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).

When combined with appropriate transport and reaction mechanisms in global chemistry transport models, these hydrocarbon emissions yield estimated ranges of global biogenically derived SOA of 13 to 24 Tg/yr (Griffin et al., 1999b) and 8 to 40 Tg/yr (Penner et al., 1999a). Figure 5.2(d) shows the global distribution of SOA production from biogenic precursors derived from the terpene sources from Guenther et al. (1995) for a total source strength of 14 Tg/yr (see Table 5.3).

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 NO_x, 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.

Table 5.4: Estimates for secondary aerosol sources (in Tg substance/yra ).
	Northern Hemisphere	Southern Hemisphere	Global	Low	High	Source
Sulphate (as NH4 HSO4 )	145	55	200	107	374	from Table 5.5
Anthropogenic	106	15	122	69	214
Biogenic	25	32	57	28	118
Volcanic	14	7	21	9	48
Nitrate (as NO3– )b
Anthropogenic	12.4	1.8	14.2	9.6	19.2
Natural	2.2	1.7	3.9	1.9	7.6
Organic compounds
Anthropogenic	0.15	0.45	0.6	0.3	1.8	see text
VOC
Biogenic VOC	8.2	7.4	16	8	40	Griffin et al. (1999b); Penner et al. (1999a)
a Total sulphate production calculated from data in Table 5.5, disaggregated into anthropogenic, biogenic and volcanic fluxes using the precursor data in Table 5.2 and the CHAM/GRANTOUR model (see Table 5.8). b Total net chemical tendency for HNO3 from UCI model (Chapter 4) apportioned as NO3 according to the model of Penner et al. (1999a). Range corresponds to range from NOx sources in Table 5.2.

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