Sources of Data

Ambient gas phase, aerosol-bound and dissolved concentrations of each hydrocarbon in the atmosphere and surface waters of each North American model segment and in the global background were estimated from the current literature. Due to the scarcity of data for the atmospheric petroleum hydrocarbons in the atmosphere bordering North America, the selection of representative distributions of PAH and n-alkanes was developed from the current available literature. For this assessment, petroleum hydrocarbons were defined as n-alkanes with carbon lengths ranging from C10 to C33. To develop an accurate assessment of the contaminant burden to the coastal waters via atmospheric deposition, the various coastal structure and representative contaminant loadings had to be determined. Five zones were assembled based on the degree of urbanization along the zone’s shoreline: (1) urban coastline 0-3 miles from shore (U0-3), (2) urban coastline 3-200 miles from shore (U3-200), (3) rural coastline 0-3 miles from shore (R0-3), (4) rural coastline 3-200 miles from shore (R3-200), and (5) background (BG) contaminant levels that would represent the open ocean. In most cases, adjoining 0-3 and 3-200 mile zones had the same designation (rural or urban) except along the west coast of North America, where are 3-200 zones were ‘rural’ to reflect the predominant westerly air flows off the Pacific Ocean.

Literature on atmospheric hydrocarbons in North America is sparse. This compilation includes those endeavors that have measured concentrations in various selected areas on the United States (Hoff and Chan, 1987; Fraser, 1997; Doskey and Andren, 1986; Foreman and Bidleman, 1990; Ligocki and Pankow, 1989; Mazurek et al., 1991; Simoneit, and Mazurek, 1984). Even fewer atmospheric n-alkane data were available for the North American coast that reported vapor phase alkanes per homologue (Fraser, 1997, 1998; Hoff, 1987). Sampling methods were somewhat consistent throughout the literature. All of the authors use a high-volume air sampler to pull ambient air at specified flow rates through a sample train that contains a glass or quartz fiber filter to retain atmospheric particles. When the vapor phase is collected, a polyurethane plug or PUF is placed beyond the filter in the sampling train. Doskey and Andren (1986) deviated from this format by using XAD-8 resin to collect the vapor phase n-alkanes. Lighter hydrocarbons can also be collected using an evacuated stainless steel canister filled with ambient air and directly injected into a gas chromatograph via sorbent thermal desorption (Fraser, 1997). Sample preparation usually involved the extraction of the filter, PUF or XAD with nonpolar organic solvent by soxhlation or sonication. Extracts may be fractionated over an alumina silicic-acid column (Foreman and Bidleman, 1990; Cotham and Bidleman, 1995) or applied to thin-layer chromatography (Daisey et al., 1981) to separate alkanes. The various forms of instrumentation used in the quantification of hydrocarbons ranged from high performance liquid chromatography coupled with a fluorescence detector (Daisey, 1981; Foreman and Bidleman, 1990) and confirmed by GC/MS (Foreman and Bidleman, 1990) or secondary spectroscopic techniques (Daisey et al., 1981). While Doskey and Andren, (1986) quantified their extracts using gas chromatography equipped with a flame ionization detector, the remainder of the most recent work relies on the resolving power of gas chromatography and the instant confirmation of mass selective detector technology (Hoff and Chan, 1987, Baker and Eisenreich, 1990; Cotham and Bidleman, 1995; Fraser et al., 1997, 1998; Offenberg, 1998; Bamford et al., 1999a; Giglitotti et al., 2000).

The data set acquired from Hoff (1987) collected along the Niagara River between Lake Erie and Ontario, contained values for C16, C22, C24, and C28 n-alkanes in the vapor and particulate phase. Fraser et al., (1997) performed a thorough analysis of the distribution of various hydrocarbons in California during a photochemical smog episode. Aerosol and vapor phase data were taken from San Nicolas Island, Long Beach, central Los Angeles, Azusa, and Claremont. Gaseous and particulate samples were taken using a standard high volume air sampler fitted with a quartz fiber filter (aerosols) and polyurethane plug (vapor). Concurrent gas samples were also taken with 6-L stainless steel canisters. Reported concentrations for the filters, PUF, and canister samples ranged from C18 to C36, from C14 to C28, and from C2 to C13 alkane homologues, respectively. There seems to be some discrepancy between the canister and PUF concentration values from C13 to C14. This may be due to breakthrough of the lighter alkanes in the PUF portion of the high-volume sampler, which consisted of a series of five PUF plugs in series. Separate analysis of the individual PUF plug series showed that the last PUF contained no more than 15 percent of the total alkanes collected on the previous four, which does not explain the factor of 10 difference between the C13 (canister) measurement and the C14 (PUF) measurement. Despite these discrepancies, this data set constitutes the most expansive coverage of the alkane distribution along the urbanized North American coast. The mean of the four coastal California cities, Long Beach, central Los Angeles, Azusa, and Claremont, has been selected to represent the highly impacted coastline of North America in terms of alkane vapor and particulate concentrations (U0-3). San Nicolas Island, which lies approximately 100 km west of the California coast, will constitute the typical atmospheric concentrations offshore (3-200 miles) of a highly urbanized coastline (U3-200).

A second, urbanized coastline alkane distribution was also selected. For the purpose of this analysis Los Angeles, California, is considered to represent an extreme example of an urbanized coastline. Thus, a second alkane distribution, representative of a less urbanized area, was needed. As no well-documented coastal setting was available, this analysis uses data obtained from an interior urban center. Denver,

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