Biogenic Terpenes

Considerable attention has been directed in determining the products from reactions of aliphatic hydrocarbons, aromatic compounds, and unsaturated compounds including biogenic terpenes that exhibit appreciable volatility. These studies have been conducted both in simulation chambers and using natural sunlight in the presence of NO. 

A range of transformation products has been identified from simulated reactions of alkanes. These include alkyl nitrates by reactions that have already been given, but also include a range of hydroxy-carbonyls that are summarized in Table 1.2 (Reisen et al. 2005). 

Monoterpenes are appreciably volatile and are produced in substantial quantities by a range of higher plants and trees. Only summary of examples are given here. 

Photochemical reactions of isoprene (references in Grosjean et al. 1993a). 

Products from reaction of a-pinene with ozone that produced a range of cyclobutane carboxylic acids (Kamens et al. 1999). 

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In addition to the important role biogenic terpenes play in gas-hase chemistry, their impact also extends to heterogeneous air chemistry. Although Went (1960) linked the formation of the blue haze over coniferous forests to the biogenic emission of 20 monoterpenes over 40 years ago, it was not until recently that terpenes received their due attention with respect to their role in secondary organic aerosol (SOA) formation. O Dowd et al. (2002) reported that nucleation events over a boreal forest were driven by the condensation of terpene oxidation products. Formaldehyde (HCHO) is a high-yield product of isoprene oxidation. The short photochemical lifetime of HCHO allows the observation of this trace gas to help constrain isoprene emissions (Shim et al. 2005). 

On a local or regional basis, terpenes may contribute significantly to oxidant problems (16,60,85,86,87, ), making control of anthropogenic hydrocarbons problematical. The question of relative controls on hydrocarbon and NOjj emissions from automobiles and industry has been a matter of longstanding controversy and debate in the US, particularly because of the potential role of biogenic hydrocarbons in oxidant production (16,89,90). This controversy is yet unresolved. 

Steroids (1) are members of a large class of lipid compounds called terpenes that are biogenically derived from the same parent compound, isoprene, C Hg Steroids contain or are derived from the pethydro-l,2-cydopentenophenanthrene ring system (1) and are foimd in a variety of different marine, terrestrial, and synthetic sources. The vast diversity of the natural and synthetic members of this class depends on variations in side-chain substitution (primatily at C17), degree of unsaturation, degree and nature of oxidation, and the stereochemical relationships at the ring jimctions. 

Biosynthesis In contrast to steroids and terpenes, there is no equivalent to the isoprene rule for alkaloids as a useful aid for structure determination and biogenetic investigations. It is generally accepted today that A. are formed in cyclization, condensation, and dimerization reactions from amino acids and biogenic amines with biogenic aldehydes and ketones. 

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