Emissions of Biogenic Organics

As discussed briefly in Chapter 2, there are substantial biogenic emissions of VOC. (We exclude a discussion of methane, since it is of low reactivity and its chemistry is quite straightforward; see Chapter f4 for a discussion of its implications for global climate change.) On a global basis, the total may exceed anthropogenic emissions by as much as an order of magnitude (see Tables 2.1 and 2.2 and the volume edited by Hewitt, 1999). In urban areas, of course, their contributions are relatively less important. For example, Benjamin et al. (1997) report that biogenic hydrocarbons in the South Coast Air Basin of southern California are ~ f 0% of the total VOC emission inventory on a typical summer day. The contribution of biogenic organics to urban 03 formation is of course variable, depending on the particular locations (e.g., Chameides et al., 1988; Roselle et al., 1991; Pierce et al., 1998; see also Chapter 16).

Several thousand different biogenic VOCs have been identified (e.g., Isidorov et al., 1985; Graedel et al., 1986; Puxbaum, 1997; Helas et al., 1997; Fall, 1999). The most well known are ethene, isoprene, and the monoterpenes emitted by terrestrial plants. As discussed in this section, it has been increasingly recognized that there are biogenic emissions of oxygen-containing organics, from small alcohols such as methanol to larger aldehydes, ketones, and alcohols such as 2-methyl-3-buten-2-ol. Figures 6.22 and 6.23 summarize some of the VOC that have been observed and possible mechanisms of production (Fall, 1999). Different compounds are produced in different parts of the plant and by different physiological processes; the

6. GAS-PHASE REACTIONS IN IRRADIATED ORGANIC-NOv-AIR MIXTURES

a-Pinene

Isoprene a-Pinene

Pinene

Pinene

Isoprene

Limonene a-Terpinene y-Terpinene Camphene

Terpinolene a-Phellandrene ß-Phellandrene

Myrcene

Ocimene

Ä3-Carene p-Cymene

Myrcene

Ocimene

Ä3-Carene p-Cymene

FIGURE 6.22 Chemical structures of some biogenically emitted hydrocarbons.

present understanding of these processes is reviewed by Fall (1999).

Ethene is a plant hormone emitted at a rate of several teragrams per year from plants; smaller amounts are emitted by soils and oceans (Rudolph, 1997). Interestingly, the emissions of ethene as well as some other VOCs (see later) increase significantly (by as much as factors of one to two orders of magnitude) when the plant is stressed, for example by mechanical means, high temperatures, or lack of water.

Isoprene is the major single, non-methane VOC emitted by plants:

Isoprene

Significant amounts of larger hydrocarbons are also generated by plants and emitted to the atmosphere. The larger hydrocarbon compounds generally fall under the classification of isoprenoids, or terpenoids, con sisting of groups of 5-carbon isoprene type units (although they are not formed from isoprene). The monoterpenes are the Cl() compounds, sesquiterpenes the C|5 compounds, diterpenes the C20 compounds, triterpenes the C30 compounds, and tetraterpenes the C40 compounds.

Table 6.24, for example, shows one estimate of the annual global emissions of isoprene, other monoterpenes, and VOCs as well as methane (Guenther, 1999). Emissions of isoprene are believed to be about four times those of the other monoterpenes and about equal to all other VOCs.

Hardwood species such as oaks, poplars, aspen, and ironwood are generally isoprene emitters. However, even within plant families, not all species are isoprene emitters. For example, while North American oaks emit isoprene, many European oak species do not. For example, Steinbrecher et al (1997) measured the emissions of isoprene and monoterpenes from five species of Mediterranean oak; two of them were strong isoprene emitters, whereas the other three did not emit significant amounts. Similarly, Kesselmeier et al (1998) measured emissions of isoprene and monoterpenes from a Holm oak and a white oak growing side by side; the white oak was a strong isoprene emitter, whereas the Holm oak was a strong monoterpene emitter.

In addition to deciduous trees such as some oaks, isoprene is also emitted from other plants, including shrubs, gorse, vines, ferns, and plants characteristic of tropical savannas and peatlands characteristic of boreal regions (e.g., Guenther et al., 1996a,b; Cao et al., 1997; Owen et al, 1998; Janson and De Serves, 1998; Guenther, 1999; Fall, 1999). Isoprene emissions have been reported to be correlated with successional patterns; for example, Klinger et al. (1998) report higher isoprene emissions in the early to mid-successional savanna ecosystems, compared to the later, rainforest development. Other sources (all believed to be relatively small) include marine phytoplankton (e.g., Bonsang et al., 1992; Moore et al., 1994; Milne et al., 1995; Broadgate et al., 1997), bacteria and fungi (e.g., Kuzma et al., 1995), the breath of humans and other mammals (e.g., Gelmont et al., 1981; Mendis et al., 1994; Phillips et al., 1994; Jones et al, 1995; Foster et al, 1996; Sharkey, 1996; Fenske and Paulson, 1999), and some industrial processes (e.g., Guenther, 1999).

Isoprene production by plants is very sensitive to light as well as temperature (e.g., Sanadze and Kalan-dadze, 1966; Tingey et al., 1979). Figure 6.24a shows the production of isoprene from an aspen leaf when light in the photosynthetically active range is turned on and off (Monson et al., 1991; Fall, 1999). When the light is switched on, isoprene emissions rise and when it is turned off, fall even more rapidly. Figure 6.24b shows

FIGURE 6.23 Schematic diagram of mechanisms of production of some biogenically emitted VOCs (adapted with permission from Fall, 1999).

the emissions as a function of this light intensity; the emissions rise rapidly and approach (but do not reach) a plateau (Monson et al., 1992; Fall, 1999). Finally, Fig. 6.25 shows the effect of temperature on the isoprene emission rate; emissions rise to a temperature of ~ 40-45 °C and then rapidly fall (e.g., Monson et al., 1992; Sharkey and Singsaas, 1995; Fall and Wilder-

TABLE 6.24 Estimated Global Annual Biogenic VOC Emissions (Tg yr " ')"

Source

Isoprene

VOCs6

Canopy foliage

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