|Working Group I: The Scientific Basis|
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Organic aerosols from the atmospheric oxidation of hydrocarbons
Atmospheric oxidation of biogenic hydrocarbons yields compounds of low volatility
that readily form aerosols. Because it is formed by gas-to-particle conversion,
this secondary organic aerosol (SOA) is present in the sub-micron size fraction.
Liousse et al. (1996) included SOA formation from biogenic precursors in their
global study of carbonaceous aerosols; they employed a constant aerosol yield
of 5% for all terpenes. Based on smog chamber data and an aerosol-producing
VOC emissions rate of 300 to 500 TgC/yr, Andreae and Crutzen (1997) provided
an estimate of the global aerosol production from biogenic precursors of 30
to 270 Tg/yr.
Recent analyses based on improved knowledge of reaction pathways and non-methane
hydrocarbon source inventories have led to substantial downward revisions of
this estimate. The total global emissions of monoterpenes and other reactive
volatile organic compounds (ORVOC) have been estimated by ecosystem (Guenther
et al., 1995). By determining the predominant plant types associated with these
ecosystems and identifying and quantifying the specific monoterpene and ORVOC
emissions from these plants, the contributions of individual compounds to emissions
of monoterpenes or ORVOC on a global scale can be inferred (Griffin, et al.,
1999b; Penner et al., 1999a).
Experiments investigating the aerosol-forming potentials of biogenic compounds
have shown that aerosol production yields depend on the oxidation mechanism.
In general, oxidation by O3 or NO3 individually yields
more aerosol than oxidation by OH (Hoffmann, et al., 1997; Griffin, et al.,
1999a). However, because of the low concentrations of NO3 and O3
outside of polluted areas, on a global scale most VOC oxidation occurs through
reaction with OH. The subsequent condensation of organic compounds onto aerosols
is a function not only of the vapour pressure of the various molecules and the
ambient temperature, but also the presence of other aerosol organics that can
absorb products from gas-phase hydrocarbon oxidation (Odum et al., 1996; Hoffmann
et al., 1997; Griffin et al., 1999a).
It should be noted that while the precursors of this aerosol are indeed of
natural origin, the dependence of aerosol yield on the oxidation mechanism implies
that aerosol production from biogenic emissions might be influenced by human
activities. Anthropogenic emissions, especially of NOx, are causing
an increase in the amounts of O3 and NO3, resulting in
a possible 3- to 4-fold increase of biogenic organic aerosol production since
pre-industrial times (Kanakidou et al., 2000). Recent studies in Amazonia confirm
low aerosol yields and little production of new particles from VOC oxidation
under unpolluted conditions (Artaxo et al., 1998b; Roberts et al., 1998). Given
the vast amount of VOC emitted in the humid tropics, a large increase in SOA
production could be expected from increasing development and anthropogenic emissions
in this region.
Anthropogenic VOC can also be oxidised to organic particulate matter. Only the oxidation of aromatic compounds, however, yields significant amounts of aerosol, typically about 30 g of particulate matter for 1 kg of aromatic compounds oxidised under urban conditions (Odum et al., 1996). The global emission of anthropogenic VOC has been estimated at 109 ± 27 Tg/yr, of which about 60% is attributable to fossil fuel use and the rest to biomass burning (Piccot et al., 1992). The emission of aromatics amounts to about 19 ± 5 Tg/yr, of which 12 ± 3 Tg/yr is related to fossil fuel use. Using these data, we obtain a very small source strength for this aerosol type, about 0.6 ± 0.3 Tg/yr.
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