Colorado was thus selected as being reflective of a more moderately impacted urban area. Foreman and Bidleman (1990) characterized the Denver atmosphere for alkanes. The authors note the major sources of airborne contaminants in the Denver area may be due to coal-driven electric power generation and wood burning, unlike Los Angeles, where the mean concentrations found by Fraser (1997) in southern California incorporate several alkane sources, from dense automobile traffic from major freeways to active shipping ports. For the purposes of this study the southern California levels will be retained as a highly impacted urban coastline with the note that nonrural, urban coastlines may more accurately be reflected in the Denver alkane distribution.

The rural sectors of the coastline are expected to have less of a petrogenic alkane signal. Doskey and Andren (1986) conducted a study in northern Wisconsin approximately 200 km from urbanization (Green Bay, Wisconsin) and 4.5 mils from any major roadway. Similar methods of sampling were performed, as mention above, using a high-volume sampler fitted with a filter and gaseous sorbent. The PUF was replaced with XAD-2 resin for the collection of vapor alkane constituents. Winter total alkanes were taken from this study to minimize the biogenic contribution via plant waxes from spring pollen that may falsely elevate hydrocarbon concentrations in a rural environment. Rural offshore (R3-200 and BG) particulate phase alkane levels were obtained from an expansive aerosol characterization endeavor performed along the western U.S. coast by Simoneit (1982). Aerosol samples were collected from Crater Lake, Oregon. These particulate values were chosen due to the low value obtained and the remote nature of the sampling site. Totals were also only reported for this particular sampling site. The distribution was back calculated using the alkane distribution from Fraser et al. (1997).

The rural areas consisted of the northern Chesapeake Bay (Offenberg, 1998; Leister and Baker, 1994; Dickhut and Gustafson, 1995) Isle Royal, Lake Superior (McVeety and Hites, 1988); and Sandy Hook, New Jersey (Gigliotti et al., 2000). Isle Royal, the main island in Lake Superior, represents a remote signal; the winds are predominantly from the west, and the nearest urban center is approximately 300 km from the sampling site. Sandy Hook, New Jersey, lies on a peninsula approximately 10 km south of New York and 30 km southeast of Newark, lending to direct urban influence and from elevated populations to the west, south, and southwest (Gigliotti et al., 2000). The rural coastal compartment (R0-3) can be best characterized by the sampling performed on the eastern shore of the northern Chesapeake Bay (Offenberg, 1998). No observable urban influence (via Baltimore) was observed from air parcels from Baltimore, the nearest urban center. Therefore this station has minimal urban influence, representative of a rural coastline. The offshore rural or background values (BG) were selected from Ellsmere Island and Alert, Canada, and Tangish in the Yukon Territories (Hallsall et al., 1997; Fellin et al., 1996); Barrow, Alaska (Daisey et al., 1981); Narwahl Island (Daisey et al., 1981); and Isle Royal, Lake Superior (McVeety and Hites, 1988).

VOC Emissions

Crude oils contain a variety of volatile organic compounds (VOC) that evaporate quickly into the atmosphere. Significant quantities of VOC can be released during cargo loading and unloading, during transport, and during crude oil washing operations on board crude oil carriers. Methane makes up approximately 80 percent of these VOC emissions. Methane released into the atmosphere will not deposit, and while it may be a “greenhouse gas” concern, does not appreciably impact the volume of oil entering the sea. Of the remaining VOC, only a small fraction is likely deposited to the sea, as detailed later in this chapter.

Precise measurement of VOC loss from tankers is difficult. The best measure currently available is derived from the fact that cargo insurance companies will typically exclude coverage for loss of 0.5 percent of a crude oil cargo as normal variation between loading and unloading ports. This is the upper range of potential uncovered loss, and it is generally assumed that average loss is probably about half that amount or 0.25 percent most of which can be attributed either to cargo tank gauging variations or, more likely, to VOC emissions.

Approximately 3.3 billion tonnes of cargo oil was moved on tank vessels in 1999. Thus, VOC emissions during crude oil shipment can be estimated as follows:

billion tonnes • 0.0025 tonnes lost/tonne shipped = 8,250,000 tonnes

The VOC emissions (heavier than butane) are therefore:

8,250,000 tonnes • 0.2 non-methane tonnes/tonne = 1,650,000 tonnes

This is a worldwide estimate from shipping. The best estimate of VOC emissions (heavier than butane) from platforms in coastal North American waters is 60,000 tonnes, and the worldwide estimate is 649,000 tonnes (see Section B).

Deposition Calculations

Deposition models were used to estimate depositional fluxes (mass deposited per unit area per year) from these concentrations, and these fluxes were integrated over the area of each model segment to calculate the annual loading. Equations used in these calculations are shown in Figure H-1 and have been used extensively to estimate exchange of semivolatile organic chemicals between the atmosphere and surface waters (Baker and Eisenreich, 1990; Iwata et al., 1993; Achman et al., 1993; Hornbuckle et al., 1994; 1995; Nelson et al., 1998; Bamford et al., 1999a,b; Zhang et al.,

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