B

Intensity of photosynthetically active radiation (ixmol rrr2 s"1)

FIGURE 6.24 Effect of light on isoprene emission rate from (a) aspen leaf and (b) velvet bean leaf (adapted from Monson et al., 1991, 1992; and Fall, 1999).

Intensity of photosynthetically active radiation (ixmol rrr2 s"1)

FIGURE 6.24 Effect of light on isoprene emission rate from (a) aspen leaf and (b) velvet bean leaf (adapted from Monson et al., 1991, 1992; and Fall, 1999).

diphosphate is a precursor for a- and /3-pinene, limonene, and myrcene, whose generation involves the enzyme limonene synthase (e.g., see Fall, 1999). In contrast to isoprene, which does not have a reservoir in leaves in many cases, monoterpenes are generated and stored in the plant prior to emission so that in general, their emission to the atmosphere is not as closely tied to short-term controls over their biosynthesis (e.g., Monson et al., 1995; Fall, 1999).

Emissions of monoterpenes have been observed from a variety of plants, including pines (e.g., Juuti et al., 1990; Guenther et al., 1994; Street et al., 1997; Staudt et al., 1997), resin in pine forests (e.g., Pio and Valente, 1998), spruce (Street et al., 1996), some deciduous trees such as oaks (e.g., Benjamin et al., 1996; Street et al., 1997; Kesselmeier et al., 1998), and gorse (e.g., Cao et al., 1997). Interestingly, as for ethene, increased emissions have been observed when plants are stressed (Fall, 1999). For example, Juuti et al. (1990) report that

FIGURE 6.25 Effect of willow leaf temperature on isoprene emission rate (adapted from Fall and Wildermuth, 1998; and Fall, 1999).

FIGURE 6.25 Effect of willow leaf temperature on isoprene emission rate (adapted from Fall and Wildermuth, 1998; and Fall, 1999).

the monoterpene emission rates from a Monterey pine increased by factors of 10-50 during rough handling.

As for isoprene, emission rates of the monoterpenes increase with temperature, although different plant species exhibit different temperature sensitivities and different compounds can also show different dependencies on temperature (e.g., Tingey et al., f 980; Loreto et al., 1996; Owen et al., 1997; Schween et al., 1997; Drewitt et al., 1998). The temperature dependence of monoterpene emissions is often taken into account by multiplying the base emission rate at a reference temperature Ts by the factor e[/3(/~ /s)I, where T is the leaf temperature and [3 is a coefficient that reflects the temperature sensitivity of emissions (e.g., Guenther et al., 1993). Light also affects monoterpene emissions but does not appear to be as significant as for isoprene (e.g., Tingey et al., 1980; Loreto et al., 1996; Guenther et al., 1996b).

Larger hydrocarbons such as the C15 sesquiterpenes also have biogenic sources such as sage (Arey et al., 1995).

As shown in Table 6.24, oceans and freshwater are not believed to be major sources of VOC to the atmosphere. As indicated earlier, isoprene is thought to be generated in small amounts in the oceans by marine phytoplankton. In addition, a variety of small hydrocarbons have been identified both in seawater and in the air above it, including alkanes (ethane, propane, n-butane, isobutane, n-pentane, isopentane, and n-hexane), alkenes (ethene, propene, 1- and 2-butene, isobutene, 1-pentene, and 1-hexene), and acetylene (e.g., Arlander et al., 1990; Rudolph and Johnen, 1990; Bonsang et al., 1991; Plass-Dulmer et al., 1993). While some of these may be due to long-range transport from the continents (Rudolph and Johnen, f990), it appears that the ocean is indeed a source of most, if not all, of these light hydrocarbons. A major organic found in ocean areas is dimethyl sulfide (DMS), whose oxidation products are believed to play a significant role in particle formation and hence radiative properties in the marine boundary layer (see Chapters 8.E.f and 14.C). DMS also plays a major role in determining the lifetime and fate of N03 (Carslaw et ai, 1997).

In addition to hydrocarbons, biogenic processes also produce a number of oxygen-containing organics. One of the most important appears to be 2-methyl-3-buten-2-ol (MBO), first identified in a forested area by Goldan et al. (1993):

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