Encyclopedia of Environmental Science and Engineering, Volume I and II

128 ATMOSPHERIC CHEMISTRY

have shown significant SOA formation from the irradiation of

simulated auto exhaust. Griffin et al. (1999) have shown that

the oxidation of biogenic hydrocarbons can also be important

contributors to SOAs. This work also investigated the role of

individual oxidation pathways, by ozone, nitrate radicals, and

hydroxyl radicals. It was found that each of these oxidants

can be quite important depending on the biogenic hydrocar-

bon with which they are reacting. Figure 7 (Seinfeld, 2002)

shows an example of the partitioning of products of the

ozone reaction with α -pinene between the gas and particulate

phases. From this figure it is clear that the partitioning can

change a lot between the various poly-functional products of

the oxidation of α -pinene.

Jang et al. (2002) suggested that acidic aerosol surfaces

may catalyze heterogeneous reactions that could lead to the

formation of additional SOAs. As we will see in the next sec-

tion, there is considerable potential for having acidic aerosols

present in the atmosphere. The authors present data that sug-

gests larger secondary-aerosol yields in the presence of an

acid seed aerosol than occurs in the presence of a non-acid

seed aerosol. The suggestion is that the acid is capable of

catalyzing the formation of lower-volatility organic products,

maybe through polymerization.

Pandis et al. (1991) have found no significant SOA forma-

tion from the photooxidation of isoprene, due to its small size

and the high volatility of its oxidation products. Significant

SOAs are formed from biogenic hydrocarbons larger than

isoprene. Claeys et al. (2004) suggest that the yield of SOAs

from the photooxidation of isoprene in the Amazonian rain

forest, where NO x is low (
100 ppt), is about 0.4% on a mass

basis. Even with its low particulate yield, since the global

annual isoprene emissions are about 500 Tg per year, the SOAs

from isoprene photooxidation alone could account for about

2 Tg/yr. This is a significant fraction of the Intergovernmental

Panel on Climate Change (Houghton et al., 2001) estimate of

between 8 and 40 Tg/yr of SOAs from biogenic sources. The

oxidation of the other biogenic hydrocarbons are expected to

have much higher SOA yields.

Alkylnitrate

Hydroxyalkylnitrate

Hydroxy carbonyl

Hydroxylalkoxy radical

Hydroxyalkylperoxy radical

as above

as above

+ HO 2

H 2 O

Alkoxy radical

O 2

O 2

O 2

decomposition

+ Alkyl radical

Alkylperoxy radical

O 2 as above

OH

Hydroperoxide

Alkylperoxy radical

NO

NO

NO 2

HO 2

+ O 2

Alkoxy radical

Self

isomerization

OH

OH

stable products with potential

to partition to the aerosol phase

or to further react

n-Alkane

hv

Carbonyl

Carbonyl

Carbonyl

=

FIGURE 6 Generalized mechanism for the photooxidation of an n-alkane. The products shown in boxes are

expected to be relatively stable organic products that might be able to partition into the particulate phase, if they

have sufficiently low vapor pressures. From Seinfeld (2002). With permission.

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