|Working Group I: The Scientific Basis|
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22.214.171.124 Primary biogenic aerosols
Primary biogenic aerosol consists of plant debris (cuticular waxes, leaf fragments,
etc.), humic matter, and microbial particles (bacteria, fungi, viruses, algae,
pollen, spores, etc.). Unfortunately, little information is available that would
allow a reliable estimate of the contribution of primary biogenic particles
to the atmospheric aerosol. In an urban, temperate setting, Matthias-Maser and
Jaenicke (1995) have found concentrations of 10 to 30% of the total aerosol
volume in both the sub-micron and super-micron size fractions. Their contribution
in densely vegetated regions, particularly the moist tropics, could be even
more significant. This view is supported by analyses of the lipid fraction in
Amazonian aerosols (Simoneit et al., 1990).
The presence of humic-like substances makes this aerosol light-absorbing,
especially in the UV-B region (Havers et al., 1998), and there is evidence that
primary biogenic particles may be able to act both as cloud droplet and ice
nuclei (Schnell and Vali, 1976). They may, therefore, be of importance for both
direct and indirect climatic effects, but not enough is known at this time to
assess their role with any confidence. Since their atmospheric abundance may
undergo large changes as a result of land-use change, they deserve more scientific
Sulphate aerosols are produced by chemical reactions in the atmosphere from
gaseous precursors (with the exception of sea salt sulphate and gypsum dust
particles). The key controlling variables for the production of sulphate aerosol
from its precursors are:
The atmospheric burden of the sulphate aerosol is then regulated by the interplay
of production, transport and deposition (wet and dry).
The two main sulphate precursors are SO2 from anthropogenic sources
and volcanoes, and DMS from biogenic sources, especially marine plankton (Table
5.2). Since SO2 emissions are mostly related to fossil fuel burning,
the source distribution and magnitude for this trace gas are fairly well-known,
and recent estimates differ by no more than about 20 to 30% (Lelieveld et al.,
1997). Volcanic emissions will be addressed in Section 126.96.36.199.
Estimating the emission of marine biogenic DMS requires a gridded database on its concentration in surface sea water and a parametrization of the sea/air gas transfer process. A 1ºx1º monthly data set of DMS in surface water has been obtained from some 16,000 observations using a heuristic interpolation scheme (Kettle et al., 1999). Estimates for data-sparse regions are generated by assuming similarity to comparable biogeographic regions with adequate data coverage. Consequently, while the global mean surface DMS concentration is quite robust because of the large data set used (error estimate ± 50%), the estimates for specific regions and seasons remain highly uncertain in many ocean regions where sampling has been sparse (error up to factor of 5). These uncertainties are compounded with those resulting from the lack of a generally accepted air/sea flux parametrization. The approach of Liss and Merlivat (1986) and that of Wanninkhof (1992) yield fluxes differing by a factor of two (Kettle and Andreae, 2000). In Table 5.2, we use the mean of these two estimates (24 Tg S(DMS)/yr).
The chemical pathway of conversion of precursors to sulphate is important
because it changes the radiative effects. Most SO2 is converted to
sulphate either in the gas phase or in cloud droplets that later evaporate.
Model calculations suggest that aqueous phase oxidation is dominant globally
(Table 5.5). Both processes produce sulphate mostly in
sub-micron aerosols that are efficient light scatterers, but the precise size
distribution of sulphate in aerosols is different for gas phase and aqueous
production. The size distribution of the sulphate formed in the gas phase process
also depends on the interplay between nucleation, condensation and coagulation.
Models that describe this interplay are in an early stage of development, and,
unfortunately, there are substantial inconsistencies between our theoretical
description of nucleation and condensation and the rates of these processes
inferred from atmospheric measurements (Eisele and McMurry, 1997; Weber et al.,
1999). Thus, most models of sulphate aerosol have simply assumed a size distribution
based on present day measurements. Because there is no general reason that this
same size should have applied in the past or will in the future, this lends
considerable uncertainty to calculations of forcing. Many of the same issues
about nucleation and condensation also apply to secondary organic aerosols.
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