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26 ppm

1-Octene

Heptanoic acid

~9 x 10 3

~12 ppm

1-Decene

Nonanoic acid

-6 x 10 4

~0.8 ppm

1-Tridecene

Dodecanoic acid

10 5

13 ppb

Cyclopentene

Glutaric acid

2 x 10 7

-0.3 ppb

Cyclohexene

Adipic acid

6 x 10 8

0.08 ppb

1,7-Octadiene

Adipic acid

6 x 10 8

0.08 ppb

3-Methylcyclohexene

Methyladipic acid

~2 x 10 8

-0.03 ppb

" From Grosjean and Friedlander (1980).

6 Lower limits calculated assuming complete conversion of the precursor to the least volatile of the possible products.

" From Grosjean and Friedlander (1980).

6 Lower limits calculated assuming complete conversion of the precursor to the least volatile of the possible products.

indeed formed. Schwartz (1974) carried out one of the first detailed product analyses of the organic component of particles formed in the oxidation of ~ 10 ppm of cyclohexene, toluene, or a-pinene in the presence of ~ 2-5 ppm NOx in air. The portion of the aerosol that was extractable into methylene chloride but that was insoluble in water was analyzed to focus on the aerosol products that had not undergone extensive secondary oxidations, in the hopes that their mechanisms of formation could be more easily related to the parent hydrocarbons. A variety of multifunctional products were tentatively identified in these studies, with structures analogous to those observed in ambient aerosols.

Since then, a number of studies of model systems have confirmed that dialkenes, cyclic alkenes, and aro-matics form substituted monocarboxylic acids, dicar-boxylic acids, and organic nitrates in the condensed phase (e.g., see O'Brien et al., 1975a; Grosjean and Friedlander, 1979; Dumdei and O'Brien, 1984; fzumi and Fukuyama, f990; and Forstner et al., f 997a, 1997b). For example, Table 9.21 shows the products identified in particles formed in the 1-octene- and 1-decene-NOx-ambient air systems. In both bases, only ~40% of the total particle mass could be identified, and the yields shown in Table 9.2f are those relative to the total identified compounds. That is, the absolute product yields are about factor of 2.5 larger. As expected from the known oxidation mechanisms (see Chapter 6.E), heptanal and heptanoic acid are the major condensed-phase oxidation products of 1-octene and nonanal and nonanoic acid from f-decene (see Problem 4). The mechanism of formation of the fura-nones, which are formed in relatively high yields, is not known. Secondary oxidation of the aldehydes is one possibility:

o2,no

C3H7(CH2)3CHO OH-C3H7(CH2)3C(0)0

cyclization

However, Forstner et al. (1997a) also point out that the formation of furanones from the cyclization of -y-hydroxycarboxylic acids (e.g., C3H7CH(OH)CH2CH2COOH) in the condensed phase is well known. If y-hydroxycarboxylic acids are formed from OH reactions with the alkenes, the furanones can be formed from this cyclization in the particles. However, Forstner and co-workers also indicated that such a reaction could occur during sample workup and hence the true yields of the furanones could be significantly smaller.

Not surprisingly, based on their complex (and not yet well understood) oxidation mechanisms (see Chapter 6.G), aromatic hydrocarbons are efficient precursors of secondary organic aerosol particles, SOA. Indeed, Odum et al. (1997a) have shown, based on smog chamber studies, that the secondary organic aerosol formation from whole gasoline vapor can be due essentially totally to the aromatic content. For example, Table 9.22 lists some of the major condensed-phase products identified in particles formed in the VOC-NOx oxidations in air of some simple aromatic hydrocarbons. Note that the yields are expressed as a percentage of the total identifiable mass. Only 15-30% of the extractable mass that was eluted through the GC could be identified in each case, so that the absolute

TABLE 9.21 Major Products Observed in the Particles Formed in the VOC - NOt Oxidations in Air of 1-Octene and 1-Decene"

Percentages of total identifiable

Reactant Products Structure products

Percentages of total identifiable

Reactant Products Structure products

1-Octene

Heptanal

CH,(CH2),CHO

0 0

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