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" Adapted from Kirstine et al. (1998).

" Adapted from Kirstine et al. (1998).

ported relatively high concentrations of methanol and acetone in the free troposphere, at least a portion of which may be due to biogenic emissions. For example, based on a comparison of measurements and model predictions, Wang et al. (1998) propose that biogenic emissions account for about 40% of the acetone on a global basis.

As is expected, there are also a variety of oxygen-containing organics found in rural and remote areas, which are oxidation products of the directly emitted biogenics. For example, in areas with significant isoprene emissions, the oxidation products methyl vinyl ketone (MVK), methacrolein (MACR), and 3-methyl-furan are also typically present (e.g., Yokouchi et al., 1993; Montzka et al., 1993, 1995; Biesenthal et al., 1998; Helmig et al., 1998b). Biesenthal and Shepson (1997) suggest that MVK and MACR may also be generated by automobile exhaust, based on the correlation of these compounds with CO in an urban area.

Indeed, separating out direct emissions and the formation by oxidation in air of other biogenics is not straightforward. For example, 6-methyl-5-hepten-2-one (Fig. 6.26) has been reported in air in different locations by a number of groups (e.g., Ciccioli et al., 1993a, 1993b; K├Ânig et al., 1995; Helmig et al., 1996). However, the reaction of 03 with organics containing the structural group (CH3)2C=CHCH2CH2C(CH3)=C-also gives this compound (e.g., Fruekilde et al., 1998), as expected from the earlier discussion of mechanisms of ozonolysis and studies of struc

Alcohols 18%

Alkanes 10%

Aromatics 2%

Alabama

Total carbon: 76 ppbC

Carbonyls 22%

Alcohols 18%

Alkanes 10%

Aromatics 2%

Carbonyls 22%

Natural NMHCs 48%

Alcohols 35%

Colorado

Total carbon: 29 ppbC

Alkanes 22%

Alkanes 22%

Alcohols 35%

Aromatics 4%

Natural NMHCs 6%

Carbonyls 33%

FIGURE 6.27 Distribution of organics observed in rural areas in Alabama and Colorado, respectively (adapted from Fehsenfeld et al., 1992).

Aromatics 4%

Natural NMHCs 6%

Carbonyls 33%

FIGURE 6.27 Distribution of organics observed in rural areas in Alabama and Colorado, respectively (adapted from Fehsenfeld et al., 1992).

turally similar compounds such as linalool, (CH 3)2C = CHCH 2CH 2C(CH 3)(OH)CH = CH 2 (e.g., Shu et al., 1997). For example, ozonolysis of squalene (Fig. 6.28) was demonstrated to form gaseous 6-methyl-5-hepten-2-one, acetone, and geranyl acetone, respectively. 4-Oxopentanal was also formed from the further oxidation of 6-methyl-5-hepten-2-one (Grosjean et al., 1996; Smith et al., 1996; Fruekilde et al., 1998). These compounds were also observed when leaves of common vegetation found in the Mediterranean were exposed to 03. Furthermore, these products could be formed from the reaction of glass wool that had been in contact with human skin, which also contains squalene as a lipid; such observations suggest the importance of avoiding contamination of samples during measurements of biogenic organics.

In short, while a variety of oxygen-containing biogenic organics have been observed to be generated from plants and most are likely direct emissions, care must be taken to distinguish such direct emissions from possible formation from oxidation of larger biogenic hydrocarbons and/or, in some cases, contamination during sample handling.

2. Chemistry a. Biogenic Hydrocarbons

Although some of the biogenic VOCs are relatively simple compounds such as ethene, most are quite complex in structure (e.g., Figs. 6.22 and 6.26). Furthermore, they tend to be unsaturated, often with multiple double bonds. As a result, they are very reactive (see Chapter 16.B) with OH, 03, N03, and CI atoms (e.g., Atkinson et al., 1995a). In addition, because they are quite large and of relatively low volatility, their polar oxidation products are even less volatile. This makes elucidating reaction mechanisms and quantifying product yields quite difficult. For a review of this area, see Atkinson and Arey (1998).

Squalene

Squalene

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