Carbon #

FIGURE 9.44 Concentrations of n-alkanoic acids (parts a-c) and n-alkanols (parts d-f) as a function of carbon number from (a, d) a particle sample collected in Jos, Nigeria, (b, e) vegetation wax in the Jos area, and (c, f) particles collected over the Atlantic Ocean (adapted from Simoneit et al., f988).

Similar biogenic organics and their distributions have been noted in a variety of locations, including the Amazon, China, and over the south Atlantic area (e.g., see Simoneit et al., 1990, 1991a, 1991b). They are also found in urban areas but generally comprise a small fraction of the total aerosol mass; for example, they are about 1-3% of the total fine particulate matter in the Los Angeles area (Hildemann et al., 1996).

Some monounsaturated fatty acids such as Cl6:l and C18:1 (the first number is the number of carbon atoms and the second number is the number of double bonds) are also produced biogenically, in addition to some diunsaturated species (e.g., see Sicre et al., 1990a). However, because they react fairly rapidly with tropo-spheric gases such as 03, OH, and NO-,, their concentrations are highly variable and also dependent on storage conditions after sampling. A variety of dicar-

boxylic acids have been observed in aerosols in the Antarctic and attributed to the oxidation of unsaturated fatty acids emitted from marine microalgae (Kawamura et al., 1996a). Also found in small amounts are iso- and anteiso-alkanoic acids (the latter have a CH3 group on the third carbon from the end, rather than the second, as is the case for the iso compounds; i.e., the anteiso compounds are 3-methyl substituted and the iso compounds are 2-methyl substituted).

Figure 9.45 shows the structures of a few of the other organics found in tropospheric aerosols and be lieved to be of biogenic origin. The reader is referred to summaries by Simoneit (1989) and Graedel et al. (1986) for more detailed lists of compounds and descriptions of sources.

In short, biogenic processes produce complicated mixtures of organics that are structurally large and have sufficiently small vapor pressures that they are found primarily or exclusively in airborne particles. While the classes of compounds discussed are typical, there are a variety of other compounds found as well, depending on the particular location, time, etc. For

6, 10, 14-Trimethylpentadecan-2-one

Stigmasterol (C29)
Cholesterol, R=H(C27) Campesterol, R=CH3(C28) ß-Sitosterol, R=C2H5(C29)

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