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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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