Biogenic hydrocarbon sources

Table I compares calculated concentrations of benzene, toluene, and ethylbenzene at several locations near the refinery with reported values for typical urban, rural, and remote settings from past EPA studies (Shah and Heyerdahl, 1988). For benzene, refinery impacts at the fenceline were similar to those observed in a rural environment. At the nearby residence, benzene concentrations were similar to those observed in a remote pristine setting. Ethylbenzene impacts were similar to benzene. Toluene impacts were somewhat higher, falling between typical rural and urban air quality. No comparable data were available for xylene. Automobiles and an adjacent power plant contribute some of these chemicals to the air. Biogenic (natural) sources also contribute. In the entire middle Atlantic region, natural sources provide about 40% of airborne hydrocarbons, with a higher percentage in more rural areas like Yorktown (Placet and Streets, 1989).

Interpretation of the aromatic/olefinic hydrocarbon distribution has recently been developed to help characterize hydrocarbon sources. Olefinic compounds are not. ibundant in petroleum thus their predominance would indicate biogenic input (4,11,20,31). 

Interpretation of Hydrocarbon Analyses. Although obtaining accurate hydrocarbon measurements in samples is often a difficult task, it is equally as difficult to determine the sources of hydrocarbons from the data. The following criteria for differentiating petroleum hydrocarbons from biogenic hydrocarbons that have been suggested and applied over the past several years were outlined in (1) and are taken from there. 

The hydrocarbons released into the water and in the sediments by biosynthesis from various life forms or issued from non-marine sources are subjected to physical, chemical and biological transformations, some of these affecting the molecular structure of the more labile compounds. There have been several studies of the modification of hydrocarbons in the ocean, initiated mainly by an attempt to evaluate the persistence of crude oil in the sea. The mechanisms affecting petroleum products are closely related to those for natural hydrocarbons with one important exception many biogenic hydrocarbons are unsaturated compounds and probably disappear rapidly in the water column. This is an important consideration in evaluating the persistent contribution of biogenic hydrocarbons to dissolved and particulate lipids of sea water and sediments. 

The reaction of ozone with alkenes produces OH and other radicals. Work in LACTOZ has provided quantitative data for the yield of OH for a variety of alkenes, including biogenic hydrocarbons. Yields of around 50 % were obtained for the reaction of ozone with typical alkenes, making this a source of atmospheric free radicals, which is especially significant at night-time when photolytic sources are absent. 

Atmospheric hydrocarbons produced by living sources are called biogenic hydrocarbons. Vegetation is the most important natural source of non-methane biogenic compounds. Several hundred different hydrocarbons are released to the atmosphere from vegetation sources. Other natural sources include microorganisms, forest fires, animal wastes, and volcanoes. 

The characterization of a novel series of biomarkers is illustrated with the g 7 2-dialkylalkanes in bitumen from a hydrothermal system on the Mid-Atlantic Ridge. The total bitumen consists of hydrocarbons, a major UCM (unresolved complex mixture of branched and cyclic compounds) and mature biomarkers (e.g. hopanes) (Fig. 13a). The bitumen contains a series of cyclopentylalkanes Cfi 2n) that range from n = 14 to 34, with only even-chained pseudohomologs and a concentration maximum (Cmax) at n = 18. Their source is biogenic, based on the presence of only even-carbon number homologs, but the precursors are unknown. 

Larger hydrocarbons such as the Cl5 sesquiterpenes also have biogenic sources such as sage (Arey et al., 1995). 

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