O-CCH3 (3Z)-Hexenyl acetate
O-CCH3 (3Z)-Hexenyl acetate
FIGURE 6.26 Structures of some oxygen-containing organics with biogenic emission sources.
hexenyl acetate, for example, are emitted by a number of plant species. For example, Arey et al. (1991a) identified emissions of these compounds from more than a dozen different agricultural plants in California as well as from Valley Oak and Whitethorn, and Kirstine et al. (1998) reported emissions from grass and clover. Such emissions are enhanced by mechanical damage. For example, Kirstine et al. (1998) reported that the emissions of (3Z)-hexenol and (3Z)-hexenyl acetate increased by three orders of magnitude when the grass and clover were mowed! A second, even larger, emission of these and other organics during the subsequent drying of the plants has been observed (de Gouw et al., 1999).These compounds as well as other volatile Cft aldehydes and alcohols are generated by the biochemical degradation of a-linolenic acid in the plants (Fall, 1999). Similarly, the breakdown of linoleic acid gives n-hexanal and n-hexanol.
Other examples of emissions of larger oxygen-containing organics include camphor, cineole, and thujone which are emitted by California sagebrush (Arey et al., 1995), cineole from pines and eucalyptus (Staudt et al., 1997), the unsaturated alcohol linalool from the blossoms of Valencia orange trees (Arey et al., 1991b) and from certain pines (Kesselmeier et al., 1997; Staudt et al., 1997), and eucalyptol from grass and clover (e.g., Kirstine et al., 1998). Ciccioli et al. (1997) measured emissions in a Mediterranean region known as the Mediterranean Pseudosteppe and found that not only isoprene but also a range of C6-Cl0 aldehydes, linalool, and acetic acid were emitted by the vegetation.
In addition to these larger VOCs, there are biogenic sources of a wide variety of small alcohols, aldehydes, ketones, and acids. For example, emissions of methanol and acetone have been reported from plant leaves, grass, and clover (e.g., MacDonald and Fall, 1993; Nemecek-Marshall et al., 1995; Fall and Benson, 1996; Kirstine et al., 1998). Table 6.25 shows some of the compounds measured in grass and clover emissions (Kirstine et al., 1998). Clearly, a wide variety of oxygen-containing species are emitted from this one source alone. Direct emissions of formaldehyde, ac-etaldehye, and formic and acetic acids have been observed from oaks and pines (Kesselmeier et al., 1997).
Consistent with the direct observation of the emissions are field measurements in remote areas. For example, Fehsenfeld et al. (1992) measured the composition of VOC at two rural locations in the United States, summarized in Fig. 6.27. Alcohols and carbonyl compounds comprise 40-70% of the total. Of these, a significant portion appear to be direct emissions, with methanol being a major contributor to the oxygen-containing portion. Similarly, Singh et al. (1995) re-
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