et al., 1996), impactors with relatively inert surfaces and low pressure drops (Saxena et al., 1995), and thermal desorption analysis to improve the accuracy of corrections from back-up filters (Novakov et al., 1997). No rigorous comparisons of different techniques are available to constrain measurement errors.

Of particular importance for the direct effect is the light-absorbing character of some carbonaceous species, such as soot and tarry substances. Modelling studies suggest that the abundance of “black carbon” relative to non-absorbing constituents has a strong influence on the magnitude of the direct effect (e.g., Hansen et al., 1997; Schult et al., 1997; Haywood and Ramaswamy, 1998; Myhre et al., 1998; Penner et al., 1998b).

Given their importance, measurements of black carbon, and the differentiation between black and organic carbon, still require improvement (Heintzenberg et al., 1997). Thermal methods measure the amount of carbon evolved from a filter sample as a function of temperature. Care must be taken to avoid errors due to pyrolysis of organics and interference from other species in the aerosol (Reid et al., 1998a; Martins et al., 1998). Other black carbon measurements use the light absorption of aerosol on a filter measured either in transmission or reflection. However, calibrations for converting the change in absorption to black carbon are not universally applicable (Liousse et al., 1993). In part because of these issues, considerable uncertainties persist regarding the source strengths of light-absorbing aerosols (Bond et al., 1998).

Carbonaceous aerosols from fossil fuel and biomass combustion
The main sources for carbonaceous aerosols are biomass and fossil fuel burning, and the atmospheric oxidation of biogenic and anthropogenic volatile organic compounds (VOC). In this section, we discuss that fraction of the carbonaceous aerosol which originates from biomass or fossil fuel combustion and is present predominantly in the sub-micron size fraction (Echalar et al., 1998; Cooke, et al., 1999). The global emission of organic aerosol from biomass and fossil fuel burning has been estimated at 45 to 80 and 10 to 30 Tg/yr, respectively (Liousse, et al., 1996; Cooke, et al., 1999; Scholes and Andreae, 2000). Combustion processes are the dominant source for black carbon; recent estimates place the global emissions from biomass burning at 6 to 9 Tg/yr and from fossil fuel burning at 6 to 8 Tg/yr (Penner et al., 1993; Cooke and Wilson, 1996; Liousse et al., 1996; Cooke et al., 1999, Scholes and Andreae, 2000; see Table 5.3). A recent study by Bond et al. (1998), in which a different technique for the determination of black carbon emissions was used, suggests significantly lower emissions. Not enough measurements are available at the present time, however, to provide an independent estimate based on this technique. The source distributions are shown in Figures 5.2(c) and 5.2(e) for organic and black carbon, respectively.

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